492 research outputs found
Fingerprints of Majorana fermions in current-correlations measurements from a superconducting tunnel microscope
We compute various current correlation functions of electrons flowing from a
topological nanowire to the tip of a superconducting scanning tunnel microscope
and identify fingerprints of a Majorana bound state. In particular, the spin
resolved cross-correlations are shown to display a clear distinction between
the presence of a such an exotic state (negative correlations) and an Andreev
bound state (positive correlations). Similarity and differences with
measurements with a normal tunnel microscope are also discussed, like the
robustness to finite temperature for instance.Comment: 7 pages, 2 figure
Spin and Charge Signatures of Topological Superconductivity in Rashba Nanowires
We consider a Rashba nanowire with proximity gap which can be brought into
the topological phase by tuning external magnetic field or chemical potential.
We study spin and charge of the bulk quasiparticle states when passing through
the topological transition for open and closed systems. We show, analytically
and numerically, that the spin of bulk states around the topological gap
reverses its sign when crossing the transition due to band inversion,
independent of the presence of Majorana fermions in the system. This spin
reversal can be considered as a bulk signature of topological superconductivity
that can be accessed experimentally. We find a similar behaviour for the charge
of the bulk quasiparticle states, also exhibiting a sign reversal at the
transition. We show that these signatures are robust against random static
disorder
Transport signatures of topological phases in double nanowires probed by spin-polarized STM
We study a double-nanowire setup proximity coupled to an -wave
superconductor and search for the bulk signatures of the topological phase
transition that can be observed experimentally, for example, with an STM tip.
Three bulk quantities, namely, the charge, the spin polarization, and the
pairing amplitude of intrawire superconductivity are studied in this work. The
spin polarization and the pairing amplitude flip sign as the system undergoes a
phase transition from the trivial to the topological phase. In order to
identify promising ways to observe bulk signatures of the phase transition in
transport experiments, we compute the spin current flowing between a local
spin-polarized probe, such as an STM tip, and the double-nanowire system in the
Keldysh formalism. We find that the spin current contains information about the
sign flip of the bulk spin polarization and can be used to determine the
topological phase transition point.Comment: 12 pages, 7 figure
Spin-dependent coupling between quantum dots and topological quantum wires
Considering Rashba quantum wires with a proximity-induced superconducting gap
as physical realizations of Majorana fermions and quantum dots, we calculate
the overlap of the Majorana wave functions with the local wave functions on the
dot. We determine the spin-dependent tunneling amplitudes between these two
localized states and show that we can tune into a fully spin polarized
tunneling regime by changing the distance between dot and Majorana fermion.
Upon directly applying this to the tunneling model Hamiltonian, we calculate
the effective magnetic field on the quantum dot flanked by two Majorana
fermions. The direction of the induced magnetic field on the dot depends on the
occupation of the nonlocal fermion formed from the two Majorana end states
which can be used as a readout for such a Majorana qubit.Comment: 18 pages, 11 figure
Poissonian tunneling through an extended impurity in the quantum Hall effect
We consider transport in the Poissonian regime between edge states in the
quantum Hall effect. The backscattering potential is assumed to be arbitrary,
as it allows for multiple tunneling paths. We show that the Schottky relation
between the backscattering current and noise can be established in full
generality: the Fano factor corresponds to the electron charge (the
quasiparticle charge) in the integer (fractional) quantum Hall effect, as in
the case of purely local tunneling. We derive an analytical expression for the
backscattering current, which can be written as that of a local tunneling
current, albeit with a renormalized tunneling amplitude which depends on the
voltage bias. We apply our results to a separable tunneling amplitude which can
represent an extended point contact in the integer or in the fractional quantum
Hall effect. We show that the differential conductance of an extended quantum
point contact is suppressed by the interference between tunneling paths, and it
has an anomalous dependence with respect to the bias voltage
Detection of finite frequency photo-assisted shot noise with a resonant circuit
Photo-assisted transport through a mesoscopic conductor occurs when an
oscillatory (AC) voltage is superposed to the constant (DC) bias which is
imposed on this conductor. Of particular interest is the photo assisted shot
noise, which has been investigated theoretically and experimentally for several
types of samples. For DC biased conductors, a detection scheme for finite
frequency noise using a dissipative resonant circuit, which is inductively
coupled to the mesoscopic device, was developped recently. We argue that the
detection of the finite frequency photo-assisted shot noise can be achieved
with the same setup, despite the fact that time translational invariance is
absent here. We show that a measure of the photo-assisted shot noise can be
obtained through the charge correlator associated with the resonant circuit,
where the latter is averaged over the AC drive frequency. We test our
predictions for a point contact placed in the fractional quantum Hall effect
regime, for the case of weak backscattering. The Keldysh elements of the
photo-assisted noise correlator are computed. For simple Laughlin fractions,
the measured photo-assisted shot noise displays peaks at the frequency
corresponding to the DC bias voltage, as well as satellite peaks separated by
the AC drive frequency
Zero-energy Andreev bound states from quantum dots in proximitized Rashba nanowires
We study an analytical model of a Rashba nanowire that is partially covered
by and coupled to a thin superconducting layer, where the uncovered region of
the nanowire forms a quantum dot. We find that, even if there is no topological
superconducting phase possible, there is a trivial Andreev bound state that
becomes pinned exponentially close to zero energy as a function of magnetic
field strength when the length of the quantum dot is tuned with respect to its
spin-orbit length such that a resonance condition of Fabry-Perot type is
satisfied. In this case, we find that the Andreev bound state remains pinned
near zero energy for Zeeman energies that exceed the characteristic spacing
between Andreev bound state levels but that are smaller than the spin-orbit
energy of the quantum dot. Importantly, as the pinning of the Andreev bound
state depends only on properties of the quantum dot, we conclude that this
behavior is unrelated to topological superconductivity. To support our
analytical model, we also perform a numerical simulation of a hybrid system
while explicitly incorporating a thin superconducting layer, showing that all
qualitative features of our analytical model are also present in the numerical
results.Comment: Accepted for publication in Phys. Rev.
Topological Phase Detection in Rashba Nanowires with a Quantum Dot
We study theoretically the detection of the topological phase transition
occurring in Rashba nanowires with proximity-induced superconductivity using a
quantum dot. The bulk states lowest in energy of such a nanowire have a spin
polarization parallel or antiparallel to the applied magnetic field in the
topological or trivial phase, respectively. We show that this property can be
probed by the quantum dot created at the end of the nanowire by external gates.
By tuning one of the two spin-split levels of the quantum dot to be in
resonance with nanowire bulk states, one can detect the spin polarization of
the lowest band via transport measurement. This allows one to determine the
topological phase of the Rashba nanowire independently of the presence of
Majorana bound states
Full Counting Statistics of the momentum occupation numbers of the Tonks-Girardeau gas
We compute the fluctuations of the number of bosons with a given momentum for
the Tonks-Girardeau gas at zero temperature. We show that correlations between
opposite momenta, which is an important fingerprint of long range order in
weakly interacting Bose systems, are suppressed and that the full distribution
of the number of bosons with non zero momentum is exponential. The distribution
of the quasi-condensate is however quasi Gaussian. Experimental relevance of
our findings for recent cold atoms experiments are discussed.Comment: 10 pages, 4 figure
On the Gender Bazaar
Denis Chevallier, curator of the exhibition “At the Bazaar of Gender, Feminine-Masculine in the Mediterranean” and Director of Research and Education at Marseille’s Museum of European and Mediterranean Civilizations (MuCEM), spoke with Leora Auslander and Michelle Zancarini-Fournel on 8 December 2013, after taking them on a tour of the exhibition. The exhibition, which closed on 6 January 2014, resulted in two publications, a volume co-authored by Denis Chevallier, Michel Bozon, Michelle Per..
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