arXiv subjects

Gad Koren

Publications and source records attributed to Gad Koren.

At least 19 recordsLinked to original sources

arXiv2025

Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced $SrTiO_3$

Since the discovery of superconductivity in nickelate thin films in 2019, the quest for enhancing their $\mathrm{T}_\mathrm{c}$ has been ongoing. Here we provide experimental evidence for $\mathrm{T}_\mathrm{c}$ enhancement in oxygen deficient films on highly reduced and conducting SrTiO$_3$ substrates. $\mathrm{T}_\mathrm{c}$ onset of 50-70 K was found in Meissner and transport measurements, which indicates superconductivity in islands or domains in our films, where $\mathrm{T}_\mathrm{c}$ of zero resistance is obtained at 20-25 K. In addition, we observed a giant paramagnetic-Meissner effect peak at about 48 K, which further supports the existence of a superconductive transition just above it. Furthermore, an asymmetric or nonreciprocal and non-hysteretic superconductive diode effect was observed. The latter effect could be fully polarized, and its polarity could be reversed. Our mixed phase films comprised of many Ruddlesden-Popper $\rm (Nd_{0.8}Sr_{0.2})_{n+1}Ni_nO_{3n+1}$ phases and includes the infinite-layer (IL) phase.

cond-mat.supr-con↗

arXiv2021

Carrier Density and Thickness Dependent Proximity Effect in Doped Topological Insulator -- Metallic Ferromagnet Bilayers

We use magneto-conductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist of a layer of Fe$_7$Se$_8$, which is a metallic ferrimagnet and a layer of Bi$_{0.8}$Sb$_{1.2}$Te$_{3}$ which is a highly hole-doped topological insulator. Using transport measurements and a modified Hikami-Larkin-Nagaoka model, we show that the ferromagnet shortens significantly the effective coherence length of the surface states, suggesting that a gap is opened at the Dirac point. We show that the magnetically induced gap persists on surface states which are separated from the magnet by a topological insulator layer as thick as 170 [nm]. Furthermore, the size of the gap is found to be proportional to the magnetization that we extract from the anomalous Hall effect. Our results give information on the ties between carrier density, induced magnetization and magnetically induced gap in topological insulator/ferromagnet bilayers. This information is important both for a theoretical understanding of magnetic interactions in topological insulators and for the practical fabrication of such bilayers, which are the basis of various suggested technologies, such as spintronic devices, far infra-red detectors etc.

cond-mat.mes-hall↗

arXiv2020

Observation of Josephson-like tunneling junction characteristics and positive magnetoresistance in Oxygen deficient Nickelate films of $Nd_{0.8}Sr_{0.2}NiO_{3-δ}$

Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of $Nd_{0.8}Sr_{0.2}NiO_2$ (obtained by reducing Sr doped $NdNiO_3$ films) and their similarity to the cuprates high temperature superconductors. Here we report on the observation of a new type of transport in oxygen poor $Nd_{0.8}Sr_{0.2}NiO_{3-δ}$ films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.

cond-mat.supr-con↗

arXiv2019

Unconventional order parameter induced by helical chiral molecules adsorbed on a metal proximity-coupled to a superconductor

Following our previous results, which provide evidence for the emergence of a chiral p-wave triplet-pairing component in superconducting Nb upon the adsorption of chiral molecules, we turned to investigate whether such an effect can take place in a proximal superconductor consisting of metal on superconductor bilayer. Note that in such proximity systems, correlated electron-hole (Andreev) pairs exist in the normal metal rather than genuine Cooper pairs. To that end, we used scanning tunneling spectroscopy (STS) on thin Au films grown in-situ on NbN (a conventional s-wave superconductor) before and after adsorbing helical chiral, alpha-helix polyalanine molecules. The tunneling spectra measured on the pristine Au surface showed conventional (s-wave like) proximity gaps. However, upon molecules adsorption the spectra significantly changed, all exhibiting a zero-bias conductance peak embedded inside a gap, indicating unconventional superconductivity. The peak reduced with magnetic field but did not split, consistent with equal-spin triplet-pairing p-wave symmetry. In contrast, adsorption of non-helical chiral cysteine molecules did not yield any apparent change in the order parameter, and the tunneling spectra exhibited only gaps free of in-gap structure.

cond-mat.supr-con↗

arXiv2018

Gating of Au-$Bi_2Se_3$-NbN ramp-type junctions with superconducting NbN gate-electrode and $SrTiO_3$ film as the gate-insulator

Ramp-type junctions of $\rm Au-Bi_2Se_3-NbN$ were prepared on top of a bottom gate comprised of a $\rm SrTiO_3$ gate-insulator film on $\rm NbN$ gate-electrode layer on (100) $\rm SrTiO_3$ wafer. Two wafers with gate-insulator thickness of 120 and 240 nm were studied, with the former showing higher gate leakage currents Ig at high gate voltages Vg, leading to heating effects and shifting of the junctions conductance spectra versus the voltage bias. At Vg=0 V, the conductance spectra of the low resistance junctions showed zero bias conductance peaks inside a tunneling gap with typical conductance drops when the critical current Ic was reached, while the high resistance ones exhibited tunneling conductance only. For Vg$>$-0.2 V ($\rm E\simeq$ -2 MV/cm) of the wafer with 120 nm thick gate-insulator linear Ig vs Vg was found, while for Vg$<$-0.2 V, Ig saturation was observed, leading to quadratic and linear heating effects at positive and negative high Vg values, respectively. This led to asymmetric conductance spectra shifts versus Vg which followed almost exactly the Ig vs Vg behavior. In the wafer with twice the gate-insulator thickness (240 nm), heating effects were strongly suppressed, and symmetric small peak shifts appeared only under the highest Vg values of Vg=$\pm$2 V ($\rm E\simeq \pm$ 10 MV/cm). Under Vg=2 V, a 5\% lower conductance was observed as compared to Vg=-2 V, indicating a small Fermi energy shift in our junctions under $\pm$10 MV/cm fields.

cond-mat.supr-con↗

arXiv2018

Strongly suppressed superconducting proximity effect and ferromagnetism in trilayers of $\rm Bi_2Se_3$ / $\rm SrRuO_3$ / underdoped $\rm YBa_2Cu_3O_x$: A possible new platform for Majorana nano-electronics

We report properties of topological insulator - ferromagnet - superconductor trilayers comprised of thin films of 20 nm thick $\rm Bi_2Se_3$ on 10 nm $\rm SrRuO_3$ on 30 nm $\rm YBa_2Cu_3O_x$. As deposited trilayers are underdoped and have a superconductive transition with $\rm T_c$ onset at 75 K, zero resistance at 65 K, $\rm T_{Cueri}$ at 150 K and $\rm T^*$ of about 200 K. Further reannealing under vacuum yields the 60 K phase of $\rm YBa_2Cu_3O_x$ which still has zero resistance below about 40 K. Only when $10\times 100$ micro-bridges were patterned in the trilayer, some of the bridges showed resistive behavior all the way down to low temperatures. Magnetoresistance versus temperature of the superconductive ones showed the typical peak due to flux flow against pinning below $\rm T_c$, while the resistive ones showed only the broad leading edge of such a peak. All this indicates clearly weak-link superconductivity in the resistive bridges between superconductive $\rm YBa_2Cu_3O_x$ grains via the topological and ferromagnetic cap layers. Comparing our results to those of a reference trilayer with the topological $\rm Bi_2Se_3$ layer substituted by a non-superconducting highly overdoped $\rm La_{1.65}Sr_{0.35}CuO_4$, indicates that the superconductive proximity effect as well as ferromagnetism in the topological trilayer are actually strongly suppressed compared to the non-topological reference trilayer. This strong suppression is likely to originate in strong proximity induced edge currents in the SRO/YBCO layer that can lead to Majorana bound states, a possible signature of which is observed in the present study as zero bias conductance peaks.

cond-mat.supr-con↗

arXiv2018

Magnetic proximity effect of a topological insulator and a ferromagnet in thin film bilayers of $BiSbTe_3$ and $SrRuO_3$

Magnetic proximity effect of a topological insulator in contact with a ferromagnet is reported in thin film bilayers of 15 nm thick $BiSbTe_3$ on either 15 or 40 nm thick $SrRuO_3$ on (100) $SrTiO_3$ wafers. Magneto transport results of the bilayers were compared with those of reference films of 15 nm $BiSbTe_3$ and 15 or 40 nm $SrRuO_3$. Comparison of the temperature coefficient of resistance [(1/R)$\times$dR/dT which is qualitatively proportional to the magnetization] of the bilayer and reference ferromagnetic film normalized above $T_c$, shows a clear suppression in the bilayer by about 50% just below $T_c$, indicating a weaker proximity magnetization in the bilayer. Resistance hysteresis loops versus field at 1.85$\pm$0.05 K in the bilayer and reference films show a clear magnetic proximity effect, where the peak resistance of the bilayer at the coercive field shifts to lower fields by $\sim$30% compared to a hypothetical bilayer of two resistors connected in parallel with no interaction between the layers. Narrowing of the coercive peaks of the bilayers as compared to those of the reference ferromagnetic films by 25-35% was also observed, which represents another signature of the magnetic proximity effect.

cond-mat.str-el↗

arXiv2016

Proximity effects and pair currents in cuprate junctions

Proximity effects and pair currents were measured in epitaxial trilayer \textit{c-axis} junctions comprise of a $PrBa_2Cu_3O_{7-δ}$ barrier sandwiched in between an overdoped $Y_{0.94}Ca_{0.06}Ba_2Cu_3O_{7-δ}$ and underdoped $YBa_2Cu_{2.7}Co_{0.3}O_y$ layers. These junctions had two $\rm T_c$ values of $\rm T_c(high)=84-86$ K and $\rm T_c(low)=50-55$ K, allowing investigation when both electrodes are superconducting, or when only one is superconducting while the other is in its pseudogap regime. For T below $\rm T_c(high)$ but much above $\rm T_c(low)$, two distinct proximity effect transitions were observed in the resistance at two temperature regimes, between 80 and 84 K, and 76 to 80 K. The first is a conventional proximity effect with the $\rm T_c(high)$ electrode, while the second is a second-order proximity effect of this electrode with uncorrelated pairs in the pseudogap regime. Conductance spectra measured between 2 and 86 K showed four different $I_c$ pair currents which were attributed to coherent pairs tunneling through the barrier below 42 K, to fluctuating pairs current up to $\sim$77 K, and to proximity pairs current between 77 and 84 K. All pair currents were suppressed under magnetic fields, with two distinct decay parameters that originated in the two different electrodes, with a significant suppression observed in the pseudogap regime.

cond-mat.supr-con↗

arXiv2016

Observation of two distinct pairs fluctuation lifetimes and supercurrents in the pseudogap regime of cuprate junctions

Pairs fluctuation supercurrents and inverse lifetimes were measured on epitaxial c-axis junctions of the cuprates in the pseudogap regime, with a $PrBa_2Cu_3O_{7-δ}$ barrier sandwiched in between two $YBa_2Cu_3O_{7-δ}$ or doped $YBa_2Cu_3O_y$ electrodes, with or without magnetic fields parallel to the a-b planes. All junctions had a $\rm T_c(high)\approx 85-90$ K and a $\rm T_c(low)\approx 50-55$ K electrodes, allowing us to study pairs fluctuation supercurrents and inverse life times in between these two temperatures. In junctions with a pseudogap electrode under zero field, an excesss current due to pair fluctuations was observed which persisted at temperatures above $\rm T_c(low)$, in the pseudogap regime, and up to about $\rm T_c(high)$. No such excess current was observed in junctions without a pseudo-gap in the electrode. The measured conductance spectra at temperatures above $\rm T_c(low)$ were fitted using a modified fluctuations model by Scalapino [Phys. Rev. Lett. \textbf{24}, 1052(1970)] of a junction with a serial resistance. We found that in the pseudo-gap regime, the conductance vs voltage consists of a narrow peak sitting on top of a very broad peak. This yielded two distinct pairs fluctuation lifetimes in the pseudogap electrode which differ by an order of magnitude up to about $\rm T_c(high)$. Under in-plane fields, these two lifetime values remain separated in two distinct groups, which varied with increasing field moderately. We also found that detection of Amperian pairing [Phys. Rev. X \textbf{4}, 031017 (2014)] in our cuprate junctions is not feasible, due to Josephson vortices penetration into the superconducting electrodes which drove the necessary field above the depairing field.

cond-mat.supr-con↗

arXiv2015

Magnetoresistance and gating effects in ultrathin NbN-$\rm Bi_2Se_3$ bilayers

Ultrathin $\rm Bi_2Se_3$-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick $\rm Bi_2Se_3$ film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR=[R(H)-R(0)]/\{[R(H)+R(0)]/2\}), and under three electric gate-fields of 0 and $\pm2$ MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the $\rm Bi_2Se_3$ cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime.

cond-mat.supr-con↗

arXiv2014

Proximity effects at the interface of a superconductor and a topological insulator in NbN - Bi_2Se_3 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 $\rm Bi_2Se_3$ 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 $\rm NbN-Bi_2Se_3$ 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 $\rm Bi_2Se_3$ suppresses the onset or mid-transition $T_c$ 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 $T_c(R=0)$, $I_c$ (the supercurrent) and the Andreev conductance, as compared to the bare NbN films. This indicates that superconductivity is induced in the $\rm Bi_2Se_3$ layer at the interface region in between the NbN grains. Thus an inverse proximity effect in the topological material is demonstrated.

cond-mat.supr-con↗

arXiv2014

Pairing and the phase diagram of the normal coherence length $ξ_N(T,x)$ above $T_c$ of $La_{2-x}Sr_xCuO_4$ thin films probed by the Josephson effect

Critical currents $I_c$ were measured in Superconducting - normal - superconducting SNS c-axis junctions, where S was optimally doped $YBa_2Cu_3O_{7-δ}$ below $T_c$ (90 K) and N was $La_{2-x}Sr_xCuO_4$ above its $T_c$ ($<$25 K) but in the pseudogap regime. $I_c$ was measured versus temperature T in junctions with various N-barrier thicknesses $d$ and doping levels $x$. From the proximity effect relation $I_c(T)\propto exp[-d/ξ_N(T)]$, the normal coherence length $ξ_N(T)$ of N was extracted and plotted versus T. Besides finding a long range proximity effect as reported also by others, we found a crossing at about 55 K above which $ξ_N(x=0.1)>ξ_N(x=0.18)]$. This unexpected result where junctions with lower carrier density in the barrier have higher conductance, is attributed to the presence of pre-formed pairs in the pseudogap regime of the cuprate barrier. Our data yields a new phase diagram of $ξ_N(T,x)$ whose contours of constant $ξ_N$ follow the trend of the superconducting dome of $La_{2-x}Sr_xCuO_4$ above its $T_c$, with $ξ_N$ enhancement in the underdoped regime above 55 K.

cond-mat.supr-con↗

arXiv2013

Signature of proximity induced triplet superconductivity in junctions made of the doped topological insulator $\rm Bi_2Se_3$ and the s-wave superconductor NbN

Thin film junctions and bilayers of the doped topological insulator $\rm Bi_2Se_3$ and the s-wave superconductor NbN were found to exhibit conductance spectra with prominent zero bias and coherence peaks. Various tunneling models with different pair potentials have failed to fit our data, except for the triplet $p_x+ip_y$ pair potential, which breaks time reversal symmetry, but yielded reasonably good fits. This provides evidence for proximity induced triplet superconductivity in the $\rm Bi_2Se_3$ layer near the interface with the NbN film.

cond-mat.supr-con↗

arXiv2012

Zero energy bound states in tunneling conductance spectra at the interface of an s-wave superconductor and a topological insulator in NbN-$Bi_2Se_3$-Au thin film junctions

Measurements of conductance spectra in a superconductor - topological insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below $\rm T_c$ of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the $\rm Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.

cond-mat.supr-con↗

arXiv2011

Measurement of the vortex mass in a superconducting film

We have combined high resolution magneto-optical imaging with an ultra-fast heating/cooling technique to measure the movement of individual vortices in a superconducting film. The motion took place while the film was heated close to $T_c$, where pinning and viscous forces are relatively small. Under these conditions, vortices move due to the magnetic repulsion between them. We found that a finite vortex mass has to be included in the analysis in order to account for the experimental results. The extent of the motion is consistent with a vortex mass being 3 orders of magnitude smaller than the mass of all the electrons in the core.

cond-mat.supr-con↗

arXiv2011

Transport and spectroscopic properties of superconductor - ferromagnet - superconductor junctions of $La_{1.9}Sr_{0.1}CuO_4$ - $La_{0.67}Ca_{0.33}MnO_3$ - $La_{1.9}Sr_{0.1}CuO_4$

Transport and Conductance spectra measurements of ramp-type junctions made of cuprate superconducting $La_{1.9}Sr_{0.1}CuO_4$ electrodes and a manganite ferromagnetic $La_{0.67}Ca_{0.33}MnO_3$ barrier are reported. At low temperatures below $T_c$, the conductance spectra show Andreev-like broad peaks superposed on a tunneling-like background, and sometimes also sub-gap Andreev resonances. The energy gap values $Δ$ found from fits of the data ranged mostly between 7-10 mV. As usual, the gap features were suppressed under magnetic fields but revealed the tunneling-like conductance background. After field cycling to 5 or 6 T and back to 0 T, the conductance spectra were always higher than under zero field cooling, reflecting the negative magnetoresistance of the manganite barrier. A signature of superparamagnetism was found in the conductance spectra of junctions with a 12 nm thick LCMO barrier. Observed critical currents with barrier thickness of 12 nm or more, were shown to be an artifact due to incomplete milling of one of the superconducting electrodes.

cond-mat.supr-con↗

arXiv2011

Critical current measurements in superconductor - ferromagnet - superconductor junctions of $YBa_2Cu_3O_y$-$SrRuO_3$-$YBa_2Cu_3O_y$: No evidence for a dominant proximity induced triplet superconductivity in the ferromagnetic barrier

Transport measurements in ramp-type junctions of $YBa_2Cu_3O_y-SrRuO_3-YBa_2Cu_3O_y$ with $T_c$ values of either 80-90 K or 60-70 K are reported. In both type of junctions but without a barrier ("shorts"), the supercurrent densities at 4.2 K reached 7.5 and 3.5 MA/cm$^2$, respectively, indicating the high quality of the fabrication process. Plots of the critical current versus thickness of the ferromagnetic barrier at 4.2 K show exponential decays with decay lengths of 1.1 nm for the 90 K phase and 1.4 nm for the 60 K phase, which are much shorter than the relevant coherence lengths $ξ_F\sim 5-6$ nm or $ξ_N\sim$16 nm of $SrRuO_3$. We thus conclude that there is no dominant proximity induced triplet superconductivity in the ferromagnet in our junctions.

cond-mat.supr-con↗

arXiv2011

Observation of two Andreev-like energy scales in $La_{2-x}Sr_{x}CuO_4$ superconductor/normal-metal/superconductor junctions

Conductance spectra measurements of highly transparent ramp-type junctions made of superconducting $La_{2-x}Sr_{x}CuO_4$ electrodes and non superconducting $La_{1.65}Sr_{0.35}CuO_4$ barrier are reported. At low temperatures below $T_c$, these junctions have two prominent Andreev-like conductance peaks with clear steps at energies $Δ_1$ and $Δ_2$ with $Δ_2 > 2Δ_1$. No such peaks appear above $T_c$. The doping dependence at 2 K shows that both $Δ_1$ and $Δ_2$ scale roughly as $T_c$. $Δ_1$ is identified as the superconducting energy gap, while a few scenarios are proposed as for the origin of $Δ_2$.

cond-mat.supr-con↗