Subgap Conductance Research Articles

Spin-polarized quasiparticle transport in ferromagnet–d-wave-superconductor junctions with a{110}interface

Within a scattering framework, a theoretical study is presented for the spin-polarized quasiparticle transport in ferromagnet--$d$-wave-superconductor junctions with ${110}$ interface. We find that the subgap conductance behaviors are qualitatively different from a nonmagnetic case, due to the modification of Andreev reflection by the exchange field in the ferromagnet, and can also be significantly different from those of a ferromagnet--$s$-wave junction because of the sign change of the d-wave order parameter along the ${110}$ direction of the crystal. For a ballistic ferromagnet--$d$-wave-superconductor junction, a zero-bias conductance minimum is achieved. In addition, a conductance maximum at finite bias can also be evolved by interfacial scattering. For a normal-metal--ferromagnet--$d$-wave-superconductor junction, conductance resonances are predicted.

Subgap conductance features ofYBa2Cu3O7−δedge Josephson junctions

The differential conductance of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ edge junctions with a ${\mathrm{PrBa}}_{2}{\mathrm{Cu}}_{2.9}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}\mathrm{}}$ barrier has been investigated in detail. One striking property of our edge junctions is the existence of a well pronounced, symmetric subharmonic gap structure which is observed in the differential conductance. These features can be explained by multiple Andreev reflections if we assume the existence of a second peak in the density of states. Furthermore, a zero-bias peak in the conductivity was observed in some of the junctions, which may be explained by Andreev bound states at the interface of a d-wave superconductor.

Strong nonequilibrium coherent states in mesoscopic superconductor-semiconductor-superconductor junctions

A biased superconductor-normal metal-superconductor junction is known to be a strong nonequilibrium system, where Andreev scattering at the interfaces creates a quasiparticle distribution function far from equilibrium. A manifestation of this is the well-known subgap structure in the I/V characteristic, with peaks in the conductance at V = 2Δ/ne. We study an SNS structure consisting of highly doped diffusive GaAs with superconducting electrodes of aluminum with one electrode configured as a flux-sensitive interferometer. This enables us to probe the coherent nature of the quasiparticle states at subgap bias voltage. Oscillations in the conductance as a function of flux are seen at zero bias voltage and disappear below experimental resolution as the bias voltage exceeds the Thouless energy E t = ħD/L 2, where L is the junction length. The oscillations reappear at higher bias in a region around the superconducting energy gap V = Δ/e corresponding to the subgap conductance peak n = 2.

A decimation method for studying transport properties of disordered systems

We present a numerical decimation method for the study of transport properties of disordered tight-binding systems. We demonstrate this method by considering two situations: 1) the problem of two interacting particles (TIP) in a quasi-one dimensional random potential and 2) the conductance of disordered normal - superconducting structures. For case 1) we compute the two particle localisation length lambda(2) presenting results for its dependence on disorder, interaction strength and system width. For case 2) we illustrate the method by presenting results for the sub-gap conductance of a normal wire connected to one normal and one superconducting reservoir and the case of a normal region in contact with two superconductors.

Finite Bias Anomaly in the Subgap Conductance of Superconductor-GaAs Junctions

We measure the subgap conductance across a superconductor-GaAs junction at low temperature. Below $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4\mathrm{K}$ we observe a zero bias conductance peak. For the first time, we observe that at even lower temperatures ( $T\ensuremath{\le}0.8\mathrm{K}$), the conductance peak is shifted to a finite voltage. Application of a magnetic field restores a zero bias conductance peak. This is consistent with theoretical predictions for superconducting-semiconducting junctions in the regime where the contact conductance is larger than the coherent semiconductor conductance.

C-axis tunneling between adjacent superconducting CuO2 bi- or trilayers

We have investigated the subgap tunneling characteristics in single crystals of Bi2Sr2CaCu2O8+δ and thin films of Tl2Ba2Ca2Cu3O10+δ with intrinsic Josephson junctions in the resistive state. The low temperature current voltage characteristics (IVC) exhibits a high subgap conductivity. The results are compared to a calculated IVC for tunnel junctions between two identical d-wave superconductors. The calculated quasiparticle branch agrees reasonable with the measured IVC. However, the presence of a small s-wave component can not be excluded. Besides the global IVC we studied fine structures which were observed in the IVC of both materials. These structures can be explained by a resonant coupling of infrared active c-axis phonons to the Josephson ac currents between adjacent pairs of superconducting layers.

Superconductor-insulator-normal tunnel junctions for on-chip measurement of the temperature

We describe SIN (superconductor-insulator-normal) tunnel junctions made by thin-films of Nb/AlOx/Cu and we report on the measurements of their I-V characteristics between 10 K and 0.3 K. The junctions show a strong dependence of the subgap conductance on the temperature, which makes them suitable for the on-chip measurement of the temperature. In this paper we also describe the design of an on-chip electronic refrigerator based on this type of junction.

Tunneling characteristics of Bi-Sr-Ca-Cu-O intrinsic Josephson junctions

Tunneling-type current-voltage (I-V) characteristics with a clear energy gap structure have been observed for very thin intrinsic Josephson junction stacks fabricated on the surface of underdoped Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ single crystals with the CuO/sub 2/ bilayer numbers of 27, 18, and 9. It is found that the nonequilibrium heating effect is significant in this layered material, and that an increase in the number of CuO/sub 2/ bilayers or a decrease in the c-axis resistivity leads to significant gap suppression and negative resistance. The tunneling properties are characterized by the three points: (1) the energy gap suppression by the nonequilibrium superconductivity effect, (2) a large subgap conductance, and (3) an extraordinarily small maximum Josephson current compared with the BCS value. Numerical calculation shows that the observed I-V curves are consistent with the two-dimensional d-wave order parameter with the gap suppression due to the nonequilibrium effect.

The effective dissipation in Nb/AlOx/Nb Josephson tunnel junctions by return current measurements

Measurements of temperature dependence of the return current in high quality Nb/AlOx/Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm2, covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation.

Phase-coherent conductance of a superconductor-normal-metal quantum interferometer.

A theory of the subgap conductance of a hybrid superconductor{endash}normal-metal ({ital S}/{ital N}) quantum interferometer consisting of two tunnel junctions ({ital S}/{ital N}) in contact with an {ital N} layer is developed. The oscillatory dependence of the conductance of the system {ital G} on the phase difference between superconductors {var_phi} is shown to be due to the proximity effect, i.e., to the penetration of the condensate into the {ital N} region. The {ital G}({var_phi}) dependence is found for the cases of both weak and strong proximity effects. This dependence can radically change its shape form qualitatively when the voltage is varied. The amplitude of the conductance oscillations appears to be large in the case of strong proximity effect. Different layouts are considered including the cases when there is the third tunnel junction or point contact between the {ital N} conductor and the {ital N} electrode. {copyright} {ital 1996 The American Physical Society.}

Symmetry breaking in the tunneling conductance of a superconducting vortex

Recent experiments on c-axis tunneling into a vortex core of YBCO have yielded a sub-gap differential conductance which is not invariant under a reversal of the applied bias. We present a new result for the differential conductanceG(E) of a normal-superconducting contact which predicts that what whatever the strength of contact, in the absence of open channels in the superconductor,G(E) is necessarily an even function ofE and that an asymetric current-voltage characteristic implies the existence of propagating bound-states within the vortex core. In the tunneling limit, asymmetries arise from the difference between particle and hole densities of states and if the Fermi energy is close to a singularity, is predicted to be sensitive to doping.

Anomalous gap structures in HgBa2Ca2Cu3O8+δ single crystals measured by scanning tunneling spectroscopy

Scanning tunneling spectroscopy on high-T c superconductors with nonideal surfaces can generate misleading results. We demonstrate that reproducible BCS-like gap structures with sharp peaks and very flat subgap conductance on HgBa2Ca2Cu3O8+δ (Hg-1223) single crystals (T c ,onset=110 K) are in fact due to a combination of charging effects (Coulomb staircase) and superconductivity.

Quasiparticle tunneling in HTS bicrystal grain boundary junctions

We have measured the temperature and voltage dependence of the quasiparticle tunneling (QPT) in YBa2Cu3O7−σ (YBCO), Bi2Sr2CaCu2O8+x (BSCCO), Nd1.83Ce0.17CuOx (NCCO) and La1.85Sr0.15CuO4−σ (LSCO) grain boundary Josephson junctions (GBJs). At voltages well above the gap voltageVg, the measured conductance vs. voltage curves show a temperature independent parabolic shape. From these curves the effective height and thickness of the insulating grain boundary barrier is derived. At voltages below the gap voltage, the conductance vs. voltage curves are well fitted by a BCS density of states and temperature dependence of the energy gap, if a large broadening parameter Γ is assumed for the gap. The large Γ value, which explains both the large subgap conductance at low temperatures and the strongly reduced conductance peak at the gap voltage, most likely is related to the high density of localized states within the grain boundary barrier.

BCS-like gap structure of HgBa/sub 2/CuO/sub 4+δ/ tunnel junctions

We report point-contact tunneling into polycrystalline HgBa/sub 2/CuO/sub 4+/spl delta// superconductors with a T/sub c/ onset of 97 K using a superconducting Nb counterelectrode. These SIS' tunnel junctions are of unusually high quality for cuprate superconductors, exhibiting low and flat sub-gap conductances and sharp conductance peaks as expected from a BCS density of states. These features are obtained reproducibly and are consistent with earlier published SIN results using a Au counterelectrode. Use of experimental data to simulate performance of a quasiparticle mixer indicates HgBa/sub 2/CuO/sub 4+/spl delta// may be suitable for use in low noise heterodyne receivers operating at a few THz. >

The proximity effect and subgap conductivity in superconductor-barrier-normal metal contacts

On the basis of microscopic equations for matrix Green's functions with boundary conditions at the interface, we calculate the conductance of S/N contacts of different types (a planar S/N junction and a one-dimensional S/N channel) as a function of bias voltage. It is shown that even at zero temperature the subgap conductance S( V) ( V < Δ) may be significant (of the order of the contact conductance in the normal state). The subgap conductance is caused by a term in the current which leads to the interference current at T < T c and to a contribution to the paraconductivity of the Maki-Thompson type at T > T c in SIS junctions. The form of S ( V) depends essentially on the relation between the barrier transmittance and the depairing rate in the N region.

Dissipative charge transport in mesoscopic SNS junctions

Ballistic charge transport through mesoscopic superconductor-normal metal-superconductor (SNS) microbridges is investigated on the basis of the Keldysh technique. At low voltages we find a peak in the conductance which is caused by multiple Andreev reflection of quasiparticles trapped in the pair potential well of the N-layer in the presence of inelastic relaxation there. For larger voltages inelastic relaxation in the N-layer plays no role, quasiparticles are accelerated out of the N-layer and relax in superconducting banks. We demonstrate that even at eV, T ⪡ Δ the system has a finite subgap conductance, i.e. dissipative current can flow through a superconducting constriction both with and without normal metal inclusion. In this case dissipation arises from pairbreaking due to acceleration of virtually created subgap quasiparticles by the electric field. In the voltage range V ∼ Δ/ e we analyze the temperature dependence of the subharmonic gap structure which turns out to be most pronounced at T ∼ Δ and vanishes at T → 0 and T → T C. This behavior is the result of a counter current induced by thermally excited quasiparticles.

Tunneling in the electron box in the nonperturbative regime

We study charging effects and tunneling in the single electron box. Tunneling mixes different charge states and in the nonperturbative regime the charge in the island may be strongly screened. When charge states are nearly degenerate the screening of the charge is strong even in the week tunneling regime. Virtual tunneling processes reduce both the level splitting Δ and the tunneling strength α. The charge on the island and the decay rates are calculated. In the strong tunneling regime also nondegenerate states are affected by tunneling. Strong-coupling scaling renormalizes the effective capacitance, a result which we confirm by Monte Carlo simulations. The tunneling strength α scales to smaller values into the regime where the weak-coupling scaling applies. We propose a two-stage scaling procedure providing the unified picture for the problem. The scaling analysis is also extended to superconducting tunnel junctions with finite subgap conductance.

Device-quality tunnel junctions on the HIGH-Tc superconductor HgBa2CuO4+δ

SIN and SIS tunnel junction device (e.g. photon detectors, logic elements) repuire quasiparticle characteristics that exhibit sharp current onsets at the gap voltage and verp lop sub-gap conductances. Progress is reported on the development of such junctions on high- T c cuprates using mechanical point contacts. In general, these contacts displap the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. Most cuprates displap a sub-gap conductance phich monotonicallp increases pith voltage about the minimum value at pero bias. Hopever, tunneling data of unusuallp high quality have been obtained for the recentlp discovered Hg-based cuprate, HgBa 2 CuO 4 ( T c = 96 K ). SIS′ tunneling data using a Nb tip are presented phich exhibit verp lop and flat sub-gap conductances and sharp conductance peaks as expected from a BCS density of states. These results are slightlp improved over earlier published results pith SIN junctions. Use of the experimental data to simulate the performance of a quasiparticle mixer demonstrates that noise temperatures approaching the puantum limit are possible for SIS and SIN mixers in the range 1–5 THp.

Low frequency noise properties of all-thin film superconducting point contacts

We report the results of low frequency voltage noise measurements of an all-thin film superconducting point contact (SPC). The SPC is prepared in liquid helium by application of a voltage pulse to a Nb-MgO-NbN edge sandwich. Increasing the amplitude of the pulse we may observe a gradual transition of a junction from the tunneling to metallic transport regime. We study the magnitude of low frequency noise during this transition and find that noise decreases quickly with decreasing ratio of subgap-to-supergap resistance. This result is contrasted with an opposite finding of C. T. Rogers and R. A. Buhrman [IEEE Trans. Magn. MAG-21, 126 (1985)] for the limit of a ‘‘leaky’’ tunnel junction. This comparison suggests that the excess subgap conductivity observed in our samples is caused by an Andreev reflection.

Sub-Coulomb-gap conductance in small tunnel junctions.

Sub-Coulomb-gap leakage currents observed in small-capacitance tunnel junctions have most often been interpreted in terms of cotunneling. We suggest another possible mechanism for this subgap conductance and address related issues on energy conservation in the context of dissipative open junctions. We show that whenever the junction is biased by the chemical-potential difference between the two electron reservoirs, the bias voltage always induces leakage currents, irrespective of other features of the setup. We also discuss the implications of this mechanism for junction arrays.