arXiv subjects

G. Wendin

Publications and source records attributed to G. Wendin.

44 records · Page 3Linked to original sources

arXiv1998

Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

We study theoretically the ac Josephson effect in voltage biased planar junctions of d-wave superconductors. For some orientations of the superconductors a current peak is found at finite voltage in the current-voltage characteristics. We pick out the relevant physical processes and write down an analytical formula for the current which clearly shows how the midgap state acts as a resonance and produces the peak. We present a possible explanation for the zero-bias conductance peak, recently found in experiments on grain boundary junctions of high-temperature superconductors, in terms of resonant transmission through midgap state of quasiparticles undergoing multiple Andreev reflections. We note that within our framework the zero-bias conductance peak appears in rather transparent Josephson junctions of d-wave superconductors.

cond-mat.supr-con↗

arXiv1998

Multiple Andreev reflections as a transport problem in energy space

We present an approach for analyzing the dc current in voltage biased quantum superconducting junctions. By separating terms from different $n$-particle processes, we find that the $n$-particle current can be mapped on the problem of wave transport through a potential structure with $n$ barriers. We discuss the relation between resonances in such structures and the subgap structures in the current-voltage characteristics. At zero temperature we find, exactly, that only processes creating real excitations contribute to the current. Our results are valid for a general SXS-junction, where the X-region is an arbitrary non-superconducting region described by an energy-dependent transfer matrix.

cond-mat.supr-con↗

arXiv1998

Resonant multiple Andreev reflections in mesoscopic superconducting junctions

We investigate the properties of subharmonic gap structure (SGS) in superconducting quantum contacts with normal-electron resonances. We find two distinct new features of the SGS in resonant junctions which distinguish them from non-resonant point contacts: (i) The odd-order structures on the current-voltage characteristics of resonant junctions are strongly enhanced and have pronounced peaks, while the even-order structures are suppressed, in the case of a normal electron resonance being close to the Fermi level. (ii) Tremendous current peaks develop at $eV=\pm 2E_0$ where $E_0$ indicates a distance of the resonance to the Fermi level. These properties are determined by the effect of narrowing of the resonance during multiple Andreev reflections and by overlap of electron and hole resonances.

cond-mat.supr-con↗

arXiv1996

Scattering theory of superconductive tunneling in quantum junctions

We present a consistent theory of superconductive tunneling in single-mode junctions within a scattering formulation of Bogoliubov-de Gennes quantum mechanics. Both dc Josephson effect and dc quasiparticle transport in voltage biased junctions are considered. Elastic quasiparticle scattering by the junction determines equilibrium Josephson current. We discuss the origin of Andreev bound states in tunnel junctions and their role in equilibrium Josephson transport. In contrast, quasiparticle tunneling in voltage biased junctions is determined by inelastic scattering. We derive a general expression for inelastic scattering amplitudes and calculate the quasiparticle current at all voltages with emphasis on a discussion of the properties of subgap tunnel current and the nature of subharmonic gap structure.

cond-mat.supr-con↗

arXiv1995

Giant Josephson current through a single bound state in a superconducting tunnel junction

We study the microscopic structure of the Josephson current in a single-mode tunnel junction with a wide quasiclassical tunnel barrier. In such a junction each Andreev bound state carries a current of magnitude proportional to the {\em amplitude} of the normal electron transmission through the junction. Tremendous enhancement of the bound state current is caused by the resonance coupling of superconducting bound states at both superconductor-insulator interfaces of the junction. The possibility of experimental observation of the single bound state current is discussed.

cond-mat↗

arXiv1995

Excess Noise in Biased Superconducting Weak Links

Non-equilibrium excess noise of a short quasi one-dimensional constriction between two superconductors is considered. A general expression for the current-current correlation function valid for arbitrary temperatures and bias voltages is derived. This formalism is applied to a current-carrying quantum channel with perfect transparency. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of noise. The source of noise is fractional Andreev scattering of quasiparticles with energies $|E|$ greater than the half-width $Δ$ of the superconducting gap. For high bias voltages, $V \gg Δ/e$, the relation between the zero-frequency limit of the noise spectrum, $S(0)$, and the excess current $I_{\text{exc}}$ reads $S(0)=(1/5)|e|I_{\text{exc}}$. As $Δ\rightarrow 0$ both the excess noise and the excess current vanish linearly in $Δ$, %$\propto Δ$, their ratio being constant.

cond-mat↗

arXiv1995

MICROWAVE-INDUCED "SOMERSAULT EFFECT" IN FLOW OF JOSEPHSON CURRENT THROUGH A QUANTUM CONSTRICTION

We consider the supercurrent flow through gated mesoscopic semiconductor hetrostructures in which a two-dimensional normal constriction is confined between superconducting electrodes. We show that for these structures the Josephson current, carried by quantized electron modes, can be strongly affected by an electromagnetic field. Photon-assisted Landau-Zener transitions between Andreev bound states in the constriction manifest themselves in one of two ways: (i) If the phase difference between the superconducting elements is fixed a series of reversals in the direction of supercurrent flow occurs (Somersault effect). (ii) If instead the junction is current biased a series of voltage spikes is seen. Both manifestations are a direct consequence of the discrete energy spectrum of the mesoscopic junction. We discuss necessary conditions for the described phenomena to be experimentally observable.

cond-mat↗

arXiv1994

Soft Disorder Effects in the Conductance Quantization in Quantum Point Contacts: Indirect Backscattering Statistics

The breakdown of conductance quantization in a quantum point contact in the presence of random long-range impurity potential is discussed. It is shown that in the linear response regime a decisive role is played by the indirect backscattering mechanism via quasilocalized states at the Fermi level; this can provide much higher backscattering rate than any direct backscattering process. For the realistic contact lengths ($\leq 2000$nm) the scattering processes prove to be independent, in spite of coherence of the electron wave. The distribution function of conductance fluctuations is obtained by direct numerical calculations as well as estimated within an analytical model for the first time. It is shown to be a generalized Poisson distribution. Estimates are made for quantum point contact performance at different choice of parameters. In particular, it is the better the larger the intermode distance is compared to the amplitude of the random impurity potential.

cond-mat↗