Normal State Resistivity Research Articles

Low-temperature transport properties of doped Ba0.57K0.43Fe2As2 superconductors in high magnetic field

For high-critical-temperature (high-${T}_{\text{c}}$) superconductors, the normal state transport properties at low temperatures are of great importance to understand the Fermi surface structure and carrier profile. In this work we studied the doped ultrathin ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ system, a member of the optimally doped 122-type multigapped iron-based superconductors with complex superconducting gap structure. To explore the normal state resistivity below the zero-field superconducting ${T}_{\text{c}}$, both Co and Zn dopants were substituted into the superconducting ${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ layer to suppress the superconductivity. The temperature dependence of in-plane resistivity below ${T}_{\text{c}}$ reveals a typical metallic behavior with the magnetic fields up to 45 T along $c$ axis. A linear dependence of Hall resistivity $({\ensuremath{\rho}}_{xy})$ is observed for ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{Ba}}_{0.57}{\mathrm{K}}_{0.43}{({\mathrm{Fe}}_{0.96}{\mathrm{Zn}}_{0.04})}_{2}{\mathrm{As}}_{2}$ at high magnetic field, indicating a conventional metal profile. Nevertheless, the Co dopants lead to a decreasing $d{\ensuremath{\rho}}_{xy}/dH$ when the field is above 20 T. Such transformation of carrier transport properties may be contributed by band shift within a two-carrier model.

Mapping of Structural Changes Induced by X-ray Nanopatterning via Nano-X-ray Diffraction and Corresponding Electrical Effects

We have investigated the modifications induced in high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals by irradiation with an X-ray nanobeam 250 × 250 nm2 in size with E = 14.85 keV and a time-averaged photon flux Φ0 = 7.53 × 108 ph s–1. Crystals were mounted onto amorphous substrates with electrical contacts to allow for both X-ray diffraction (XRD) and electrical measurements on the same sample. Changes in the domain structure were monitored via space-resolved nano-XRD mapping, whereas electrical properties were measured off-line at different levels of irradiation dose. The results show that irradiation induces both an increase of the mosaicity spread and a localized release of the mechanical stress of the crystals. This implies a transient local change of mechanical properties that could be due to thermal fatigue or nonthermal melting. Analysis of the electrical properties of the irradiated regions indicates a strong underdoping effect by the nanobeam and an exponential increase of the normal-state resistivity with the irradiation dose, according to the average rate α = (1.48 ± 0.12) × 10–11 Gy–1. These results represent an important step toward the full development of the X-ray nanopatterning technique.

Superconductivity in amorphous RexZr (x≈6) thin films

We report the growth, characterization and superconducting properties of thin films of a new amorphous superconductor, RexZr ( x≈6). Films were grown by pulsed laser deposition with the substrate kept at room temperature. Films with thicknesses larger than 40 nm showed a superconducting transition temperature (Tc) of 5.9 K. Superconducting properties were measured for films with varying thickness from 120 to 3 nm. The normal state resistance scaled linearly with the inverse of thickness. The transition temperature, critical field, coherence length, penetration depth and superconducting energy gap changed marginally with decreasing film thickness down to 8 nm. Scanning tunneling spectroscopy and penetration depth measurements provide evidence for a single gap strong coupling s-wave superconductor.

Enhancing Transport Performance in 7-filamentary Ba0.6K0.4Fe2As2 Wires and Tapes via Hot Isostatic Pressing

Iron-based superconductors (IBS) are considered as potential materials for manufacturing high-field magnets, for which developing multi-filamentary conductors with high performance and high strength is essential. Herein, 7-filamentary Cu/Ag composite sheathed Ba0.6K0.4Fe2As2 (Ba122) round wires and tapes were successfully prepared through the ex situ powder-in-tube (PIT) method and treated by hot isostatic pressing (HIP) process. Pure Ba122 phase with roughly homogeneous element distribution was obtained in the superconducting filaments. The wires and tapes show very small low-temperature normal-state resistivity of 0.12 μΩ cm and 0.17 μΩ cm respectively owing to the high electrical conductivity of copper and silver. The HIP process greatly enhances the mass density of the superconducting filaments and promotes the formation of well-grown plate-like Ba122 grains. Vickers hardness measurements on the cross sections of these filaments reveal a rather good uniformity of the mass density. The transport critical current density (Jc) in the 7-filamentary Ba122/Ag/Cu round wires and tapes reached 1.3 × 104 A cm−2 and 4.8 × 104 A cm−2 at 4.2 K in 10 T respectively. These results indicate that hot isostatic pressing is advantageous in developing high-performance multi-filamentary iron-based superconductors.

Two superconductor-insulator phase transitions in the spinel oxide Li1±xTi2O4−δ induced by ionic liquid gating

The associations between emergent physical phenomena (e.g., superconductivity) and orbital, charge, and spin degrees of freedom of $3d$ electrons are intriguing in transition metal compounds. Here, we successfully manipulate the superconductivity of spinel oxide Li$_{1\pm x}$Ti$_2$O$_{4-\delta}$ (LTO) by ionic liquid gating. A dome-shaped superconducting phase diagram is established, where two insulating phases are disclosed both in heavily electron-doping and hole-doping regions. The superconductor-insulator transition (SIT) in the hole-doping region can be attributed to the loss of Ti valence electrons. In the electron-doping region, LTO exhibits an unexpected SIT instead of a metallic behavior despite an increase in carrier density. Furthermore, a thermal hysteresis is observed in the normal state resistance curve, suggesting a first-order phase transition. We speculate that the SIT and the thermal hysteresis stem from the enhanced $3d$ electron correlations and the formation of orbital ordering by comparing the transport and structural results of LTO with the other spinel oxide superconductor MgTi$_2$O$_4$, as well as analysing the electronic structure by first-principles calculations. Further comprehension of the detailed interplay between superconductivity and orbital ordering would contribute to the revealing of unconventional superconducting pairing mechanism.

Fabrication of Corner-Like Josephson Junctions Based on Pnictide Single crystals

In order to study fundamental properties of pnictide superconductors such as the pairing symmetry, we have fabricated corner-like Josephson devices. Their configuration contains two SNS* (superconductor/barrier/superconductor*) thin-film junctions in different crystal directions. We have used multi-band Co-doped Ba-122 single crystals as the base electrode (S), evaporated lead films as a counter electrode (S*) made of a conventional s-wave superconductor and normal-metal (N) layers as the barrier. The junctions exhibit symmetric I-V characteristics with IcRN values about 0.975 μV. Ic is the critical current, and RN is the normal-state resistance. The observed Fraunhofer pattern-like dependence of Ic on the magnetic field indicates homogeneity of the junctions studied.

Hydrothermal synthesis and complete phase diagram of FeSe1−xSx (0≤x≤1) single crystals

We report the successful synthesis of FeSe$_{1-x}$S$_{x}$ single crystals with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase diagram of FeSe$_{1-x}$S$_{x}$ has been obtained based on resistivity and magnetization measurements. The nematicity is suppressed with increasing $x$, and a small superconducting dome appears within the nematic phase. Outside the nematic phase, the superconductivity is continuously suppressed and reaches a minimum $T_c$ at $x$ = 0.45; beyond this point, $T_c$ slowly increases until $x$ = 1. Intriguingly, an anomalous resistivity upturn with a characteristic temperature $T^*$ in the intermediate region of $0.31 \leq x \leq 0.71$ is observed. $T^{*}$ shows a dome-like behavior with a maximum value at $x$ = 0.45, which is opposite the evolution of $T_c$, indicating competition between $T^*$ and superconductivity. The origin of $T^*$ is discussed in detail. Furthermore, the normal state resistivity evolves from non-Fermi-liquid to Fermi-liquid behavior with S doping at low temperatures, accompanied by a reduction in electronic correlations. Our study addresses the lack of single crystals in the high-S doping region and provides a complete phase diagram, which will promote the study of relations among nematicity, superconductivity, and magnetism.

Process Control Monitoring for Fabrication Technology of Superconducting Integrated Circuits

We have developed a 6-kA/cm superconducting integrated circuit fabrication process toward large-scale single flux quantum digital circuits. To evaluate the process reliability and controllability, a set of process control monitors (PCMs) for monitoring fabrication and design parameters was designed and tested. Fabrication parameters, for example, film thicknesses and etched depths of the layers, were measured by testing specific patterns during fabrication. Design parameters such as the critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , normal-state resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> and actual sizes of Josephson junctions, the sheet resistance of Mo resistor R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> , and the penetration depth of Nb films were evaluated by electrically characterizing PCM circuits after fabrication. We also monitored superconducting critical currents for vias and Nb lines, and insulation properties between the metal layers systematically.

Scaling laws for ion irradiation effects in iron-based superconductors

We report on ion irradiation experiments performed on compounds belonging to the hbox {BaFe}_2hbox {As}_2 family, each one involving the partial substitution of an atom of the parent compound (K for Ba, Co for Fe, and P for As), with an optimal composition to maximize the superconducting critical temperature T_c. Employed ion beams were 3.5-MeV protons, 250-MeV Au ions, and 1.2-GeV Pb ions, but additional data from literature are also considered, thus covering a wide range of ions and energies. Microwave characterization based on the use of a coplanar waveguide resonator allowed us to investigate the irradiation-induced T_c degradation, as well as the increase of normal state resistivity and London penetration depth. The damage was quantified in terms of displacements per atom (dpa). From this broad and comprehensive set of experimental data, clear scaling laws emerge, valid in the range of moderate irradiation-induced disorder (dpa up to 5 times 10 ^{-3} were investigated). In these conditions, linear trends with dpa were found for all the modification rates, while a power law dependence on the ion energy was found for heavy-ion irradiation. All these scaling laws are reported and discussed throughout the paper.

Dense rotational twins in superconducting (111)-orientated δ-NbN epitaxial films on 4H-SiC substrates

In order to confirm the superconductivity observed in hexagonal ϵ-NbN reported recently, we have deposited NbN film on (0001) 4H-SiC substrate, with good lattice match, intentionally to grow hexagonal NbN phase. The detailed structural analysis show that the deposited films are (111)-orientated δ-NbN phase with dense rotational (60°) twins. Double diffraction effect in selected area electron diffraction and Moiré fringes in high resolution transmission electron microscope images confirm the existence of rotational twins in nanometer scale. The growth of highly twined film results from no energy preference for the two rotational twins of (111)-orientated δ-NbN to bond with the Si-faced (0001) 4H-SiC substrate. These highly twined δ-NbN epitaxial films exhibit high normal state resistivity, lower T C, but good spatial uniformity of superconducting energy gap. No ϵ-NbN phase was observed in this study which is might due to a low growth temperature of thin film.

Very high upper critical fields and enhanced critical current densities in Nb3Sn superconductors based on Nb–Ta–Zr alloys and internal oxidation

The inhibition of Nb3Sn grain growth in the presence of ZrO2 nanoparticles appears to be one of the most promising method for pushing the critical current densities of Nb3Sn superconducting wires to levels that meet the requirements set for the Future Circular Collider. We have investigated the effect of ZrO2 nanoparticles formed by the internal oxidation of Zr on the superconducting properties and microstructure of Nb3Sn formed from Nb-1 wt%Zr, Nb-7.5 wt%Ta, Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr alloys. A monofilamentary wire configuration was used, with a 0.22 mm outer diameter Nb-alloy tube containing a core of powdered metal oxide (SnO2, CuO or MoO3) as oxygen source and successive deposits of Cu, Sn and Cu on the outer surface. As determined from inductive measurements, the layer critical current densities of the samples based on Nb alloys with internally oxidized Zr were superior to those based on Nb-7.5 wt%Ta. The samples based on Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt%Zr showed higher critical current densities at high magnetic fields (above 10–15 T), and upper critical fields exceeding 28.5 T at 4.2 K (99% normal state resistivity criterion). A record value of 29.2 T of the upper critical field at 4.2 K was obtained on samples based on Nb-7.5 wt%Ta-2 wt%Zr. Hypotheses are proposed and discussed for explaining this unexpected increase of the upper critical field, by considering the possible effects of non-oxidized Zr on the superconducting properties of Nb3Sn and of the oxidized Zr on the formation and microchemistry of Nb3Sn. Regardless of sample type the Nb3Sn grains observed in our samples have an aspect ratio of 1.5–1.7. When compared in the short axis direction, the mean distance between grain boundary intercepts (lineal intercept method) is ∼40% smaller in the samples with internally oxidized Zr than in the reference samples based on Nb-7.5 wt%Ta. In the long axis direction the reduction is of 20%–30%.

Dependence of Photon Detection Efficiency on Normal-State Sheet Resistance inMarginallySuperconducting Films of NbN

We present an extensive set of data on nanowire-type superconducting single-photon detectors based on niobium-nitride (NbN) to establish the empirical correlation between performance and the normal-state resistance per square. We focus, in particular, on the bias current, compared to the expected depairing current, needed to achieve a near-unity detection efficiency for photon detection. The data are discussed within the context of a model in which the photon energy triggers the movement of vortices <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i.e.</i> superconducting dissipation, followed by thermal runaway. Since the model is based on the non-equilibrium theory for conventional superconductors deviations may occur, because the efficient regime is found when NbN acts as a marginal superconductor in which long-range phase coherence is frustrated.

Effect of grain size selection in ex-situ made MgB2 wires

Commercial MgB2 powder manufactured by Alfa Aesar was mechanically sieved in order to obtain powder samples with different particle size. As-sieved powders were used for single-core wires with FeNiCo sheath made by ex-situ process and heat treated at the same conditions, 950 °C/30 min. Resistive transitions of extracted MgB2 cores allowed to obtain the critical temperature and normal state resistivity for different particle size and estimate the connectivity by the Rowell approach. In field critical currents were also affected by the particle size of sieved MgB2 powder. The highest current densities were measured for the wire with particles 15-25 µm, which is explained by the dominant effect of grain-boundary pinning.

Performance and uniformity of a kilo-pixel array of Ti/Au transition-edge sensor microcalorimeters.

Uniform large transition-edge sensor (TES) arrays are fundamental for the next generation of x-ray space observatories. These arrays are required to achieve an energy resolution ΔE < 3eV full width at half maximum (FWHM) in the soft x-ray energy range. We are currently developing x-ray microcalorimeter arrays for use in the future laboratory and space-based x-ray astrophysics experiments and ground-based spectrometers. In this contribution, we report on the development and the characterization of a uniform 32 × 32 pixel array with 140 × 30 μm2 Ti/Au TESs with the Au x-ray absorber. We report on extensive measurements on 60 pixels in order to show the uniformity of our large TES array. The averaged critical temperature is Tc = 89.5 ± 0.5 mK, and the variation across the array (∼1cm) is less than 1.5 mK. We found a large region of detector's bias points between 20% and 40% of the normal-state resistance where the energy resolution is constantly lower than 3eV. In particular, results show a summed x-ray spectral resolution ΔEFWHM = 2.50 ± 0.04eV at a photon energy of 5.9 keV, measured in a single-pixel mode using a frequency domain multiplexing readout system developed at SRON/VTT at bias frequencies ranging from 1MHz to 5MHz. Moreover, we compare the logarithmic resistance sensitivity with respect to temperature and current (α and β, respectively) and their correlation with the detector's noise parameter M, showing a homogeneous behavior for all the measured pixels in the array.

Internal and External Pressure Effects on Superconductivity in FeTexSe1-x (x = 0.46, 0.54) Single Crystals

We have synthesized and studied the transport properties of FeTexSe1-x (x = 0.4, 0.46, 0.5, 0.54, and 0.58) single crystals grown by slow cooling method and found a sharp increase of Tc up to x = 0.46 followed by a moderate decrease of Tc up to x = 0.58. We present the temperature dependence of electrical resistivity under various pressures up to ~ 2.5 GPa, and the pressure dependence of Tc shows positive pressure coefficients of ~ 2.7 K/GPa and ~ 3.33 K/GPa for x = 0.46 and x = 0.54, respectively, up to 2 GPa followed by a decreasing trend with marginal variation of Tc with negative pressure coefficients of − 0.602 K GPa−1 (x = 0.46) and − 0.7 K GPa−1 (x = 0.54) at maximum pressure of 2.5 GPa. Furthermore, both normal state resistivity and activation energy decrease sharply, and it suggests that the reduction of net electron-phonon scattering and densities of states (DOS) at the Fermi surface with application of pressure leads to enhancement of metallic behavior in FeSe systems.

Irreversibility line and thermally activated flux flow in Micro-bridges of high-temperature superconductor

We have studied the electrical resistivity as a function of temperature in Micro-bridges of YBa2Cu3O7-δ deposited by laser ablation on the SrTiO3 substrate face, around superconducting transition region in different magnetic fields. The activation energy U0 was determined and discussed using the Arrhenius relation. The irreversibility line Birr and the upper critical field Bc2were obtained using 10% and 90% criteria of the normal-state resistivity value from ρ (T) curves. A phase diagram of the studied sample is constructed showing the Tg glass line and a very broad vortex-liquid phase regime.

Magnetic doping effects on the superconductivity of Y1-xMxBa2Cu3O7-δ (M = Fe, Co, Ni)

The discovery of superconductivity in copper oxide compounds has attracted considerable attention over the past three decades. The high transition temperature (Tc) in these compounds, exhibiting proximity to an antiferromagnetic order in their phase diagrams, remains one of the main areas of research. It is believed that magnetic fluctuations provide substance for the exotic superconductivity observed in these compounds. The present study attempts to introduce Fe, Co and Ni magnetic impurities into the superconducting cuprate YBa2Cu3O7-δ with the aim of exploring the Tc behavior. The solid-state synthesis method is exploited to prepare fully oxygenated Y1-xMxBa2Cu3O7-δ (Y1-xMx-123) (M = Co, Fe, Ni) samples with low levels of doping (0.00000 ≤ x ≤ 0.03000). Systematic measurements are then employed to assess the synthesized samples using AC magnetic susceptibility, electrical resistivity and X-ray diffraction (XRD). The measurements revealed an increase in Tc as a result of magnetic substitution for Y. However, the study of non-magnetic dopings on the fully oxygenated Y1-xM'xBa2Cu3O7-δ (Y1-xM'x-123) (M' = Ca, Sr) samples showed a decrease in Tc. Quantitative XRD analysis further suggested that the internal pressure could have minor effects on the increase in Tc. The normal state resistivity vs temperature showed a linear profile, confirming that the samples are at an optimal doping of the carrier concentration.

Phase diagram of infinite layer praseodymium nickelate Pr1−xSrxNiO2 thin films

We report the phase diagram of infinite layer Pr$_{1-x}$Sr$_{x}$NiO$_2$ thin films synthesized via topotactic reduction from the perovskite precursor phase using CaH$_2$. Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between $x$ = 0.12 and 0.28, with a maximum superconducting transition temperature $T_{\rm{c}}$ of 14 K at $x$ = 0.18, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in Nd$_{1-x}$Sr$_{x}$NiO$_2$, a local $T_{\rm{c}}$ suppression near $x$ = 0.2 was not observed for the Pr$_{1-x}$Sr$_{x}$NiO$_2$ system. Normal state Hall effect measurements indicate mixed carrier contributions of both electrons and holes, and show a sign change in the Hall coefficient as functions of temperature and $x$, quite similar to that in Nd$_{1-x}$Sr$_{x}$NiO$_2$. Also similar is the observation of a minimum in the normal state resistivity associated with the superconducting compositions. These findings indicate an infinite layer nickelate phase diagram that is relatively insensitive to the rare-earth element, but suggest that disorder arising from the variations of the ionic radii on the rare-earth site affects the superconducting dome.

Effect of grain boundary deformation on the critical temperature degradation of superconducting Nb3Sn under hydrostatic pressure

Variation of the critical superconducting properties of Nb3Sn material in terms of mechanical deformation has not been well determined. Fundamental understanding of electromechanical coupling effects is significant in the analysis and construction of high magnetic field superconducting magnet. Considering the microstructure deformation and its inner relationship with normal state resistivity evolution, we developed a consistent description of critical temperature degradation responses of single- and polycrystal Nb3Sn under hydrostatic pressure. The model simulations qualitatively agree with the experimental observations. Based on the polycrystalline finite element method, the analysis carefully examines the role of grain boundary deformation in the critical temperature degradation of superconducting Nb3Sn under high pressure. The grain boundary deformation-related effect is responsible for the obvious degradation differences between polycrystal and single crystal Nb3Sn under the same level of pressure. The results are helpful for understanding the empirical relation presented by the experiment and opens the way for the parameterization of the strain sensitivity of the superconducting Nb3Sn material.

Magnetoresistance of a Ferromagnet/Superconductor/Ferromagnet Trilayer Microbridge Based on Diluted PdFe Alloy

A negative magnetoresistive effect has been observed for ferromagnet/superconductor/ferromagnet (FSF) microbridges based on diluted ferromagnetic PdFe alloy containing as small as 1% of magnetic atoms. The effect is represented by sharp negative peaks in magnetoresistance at magnetic fields opposite in sign to the initial saturated magnetizations. Microstructuring of the FSF trilayers does not suppress the effect: the most pronounced dips were obtained for the smallest bridges 6–8 µm wide and 10–15 µm long. The negative magnetoresistance peak was observed at temperatures within the superconducting transition and reaches a noticeable value of up to 1.3% of the normal state resistance.