Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.     

Simulated performance of SIS and SIN junctions as heterodyne receivers in waveguide and open antenna mixer mounts

Tucker's quantum theory of mixing (in the 3-port approximation) is employed to calculate the gain over a wide range of frequencies of model mixers employing SIS and SIN junctions with both real and idealI–V characteristics. A comparison is made between the performance of junctions in waveguide and open antenna mounts. It is concluded that ideal junctions give gain 1.5 to 2 times higher than real ones, SIS junctions have gain approximately three times greater than otherwise similar SIN junctions, and that junction areas need to be typically three times smaller in open antenna structures to provide comparable gain to those in waveguide mounts.     

A comparison of barrier type tunnel junction and point contact tunnel junction formed on the same highT c material

By making a combination of both point contact and barrier type tunnel junctions on a single sample of the highT _c superconductor BSCCO (2212) single crystal, we have shown that as the tunneling tip is slowly retracted from the surface a point contact junction gradually evolves from a N-S short to a high resistance tunnel junction. The scaled dynamic conductance (dI/dV) of this point contact tunnel junction becomes almost identical to that of a conventional barrier type tunnel junction and both show a linear dI/dV —V curve. The observation implies that at high resistance a point contact junction behaves in the same way as a barrier type tunnel junction. We suggested that the almost linear tunneling conductance obtained in both the cases most likely arises due to an intrinsic characteristic of the surface of the crystal comprising of a mosaic of superconducting regions of the order of a few nanometers. We also conclude that the barrierless (N-S) point contact obtained by piercing the surface oxide layer of the crystal shows Andreev reflection which we suggest as the origin of the zero bias anomaly often observed in point contact junctions.     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

正常金属/超导/正常金属双垒隧道结中的量子相干输运

考虑到量子相干效应和界面散射效应 ,利用 L ambert理论模型 ,计算正常金属 /绝缘层 /超导 /绝缘层 /正常金属双垒隧道结中的准粒子输运系数和隧道谱。研究表明 :( 1)所有的准粒子输运系数和电导谱在超导能隙之上都随能量作周期性振荡 ,其振荡周期依赖于超导层的厚度 ;( 2 )在超导能隙之上 Andreev反射系数随能量呈现周期性消失现象 ;( 3)在绝缘层势垒强度取很大的隧道极限下 ,超导层中会形成一系列的准粒子束缚态 ,其位置由量子化条件决定 ;( 4)界面散射效应不仅能压低各子能隙电导峰 ,还能使子能隙电导峰劈裂为两个峰。     

Coherent transmission of nodal Dirac fermions through a graphene-based superconducting double barrier junction

Transport characteristics of relativistic electrons through graphene-based d-wave superconducting double barrier junction and ferromagnet/d-wave superconductor/normal metal double junction have been investigated based on the Dirac–Bogoliubov–de Gennes equation. We have first presented the results of superconducting double barrier junction. In the subgap regime, both the crossed Andreev and nonlocal tunneling conductance all oscillate with the bias voltage due to the formation of Andreev bound states in the normal metal region. Moreover, the critical voltage beyond which the crossed Andreev conductance becomes to zero decreases with increasing value of superconducting pair potential α. In the presence of the ferromagnetism, the MR through graphene-based ferromagnet/ d-wave superconductor/normal metal double junction has been investigated. It is shown that the MR increases from exchange splitting h ₀=0 to h ₀=E _F (Fermi energy), and then it goes down. At h ₀=E _F, MR reaches its maximum 100. In contrast to the case of a single superconducting barrier, Andreev bound states also manifest itself in the zero bias MR, which result in a series of peaks except the maximum one at h ₀=E _F. Besides, the resonance peak of the MR can appear at certain bias voltage and structure parameter. Those phenomena mean that the coherent transmission can be tuned by superconducting pair potential, structure parameter, and external bias voltage, which benefits the spin-polarized electron device based on the graphene materials.     

Coherent quantum transport in normal-metal/<Emphasis Type="Italic">d</Emphasis>-wave superconductor/normal-metal double tunnel junctions

Taking into account the effects of quantum interference and interface scattering, combining the electron current with hole current contribution to tunnel current, we study the coherent quantum transport in normal-metal/d-wave superconductor/ normal-metal (NM/d-wave SC/NM) double tunnel junctions by using extended Blonder-Tinkham-Klapwijk (BTK) approach. It is shown that all quasiparticle transport coefficients and conductance spectrum exhibit oscillating behavior with the energy, in which periodic vanishing of Andreev reflection (AR) above superconducting gap is found. In tunnel limit for the interface scattering strength taken very large, there are a series of bound states of quasiparticles formed in SC.     

Spin triplet Andreev reflection induced by interface spin-orbit coupling in half-metal/superconductor junctions

We show that with interface spin-orbit coupling, triplet pairing can occur in the half-metal/superconductor junction. The tunneling conductance is different from the usual Andreev reflection and strongly depends on the polarisation orientation. The probability of triplet pairing for different incident angles and zero-biased conductance are also calculated. The mechanism for the formation of the triplet pairing is that the interface spin-orbit coupling provides an effective spin-flip barrier, which couples all the transport modes in spin Nambu space. Because of its unique particle hole symmetry, this spin-orbit coupling interface effect is different from the spin-flip ferromagnetic barrier which induces zero-bias conductance vanishing and finite V-shape conductance within the energy gap.     

Andreev reflection and bound states in topological insulator based planar and step Josephson junctions

A superconductor-topological insulator-superconductor (S/TI/S) junction having normal region at angle θ is studied theoretically to investigate the junction angle dependency of the Andreev reflection and the formation of the Andreev bound states in the step and planar S/TI/S structures. It is found that the Andreev reflection becomes θ dependent only in the presence of the potential barrier at the TI/S interface. In particular, the step and planar TI/S junction have totally different conductive behavior with bias voltage and potential barrier in the regime of retro and specular Andreev reflection. Interestingly, we find that the elliptical cross section of Dirac cone, an important feature of topological insulator with step surface defect, affects the Fabry-Perot resonance of the Andreev reflection induced Andreev bound states (which become Majorana zero energy states at low chemical potential) in the step S/TI/S structure. Unlike the usual planar S/TI/S structures, we find these ellipticity affected Andreev bound states lead to non-monotonic Josephson super-current in the step S/TI/S structure whose non-monotonicity can be controlled with the use of the potential barrier, which may find applications in nanoelectronics.     

Suppression of Andreev conductance in a topological insulator–superconductor nanostep junction

When two three-dimensional topological insulators(TIs) are brought close to each other with their surfaces aligned,the surfaces form a line junction. Similarly, three TI surfaces, not lying in a single plane, can form an atomic-scale nanostep junction. In this paper, Andreev reflection in a TI–TI–superconductor nanostep junction is investigated theoretically. Because of the existence of edge states along each line junction, the conductance for a nanostep junction is suppressed. When the incident energy(ε) of an electron is larger than the superconductor gap(?), the Andreev conductance in a step junction is less than unity while for a plane junction it is unity. The Andreev conductance is found to depend on the height of the step junction. The Andreev conductance exhibits oscillatory behavior as a function of the junction height with the amplitude of the oscillations remaining unchanged when ε = 0, but decreasing for ε = ?, which is different from the case of the plane junction. The height of the step is therefore an important parameter for Andreev reflection in nanostep junctions, and plays a role similar to that of the delta potential barrier in normal metal–superconductor plane junctions.     

Spin-polarized quasiparticle transport in ferromagnet/insulator/p-wave superconductor junction

We have studied the tunneling conductance in ferromagnet/insulator/p-wave superconductor junctions, taking into account the rough interface scattering effect. We find that there exist zero-bias conductance peaks and single-minimum structure in tunneling spectroscopy. As the exchange energy increases, the Andreev reflection is always suppressed and the differential conductance decreases. The differential conductance depends on the barrier strength and the roughness at the interface. Supported by the Natural Science Foundation of Jiangsu Higher Education Institutions, China (Grant No. 06KJB140009)     

Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.     

Evolution of edge states and critical phenomena in the Rashba superconductor with magnetization

We study Andreev bound states (ABS) and the resulting charge transport of a Rashba superconductor (RSC) where two-dimensional semiconductor (2DSM) heterostructures are sandwiched by spin-singlet s-wave superconductor and ferromagnet insulator. ABS becomes a chiral Majorana edge mode in the topological phase (TP). We clarify two types of quantum criticality about the topological change of ABS near a quantum critical point (QCP), whether or not ABS exists at QCP. In the former type, ABS has an energy gap and does not cross at zero energy in the nontopological phase. These complex properties can be detected by tunneling conductance between normal metal-RSC junctions.     

Effect of spin-dependent phases of tunneling electrons on the conductance of a point ferromagnet/isolator/superconductor contact

The Andreev reflection probability for a ferromagnet/isolator/superconductor (FIS) contact at the arbitrary spin-dependent amplitudes of the electron waves transmitted through and reflected from the potential barrier is found. It is shown that the Andreev reflection probabilities of electron and hole excitations in the FIS contact are different. The energy levels of the Andreev bound states are found. The ballistic conductance of the point FIS contact is calculated. The text was submitted by the author in English.     

Tunneling conductance of the graphene SNS junction with a single localized defect

Using the Dirac-Bogoliubov-de Gennes equation, we study the electron transport in a graphene-based superconductor-normal(graphene)-superconductor (SNS) junction. We consider the properties of tunneling conductance through an undoped strip of graphene with heavily doped superconducting electrodes in the dirty limit l _def ≪ L ≪ ξ. We find that the spectrum of Andreev bound states is modified in the presence of a single localized defect in the bulk. The minimum tunneling conductance remains the same, and this result is independent of the actual location of the imperfection.     

Effect of surface Andreev bound states on the Bean-Livingston barrier in d-wave superconductors

We study the influence of surface Andreev bound states in d-wave superconductors on the Bean-Livingston surface barrier for entry of a vortex line into a strongly type-II superconductor. Starting from Eilenberger theory, we derive a generalization of London theory to incorporate the anomalous surface currents arising from the Andreev bound states. This allows us to find an analytical expression for the modification of the Bean-Livingston barrier in terms of a single parameter describing the influence of the Andreev bound states. We find that the field of first vortex entry is significantly enhanced. Also, the depinning field for vortices near the surface is renormalized. Both effects are temperature dependent and depend on the orientation of the surface relative to the d-wave gap.     

Spectroscopic evidence for unconventional superconductivity in UBe13

We report on measurements of the differential conductivity G of UBe13-Au contacts, which reveal the existence of low-energy Andreev surface bound states. These bound states are identified via huge conductance peaks at zero bias that may form only in superconductors with nontrivial energy-gap functions. From the voltage dependence of G at T6.7, much in excess of the weak coupling BCS value of 3.5, and directly indicating strong coupling effects in superconducting UBe13.     

Andreev states and shot noise in bicrystal junctions of cuprate superconductors

Experimentally observed features of the electrical and noise characteristics of bicrystal junctions of cuprate superconductors, such as linearity of the critical current density versus square root of the junction transparency and increase in the spectral density of shot noise for small bias voltages (below the superconducting gap), indicate that the superconducting current in cuprate bicrystal junctions is determined by the passage of quasi-particles through a potential barrier at the superconductor boundaries. This process involves bound states appearing as a result of multiple Andreev reflections in superconductors with dominant wavefunction components of the d _x ² ? _y ² symmetry type. At the same time, interpretation of the experimental current-phase and current-magnetic field curves requires that the character of faceting at the bicrystal junctions would be also taken into account.     

c-axis tunneling on YBCO films

We have measured I(V) characteristics of c-axis planar tunnel junctions on Y₁Ba₂Cu₃O _{7 - δ} films. Our results and their analysis provide experimental support for the importance of the two-dimensional character of the YBCO band structure, and a method to measure the ratio between the Fermi energy of YBCO and the barrier height. The analysis is based on the relation between the linear conductance background, related to the inelastic tunneling component, and the zero bias conductance, related to the elastic one. Received 24 September 2000 and Received in final form 15 November 2000     

Phase modulated thermal conductance of Josephson weak links

We present a theory for quasiparticle heat transport through superconducting weak links. The thermal conductance depends on the phase difference (phi) of the superconducting leads. Branch-conversion processes, low-energy Andreev bound states near the contact, and the suppression of the local density of states near the gap edge are related to phase-sensitive transport processes. Theoretical results for the influence of junction transparency, temperature, and disorder, on the conductance, are reported. For high-transmission weak links, D-->1, the formation of an Andreev bound state leads to suppression of the density of states for the continuum excitations, and thus, to a reduction in the conductance for phi approximately pi. For low-transmission (D<1) barriers resonant scattering leads to an increase in the thermal conductance as T drops below T(c) (for phase differences near phi=pi).