Gerson J. Ferreira


Departamento de Física e Informática
Instituto de Física de São Carlos
Universidade de São Paulo, Brazil


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1.  Magnetically-Defined Qubits on 3D Topological Insulators
Gerson J. Ferreira and Daniel Loss.

We explore time-reversal-symmetry-breaking potentials to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect (QAHE). Here we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits.

2.  Helical States in Curved Bilayer Graphene
Jelena Klinovaja, Gerson J. Ferreira, and Daniel Loss.
Phys. Rev. B 86, 235416 (2012); arXiv:1208.2601.

We study spin effects of quantum wires formed in bilayer graphene by electrostatic confinement. With a proper choice of the confinement direction, we show that in the presence of magnetic field, spin-orbit interaction induced by curvature, and intervalley scattering, bound states emerge that are helical. The localization length of these helical states can be modulated by the gate voltage which enables the control of the tunnel coupling between two parallel wires. Allowing for proximity effect via an s-wave superconductor, we show that the helical modes give rise to Majorana fermions in bilayer graphene.

3.  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 Landauer-B\"uttiker 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 band gaps is strongly suppressed, leading to alternating regions of nearly unity and zero transmission probabilities as a function of V_SD due to crossing of band gaps 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 Wannier-Stark ladder regime with current peaks occurring at the crossings of Wannier-Stark 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.

4.  Many-body effects on the $\rho_{xx}$ ringlike structures in two-subband wells
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues.
Phys. Rev. Lett. 104, 066803 (2010); arXiv:0909.2175.

The longitudinal resistivity $\rho_{xx}$ of two-dimensional electron gases formed in wells with two subbands displays ringlike structures when plotted in a density--magnetic-field diagram, due to the crossings of spin-split Landau levels (LLs) from distinct subbands. Using spin density functional theory and linear response, we investigate the shape and spin polarization of these structures as a function of temperature and magnetic-field 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 ($\sim 50 $%) within, consistent with the NMR data of Zhang \textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for increasing tilting angles the interplay between the anticrossings due to inter-LL couplings and the exchange-correlation (XC) effects leads to a collapse of the rings at some critical angle $\theta_c$, in agreement with the data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].

5.  Collapse of $\rho_{xx}$ ringlike structures in 2DEGs under tilted magnetic fields
Gerson J. Ferreira and J. Carlos Egues.
J. Supercond. Novel Magnetism 23, 19 (2010); arXiv:1003.1761.

In the quantum Hall regime, the longitudinal resistivity $\rho_{xx}$ plotted as a density--magnetic-field ($n_{2D}-B$) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [e.g., Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801 (2005)]. For tilted magnetic fields, some of these ringlike structures "shrink" as the tilt angle is increased and fully collapse at $\theta_c \approx 6^\circ$. Here we theoretically investigate the topology of these structures via a non-interacting model for the 2DEG. We account for the inter Landau-level coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anti-crossings 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 anti-crossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the $\theta = 0^\circ$ data, we find a collapsing angle $\theta_c \approx 3.6^\circ$. Despite this factor-of-two discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.

6.  Ringlike structures in the density--magnetic-field $\rho_{xx}$ diagram of two-subband quantum Hall systems
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues.
Phys. Stat. Sol. (c) 3, 4364 (2006); arXiv:cond-mat/0607456.

Motivated by recent experiments [Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801 (2005) and Ellenberger \textit{et al.}, cond-mat/0602271] reporting novel ringlike structures in the density--magnetic-field ($n_{2D}$\emph{--B}) diagrams of the longitudinal resistivity $\rho_{xx}$ of quantum wells with two subbands, we investigate theoretically here the magneto-transport properties of these quantum-Hall systems. We determine $\rho_{xx}$ via both the Hartree and the Kohn-Sham self-consistent schemes plus the Kubo formula. While the Hartree calculation yields diamond-shaped structures in the $n_{2D}$\emph{--B} diagram, the calculation including exchange and correlation effects (Kohn-Sham) more closely reproduces the ringlike structures in the experiments.

7.  Preface: Proceedings of the 20th Latin American Symposium on Solid State Physics (SLAFES XX) held in March 2011 in Maragogi/AL, Brazil.
Poliana H. Penteado, Gerson J. Ferreira, Guilheme M. Sipahi, and J. Carlos Egues.
J. Supercond. Novel Magnetism (online first)

8.  Preface: Proceedings of the PASPS V Conference Held in August 2008 in Foz do Iguaçu, Brazil
Gerson J. Ferreira, Guilherme M. Sipahi, Yara G. Gobato, and J. Carlos Egues.
J. Supercond. Novel Magnetism 23, 1 (2010)

From the issue entitled "5th International Conference on Physics and Applications of Spin-related Phenomena in Semiconductors (PASPS V), Foz do Iguaçu, Brazil, August 3-6, 2008. Guest Editors: Gerson J. Ferreira, Guilherme M. Sipahi, Yara G. Gobato, and J. Carlos Egues