Vortex Liquid Regime Research Articles

Angle-dependent Hall resistivity and longitudinal resistivity of type-II superconductor

We use time-dependent Ginzburg–Landau theory in a three-dimensional model, including thermal noise, to analyze angle-dependent Hall resistivity and longitudinal resistivity of type-II superconductor. The Hall resistivity and longitudinal resistivity are calculated as functions of temperature, magnetic field and the angle θ between the magnetic field and the ab-plane in the vortex-liquid regime. Our theoretical calculations within a self-consistent fluctuation approximation for MgB2 and HgBa2CaCu2O6 materials are in good agreements with the experimental findings for both below and above the critical temperature Tc. We observe that when the field angle decreases, the transition temperature increases and the magnitude of longitudinal resistivity decreases, which is qualitatively comparable to decrease of the perpendicular field component. However, when the magnetic field direction approaches the layer surface, it shows a clear different effect from that of a perpendicular field with the same normal component.

Effect of thermal fluctuations on the flux motion and vortex dynamic in (Li0.8Fe0.2)OHFeSe single crystals

We present a qualitatively investigation on the influence of thermal fluctuation on flux motion and vortex dynamic of (Li0.8Fe0.2)OHFeSe single crystals. The importance of thermal fluctuations has been assessed through the Ginzburg number Gi. The resistivity in the mixed state for (Li0.8Fe0.2)OHFeSe exhibits a thermally activated behavior investigated by TAFF theory. The field-dependent activation energy follows the relation U0 (H) ∼ H-α. The appearance of crossover at 2 T is related to the different pinning mechanisms. Single-vortex pinning occurs in the low fields range at H < 2 T, and collective pinning dominates in the region of field higher than 2 T. The vortex phase diagram from glass state to liquid state has been discussed based on the vortex-glass theory. Due to the influence of thermal fluctuations, there is a broad vortex liquid regime in the vortex phase diagram.

Electric Tuning of Vortex Ratchet Effect in NbSe2.

Inversion symmetry breaking has played an important role in recent discoveries of nonreciprocal charge transport. Niobium diselenide, for example, lacks an inversion center in the monolayer form and can host prominent nonreciprocal transport property. Here, however, we observe a nonreciprocal transport signal in the second-harmonic channel of bulk-like NbSe2, in which inversion symmetry of the lattice seems preserved. The second-harmonic signal occurs along different in-plane current orientations and appears not only in the vortex-liquid regime but also even in the superconducting fluctuation regime without an applied magnetic field. By adding a direct current (DC) bias, we quantify the symmetry breaking effect in the vortex-liquid regime. The DC bias also suggests that the rectification effect at the contacts may account for the seemingly nonreciprocal transport at zero magnetic field. Our results demonstrate that DC biasing is a useful knob for addressing nonreciprocal charge transport in a wide range of materials.

Vortex lattice melting line in superconductors with paramagnetic pair breaking

Recent experiments on the iron-based superconductor FeSe in a high magnetic field have suggested the presence of both the fluctuation-induced vortex liquid regime and a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice. To get a general picture of the magnetic phase diagram in type II superconductors with strong superconducting (SC) fluctuation and strong paramagnetic pair breaking (PPB) such as FeSe, the vortex lattice melting curve ${H}_{m}(T)$ is theoretically investigated in situations where a FFLO state is expected to occur. In general, PPB tends to narrow the vortex liquid regime intervening between ${H}_{c2}(T)$ and ${H}_{m}(T)$. In particular, the vortex liquid regime is found to rapidly shrink upon entering; by cooling, the temperature range in which the FFLO state with a periodic modulation parallel to the magnetic field is stable in mean-field theory. Based on the present results, the high-field SC phase diagrams of FeSe in the parallel and perpendicular field configurations are discussed.

Irreversibility line and thermally activated flux flow in Micro-bridges of high-temperature superconductor

We have studied the electrical resistivity as a function of temperature in Micro-bridges of YBa2Cu3O7-δ deposited by laser ablation on the SrTiO3 substrate face, around superconducting transition region in different magnetic fields. The activation energy U0 was determined and discussed using the Arrhenius relation. The irreversibility line Birr and the upper critical field Bc2were obtained using 10% and 90% criteria of the normal-state resistivity value from ρ (T) curves. A phase diagram of the studied sample is constructed showing the Tg glass line and a very broad vortex-liquid phase regime.

The Nernst effect in layered superconductors under a magnetic field

We calculated the Nernst signal [Formula: see text], describing the Nernst effect in type-II superconductor in the vortex–liquid regime, by using the time-dependent Ginzburg–Landau (TDGL) equation with thermal noise. The nonlinear interaction term in the TDGL equation is treated within self-consistent Gaussian approximation. The expression of the Nernst signal [Formula: see text] including all the Landau levels is presented in explicit form which is applicable essentially to the whole phase. Our results are compared with the recent experimental data on high-[Formula: see text] superconductor.

Transverse thermoelectric conductivity and magnetization in high-Tc superconductors

We use the time-dependent Ginzburg-Landau (TDGL) equation with thermal noise to calculate the transverse thermoelectric conductivity αxy describing the Nernst effect and magnetization Mz in type-II superconductor in the vortex-liquid regime. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation. The expressions of the transverse thermoelectric conductivity and magnetization including all the Landau levels are presented in explicit form which are applicable essentially to the whole phase. Our results are compared to recent simulation data on high-Tc superconductor.

Nernst Signal in High-T c Superconductors

The time-dependent Ginzburg-Landau equation with thermal noise is used to calculate the Nernst signal e N , describing the Nernst effect, in type-II superconductor in the vortex-liquid regime. The Gaussian method used is an elaboration of the Hartree-Fock method. An additional assumption often made in analytical calculations that only the lowest Landau level significantly contributes to physical quantities of interest in the high-field limit is lifted by including all the Landau levels. The values of e N are in good quantitative agreement with experimental data for temperature close to T c on Bi 2 Sr 2 CaCu 2 O 8+δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ .

Hall effect and pinning regimes in [formula omitted] thin films doped with [formula omitted] nanoparticles

Hall effect and flux pinning in YBa2Cu3O6+x (YBCO) thin films doped with BaZrO3 (BZO) nanoparticles is investigated. The results show that sign reversal of the Hall coefficient from positive hole-like to negative electron-like occurs in vortex-liquid regime of undoped and BZO-doped YBCO films. With increasing BZO concentration the amplitude of the negative Hall effect is suppressed while the temperature position of the anomalous Hall effect does not depend significantly on doping level. In addition, it is shown that Hall conductivity increases non-monotonically with increasing BZO doping. These results support a model where BZO at low doping concentration induces point pinning centres turning to strong columnar pinning defects in films doped with 4% BZO.

Theory of Nernst effect in layered superconductors

We calculate, using the time-dependent Ginzburg-Landau (TDGL) equation with thermal noise, the transverse thermoelectric conductivity αxy, describing the Nernst effect, in type-II superconductor in the vortex-liquid regime. The method is an elaboration of the Hartree-Fock. An often made in analytical calculations additional assumption that only the lowest Landau level significantly contributes to αxy in the high field limit is lifted by including all the Landau levels. The resulting values in two dimensions (2D) are significantly lower than the numerical simulation data of the same model, but are in reasonably good quantitative agreement with experimental data on La2SrCuO4 above the irreversibility line (below the irreversibility line at which αxy diverges and theory should be modified by including pinning effects).

A vortex solid-to-liquid transition with fully anisotropic scaling

The vortex solid-to-liquid transition has been studied in heavy ion irradiated untwinned single crystals of YBa2Cu3O7-δ with an inclined applied magnetic field. For magnetic fields tilted at angles about 45° away from the columnar defects, we find that the electric resistivity in the vortex liquid regime approaches zero with power laws in the reduced temperature T — Tc that have different exponents in all three spatial directions. Since the symmetry in the problem has been broken in two non-collinear directions by i) the direction of the columnar defects and ii) the direction of the applied magnetic field, our findings give evidence for a new type of critical behavior with fully anisotropic critical exponents. A possible view of the vortex topology for the transition is also suggested.

Theory of Nernst effect in high-Tcsuperconductors

We calculated, using the time-dependent Ginzburg-Landau equation with thermal noise, the transverse thermoelectric conductivity ${\ensuremath{\alpha}}_{xy}$ and the Nernst signal ${e}_{N}$, describing the Nernst effect, in type-II superconductor in the vortex-liquid regime. The Gaussian method used is an elaboration of the Hartree-Fock utilized by Ullah and Dorsey [Phys. Rev. Lett. 65, 2066 (1990)]. An additional assumption often made in analytical calculations that only the lowest Landau level significantly contributes to physical quantities of interest in the high-field limit is lifted by including all the Landau levels. The resulting values in two dimensions are significantly lower than the numerical simulation data of the same model but are in reasonably good quantitative agreement with experimental data on ${\text{La}}_{2}{\text{SrCuO}}_{4}$ above the irreversibility line (below the irreversibility line at which ${\ensuremath{\alpha}}_{xy}$ diverges and theory should be modified by including pinning effects). The values of ${e}_{N}$ calculated in three dimensions are also in good quantitative agreement with experimental data for temperature close to ${T}_{c}$ on ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$. For each of the materials we consider, the melting and the irreversibility lines are also fitted with the same set of parameters using a recent quantitative Ginzburg-Landau theory.

Magnetic flux penetration into superconductor in vortex-liquid and vortex-glass regimes

Magnetic flux penetration into a half-space occupied by a type-II superconductor occurring in a vor- tex-liquid or vortex-glass state is considered under the assumption that magnetic field exhibits a sharp increase with time (in the blow-up regime) at the interface. It is shown that each phase admits effective localization of magnetic flux in the form of a magnetic wave front, which is stopped at a depth of x s ∝ e - σ σ in a vortex-liquid regime and at x s ∝ (2 σ ) 1/( σ + 2) in a vortex-glass regime. Here, the parameter σ = U 0 / T depends on the thermal activation barrier height U 0 and on the temperature T in the vicinity of the melting curve T m ( B ) at a given mag- netic field induction B. © 2005 Pleiades Publishing, Inc.

Nernst effect in the vortex-liquid regime of a type-II superconductor

We measure the transverse thermoelectric coefficient $\alpha_{xy}$ in simulations of type-II superconductors in the vortex liquid regime, using the time-dependent Ginzburg-Landau (TDGL) equation with thermal noise. Our results are in reasonably good quantitative agreement with experimental data on cuprate samples, suggesting that this simple model of superconducting fluctuations contains much of the physics behind the large Nernst effect observed in these materials.

Quantum Resistive Behaviors in Vortex Liquid Regimes at Finite Temperatures

Motivated by a {\it mean field-like} resistive behavior in magnetic fields commonly seen in various superconducting (SC) cuprates and organics with strong fluctuation, {\it quantum} SC fluctuation effects on resistive behaviors are reexamined by putting emphasis on their roles in the so-called {\it thermal} vortex liquid regime. By incorporating the quantum fluctuation and a vortex pinning effect in the GL fluctuation theory, it is found that the resistivity curve shows not a fan-shaped broadening but a sharp drop at a 3d vortex-glass transition point far below an apparent upper critical field H_{c2}^*(T) as a result of a quantm fluctuation enhanced by an adequately small condensation energy or by a strong field. Fittings to data of cuprate and organic superconductors are performed by including effects of SC pseudogap region. It is argued on the cuprate materials that, irrespective of the presence of fluctuations of competing non-SC orders, the in-plane coherence length of hole-doped cuprates decreases on approaching the underdoped limit and that the condensation energy density to be measured from the heat capacity data is maximum near the optimal doping. The case of disordered quasi 2d s-wave films showing the field-tuned superconductor-insulator transition (FSIT) behavior is also studied for comparison, and an agreement with available data is found.

Theory of resistive behaviors in vortex states induced by strong quantum fluctuation in type II superconductors

Transport data in the vortex liquid regime of underdoped cuprates and organic materials show evidences of strong quantum superconducting fluctuation such as a field-tuned superconductor–insulator transition (FSIT) behavior. Two typical examples of strong quantum fluctuation effects, the FSIT behaviors in dirty films of s-wave pairing materials and the field-induced quantum fluctuation effects in κ-(ET) 2 organic superconductors, are discussed through comparison between resistivity data and theoretical curves.

Magnetic field dependence of the transverse plasmon in SmLa(0.8)Sr(0.2)CuO(4-delta).

The magnetic field and temperature dependence of the transverse and longitudinal plasmons in SmLa(0.8)Sr(0.2)CuO(4-delta) have been investigated. A transition between a vortex-glass and a vortex-liquid regime, which revealed different field dependencies of the resonance frequencies, could be clearly observed. The positions and the spectral weights of the plasmons were successfully described using the multilayer model [Phys. Rev. B 64, 024530 (2001)], which takes the compressibility of the electronic liquid into account. The absolute value of the compressibility is close to that of a two-dimensional noninteracting electron gas.

Anomalous static and dynamic behaviors of the vortex liquid phase in Bi 2Sr 2CaCu 2O 8+ δ

We have studied static and dynamic properties of Bi 2Sr 2CaCu 2O 8+ δ single crystals in the vortex liquid regime by means of local ac permeability and static magnetization measurements in magnetic fields parallel to the c-axis. A weak pinning phenomenon was observed even in the vortex liquid state below a characteristic field, H F( T), above which there is no pinning, resulting in a reversible magnetization behavior. The local permeability is rapidly reduced around H F( T) as temperature is decreased and turns negative before becoming zero at the vortex glass transition line. These results strongly suggest that the vortex liquid state below H F( T) may be distinguished as a new vortex phase obviously different from that above H F( T).

Coupling between the layers in vortex liquid regime of YBa 2Cu 3O 7−δ single crystals

We have measured the temperature dependence of voltages on both sides of MTG-YBCO single crystals in case of the applied magnetic field up to 7.5T parallel and perpendicular to the c-axis, respectively. The results show that in the dissipative regime the voltage signal on the side of the applied current contacts is more than that on the opposite side. There is a maximum and a minimum in the ratio curves of voltage signals. The layers coupling destroyed by the applied magnetic field was observed. The field of layers-coupling temperature follows the formula HH 0(1-T/T c0) 1.44 consistent with the “irreversibility line”.

Interplay between the vortex phase coherence and extended disorder defects in the vortex-liquid regime of thin films and superlattices of 123 superconductors

a-axis oriented (CuO_2 planes perpendicular to the substrate) films and superlattices grown on cubic substrates show a structure with 90° microdomains. The microdomains are grains with a 90° rotation of the c axis. In the vortex liquid region, this microstructure induces a change from the usual three-dimensional (3D) behavior of the anisotropy toward a 2D behavior, in a temperature range above the usual crossover from 2D to 3D. Moreover, when the thickness of the superconducting part of the multilayers becomes smaller (or of the same order as) than the microdomain size the 3D dimensional anisotropy behavior is recovered.