Superconductors In Mixed State Research Articles

Mixed superconducting state without applied magnetic field

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field Hc2 is higher than the thermodynamic critical field Hc. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ringshaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.

Topological superconductivity in Janus monolayer transition metal dichalcogenides

The Janus monolayer transition metal dichalcogenides (TMDs) MXY (M = Mo, W, etc. and X, Y = S, Se, etc.) have been successfully synthesized in recent years. The Rashba spin splitting in these compounds arises due to the breaking of out-of-plane mirror symmetry. Here we study the pairing symmetry of superconducting Janus monolayer TMDs within the weak-coupling framework near critical temperature T c, of which the Fermi surface (FS) sheets centered around both Γ and K (K′) points. We find that the strong Rashba splitting produces two kinds of topological superconducting states which differ from that in its parent compounds. More specifically, at relatively high chemical potentials, we obtain a time-reversal invariant s + f + p-wave mixed superconducting state, which is fully gapped and topologically nontrivial, i.e., a ℤ2 topological state. On the other hand, a time-reversal symmetry breaking d + p + f-wave superconducting state appears at lower chemical potentials. This state possess a large Chern number |C| = 6 at appropriate pairing strength, demonstrating its nontrivial band topology. Our results suggest the Janus monolayer TMDs to be a promising candidate for the intrinsic helical and chiral topological superconductors.

Induced superconductivity in linear carbon chains and engineering of zero-bias peaks with geometric angles: a new approach

We investigated the effect of slip angle, a geometric parameter named as α in superconducting quasi-one-dimensional carbon structures, by engendering resonant peak of linear carbon chains. We studied also the density of states and the effect of different slip angles on this structure. We found the interesting role of the slip angle, similar to the mixed superconducting state, creating a mass-like gap in the electron-hole excitation spectrum of the superconducting quasi-one-dimensional carbon structure. In the framework of the Dirac-Bogoliubov–de Gennes formalism, we investigated the transport properties in the superconducting linear carbon chain. The superconductor gap is taken to be a spin-singlet and spin-triplet superconducting state.

Enhancement of the Electric Current in High-Tc Superconductor Interacting with an Artificial Pinning Array

The dynamics of the flux flow in the mixed state of high-Tc superconductor interacting with an artificial pinning array is considered. We calculate analytically the current density for the case of when the number of vortices outnumbers that of the pinning centers using the time-dependent Ginzburg-Landau equations in a magnetic field in the presence of strong thermal fluctuations. Our results show that the current density increases considerably with increasing pinning strength. It means that an original property of superconductor is recovered even in strong thermal fluctuations under a magnetic field by adding a pinning array to the mixed state of superconductor.

Large enhancement of conductivity in a strongly layered type-II superconductor with an artificial pinning array

Abstract We use the time-dependent Ginzburg–Landau equation to describe a type-II superconductor in a magnetic field in the presence of both strong thermal fluctuations and an artificial pinning array. Thermal fluctuations are represented by the Langevin white noise. The layered structure of the superconductor is taken into accounted with the Lawrence–Doniach model. The self-consistent Gaussian approximation is used to treat the nonlinear interaction term in the time-dependent Ginzburg–Landau equation. In the case of the $\delta $-function model for the pinning centers and the matching field, analytic expressions for the fluctuation electrical and thermoelectric conductivity are obtained. It is found that the fluctuations in electrical and thermoelectric conductivities increase with increasing pinning strength, and when the pinning strength comes near a critical value, the fluctuation conductivity is greatly enhanced. Our result shows that if a pinning array is added to a mixed state superconductor, the original properties of the superconductor are recovered. Physically, in the presence of thermal fluctuations, when the energy scale of the vortex lattice shear fluctuations becomes comparable to the pinning energy scale there is a large enhancement of the fluctuation conductivity in the presence of pinning.

The nature of voltage in superconductors with dynamic intermediate-mixed state

Aim: to describe scientific achievement providing the essence of the phenomenon; to confirm reliability of existence of DC electromagnetic induction using direct evidence.
 Methods: experimental research into magnetizing superconducting samples. Visualisation of intermediate and mixed state of superconductors and movement of quantised threads of magnetic flux, the beams of such threads. Development of mathematical theory on the basis of fractal geometry and functional analysis.
 Results: The essence of the scientific discovery consists in the proof that the direct consequence of the formation of an intermediate-mixed state of superconductors and the movement of quantum magnetic fluxes threads, beams of such filaments and macroscopic normal zones is the phenomenon of direct current electromagnetic induction.
 Conclusion: the discovery of DC electromagnetic induction has been made, superconducting topological electrical generators on the basis of DC electromagnetic induction have been built, that achieve high nominal parameters: 10-100 kA current, specific current load - 1 kA/kg.

Дислокаційний гістерезис у змішаному стані надпровідника ІІ роду

Дислокаційний гістерезис у змішаному стані надпровідника ІІ роду

Thermal properties and electronic structure of superconducting germanide skutterudites and : a multi-band perspective

We present the temperature (T) and magnetic field (H) dependence of heat capacity (C) of the superconducting skutterudite compounds and along with their electronic band structure. We show that the zero-field C(T) in the superconducting state of both the samples cannot be explained using an isotropic single gap or an anisotropic single gap perspective. We find that it can only be explained in terms of two superconducting gaps. The Sommerfeld coefficient of electronic heat capacity in the mixed superconducting state of these compounds obtained from the C(T) closely follows the measured C(H) at 2 K. Observation of two distinct linear regions in with a change of slope in between for both the samples supports the fact that the two gap superconductivity is intrinsic to this family of skutterudites. A large observed in the limit of zero magnetic field for both the samples indicates the possibility of anisotropy in the superconducting gaps of these compounds. Apart from supporting the evidence for the above experimental observations, the band structure calculations also show that the anisotropy of the gaps in is different from that of .

Strain-induced time-reversal odd superconductivity in graphene

Time-reversal symmetry breaking superconductors are exotic phases of matter with fascinating properties, which are, however, encountered rather sparsely. Here we identify the possibility of realizing such a superconducting ground state that exhibits an $f+is$ pairing symmetry in strained graphene. Although the underlying attractive interactions need to be sufficiently strong and comparable in pristine graphene to support such pairing state, we argue that strain can be conducive for its formation even for weak interactions. We show that quantum-critical behavior near the transition is controlled by a multicritical point, characterized by various critical exponents computed here in the framework of an $\epsilon$-expansion near four spacetime dimensions. Furthermore, a vortex in this mixed superconducting state hosts a pair of Majorana fermions supporting a quartet of insulating and superconducting orders, among which topologically nontrivial quantum spin Hall insulator. These findings suggest that strained graphene could provide a platform for the realization of exotic superconducting states of Dirac fermions.

Control of dissipation in superconducting films by magnetic stray fields

Hybrid superconducting/magnetic nanostructures on Si substrates have been built with identical physical dimensions but different magnetic configurations. By constructing arrays based on Co-dots with in-plane, out-of-plane, and vortex state magnetic configurations, the stray fields are systematically tuned. Dissipation in the mixed state of superconductors can be decreased (increased) by several orders of magnitude by decreasing (increasing) the stray magnetic fields. Furthermore, ordering of the stray fields over the entire array helps to suppress dissipation and enhance commensurability effects increasing the number of dissipation minima.

Vortex-induced strain and magnetization in type-II superconductors

It is argued that the stress caused by vortex cores in the mixed state of superconductors may result in a field dependent contribution to the free energy and magnetization of measurable levels. For sufficiently strong stress dependence of the critical temperature, $\partial T_c/\partial p$, this contribution may result in the so-called "second peak" in the field dependence of the reversible magnetization, the effect often masked by vortex pinning and creep.

Magnetization Study of Hg and Tl based High Temperature Superconductors in Mixed State

A characteristic study of the temperature and field dependence magnetization of high temperature superconductors (HTSCs) HgBa2CuO4 and Tl2Ba2Ca2Cu3O10 have been carried out in this present work. The field dependence of dM/dlnB has been discussed and compared using few existing models. The findings of variation of magnetization with temperature for different applied fields are found to give correct behavior as observed in experiments.

Ac susceptibility investigation of vortex dynamics in nearly optimally dopedRFeAsO1−xFxsuperconductors (R= La, Ce, Sm)

Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$ superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{cr} \lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth $\lambda_{ab}(0)$ and the anisotropy parameter $\gamma$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{\pm}$ two-gap scenario.

INVESTIGATION INTO SUPERCONDUCTING Y-BA-CU-O FILMS CONTAINING REGIONS OF WEAK SUPERCONDUCTIVITY AND BIASED AT I >> IC / STIPRIOS I >> IC ELEKTROS SROVĖS TANKIO PASISKIRSTYMAS PLONUOSE Y-BA-CU-O SLUOKSNIUOSE, TURINČIUOSE SILPNOJO SUPERLAIDUMO SRITIS

Current distribution in YBa2Cu3O7-x superconducting thin film micro-bridges containing laser–written -shaped channel for easy vortex motion have been investigated in a narrow range of temperatures ranging from 85.7 to 90.2 K, which are below the critical temperature Tc of the material. In the range of the before mentioned temperatures the current-self magnetic field produced, Abrikosov magnetic vortices/anti-vortices appear in the partially deoxygenated (x ~ 0.2) channel and move coherently identifying itself as Josephson–like voltage “steps” in nonlinear current–voltage dependences. Bias current creates Lorentz force FL, pushing vortices along the LW-channel from its edges towards the centre. As follows from current-voltage dependences, vortices start moving at I = Ic. This current already satisfies condition FL >> Fp (where Fp – is pinning force in the film). Our results confirm that coherent vortex motion in the channel holds even at high biasing current when I ~ 200Ic. However, at these biasing currents, the width of voltage steps appears to be shorter than at lower current. This phenomenon has been explained by means of the onset of additional vortex motion in the laser untreated regions of the superconducting device. These regions are located in between the Π-shaped channel for easy vortex motion. Our results also demonstrate the possibility of investigating the electric properties of the mixed state superconductor by means of current dissipation due to the onset of coherent vortex motion. Santrauka Nagrinėjamas elektros srovės pasiskirstymas YBa2Cu3O7-x superlaidininko plonuose sluoksniuose, kuriuose lazerio spinduliu suformuoti Π formos sūkurių kanalai. Superlaidžiajame darinyje atsiradus sūkurių kanalui, srovės tankis lokaliose darinio vietose gali skirtis 5 ir daugiau kartų, priklausomai nuo kanalo geometrijos. Darbe eksperimentiškai nustatyta, kad Abrikosovo magnetinių sūkurių koherentinis judėjimas sūkurių kanale siaurame temperatūrų ruože nuo 85,1 K iki 90,2 K prasideda superlaidžiuoju dariniu tekant I ~ Ic ir išlieka iki ypač stiprių srovių I > 200I1st step. Čia I1st step – srovės stipris, kuriam esant sūkurių kanale vyksta koherentinis sūkurių judėjimas, o Ic – kritinė superlaidininko srovė. Koherentinis sūkurių judėjimas kanalo srityje eksperimentuose pasireiškia darinių voltamperinėse charakteristikose elektros įtampos „laipteliais“, kurių aukštis beveik nepriklauso nuo dariniu tekančios srovės stiprio. Eksperimentiniai rezultatai paaiškinti koherentinio sūkurių judėjimo ypatumais Π formos sūkurių kanalu.

μSR study of CeRhIn5 under applied pressure

At ambient pressure CeRhIn5 is an incommensurate antiferromagnet with Néel temperature TN = 3.8 K. The application of pressure reduces TN and yields (1) a state in which AFM coexists with superconductivity, and then (2) a superconducting state in the absence of AFM. We report transverse-field muon spin relaxation (μSR) measurements at pressures 2.07 and 2.26 GPa in the pure superconducting state of single-crystalline CeRhIn5, which has a tetragonal structure. μSR is sensitive to the local field distribution produced by the vortex lattice in a mixed superconducting state, allowing a measurement of the magnetic field penetration depth λ(T). We measured λab(T) for applied field along the c-axis as a function of temperature T, and find λab(0) = 372(5) nm and 338(6) nm for P = 2.07 GPa and 2.26 GPa, respectively. The temperature dependence of the superfluid density ρs(T) ∝ λ−2(T) was found to be ρs(T) ∝ Tn with n = 1.9(2).

Anisotropy in mixed superconducting state of two-band MgB2 superconductor

The multi-band superconductor MgB2 exhibits two-dimensional σ-band superconductivity and three-dimensional π-band superconductivity. We investigate the electronic anisotropy in MgB2 by doing systematic magnetic torque measurements under various different temperatures and magnetic fields. The Kogan theory for single-band superconductors explains the properties of a variety of anisotropic superconductors in terms of the anisotropy γλ in penetration depth λ, but it fails to explain the MgB2 torque successfully in some cases. The two-band version of the Kogan torque theory is needed to give a better description of the MgB2 torque curve. The γξ value coming from the anisotropy in coherence length ξ plays an important role in a multi-band superconductor. We consider that the suppression of γξ in MgB2 derived here comes from enhanced σ–π mixing between two-band carriers.

Double-stage continuous-discontinuous superconducting phase transition in the Pauli paramagnetic limit of a 3D superconductor: the URu2Si2 case

The sharp suppression of the de-Haas van-Alphen oscillations observed in the mixed superconducting (SC) state of the heavy fermion compound URu$_{2}$Si$% _{2}$ is shown to confirm a theoretical prediction of a narrow double-stage SC phase transition, smeared by fluctuations, in a 3D paramagnetically-limitted superconductor. The predicted scenario of a second order transition to a nonuniform (FFLO) state followed by a first order transition to a uniform SC state, obtained by using a non-perturbative approach, is also found to be consistent with recent thermal conductivity measurements performed on this material.

Macroscopic Quantum Tunnelling of Josephson Vortex in Antiferromagnetic Superconductor

The specific feature caused by the long-range antiferromagnetic order in the mixed state of superconductor might be the creation of spin-flop (or metamagnetic) domain along each vortex core. Let us consider an antiferromagnet with two magnetic sublattices as an example. An infinitesimal magnetic field applied perpendicular to the easy axis makes the ground antiferromagnetic state unstable against the phase transformation to the canted phase (spin-flop, SF). On the contrary, if the magnetic field is applied parallel to the easy axis, the antiferromagnetic configuration is stable up to the thermodynamic critical field HT. When the field is further increased a canted phase develops in the system. Let us assume that in the antiferromagnetic superconductor the lower critical field fulfils the relation Hc1 < 1 2HT and that the external field, Hc1 < H < 1 2HT, is applied parallel to the easy axis. Then the superconducting vortices appear in the ground antiferromagnetic state. If the field is increased above 12HT, the phase transition to the canted phase originates in the vortex core because the field intensity in the core doubles the external one. The spatial distribution of the field around the vortex is a decreasing function of the distance from its centre. Hence the magnetic field intensity in the neighbourhood of the core is less than HT. Therefore, the rest of the vortex remains in the antiferromagnetic configuration. The radius of spin-flop domain grows as the field is increased. Thus, in the considered model there are two distinct types of vortices. Possible candidate of such system might be ErBa2Cu3O7 [1, 2]. This compound has tetragonal unit cell with small orthorhombic distortion

Anisotropic Hall effect in single domain YBaCuO superconductor in mixed state

The Hall resistance has been measured by the van der Pauw method on single domain YBa2Cu3Ox samples in magnetic fields up to 5T. Two transversal resistances were measured at two directions approximately perpendicular to each other. Both Hall resistances have typical temperature dependences with negative minimum below the critical temperature, but the anisotropy of the Hall resistance has been observed there. The Hall effect anisotropy can be observed only as a consequence of the violation of the Onsager relations, which include only magnetic field inversion. These relations are connected with corresponding reciprocity theorem, which is not valid below superconducting transition in this material. The observed effects are discussed in term of vortex dynamics in the presence of planar pinning defects. The Hall effect anisotropy can be explained through a model, which combine the guiding pinning with equation of motion in WDT model.

New quantum phase in the heavy fermion superconductor CeRhIn5

Pressure dependence on the specific heat of CeRhIn5 has been studied for magnetic field perpendicular to the ab-plane. Similar to the field parallel to the ab-plane [T. Park et al., Nature 440 (2006) 65], magnetic field reveals hidden magnetism in the mixed superconducting state above 1.75GPa where antiferromagnetism disappears at zero field. The specific heat anomaly at Tc is initially suppressed with increasing field, but becomes enhanced with further increasing field, indicating that the nature of the superconducting (SC) transition may have changed from second to first order. Together with the coexisting phase of the field-induced magnetism and superconductivity, the first-order like SC transition in CeRhIn5 may provide insight to the inhomogeneous SC state (or FFLO state) observed in CeCoIn5.