Critical Field Behavior Research Articles

Evolution of vortex matter, phase diagram, and upper critical field in the FeSe1−x S x system

We present a comprehensive study of vortex matter in the FeSe1−x S x system with various doping degree. The influence of sulphur substitution on peak effect occurrence is studied. Additionally, we study a possible vortex liquid–vortex glass/lattice transition and find an evidence that the vortex liquid–vortex glass phase transition in FeSe1−x S x has a quasi two–dimensional nature. We investigate the upper critical field behaviour in FeSe1−x S x system, and found that the upper critical field is higher than that predicted by the Werthamer–Helfand–Hohenberg (WHH) model, whereas its temperature dependence could be fitted within a two-band framework. Finally, a detailed H–T phase diagram is presented.

Upper critical field in ferromagnetic metals with triplet pairing

The theory of triplet superconductivity in ferromagnetic metals based on electron–electron interaction by spin fluctuation exchange is developed. The equations for the upper critical field temperature dependence are derived. In contrast to the similar equations for the superconductivity in two band metals they contain the pairing amplitudes and the Fermi velocities depending on magnetic field. The critical field behaviour near the critical temperature and at T=0 is established analytically.

Critical field behavior of a multiply connected superconductor in a tilted magnetic field

We report magnetotransport measurements of the critical field behavior of thin Al films deposited onto multiply connected substrates. The substrates were fabricated via a standard electrochemical process that produced a triangular array of 66 nm diameter holes having a lattice constant of 100 nm. The critical field transition of the Al films was measured near $T_c$ as a function of field orientation relative to the substrate normal. With the field oriented along the normal ($\theta=0$), we observe reentrant superconductivity at a characteristic matching field $H_m=0.22\,\mathrm{T}$, corresponding to one flux quantum per hole. In tilted fields, the position $H^*$ of the reentrance feature increases as $\sec(\theta)$, but the resistivity traces are somewhat more complex than those of a continuous superconducting film. We show that when the tilt angle is tuned such that $H^*$ is of the order of the upper critical field $H_c$, the entire critical region is dominated by the enhanced dissipation associated with a sub-matching perpendicular component of the applied field. At higher tilt angles a local maximum in the critical field is observed when the perpendicular component of the field is equal to the matching field.

Superconducting properties of molybdenum ruthenium alloy Mo0.63Ru0.37

Resistance, magnetization and specific heat measurements were performed on Mo0.63Ru0.37 alloy. All of them confirm that Mo0.63Ru0.37 becomes superconducting at about 7.0 K with bulk nature. Its upper critical field behavior fits to Werthamer-Helfand-Hohenberg (WHH) model quite well, with an upper critical field of μ0Hc2(0) = 8.64 T, less than its Pauli limit. Its electronic specific heat is reproduced by Bardeen-Cooper-Schriffer (BCS)-based α-model with a gap ratio Δ0 = 1.88k B T c , which is a little larger than the standard BCS value of 1.76. We concluded that Mo0.63Ru0.37 is a fully gapped isotropic s-wave superconductor, with its features are mostly consistent with the conventional theory.

Magnetocaloric effect and critical field analysis in Eu substituted La0.7-xEuxSr0.3MnO3 (x = 0.0, 0.1, 0.2, 0.3) manganites

In this study, we have investigated magnetic behavior, magnetocaloric effect and critical exponent analysis of La0.7-xEuxSr0.3MnO3 (x=0.0, 0.1, 0.2, 0.3) manganites synthesized through solid state reaction route. The crystallographic data obtained from refinement of X-ray diffraction patterns reveal that crystal structure changes from rhombohedral (for x=0.0) to orthorhombic (for x≥0.1). The average ionic radius of A-site is decreased from 1.384Å (for x=0.0) to 1.360Å (for x=0.3) with Eu3+ substitution which in turn decreases the Mn-O-Mn bond angles. Magnetization measurements are performed in the vicinity of TC to determine magnetocaloric effect (MCE) and critical field behavior. The maximum magnetic entropy change (ΔSMmax) (for μ0ΔH=6T) increases with the Eu3+ substitution from 3.88J/kgK (for x=0.0) to 5.03J/kgK (for x=0.3) at the transition temperature. The critical field behaviour of compounds was analysed using various methods such as modified Arrott plots, Kouvel-Fisher method and critical isotherm to determine critical temperature and critical exponents (β, γ and δ). The obtained critical exponents are in good accordance with scaling relation. The temperature dependence of the order parameter n, for different magnetic fields, is studied using the relation ΔSMαHn. The values of n are found to obey the Curie-Weiss law for temperatures above the transition temperature. The rescaled change in entropy data for all compounds collapses into the same universal curve, revealing a second order phase transition.

Multiband character of β-FeSe: Angular dependence of the magnetoresistance and upper critical field

We studied ab-plane transport properties in single crystals of the superconductor β-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (Tc=8.87(5) K) the upper critical field anisotropy Hc2||ab/Hc2||c changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.

Evidence that the upper critical field of Nb3Sn is independent of whether it is cubic or tetragonal

Although 2011 marks the 50th anniversary of Nb3Sn as the first high field superconductor, real understanding of its upper critical field behavior Hc2 is incomplete. Here, we show surprising Hc2 data on highly homogeneous bulk samples that exhibit identical upper critical field Hc2(0.3 K) ∼ 29 ± 0.2 T with or without undergoing the cubic-to-tetragonal transition, a result in strong contrast to widely used multiple-source data compilations that show a strong depression of Hc2(0 K) from 29 T to 21.4 T in the tetragonal state.

Edge saturation effects on the magnetism and band gaps in multilayer graphene ribbons and flakes

Using a density functional theory based electronic structure method and semi-local density approximation, we study the interplay of geometric confinement, magnetism and external electric fields on the electronic structure and the resulting band gaps of multilayer graphene ribbons whose edges are saturated with molecular hydrogen (H$_2$) or hydroxyl (OH) groups. We discuss the similarities and differences of computed features in comparison with the atomic hydrogen (or H-) saturated ribbons and flakes. For H$_2$ edge-saturation, we find \emph{shifted} labeling of three armchair ribbon classes and magnetic to non-magnetic transition in narrow zigzag ribbons whose critical width changes with the number of layers. Other computed characteristics, such as the existence of a critical gap and external electric field behavior, layer dependent electronic structure, stacking-dependent band gap induction and the length confinement effects remain qualitatively same with those of H-saturated ribbons.

Critical field behavior and antiband instability under controlled surface electromigration on Si(111)

In this study we investigate step bunching and antiband surface instabilities on Si(111). We experimentally study the effects of a controlled electromigration field on the onset of antibands. We analyze the initial stage of antiband formation on step bunched surfaces under conditions of constant temperature of 1270 \ifmmode^\circ\else\textdegree\fi{}C, while systematically varying the applied electromigration field. The relationship between the electromigration field and minimum terrace width required to initiate the antiband formation has been established. Also, we systematically measured values of the critical electromigration field, which is required to initiate the step-bunching process on Si(111) at 1130 \ifmmode^\circ\else\textdegree\fi{}C (regime II) and 1270 \ifmmode^\circ\else\textdegree\fi{}C (regime III). The dependence of the critical field on the mean atomic terrace width has been investigated and discussed.

Evolution of ground state and upper critical field inR1−xGdxNi2B2C(R=Lu,Y): Coexistence of superconductivity and spin-glass state

We report effects of local magnetic moment, ${\text{Gd}}^{3+}$, doping $(x\ensuremath{\lesssim}0.3)$ on superconducting and magnetic properties of the closely related ${\text{Lu}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Ni}}_{2}{\text{B}}_{2}\text{C}$ and ${\text{Y}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Ni}}_{2}{\text{B}}_{2}\text{C}$ series. The superconducting transition temperature decreases and the heat capacity jump associated with it drops rapidly with Gd doping; qualitative changes with doping are also observed in the temperature-dependent upper critical field behavior, and a region of coexistence of superconductivity and spin-glass state is delineated on the $x\text{\ensuremath{-}}T$ phase diagram. The evolution of superconducting properties can be understood within Abrikosov-Gor'kov theory of magnetic impurities in superconductors taking into account the paramagnetic effect on upper critical field with additional contributions particular for the family under study.

Coupling phenomena and vortex transitions in superconducting Ni/Nb multilayers

We have studied some structural and superconducting properties of Ni/Nb multilayered films prepared by magnetron sputtering. Magnetization measurements allowed us to establish a rich H × T diagram that indicates a 3D-2D dimensional crossover of the vortex system, as revealed by the parallel upper critical field behavior. Consistently, it was identified a possible decoupling line associated with the transition of Abrikosov vortex lines into vortex pancakes, starting around the same crossover region. An irreversibility line was also determined and it suggests different regimes of the vortex matter, where the role played by the Ni ferromagnetic layers might be relevant.

Effects of neutron irradiation on polycrystallineMg11B2

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour has been carried out to identify what scattering and pinning mechanisms come into play. Finally the correlation between some characteristic lengths and the transition widths is analysed.

Spin Proximity Effect in Ultrathin Superconducting Be-Au Bilayers

We present a detailed study of the effects of interface spin-orbit coupling on the critical field behavior of ultrathin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of d=2-30 nm and Au coverages of 0.5 nm. Though the Au had little effect on the superconducting gap, it produced profound changes in the spin states of the system. In particular, the parallel critical field exceeded the Clogston limit by an order of magnitude in the thinnest films studied. In addition, the parallel critical field unexpectedly scaled as [FORMULA: SEE TEXT], suggesting that the spin-orbit coupling energy was proportional to Delta0/d2. Tilted field measurements showed that, contrary to recent theory, the interface spin-orbit coupling induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.

HIGH QUALITY FULLY IN-SITU MgB2 THIN FILMS OBTAINED BY DC MAGNETRON SPUTTERING

MgB 2 thin films were grown in-situ at INFM- University of Naples by a magnetron sputtering technique in a UHV system (10-7 Pa) equipped with 3 focused 2′′ magnetron sources (a stoichiometric MgB 2 and metallic Mg and B targets by Superconducting Components Inc.). The substrates (sapphire or MgO) were placed "on axis" at 7 cm from the target surface on the surface of a molybdenum heater that could be operated up to 1000°C under vacuum. Best results were obtained codepositing MgB 2 and Mg at equal sputtering power (500W) for 10 min on cold substrates, resulting in a Mg rich Mg-B precursor film. The films were then annealed inhyphen;situ at 830°C for 10 min in a In sealed Nb box in presence of saturated Mg vapor. The process is highly reproducible and can be easily scaled to produce large area films. The resulting films were about 1μm thick, with 100nm surface roughness as measured by AFM Resistive transition showed a maximum T c of 35 K and a transition width lower than 0.5 K. The residual resistivity ratio was 1.6 for the best sample. Resistivity measurements in external magnetic field up to 8 T have been performed both in parallel and perpendicular configuration. The upper critical magnetic field vs. temperature behavior has been determined from the experimental data and the superconducting anisotropy has been calculated for samples with different T c .

Anisotropy in c-oriented MgB 2 thin films grown by pulsed laser deposition

The electronic anisotropy in MgB 2 is still a not completely clear topic; high quality c-oriented films are suitable systems to investigate this property. In this work we present our results on MgB 2 superconducting thin films grown on MgO and sapphire substrates. The films are deposited in high vacuum, at room temperature, by laser ablation, starting from two different targets: pure boron and stoichiometric MgB 2. In both cases, to obtain and crystallize the superconducting phase, an ex situ annealing in magnesium vapor is needed. The films were characterized by synchrotron radiation diffraction measurements; the films turned out to be strongly c-oriented, with the c-axis perpendicular to the film surface and an influence of the substrate on crystallographic parameters is observed. Resisivity measurements with the magnetic field perpendicular and parallel directions to the film surface evidenced an anisotropic upper critical field behavior. The H c2 ratios ( η) resulted in the range 1.2–1.8; this difference will be discussed also in comparison with the literature data.

Critical Magnetic Field and Asymptotic Behavior of Superconducting Thin Films

In this paper, we discuss the vortex structure of the superconducting thin films placed in a magnetic field. The discussion is based on a system of simplified Ginzburg--Landau equations. We obtain the estimate for the lower critical magnetic field Hc1 , in the sense that it is the first critical value of hex, the applied field, for which the minimal energy among vortexless configurations is equal to the minimal energy among single-vortex configurations; moreover, it corresponds to the first phase transition in which vortices appear in the superconductor. We also discuss the location of these vortices and the asymptotic behavior of the local minimizers.

PHASE EQUILIBRIA IN THIN POLYMER FILMS

Within self-consistent field theory and Monte Carlo simulations the phase behavior of a symmetrical binary AB polymer blend confined into a thin film is studied. The film surfaces interact with the monomers via short ranged potentials. One surface attracts the A component and the corresponding semi-infinite system exhibits a first order wetting transition. The surface interaction of the opposite surface is varied as to study the crossover from capillary condensation for symmetric surface fields to interface localization/delocalization transition for antisymmetric surface fields. In the former case the phase diagram has a single critical point close to the bulk critical point. In the latter case the phase diagram exhibits two critical points which correspond to the prewetting critical points of the semi-infinite system. Only below a triple point there is a single two-phase coexistence region. The crossover between these qualitatively different limiting behaviors occurs gradually, however, the critical temperature and the critical composition exhibit a non-monotonic dependence on the surface field. The dependence of the phase behavior for antisymmetric boundaries is studied as a function of the film thickness and the strength of the surface interactions. Upon reducing the film thickness or decreasing the strength of the surface interactions we can change the order of the interface localization/delocalization transition from first to second. The role of fluctuations is explored via Monte Carlo simulations of a coarse grained lattice model. Close to the (prewetting) critical points we observe 2D Ising critical behavior. Also, there is a rich crossover behavior between Ising critical, tricritical and mean field behavior. At lower temperatures capillary waves of the AB interface lead to a pronounced dependence of the effective interface potential on the lateral system size.

The resistive anomaly and upwardcurvature of the perpendicular upper critical field innon-homogeneous superconductors

A peak in the resistance versus temperature measurements, justabove Tc, has been observed for perforated superconductingNb/CuMn multilayers. The peak is present in zero magnetic fieldand its height becomes smaller as the perpendicular field isincreased. In addition, the perpendicular critical magnetic fieldversus temperature curve shows an upward positive curvature. Byconsidering a simple concentrated constant-equivalent circuit wehave reproduced the observed resistive transitions and the upperperpendicular critical magnetic field behaviour.

Effect of the microscopic correlated-pinning landscape on the macroscopic critical current density in YBCO films

This paper deals with the mechanisms controlling the critical current density vs. field behavior in YBCO films. We base our analysis on a suitable model concerning the existence of a network of intergrain Josephson junctions whose length is modulated by defects. Irradiation with 0.25 GeV Au ions provide a useful tool to check the texture of the sample, in particular to give a gauge length reference to separate “weak” links and high- J c links.

Critical current versus field behaviour in Ag/BSCCO-2223 tapes with different critical currents

The dependence of J c upon applied magnetic field has been analysed in BSCCO-2223 tapes with different geometrical structures of the superconducting core. The samples, prepared by the PIT method, and rolled down to tapes of different thickness, were annealed up to three times with intermediate mechanical working. All measurements were carried out at 77 K, in magnetic field up to l T, applied both parallel and perpendicular with respect to the transport current. Several samples with the same J c and different I c and superconducting core thickness were measured and compared. All samples showed a qualitatively similar “memory effect”, with higher values of J c measured with decreasing magnetic field, but remarkable differences in the low fields regime, where thinner tapes (lower I c ) showed a faster decay of J c with B compared with the thicker ones. A simple semi-quantitative explanation, taking into account the self field of tapes, was well supported by tests carried out in a special experimental arrangement so designed as to allow to characterize the “true” zero self-field behaviour of the material.