Critical Magnetic Field Hc2 Research Articles

Effect of High Valancy Cations on the Intergranular Pinning Energies of (Bi-Pb)2Sr2Ca2Cu3O10+δ Samples

In the present work, Bi-Pb-V-Sr-Ca-Cu-Ti-O bulk samples with nominal composition (BiPb)2V x Sr2Ca3Cu4−y Ti y O12+δ with x=0.2 and y=0.1, 0.2 and 0.3 compounds have been prepared by the melt-quenching method. The magnetoresistance of the samples has been measured for different values of the applied magnetic field. Using the resistivity data, the upper critical magnetic field H c2(0) at T=0 K was calculated. The coherence lengths at T=0 K were calculated from H c2(0) values. The thermally activated flux creep model has been studied in order to calculate the flux pinning energies. The results showed that H c2(0) varied from 152.5 to 60.4 T and the flux pinning energies varied from 515 to 184 meV at 0 T, with the content y.

Stability of Abrikosov lattices under gauge-periodic perturbations

We consider Abrikosov-type vortex lattice solutions of the Ginzburg–Landau equations of superconductivity, consisting of single vortices, for magnetic fields close to the second critical magnetic field Hc2 = κ2 and for superconductors filling the entire . Here κ is the Ginzburg–Landau parameter. The lattice shape, parametrized by τ, is allowed to be arbitrary (not just triangular or rectangular). Within the context of the time-dependent Ginzburg–Landau equations, called the Gorkov–Eliashberg–Schmid equations, we prove that such lattices are asymptotically stable under gauge-periodic perturbations for and unstable for , where β(τ) is the Abrikosov constant depending on the lattice shape τ. This result goes against the common belief among physicists and mathematicians that Abrikosov-type vortex lattice solutions are stable only for triangular lattices and . (There is no real contradiction though as we consider very special perturbations.)

Properties and order-disorder competition of spin-1/2 XY model in the ferrimagnetic diamond chain

The elementary excitation spectra of the one-dimensional spin-1/2 XY model in the ferrimagnetic diamond chain at low temperature are calculated by using invariant eigenvector in this paper. And the elementary excitation energies in different cases are discussed. Therefore, analytic solutions of the three critical magnetic field intensities HC1, HC2 and Hpeak in the system are given. It is found that the analytic solutions of three critical magnetic field intensities are correct from the law of the magnetization changing with temperature under different external magnetic fields, and it is explained by the contributions of the three elementary excitations to the magnetization. The external magnetic field dependent magnetization presents a 1/3 magnetized plateau at low temperature. The variation of magnetic susceptibility either with temperature or with external magnetic field shows a double peak structure, this phenomenon indicates that the double peak structure originates from the competition among the ferromagnetic exchange interaction energy of intramolecular electronic spin parallel arrangement in dimer, the antiferromagnetic exchange interaction energy of intermolecular electronic spin antiparallel arrangement in dimer-monomer, the thermal disorder energy and the spin magnetic moment potential energy related to external magnetic field.

Degradation of the second critical magnetic field in multilayered composites with nanodimensional layers of niobium and Nb-Ti superconducting alloy

Multilayered tapes containing layers of superconducting niobium alloys with 30 and 31 wt % Ti and separated by niobium layers were investigated. The layers were ∼140 to ∼10 nm thick. Effective pinning of superconducting vortex filaments was observed on the interlayer Nb-NbTi interfaces. It was established that second critical magnetic field Hc2 decreased as the layer thickness decreased. With thin layers, its magnitude depended on the orientation of rolling the tape with respect to the external magnetic field. Results are explained by the proximity effect.

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb0.3Sn0.7Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature Tc and the second critical magnetic field Hc2, in the (Pb0.3Sn0.7)0.95In0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at Tc ∼ 2.3 K, i.e., close to the maximum value Tc ∼ 2.9 K found for the (PbzSn1 − z)0.95In0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence Tc(P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level EF with increasing pressure within the impurity band of the In quasi-local states. In this case, EF passes through a maximum in the density of impurity states at P = 3–5 kbar.

The role of antiferromagnetic La1/3Ca2/3MnO3 barriers in superconductor/insulator/ferromagnet tunnel junctions

Abstract Current transport through thin antiferromagnetic (AF) barriers of the perovskite manganite La1/3Ca2/3MnO3 (LCMO) was studied with respect to its dependence on temperature and voltage. Planar-type La2/3Ca1/3MnO3(∼80 nm)/La1/3Ca2/3MnO3(∼7 nm)/YBa2Cu3O7−δ(∼100 nm) heterojunctions were used as basic structures. The current–voltage (I–V) measurements were carried out on test junctions with a standard area of 20 × 40 μm2 in a four-terminal configuration. In spite of the carefully controlled growth conditions, barriers with the same nominal thickness showed different electrical behavior varying from elastic tunneling to Mott variable range hopping (VRH) via localized states. Fitting the VRH model to the experimental data, allowed for estimating important physical parameters of the barrier as the density of states at the Fermi level N(EF) and with this the average distance between two localized states l 0 . The different transport characteristics seem to be related to intrinsic difference in microstructure as the average surface roughness of the constituent layers may already be larger than the thickness of the barrier itself. Independent of the barrier quality, the active presence of the diamagnetic and ferromagnetic phases in the heterostructure was corroborated by transport measurements in magnetic fields and in-plane/out-of-plane magnetization hysteresis loops below the superconducting critical temperature, Tc (∼80 K). The values of the critical magnetic field Hc1 estimated from these experiments were in good accordance with those reported in the literature.

Two-Dimensional Regime in the Angular Dependence of the Upper Critical Field of Superconducting/Normal Metal Hybrids

The temperature and angular dependence of the upper critical magnetic field Hc2 in Nb/Pd superconducting multilayers were investigated with a particular attention given to the region where the system is in the two-dimensional (2D) regime. The influence of the proximity effect on the 2D region as obtained from the Hc2 angular behavior was explicitly analyzed.

Observation of a multiferroic critical end point

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a "metaelectric" critical end point, has been rarely studied. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (H) as a tuning parameter. Herein, we report the discovery of a magnetic-field-induced metaelectric transition in multiferroic BiMn(2)O(5), in which the electric polarization (P) switches polarity along with a concomitant Mn spin-flop transition at a critical magnetic field H(c). The simultaneous metaelectric and spin-flop transitions become sharper upon cooling but remain a continuous cross-over even down to 0.5 K. Near the P = 0 line realized at mu(0)H(c) approximately 18 T below 20 K, the dielectric constant (epsilon) increases significantly over wide field and temperature (T) ranges. Furthermore, a characteristic power-law behavior is found in the P(H) and epsilon(H) curves at T = 0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn(2)O(5) near zero temperature.

Upper Critical Fields and Anisotropy of BaFe1.9 Ni0.1As2 Single Crystals

Temperature dependence of the upper critical magnetic field (Hc2) near Tc of 20 K in a BaFe1.9Ni0.1As2 single crystal is determined via magneto-resistance measurements, for the out-plane (H⊥ab) and in-plane (H || ab) directions in magnetic fields of up to 8 T. The upper critical fields at zero temperature estimated by the Werthamer-Helfand-Hohenberg (WHH) formula are μ0H||c2(0) = 137 T and μ0H⊥c2(0) = 51 T, both exceeding the weak-coupling Pauli paramagnetic limit (μ0Hp = 1.84Tc). However, the WHH formula could overestimate the μ0H||c2(0) value. The anisotropy of upper critical fields is around 3 in the temperature range close to Tc. The result is very similar to the Co-doped 122 superconductor BaFe2–xCoxAs2, indicating that electron-doped 122 superconductors exhibit similar superconducting properties.

Anisotropy of the Upper Critical Field in a Co-Doped BaFe2As2 Single Crystal

The temperature dependence of the upper critical magnetic field (Hc2) in a BaFe1.84Co0.16As2 single crystal was determined via resistivity, for the inter-plane (H^ab) and in-plane (H//ab) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at 3He temperatures permits us to construct an H-T phase diagram from quantitative Hc2(0) values and determine its behavior in low temperatures. Hc2(0) with H//ab (Hc2//(0)) and H^ab (Hc2^(0)) are ~ 55 T and 50 T respectively. These values are ~ 1.2 - 1.4 times larger than the weak-coupling Pauli paramagnetic limit (Hp = 1.84 Tc), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While Hc2//ab is saturated at low temperature, Hc2 with H^ab (Hc2^) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of Hc2 in low temperature. The anisotropy of Hc2 was ~ 3.4 near Tc, and decreases rapidly with lower temperatures reaching ~ 1.1 at T = 0.7 K.

Upper critical field H c2 in anisotropic superconductors with varying charge carrier density: Application of the theory to doped MgB2

A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed. The upper critical field Hc2(ab) parallel to the ab plane and field Hc2(c) parallel to the c axis are determined in the entire temperature range 0 < T < Tc. A considerable increase in upper critical field Hc2(ab) as compared to Hc2(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γH = Hc2(ab) / Hc2(c) is obtained as a function of temperature for pure MgB2 and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential and critical magnetic fields Hc2(ab) and Hc2(c) is observed. The effect of doping on magnetic anisotropy is also determined.

Superconducting and magnetic properties of a new superconductor: EuAsFeO0.85F0.15

Polycrystalline samples of the new superconducting compound EuAsFeO0.85F0.15, with critical temperature Tc=11K, are prepared by solid state synthesis. Its electric and magnetic properties are investigated in magnetic fields from 0.1to140000Oe. The critical magnetic fields Hc1 and Hc2 are measured, and thence the magnetic penetration depth λ and the coherence length ξ are estimated. The temperature dependence Hc2(T) exhibits clear hyperbolic-type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare earths having small atomic radii.

Superconductors in Applications; Some Practical Aspects

The first blush of success in the search for a new superconductor is usually a high transition temperature, Tc. However, all power applications of superconductors and most other applications requires good current carrying capacity, usually characterized by a critical current Ic, within substantial magnetic fields, usually characterized by a critical magnetic field Hc. Furthermore, a number of other characteristics must be satisfied before commercial success can be obtained, such as acceptable cost, mechanical strength, stabilizers, and appropriate insulation materials. I examine a number of superconductors, starting with the workhorse NbTi, and look at the hard road to success for the successful commercialization of a new superconductor. I also review the applications, potential and actual, in which superconductors might be used.

Interrelation of the critical magnetic field H c2 and the residual resistivity for the two-band superconductor MgB2

An expression that describes the upper critical magnetic field Hc2 and generalizes the Gor’kov relation has been derived for the two-band two-gap superconductor MgB2. The expression relates the upper critical magnetic field Hc2 to the residual resistivity and the parameters of the band structure and holds in the range from the clean limit to the dirty limit. The ratios of the relaxation times τπ/τσ and the mean free paths of π- and σ-band electrons for MgB2 samples with a low defect level and Mg(B1 − xCx)2 samples with a partial substitution of carbon for boron are determined from experimental data.

Superconductivity in In2O3-ZnO crystalline films

Thin In2O3-ZnO (IZO) films were prepared by DC-magnetron sputtering with changing the weight concentration x of ZnO, 0 ≤ x ≤ 0.07. As-grown amorphous IZO films show the metallic characteristic but do not show superconductivity. We have investigated the T and H dependence of the resistivity ρ and Hall coefficient for annealed films with thickness d = 350 nm in air. It is found that the films with 0 ≤ x ≤ 0.03 show superconductivity. Transition temperature Tc and the carrier density n of these films are ≈ 2.0 K and 1025 -1026 m-3, respectively. The annealed In2O3-ZnO films were examined by a high resolution transparent electron microscopy. This investigation reveals that films annealed at 300 °C are sufficient crystalline. There is a difference of crystal structures between the superconducting film and normal film. We studied the upper critical magnetic field HC2 (T) for the film with x = 0.005. From the slope of dHc2/dT, we obtain the coherent length ζ(0) ≈ 100 A at T = 0 K and the coefficient of the electronic heat capacity γ ≈ 3.2 (J/m3K2) which is small compared with that of other oxide materials.

Superconductivity in transition metal-silicide W5Si3

We report here the superconducting properties of the tungsten silicide W5Si3. Electrical resistivity and magnetic susceptibility measurements as functions of temperature and magnetic field show a bulk superconductivity with Tc = 2.7 K. The magnetization versus magnetic field (M — H) curve shows a typical type-II superconducting behavior. Furthermore, we present the H — T phase diagram and estimate the upper critical field HC2(0) to be about 5200 Oe. We also discuss the superconducting properties in a Ginzburg-Landau framework and estimate the lower critical magnetic field HC1 (T), penetration depth λ and coherence length ξ to be about 33 Oe, 450 nm, and 25 nm, respectively.

An improved model for the strain dependence of the superconducting properties ofNb3Sn

We propose an improved model for the strain dependence of the superconducting properties ofNb3Sn. The model is based on the three-dimensional strain tensor and is derived in terms of thefirst, second and third invariants; it improves an existing model that only includes thesecond invariant. The axial form of the new model accurately accounts for theexperimentally observed dependence of the effective upper critical magnetic field(Hc2*) on axial strain, i.e., a quasi-parabolic strain dependence, asymmetry, and an upturn atlarge compressive axial strain. An accurate model that accounts for the three-dimensionalnature of strain is important for scaling relations for the critical current that are used tomodel magnet performance based on wire measurements.

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Thin polycrystalline zinc-doped indium oxide (In2O3–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature Tc is below 3.3 K and the carrier density n is about 1025–1026 m−3. The annealed In2O3–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field Hc2 (T) for the film with x = 0.01. From the slope of dHc2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.

Interfacial superconductivity in bilayer and multilayer IV–VI semiconductor heterostructures

A comprehensive investigation and comparison of the superconducting properties of bilayer and multilayer epitaxial heterostructures of IV–VI semiconductors exhibiting superconductivity at critical temperatures Tc⩽6.5K is carried out. The superconductivity of these systems is due to inversion of the bands in the narrow-gap semiconductors on account of the nonuniform stresses created by the grids of misfit dislocations arising at the interfaces during the epitaxial growth. It is found that Tc and the character of the superconducting transition of bilayer PbTe∕PbS heterostructures depend on the thickness d of the semiconductor layers and are directly related to the quality of the grids of misfit dislocations at the interfaces (the number and type of structural defects in the grids). Substantial differences in the behavior of bilayer sandwiches and superlattices are found. The minimum thickness d at which superconductivity appears is several times larger for bilayer than for multilayer systems. The upper critical magnetic fields Hc2 of the bilayer systems are more anisotropic. For superlattices 3D behavior is observed in the temperature region close to Tc, and with decreasing temperature a 3D–2D crossover occurs. For the bilayer structures 2D behavior starts immediately from Tc, and a 2D–1D crossover is observed, with the sharp divergence of Hc2 that is characteristic of superconducting nets.

Rigorous solution of a Hubbard model extended by nearest‐neighbour Coulomb and exchange interaction on a triangle and tetrahedron

AbstractThe Hubbard model extended by both nearest‐neighbour (nn) Coulomb correlation and nearest‐neighbour Heisenberg exchange is solved rigorously for a triangle and tetrahedron. All eigenvalues and eigenvectors are given as functions of the model parameters in a closed analytical form. For fixed electron numbers we found a multitude of level crossings, both in the ground state and in the excited states in dependence on the various model parameters. By coupling an ensemble of clusters to an electron bath we get the cluster gas model or the cluster gas approximation, if an extended array of weak‐interacting clusters is considered. The grand‐canonical potential Ω (μ, T, h) and the electron occupation N (μ, T, h) of the related cluster gases were calculated for arbitrary values (attractive and repulsive) of the three interaction constants. For the cluster gases without the additional interactions we found various steps in N (μ, T = 0, h = 0) higher than one. The reason is the degeneration of ground states differing in their electron occupation by more than one electron. For the triangular cluster gas we have one such degeneration point. For the tetrahedral cluster gas two. As a consequence, we do not find areas with one electron in the μ‐U ground‐state phase diagram of the triangular cluster gas or with one, two and five electrons in the case of the tetrahedral cluster gas. The degeneration point of the triangular cluster gas can not be destroyed by an applied magnetic field. This holds also for the lower degeneration point of the tetrahedral cluster gas. Otherwise, the upper degeneration point breaks down at a critical magnetic field hc. The dependence of hc on U shows a maximum for strong on‐site correlation. The influence of nn‐exchange and nn‐Coulomb correlation on the ground‐state phase diagrams is calculated. Whereas antiferromagnetic nn‐exchange breaks the degeneration points of the tetrahedral cluster gas partially only, a repulsive nn‐Coulomb correlation lifts the underlying degeneracies completely. Otherwise both ferromagnetic nn‐exchange and attractive nn‐Coulomb interaction stabilise the degeneration points. The consequences of the cluster gas results for extended cluster arrays are discussed.