Arijit Saha
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

CV
10/2012  Present  Postdoctoral Research Associate at the University of Basel (Switzerland), in the group of Prof. Dr. Daniel Loss 
09/2009  08/2012  Postdoctoral Fellow at the Weizmann Institute of Science (Israel), in the group of Prof. Yuval Gefen 
08/2003  08/2009  Ph.D student at the Harish Chandra Research Institute (India), under the supervision of Prof. Sumathi Rao 
Research Interests
 Quantum dots and Quantum wires
 Mesoscopic Superconductivity
 Quantum charge and Spin pumping
 Hybrid junctions with Graphene and Topological Insulator
 Spintronics, Spin decoherence in Quantum dots
Publications
Show all abstracts.1.  AmbegaokarEckernSchon theory for a collective spin: geometric Langevin noise 
Alexander Shnirman, Yuval Gefen, Arijit Saha, Igor S. Burmistrov, Mikhail N. Kiselev, and Alexander Altland. arXiv:1409.0150; .  
2.  Quantum charge pumping through fractional Fermions in charge density modulated quantum wires and Rashba nanowires 
Arijit Saha, Diego Rainis, Rakesh P. Tiwari, and Daniel Loss. Phys. Rev. B 90, 035422 (2014); arXiv:1405.5719; ; .
We study the phenomenon of adiabatic quantum charge pumping in systems supporting fractionally charged fermionic bound states, in two different setups. The first quantum pump setup consists of a chargedensitymodulated quantum wire, and the second one is based on a semiconducting nanowire with Rashba spinorbit interaction, in the presence of a spatially oscillating magnetic field. In both these quantum pumps transport is investigated in a NXN geometry, with the system of interest (X) connected to two normalmetal leads (N), and the two pumping parameters are the strengths of the effective wirelead barriers. Pumped charge is calculated within the scattering matrix formalism. We show that quantum pumping in both setups provides a unique signature of the presence of the fractionalfermion bound states, in terms of asymptotically quantized pumped charge. Furthermore, we investigate shot noise arising due to quantum pumping, verifying that quantized pumped charge corresponds to minimal shot noise.
 
3.  Transport signature of fractional Fermions in Rashba nanowires 
Diego Rainis, Arijit Saha, Jelena Klinovaja, Luka Trifunovic, and Daniel Loss. Phys. Rev. Lett. 112, 196803 (2014); arXiv:1309.3738.
We theoretically study transport through a semiconducting Rashba nanowire (NW) in the presence of uniform and spatially modulated magnetic fields. The system is fully gapped, and the interplay between the spin orbit interaction and the magnetic fields leads to fractionally charged fermion (FF) bound states of the JackiwRebbi type at each end of the nanowire. We investigate the transport and noise behavior of a
N=NW=N system, where the wire is contacted by two normal leads (N), and we look for possible signatures that could help in the experimental detection of such states. We find that the differential conductance and the shot noise exhibit a subgap structure which fully reveals the presence of the FF state. Alternatively, another confirmation of the presence of the FFs is provided by a conductance measurement in an AharonovBohm setup, where the FFs are responsible for oscillations with double period. Our predictions can be tested in InSb/InAs nanowires and are within reach of the present technology.
 
4.  Persistent current of relativistic electrons on a Dirac ring in presence of impurities 
Sumit Ghosh and Arijit Saha. Eur. Phys. J. B, 87 8 (2014) 167; arXiv:1308.4071.
We study the behavior of persistent current of relativistic electrons on a one dimensional ring in presence of attractive/repulsive scattering potentials.
In particular, we investigate the persistent current in accordance with the strength as well as the number of the scattering potential. We find that in presence
of single scatterer the persistent current becomes smaller in magnitude than the scattering free scenario. This behaviour is similar to the nonrelativistic case.
Even for a very strong scattering potential, finite amount of persistent current remains for a relativistic ring. In presence of multiple scatterer we observe that
the persistent current is maximum when the scatterers are placed uniformly compared to the current averaged over random configurations. However if we increase the
number of scatterers, we find that the random averaged current increases with the number of scatterers. The latter behaviour is in contrast to the nonrelativistic case.
 
5.  Electronelectron interaction effects on transport through mesoscopic superconducting hybrid junctions 
Arijit Saha Int. J. Mod. Phys. B, 27, 1330015 (2013); arXiv:1308.0138.
Effects due to the proximity of a superconductor has motivated a lot of
research work in the last several decades both from theoretical and
experimental point of view. In this review we are going to describe the physics
of systems containing normal metalsuperconductor interface. Mainly we discuss
transport properties through such hybrid structures. In particular, we describe
the effects of electron electron interaction on transport through such
superconducting junction of multiple onedimensional quantum wires. The latter
can be described in terms of a nonFermi liquid theory called Luttinger liquid.
In this review, from the application point of view, we also demonstrate the
possible scenarios for production of pure spin current and large tunnelling
magnetoresistance in such hybrid junctions and analyze the influence of
electronelectron interaction on the stability of the production of pure spin
current.
 
6.  A quantum dot close to Stoner instability: the role of Berry's Phase 
Arijit Saha, Yuval Gefen, Igor Burmistrov, Alexander Shnirman, and Alexander Altland. Annals of Physics 327 (2012) 2543  2559; arXiv:1203.4929.
The physics of a quantum dot with electron electron interactions is well
captured by the so called Universal Hamiltonian if the dimensionless
conductance of the dot is much higher than unity. Within this scheme
interactions are represented by three spatially independent terms which
describe the charging energy, the spinexchange and the interaction in the
Cooper channel. In this paper we concentrate on the exchange interaction and
generalize the functional bosonization formalism developed earlier for the
charging energy. This turned out to be challenging as the effective bosonic
action is formulated in terms of a vector field and is nonabelian due to the
noncommutativity of the spin operators. Here we develop a geometric approach
which is particularly useful in the mesoscopic Stoner regime, i.e., when the
strong exchange interaction renders the system close the the Stoner
instability. We show that it is sufficient to sum over the adiabatic paths of
the bosonic vector field and, for these paths, the crucial role is played by
the Berry phase. Using these results we were able to calculate the magnetic
susceptibility of the dot. The latter, in close vicinity of the Stoner
instability point, matches very well with the exact solution (Pis'ma v ZhETF
92, 202 (2010)).
 
7.  Quantum charge pumping through a superconducting double barrier structure in graphene 
Arijit Kundu, Sumathi Rao, and Arijit Saha. Phys. Rev. B 83, 165451 (2011); arXiv:1102.1447.
We consider the phenomenon of quantum charge pumping of electrons across a
superconducting double barrier structure in graphene in the adiabatic limit. In
this geometry, quantum charge pumping can be achieved by modulating the
amplitudes (Delta_1 and Delta_2) of the gaps associated with the two
superconducting strips. We show that the superconducting gaps give rise to a
transmission resonance in the Delta_1Delta_2 plane, resulting in a large value
of pumped charge, when the pumping contour encloses the resonance. This is in
sharp contrast to the case of charge pumping in a normal double barrier
structure in graphene, where the pumped charge is very small, due to the
phenomenon of Klein tunneling. We analyse the behaviour of the pumped charge
through the superconducting double barrier geometry as a function of the
pumping strength and the phase difference between the two pumping parameters,
for various angles of the incident electron.
 
8.  Resonant tunneling through superconducting double barrier structures in graphene 
Arijit Kundu, Sumathi Rao, and Arijit Saha. Phys. Rev. B 82, 155441 (2010); arXiv:1007.3716.
We study resonant tunneling through a superconducting double barrier
structure in graphene as a function of the system parameters. At each barrier,
due to the proximity effect, an incident electron can either reflect as an
electron or a hole (specular as well as retro Andreev reflection in graphene).
Similarly, transport across the barriers can occur via electrons as well as via
the crossed (specular and/or retro) Andreev channel, where a hole is
transmitted nonlocally to the other lead. In this geometry, in the subgap
regime, we find resonant suppression of Andreev reflection at certain energies,
due to the formation of Andreev bound levels between the two superconducting
barriers, where the transmission probability T for electrons incident on the
double barrier structure becomes unity. The evolution of the transport through
the superconducting double barrier geometry as a function of the incident
energy for various angles of incidence shows the damping of the resonance as
normal reflection between the barriers increases.
 
9.  Resonant spin transport through a superconducting double barrier structure 
Arijit Kundu, Sumathi Rao, and Arijit Saha. Europhys. Lett. 88, 57003 (2009); arXiv:0906.3679.
We study resonant transport through a superconducting double barrier
structure. At each barrier, due to the proximity effect, an incident electron
can either reflect as an electron or a hole (Andreev reflection). Similarly,
transport across the barrier can occur via direct tunneling as electrons as
well as via the crossed Andreev channel, where a hole is transmitted. In the
subgap regime, for a symmetric double barrier system (with low transparency for
each barrier), we find a new T=1/4 resonance (T is the transmission probability
for electrons incident on the double barrier structure) due to interference
between electron and hole wavefunctions between the two barriers, in contrast
to a normal double barrier system which has the standard transmission resonance
at T=1. We also point out as an application that the resonant value of T=1/4
can produce pure spin current through the superconducting double barrier
structure.
 
10.  A systematic stability analysis of the renormalisation group flow for the normalsuperconductornormal junction of Luttinger liquid wires 
Sourin Das, Sumathi Rao, and Arijit Saha. Phys. Rev. B 79, 155416 (2009); arXiv:0901.0126.
We study the renormalization group flows of the two terminal conductance of a
superconducting junction of two Luttinger liquid wires. We compute the power
laws associated with the renormalization group flow around the various fixed
points of this system using the generators of the SU(4) group to generate the
appropriate parameterization of a Smatrix representing small deviations from a
given fixed point Smatrix (obtained earlier in Phys. Rev. B 77, 155418
(2008)), and we then perform a comprehensive stability analysis. In particular,
for the nontrivial fixed point which has intermediate values of transmission,
reflection, Andreev reflection and crossed Andreev reflection, we show that
there are eleven independent directions in which the system can be perturbed,
which are relevant or irrelevant, and five directions which are marginal. We
obtain power laws associated with these relevant and irrelevant perturbations.
Unlike the case of the twowire chargeconserving junction, here we show that
there are power laws which are nonlinear functions of V(0) and V(2k_{F})
(where V(k) represents the Fourier transform of the interelectron interaction
potential at momentum k). We also obtain the power law dependence of linear
response conductance on voltage bias or temperature around this fixed point.
 
11.  Resonant charge and spin transport in a tstub coupled to a superconductor 
Sourin Das, Sumathi Rao, and Arijit Saha. Eur. Phys. J. B 72, 139144 (2009); arXiv:0811.0660.
We study transport through a single channel tstub geometry strongly coupled
to a superconducting reservoir. In contrast to the standard stub geometry which
has both transmission resonances and antiresonances in the coherent limit, we
find that due to the proximity effect, this geometry shows neither a T=1
resonance (T is the transmission probability for electrons incident on the
tstub) nor a T=0 antiresonance as we vary the energy of the incident
electron. Instead, we find that there is only one resonant value at T=1/4,
where charge transport vanishes while the spin transport is perfect.
 
12.  Quantized charge pumping in superconducting double barrier structure : Nontrivial correlations due to proximity effect 
Arijit Saha and Sourin Das. Phys. Rev. B 78, 075412 (2008); arXiv:0711.3216.
We consider quantum charge pumping of electrons across a superconducting
double barrier structure in the adiabatic limit. The superconducting barriers
are assumed to be reflectionless so that an incident electron on the barrier
can either tunnel through it or Andreev reflect from it. In this structure,
quantum charge pumping can be achieved (a) by modulating the amplitudes,
Delta_1 and Delta_2 of the two superconducting gaps or alternatively, (b)
by a periodic modulation of the order parameter phases, phi_1 and \phi_2
of the superconducting barriers. In the former case, we show that the
superconducting gap gives rise to a very sharp resonance in the transmission
resulting in quantization of pumped charge, when the pumping contour encloses
the resonance. On the other hand, we find that quantization is hard to achieve
in the latter case. We show that inclusion of weak electronelectron
interaction in the quantum wire leads to renormalisation group evolution of the
transmission amplitude towards the perfectly transmitting limit due to
proximity effects. Hence as we approach the zero temperature limit, we get
destruction of quantized pumped charge. This is in sharp contrast to the case
of charge pumping in a double barrier in a Luttinger liquid where quantized
charge pumping is actually achieved in the zero temperature limit. We also
propose an experimental setup to study these effects.
 
13.  Renormalization group study of transport through a superconducting junction of multiple onedimensional quantum wires 
Sourin Das, Sumathi Rao, and Arijit Saha. Phys. Rev. B 77, 155418 (2008); arXiv:0711.1324.
We investigate transport properties of a superconducting junction of many ($N
\ge 2$) onedimensional quantum wires. We include the effect of electron electron
interaction within the onedimensional quantum wire using a weak interaction
renormalization group procedure. Due to the proximity effect, transport across
the junction occurs via direct tunneling as well as via the crossed Andreev
channel. We find that the fixed point structure of this system is far more rich
than the fixed point structure of a normal metal$$superconductor junction ($N
= 1$), where we only have two fixed points  the fully insulating fixed point
or the Andreev fixed point. Even a two wire (N=2)system with a superconducting
junction i.e. a normalmetal$$superconductor$$normal metal structure, has
nontrivialfixed points with intermediate transmissions and reflections. We
also include electronelectron interaction induced backscattering in the
quantum wires in our study and hence obtain nonLuttinger liquid behaviour. It
is interesting to note that {\textsl{(a)}} effects due to inclusion of
electronelectron interaction induced backscattering in the wire, and
{\textsl{(b)}} competition between the charge transport via the electron and
hole channels across the junction, give rise to a nonmonotonic behavior of
conductance as a functionof temperature. We also find that transport across the
junction depends on two independent interaction parameters. The first one is
due to the usual correlations coming from Friedel oscillations for spinfull
electrons giving rise to the wellknown interaction parameter (${{\alpha =
(g_22g_1)/2 \pi \hbar v_F}}$). The second one arises due to the scattering
induced by the proximity of the superconductor and is given by(${{\alpha^\prime
= (g_2 + g_1)/2 \pi \hbar v_F}}$).
 
14.  Spintronics with NSN Junction of onedimensional quantum wires : A study of Pure Spin Current and Magnetoresistance 
Sourin Das, Sumathi Rao, and Arijit Saha. Europhys. Lett. 81, 67001 (2008); arXiv:0710.5240.
We demonstrate possible scenarios for production of pure spin current and
large tunnelling magnetoresistance ratios from elastic cotunnelling and
crossed Andreev reflection across a superconducting junction comprising of
normal metalsuperconductornormal metal, where, the normal metal is a
onedimensional interacting quantum wire. We show that there are fixed points
in the theory which correspond to the case of pure spin current. We analyze the
influence of electronelectron interaction and see how it stabilizes or
destabilizes the production of pure spin current. These fixed points can be of
direct experimental relevance for spintronics application of normal
metalsuperconductornormal metal junctions of onedimensional quantum wires.
We also calculate the power law temperature dependence of the crossed Andreev
reflection enhanced tunnelling magnetoresistance ratio for the normal
metalsuperconductornormal metal junction.
 
15.  Adiabatic charge pumping through a dot at the junction of N quantum wires 
Shamik Banerjee, Anamitra Mukherjee, Sumathi Rao, and Arijit Saha. Phys. Rev. B 75, 153407 (2007); arXiv:condmat/0608267.
We study adiabatic charge pumping through a quantum dot placed at the
junction of $N$ quantum wires. We explicitly map out the pattern of pumped
charge as a function of the timevarying tunneling parameters coupling the
wires to the dot and the phase between any two time varying parameters
controlling the shape of the dot. We find that with $N2$ timeindependent
wellcoupled leads, the maximum pumped charge in the remaining two leads is
strongly suppressed with increasing $N$, leading to the possibility of tuning
of the pumped charge, by modulating the coupling of the $N2$ leads.
