Superconductor Nb Research Articles

Proximity effect and electron transport in oxide hybrid heterostructures withsuperconducting/magnetic interfaces

We report on electron transport in oxide heterostructures with superconducting/magnetic(S/M) thin film interfaces. The investigated hybrid mesa-heterostructuresconsist of a cuprate superconductor, a nonsuperconducting cuprate(antiferromagnetic) or manganite (ferromagnetic) interlayer with thicknessdM = 5–50 nm and a conventional superconductor (Nb). The superconducting critical current(IC) with a criticalcurrent density jc = 103 A cm − 2 (for dM = 10 nm) and acharacteristic voltage ICRN = 100–200 µV (RN is normal resistance) are observed at liquid helium temperature for aCaXSr1 − XCuO2 antiferromagnetic cuprate interlayer with a thickness ofdM = 10–50 nm. The superconducting current-phase relation of heterostructures deviates from theregular sine type, demonstrating a second harmonic component. These hybridheterostructures with S/M interfaces show unusually high sensitivity to external magneticfields. When substituting the cuprate interlayer by a manganite film, no critical current wasobserved although the manganite interlayer was made several times thinner (down todM ≤ 5 nm).

Spin-triplet supercurrent in Co-based Josephson junctions

In the past year several groups have reported experimental evidence for spin-tripletsupercurrents in Josephson junctions containing strong ferromagnetic materials. In thispaper we present several new experimental results that follow up on our previous work. Westudy Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor(Nb), N is a normal metal, SAF is a synthetic antiferromagnet of the form Co/Ru/Co andX is an ferromagnetic layer necessary to induce spin-triplet correlations in the structure.Our work is distinguished by the fact that the generation of spin-triplet correlations istuned by the type and thickness of the X layers. The most important new result reportedhere is the discovery that a conventional, strong ferromagnetic material, Ni, performs wellas the X layer, if it is sufficiently thin. This discovery rules out our earlier hypothesis thatout-of-plane magnetocrystalline anisotropy is an important attribute of the Xlayers. These results suggest that the spin-triplet correlations are most likelyinduced by noncollinear magnetization between the X layers and adjacent Co layers.

Contactless Estimation of Critical Current Density and Its Temperature Dependence Using Magnetic Measurements

Contactless Estimation of Critical Current Density and Its Temperature Dependence Using Magnetic Measurements A. Youssef, L. Banicova, Z. Svindrych and Z. Janůb,∗ Charles University in Prague, Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic Institute of Physics of the AS CR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic We have measured magnetic response of second-generation high temperature superconductor YBa2Cu3Ox wire and Nb thin film in perpendicular ac field as a function of temperature. We compare experimental complex ac susceptibility to the calculated susceptibility based on the model of Bean’s critical-state response in two-dimensional disk in perpendicular field. The harmonic analysis is needed for comparison between the model and the experimental data. We present a method of linking model and experimental susceptibility. Good agreement of experimental susceptibility with model susceptibility of 2D disk allows contactless estimation of critical depinning current density and its temperature dependence.

KOHN ANOMALY ENERGY IN CONVENTIONAL SUPERCONDUCTORS EQUALS TWICE THE ENERGY OF THE SUPERCONDUCTING GAP: HOW AND WHY?

Kohn anomaly occurs in metals as a weak but discernible kink in the phonon spectrum around 2kF arising out of screened Coulombic interaction. Over the years this has been observed in a number of normal metallic systems. As a major surprise however, the recent neutron spin-echo experiments on elemental (conventional) superconductors Pb and Nb reveal a very important and striking relation that Kohn (anomaly) energy equals twice the energy of the superconducting gap. In this paper we explore the microscopic origin of this novel phenomenon and discuss its implication to the standard model BCS Theory.

Non-local Quasi-Particles Current in the Lateral SNS Junction with Transparent Interfaces

We have fabricated multi-terminal superconductor/normal metal/superconductor junctions comprising superconductor Nb and normal metal Cu. By means of the local and non-local measurements of the junction, we successfully detected the non-existence of a bias current in the Cu wire and the non-local voltage. These results indicate that the current flowing non-locally over the junction is due to the proximity effects and the conversion of the supercurrent and normal current.

Second superconducting energy gap ofNb3Snobserved by breakjunction point-contact spectroscopy

We report on investigations of the superconducting energy gap of the A15 superconductor Nb_3Sn by point-contact spectroscopy of breakjunctions. The voltage-dependent differential conductance dI/dV reveals features of a second energy gap besides the energy gap known from previous tunnel measurements with maxima at \Delta_1=3.92 +/- 0.16 meV and \Delta_2=0.85 +/- 0.17 meV as derived from a histogram summarizing the data of more than 60 contacts. These findings are the first spectroscopic evidence that Nb_3Sn belongs to the class of two-band superconductors and they are in line with low-temperature specific-heat measurements on Nb_3Sn.

Comparative study of mono- and multi-filamentMgB2 wires with different boron powders and malic acid addition

We evaluated the effects of different boron (B) powders and of malic acid(C4H6O5) addition on the criticalcurrent density (Jc) ofmono- and multi-filament MgB2 wires. We found that Jc of the wires made from the amorphous B showed the best performance. Lowsintering processing keeps the grain size small, which leads to strong fluxpinning. From transmission electron microscopy (TEM) observations, weconfirmed that the crystalline B powders supplied by SMI and Tangshan had aβ-rhombohedral structure as a main phase. These two powders also had numerousdefects, such as twin and stacking faults inside the powder particles. However, thesedefects are not enough to promote the full reaction between magnesium (Mg) andB crystalline powder when sintered at low temperatures, and brings about lowJc values. Even though the morphologies of the two crystalline B powders appeared somewhat different,Jc for the two wires had similar quantitative values. For the malic acid doped wires,Jc of the wire made from the amorphous B was estimated to be above 100 000 A cm − 2 at6 T and 4.2 K. This value is comparable to the low temperature superconductor Nb–Ti. In addition, theJc was25 000 A cm − 2 at 10 T and 4.2 K. This performance is the bestJc reported so far in in situprocessed MgB2 wires.Interestingly, the low-field Jc performance of malic acid doped wire fabricated with crystalline B is higher than that ofthe pure wire fabricated with amorphous B. At 20 K, enhanced performance ofJc by malic acid doping is much higher than that of the pureMgB2 wire. TheJc at 5 T and 20 K wasestimated to be 13 000 A cm − 2.

Nonlocal injection of spin current into a superconducting Nb wire

Pure spin current injection into a superconductive Nb was demonstrated experimentally by employing a nonlocal spin absorption technique in a multiterminal lateral structure. From the reduction in spin signals, we found the absorption efficiency remained almost unchanged even after superconducting transition. The Nb/Cu interface properties were also complementarily examined by using a Nb/Cu/Nb junction incorporated in the same device, revealing superconductivity below the transition temperature. These experimental results support that the pure spin current can be absorbed into a superconductive Nb wire with almost the same efficiency as in a normal state.

Magnetic field dependence of the coupling efficiency of a superconducting transmission line due to the proximity effect

The coupling efficiency of a Nb superconducting transmission line has been measured using a Fourier transform spectrometer for different magnetic fields. It is found that the coupling decreases with increasing magnetic field when the frequency is close to the gap of the Nb superconductor. This is attributed to the changes of the surface impedance of the proximity-coupled superconductor/normal-metal bilayers in the transmission line.

Measuring the dependence of weight on temperature in the low-temperature regime using a magnetic suspension balance

A novel set-up was developed that allows us to cool samples close to liquid helium temperatures, measure their exact temperature and determine their weight in a buoyancy-free environment along a wide temperature range using a magnetic suspension balance. This allows for the first time to accurately determine the weight of both high-Tc (BSCCO and YBCO) and low-Tc (Nb) superconductors during their phase transition. Our data allow us to put limits on possible weight changes over temperature (α < 2 × 10−8 K−1 for copper) as well as violations of the weak equivalence principle for superconductors while passing their critical temperature (η < 2 × 10−3).

Rectified vortex motion in an Nb film with a spacing-graded array of holes

Superconducting Nb thin films with a spacing-graded array of holes were prepared by electron beam lithography. Two films with different hole gradients were fabricated. The ac-driven vortices were investigated in Nb superconductors with a spacing-graded array of holes. The measurements revealed pronounced rectified voltage when the vortex lattice is driven by an ac injected current. The rectified voltage is mainly caused by the strength of the vortex–vortex interaction. The rectified motion of a vortex is affected by the pinning potential of the spacing-graded array and the applied magnetic field. The vortex–vortex interaction strength changes the effective pinning landscape of the vortices and an asymmetric potential is formed. Vortices depin easily from high concentration to low concentration of pinning sites. In both samples, the ac-driven vortices exhibit a variety of dynamical responses and the rectified voltage is tunable with the applied magnetic field.

Progress in the Development of Cryocooled Digital Channelizing RF Receivers

HYPRES is developing a class of digital receivers featuring direct digitization at radio frequency. The complete system, consisting of a cryopackaged Nb superconductor all-digital receiver (ADR) chip followed by room-temperature interface electronics and a field-programmable gate array (FPGA) based post-processing module, has been developed. Depending on the targeted application the ADR chip comprised either a low-pass delta with phase modulation-demodulation architecture or X-band band-pass sigma-delta modulators together with digital in-phase and quadrature mixer and a pair of digital decimation filters. The chips were fabricated using a 4.5-kA/cm2 HYPRES process and were cryopackaged using a commercial-off-the-shelf cryocooler. Recently, with significant improvements in chip cryopackage, room-temperature electronics and FPGA programming we were able to achieve stable operation of a low-pass ADR at 28.16 GHz and X-band ADR at 30.72 GHz clock frequencies. Experimental results are presented and discussed.

Novel superconducting proximized heterostructures for ultrafast photodetection

Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R/ R) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.

Superconductivity-induced magnetoresistance suppression in hybrid superconductor/magnetic tunnel junctions

Hybrid superconductor/magnetic tunnel junctions of Nb/CoFe/AlO/CoFe/NiFe have been implemented to investigate the impact of the superconducting Nb layer on the proximal ferromagnetic CoFe layer. A suppression of tunneling magnetoresistance effect is observed as the Nb layer undergoes the superconducting transition. This suppression is thought to be due primarily to the coupling of the magnetic inhomogeneity of CoFe to the superconductivity of Nb. The magnetic structure-vortex interactions interplay each other, inducing a complex domain evolution in the magnetization process and consequently degrading the junction’s antiparallel magnetization alignment.

Resistive transitions in Nb/Cu0.41Ni0.59/Nb trilayers

The superconducting phase transition in Nb/Cu0.41Ni0.59/Nb trilayers, with superconducting (S) Nb and ferromagnetic (F) Cu0.41Ni0.59, has been experimentally studied as a function of the F-layer thickness by measuring the temperature dependence of the electrical resistance R(T). It is shown that the shape and the width of the R(T) curves depends on the Cu0.41Ni0.59 thickness, in particular in the regime where π is the coupling between the S layers, which can be expected. To explain the data, we developed a qualitative model which makes the interconnection between the superconducting phase transition and the 0 to π transition in SFS structures are more evident.

Onset of phase correlations inYBa2Cu3O7−xas determined from reversible-magnetization measurements

Isofield magnetization curves are obtained and analyzed for three single crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as well as the BCS superconductor Nb, in the presence of magnetic fields applied both parallel and perpendicular to the $ab$ planes. Near Tc, the magnetization exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with recent theories, we associated Ta(H) with the onset of coherent phase fluctuations of the superconducting order parameter. For Nb and optimally doped YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H). The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis, suggesting that the magnetization is probing a region of temperatures above Tc where phase correlations persist. In this region, the fitting exponent falls in the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab planes. The results are interpreted in terms of an anisotropic pairing symmetry of the order parameter: d-wave along the ab planes and s-wave along the c axis.

Magnetic state modification induced by superconducting response in ferromagnet/superconductor Nb/Co superlattices

Magnetization measurements in superconductor/ferromagnet $\mathrm{Nb}∕\mathrm{Co}$ superlattices show a complex behavior as a function of temperature, applied field, and sample history. Based on a simple model, it is shown that this behavior is due to an interplay between the superconductor magnetization temperature dependence, the ferromagnet magnetization time dependence, and the stray fields of both materials. It is also shown that the magnetic state of the Co layers is modified by the Nb superconducting response, implying that the problem of a superconductor/ferromagnetic heterogeneous sample has to be solved in a self-consistent manner.

Energy Spectra of Field Emitted Electrons

Energy spectra of field emission electrons from some materials at different temperatures are reviewed in the practical points of views; they are conventional refractory metals, multi-walled carbon nano-tubes, Nb superconductors and single-atom electron sources. While the spectra of conventional refractory metals are explained well on the basis of Folwer-Nordheim theory, those of the other systems are not; the main peak with a different shape appears in the spectra of CNT, the extra peaks overlap in addition to the main peak of CNT and the single-atom electron sources. An extra sharp peak appears at the Fermi level of the spectra of a Nb superconductor.

紫外レーザー励起高分解能光電子分光の開発と応用

In this review, we have presented recent high-resolution photoemission results by using the ultra-violet laser as an excitation source. Due to the narrow band width of the light source and the improvement of the detection system, the overall instrumental resolution was improved up to 0.35 meV. Low energy excitation allows us obtaining the more bulk-sensitive measurements than the conventional high-resolution photoemission spectroscopy. These advantages appeared on some recent results on superconductors (Nb and Mg(B1-xCx)2) and other correlated materials (FeSi and LiV2O4). Angle-resolved photoemission of Bi2Sr2CaCu2O8+δ shows very sharp spectra, which allows us to discuss much more intrinsic electronic structure. These results demonstrate the promising future of the high-resolution and low energy excitation photoemission spectroscopy.

Synergy of exchange bias with superconductivity in ferromagnetic–superconductinglayered hybrids: the influence of in-plane and out-of-plane magnetic order onsuperconductivity

It is generally believed that superconductivity and magnetism are two antagonisticlong-range phenomena. However, as was preliminarily highlighted in Stamopoulos et al (2007 Phys. Rev. B 75 014501), and extensively studied in this work, under specificcircumstances these phenomena instead of being detrimental to each other may evenbecome cooperative so that their synergy may promote the superconducting properties of ahybrid structure.Here, we have studied systematically the magnetic and transport behaviorof such exchange biased hybrids that are comprised of ferromagnetic (FM)Ni80Fe20 andlow-Tc superconducting (SC) Nb for the case where the magnetic field is applied parallel to thespecimens. Two structures have been studied: FM–SC–FM trilayers (TLs) and FM–SCbilayers (BLs). Detailed magnetization data on the longitudinal and transversemagnetic components are presented for both the normal and superconductingstates. These data are compared to systematic transport measurements includingI–V characteristics. The comparison of the exchange biased BLs and TLs that are studied herewith the plain ones studied in Stamopoulos et al (2007 Phys. Rev. B 75 184504) enable usto reveal an underlying parameter that may falsify the interpretation of the transportproperties of relevant FM–SC–FM TLs and FM–SC BLs investigated in the recentliterature: the underlying mechanism motivating the extreme magnetoresistancepeaks in the TLs relates to the suppression of superconductivity mainly due tothe magnetic coupling of the two FM layers as the out-of-plane rotation of theirmagnetizations takes place across the coercive field where stray fields emerge in theirwhole surface owing to the multidomain magnetic state that they acquire. Therelative in-plane magnetization configuration of the outer FM layers exerts asecondary contribution on the SC interlayer. Since the exchange bias directlycontrols the in-plane magnetic order it also controls the out-of-plane rotation of theferromagnets’ magnetizations so that the magnetoresistance peaks may be tuned at will.All the contradictory experimental data reported in the recent literature are discussedfairly in the light of our results; based on a specific prerequisite we propose aphenomenological stray-field mechanism that efficiently explains the evolution of themagnetoresistance effect in TLs. Our experiments not only point out the need for a newtheoretical treatment of FM–SC hybrids but also direct us toward the design of efficientsupercurrent-switch elemental devices.