Superconductors In High Magnetic Fields Research Articles

Huge magnetostriction in superconducting single-crystalline BaFe1.908Ni0.092As2

The performance of iron-based superconductors in high magnetic fields plays an important role for their practical application. In this work, we measured the magnetostriction and magnetization of BaFe1.908Ni0.092As2 single crystals using pulsed magnetic fields up to 60 T and static magnetic fields up to 33 T, respectively. A huge longitudinal magnetostriction (of the order of 10–4) was observed in the direction of twin boundaries. The magnetization measurements evidence a high critical-current density due to strong bulk pinning. By using magnetization data with an exponential flux-pinning model, we can reproduce the magnetostriction curves qualitatively. This result shows that the magnetostriction of BaFe1.908Ni0.092As2 can be well explained by a flux-pinning-induced mechanism.

Critical current density of superconducting-normal-superconducting Josephson junctions and polycrystalline superconductors in high magnetic fields

We investigate the in-field critical current density ${J}_{\text{c}}(B)$ of superconducting-normal-superconducting (SNS) Josephson junctions (JJs) and polycrystalline superconducting systems with grain boundaries modeled as Josephson-type planar defects, both analytically and through computational time-dependent Ginzburg-Landau (TDGL) simulations in two and three dimensions. For very narrow SNS JJs, we derive analytic expressions for ${J}_{\text{c}}(B)$ that are high-field solutions for ${J}_{\text{c}}(B)$ for JJs across the entire applied field range up to the effective upper critical field ${B}_{\text{c2}}^{*}$. They generalize the well-known (low-field) exponential junction thickness dependence for ${J}_{\text{c}}$ from de Gennes, often used in the Josephson relation. We then extend our analytic expressions to describe wider junctions using physical arguments, and we confirm their agreement with TDGL simulations. These results are then compared with the current densities found in superconductors optimized for high-field applications. They provide an explanation for the Kramer field dependence and inverse power-law grain size dependence widely found in many low-temperature superconductors, and the power-law field dependence ${J}_{\text{c}}(B)\ensuremath{\sim}{B}^{\ensuremath{-}0.6}$ found at intermediate fields in some high-temperature superconductors including powder-in-tube ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ and $R{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$ tapes ($R=\text{rare}\phantom{\rule{4.pt}{0ex}}\text{earth}$). By reanalyzing critical current density data using the mathematical framework derived here and confirmed using TDGL, we enable an analysis of ${J}_{\text{c}}$ data that provides the local properties of grain boundaries in high-field superconductors and hence a deeper understanding of how grain boundaries influence ${J}_{\text{c}}$ in high magnetic fields.

Surface Impedance Measurements in Superconductors in DC Magnetic Fields: Challenges and Relevance to Particle Physics Experiments

Particle physics and radio-frequency (RF) superconductivity have driven each other on since the 1970s. The unique properties of superconductors (SC) have been the enabling keys for the realization of accelerators with always increased performances thanks to the realization of all-superconducting cavities. The use of increasingly pure superconducting coatings for accelerating cavities, with lower and lower RF losses, determined such high-quality factors [1] that the need to operate at low temperatures (below the superconducting transition temperature <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{c}$</tex> ) was well paid for. Recently, with respect to the path followed by the high frequency superconductivity [2], a new field opened since SCs are being considered for GHz operation in high dc magnetic fields, and measurements (and optimization) of totally different quantities are needed. The possibility of successfully using SCs in high magnetic fields for these purposes is far from obvious and it depends on the outcome of accurate measurements of usually overlooked quantities.

The magnetic and levitation characteristics of single-grain YBaCuO and GdBaCuO-Ag bulk superconductors in high magnetic fields

In principle, high-temperature superconducting (HTS) flux-pinning maglev has the advantage of self-stable levitation and is therefore a good development prospect as a new rail transit technology. The magnetic field intensity is one of the key factors affecting the levitation performance of single-grain bulk superconductors in the HTS maglev system. To date, however, most researchers have focused on the levitation performance of HTS bulk superconductors in magnetic fields within a permanent magnet guideway (PMG) configuration. The magnitude of the external working magnetic field provided by a PMG is limited to about 0.5 T at 77 K, given that the superconductor usually levitates at a typical working height of 10 mm above the PMG. Therefore, the weak magnetic fields generated by the PMG significantly limits the levitation performance of HTS maglev systems to that of the permanent magnet (PM). In this article, a superconducting magnet test bench was utilized to provide a 0–3 T external magnetic field (B) environment for the study of the levitation force in HTS single grains. The difference in levitation performance between YBaCuO (YBCO) and GdBaCuO-Ag (GdBCO-Ag) bulk superconductors under large, changing magnetic fields was studied systematically. The relationships between levitation force and magnetic field characteristics, including vertical magnetic field component (Bz), transverse magnetic field component (Br), and vertical magnetic field variation (ΔBz) were investigated. The result of variation curves about YBCO and GdBCO bulks' levitation forces under different ΔBz shows that the levitation force of the bulk, single-grain superconductors will reach saturation under a varying ΔBz environment (0.72 and 1.5 T). The values of ΔBz corresponding to the saturation positions are found to be independent of the sizes of the HTS samples. At the same time, GdBCO-Ag exhibits a levitation force that is 112.8% greater than YBCO in B = 3 T magnetic environment. As a result, it is demonstrated that the levitation potential of single-grain HTS bulks can be better exploited by optimizing the distribution of the magnetic field.

Direct observation of the scaling relation between density of states and pairing gap in a dirty superconductor* *Project supported by the National Key Research and Development Program of the Ministry of Science and Technology of China (Grant Nos. 2017YFA0302903, 2016YFA0300602, 2016YFA0300604, and 2016YFJC010282), the National Natural Science Foundation of China (Grant Nos. 11227903, 12004416, and U2032204), the Beijing Municipal Science

Theories and experiments on dirty superconductors are complex but important in terms of both theoretical fundamentals and practical applications. These activities are even more challenging when magnetic fields are present because the field distribution, electron density of states, and superconducting pairing potentials become nonuniform. Here, we present tunneling microspectroscopic experiments on NbC single crystals and demonstrate that NbC is a homogeneous dirty superconductor. When applying magnetic fields to the samples, we found that the zero-energy local density of states and the pairing energy gap followed the explicit scaling relation proposed by de Gennes for homogeneous dirty superconductors in high magnetic fields. More significantly, our experimental findings indicate that the validity of the scaling relation extends to magnetic field strengths far below the upper critical field, calling for a new nonperturbative understanding of this fundamental property in dirty superconductors. On the practical side, we used the observed scaling relation to derive a simple and straightforward experimental scheme for estimating the superconducting coherence length of a dirty superconductor in magnetic fields.

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.

Charge carrier drop at the onset of pseudogap behavior in the two-dimensional Hubbard model

We show that antiferromagnetic spin-density wave order in the two-dimensional Hubbard model yields a drop of the charge carrier density as observed in recent transport measurements for cuprate superconductors in high magnetic fields upon entering the pseudogap regime. The amplitude and the (generally incommensurate) wave vector of the spin-density wave is obtained from dynamical mean-field theory (DMFT). An extrapolation of the finite temperature results to zero temperature yields an approximately linear doping dependence of the magnetic gap $\Delta(p) \propto p^*-p$ in a broad doping range below the critical doping $p^*$. The magnetic order leads to a Fermi surface reconstruction with electron and hole pockets, where electron pockets exist only in a restricted doping range below $p^*$. DC charge transport properties are computed by combining the renormalized band structure as obtained from the DMFT with a doping-independent phenomenological scattering rate. A pronounced drop of the longitudinal conductivity and the Hall number in a narrow doping range below $p^*$ is obtained.

Longitudinal conductivity and Hall coefficient in two-dimensional metals with spiral magnetic order

We compute the longitudinal dc conductivity and the Hall conductivity in a two-dimensional metal with spiral magnetic order. Scattering processes are modeled by a momentum-independent relaxation rate $\Gamma$. We derive expressions for the conductivities, which are valid for arbitrary values of $\Gamma$. Both intraband and interband contributions are fully taken into account. For small $\Gamma$, the ratio of interband and intraband contributions is of order $\Gamma^2$. In the limit $\Gamma \to 0$, the conductivity formulas assume a simple quasiparticle form, as derived by Voruganti et. al. [Phys. Rev. B 45, 13945 (1992)]. Using the complete expressions, we can show that relaxation rates in the regime of recent transport experiments for cuprate superconductors in high magnetic fields are sufficiently small to justify the application of these simplified formulas. The longitudinal conductivity exhibits a pronounced nematicity in the spiral state. The drop of the Hall number as a function of doping observed recently in several cuprate compounds can be described with a suitable phenomenological ansatz for the magnetic order parameter.

Upgrade of SULTAN/EDIPO for HTS Cable Test

CRPP hosts two unique conductor test facilities SULTAN (SUpraLeiter TestANlage) and EDIPO (European DIPOle). They allow the test of high current superconductors in high magnetic fields (SULTAN 11 T, EDIPO 12.5 T). In both facilities sample currents up to 100 kA can be supplied by means of a NbTi transformer. Presently the facilities are upgraded for the test of high current high-temperature superconductor (HTS) samples. For HTS conductor testing at temperatures between 20 and 50K, the heat flux between the HTS sample under test and the NbTi transformer needs to be limited to around 10W per conductor leg by means of an HTS adapter connecting them. The second required upgrade is the supply of intermediate temperature helium (20-50 K) to the HTS test conductor. It is mandatory that the helium gas coming from the HTS conductor under test can be returned to the cryoplant as cold gas (T < 20K). To reach this goal a tube-in-tube heat exchanger has been manufactured in which 4.5K helium coming from the cryoplant is in counter flow with the warm gas leaving the HTS test conductor.

The critical current density of an SNS Josephson-junction in high magnetic fields

Although the functional form of the critical current density (Jc) of superconducting–normal–superconducting (SNS) Josephson-junctions (J-Js) has long been known in the very low field limit (e.g. the sinc function), includes the local properties of the junction and has been confirmed experimentally in many systems, there have been no such general solutions available for high fields. Here, we derive general analytic equations for Jc in zero field and in high fields across SNS J-Js for arbitrary resistivity of the superconductor and the normal layer which are consistent with the literature results available in limiting cases. We confirm the validity of the approach using both computational solutions to time-dependent Ginzburg–Landau (TDGL) theory applied to SNS junctions and experimental Jc data for an SNS PbBi–Cd–PbBi junction. We suggest that since SNS junctions can be considered the basic building blocks for the description of the grain boundaries of polycrystalline materials because they both provide flux-flow channels, this work may provide a mathematical framework for high Jc technological polycrystalline superconductors in high magnetic fields.

The shock‐wave application for increasing of a critical current in composite HTSC

AbstractThis paper aims to find methods for effective pinning centres generation using shock waves in order to increase the current‐carrying capability of high temperature superconductors in high magnetic fields. It presents the results of the effect of a shock wave on properties of composite high temperature superconductor tapes (HTSC): YBCO(123) and Bi(2223). To generate the shock wave, authors used the interaction of a solid state target with a high speed cumulative plasma jet generated in a Plasma Focus setup. Shock waves have a front‐edge pressure of 1011 Pa. The sample was exposed to the plasma pulse for about 50 ns. A significant and lasting increase of the critical current density of composite HTSC tapes was observed as a result of the effect of shock waves, also in high magnetic fields. This paper discusses the presumed reasons for the increase of the critical current. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Iron-based superconductors in high magnetic fields

Abstract Here we review measurements of the normal and superconducting state properties of iron-based superconductors using high magnetic fields. We discuss the various physical mechanisms that limit superconductivity in high fields, and the information on the superconducting state that can be extracted from the upper critical field, but also how thermal fluctuations affect its determination by resistivity and specific heat measurements. We also discuss measurements of the normal state electronic structure focusing on measurement of quantum oscillations, particularly the de Haas–van Alphen effect. These results have determined very accurately, the topology of the Fermi surface and the quasi-particle masses in a number of different iron-based superconductors, from the 1111, 122 and 111 families.

STM differential conductance of a disordered extreme type-II superconductor at high magnetic fields

We calculate the tunneling conductance between a scanning tunneling microscope tip and the surface of an extreme type-II superconductor in a high magnetic field and at zero temperature. In a clean system, and with electrons tunneling only along vortex lines, we find that the spatially averaged differential conductance $\ensuremath{\sigma}(V)$ has a V-shape dependence on the bias voltage $V$, reflecting the presence of gapless points and near gapless regions in the quasiparticle excitation spectrum of a superconductor in high magnetic fields. Within a $T$-matrix approximation for a homogeneous superconductor, we investigate the influence of nonmagnetic impurities on the differential conductance. We find that in the presence of disorder the differential conductance becomes finite at zero bias and develops a nonlinear dependence on the bias voltage. We apply our theory to calculate the differential conductance $\ensuremath{\sigma}(V)$ of the superconductor ${\text{LuNi}}_{2}{\text{B}}_{2}\text{C}$ in the mixed state.

Superconducting magnetic bearing for a flywheel energy storage system using superconducting coils and bulk superconductors

Stable levitation or suspension of a heavy object in mid-air can be realized using a combination of a permanent magnet and a bulk superconductor with high critical current density, in that the force density has reached 100 kN/m 2. The superconducting flywheel system for energy storage is attractive due to a great reduction in the rotational loss of the bearings. So long as a permanent magnet is used as a magnetic source, however, the electromagnetic force (EMF) is essentially limited by its field strength. In order to overcome such a limitation, we employed a superconducting coil/magnet as a magnetic source and studied the EMF characteristics of bulk superconductors in high-magnetic fields. We also measured the EMF between a bulk superconductor and a specially designed superconducting coil having a high field gradient, and confirmed that the EMF reached 9450 N (force density: 2507 kN/m 2).

Development of high-field STM for 18 T cryocooled superconducting magnet

To study the nanoscale electronic order in strongly correlated electron systems and vortex states in high-Tc superconductors in high magnetic fields, we have developed scanning tunneling microscopy (STM) for the 18 T cryocooled superconducting magnet (18T-CSM). The test results of the STM operation in the 18T-CSM at room temperature indicate that our STM has a good atomic resolution up to 18 T when we use the nonmagnetic vibration-isolation table which reduce the vibration noise from the cryocoolers of the 18T-CSM. In this paper, we report on the design of the high-field STM system for large-scale magnets and its performance.

Fermi pockets and quantum oscillations of the Hall coefficient in high-temperature superconductors

Recent quantum oscillation measurements in high-temperature superconductors in high magnetic fields and low temperatures have ushered in a new era. These experiments explore the normal state from which superconductivity arises and provide evidence of a reconstructed Fermi surface consisting of electron and hole pockets in a regime in which such a possibility was previously considered to be remote. More specifically, the Hall coefficient has been found to oscillate according to the Onsager quantization condition, involving only fundamental constants and the areas of the pockets, but with a sign that is negative. Here, we explain the observations with the theory that the alleged normal state exhibits a hidden order, the d-density wave, which breaks symmetries signifying time reversal, translation by a lattice spacing, and a rotation by an angle pi/2, while the product of any two symmetry operations is preserved. The success of our analysis underscores the importance of spontaneous breaking of symmetries, Fermi surface reconstruction, and conventional quasiparticles. We primarily focus on the version of the order that is commensurate with the underlying crystalline lattice, but we also touch on the consequences if the order were to incommensurate. It is shown that whereas commensurate order results in two independent oscillation frequencies as a function of the inverse of the applied magnetic field, incommensurate order leads to three independent frequencies. The oscillation amplitudes, however, are determined by the mobilities of the charge carriers comprising the Fermi pockets.

Transport Critical Current of Filamentary Zr-Doped Gd-Ba-Cu-O Superconductors in High Magnetic Fields

Filamentary Gd-Ba-Cu-O superconductors doped with nominal Zr concentrations between 0 and 0.5 at% relative to Gdl23 were prepared by a solution spinning method. Samples were partially melted in flowing 20% O2 + Ar atmosphere gas at various partial melting temperatures and oxygenated in pure oxygen gas. High J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value of 2 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> A/cm at 77 K and 0 T were obtained for the Gd123 + 0.1at%Zr sample optimally processed. In the case of doping levels less than 0.5 at %, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values, which were about 92 K with sharp transitions, were hardly influenced by Zr concentration. From transport critical current measurements in magnetic fields, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value of the pure Gd123 filament sharply decreased with increasing applied field. Gd123 with 0.5 at%Zr doping processed under optimum conditions showed little deterioration of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values in low magnetic fields and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values higher than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/cm was maintained up to 14 T. It was also found that J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> s in high magnetic field and the irreversibility field were improved by a small amount of Zr doping.

Force Density of Magnetic Bearings Using Superconducting Coils and Bulk Superconductors

Stable levitation or suspension of a heavy object in mid-air can be realized using a combination of a permanent magnet and a bulk superconductor with high critical current density, in that the force density has reached 100 kN/m2. The superconducting flywheel system for energy storage is attractive due to a great reduction in the rotational loss of the bearings So long as a permanent magnet is used as a magnetic source, however, the electromagnetic force (EMF) is essentially limited by its field strength. In order to overcome such a limitation, we employed a superconducting coil/magnet as a magnetic source and studied the EMF characteristics of bulk superconductors in high magnetic fields. We also measured the EMF between a bulk superconductor and a specially designed superconducting coil having a high field gradient, and confirmed that the EMF reached 9,000 N (force density: 1,190 k N/m2).

Critical current of a textured granular superconductor in high magnetic fields

The critical current density J c is studied theoretically using the cluster model of a granular superconducting structure for the case where Abrikosov vortices are present in the grains. It is found that the dependence of J c on the effective ratio of the sizes of grains forming an intergrain Josephson junction is Gaussian. The dependence of J c on the anisotropy of the penetration of a magnetic field into the grains is shown to reduce to the dependence on the coupling strength between the crystallites.

Measurements of the levitation forces for a bulk Gd–Ba–Cu–O superconductor in high magnetic fields

We studied the effects of magnetic field and the temperature on the levitation force for a bulk Gd–Ba–Cu–O superconductor 60mm in diameter and 20mm in thickness. Magnetic field distribution in the bulk body was monitored with Hall sensors placed on the sample surface. The sample was placed 10mm below the magnetic center of a superconducting solenoid coil. The sample was cooled in zero-field mode, and the magnetic field was increased from 0 to 5T. Therefore, the levitation force which acts downward was measured at various temperatures. After measuring the levitation force, its time relaxation was also measured for 1h while applying 5T. The levitation force could be increased with increasing magnetic field in all the temperature range. Lowering temperature was effective in increasing the levitation force and reducing its time relaxation.