Investigation of Supercurrent in the Quantum Hall Regime in Graphene Josephson Junctions

In this study, we examine multiple encapsulated graphene Josephson junctions to determine which mechanisms may be responsible for the supercurrent observed in the quantum Hall (QH) regime. Rectangular junctions with various widths and lengths were studied to identify which parameters affect the occurrence of QH supercurrent. We also studied additional samples where the graphene region is extended beyond the contacts on one side, making that edge of the mesa significantly longer than the opposite edge. This is done in order to distinguish two potential mechanisms: (a) supercurrents independently flowing along both non-contacted edges of graphene mesa, and (b) opposite sides of the mesa being coupled by hybrid electron–hole modes flowing along the superconductor/graphene boundary. The supercurrent appears suppressed in extended junctions, suggesting the latter mechanism.     

Giant Tunneling Magnetoresistance Effect in Ferromagnet/Spin-triplet Superconductor Junctions Caused by Andreev Reflection

We study theoretically the tunneling magnetoresistance effect in ferromagnet/chiral triplet superconductor junctions using the generalized Blonder-Tinkham-Klapwijk theory. This effect rely entirely on the dependence of Andreev reflection on the relative orientation between the magnetic moment in ferromagnet and the d-vector in spin-triplet superconductor, The tunneling differential conductance and magnetoresistance are calculated for three types of chiral p-wave gap functions, and huge tunneling magnetoresistance effect is obtained in the strong spin-polarization and low-temperature regime. This effect might provide a valuable new source of device application.     

Andreev Reflection and Proximity effect

The Andreev Reflection is the key mechanism for thesuperconducting proximity effect. It provides phasecorrelations in a system of non-interacting electrons atmesoscopic scales, i.e. over distances much larger than themicroscopic lengths: Fermi wavelength and elastic electronmean free path. This field of research has attracted anincreasing interest in the recent years in part because of thetremendous development of nanofabrication technologies, andalso because of the richness of the involved quantum effects.In this paper we review some recently achieved advances. Wealso discuss new open questions, in particular non-equilibriumeffects and proximity effect in systems with ferromagneticelements.     

Modeling the Microwave Impedance of High-Tc Long Josephson Junctions

A circuit model is presented for Josephson junctions (JJs) that solves the nonlinear long-junction equation, driven by a nonuniform current distribution. This extended resistively shunted junction (ERSJ) model consists of a parallel array of ideal resistively shunted JJs coupled by inductors. The junction array is connected to an array of current sources that simulate the time- and space-dependent current distribution in a stripline. The rf-current dependent complex impedance of a long JJ calculated using this model agrees with measured data on a YBCO grain-boundary JJ and provides an explanation of the measured steps in the resistance resulting from the creation, annihilation, and motion of Josephson vortices under the influence of rf currents. This model contributes to a better understanding of the power-handling characteristics of high-T _c microwave devices, in which the power losses are believed to result from JJ effects associated with imperfections in the films. The model also predicts second-harmonic generation with a highly nonlinear and nonmonotonic power dependence. Details of the dynamics of Josephson vortices are presented and discussed.     

Surface Andreev Bound States and Odd-Frequency Pairing in Topological Superconductor Junctions

In this review, we summarize the achievement of the physics of surface Andreev bound states (SABS) up to now. The route of this activity has started from the physics of SABS of unconventional superconductors where the pair potential has a sign change on the Fermi surface. It has been established that SABS can be regarded as a topological edge state with topological invariant defined in the bulk Hamiltonian. On the other hand, SABS accompanies odd-frequency pairing like spin-triplet s-wave or spin-singlet p-wave. In a spin-triplet superconductor junction, induced odd-frequency pairing can penetrate into a diffusive normal metal (DN) attached to the superconductor. It causes so called anomalous proximity effect where the local density of states of quasiparticle in DN has a zero energy peak. When bulk pairing symmetry is spin-triplet \(p_{x}\)-wave, the anomalous proximity effect becomes prominent and the zero bias voltage conductance is always quantized independent of the resistance in DN and interface. Finally, we show that the present anomalous proximity effect is realized in an artificial topological superconducting system, where a nanowire with spin-orbit coupling and Zeeman field is put on the conventional spin-singlet s-wave superconductor.     

Supercurrent and multiple Andreev reflections in an InSb nanowire Josephson junction

Epitaxially grown, high quality semiconductor InSb nanowires are emerging material systems for the development of high performance nanoelectronics and quantum information processing and communication devices and for the studies of new physical phenomena in solid state systems. Here, we report on measurements of a superconductor-normal conductor-superconductor junction device fabricated from an InSb nanowire with aluminum-based superconducting contacts. The measurements show a proximity-induced supercurrent flowing through the InSb nanowire segment with a critical current tunable by a gate in the current bias configuration and multiple Andreev reflection characteristics in the voltage bias configuration. The temperature dependence and the magnetic field dependence of the critical current and the multiple Andreev reflection characteristics of the junction are also studied. Furthermore, we extract the excess current from the measurements and study its temperature and magnetic field dependences. The successful observation of the superconductivity in the InSb nanowire-based Josephson junction device indicates that InSb nanowires provide an excellent material system for creating and observing novel physical phenomena such as Majorana fermions in solid-state systems.     

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

We investigate the iron-based superconductor Ba(Fe _1?x Co _x ) ₂As ₂ (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I _c R _n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I _c R _n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I _c R _n to 90 μV for planar TiO _x barrier junctions.     

Quantum Computation and Quantum Coherence in Mesoscopic Josephson Junctions

Macroscopic quantum dynamics of Josephson junctions hasattracted recently renewed interest in the context of quantumcomputation. Quantum coherence of the charge states in asingle-Cooper-pair box or the flux states in an rf SQUID canbe used to create charge and flux qubits for quantumcomputation. The aim of this paper is to give a briefintroduction to the basic concepts of quantum computation withJosephson junctions and to discuss the theory of quantumcoherence in the rf-stimulated macroscopic resonanttunneling of flux in SQUID's. The results for resonant fluxtunneling show that the quantum coherent flux oscillationsshould lead to splitting of resonant tunneling peaks,indicating that the resonant tunneling can be a convenienttool for studying macroscopic quantum coherence of flux.     

Andreev Reflection in Unitary and Non-Unitary Triplet States

The quasiparticle reflection and transmission properties at normal conductor-superconductor interfaces are examined for unitary and non-unitary spin triplet pairing states recently discussed in connection with Sr ₂ RuO ₄ . We find resonance peaks in the Andreev reflection amplitude, which are related to surface bound states in the superconductor. They lead to conductance peak features below the quasiparticle gap in the superconductor. The symmetry of the pairing state determines the specific dependence of the peak on the angle of incidence. Based on this observation we propose a possible experiment which allows to distinguish between different superconducting states.     

Testing the Zurek-Kibble Causality Bounds with Annular Josephson Tunnel Junctions

Zurek has provided simple causal bounds for the onset of phase transitions in condensed matter, that mirror those proposed by Kibble for relativistic quantum field theory. In this paper we show how earlier experiments with annular Josephson tunnel junctions are consistent with this scenario, and suggest how further experiments might confirm it.     

Nonlinear Effects in the Josephson Plasma Resonance

Experimental and theoretical studies are reported of the stability of the zero-voltage state of Josephson tunnel junctions in the simultaneous presence of dc bias current and rf field at a frequency comparable to the Josephson plasma frequency. The observations can be qualitatively understood in terms of large amplitude parametric excitation of the plasma mode at fractional harmonics of the plasma frequency. The quasiparticle-pair interference or cos φ current appears to play a significant role in the observed phenomena. Comments are made on nonlinear phenomena in the longitudinal nuclear magnetic resonance in superfiuid ³ He A on the basis of a previously proposed analogy between the two systems. *The experimental work reported here was supported at the University of Pennsylvania by the Army Research Office (Durham) and the Advanced Research Projects Agency. Subsequent analysis by AJD was supported by the U. S. Atomic Energy Commission under Grant No. AT(11-1)-2027 and the National Science Foundation under Grant No. GH-40960, and by DNL by the Army Research Office (Durham) and the National Science Foundation. © 1975 Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission of the publisher.     

Microwave-Induced Constant Voltage Steps at Series Arrays of Josephson Tunnel Junctions with Near-Zero Current Bias

Microwave-induced, constant-voltage steps in the current-voltage characteristics of Josephson tunnel junctions are used as a reference voltage in cryogenic voltage standards. This relatively low reference voltage of a few millivolts could be increased by using series arrays of tunnel junctions instead of a single junction. Series arrays consisting of 2, 6, 14, and 54 tunnel junctions were tested at a frequency of 70 GHz. The use of this relatively high frequency and of lead-alloy base electrodes allows zero crossing steps of up to 34 mV to be observed for an array with only 54 junctions when irradiated by an RF power of 32 mW. The measurements were performed at 4.2 K.     

Modeling the Microwave Impedance of High-Tc Long Josephson Junctions

A circuit model is presented for Josephson junctions (JJs) that solves the nonlinear long-junction equation, driven by a nonuniform current distribution. This extended resistively shunted junction (ERSJ) model consists of a parallel array of ideal resistively shunted JJs coupled by inductors. The junction array is connected to an array of current sources that simulate the time- and space-dependent current distribution in a stripline. The rf-current dependent complex impedance of a long JJ calculated using this model agrees with measured data on a YBCO grain-boundary JJ and provides an explanation of the measured steps in the resistance resulting from the creation, annihilation, and motion of Josephson vortices under the influence of rf currents. This model contributes to a better understanding of the power-handling characteristics of high-T _c microwave devices, in which the power losses are believed to result from JJ effects associated with imperfections in the films. The model also predicts second-harmonic generation with a highly nonlinear and nonmonotonic power dependence. Details of the dynamics of Josephson vortices are presented and discussed.     

Photodoping Effects in High Critical Temperature Superconducting Films and Josephson Junctions

Persistent photoconductivity is an interesting and unusual property of high temperature superconductors. Illumination of these compounds can lead to a change in doping that is persistent at low temperature and relaxes back to its initial value at elevated temperatures. This photodoping can give rise to an improvement of superconducting properties, which is contrary to the illumination effects in conventional superconductors. Furthermore, these photo-induced effects can also be used to probe and modify the properties of grain boundaries in grain boundary Josephson junctions. This paper reviews the experimental data and proposed theoretical models related to these photoinduced effects.     

Coulomb Blockade and Multiple Andreev Reflection in a Superconducting Single-Electron Transistor

In superconducting quantum point contacts, multiple Andreev reflection (MAR), which describes the coherent transport of m quasiparticles each carrying an electron charge with \(m\ge 3\), sets in at voltage thresholds \(eV = 2\Delta /m\). In single-electron transistors, Coulomb blockade, however, suppresses the current at low voltage. The required voltage for charge transport increases with the square of the effective charge \(eV\propto \left( me\right) ^2\). Thus, studying the charge transport in all-superconducting single-electron transistors (SSETs) sets these two phenomena into competition. In this article, we present the fabrication as well as a measurement scheme and transport data for a SSET with one junction in which the transmission and thereby the MAR contributions can be continuously tuned. All regimes from weak to strong coupling are addressed. We extend the Orthodox theory by incorporating MAR processes to describe the observed data qualitatively. We detect a new transport process the nature of which is unclear at present. Furthermore, we observe a renormalization of the charging energy when approaching the strong coupling regime.     

Solution Spectra and Stability of Current Biased Josephson Junctions in a Magnetic Field

We examine the solutions of the non-linear equations governing the behavior of a current biased Josephson junction. Both inline and overlap current bias geometries are considered. The solution space is investigated analytically and using numerical techniques. We characterize the types of solutions expected analytically and find good approximations for large magnetic fields. For small magnetic fields the solution space is large and its stability is entangled. We study this space and its stability as a function of magnetic field and applied bias current. Selective results are presented that characterize the general behavior of the solution space.     

Spin current,spin accumulation and spin Hall effect

Abstract

Nonlocal spin transport in nanostructured devices with ferromagnetic injector (F1) and detector (F2) electrodes connected to a normal conductor (N) is studied. We reveal how the spin transport depends on interface resistance, electrode resistance, spin polarization and spin diffusion length, and obtain the conditions for efficient spin injection, spin accumulation and spin current in the device. It is demonstrated that the spin Hall effect is caused by spin–orbit scattering in nonmagnetic conductors and gives rise to the conversion between spin and charge currents in a nonlocal device. A method of evaluating spin–orbit coupling in nonmagnetic metals is proposed.     

Annular Long Josephson Junctions in a Magnetic Field: Engineering and Probing the Fluxon Interaction Potential

The interaction of a Josephson fluxon with an externalmagnetic field-induced potential in a long Josephson junctionis investigated experimentally. The thermal activation of thefluxon from a potential well is observed and experimentsprobing its predicted quantum properties are discussed. Amethod for engineering a magnetic double-well potential for afluxon is proposed and the use of the coupled fluxon statesfor quantum computation is suggested.     

Recent Achievements on the Physics of High-T C Superconductor Josephson Junctions: Background, Perspectives and Inspiration

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a π-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.