Carlos Egues
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Research Interests
 Semiconductor Spintronics
 Density Functional Theory + k.p approaches
 Spin orbit interactions
Permanent Adress
Departamento de Fisica e InformaticaInstituto de Fisica de Sao Carlos
Universidade de Sao Paulo
Sao Carlos/Sao Paulo
Brazil
Publications
Show all abstracts.1.  Closedform weak localization magnetoconductivity in quantum wells with arbitrary Rashba and Dresselhaus spinorbit interactions 
D. Catalina Marinescu, Pirmin J. Weigele, Domink Zumbuhl, and J. Carlos Egues. (submitted) arXiv:1811.04488
We derive a closedform expression for the weak localization (WL) corrections to the magnetoconductivity of a 2D electron system with arbitrary Rashba α and Dresselhaus β (linear) and β3 (cubic) spinorbit interaction couplings, in a perpendicular magnetic field geometry. In a system of reference with an inplane ẑ axis chosen as the high spinsymmetry direction at α=β, we formulate a new algorithm to calculate the three independent contributions that lead to WL. The antilocalization is counterbalanced by the term associated with the spinrelaxation along ẑ , dependent only on α−β. The other term is generated by two identical scattering modes characterized by spinrelaxation rates which are explicit functions of the orientation of the scattered momentum. Excellent agreement is found with data from GaAs quantum wells, where in particular our theory correctly captures the shift of the minima of the WL curves as a function of α/β. This suggests that the anisotropy of the effective spin relaxation rates is fundamental to understanding the effect of the SO coupling in transport.
 
2.  Zitterbewegung and bulkedge LandauZener tunneling in topological insulators 
Gerson J. Ferreira, Renan P. Maciel, Poliana H. Penteado, and J. Carlos Egues. Physical Review B 98, 165120 (2018); arXiv:1807.07390.
We investigate the ballistic Zitterbewegung dynamics and the LandauZener tunneling between edge and bulk states of wave packets in twodimensional topological insulators. In bulk, we use the Ehrenfest theorem to show that an external inplane electric field not only drifts the packet longitudinally, but also induces a transverse finite sidejump for both trivial and topological regimes. For finite ribbons of width W, we show that the LandauZener tunneling between bulk and edge states vanishes for large W as their electric fieldinduced coupling decays with W−3/2. This is demonstrated by expanding the timedependent Schr\"odinger equation in terms of Houston states. Hence we cannot picture the quantum spin Hall states as arising from the Zitterbewegung bulk trajectories `leaking' into the edge states as proposed in Phys. Rev. B 87, 161115 (2013).
 
3.  Paradoxical extension of the edge states across the topological phase transition due to emergent approximate chiral symmetry in a quantum anomalous Hall system (Editors' Suggestion) 
Denis R. Candido, Maxim Kharitonov, J. Carlos Egues, and Ewelina Hankiewicz. Physical Review B 98, 161111(R) (2018); arXiv:1807.05111.
We present a paradoxical finding that, in the vicinity of a topological phase transition in a quantum anomalous Hall system (Chern insulator), topology nearly always (except when the system obeys chargeconjugation symmetry) results in a significant extension of the edgestate structure beyond the minimal one required to satisfy the Chern numbers. The effect arises from the universal gapless linearinmomentum Hamiltonian of the nodal semimetal describing the system right at the phase transition, whose form is enforced by the change of the Chern number. Its emergent approximate chiral symmetry results in an edgestate band in the vicinity of the node, in the region of momenta where such form is dominant. Upon opening the gap, this edgestate band is modified in the gap region, becoming "protected" (connected to the valence bulk band with one end and conduction band with the other) in the topologically nontrivial phase and "nonprotected" (connected to either the valence or conduction band with both ends) in the trivial phase. The edgestate band persists in the latter as long as the gap is small enough.
 
4.  Blurring the boundaries between topological and nontopological phenomena in dots 
Denis R. Candido, Michael E. Flatte, and J. Carlos Egues. Physical Review Letters 121, 256804 (2018); arXiv:1803.02936.
We investigate the electronic and transport properties of topological and trivial InAs1−xBix quantum dots (QDs). By considering the rapid band gap change within valence band anticrossing theory for InAs1−xBix, we predicted that Bialloyed quantum wells become ∼30meV gapped 2D topological insulators for well widths d>6.9nm (x=0.15) and obtain the k.p parameters of the corresponding BernevigHughesZhang (BHZ) model. We analytically solve this model for cylindrical confinement via modified Bessel functions. For nontopological dots we find "geometrically protected" discrete helical edgelike states, i.e., Kramers pairs with spinangularmomentum locking, in stark contrast with ordinary InAs QDs. For a conduction window with four edge states, we find that the twoterminal conductance G vs. the QD radius R and the gate Vg controlling its levels shows a double peak at 2e^2/h for both topological and trivial QDs. In contrast, when bulk and edgestate Kramers pairs coexist and are degenerate, a singlepeak resonance emerges. Our results blur the boundaries between topological and nontopological phenomena for conductance measurements in small systems such as QDs. Bibased BHZ QDs should also prove important as hosts to edge spin qubits.
 
5.  Symmetry Breaking of the Persistent Spin Helix in Quantum Transport 
Pirmin J. Weigele, D. C. Marinescu, Florian Dettwiler, Jiyong Fu, Shawn Mack, J. Carlos Egues, David D. Awschalom, and Dominik M. Zumbuhl. (submitted) arXiv:1801.05657
We exploit the highsymmetry spin state obtained for equal Rashba and linear Dresselhaus interactions to derive a closedform expression for the weak localization magnetoconductivity  the paradigmatic signature of spinorbit coupling in quantum transport. The small parameter of the theory is the deviation from the symmetry state introduced by the mismatch of the linear terms and by the cubic Dresselhaus term. In this regime, we perform quantum transport experiments in GaAs quantum wells. Top and back gates allow independent tuning of the Rashba and Dresselhaus terms in order to explore the brokensymmetry regime where the formula applies. We present a reliable twostep method to extract all parameters from fits to the new expression, obtaining excellent agreement with recent experiments. This provides experimental confirmation of the new theory, and advances spinorbit coupling towards a powerful resource in emerging quantum technologies.
 
6.  Stretchable persistent spin helices in GaAs quantum wells 
Florian Dettwiler, Jiyong Fu, Pirmin J. Weigele, Shawn Mack, J. Carlos Egues, David D. Awschalom, and Dominik Zumbuhl. Physical Review X 7, 031010 (2017); arXiv:1702.05190.
The Rashba and Dresselhaus spinorbit (SO) interactions in 2D electron gases act as effective magnetic fields with momentumdependent directions, which cause spin decay as the spins undergo arbitrary precessions about these randomlyoriented SO fields due to momentum scattering. Theoretically and experimentally, it has been established that by finetuning the Rashba α and Dresselhaus β couplings to equal {\it fixed} strengths α=β, the total SO field becomes unidirectional thus rendering the electron spins immune to dephasing due to momentum scattering. A robust persistent spin helix (PSH) has already been experimentally realized at this singular point α=β. Here we employ the suppression of weak antilocalization as a sensitive detector for matched SO fields together with a technique that allows for independent electrical control over the SO couplings via top gate voltage VT and back gate voltage VB. We demonstrate for the first time the gate control of β and the {\it continuous locking} of the SO fields at α=β, i.e., we are able to vary both α and β controllably and continuously with VT and VB, while keeping them locked at equal strengths. This makes possible a new concept: "stretchable PSHs", i.e., helical spin patterns with continuously variable pitches P over a wide parameter range. The extracted spindiffusion lengths and decay times as a function of α/β show a significant enhancement near α/β=1. Since within the continuouslocking regime quantum transport is diffusive (2D) for charge while ballistic (1D) for spin and thus amenable to coherent spin control, stretchable PSHs could provide the platform for the much heralded longdistance communication ∼8−25 μm between solidstate spin qubits.
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7.  Giant edge spin accumulation in a symmetric quantum well with two subbands (Editor's choice) 
Alexander Khaetskii and J. Carlos Egues. Europhysics Letters 118, 57006 (2017); arXiv:1602.00026.
We have studied the edge spin accumulation in a high mobility twodimensional electron gas formed in a symmetric well with two subbands. This study is strongly motivated by the recent experiment of Hernandez et al. [Phys. Rev. B {\bf 88}, 161305(R) (2013)] who demonstrated the spin accumulation near the edges of a bilayer symmetric GaAs structure in contrast to no effect in a singlelayer configuration. The intrinsic mechanism of the spinorbit interaction we consider arises from the coupling between two subband states of opposite parities. We obtain a parametrically large magnitude of the edge spin density for the twosubband sample as compared to the usual singlesubband structure. We show that the presence of a gap in the system, i.e., the energy separation Δ between the two subband bottoms, changes drastically the picture of the edge spin accumulation. Thus one can easily proceed from the regime of weak spin accumulation to the regime of strong one by varying the Fermi energy (electron density) and/or Δ. We estimate that by changing the gap Δ from zero up to 1÷2 K, the magnitude of the effect changes by three orders of magnitude. This opens up the possibility for the design of new spintronic devices.
 
8.  Persistent Skyrmion Lattice of NonInteracting Electrons with SpinOrbit Coupling 
Jiyong Fu, Poliana H. Penteado, Marco Hachiya, Daniel Loss, and J. Carlos Egues. Physical Review Letters 117, 226401 (2016); arXiv:1507.00811.
A persistent spin helix (PSH) is a robust helical spindensity pattern arising in disordered 2D electron gases with Rashba α and Dresselhaus β spinorbit (SO) tuned couplings, i.e., α=±β. Here we investigate the emergence of a Persistent Skyrmion Lattice (PSL) resulting from the coherent superposition of PSHs along orthogonal directions  crossed PSHs  in wells with two occupied subbands ν=1,2. Our calculation shows that the Rashba αν and Dresselhaus βν couplings can be simultaneously tuned to equal strengths but opposite signs, e.g., α1=β1 and α2=−β2. In this regime and away from band anticrossings, our noninteracting electron gas sustains a topologically nontrivial skyrmionlattice spindensity excitation, which inherits the robustness against timereversal conserving perturbations from its underlying crossed PSHs. We find that the spin relaxation rate due to the interband SO coupling is comparable to that of the cubic Dresselhaus term as a mechanism for the PSL decay. Near the anticrossing, the strong interbandinduced spin mixing leads to unusual spin textures along the energy contours beyond those of the RahsbaDresselhaus bands. We consider realistic GaAs and InSb wells for possible experiments.
 
9.  Trappedion Lissajous trajectories by engineering Rashba and Dresselhaustype spinorbit interactions in a Paul trap 
R. F. Rossetti, G. D. de Moraes Neto, J. C. Egues, and M.H.Y. Moussa. Europhys. Lett. 115, 53001 (2016); arXiv:1502.07298.
Here we present a protocol for generating Lissajous curves with a trapped ion by engineering Rashba and the Dresselhaustype spinorbit interactions in a Paul trap. The unique anisotropic Rashba αx, αy and Dresselhaus βx, βy couplings afforded by our setup also enables us to obtain an "unusual" Zitterbewegung, i.e., the semiconductor analog of the relativistic trembling motion of electrons, with cycloidal trajectories in the absence of magnetic fields. We have also introduced bounded SO interactions, confined to an upperbound vibrational subspace of the Fock states, as an additional mechanism to manipulate the Lissajous motion of the trapped ion. Finally, we accounted for dissipative effects on the vibrational degrees of freedom of the ion and find that the Lissajous trajectories are still robust and well defined for realistic parameters.
 
10.  Interaction effects on a Majorana zero mode leaking into a quantum dot 
David A. RuizTijerina, Edson Vernek, Luis G. G. V. Dias da Silva, and J. Carlos Egues. Phys. Rev. B 91, 115435 (2015); arxiv:1412.1851.
We have recently shown [Phys. Rev. B {\bf 89}, 165314 (2013)] that a noninteracting quantum dot coupled to a 1D topological superconductor can sustain a Majorana mode even when the dot charge is removed by a gate voltage. This is due to the Majorana bound state of the wire leaking into the quantum dot. Here we extend this previous work by investigating the lowtemperature quantum transport through an {\it interacting} quantum dot connected to source and drain leads and sidecoupled to a topological wire. We explore the signatures of a Majorana zeromode leaking into the quantum dot for a wide range of dot parameters, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. We observe the interplay between the Majorana mode and the Kondo effect for different dotwire coupling strengths, gate voltages and Zeeman fields. Our results show that a "0.5" conductance signature appears in the dot due to the presence of the Majorana mode, and that it persists for a wide range of dot parameters. The Kondo effect, on the other hand, is suppressed by both Zeeman fields and gate voltages. We show that the zerobias conductance as a function of the magnetic field follows a wellknown universality curve. This can be measured experimentally, and we propose that the universal conductance drop followed by a persistent conductance of 0.5e2/h are evidence for the presence of MajoranaKondo physics. These results confirm that the Majorana signature in the dot remains even in the presence of the Kondo effect.
 
11.  Spinorbit interaction in GaAs wells: from one to two subbands 
Jiyong Fu and J. Carlos Egues. Phys. Rev. B 91, 075408 (2015); arxiv:1410.7358.
We investigate the Rashba and Dresselhaus spinorbit (SO) couplings in GaAs quantum wells in the range of well widths w allowing for a transition of the electron occupancy from one to two subbands. By performing a detailed PoissonSchr\"odinger selfconsistent calculation, we determine all the intra and intersubband Rashba (α1, α2, η) and Dresselhaus (β1, β2, Γ) coupling strengths. For relatively narrow wells with only one subband occupied, our results are consistent with the data of Koralek \emph{et al.} [Nature \bfs{48}, 610 (2009)], i.e., the Rashba coupling α1 is essentially independent of w in contrast to the decreasing linear Dresselhaus coefficient β1. When we widen the well so that the second subband can also be populated, we observe that α2 decreases and α1 increases, both almost linearly with w. Interestingly, we find that in the parameter range studied (i.e., very asymmetric wells) α2 can attain zero and change its sign, while α1 is always positive. In this doubleoccupancy regime of w's, β1 is mostly constant and β2 decreases with w (similarly to β1 for the singleoccupancy regime). On the other hand, the intersubband Rashba coupling strength η decreases with w while the intersubband Dresselhaus Γ remains almost constant. We also determine the persistentspinhelix symmetry points, at which the Rashba and the renormalized (due to cubic corrections) linear Dresselhaus couplings in each subband are equal, as a function of the well width and doping asymmetry. Our results should stimulate experiments probing SO couplings in multisubband wells.
 
12.  Electrical spin protection and manipulation via gatelocked spinorbit fields 
Florian Dettwiler, Jiyong Fu, Pirmin J. Weigele, Shawn Mack, J. Carlos Egues, David D. Awschalom, and Dominik Zumbuhl. arXiv:1403.3518
The spinorbit (SO) interaction couples electron spin and momentum via a relativistic, effective magnetic field. While conveniently facilitating coherent spin manipulation in semiconductors, the SO interaction also inherently causes spin relaxation. A unique situation arises when the Rashba and Dresselhaus SO fields are matched, strongly protecting spins from relaxation, as recently demonstrated. Quantum computation and spintronics devices such as the paradigmatic spin transistor could vastly benefit if such spin protection could be expanded from a single point into a broad range accessible with insitu gatecontrol, making possible tunable SO rotations under protection from relaxation. Here, we demonstrate broad, independent control of all relevant SO fields in GaAs quantum wells, allowing us to tune the Rashba and Dresselhaus SO fields while keeping both locked to each other using gate voltages. Thus, we can electrically control and simultaneously protect the spin. Our experiments employ quantum interference corrections to electrical conductivity as a sensitive probe of SO coupling. Finally, we combine transport data with numerical SO simulations to precisely quantify all SO terms.
 
13.  Allelectron topological insulator in InAs double wells 
Sigurdur I. Erlingsson and J. Carlos Egues. Phys. Rev. B 91, 035312 (2015); arXiv:1312.2034.
We show that electrons in ordinary IIIV semiconductor double wells with an inplane modulating periodic potential and inter well spinorbit interaction are tunable Topological Insulators (TIs). Here the essential TI ingredients, namely, band inversion and the opening of an overall bulk gap in the spectrum arise, respectively, from (i) the combined effect of the double well evenodd state splitting $\Delta_{SAS}$ together with the superlattice potential and (ii) the interband Rashba spinorbit coupling $\eta$. We corroborate our exact diagonalization results by an analytical nearlyfree electron description that allows us to derive an effective BernevigHughesZhang (BHZ) model. Interestingly, the gatetunable $\Delta_{SAS}$ drives a topological phase transition featuring a discontinuous Chern number at $\Delta_{SAS}\sim 5.4$\, meV. Finally, we explicitly verify the bulkedge correspondence by considering a strip configuration and determining not only the bulk bands in the nontopological and topological
phases but also
the
edge states and their Diraclike spectrum in the topological phase. The edge electronic densities exhibit peculiar spatial oscillations as they decay away into the bulk. For concreteness, we present our results for InAsbased wells with realistic parameters.
 
14.  Subtle leakage of a Majorana mode into a quantum dot (Editors' Suggestion) 
E. Vernek, P.H. Penteado, A. C. Seridonio, and J. C. Egues. (former title: Majorana sneakily leaking into a quantum dot connected to a Kitaev wire) Phys. Rev. B 89, 165340 (2014); arXiv:1308.0092.
We investigate quantum transport through a quantum dot connected to source
and drain leads and side coupled to a topological superconducting nanowire
(Kitaev chain) sustaining Majorana end modes. Using a recursive Green's
function approach, we determine the local density of states of the system and
find that the end Majorana mode of the wire leaks into the dot thus emerging as
a unique dot level pinned to the Fermi energy $\e_F$ of the leads. Quite
surprisingly, this resonance pinning, resembling in this sense a "Kondo
resonance", occurs even when the gatecontrolled dot level $\e_\text{dot}(V_g)$
is far above or far below $\e_F$. The calculated conductance $G$ of the dot
exhibits an unambiguous signature for the Majorana end mode of the wire: in
essence, an offresonance dot [$\e_\text{dot}(V_g)\neq \e_F$], which should
have G=0, shows instead a conductance $e^2/2h$ over a wide range of $V_g$, due
to this leaked mode into the dot. Interestingly, this pinning effect only
occurs when the dot level is coupled to a Majorana mode; ordinary fermionic
modes in the wire simply split and broaden (if a continuum) the dot level. We
discuss three experimental scenarios to probe the Majorana modes in wires via
these leaked/pinned dot modes.
 
15.  Ballistic spin resonance in multisubband quantum wires (Editors' suggestion) 
Marco O. Hachiya, Gonzao Usaj, and J. Carlos Egues. Phys. Rev. B 89, 125310 (2014); arXiv:1310.3707.
Ballistic spin resonance was experimentally observed in a quasionedimensional wire by Frolov et al. [Nature (London) 458, 868 (2009)]. The spin resonance was generated by a combination of an external static magnetic field and the oscillating effective spinorbit magnetic field due to periodic bouncings of the electrons off the boundaries of a narrow channel. An increase of the D'yakonovPerel spin relaxation rate was observed when the frequency of the spinorbit field matched that of the Larmor precession frequency around the external magnetic field. Here we develop a model to account for the D'yakonovPerel mechanism in multisubband quantum wires with both the Rashba and Dresselhaus spinorbit interactions. Considering elastic spinconserving impurity scatterings in the timeevolution operator (Heisenberg representation), we extract the spin relaxation time by evaluating the time dependent average of the spin operators. The magnetic field dependence of the nonlocal voltage, which is related to the spin relaxation time behavior, shows a wide plateau, in agreement with the experimental observation. This plateau arises due to injection in higher subbands and smallangle scattering. In this quantum mechanical approach, the spin resonance occurs near the spinorbit induced energy anticrossings of the quantum wire subbands with opposite spins. We also predict anomalous dips in the spin relaxation time as a function of the magnetic field in systems with strong spinorbit couplings.
 
16.  Nonmonotonic spin relaxation and decoherence in graphene quantum dots with spinorbit interactions 
Marco O. Hachiya, Guido Burkard, and J. Carlos Egues. Phys. Rev. B 89, 115427 (2014); arXiv:1307.4668.
We investigate the spin relaxation and decoherence in a singleelectron
graphene quantum dot with Rashba and intrinsic spinorbit interactions. We
derive an effective spinphonon Hamiltonian via the SchriefferWolff
transformation in order to calculate the spin relaxation time T_1 and
decoherence time T_2 within the framework of the BlochRedfield theory. In this
model, the emergence of a nonmonotonic dependence of T_1 on the external
magnetic field is attributed to the Rashba spinorbit couplinginduced
anticrossing of opposite spin states. A rapid decrease of T_1 occurs when the
spin and orbital relaxation rates become comparable in the vicinity of the
spinmixing energylevel anticrossing. By contrast, the intrinsic spinorbit
interaction leads to a monotonic magnetic field dependence of the spin
relaxation rate which is caused solely by the direct spinphonon coupling
mechanism. Within our model, we demonstrate that the decoherence time T_2 ~ 2
T_1 is dominated by relaxation processes for the electronphonon coupling
mechanisms in graphene up to leading order in the spinorbit interaction.
Moreover, we show that the energy anticrossing also leads to a vanishing pure
spin dephasing rate for these states for a superOhmic bath.
 
17.  Dimensionality effects in the LDOS of ferromagnetic hosts probed via STM: spinpolarized quantum beats and spin filtering 
A. C. Seridonio, S. C. Leandro, L. H. Guessi, E. C. Siqueira, F. M. Souza, E. Vernek, M. S. Figueira, and J. C. Egues. Phys. Rev. B 87, 125104 (2013); arXiv:1211.3406.
We theoretically investigate the local density of states (LDOS) probed by a
STM tip of ferromagnetic metals hosting a single adatom and a subsurface
impurity. We model the system via the twoimpurity Anderson Hamiltonian. By
using the equation of motion with the relevant Green functions, we derive
analytical expressions for the LDOS of two host types: a surface and a quantum
wire. The LDOS reveals Friedellike oscillations and Fano interference as a
function of the STM tip position. These oscillations strongly depend on the
host dimension. Interestingly, we find that the spindependent Fermi wave
numbers of the hosts give rise to spinpolarized quantum beats in the LDOS.
While the LDOS for the metallic surface shows a damped beating pattern, it
exhibits an opposite behavior in the quantum wire. Due to this absence of
damping, the wire operates as a spatially resolved spin filter with a high
efficiency.
 
18.  Helical edge states in multiple topological mass domains 
P. Michetti, P. H. Penteado, J. C. Egues, and P. Recher. Semicond. Sci. Technol. 27, 124007 (2012); arXiv:1209.2313.
The twodimensional topological insulating phase has been experimentally
discovered in HgTe quantum wells (QWs). The lowenergy physics of
twodimensional topological insulators (TIs) is described by the
BernevigHughesZhang (BHZ) model, where the realization of a topological or a
normal insulating phase depends on the Dirac mass being negative or positive,
respectively. We solve the BHZ model for a mass domain configuration, analyzing
the effects on the edge modes of a finite Dirac mass in the normal insulating
region (softwall boundary condition). We show that at a boundary between a TI
and a normal insulator (NI), the Dirac point of the edge states appearing at
the interface strongly depends on the ratio between the Dirac masses in the two
regions. We also consider the case of multiple boundaries such as NI/TI/NI,
TI/NI/TI and NI/TI/NI/TI.
 
19.  Rashba spin orbit interaction in a quantum wire superlattice 
Gunnar Thorgilsson, J. Carlos Egues, Daniel Loss, and Sigurdur I. Erlingsson. Phys. Rev. B 85, 045306 (2012); arXiv:1111.1534.
In this work we study the effects of a longitudinal periodic potential on a
parabolic quantum wire defined in a twodimensional electron gas with Rashba
spinorbit interaction. For an infinite wire superlattice we find, by direct
diagonalization, that the energy gaps are shifted away from the usual Bragg
planes due to the Rashba spinorbit interaction. Interestingly, our results
show that the location of the band gaps in energy can be controlled via the
strength of the Rashba spinorbit interaction. We have also calculated the
charge conductance through a periodic potential of a finite length via the
nonequilibrium Green's function method combined with the Landauer formalism.
We find dips in the conductance that correspond well to the energy gaps of the
infinite wire superlattice. From the infinite wire energy dispersion, we derive
an equation relating the location of the conductance dips as a function of the
(gate controllable) Fermi energy to the Rashba spinorbit coupling strength. We
propose that the strength of the Rashba spinorbit interaction can be extracted
via a charge conductance measurement.
 
20.  Scanning Tunneling Microscope Operating as a Spindiode 
P. H. Penteado, F. M. Souza, A. C. Seridonio, E. Vernek, and J. C. Egues. Phys. Rev. B 84, 125439 (2011); arXiv:1106.4331.
We theoretically investigate spinpolarized transport in a system composed of
a ferromagnetic Scanning Tunneling Microscope (STM) tip coupled to an adsorbed
atom (adatom) on a host surface. Electrons can tunnel directly from the tip to
the surface or via the adatom. Since the tip is ferromagnetic and the host
surface (metal or semiconductor) is nonmagnetic we obtain a spindiode effect
when the adatom is in the regime of single occupancy. This effect leads to an
unpolarized current for direct bias (V > 0) and polarized current for reverse
(V < 0) bias voltages, if the tip is nearby the adatom. Within the
nonequilibrium Keldysh technique we analyze the interplay between the lateral
displacement of the tip and the intra adatom Coulomb interaction on the
spindiode effect. As the tip moves away from the adatom the spindiode effect
vanishes and the currents become polarized for both V > 0 and V < 0. We also
find an imbalance between the up and down spin populations in the adatom, which
can be tuned by the tip position and the bias. Finally, due to the presence of
the adsorbate on the surface, we observe spinresolved Friedel oscillations in
the current, which reflects the oscillations in the calculated LDOS of the
subsystem surface+adatom.
 
21.  Low Bias Negative Differential Resistance in Graphene Nanoribbon Superlattices 
Gerson J. Ferreira, Michael N. Leuenberger, Daniel Loss, and J. Carlos Egues. Phys. Rev. B 84, 125453 (2011); arXiv:1105.4850.
We theoretically investigate negative differential resistance (NDR) for
ballistic transport in semiconducting armchair graphene nanoribbon (aGNR)
superlattices (5 to 20 barriers) at low bias voltages V_{SD} < 500 mV. We
combine the graphene Dirac hamiltonian with the LandauerBüttiker formalism
to calculate the current I_{SD} through the system. We find three distinct
transport regimes in which NDR occurs: (i) a "classical" regime for wide
layers, through which the transport across bandgaps is strongly suppressed,
leading to alternating regions of nearly unity and zero transmission
probabilities as a function of V_{SD} due to crossing of bandgaps from
different layers. (ii) a quantum regime dominated by superlattice miniband
conduction, with current suppression arising from the misalignment of miniband
states with increasing V_{SD} and (iii) a WannierStark ladder regime with
current peaks occurring at the crossings of WannierStark rungs from distinct
ladders. We observe NDR at voltage biases as low as 10 mV with a high current
density, making the aGNR superlattices attractive for device applications.
 
22.  Kondo screening regimes of a quantum dot with a single Mn ion 
E. Vernek, Fanyao Qu, F. M. Souza, J. C. Egues, and E. V. Anda. Physical Review B 83, 205422 (2011); arXiv:1102.1154.
We study the Kondo and transport properties of a quantum dot with a single
magnetic Mn ion connected to metallic leads. By employing a numerical
renormalization group technique we show that depending on the value of
ferromagnetic coupling strength between the local electronic spin and the
magnetic moment of the Mn, two distinct Kondo regimes exist. In the weak
coupling limit, the system can be found in a completely screened Kondo state
describing a local magnetic moment decoupled from the rest of the system. In
contrast, in the strong coupling regime the quantum dot spin and the local
magnetic moment form a single largespin entity partially Kondo screened. A
crossover between these two regimes can be suitably tuned by varying the tunnel
coupling between the quantum dot electron and the leads. The model investigated
here is also suitable to study magnetic molecules adsorbed on a metallic
surface. The rich phenomenology of these systems is reflected in the
conductance across the system.
 
23.  Energy spectra for quantum wires and 2DEGs in magnetic fields with Rashba and Dresselhaus spinorbit interactions 
Sigurdur I. Erlingsson, J. Carlos Egues, and Daniel Loss. Phys. Rev. B 82, 155456 (2010); arXiv:1008.1317.
We introduce an analytical approximation scheme to diagonalize parabolically
confined two dimensional electron systems with both the Rashba and Dresselhaus
spinorbit interactions. The starting point of our perturbative expansion is a
zerothorder Hamiltonian for an electron confined in a quantum wire with an
effective spinorbit induced magnetic field along the wire, obtained by
properly rotating the usual spinorbit Hamiltonian. We find that the
spinorbitrelated transverse coupling terms can be recast into two parts W and
V, which couple crossing and noncrossing adjacent transverse modes,
respectively. Interestingly, the zerothorder Hamiltonian together with W can
be solved exactly, as it maps onto the JaynesCummings model of quantum optics.
We treat the V coupling by performing a SchriefferWolff transformation. This
allows us to obtain an effective Hamiltonian to third order in the coupling
strength k_Rl of V, which can be straightforwardly diagonalized via an
additional unitary transformation. We also apply our approach to other types of
effective parabolic confinement, e.g., 2D electrons in a perpendicular magnetic
field. To demonstrate the usefulness of our approximate eigensolutions, we
obtain analytical expressions for the n^th Landaulevel g_nfactors in the
presence of both Rashba and Dresselhaus couplings. For small values of the bulk
gfactors, we find that spinorbit effects cancel out entirely for particular
values of the spinorbit couplings. By solving simple transcendental equations
we also obtain the band minima of a Rashbacoupled quantum wire as a function
of an external magnetic field. These can be used to describe Shubnikovde Haas
oscillations. This procedure makes it easier to extract the strength of the
spinorbit interaction in these systems via proper fitting of the data.
 
24.  Manybody effects on the ρ_{xx} ringlike structures in twosubband wells 
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues. Physical Review Letters 104 , 066803 (2010); arXiv:0909.2175.
The longitudinal resistivity ρ_{xx} of twodimensional electron gases formed in wells with two subbands displays ringlike structures when plotted in a densitymagneticfield diagram, due to the crossings of spinsplit Landau levels (LLs) from distinct subbands. Using spin density functional theory, we investigate the shape and spin polarization of these structures as a function of the temperature and the magneticfield tilt angle. We find that (i) some of the rings "break" at sufficiently low temperatures due to a quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of spin polarization (∼ 50%) within, consistent with the NMR data of Zhang et al. [Phys. Rev. Lett. 98, 246802 (2007)], and (ii) for increasing tilting angles the interplay between the anticrossings due to interLL couplings and the exchangecorrelation (XC) effects leads to a collapse of the rings at some critical angle θ_{c}, in agreement with the data of Guo et al. [Phys. Rev. B 98, 246802 (2008)].
 
25.  Proceedings of the PASPS V Conference Held in August 2008 in Foz do Iguacu, Brazil 
Gerson J. Ferreira, Guilherme M. Sipahi, Yara G. Gobato, and J. Carlos Egues. (Guest Editors) J. Supercond. Novel Magnetism 23, , 1 (2010)
In recent years, spinrelated effects have emerged as the key
ingredient underlying many fundamental spindependent
phenomena in nanoscale condensedmatter systems. In this
context, the 5th International Conference on Physics and
Applications of Spinrelated Phenomena in Semiconductors
(PASPS V) took place in the city of Foz do Iguaçu, Brazil, on
August 36, 2008. PASPS V followed the successful series
of conferences held in Japan (Sendai, 2000, 2006), Germany
(Wurzburg, 2002), and The United States (Santa Barbara,
2004).
 
26.  Collapse of ρ_{xx} Ringlike Structures in 2DEGs Under Tilted Magnetic Fields 
Gerson J Ferreira and J. Carlos Egues. Proceedings of PASPS V (www.paspsv.com.br) J. Supercond. Novel Magnetism 23, 19 (2010).
In the quantum Hall regime, the longitudinal resistivity \rho_xx plotted as a densitymagneticfield (n_2DB) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [see, e.g., Zhang et al., in Phys. Rev. Lett. 95:216801, 2005. For tilted magnetic fields, some of these ringlike structures 'shrink' as the tilt angle is increased and fully collapse at \theta_c ~ 6 degrees. Here we theoretically investigate the topology of these structures via a noninteracting model for the 2DEG. We account for the inter Landaulevel coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anticrossings of Landau levels with parallel spins. With the new energy spectrum, we calculate the corresponding n_2D  B diagram of the density of states (DOS) near the Fermi level. We argue that the DOS displays the same topology as \rho_xx in the n_2D  B diagram. For the ring with filling factor \nu = 4, we find that the anticrossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the theta=0 data, we find a collapsing angle \theta_c ~ 3.6 degrees. Despite this factoroftwo discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.
 
27.  Spin Hall Effect in Symmetric Wells with Two Subbands 
M. O. Hachiya, M. Lee, E. Bernardes, J. C. Egues, and D. Loss. Proceedings of PASPS V (www.paspsv.com.br) J. Supercond. Novel Magnetism 23, , 65 (2010)
We investigate the spin Hall conductivity \sigma_xy^z of a clean 2D electron gas formed in a twosubband well. We determine \sigma_xy^z as arising from the intersubband induced spinorbit (SO) coupling \eta (Calsaverini et al., Phys. Rev. B 78:155313, 2008) via a linearresponse approach due to Rashba. By selfconsistently calculating η for realistic wells, we find that \sigma_xy^z presents a nonmonotonic (and nonuniversal) behavior and a sign change as the Fermi energy varies between the subband edges. Although our σ xy z is very small (i.e., < < e/4Pi), it is nonzero as opposed to linearink SO models.
 
28.  Shot Noise in a SpinDiode Geometry 
F. M. Souza, J. Del Nero, and J. C. Egues. Proceedings of PASPS V (www.paspsv.com.br) J. Supercond. Novel Magnetism 23, , 45 (2010)
We apply the master equation technique to calculate shot noise in a system composed of single level quantum dot attached to a normal metal lead and to a ferromagnetic lead (NMQDFM). It is known that this system operates as a spindiode, giving unpolarized currents for forward bias and polarized current for reverse bias. This effect is observed when only one electron can tunnel at a time through the dot, due to the strong intradot Coulomb interaction. We find that the shot noise also presents a signature of this spindiode effect, with a superPoissonian shot noise for forward and a subPoissonian shot noise for reverse bias voltages. The shot noise thus can provide further experimental evidence of the spinrectification in the NMQDFM geometry.
 
29.  Spin Hall effect due to intersubbandinduced spinorbit interaction in symmetric quantum wells 
Minchul Lee, Marco O. Hachiya, E. Bernardes, J. Carlos Egues, and Daniel Loss. Physical Review B 80, 155314 (2009); http://arxiv.org/abs/0907.4078.
We investigate the intrinsic spin Hall effect in twodimensional electron gases in quantum wells with two subbands, where a new intersubbandinduced spinorbit coupling is operative. The bulk spin Hall conductivity $\sigma_{xy}^z$ is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field $B$ and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated $\sigma_{xy}^z$ exhibits a nonmonotonic behavior and can change its sign as the Fermi energy (the carrier areal density $n_{2D}$) is varied between the subband edges. We determine the magnitude of $\sigma_{xy}^z$ for realistic InSb quantum wells by performing a selfconsistent calculation of the intersubbandinduced spinorbit coupling.
 
30.  Intersubbandinduced spinorbit interaction in quantum wells 
Rafael S. Calsaverini, Esmerindo Bernardes, J. Carlos Egues, and Daniel Loss. Phys. Rev. B 78, 155313 (2008); http://arxiv.org/abs/0807.0771.
Recently, we have found an additional spinorbit (SO) interaction in quantum wells with two subbands [Phys. Rev. Lett. 99, 076603 (2007)]. This new SO term is nonzero even in symmetric geometries, as it arises from the intersubband coupling between confined states of distinct parities, and its strength is comparable to that of the ordinary Rashba. Starting from the $8 \times 8$ Kane model, here we present a detailed derivation of this new SO Hamiltonian and the corresponding SO coupling. In addition, within the selfconsistent Hartree approximation, we calculate the strength of this new SO coupling for realistic symmetric modulationdoped wells with two subbands. We consider gated structures with either a constant areal electron density or a constant chemical potential. In the parameter range studied, both models give similar results. By considering the effects of an external applied bias, which breaks the structural inversion symmetry of the wells, we also calculate the strength of the resulting induced Rashba couplings within each subband. Interestingly, we find that for double wells the Rashba couplings for the first and second subbands interchange signs abruptly across the zero bias, while the intersubband SO coupling exhibits a resonant behavior near this symmetric configuration. For completeness we also determine the strength of the Dresselhaus couplings and find them essentially constant as function of the applied bias.
 
31.  Voltage induced spin density in a double quantum well with inversion asymmetry 
S. I. Erlingsson, J. C. Egues, and D. Loss. Physica E. LowDimensional Systems and Nanostructures, v. 40, p. 14841486, (2008). (Proceedings of the 17th International Conference on Electronic Properties of 2D Systems)
We study spin polarization induced by an applied bias in a bilayer quantum well system with interlayer spinorbit coupling. The bias is incorporated via the nonequilibrium Green's function formalism, which allows us to handle a variety of system configurations. We shall focus on the component of the spin density perpendicular to the bilayer and compare our results to those obtained for a single layer system.
 
32.  Spinpolarized current and shot noise in the presence of spin flip in a quantum dot via nonequilibrium Green's functions 
Fabricio M. Souza, AnttiPekka Jauho, and J. Carlos Egues. Phys. Rev. B 78, 155303 (2008); http://arXiv.org/abs/0802.0982.
Using nonequilibrium Green functions we calculate the spinpolarized current and shot noise in a ferromagnetquantumdotferromagnet (FMQDFM) system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and spinflip are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a biasdependent current polarization $\wp$. In particular, $\wp$ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The spinflip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5 due to spinflip.
 
33.  Weak measurement: the effect of the detector dynamics 
Antonio Di Lorenzo and J. Carlos Egues. Phys. Rev. A 77, 042108 (2008); http://arxiv.org/abs/0801.1814.
A general approach to the measurement of an observable with pre and postselection is presented. The limit of weak measurement is studied in detail, and it is shown that the phase of the probe, including a Hamiltonian contribution to it, gives rise to observable effects, since the coherence of the probe is essential for the concept of complex weak value to be meaningful. As a particular example, the measurement of a spin component is considered. We find that the contribution of the imaginary part of the weak value is sizeable in this case.
 
34.  Spin orbit interaction and zitterbewegung in symmetric wells 
E. Bernardes, J. Schliemann, J. C. Egues, and D. Loss. Physica Status Solidi (c) 3, 4330 (2006); PASPS IV Proceedings; arXiv:0708.3091.
Recently, we have introduced a novel intersubbandinduced spinorbit (so) coupling (Phys. Rev. Lett. 99, 076603 (2007); condmat/0607218) arising in symmetric wells with at least two subbands. This new so coupling gives rise to an usual zitterbewegung  i.e. the semiconductor analog to the relativistic trembling motion of electrons  with cycloidal motion without magnetic fields. Here we complement these findings by explicitly deriving expressions for the corresponding zitterbewegung in spin space.
 
35.  Spin densities in parabolic quantum wires with Rashba spinorbit interaction 
S. I. Erlingsson, J. C. Egues, and D. Loss. Physica Status Solidi (c) 3, 4317 (2006); condmat/0701564 (PASPS IV Proceedings).
Using canonical transformations we diagonalize approximately the Hamiltonian of a gaussian wire with Rashba spinorbit interaction. This proceedure allows us to obtain the energy dispersion relations and the wavefunctions with good accuracy, even in systems with relatively strong Rashba coupling. With these eigenstates one can calculate the nonequilibrium spin densities induced by applying bias voltages across the sample. We focus on the $z$component of the spin density, which is related to the spin Hall effect.
 
36.  Quantum Dot as a SpinCurrent Diode: a master equation approach 
F. M. Souza, J. C. Egues, and A. P. Jauho. Phys. Rev. B 75, 165303 (2007); condmat/0611336.
We report a study of spin dependent transport in a system composed of a quantum dot coupled to a normal metal lead and a ferromagnetic lead (NMQDFM). We use the master equation approach to calculate the spinresolved currents in the presence of an external bias and an intradot Coulomb interaction. We find that for a range of positive external biases (current flow from the normal metal to the ferromagnet) the current polarization $\wp=(I_\uparrowI_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to zero, while for the corresponding negative biases (current flow from the ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The system thus operates as a rectifier for spincurrent polarization. This effect follows from an interplay between Coulomb blockade and nonequilibrium spin accumulation in the dot. In the parameter range considered, we also show that the above results can be obtained via nonequilibrium Green functions within a HartreeFock type approximation.
 
37.  Ringlike structures in the densitymagneticfield $\rho_{xx}$ diagram of twosubband quantum Hall systems 
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues. Physica Status Solidi (c) 3, 4364 (2006); condmat/0607456 (PASPS IV Proceedings).
Motivated by recent experiments [Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801 (2005) and Ellenberger \textit{et al.}, condmat/0602271] reporting novel ringlike structures in the densitymagneticfield ($n_{2D}$\emph{B}) diagrams of the longitudinal resistivity $\rho_{xx}$ of quantum wells with two subbands, we investigate theoretically here the magnetotransport properties of these quantumHall systems. We determine $\rho_{xx}$ via both the Hartree and the KohnSham selfconsistent schemes plus the Kubo formula. While the Hartree calculation yields diamondshaped structures in the $n_{2D}$\emph{B} diagram, the calculation including exchange and correlation effects (KohnSham) more closely reproduces the ringlike structures in the experiments.
 
38.  Spinorbit interaction in symmetric wells and cycloidal orbits without magnetic fields. [Title changed to: Spinorbit interaction in symmetric wells with two subbands] 
Esmerindo Bernardes, John Schliemann, Minchul Lee, J. Carlos Egues, and Daniel Loss. Phys. Rev. Lett. 99, 076603 (2007); condmat/0607218.
We investigate the spinorbit (so) interaction in twodimensional electron gases (2DEGs) in quantum wells with two subbands. From the $8\times 8$ Kane model, we derive a new intersubbandinduced so term which resembles the functional form of the Rashba so  but is nonzero even in \emph{symmetric} structures. This follows from the distinct parity of the confined states (even/odd) which obliterates the need for asymmetric potentials. We selfconsistently calculate the new so coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new so term gives rise to a nonzero ballistic spinHall conductivity, which changes sign as a function of the Fermi energy ($\varepsilon_F$), and can induce an unusual \emph{zitterbewegung} with cycloidal trajectories \textit{without} magnetic fields.
 
39.  Measurement, control, and decay of quantumdot spins 
W. A. Coish, Vitaly N. Golovach, J. Carlos Egues, and Daniel Loss. Physica Status Solidi (b) 243, 3658 (2006); condmat/0606782.
In this review we discuss a recent proposal to perform partial Bellstate (parity) measurements on twoelectron spin states for electrons confined to quantum dots. The realization of this proposal would allow for a physical implementation of measurementbased quantum computing. In addition, we consider the primary sources of energy relaxation and decoherence which provide the ultimate limit to all proposals for quantum information processing using electron spins in quantum dots. We give an account of the Hamiltonians used for the most important interactions (spinorbit and hyperfine) and survey some of the recent work done to understand dynamics, control, and decoherence under the action of these Hamiltonians. We conclude the review with a table of important decay times found in experiment, and relate these time scales to the potential viability of measurementbased quantum computing.
 
40.  Shot noise and spinorbit coherent control of entangled and spin polarized electrons 
J. Carlos Egues, Guido Burkard, D. Saraga, John Schliemann, and Daniel Loss. Phys. Rev. B 72, 235326 (2005); condmat/0509038.
We extend our previous work on shot noise for entangled and spin polarized electrons in a beamsplitter geometry with spinorbit (\textit{so}) interaction in one of the incoming leads (lead 1). Besides accounting for both the Dresselhaus and the Rashba spinorbit terms, we present general formulas for the shot noise of singlet and triplets states derived within the scattering approach. We determine the full scattering matrix of the system for the case of leads with \textit{two} orbital channels coupled via weak \textit{so} interactions inducing channel anticrossings. We show that this interband coupling coherently transfers electrons between the channels and gives rise to an additional modulation angle  dependent on both the Rashba and Dresselhaus interaction strengths  which allows for further independent coherent control of the electrons traversing the incoming leads. We derive explicit shot noise formulas for a variety of correlated pairs (e.g., Bell states) and lead spin polarizations. Interestingly, the singlet and \textit{each} of the triplets defined along the quantization axis perpendicular to lead 1 (with the local \textit{so} interaction) and in the plane of the beam splitter display distinctive shot noise for injection energies near the channel anticrossings; hence, one can tell apart all the triplets, in addition to the singlet, through noise measurements. We also find that spinorbit induced backscattering within lead 1 reduces the visibility of the noise oscillations, due to the additional partition noise in this lead. Finally, we consider injection of twoparticle wavepackets into leads with multiple discrete states and find that twoparticle entanglement can still be observed via noise bunching and antibunching.
 
41.  Fingerprinting Spin Qubits 
J. Carlos Egues Science 309, 565567 (2005) (Perspective)
Editorial summary. Electrons not only have mass and charge, they also have magnetic properties directly related to their intrinsic spin. These spins can combine into quantum states of different spin parity, and such states may be useful as qubits in future quantum computers. As Egues discusses in his Perspective, Engel and Loss [ Science 309, 586 (2005)] report in this issue a method for "fingerprinting" the spin states of electrons contained in quantum dot structures. By allowing the electrons to leak from one dot to another, and then using a nanowire to sense the presence of the spin states, the authors were able to perform a nondestructive spinparity measurement. Such a spinparity detector should permit the manipulation of quantum dot qubits for quantum computation.
 
42.  Spin injection revisited (Title changed to "Spin injection across magnetic/nonmagnetic interfaces with finite magnetic layers") 
Alexander Khaetskii, J. Carlos Egues, Daniel Loss, Charles Gould, Georg Schmidt, and Laurens W. Molenkamp. Physical Review B 71, 235327 (2005); condmat/0312705.
We have reconsidered the relevant problem of spin injection across ferromagnet/nonmagneticsemiconductor (FM/NMS) and dilutemagneticsemiconductor/nonmagneticsemiconductor interfaces, for structures with \textit{finite} magnetic layers (FM or DMS). By using appropriate physical boundary conditions, we find new expressions for the resistances of these structures which are in general different from previous results in the literature. The results obtained can be important for the interpretation of the experimental data in the case of DMS/NMS structures.
 
43.  Hysteretic resistance spikes in quantum Hall ferromagnets without domains 
Henrique J.P. Freire and J. Carlos Egues. Phys. Rev. Lett. 99, 026801 (2007); condmat/0412491.
We use spindensityfunctional theory to study recently reported hysteretic magnetoresistance $\rho_{xx}$ spikes in Mnbased 2D electron gases [Jaroszy\'{n}ski \textit{et al.} Phys. Rev. Lett. \textbf{89}, 266802 (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantumHallferromagnet phase transitions. Spindependent exchangecorrelation effects are crucial to stabilize the relevant magnetic phases arising from \emph{distinct}symmetrybroken excited and groundstate solutions of the KohnSham equations. Besides hysteretic spikes in $\rho _{xx}$, we predict \textit{hysteretic dips} in the Hall resistance $ \rho _{xy}$. Our theory, \textit{without} domain walls, satisfactorily explains the recent data.
 
44.  Shubnikovde Haas oscillations in digital magnetic heterostructures 
Henrique J. P. Freire and J. Carlos Egues. Proceedings of the 11th Brazilian Workshop on Semiconductor Physics, Fortaleza/CE, March/2003 Braz. J. Phys. 34, 614 (2004)
In this paper we theoretically investigate the magneticfield and temperature dependences of the Shubnikovde Haas oscillations in group IIVI modulationdoped Digital Magnetic Heterostructures. We selfconsistently solve the effectivemass Schroedinger equation within the Hartree approximation and calculate the electronic structure and the magnetotransport properties. Our results show i) a shift of the Shubnikovde Haas minima to lower magnetic fields with increasing temperature, and ii) an anomalous oscillation which develops when two opposite Landau levels cross near the Fermi energy. Both of these are consistent with recent magnetotransport measurements in such heterostructures [R. Knobel et al., Phys. Rev. B 65, 235327 (2002)].
 
45.  TMR Effect in a FMQDFM System 
F. M. Souza, J. Carlos Egues, and A. P. Jauho. Proceedings of the 11th Brazilian Workshop on Semiconductor Physics, Fortaleza/CE, March/2003 Braz. J. Phys. 34, 565 (2004).
Using the Keldysh nonequilibrium technique, we study current and the tunnelling magnetoresistance (TMR) in a quantum dot coupled to two ferromagnetic leads (FMdotFM). The current is calculated for both parallel and antiparallel lead alignments. Coulomb interaction and spinflip scattering are taken into account within the quantum dot. Interestingly, we find that these interactions play a contrasting role in the TMR: there is a parameter range where spin flip suppresses the TMR, while Coulomb correlations enhance it, due to Coulomb blockade.
 
46.  Towards Quantum Communication with Electron Spins 
D.S. Saraga, G. Burkard, J.C. Egues, H.A. Engel, P. Recher, and D. Loss. Proceedings of the Quantum Computation at the Atomic Scale Conference, (Istanbul, 111 June, 2003) Turk J Phys 27, 427 (2003)
We review our recent work towards quantum communication in a solidstate environment with qubits carried by electron spins. We propose three schemes to produce spinentangled electrons, where the required separation of the partner electrons is achieved via Coulomb interaction. The nonproduct spinstates originate either from the Cooper pairs found in a superconductor, or in the ground state of a quantum dot with an even number of electrons. In a second stage, we show how spinentanglement carried by a singlet can be detected in a beamsplitter geometry by an increased (bunching) or decreased (antibunching) noise signal. We also discuss how a local spinorbit interaction can be used to provide a continuous modulation of the noise as a signature of entanglement. Finally, we review how one can use a quantum dot as a spin lter, a spinmemory readout, a probe for singlespin decoherence and ultimately, a singlespin measurement apparatus.
 
47.  'Reservoir Model' for Shallow ModulationDoped Digital Magnetic Quantum Wells 
H. J. P. Freire and J. C. Egues. J. Supercond. 16, 299 (2003)
Digital magnetic heterostructures (DMH) are semiconductor structures with magnetic monolayers. Here we study electronic and magnetotransport properties of shallow modulationdoped (ZnSe/ZnCdSe) DMHs with spin5/2 Mn impurities. We compare the 'reservoir' model, possibly relevant to shallow geometries, to the usual 'constantdensity' model. Our results are obtained by solving the KohnSham equations within the local spin density approximation (LSDA). In the presence of a magnetic field, we show that both models exhibit characteristic behaviors for the electronic structure, twodimensional carrier density, Fermi level and transport properties. Our results illustrate the relevance of exchange and correlation effects in the study of shallow heterostructures of the group IIVI.
 
48.  Noise of SpinPolarized Currents at a Beam Splitter with Local SpinOrbit Interaction 
G. Burkard, J. C. Egues, and D. Loss. J. Supercond. 16, 237 (2003)
An electronic beam splitter with a local Rashba spinorbit coupling can serve as a detector for spinpolarized currents. The spinorbit coupling plays the role of a tunable spin rotator and can be controlled via a gate electrode on top of the conductor. We use spinresolved scattering theory to calculate the zerotemperature current fluctuations (shot noise) for such a fourterminal device and show that the shot noise is proportional to the spin polarization of the source. Moreover, we analyze the effect of spinorbitinduced intersubband coupling, leading to an additional spin rotation.
 
49.  Nonballistic spin fieldeffect transistor 
John Schliemann, J. Carlos Egues, and Daniel Loss. Phys. Rev. Lett. 90, 146801 (2003); condmat/0211603.
We propose a spin fieldeffect transistor based on spinorbit (so) coupling of both the Rashba and the Dresselhaus types. Differently from earlier proposals, spin transport through our device is tolerant against spinindependent scattering processes. Hence the requirement of strictly ballistic transport can be relaxed. This follows from a unique interplay between the Dresselhaus and the (gatecontrolled) Rashba interactions; these can be tuned to have equal strengths thus yielding kindependent eigenspinors even in two dimensions. We discuss implementations with twodimensional devices and quantum wires. In the latter, our setup presents strictly parabolic dispersions which avoids complications arising from anticrossings of different bands.
 
50.  Shot noise for entangled and spinpolarized electrons 
J. C. Egues, P. Recher, D. S. Saraga, V. N. Golovach, G. Burkard, E. V. Sukhorukov, and D. Loss. "Quantum Noise in Mesoscopic Physics", pp 241274, Kluwer, 2003, The Netherlands; condmat/0210498.
We review our recent contributions on shot noise for entangled electrons and spinpolarized currents in novel mesoscopic geometries. We first discuss some of our recent proposals for electron entanglers involving a superconductor coupled to a double dot in the Coulomb blockade regime, a superconductor tunnelcoupled to Luttingerliquid leads, and a tripledot setup coupled to Fermi leads. We calculate current and shot noise for spinpolarized currents and entangled/unentangled electron pairs in a beamsplitter geometry with a \textit{local} Rashba spinorbit (so) interaction in the incoming leads. We find \textit{continuous} bunching and antibunching behaviors for the \textit{entangled} pairs  triplet and singlet  as a function of the Rashba rotation angle. In addition, we find that unentangled triplets and the entangled one exhibit distinct shot noise. Shot noise for spinpolarized currents shows sizable oscillations as a function of the Rashba phase. This happens only for electrons injected perpendicular to the Rashba rotation axis; spinpolarized carriers along the Rashba axis are noiseless. We find an additional spin rotation for electrons with energies near the crossing of the bands where so induced interband coupling is relevant. This gives rise to an additional modulation of the noise for both electron pairs and spinpolarized currents. Finally, we briefly discuss shot noise for a double dot near the Kondo regime.
 
51.  DattaDas transistor with enhanced spin control 
J. Carlos Egues, Guido Burkard, and Daniel Loss. Appl. Phys. Lett. 82, 2658 (2003); condmat/0209682.
We consider a twochannel spin transistor with weak spinorbit induced interband coupling. We show that the coherent transfer of carriers between the coupled channels gives rise to an \textit{additional} spin rotation. We calculate the corresponding spinresolved current in a DattaDas geometry and show that a weak interband mixing leads to enhanced spin control.
 
52.  Current and Noise in a FM/quantum dot/FM System 
F. M. Souza, J. C. Egues, and A. P. Jauho. condmat/0209263
Using the Keldysh nonequilibrium technique we calculate current, noise and Fano factor in a ferromagnetic(FM)quantum dotferromagnetic(FM) system with Coulomb interaction and spinflip scattering in the dot. The lead polarizations are considered in both parallel P and antiparallel AP alignments. We show that spinflip can increase both APcurrent and APnoise, while the Pcurrent and Pnoise are almost insensitive to it. This fact leads to a suppression of the tunnelling magnetoresistance with increasing spinflip rate.
 
53.  Probing entanglement via Rashbainduced shot noise oscillations 
J. Carlos Egues, Guido Burkard, and Daniel Loss. J. Superconductivity, 16, 711 (2003); condmat/0207392 (special issue in honor of E. I. Rashba.).
We have recently calculated shot noise for entangled and spinpolarized electrons in novel beamsplitter geometries with a local Rashba so interaction in the incoming leads. This interaction allows for a gatecontrolled rotation of the incoming electron spins. Here we present an alternate simpler route to the shot noise calculation in the above work and focus on only electron pairs. Shot noise for these shows continuous bunching and antibunching behaviors. In addition, entangled and unentangled triplets yield distinctive shot noise oscillations. Besides allowing for a direct way to identify triplet and singlet states, these oscillations can be used to extract so coupling constants through noise measurements. Incoming leads with spinorbit interband mixing give rise an additional modulation of the current noise. This extra rotation allows the design of a spin transistor with enhanced spin control.
 
54.  Variational study of the nu=1 quantum Hall ferromagnet in the presence of spinorbit interaction 
John Schliemann, J. Carlos Egues, and Daniel Loss. Phys. Rev. B 67, 085302 (2003); condmat/0209185.
We investigate the nu=1 quantum Hall ferromagnet in the presence of spinorbit coupling of the Rashba or Dresselhaus type by means of HartreeFocktyped variational states. In the presence of Rashba (Dresselhaus) spinorbit coupling the fully spinpolarized quantum Hall state is always unstable resulting in a reduction of the spin polarization if the product of the particle charge $q$ and the effective $g$factor is positive (negative). In all other cases an alternative variational state with O(2) symmetry and finite inplane spin components is lower in energy than the fully spinpolarized state for large enough spinorbit interaction. The phase diagram resulting from these considerations differs qualitatively from earlier studies.
 
55.  Rashba spinorbit interaction and shot noise for spinpolarized and entangled electrons 
J. Carlos Egues, Guido Burkard, and Daniel Loss. Phys. Rev. Lett. 89, 176401 (2002); condmat/0204639.
We study shot noise for spinpolarized currents and entangled electron pairs in a fourprobe (beam splitter) geometry with a local Rashba spinorbit (so) interaction in the incoming leads. Within the scattering formalism we find that shot noise exhibits Rashbainduced oscillations with continuous bunching and antibunching. We show that entangled states as well as triplet states can be identified via their Rashba phase in noise measurements. For twochannel leads we find an additional spin rotation due to so induced interband coupling which provides additional spin control. We show that the so interaction determines the Fano factor which provides a direct way to measure the Rashba coupling constant via noise.
 
56.  Universal spinpolarization fluctuations in 1D wires with magnetic impurities 
N.A. Mortensen and J.C. Egues. Phys. Rev. B 66, 153306 (2002); condmat/0112376.
We study conductance and spinpolarization fluctuations in onedimensional wires with spin5/2 magnetic impurities. Our tightbinding Green function approach goes beyond mean field thus including sd exchangeinduced spinflip scattering exactly. In a certain parameter range, we find that spin flip suppresses conductance fluctuations while enhancing spinpolarization fluctuations. More importantly, spinpolarization fluctuations attain a universal value 1/3 for large enough spin flip strengths.
 
57.  Screening ineffectiveness and THz emission at bare LO phonon frequencies 
F. M. Souza and J. C. Egues. Phys. Rev. B 66, R060301 (2002); condmat/0105529.
Within a hydrodynamic approach we investigate the dynamics of an inhomogeneous electronhole gas coupled to phonons in Te and the corresponding THz emission. We find that the {\em longitudinal} {\em % inhomogeneity} of the photogenerated electronhole gas  due to short absorption lengths in Te  gives rise to {\em screening ineffectiveness} for nonzero wavevector modes. This allows for THz dynamics and emission at the bare LO phonon frequency $\nu _{LO}$ even at high carrier densities. Screening ineffectiveness thus provides an appealingly simple explanation for the existence of bare modes at $\nu _{LO}$ in longitudinally inhomogeneous systems such as Te; no lateral inhomogeneity of the excitation spot is needed here.
 
58.  Bare LOPhonon Peak in THzEmission Signals: a DielectricFunction Analysis 
Fabricio M. Souza and J. C. Egues. Proceedings of the 10th Brazilian Workshop on Semiconductor Physics, Guaruja/SP, April/2001 Braz. J. Phys. 32, 415(2002); condmat/0210049..
We present a normalmode analysis of coupled photocarrierphonon dynamics in Te. We consider a dielectric function which accounts for LO phonons and the electronhole gas within the DebyeHuckel model and RPA. Our main finding is the existence of a bare LO phonon mode in the system even at high carrier density. This oscillation is an unscreened L mode arising from ineffective screening at large wave vectors. This mode is consistent with the bare LOphonon peak in recent THzemission spectra of Te.
 
59.  Subband structure of IIVI modulationdoped magnetic quantum wells 
Henrique J. P. Freire and J. Carlos Egues. Proceedings of the 10th Brazilian Workshop on Semiconductor Physics, Guaruja/SP, April/2001 Braz. J. Phys 32, 327 (2002); condmat/0112263..
Here we investigate the spindependent subband structure of newlydeveloped Mnbased modulationdoped quantum wells. In the presence of an external magnetic field, the sd exchange coupling between carriers and localized d electrons of the Mn impurities gives rise to large spin splittings resulting in a magneticfield dependent subband structure. Within the framework of the effectivemass approximation, we selfconsistently calculate the subband structure at zero temperature using Density Functional Theory (DFT) with a Local Spin Density Approximation (LSDA). We present results for the magneticfield dependence of the subband structure of shallow ZnSe/ZnCdMnSe modulation doped quantum wells. Our results show a significant contribution to the selfconsistent potential due to the exchangecorrelation term. These calculations are the first step in the study of a variety of interesting spindependent phenomena, e.g., spinresolved transport and manybody effects in polarized twodimensional electron gases.
 
60.  Shot noise in the presence of spin flip scattering 
F. Brito and J. C. Egues. Proceedings of the 10th Brazilian Workshop on Semiconductor Physics, Guaruja/SP, April/2001 Braz. J. Phys. 32, 324 (2002).
Shot noise is a timedependent current fluctuation due to the discrete character of the electron charge. Here we investigate shot noise in a spinresolved tunneling system under the influence of spinflip scattering. We find that the average current $\langle I\rangle $ and Fano factor $\gamma $ (``normalized noise'') present contrasting behavior for differing spinflip time ratios: $\langle I\rangle $\ decreases while $\gamma $\increases for $\tau_{{\uparrow }{\downarrow }}>\tau_{{\downarrow }{\uparrow }}$\ as compared to the $\tau _{{\uparrow }{\downarrow }}=\tau_{{\downarrow }{\uparrow }}$ case and vice versa for $\tau_{{\uparrow }{\downarrow }}<\tau_{{\downarrow }{\uparrow}}$.
 
61.  Spin filtering and magnetoresistance in ballistic tunnel junctions 
J.C. Egues, C. Gould, G. Richter, and L. W. Molenkamp. Phys. Rev. B 64, 195319 (2001); condmat/0103442.
We theoretically investigate magnetoresistance (MR) effects in connection with spin filtering in quantumcoherent transport through tunnel junctions based on nonmagnetic/semimagnetic heterostructures. We find that spin filtering in conjunction with the suppression/enhancement of the spindependent Fermi seas in semimagnetic contacts gives rise to (i) spinsplit kinks in the MR of single barriers and (ii) a robust beating pattern in the MR of double barriers with a semimagnetic well. We believe these are unique signatures for quantum filtering.
 
62.  Shot noise in spinpolarized currents 
F. G. Brito, J. F. Estanislau, and J. C. Egues. J. Magn. Magn. Mater. 226230, 457 (2001)
We investigate spindependent fluctuations in spinpolarized electronic currents in a semimagnetic doublebarrier tunneling structure. We use both quantumcoherent and semiclassical approaches to study the effects of spinflip scattering on shot noise. In a limited parameter range, we find that for a fully spinpolarized incoming beam both descriptions yield shotnoise suppression.
 
63.  Dynamics of internal electric and phonon fields in nGaAs pumped with ultrashort pulses 
F. M. Souza and J. C. Egues. Proceedings of the 9th Brazilian Workshop on Semiconductor Physics, Belo Horizonte/MG, February/1999 Braz. J. Phys. 29, 831(1999)
We investigate the ultrafast dynamics of an electronhole plasma coupled to phonons in bulk GaAs excited with femtosecond laser pulses. Our approach is based on balance equations directly derived from the Boltzmann equation within the relaxationtime approximation. Poisson's equation together with a phenomenological drivenharmonicoscillator equation supplements our description by accounting for timedependent electric and vibrational fields. Our calculated internal fields show oscillations at frequencies characteristic of those of coupled plasmonphonon modes. Our results are consistent with recent experimental data.
 
64.  Spindependent perpendicular magnetotransport through a tunable ZnSe/Zn1xMnxSe heterostructure: A possible spin filter? 
J. C. Egues Phys. Rev. Lett. 80, 4578 (1998)
This work addresses spindependent magnetotransport in a bandgapmatched ZnSe/Zn1xMnxSe heterostructure. In an external magnetic field the paramagnetic layer behaves as a potential well for spindown electrons and a potential barrier for spinup ones. My currentdensity calculation shows a strong suppression of the spinup component of the current density for increasing magnetic fields; the total current density is dominated by the spindown component for B > 2 T. This result gives rise to the possibility of devising spin filters relevant for spindependent optoelectronics.
 
65.  Spindependent phenomena in digitalmagnetic heterostructures: Clustering and phasespace filling effects 
J. C. Egues and J. W. Wilkins. Phys. Rev. B 58, R16012 (1998)
Digitalmagnetic heterostructures (DMH's), IIVI quantum wells with "planes" of Mn, exhibit strongly spindependent physics. We investigate the magneticfield (B) dependence of exchangeinduced energy splittings and spinflip scattering in DMH's. We find that Mn clustering is relevant to explain the magnitude and the concentration dependence of recently observed splittings. Our calculated electron spinRip times show "branching," i.e., tau(sf)(up>down) decreases and tau(sf)(down>up) increases with increasing B. This feature, consistent with recent data, is due to the Bfield dependence of the available phase space.
 
66.  Effect of incoherence on current and shot noise in resonant tunneling: An exactly solvable model 
J. H. Davies, J. Carlos Egues, and J. W. Wilkins. Phys. Rev. B 52, 11259 (1995)
We describe an exactly solvable model of a resonanttunneling diode in which incoherence is
introduced by adding a small random phase to the wave function on each round trip between the
barriers —a FabryPerot picture with dephasing. We find that this form of incoherence broadens
and lowers the resonant transmission peak while conserving its area (current conservation), in
agreement with previous studies. The autocorrelation function of the transmission coefficient cannot
be characterized by a single width but has two energy scales, which we speculate may identify
"coherent" and "incoherent" fractions of the current. The shot noise is raised from the suppressed
value found for purely coherent transport towards its classical value. This behavior is supported by
a picture using wave packets with differing transmission coefficients impinging on a barrier.
 
67.  Zerofrequency shot noise for tunneling through a system with internal scattering 
J. Carlos Egues, S. Hershfield, and J. W. Wilkins. Phys. Rev. B 49, 13517 (1994)
Within the sequential tunneling approach we calculate the zerofrequency shot noise for electrons tunneling through a mesoscopic system with internal scattering. This scattering is included by coupling the resonant level to another set of ‘‘internal’’ states. Our results show that the internal scattering has no effect on the noise provided that the internal states are not coupled to the collector. On the other hand, when electrons can hop from the internal states to the collector, the noise is affected. In some cases the zerofrequency noise can be suppressed below onehalf of full shot noise, 2eI. All limiting cases for the ratio of noise to the current, S/2eI, are presented and discussed.
