1,331 research outputs found
Josephson effect through a multilevel dot near a singlet-triplet transition
We investigate the Josephson effect through a two-level quantum dot with an
exchange coupling between two dot electrons. We compute the superconducting
phase relationship and construct the phase diagram in the superconducting
gap--exchange coupling plane in the regime of the singlet-triplet transition
driven by the exchange coupling. In our study two configurations for the
dot-lead coupling are considered: one where effectively only one channel
couples to the dot, and the other where the two dot orbitals have opposite
parities. Perturbative analysis in the weak-coupling limit reveals that the
system experiences transitions from 0 to (negative critical current)
behavior, depending on the parity of the orbitals and the spin correlation
between dot electrons. The strong coupling regime is tackled with the numerical
renormalization group method, which first characterizes the Kondo correlations
due to the dot-lead coupling and the exchange coupling in the absence of
superconductivity. In the presence of superconductivity, many-body correlations
such as two-stage Kondo effect compete with the superconductivity and the
comparison between the gap and the relevant Kondo temperature scales allows to
predict a rich variety of phase diagrams for the ground state of the system and
for the Josephson current. Numerical calculations predicts that our system can
exhibit Kondo-driven 0--0 or -0- double transitions and, more
interestingly, that if proper conditions are met a Kondo-assisted
-junction can arise, which is contrary to a common belief that the Kondo
effect opens a resonant level and makes the 0-junction. Our predictions could
be probed experimentally for a buckminster fullerene sandwiched between two
superconductors.Comment: 19 pages, 15 figure
Transport through a band insulator with Rashba spin-orbit coupling: metal-insulator transition and spin-filtering effects
We calculate the current-voltage characteristic of a one-dimensional band
insulator with magnetic field and Rashba spin-orbit coupling which is connected
to nonmagnetic leads. Without spin-orbit coupling we find a complete
spin-filtering effect, meaning that the electric transport occurs in one spin
channel only. For a large magnetic field which closes the band gap, we show
that spin-orbit coupling leads to a transition from metallic to insulating
behavior. The oscillations of the different spin-components of the current with
the length of the transport channel are studied as well
Interactions and charge fractionalization in an electronic Hong-Ou-Mandel interferometer
We consider an electronic analog of the Hong-Ou-Mandel (HOM) interferometer,
where two single electrons travel along opposite chiral edge states and collide
at a Quantum Point Contact. Studying the current noise, we show that because of
interactions between co-propagating edge states, the degree of
indistinguishability between the two electron wavepackets is dramatically
reduced, leading to reduced contrast for the HOM signal. This decoherence
phenomenon strongly depends on the energy resolution of the packets. Insofar as
interactions cause charge fractionalization, we show that charge and neutral
modes interfere with each other, leading to satellite dips or peaks in the
current noise. Our calculations explain recent experimental results [E.
Bocquillon, et al., Science 339, 1054(2013)] where an electronic HOM signal
with reduced contrast was observed.Comment: 5 pages, 2 figure
Dynamic response of a mesoscopic capacitor in the presence of strong electron interactions
We consider a one dimensional mesoscopic capacitor in the presence of strong
electron interactions and compute its admittance in order to probe the
universal nature of the relaxation resistance. We use a combination of
perturbation theory, renormalization group arguments, and quantum Monte Carlo
calculation to treat the whole parameter range of dot-lead coupling. The
relaxation resistance is universal even in the presence of strong Coulomb
blockade when the interactions in the wire are sufficiently weak. We predict
and observe a quantum phase transition to an incoherent regime for a Luttinger
parameter . Results could be tested using a quantum dot coupled to an
edge state in the fractional quantum Hall effect.Comment: 4 pages, 4 figures, submitted to PR
Electron polarizability of crystalline solids in quantizing magnetic fields and topological gap numbers
A theory of the static electron polarizability of crystals whose energy
spectrum is modified by quantizing magnetic fields is presented. It is argued
that The polarizability is strongly affected by non-dissipative Hall currents
induced by the presence of crossed electric and magnetic fields: these can even
change its sign. Results are illustrated in detail for a two dimensional square
lattice. The polarizability and the Hall conductivity are respectively linked
to the two topological quantum numbers entering the so--called Diophantine
equation. These numbers could in principle be detected in actual experiments
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
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
Non-equilibrium supercurrent through a quantum dot: current harmonics and proximity effect due to a normal metal lead
We consider a Hamiltonian model for a quantum dot which is placed between two
superconducting leads with a constant bias imposed between these leads. Using
the non-equilibrium Keldysh technique, we focus on the subgap current, where it
is known that multiple Andreev reflections (MAR) are responsible for charge
transfer through the dot. Attention is put on the DC current and on the first
harmonics of the supercurrent. Varying the energy and width of the resonant
level on the dot, we first investigate a cross-over from a quantum dot regime
to a quantum point contact regime when there is zero coupling to the normal
probe. We then study the effect on the supercurrent of the normal probe which
is attached to the dot. This normal probe is understood to lead to dephasing,
or alternatively to induce reverse proximity effect. We describe the full
crossover from zero dephasing to the incoherent case. We also compute the
Josephson current in the presence of the normal lead, and find it in excellent
agreement with the values of the non-equlibrium current extrapolated at zero
voltage
Josephson Effect through an isotropic magnetic molecule
We investigate the Josephson effect through a molecular quantum dot magnet
connected to superconducting leads. The molecule contains a magnetic atom,
whose spin is assumed to be isotropic. It is coupled to the electron spin on
the dot via exchange coupling. Using the numerical renormalization group method
we calculate the Andreev levels and the supercurrent and examine intertwined
effect of the exchange coupling, Kondo correlation, and superconductivity on
the current. Exchange coupling typically suppresses the Kondo correlation so
that the system undergoes a phase transition from 0 to state as the
modulus of exchange coupling increases. Antiferromagnetic coupling is found to
drive exotic transitions: the reentrance to the state for a small
superconducting gap and the restoration of 0 state for large antiferromagnetic
exchange coupling. We suggest that the asymmetric dependence of supercurrent on
the exchange coupling could be used as to detect its sign in experiments
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