6 research outputs found
Excitation gap of a graphene channel with superconducting boundaries
We calculate the density of states of electron-hole excitations in a
superconductor/normal-metal/superconductor (SNS) junction in graphene, in the
long-junction regime that the superconducting gap is much larger than the
Thouless energy. If the normal region is undoped, the excitation spectrum
consists of neutral modes that propagate along the boundaries - transporting
energy but no charge. These ``Andreev modes'' are a coherent superposition of
electron states from the conduction band and hole states from the valence band,
coupled by specular Andreev reflection at the superconductor. The lowest
Andreev mode has an excitation gap, which depends on the superconducting phase
difference across the SNS graphene channel. At high doping the excitation gap
vanishes and the usual gapless density of states of Andreev levels is
recovered. We use our results to calculate the superconducting phase dependence
of the thermal conductance of the graphene channel.Comment: 8 pages, 10 figure
Excess Resistance Effect in a Normal Metal Contacting a Superconductor
In relatively pure normal samples contacting a superconductor we consider the
excess resistance effect (that is a decrease of the total electrical resistance
of the sample after transition of the superconducting part into the normal
state) and determine conditions under which the effect arises.Comment: 3 pages, 2 postscript figures all in uuencoded forma
Nonlinear Transport through NS Junctions due to Imperfect Andreev Reflection
We investigate a normal metal -- superconductor (point) contact in the limit
where the number of conducting channels in the metallic wire is reduced to few
channels. As the effective Fermi energy drops below the gap energy, a
conducting band with a width twice the Fermi energy is formed. Depending on the
mode of operation, the conduction band can be further squeezed, leading to
various non-linear effects in the current-voltage characteristics such as
current saturation, a N-shaped negative differential resistance, bistability,
and hysteresis.Comment: 4 pages, RevTeX, three postscript figure
Dissipative Electron Transport through Andreev Interferometers
We consider the conductance of an Andreev interferometer, i.e., a hybrid
structure where a dissipative current flows through a mesoscopic normal (N)
sample in contact with two superconducting (S) "mirrors". Giant conductance
oscillations are predicted if the superconducting phase difference is
varied. Conductance maxima appear when is on odd multiple of due
to a bunching at the Fermi energy of quasiparticle energy levels formed by
Andreev reflections at the N-S boundaries. For a ballistic normal sample the
oscillation amplitude is giant and proportional to the number of open
transverse modes. We estimate using both analytical and numerical methods how
scattering and mode mixing --- which tend to lift the level degeneracy at the
Fermi energy --- effect the giant oscillations. These are shown to survive in a
diffusive sample at temperatures much smaller than the Thouless temperature
provided there are potential barriers between the sample and the normal
electron reservoirs. Our results are in good agreement with previous work on
conductance oscillations of diffusive samples, which we propose can be
understood in terms of a Feynman path integral description of quasiparticle
trajectories.Comment: 24 pages, revtex, 12 figures in eps forma
Coherent quantum phenomena in a normal cylindrical conductor with a superconducting coating
The thermodynamic properties of a mesoscopic-size, simply connected cylindrical normal metal in good metallic contact with superconducting banks are studied theoretically. It is commonly accepted that if the superconductor thickness is quite small (of the order of the coherence length), as is assumed to be the case here, a vector potential field, whose value can be varied, exists inside the normal layer. It is further assumed that the quasiparticles with energy E<Δ (2Δ is the superconducting gap) move ballistically through the normal metal and undergo Andreev scattering caused by the off-diagonal potential of the superconductor. An equation is obtained within the multidimensional quasiclassical method which permits us to determine the spectrum of the Andreev levels and to calculate the density of states of the system in question. It is shown that the Andreev levels shift as the trapped flux Φ changes inside the normal conductor. At a certain flux value they coincide with the Fermi level. A resonance spike in the density of states ν(E) appears in this case, since near E=0 there is strong degeneracy of the quasiparticle states in respect to the quantum number q characterizing their motion along the cylinder axis. As a result, a macroscopic number of q states contribute to the amplitude of the effect. As the flux is increased, the density of states v(E) behaves as a stepwise function of Φ. The distance between the steps is equal to the superconducting flux quantum hc/2e