Inverse Proximity Effect Research Articles

Proximity effects at the interface of a superconductor and a topological insulator in NbN-Bi2Se3 thin film bilayers

In a search for a simple proximity system of a topological insulator and a superconductor for studying the role of surface versus bulk effects by gating, we report here on a first step toward this goal, namely the choice of such a system and its characterization. We chose to work with thin film bilayers of grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick topological layer of and compare the transport results to those obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with ex situ and in situ prepared NbN- interfaces were studied and two kinds of proximity effects were found. At high temperatures just below the superconducting transition, all bilayers showed a conventional proximity effect where the topological suppresses the onset or mid-transition Tc of the superconducting NbN films by about 1 K. At low temperatures, a cross-over of the resistance versus temperature curves of the bilayer and reference NbN film occurs, where the bilayers show enhancement of , Ic (the supercurrent) and the Andreev conductance, as compared to the bare NbN films. This indicates that superconductivity is induced in the layer at the interface region in between the NbN grains. Thus an inverse proximity effect in the topological material is demonstrated.

Soft superconducting gap in semiconductor-based Majorana nanowires

We develop a theory for the proximity effect in superconductor-semiconductor-normal metal tunneling structures, which have recently been extensively studied experimentally, leading to the observation of transport signatures consistent with the predicted zero-energy Majorana bound states. We show that our model for the semiconductor nanowire having multiple occupied subbands with different transmission probabilities through the barrier reproduces the observed "soft-gap" behavior associated with substantial subgap tunneling conductance. We study the manifestations of the soft gap phenomenon both in the tunneling conductance and in local density of states measurements and discuss the correlations between these two quantities. We emphasize that the proximity effect associated with the hybridization between low-lying states in the multiband semiconductor and the normal metal states in the lead is an intrinsic effect leading to the soft gap problem. In addition to the intrinsic contribution, there may be extrinsic effects, such as, for example, interface disorder, exacerbating the soft gap problem. Our work establishes the generic possibility of an ubiquitous presence of an intrinsic soft gap in the superconductor-semiconductor-normal metal tunneling transport conductance induced by the inverse proximity effect of the normal metal.

Weak links in proximity-superconducting two-dimensional electron systems

We report a giant inverse proximity effect that arises in the low-dimensional devices and is crucially different from proximity in the standard superconductor-normal-superconductor (S/N/S) junctions. The normal conductors induce a giant back-action on the proximity superconducting regions even when the dimensions of the normal parts are smaller than the proximity coherence length. This essentially suppresses the superconducting characteristics of the entire system, including the critical current, well below those for the usual S/N/S structures.

On the feasibility to study inverse proximity effect in a single S/F bilayer by Polarized Neutron Reflectometry

Here we report on a feasibility study aiming to explore the potential of Polarized Neutron Reflectometry (PNR) for detecting the inverse proximity effect in a single superconducting/ferromagnetic bilayer. Experiments, conducted on the V (40 nm)/Fe (1 nm) S/F bilayer, have shown that experimental spin asymmetry measured at T = 0.5T C is shifted towards higher Q values compared to the curve measured at T = 1.5T C . Such a shift can be described by the appearance in superconducting vanadium of magnetic sublayer with a thickness of 7 nm and a magnetization of +0.8 kG.

Nonequilibrium and proximity effects in superconductor–normal metal junctions

We study the consequences of non-equilibrium heating and inverse proximity effect in normal metal - insulator - superconductor - insulator - normal metal (NISIN) junctions with a simple quasi-one-dimensional model. We especially focus on observables and parameter regions that are of interest in the design of SINIS coolers with quasiparticle traps. We present numerical results calculated by solving the Usadel equation and also present analytical approximations in two limiting cases: a short junction with a non-negligible resistance in both ends and a long junction with a transparent contact at one end.

Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets

We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important short-coming of previous work within the framework of Usadel theory. We show that non-magnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin-current as a function of temperature.

Effect of Mo/Cu Superconducting Bilayer Geometry on Ultra-Sensitive Transition-Edge Sensor Performance

We built membrane-isolated transition-edge sensors (TESs) for the background-limited infrared/sub-mm spectrograph using Mo/Cu superconducting bilayer thermistors of varying geometry and found that undesired proximity effects, including the so-called longitudinal proximity effect (LoPE) and the latitudinal inverse proximity effect, affect both the superconducting transition temperature <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and the sharpness of the transition α = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> . The LoPE and latitudinal inverse proximity effect arise because of unintentional proximity effects between the bilayer thermistors, the superconducting wiring of the TES circuitry, and normal metal decorations added to mitigate the LoPE. We examined Mo/Cu bilayer films with widths of 120 μm and lengths of 5, 10, 20, 40, and 120 μm, and studied the variation of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , α, and approximate 80% resistance per square ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub> ) with Ti ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ~ 500 mK) and TiN ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ~ 3.8 K) wiring to the devices. We found larger α values in general for the Ti wiring, where α was as high as 90 for 20-120 μm devices and decreased to 20 for 5-μm-wide devices. We then built arrays of TESs with bilayer thermistor lengths of 10 μm, Ti contacts, TiN wiring, and Au borders. The devices were expected to demonstrate a noise equivalent power less than or equal to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> W/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . We report a measured noise equivalent power at 87 mK of (0.95 ±0.2) × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> W/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> and a response time τ of (360 ± 30)ms on our best device with a thermal conductance x <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> = (15 ±5)fW/K, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> = (120.5 ±3.5)mK, and stray power <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> = (135 ±85)aW. The thermistor had a value of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> = 6 mΩ and value of α = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> between 10 and 60 in the transition. We compare our measured performance with the performance specifications needed for ultrasensitive TESs on the Background-Limited Infrared/Sub-mm Spectrograph (BLISS) and discuss paths forward.

Inverse magnetic proximity effects in superconducting In-Ni and Sn-Ni nanoparticle assemblies

We report on experimental evidence of non-s-wave pairing in In and Sn nanoparticle assemblies. The superconducting transition temperature TC of the In and Sn nanoparticles varies noticeably with the particle diameter. Surprisingly, TC shifts to a noticeably higher temperature when magnetic Ni nanoparticles are introduced into the vicinity of the superconducting nanoparticles. The TC increases further when the magnetic Ni nanoparticles are brought even closer to the superconducting nanoparticles. There is a critical Ni composition and a critical spatial separation between the magnetic and superconducting nanoparticles that must be reached before their magnetic proximity will suppress the superconductivity. A qualitative mechanism is proposed to understand the present observations.

Enhanced Superconducting Parameters of Zero-Dimensional Sn Nanoparticles by Magnetic Proximities

We report on the results of ac magnetic susceptibility measurements made on Sn and Sn–Ni nanoparticle assemblies. The thermal profiles of the magnetic susceptibility of the nanoparticle assemblies can be described by Scalapino’s expression. Inverse magnetic proximity effect is observed, as the T C of the superconducting Sn nanoparticles is found to be noticeably increased, when magnetic Ni nanoparticles are introduced into the vicinity of the superconducting Sn nanoparticles. Closing up the spatial separations between Sn and Ni nanoparticles leads to a further increase in T C.

Current-controllable planar S-(S/F)-S Josephson junction

We report on the experimental realization of a current-controllable lateral S-(S/F)-S Josephson junction based on the inverse proximity effect in the superconductor-ferromagnet bilayer (S/F). The dependence of the critical current on the magnetic field Ic(B) shows a Fresenel-like pattern, which could qualitatively be understood with the theory of Josephson junctions in a magnetic field gradient. The amplitude and the period of the Ic(B) pattern can be controlled by spin-polarized quasiparticles injection into the weak link. The period change suggests controllability of effective area of the Josephson junction. Furthermore, a temperature-induced transition from a weak-link behavior to a strong coupling between the superconducting banks is also observed in these lateral Josephson junctions.

Nb Lateral Josephson Junction Induced by Inverse Proximity Effect With NiFe

We utilize the inverse proximity effect in superconductor-ferromagnet bilayer to generate lateral Josephson junctions. The weak link is created by a ferromagnetic NiFe strip across a superconductor Nb bridge due to the inverse proximity effect. The critical currents of the junctions were measured at different temperatures and magnetic fields. The junctions exhibit a modulation of the critical current in perpendicular magnetic field similar to a Fraunhofer interference pattern which proves the dc Josephson effect. The weak link region induced by the NiFe in Nb persists down to 2 K.

Proximity effects and nonequilibrium superconductivity in transition-edge sensors

We have recently shown that normal-metal/superconductor (N/S) bilayer TESs (superconducting Transition-Edge Sensors) exhibit weak-link behavior.1 Here we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We find TESs with added Au structures also exhibit weak-link behavior as evidenced by exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We explain our results in terms of an effect converse to the longitudinal proximity effect (LoPE)1, the lateral inverse proximity effect (LaiPE), for which the order parameter in the N/S bilayer is reduced due to the neighboring N structures. Resistance and critical current measurements are presented as a function of temperature and magnetic field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130 {\mu}m with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperatures scale approximately as the inverse square of the in- plane N-structure separation distance, without appreciable broadening of the transition width. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 \times 10-10 s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayer's steep resistive transition.

Laterally proximized aluminum tunnel junctions

This letter presents experiments on junctions fabricated by a technique that enables the use of high-quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures. Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot through a clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of this method, fully normal-state single electron transistors (SETs) and normal metal-insulator-superconductor (NIS) junctions applying proximized Al junctions were fabricated. The transport characteristics of the junctions were similar to those obtained from standard theoretical models of regular SETs and NIS junctions.

Evaluation of spin polarization inp-In0.96Mn0.04As using Andreev reflection spectroscopy including inverse proximity effect

We report on carrier transport across a superconductor/ferromagnetic semiconductor junction with a highly transparent metallic contact, Nb/$p$-In${}_{0.96}$Mn${}_{0.04}$As. Below $~$10 K, $p$-In${}_{0.96}$Mn${}_{0.04}$As becomes ferromagnetic, as evidenced by the hysteretic transverse resistance caused by the anomalous Hall effect. Below the superconducting critical temperature $T$${}_{c}$ of the Nb electrodes (8.2 K), a conductance reduction occurs within the bias voltage that is comparable to the Nb superconducting energy gap. A rather moderate slope in the differential conductance curves within the gap region indicates the partial suppression of the Andreev reflection caused by spin-polarized carriers in $p$-In${}_{0.96}$Mn${}_{0.04}$As. Spin polarization $P$ in $p$-In${}_{0.96}$Mn${}_{0.04}$As has been extracted by fitting the measured differential conductance curves with a newly modified Blonder-Tinkham-Klapwijk model with both spin polarization and the inverse proximity effect (mod2-BTK model). The extracted $P$ value is $P$ $=$ 0.725 at 0.5 K, and it decreases gradually with increasing temperature.

Influence of spin relaxation length on lateral double superconductor/ferromagnet/superconductor junctions

We study the spin dependent transport at the junctions between two superconducting Nb leads connected by two ferromagnetic NiFe bars in parallel, which have various separations. The separation distance l between NiFe bars was varied from 150 to 700 nm to study the spin relaxation effect in Nb. In small l (150 and 250 nm) samples, a spin-valve-like behavior related to the inverse proximity effect was observed, which manifest itself as charge accumulation due to spin imbalance near the interface. However, for samples with large l, the spin-valve behavior vanishes and the anisotropic magnetoresistance effect dominates.

Thermal Conductance by the Inverse Proximity Effect in a Superconductor

We study heat transport in hybrid lateral normal-metal-superconductor-normal-metal structures. We find the thermal conductance of a short superconducting wire to be strongly enhanced beyond the BCS value due to the inverse proximity effect, resulting from contributions of elastic cotunneling and crossed Andreev reflection of quasiparticles. Our measurements agree with a model based on the quasiclassical theory of inhomogeneous superconductivity in the diffusive limit.

Nanocrystalline boron‐doped diamond films, a mixture of BCS‐like and non‐BCS‐like superconducting grains

AbstractScanning tunneling topography and spectroscopy are performed below 100 mK on granular nanocrystalline boron‐doped diamond (BDD) films. We found the superconductivity behavior to follow mainly the granular features of the BDD films. The temperature dependence of the local differential conductance spectra shows our nanocrystalline BDD films as made of grains with a supercondutivity either BCS‐like or non‐BCS‐like. Such a distribution is not discernible in transport measurements, which present a sharp macroscopic superconducting transition at a temperature of a few Kelvins. Our local scanning tunneling microscopies also confirm the good coupling between these grains: only a few opaque interfaces are detected. Such a transparency of intergrain interfaces is responsible for a proximity effect in weakly superconductive grains and an inverse proximity effect in strongly superconducting grains.

Coexistence of ferromagnetism and superconductivity in normal mental/superconductor/ferromagnet structures

We extend the Blonder, Tinkham and Klapwijk theory to the study of the inverse proximity effects in the normal mental/superconductor/ferromagnet structures. In the superconducting film, there are the gapless superconductivity and the spin-dependent density of states both within and without the energy gap. It indicates an appearance of the inverse-proximity-effect-induced ferromagnetism and a coexistence of ferromagnetism and superconductivity near the interface. The influence of exchange energy in the ferromagnet and barrier strength at the superconductor/ferromagnet interface on the inverse proximity effects is discussed.

Signature of odd-frequency pairing correlations induced by a magnetic interface

We investigate the mutual proximity effect in a normal metal contacted to a superconductor through a magnetic interface. Analytical and self-consistent numerical results are presented, and we consider both the diffusive and ballistic regimes. We focus on the density of states in both the normal and superconducting region, and find that the presence of spin-dependent phase-shifts occurring at the interface qualitatively modifies the density of states. In particular, we find that the proximity-induced pairing amplitudes in the normal metal region undergo a conversion at the Fermi level from pure even-frequency to odd-frequency. Above a critical value of the interface spin-polarization (or, equivalently, for fixed interface spin-polarization, above a critical interface resistance), only odd frequency correlations remain. This is accompanied by the replacement of the familiar proximity minigap or pseudogap in the normal layer by an enhancement of the density of states above its normal state value for energies near the chemical potential. The robustness of this effect towards inelastic scattering, impurity scattering, and the depletion of the superconducting order parameter close to the interface is investigated. We also study the inverse proximity effect in the diffusive limit. We find that the above-mentioned conversion persists also for thin superconducting layers comparable in size to the superconducting coherence length $\xi_\text{S}$, as long as the inverse proximity effect is relatively weak. Concomitantly, we find a shift in the critical interface resistance where the pairing conversion occurs. Our findings suggest a robust and simple method for producing purely odd-frequency superconducting correlations, that can be tested experimentally.

Evaluation of spin polarization in p-In0.96Mn0.04As using Andreev reflection spectroscopy

We report on the transport behavior and spin polarization P of carriers for superconductor/ferromagnetic semiconductor (S-F) junctions. We fabricated Nb/ferromagnetic semiconductor p-In0.96Mn0.04As junctions with Hall voltage probes. Below ~15 K, p-In0.96Mn0.04As becomes ferromagnetic and then the anomalous Hall effect is observed. Below the TC of the Nb electrodes (~8.2 K), we observed a conductance reduction within the Nb superconducting energy gap voltage owing to the suppression of Andreev reflection by spin polarization in p-In0.96Mn0.04As. We evaluated the degree of spin polarization in p-In0.96Mn0.04As experimentally by comparing the measured differential conductance with that obtained with Strijkers' model extended for spin-polarized Andreev reflection including the inverse proximity effect. Consequently, the P value extracted experimentally for p-In0.96Mn0.04As at 0.5 K was 0.795 assuming Z = 0.