1,189 research outputs found

### Cooper Instability in the Occupation Dependent Hopping Hamiltonians

A generic Hamiltonian, which incorporates the effect of the orbital
contraction on the hopping amplitude between the nearest sites, is studied both
analytically at the weak coupling limit and numerically at the intermediate and
strong coupling regimes for finite atomic cluster. The effect of the orbital
contraction due to hole localization at atomic sites is specified with two
coupling parameters V and W (multiplicative and additive contraction terms).
The singularity of the vertex part of the two-particle Green's function
determines the critical temperature Tc and the relaxation rate Gamma(T) of the
order parameter at temperature above Tc. Unlike in conventional BCS
superconductors, Gamma has a non-zero imaginary part which may influence the
fluctuation conductivity of superconductor above Tc. We compute the ground
state energy as a function of the particle number and magnetic flux through the
cluster, and show the existence of the parity gap Delta appearing at the range
of system parameters consistent with the appearance of Cooper instability.
Numeric calculation of the Hubbard model (with U>0) at arbitrary occupation
does not show any sign of superconductivity in small cluster.Comment: 13 pages, 12 figure

### Persistent Currents in Helical Structures

Recent discovery of mesoscopic electronic structures, in particular the
carbon nanotubes, made necessary an investigation of what effect may helical
symmetry of the conductor (metal or semiconductor) have on the persistent
current oscillations. We investigate persistent currents in helical structures
which are non-decaying in time, not requiring a voltage bias, dissipationless
stationary flow of electrons in a normal-metallic or semiconducting cylinder or
circular wire of mesoscopic dimension. In the presence of magnetic flux along
the toroidal structure, helical symmetry couples circular and longitudinal
currents to each other. Our calculations suggest that circular persistent
currents in these structures have two components with periods $\Phi_0$ and
$\Phi_0/s$ ($s$ is an integer specific to any geometry). However, resultant
circular persistent current oscillations have $\Phi_0$ period.
\pacs{PACS:}PACS:73.23.-bComment: 4 pages, 2 figures. Submitted to PR

### Spin Current in p-wave Superconducting Rings

A formula of the spin current in mesoscopic superconductors is derived from
the mean-field theory of superconductivity. The spin flow is generated by the
spatial fluctuations of $\vec{d}$ which represents a spin state of spin-triplet
superconductors. We discuss a possibility of the circulating spin current in
isolated p-wave superconducting rings at the zero magnetic field. The direction
of the spin current depends on topological numbers which characterize the
spatial configuration of $\vec{d}$ on the ring.Comment: 4page

### Non-adiabatic Josephson Dynamics in Junctions with in-Gap Quasiparticles

Conventional models of Josephson junction dynamics rely on the absence of low
energy quasiparticle states due to a large superconducting gap. With this
assumption the quasiparticle degrees of freedom become "frozen out" and the
phase difference becomes the only free variable, acting as a fictitious
particle in a local in time Josephson potential related to the adiabatic and
non-dissipative supercurrent across the junction. In this article we develop a
general framework to incorporate the effects of low energy quasiparticles
interacting non-adiabatically with the phase degree of freedom. Such
quasiparticle states exist generically in constriction type junctions with high
transparency channels or resonant states, as well as in junctions of
unconventional superconductors. Furthermore, recent experiments have revealed
the existence of spurious low energy in-gap states in tunnel junctions of
conventional superconductors - a system for which the adiabatic assumption
typically is assumed to hold. We show that the resonant interaction with such
low energy states rather than the Josephson potential defines nonlinear
Josephson dynamics at small amplitudes.Comment: 9 pages, 1 figur

### Stability of dynamic coherent states in intrinsic Josephson-junction stacks near internal cavity resonance

Stacks of intrinsic Josephson junctions in the resistive state can by
efficiently synchronized by the internal cavity mode resonantly excited by the
Josephson oscillations. We study the stability of dynamic coherent states near
the resonance with respect to small perturbations. Three states are considered:
the homogeneous and alternating-kink states in zero magnetic field and the
homogeneous state in the magnetic field near the value corresponding to half
flux quantum per junction. We found two possible instabilities related to the
short-scale and long-scale perturbations. The homogeneous state in modulated
junction is typically unstable with respect to the short-scale alternating
phase deformations unless the Josephson current is completely suppressed in one
half of the stack. The kink state is stable with respect to such deformations
and homogeneous state in the magnetic field is only stable within a certain
range of frequencies and fields. Stability with respect to the long-range
deformations is controlled by resonance excitations of fast modes at finite
wave vectors and typically leads to unstable range of the wave-vectors. This
range shrinks with approaching the resonance and increasing the in-plane
dissipation. As a consequence, in finite-height stacks the stability frequency
range near the resonance increases with decreasing the height.Comment: 15 pages, 8 figures, to appear in Phys. Rev.

### Josephson effect in mesoscopic graphene strips with finite width

We study Josephson effect in a ballistic graphene strip of length $L$ smaller
than the superconducting coherence length and arbitrary width $W$. We find that
the dependence of the critical supercurrent $I_{c}$ on $W$ is drastically
different for different types of the edges. For \textit{smooth} and
\textit{armchair} edges at low concentration of the carriers $I_{c}$ decreases
monotonically with decreasing $W/L$ and tends to a constant minimum for a
narrow strip $W/L\lesssim1$. The minimum supercurrent is zero for smooth edges
but has a finite value $e\Delta_{0}/\hbar$ for the armchair edges. At higher
concentration of the carriers, in addition to this overall monotonic variation,
the critical current undergoes a series of peaks with varying $W$. On the other
hand in a strip with \textit{zigzag} edges the supercurrent is half-integer
quantized to $(n+1/2)4e\Delta_{0}/\hbar$, showing a step-wise variation with
$W$.Comment: 4 pages, 3 figure

### Josephson effect in ballistic graphene

We solve the Dirac-Bogoliubov-De-Gennes equation in an impurity-free
superconductor-normal-superconductor (SNS) junction, to determine the maximal
supercurrent that can flow through an undoped strip of graphene with heavily
doped superconducting electrodes. The result is determined by the
superconducting gap and by the aspect ratio of the junction (length L, small
relative to the width W and to the superconducting coherence length). Moving
away from the Dirac point of zero doping, we recover the usual ballistic result
in which the Fermi wave length takes over from L. The product of critical
current and normal-state resistance retains its universal value (up to a
numerical prefactor) on approaching the Dirac point.Comment: 4 pages, 2 figure

### Phase diagram of geometric d-wave superconductor Josephson junctions

We show that a constriction-type Josephson junction realized by an epitactic
thin film of a d-wave superconductor with an appropriate boundary geometry
exhibits intrinsic phase differences between 0 and pi depending on geometric
parameters and temperature. Based on microscopic Eilenberger theory, we provide
a general derivation of the relation between the change of the free energy of
the junction and the current-phase relation. From the change of the free
energy, we calculate phase diagrams and discuss transitions driven by geometric
parameters and temperature.Comment: 9 pages, 11 figures. Phys. Rev. B, accepte

### Electron-electron interactions in antidot-based Aharonov-Bohm interferometers

We present a microscopic picture of quantum transport in quantum antidots in
the quantum Hall regime taking electron interactions into account. We discuss
the edge state structure, energy level evolution, charge quantization and
linear-response conductance as the magnetic field or gate voltage is varied.
Particular attention is given to the conductance oscillations due to
Aharonov-Bohm interference and their unexpected periodicity. To explain the
latter we propose the mechanisms of scattering by point defects and Coulomb
blockade tunneling. They are supported by self-consistent calculations in the
Hartree approximation, which indicate pinning and correlation of the
single-particle states at the Fermi energy as well as charge oscillation when
antidot-bound states depopulate. We have also found interesting phenomena of
anti-resonance reflection of the Fano type.Comment: 12 pages, 8 figure

### Transport and magnetization dynamics in a superconductor/single-molecule magnet/superconductor junction

We study dc-transport and magnetization dynamics in a junction of arbitrary
transparency consisting of two spin-singlet superconducting leads connected via
a single classical spin precessing at the frequency $\Omega$. The presence of
the spin in the junction provides different transmission amplitudes for spin-up
and spin-down quasiparticles as well as a time-dependent spin-flip transmission
term. For a phase biased junction, we show that a steady-state superconducting
charge current flows through the junction and that an out-of-equilibrium
circularly polarized spin current, of frequency $\Omega$, is emitted in the
leads. Detailed understanding of the charge and spin currents is obtained in
the entire parameter range. In the adiabatic regime, $\hbar \Omega \ll 2\Delta$
where $\Delta$ is the superconducting gap, and for high transparencies of the
junction, a strong suppression of the current takes place around \vp \approx
0 due to an abrupt change in the occupation of the Andreev bound-states. At
higher values of the phase and/or precession frequency, extended
(quasi-particle like) states compete with the bound-states in order to carry
the current. Well below the superconducting transition, these results are shown
to be weakly affected by the back-action of the spin current on the dynamics of
the precessing spin. Indeed, we show that the Gilbert damping due to the
quasi-particle spin current is strongly suppressed at low-temperatures, which
goes along with a shift of the precession frequency due to the condensate. The
results obtained may be of interest for on-going experiments in the field of
molecular spintronics.Comment: 19 pages, 13 figures (v3) Minor modifications per referee's comments.
No change in results. (v2) 2 authors added, 1 reference added (Ref. 25), no
change in the text and result

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