Normal State Resistivity Research Articles

Controlled dc Monitoring of a Superconducting Qubit.

Creating a transmon qubit using semiconductor-superconductor hybrid materials not only provides electrostatic control of the qubit frequency, it also allows parts of the circuit to be electrically connected and disconnected insitu by operating a semiconductor region of the device as a field-effect transistor. Here, we exploit this feature to compare in the same device characteristics of the qubit, such as frequency and relaxation time, with related transport properties such as critical supercurrent and normal-state resistance. Gradually opening the field-effect transistor to the monitoring circuit allows the influence of weak-to-strong dc monitoring of a "live" qubit to be measured. A model of this influence yields excellent agreement with experiment, demonstrating a relaxation rate mediated by a gate-controlled environmental coupling.

The Josephson Effect in PbIn/Fe1 –ySe1 –xTex Point Contacts: Probing the Order Parameter Symmetry

Characteristics of the Josephson effect in Pb0.6In0.4/FeSe0.4Te0.6 point contacts with the current flowing parallel to ab plane of the iron chalcogenide crystal (with critical temperature Tc ≈ 15 K) have been studied. Dependences of the critical current Ic and amplitudes In of the first steps of current (Shapiro steps) in the current–voltage (I–V) characteristics (n = 0, 1, 2; I0 = Ic) on the microwave power P (at frequency f = 7.6 GHz) and dependences of the characteristic voltage Vc(T) = IcRN (RN being the normal state resistance) on the temperature T have been measured. It is found that the characteristics of contacts (Ic, RN, Vc) obtained for initial parts of the I–V curves do not adequately describe the oscillations of current steps in the microwave radiation field. A method is proposed for determining the normalized microwave frequency Ω = 2πf(2eVc/$$\hbar $$)–1 that provides quantitative description of the current oscillation steps in the framework of a resistive contact model and allows one to check that the Josephson superconducting current Is is proportional to sin(aφ) (a = 1, 2), where φ is the phase difference of the order parameters. Comparison of the measured In($$\sqrt P $$) curves to those calculated using the resistive model showed that Is = Icsinφ, in agreement with the conventional s and s++ symmetry of the order parameter in FeSe0.4Te0.6. It is established that Vc of the contacts studied is proportional to T in a wide temperature interval.

Recovery performance of partially-joined porous-stabilized REBCO tape for resistive type superconducting fault current limiters

In the case of using rare-earth barium copper oxide (REBCO) tape as a current-limiting element, there are three requirements: high normal state resistance, high thermal stability, and quick recovery to the superconducting state. In this study, a new type of REBCO tape with a metal porous medium as a stabilizer (a porous-stabilized REBCO tape) was proposed, which has high resistance compared to bulk material and prevents film boiling due to its strong capillary force. First, a numerical simulation was performed to evaluate the current-limiting performance of the porous-stabilized REBCO tape with a Ni–Cr porous medium. The results showed that the current-limiting performance was dramatically improved by placing indium partially on the REBCO tape to join the porous medium compared to placing indium wholly. Then, the porous-stabilized REBCO tapes with various arrays of indium were manufactured, and tested to confirm its recovery characteristics. The experimental results indicated that the partially-joined Ni–Cr porous-stabilized REBCO tape reduced the recovery time by 69% compared to that of the stabilizer-free REBCO tape.

Hybrid Nb/Al/Ba0.5K0.5Fe2As2 sandwich Josephson junctions

We fabricated hybrid Josephson junctions between a Ba0.5K0.5Fe2As2 (BKFA) single crystal and the conventional superconductor Nb with normal metal Al as the barrier. An in situ process was used to create clean BKFA/Al/Nb interfaces. Current transport is along the c-axis of BKFA. The current–voltage characteristics are slightly hysteretic at low temperatures. Except for some features indicative of resonant modes the shape of the current–voltage characteristics can be well described by the resistively and capacitvely shunted junction (RCSJ) model, with a product of the Josephson critical current and the normal state resistance of the junction exceeding 180 μV at low temperatures. The temperature dependence of the Josephson current is linear for temperatures down to 2 K. Under 40 GHz microwave irradiation integer Shapiro steps are observed. The power dependence of the Shapiro step-height is in good agreement with RCSJ simulations. Possible relations of our data with the symmetry of the superconducting order parameter of BKFA are discussed.

Vortex state study of superconducting Fe(Te, Se) thin films deposited on SrTiO3 substrate by pulsed laser deposition technique

Superconducting thin films of two thicknesses have been fabricated on (100) oriented SrTiO3 (STO) substrates using the target of composition Fe1.05Te0.50Se0.50 by pulsed laser deposition technique. The structural and transport properties of the fabricated thin films have been investigated and the results indicate the enhancement in the superconducting properties with increasing thickness of the thin films. The onset of the superconducting transition temperature of the grown thin films of thicknesses ∼78 nm and ∼177 nm are ∼12.10 and 12.62 K at 0 T magnetic field, respectively. To estimate the upper critical fields HC2(0), thermally activated energy (TAE) and vortex phase diagram, the magnetoresistance measurements have been performed in the magnetic field range of 0 - 8 T. HC2(0) have been calculated by Ginzburg Landau (GL) theory and Werthamer-Helfand-Hohenberg model by taking the criterion of 90%, 50% and 10% of normal state resistivity and the corresponding GL coherence lengths have also been calculated. In the present work, the TAE has been estimated by conventional Arrhenius relation and modified thermally activated flux flow (TAFF) theory. The power law dependence of TAE, shows prominently the possible planer defects in the system. From the modified TAFF model, the values of fitting parameter ‘q’ suggests the 3 dimensional behaviour of the vortices for both the grown thin films. The vortex phase diagram study reveals the transition from the vortex liquid to vortex glass state.

Design and performance of hafnium optical and near-IR kinetic inductance detectors

We report on the design and performance of microwave kinetic inductance detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 μΩ cm with RRR=1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process, which resulted in pixels with decay times of about 40 μs and resolving powers of ∼9 at 800 nm.

Electronic structure, phonon and superconductivity for WP 5d-transition metal

We have reported the electronic structure, elastic, mechanical, phononic, and superconductivity properties for the orthorhombic WP single crystal, which has very recently been discovered to be the first superconductor among 5d-transition metal pnictides using density functional theory. The calculated electronic band structure and density of states reveal that WP is semimetallic in nature and the bands are mainly strengthened by the d-orbital of W atoms as well as determined several semi-Dirac-like points near the Fermi level. Elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and the brittle/ductile nature were determined. Some of these values were found to be compatible with other theoretical values that we found. The phonon spectrum shows that the orthorhombic MnP-type WP structure is dynamically stable. The calculated Debye temperature is comparable to the fitting experimental measurement of the normal state resistivity into the Bloch–Grüneisen function. The electron-phonon coupling parameter shows that WP is weakly coupled. We have also calculated the critical temperature (Tc) value of ∼0.81 K, which matches the experimental estimated value using electrical resistance, ac magnetic sensitivity, and specific temperature measurements.

Onset of phase diffusion in high kinetic inductance granular aluminum micro-SQUIDs

Superconducting granular aluminum is attracting increasing interest due to its high kinetic inductance and low dissipation, favoring its use in kinetic inductance particle detectors, superconducting resonators or quantum bits. We perform switching current measurements on DC-SQUIDs, obtained by introducing two identical geometric constrictions in granular aluminum rings of various normal-state resistivities in the range from ρn = 250–5550 μΩ cm. The relative high kinetic inductance of the SQUID loop, in the range of tens of nH, leads to a suppression of the modulation in the measured switching current versus magnetic flux, accompanied by a distortion towards a triangular shape. We observe a change in the temperature dependence of the switching current histograms with increasing normal-state film resistivity. This behavior suggests the onset of a diffusive motion of the superconducting phase across the constrictions in the two-dimensional washboard potential of the SQUIDs, which could be caused by a change of the local electromagnetic environment of films with increasing normal-state resistivities.

Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO$_3$ thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped SrTiO$_3$ films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in SrTiO$_3$, but call into question the role played by ferroelectric fluctuations.

Effect of Indium doping on the superconductivity of layered oxychalcogenide La2O2Bi3Ag1-xInxS6

We report on the substitution effect of Indium (In) at the Ag site of layered oxychalcogenide La2O2Bi3Ag1−xInxS6. The Tc decreases with increasing In concentration. A hump in the normal state resistivity at an anomaly temperature (T*) near 180 K was observed for all the samples. The anomaly in the resistivity at T* is indicating the possible occurrence of a charge-density-wave (CDW) transition. The T* does not markedly change by In doping. The x dependence of Seebeck coefficient suggests that carrier concentration does not change by In doping. The EDX analysis indicates small amount of Bi deficiency, which suggests that the Bi site is slightly substituted by In. The CDW transition is robust against the In substitution at Ag site, while Tc is decreasing due to the Bi site substitution by In. On the basis of those analyses, we propose that the suppression of superconductivity in the In-doped La2O2Bi3Ag1-xInxS6 system is caused by negative in-plane chemical pressure effect and partial substitution of In for the inplane Bi site.

Thermally Activated ReBCO Switches for Charging High-Current Magnets

Aiming at design improvements for the International Axion Observatory, alternative options for powering of the 20-kA magnet are under consideration. Use of high-Temperature superconductors in a transformer-rectifier system cooled by a single stage cryocooler at 40 K-50 K level is a promising option, which allows to avoid massive movable bus bars, current leads, and related services. The overall efficiency of such a magnet powering system is determined by the characteristics of high-current switches, which have to operate in a fast synchronous manner. We studied the performance of thermally activated superconducting switches made of adjusted 4-mm-wide ReBCO tapes, using etching of the tapes in a FeCl-3 bath to increase their resistance in normal state and to prepare low-mass heaters perfectly matching the tape surface. First experimental results on the 'off' state performance and time constants of the switches, when toggling between 'on' and 'off' states, are presented. Finally, we discuss a conceptual design of an all-ReBCO rectifier system comprising a superconducting transformer and thermally activated switches, which can be scaled up for charging large magnet systems like particle detector magnets where slow charging is feasible.

Superconductivity Dependence on Epitaxial NbN Film Thickness

A series of NbN films (2.2–200 nm thick) were epitaxially grown on (100) oriented single crystal MgO substrates using reactive dc magnetron sputtering. As the film thickness decreased, the superconducting critical temperature ( T C) and the residual resistivity ratio both decreased, the normal state resistivity increased, while the zero temperature coherence length remained basically unchanged. In addition, a negative linear relationship existed between the T C and the normal state resistivity at 20 K. Hall Effect measurements showed that the carrier density of the 200-nm-thick NbN film that exhibited the highest T C (16.63 K) decreased from 1.12 × 1023 to 5.56 × 1022 e /cm−3 in going to the 2.2-nm-thick NbN film that exhibited the lowest T C (9.28 K). By fitting the data using McMillan theory for strong coupling superconductors, the T C for the NbN films was determined directly from the thickness-dependent carrier density.

Growth and Characterization of NbTiN Films Synthesized by Reactive Bias Target Ion Beam Deposition (RBTIBD)

High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ∼670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology—reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.

Normal-state and superconducting properties of Co-doped BaFe2As2 and MgB2 thin films after focused helium ion beam irradiation

We have investigated the normal-state and superconducting properties of Co-doped BaFe2As2 (Ba122) and MgB2 thin films irradiated at room temperature using a 30 keV focused He+ ion beam with doses between 1012 and 1017 cm2. We show that superconductivity is suppressed and the normal-state resistivity is increased upon irradiation. The critical dose for the complete suppression of superconductivity is ∼5 × 1014 cm−2 for Ba122 and ∼8 × 1015 cm−2 for MgB2. The dependence of the normal-state and superconducting properties on irradiation dose is discussed, taking into account the spatial distribution of ion-induced damage. Hillock formation due to the substrate swelling, arising from the He+ ions stopped in the substrate, is also observed. The findings provide guidelines for exploiting focused He+ ion beam irradiation in fabricating iron pnictide and MgB2 planar Josephson junctions.

Superconducting properties of tungsten nanowires fabricated using focussed ion beam technique

We report superconducting properties of tungsten meander structures fabricated using the focussed ion beam (FIB) induced technique. Three meander structures with individual line widths of ∼70, ∼300 and ∼450 nm were fabricated for evaluation and comparison of the superconducting properties. The resistance-temperature characteristics of the meanders were measured and analysed down to a temperature of 100 mK. The superconducting properties such as critical temperature (TC) and upper-critical field (HC2) of these wires are in comparison to the reported values of FIB deposited tungsten available in literature. While the normal state resistance increases sharply as the width of the wire decreases, the superconducting transition temperature registered a slight decrease. Significant amount of residual resistance (3.8% of normal state value at 100 mK) was observed for the sample with the lowest width (70 nm). The residual resistance trails as function of temperature was analysed invoking theoretical models governing the phase slip induced dissipations in superconducting nanowires. The results indicated signature of phase slips as the width of the wire decreases: thermally activated phase slips dominant near to the TC and quantum phase slip (QPS) near to TC as well as much below to the TC. The magneto-resistance isotherms indicated quantum phase transitions (QPT); typical of a superconductor-to-insulator transition (SIT) driven by magnetic field. The SIT transition which originates from the intrinsic disorder present in the sample can be tuned by an external parameter such as magnetic field, and can be modelled by standard theories of QPT for quasi 2D or (2 + 1) D XY models. The successful fabrication of meander structures of W using FIB and the demonstration of superconductivity suggest that FIB deposited W can be exploited for many of the technological applications of superconducting nanowires such as superconducting nanowire single photon detectors, bolometers, transition edge sensors and even for quantum current standard employing the QPS phenomenon.

Jahn-Teller Distortions and Infield Superconductivity of CuTl-1223 Phase

The strong inter-planes coupling in cuprates is of immense importance in the field of high Tc superconductivity. For this purpose, Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (CuTl-1223) superconducting phase was selected and Cu 3d9 was partially replaced with Zn 3d10 in CuO2 planes. The decrease in c-axis length and improvement in infield superconducting transport properties were observed after Zn doping at Cu sites in Cu0.5Tl0.5Ba2Ca2Cu3-xZnxO10−δ; x = 0, 1, and 2 superconducting phase. A continuous decrease in normal state resistivity and increase in Tc (0) values under applied magnetic field (i.e., from H = 0 to 5 T) was observed, which was an evidence of diminishing the anisotropic behavior, enhancement in coherence length, Fermi wave vector, and Fermi velocity of the carriers due to the improvement of inter-plane coupling associated with the reduction of Jahn-Teller distortions after Zn doping in CuO2 planes of CuTl-1223 system. The suppression of active Jahn-Teller distortions with Zn doping has reduced the volume of unit cell and enhanced the three dimensional (3D) superconducting properties of CuTl-1223 phase. The improvement in infield superconducting transport properties can also be attributed to the homogeneous and optimal charge distribution in conducting CuO2/ZnO2 planes of CuTl-1223 superconductor.

Role of interlayer coupling in alkaline-substituted (Bi, Pb)-2223 superconductors

In this work, we investigated a correlation between superconductivity and interlayer coupling of two different alkaline (Na and K)-substituted Bi1·6Pb0·4Sr2Ca2Cu3O10+δ (BSCCO) polycrystalline samples. The excess conductivity analysis by the Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) theories showed that Na substitution at the Ca site induced a gradual broadening of 3D fluctuation region with increasing interlayer coupling strength, which explains a systematic increase of Tc and a decrease of normal state resistivity. On the other hand, exactly the opposite results were observed in the K-substituted samples in place of Sr. Extended x-ray absorption fine structure (EXAFS) studies revealed that substitution of Na and K generated completely different effects on the local structure around Cu atoms. It is noticeable that the Cu–O bond distance was found to decrease monotonically with the varying amounts of Na, which indicates that the CuO2 layer is stabilized. On the while, the opposite was observed to occur with the varying amounts of K. Unlike the Cu–Ca bond which was the least affected by the substitution, the Cu–Sr bond distance increased drastically with K substitution. All these findings indicate that Na substitution at the Ca site enhances superconductivity with no loss of interlayer interaction, while K substitution at the Sr site weakens superconductivity due to the diminished interlayer interaction.

Effects of Ageing on the Microwave Surface Resistance of MgB2 Superconductor Films Stored in Low Vacuum

Nb is a very popular material in superconductive electronic devices such as superconducting single-photon detectors and radio-frequency (RF) cavities for particle accelerators. Before MgB2 can be used as a substitute material, the stability of its properties over time must be confirmed. We studied how ageing over a period of 514 weeks affected the microwave surface resistance (RS) and the normal-state resistivity (ρ) of epitaxially-grown MgB2 films. MgB2 films with thicknesses of 1.0 μm and 1.7 μm were prepared using the hybrid physical CVD method, and exhibited a critical temperature (TC) of 39 K. They were stored in low-vacuum to prevent reactions with humidity, except when the films were taken out of storage for measurement purposes. Variation in RS with the ageing period appeared to be significant at very low temperatures. The RS values of 1.0 μm-thick and 1.7 μm-thick MgB2 films increased by ~ 8 times and ~ 3 times, respectively, over ~ 10 years. The aged MgB2 films appeared to have reduced π-band gap energy values. The pristine MgB2 films value of 2.3 meV was reduced to 1.3–1.7 meV over ~ 10 years. The TC of the MgB2 films remained almost the same regardless of the ageing time. The enhanced RS and ρ in the aged MgB2 films were attributed to increased intraband scattering within the π-band and the σ-band.

Optical conductivity of granular aluminum films near the Mott metal-to-insulator transition

We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2\Delta(0)/k_{B}T_{c}$ increases above the BCS weak coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of $\sigma_{1,s}/\sigma_{1,n}$ for all samples up to very high normal state resistivities.

Resistivity anisotropy in YBCO single crystals irradiated with fast electrons

Temperature dependences of the longitudinal and transverse resistivity of YBa2Cu3O7−δ single crystals irradiated with fast electrons are investigated. It is revealed that the ratio of the normal-state resistivity along the c-axis and in the ab-plane, ρc∕ρab, markedly decreases with increase of the number of structural defects induced by the electron irradiation. Importantly, the temperature dependence of the ratio ρc∕ρab can be described well by both, the universal “law of 1/2” for the variable-range hopping conductivity as well as the conventional exponential expression for the thermally activated conductivity.