Critical Field Bc2 Research Articles

3D modeling of coupling between superconducting filaments via resistive matrix in AC magnetic field

The ac loss of superconducting composite depends strongly on coupling between superconducting filaments via the resistive matrix. The established technique for loss reduction using twisted filaments relies on the decoupling of the filaments below a critical coupling field Bc, which increases with the reduction of the twist pitch and the matrix conductivity. Although the concept of Bc may be clearly demonstrated using two infinite slabs of finite length, further details on its correlation with the filament/conductor geometry are not yet available. The main obstacle is due to the fact that any accurate analysis of such a problem must be carried out in 3d. In this paper, we describe the initial results from 3d modeling using Cedrat's Flux3D, for which a superconductor module for handling power-law E-J characteristics was developed. Using a simple model of two rectangular superconductors connected through a normal metal, we demonstrate the feasibility for quantitative modeling of their coupling behavior over a wide range of field sweep rates for different conductor geometries. Typical examples were given for cases not addressed by the existing approximate theory, as well as for the evolution of field profile for varying field sweep rate.

Influence of pressure on itinerant electron metamagnetic transition in La(FexSi1−x)13 compounds

The influence of pressure on the itinerant electron metamagnetic (IEM) transition has been investigated for a La(FexSi1−x)13 compound. The critical temperature T0 defined as the disappearance point of the IEM transition decreases with increasing hydrostatic pressure. Since the pressure dependence of T0 is smaller than that of TC, the temperature range for appearance of the IEM transition becomes wider with increasing hydrostatic pressure. These results are consistent with the theoretical model based on the Landau expansion of free energy including the influence of spin fluctuations and the magnetovolume effect. On applying pressure, the critical field Bc for the IEM transition increases proportionally with temperature in low pressures, whereas it exhibits a quadratic temperature dependence with lowering the Curie temperature in high pressures. The change in the temperature dependence of Bc is explained by a change in the thermal growth rate of spin fluctuations.

Critical fields and flux-flow resistances in strongly disordered ultra-thin superconducting films

Strongly disordered ultra-thin films of Bi and Sn (<100 Å) produced by quench-condensation, are well known systems that show insulator to superconductor transition. Some aspects of the transition and the nature of the superconducting state are weakly dependent on the material and substrate, but we find that the critical field (Bc2) of Bi and Sn films of comparable resistance show different temperature dependences. For Sn the mean field Bc2 is seen to vary with temperature as Bc2(T)=Bc2(0)(1−(T/Tc)2), whereas for Bi it is found to be Bc2(T)=Bc2(0)(1−(T/Tc))α with α≈1.14. In films with low sheet resistance we find a dissipationless vortex-solid regime. The flux-flow resistance calculated from the I–V traces taken in several magnetic fields show a much faster field dependence than existing theories predict.

Possibility of unconventional superconductivity of SrTiO3−δ

SrTiO3−δ can show metallic behavior and superconductivity at T<500 mK. Due to its low carrier concentration (≃1020 cm−1) and huge dielectric polarisability (ϵ≃300) this compound is considered as a candidate for a polaronic superconducting pairing mechanism.It is demonstrated that by variation of the annealing temperature in vacuum the transport properties of SrTiO3−δ can be tuned continuously from semiconducting to metallic. We present measurements of the upper critical field Bc2(T) which show near Tc a positive curvature. This unusual temperature dependence is consistent with a model of weakly interacting charged bosons which condense in the superconducting state (local pairing). However, measurements of current–voltage curves reveal only small critical currents of our samples. This observation is discussed in the framework of doping inhomogeneities.

Vortices and the mixed state of ultrathin Bi films

Current–voltage (I–V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These I–Vs show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor–insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (Bc2) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg–Landau theory, after modifications for disorder.

Full ferromagnetic saturation of a two-dimensional quantum antiferromagnet

Cs2CuCl4 is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of 1/2, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field Bc=8.44(1) T∥a. Above Bc the measured excitations lineshapes show well-defined, almost resolution-limited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at Bc, below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations.

Spectrum of low-energy excitations in the vortex state: Comparison of the Doppler-shift method to a quasiclassical approach

We present a detailed comparison of numerical solutions of the quasiclassical Eilenberger equations with several approximation schemes for the density of states of s- and d-wave superconductors in the vortex state, which have been used recently. In particular, we critically examine the use of the Doppler shift method, which has been claimed to give good results for d-wave superconductors. Studying the single vortex case we show that there are important contributions coming from core states, which extend far from the vortex cores into the nodal directions and are not present in the Doppler shift method, but significantly affect the density of states at low energies. This leads to sizeable corrections to Volovik's law, which we expect to be sensitive to impurity scattering. For a vortex lattice we also show comparisons with the method due to Brandt, Pesch, and Tewordt and an approximate analytical method, generalizing a method due to Pesch. These are high field approximations strictly valid close to the upper critical field Bc2. At low energies the approximate analytical method turns out to give impressively good results over a broad field range and we recommend the use of this method for studies of the vortex state at not too low magnetic fields.

Single crystal growth and superconductivity of Mo3Sb7

Single crystals of Mo3Sb7 have been grown from molten antimony in a sealed molybdenum crucible. The crystals were characterized by means of scanning electron microscopy, X-ray diffraction, electrical resistivity and magnetization measurements. This compound exhibits metallic properties with the electrical resistivity decreasing linearly from a value of 137μΩcm at room temperature to 95μΩcm at 4.2K. Superconductivity was found below Tsc=2.08K. Magnetization measurements revealed that Mo3Sb7 is a bulk type-II superconductor with critical fields Bc1∼1mT and Bc2∼0.6T at 1.72K.

Superconducting properties andc-axis superstructure ofMg1−xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.

Magnetic-field-induced irreversibility in specific heat of UNiAl

Results of low-temperature specific-heat (Cp) measurements of UNiAl single crystal in magnetic fields B⩽14T are presented. At temperatures <7K, this antiferromagnet exhibits a first-order metamagnetic transition at the critical field Bc≈11.4T (when applied parallel to the c-axis). In the same temperature range, different Cp vs. T curves are obtained in zero magnetic field depending on the magnetic history of the sample (zero-field-cooled (ZFC)=cooling to T<7K in zero field, field cooled (FC)=cooling in a field of 14T parallel to c). The magnetic-field-induced irreversibility is clearly evidenced on the Cp/T vs. B data obtained at 3.2K. For a measurement cycle; 0 (starting from ZFC state)→14→0T the “increasing field” Cp,ZFC/T and “decreasing field” Cp,FC/T data clearly differ in fields <9T and (Cp,ZFC/T)0T>(Cp,FC/T)0T. However in both cases, we have observed (for B<9T) a Cp/T∼κcB2 dependence with different κc values. The results are discussed in the context of earlier-reported magnetic-history effects in magnetic and other electronic properties.

Itinerant-electronmetamagnetism and magneto-volume effects in Lu(Co1-xAlx)2Laves phase compounds

The itinerant-electron metamagnetic transition (MT) and the effectsof hydrostatic pressure on the critical transition field BC ofthe MT, on the spontaneous magnetization MS and on the Curietemperature TC have been investigated for well homogenizedLu(Co1-xAlx)2 Laves phase compounds.The critical field BC decreases with increasing x, maintaining alinear relationship with the inverse susceptibility at thetemperature where the susceptibility exhibits a maximum value, χ-1(Tmax). On applying pressure, the magnetization M ofthe ferromagnetic compound with x = 0.100 is drastically decreasedat a critical pressure, resulting in a paramagnetic state. Inaddition, the metamagnetic transition from the paramagnetic to theferromagnetic state is induced by applying an external magnetic field.The effect of pressure on the Curie temperature TC is extremelylarge and negative in the vicinity of the criticalconcentration for the onset of ferromagnetism. The pressurecoefficient of the Curie temperature, ∂ln TC/∂P, is much larger than that of the spontaneous magnetization,∂ln MS/∂P, below x = 0.150. These results canbe explained by the theory for itinerant ferromagnets having anegative coefficient b of the fourth-order term in the Landauexpansion. The Landau expansion coefficients estimated from theexperimental results are in accord with the theories. From theseestimated values, it is concluded that the magneto-volume effectdecreases the critical transition field BC. It has beenconfirmed that the results for Lu(Co1-xAlx)2 are verymuch analogous to those for Lu(Co1-xGax)2.

Magnetic phase transitions in CePtSn

Magnetization (M) and specific-heat measurements of a CePtSn single crystal in magnetic fields up to 14 T applied along principal crystallographic axes are reported. CePtSn becomes antiferromagnetic at TN=7.5 K and undergoes an order–order transition at 5 K (in zero field). In a field applied along the a axis (B//a) a metamagnetic transition (MT) is induced that is manifest by a cusp on the M(B) curve at a critical field Bc, an exponential increase of the low-field magnetization, and a linear increase for B&amp;gt;Bc(=12.5 T) at 2 K; Bc decreases with increasing temperature. For B//b, qualitatively different M(B) curves are observed in two temperature regimes. For T&amp;lt;3 K, two MTs are found at 4 and 11 T, respectively, the former one irreversible. For T⩾3 K only the “11 T” transition remains with modified character and Bc is rapidly decreasing with increasing temperature. These MTs are associated with previously reported GMR effects. Microscopic aspects of these phenomena are discussed and updated magnetic-phase diagrams for B//a and B//b, respectively, are presented.

Influence of the inhomogeneous distribution of components on itinerant-5f-electron metamagnetism in UCoAl under high pressures

Abstract We have performed a comparative study of the magnetization process and the magnetic susceptibility under pressure on a stoichiometric UCoAl single crystal and an off-stoichiometric U09Co1.05Al1.05 single crystal of the anisotropic itinerant-5f-electron metamagnet UCoAl. Both UCoAl and U0.9Co1.05Al1. 05 exhibit a metamagnetic transition to the ferromagnetic state with a magnetic moment M ≍ 0.3μB (U atom)−1 in magnetic fields applied along the c axis. The critical field Bc of the transition increases with increasing pressure and temperature. The temperature dependence of the susceptibility shows a broad maximum at a temperature T max that gradually increases with increasing pressure. At ambient pressure, the transition in UCoAl is of first order below 12 K and of second order at 12–20 K. Broadeningof the transition inU0.9Co1.05Al1.05 at low temperatures is attributed rather to a variation in the B c value over the crystal (which can be described by Gaussian distribution with a half-width of 0.7 T a...

IQHE and FQHE in a wire with incompressible and compressible strips

We report the study of the longitudinal (RL) and Hall (RH) resistances in a wide ballistic wire (LH⪡W⪡l, l is mean free path, W is the width of wire, LH is the magnetic length in QHE regime) in the IQHE and FQHE regimes. It was observed a partial suppression of the RL minima in the IQHE and their practically total disappearance in the FQHE while the Hall resistance plateaus corresponding to these minima remained well pronounced in both the IQHE and FQHE regimes. At lower filling factors (ν<1/3) we have observed a quantum Hall liquid–quantized Hall insulator transition that takes place at a certain critical magnetic field Bc and a critical resistance value close to h/e2. Finally, at ν<1/2 and at the lowest experimental temperatures T<0.3K there have been observed correlated fluctuations in the longitudinal and in the Hall resistance both as a function of magnetic field and gate voltage. It is supposed that the observed features are due to interplay between wide compressible and narrow incompressible strips in the wire.

Advances in Bi-Based High-Tc Superconducting Tapes and Wires

Since the discovery of high-Tc superconductors (HTSs), especially the Bi-based oxide superconductors (Bi-HTSs) in 1989, much effort has been concentrated on the fabrication of Bi-HTS wires and tapes. Bi-HTSs are interesting materials from the viewpoint of practical applications. One of the interesting applications of Bi-HTSs is the cryogen-free conduction-cooled magnet. Besides having a high transition tem- perature (Tc), Bi-HTSs have high upper critical fields (Bc2) or high irreversibility fields (Birr) at temperatures below ∼30 K. Because ∼30 K can be efficiently main- tained by a cryocooler, a Bi-HTS magnet cooled with a cryocooler has great potential in many technological applications. Another promising application of Bi-HTSs is the high-field magnet. When the temperature is reduced to ∼5 K, Bi-HTSs show a much higher Bc2 or Birr than do conventional metallic superconductors (low-temperature superconductors, LTSs) such as Nb3Sn. This indicates that Bi-HTSs also have great potential for use in a high- field superconducting magnet if the magnet is operated at low temperature.

Magnetic pair breaking in superconductingBa1−xKxBiO3investigated by magnetotunneling

The de Gennes and Maki theory of gapless superconductivity for dirty superconductors is used to interpret the tunneling measurements on the strongly type-II high-Tc oxide-superconductor Ba1-xKxBiO3 in high magnetic fields up to 30 Tesla. We show that this theory is applicable at all temperatures and in a wide range of magnetic fields starting from 50 percent of the upper critical field Bc2. In this magnetic field range the measured superconducting density of states (DOS) has the simple energy dependence as predicted by de Gennes from which the temperature dependence of the pair-breaking parameter alpha(T), or Bc2(T), has been obtained. The deduced temperature dependence of Bc2(T) follows the Werthamer-Helfand-Hohenberg prediction for classical type-II superconductors in agreement with our previous direct determination. The amplitudes of the deviations in the DOS depend on the magnetic field via the spatially averaged superconducting order parameter which has a square-root dependence on the magnetic field. Finally, the second Ginzburg-Landau parameter kappa2(T) has been determined from the experimental data.

Direct evidence for filamentary and channel vortex flow in Pb/In superconducting films

Filamentary vortex flow in thin Pb/~14 at. %!In films is revealed by multiple voltage jumps in currentvoltage (IV) characteristics below a critical magnetic field much lower than the upper critical field Bc2 . The voltage jumps exhibit large counterclockwise hysteresis which diminishes with increasing field leading to bistable behavior close to the onset of free-flux flow. From the magnetic field and temperature dependence of the jump heights and currents at which jumps occur we infer that the number of flux lines in each channel or filament remains constant. In this regime, where the shear modulus of the vortex lattice is negligible, we find a nearly parabolic dependence of the depinning force on magnetic field, which is a signature of channel vortex motion. @S0163-1829~99!50710-8#

Anisotropy of the Upper Critical Field of the Organic Superconductor λ-(BETS)2GaCl4

The upper critical field Bc2 of the organic superconductor λ-(BETS)2GaCl4 has been determined from resistance measurements for orientation of the magnetic field along three perpendicular crystallographic directions. The Ginzburg–Landau coherence length was estimated from the slope of Bc2(T) curve near Tc as 12.5 nm, 1.6 nm, and 12.5 nm for the a*-, b*-, and c-directions. Angular dependence of the critical field within highly conducting ac plane at 1.5 K was shown to possess two-fold symmetry, which can be related to the band structure anisotropy.

Effect of Hydrogen Absorption on the Magnetic Properties of Itinerant-Electron Metamagnetic Compounds Y(Co1-xAlx)2

The high-field magnetizations of Y(Co1-xAlx)2Hy have been measured for 0≤x≤0.075 and 0≤y≤3.44 in pulsed high magnetic fields up to 42 T. The itinerant metamagnetic transition is broadened, but clearly observed in Y(Co0.925Al0.075)2Hy with y≤0.72. The value of the critical field Bc of the transition decreases nearly linearly with increasing hydrogen content in the region y<0.28, which is consistent with the decrease of the transition field due to Al substitution for Co in the parent compound. A weak spontaneous moment appears in all the hydrides, which originates from a partial structural disordering of the lattice induced by hydrogen absorption.

Point contact studies of the superconducting gap of CeRu2

The measured dV/dI(V) curves of point contacts between the intermetallic compound CeRu2 in the superconducting state and the normal metal Cu correspond fairly well to the Blonder–Tinkham–Klapwijk model. It is used to obtain temperature and magnetic field dependence of the superconducting gap Δ. The Δ(T) dependence corresponds well to the BCS theory with 2Δ(0)/kBTc*=3.1±0.1. Additionally, a region of gapless superconductivity between Tc*≃4.4−5.4 K and Tcbulk≃6.2 K is found. The gap decreases approximately linearly in a magnetic field and vanishes in a field Bc*≃3.5 T, which is well below the upper critical field Bc2≃6 T and close to the irreversibility field.