Determination of two-dimensional zero-magnetic-fieldI-Vexponent inBi2Sr2CaCu2O8+δ

Origin of the fast magnetization relaxation at low temperatures in HTS with strong pinning

The temperature T variation of the normalized magnetization relaxation rate S in high-temperature superconductors (HTS) with strong vortex pinning exhibits a maximum in the low-T range. This was reported for various HTS, and the origin of the faster relaxation at low T appearing in standard magnetization relaxation measurements was usually related to specific pinning properties of the investigated specimens. Since the observed behaviour seems to be characteristic to all HTS with enhanced pinning (generated by random and/or correlated disorder), we show that the S(T) maximum can be explained in terms of classic collective vortex creep. The influence of thermo-magnetic instabilities in the lo…

Vortex–antivortex unbinding in oxygen-deficient YBa2Cu3O7− films

Abstract In-plane zero-magnetic-field current–voltage ( I – V ) characteristics of oxygen-deficient YBa 2 Cu 3 O 7− δ films (7 −  δ  ∼6.5, 6.55, and 6.65) were thoroughly investigated. A good agreement with the quasi two-dimensional (2D) vortex–antivortex unbinding scenario was observed, similar to the behaviour of highly anisotropic Bi 2 Sr 2 CaCu 2 O 8+ δ films. The temperature variation of the I – V exponent allows the determination of the Berezinskii–Kosterlitz–Thouless transition temperature T KT at the superconducting Cu–O layer level, and the mean-field critical temperature T c0 . We found that both T KT and T c0 increase with increasing doping, and T c0 remains in the region of the …

Location of the mean-field critical temperature of underdopedYBa2Cu3Oyfilms

We determined the Berezinskii-Kosterlitz-Thouless transition temperature ${T}_{\mathit{KT}}$ at the superconducting layer level and the mean-field critical temperature ${T}_{c0}$ in oxygen-deficient $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ films ($y\ensuremath{\sim}6.5$, 6.55, and 6.65). We used the $T$ dependence of the quasi-two-dimensional $I\text{\ensuremath{-}}V$ exponent for $T\ensuremath{\leqslant}{T}_{\mathit{KT}}$, and the analysis of the resistive transition in the framework of the Ginzburg-Landau two-dimensional Coulomb gas model for $Tg{T}_{\mathit{KT}}$. Both procedures give essentially the same results, with ${T}_{c0}$ remaining in the domain of the elect…

Anomalous behavior of the second magnetization peak inLa1.81Sr0.19CuO4single crystals: Possible influence of two-band superconductivity

We report an anomalous temperature $T$ variation of the field ${H}_{\text{on}}$ for the onset of the second magnetization peak in ${\text{La}}_{1.81}{\text{Sr}}_{0.19}{\text{CuO}}_{4}$ single crystals with the external magnetic field $H$ oriented parallel to the $c$ axis. While the peak field ${H}_{p}$ has a continuous decrease with increasing $T$, ${H}_{\text{on}}$ exhibits a sudden decrease for $T\ensuremath{\sim}11--15\text{ }\text{K}$. This behavior appears to be related to the particular $T$ dependence of the superfluid density in the case of two-band superconductivity affecting the $T$ variation of the elastic energy of the vortex system at low $H$.

Magnetization relaxation in YBCO films with improved supercurrent transport properties

The relaxation of the irreversible magnetization in optimally doped YBCO films with natural and artificial pinning centres was measured in zero-field cooling conditions using SQUID magnetometry. The external magnetic field H was oriented along the c axis. An appropriate method for the determination of the characteristic vortex pinning energy from the normalized vortex-creep activation energy is discussed. This is based on the existence of a crossover elastic (collective) vortex creep at low temperatures T – plastic vortex creep at high T, caused by the T dependent macroscopic currents induced in the sample during magnetization measurements.

Vortex-system ordering during magnetisation measurements in YBa2Cu3O7−δ films at low temperatures

Abstract Zero-field-cooling dc magnetisation relaxation measurements performed on optimally doped epitaxial YBa 2 Cu 3 O 7− δ films with the external magnetic field H oriented parallel to the c -axis reveal the ordering of the creeping vortex-system in the low temperature T domain, due to the macroscopic currents induced in the sample. This dynamic ordering manifests itself through the appearance of a maximum in the T variation of the normalised vortex-creep activation energy. The location of this maximum in the H – T plane describes a well-defined current induced order–disorder line. Our results suggest the absence of a static elastic vortex glass in disordered high-temperature superconduc…

Analysis of magnetization relaxation in MgB2 bulk samples obtained by electric-field assisted sintering

Abstract The relaxation of the irreversible magnetization of MgB2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H ∝ T−2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurren…

Relaxation of remnant magnetisation in YBa2Cu3O7−δ films

Abstract The relaxation of the remnant magnetisation in optimally doped disk-shaped YBa2Cu3O7−δ films with the initially applied magnetic field H oriented along the c axis was measured using dc SQUID magnetometry. The temperature (T) dependence of the experimentally observed normalised magnetisation relaxation rate S exhibits three distinct regions. At high T, S(T) increases with increasing T, which can be explained in terms of plastic vortex creep. The well known plateau in the S(T) variation at intermediate T appears to be caused by collective (elastic) creep in a dynamically ordered vortex system. At low T, where S increases again with increasing T, the magnetisation relaxation is not in…

Inhibition of the detrimental double vortex-kink formation in thick YBa2Cu3O7films with BaZrO3nanorods

We investigated the temperature (T) variation of the normalized magnetization relaxation rate S and of the corresponding normalized vortex-creep activation energy U* = T/S for YBa2Cu3O7 films containing BaZrO3 nanorods, with the external magnetic field H oriented perpendicular to the film surface. It was found that by increasing the film thickness and using nanodot decorated substrates the high-T S(T) maximum appearing at low H is substituted by a minimum in S(T). As revealed by the analysis of the current density dependence of U*, this behaviour is due to the inhibition of vortex excitations involving double vortex-kinks and superkinks formation in the investigated thick films, owing to th…

Origin of the plateau in the temperature dependence of the normalized magnetization relaxation rate in disordered high-temperature superconductors

The temperature $T$ dependence of the normalized magnetization relaxation rate $S$ in optimally doped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films with the external dc magnetic field $H$ oriented along the $c$ axis exhibits the well-known plateau in the intermediate $T$ range, associated with the presence of elastic (collective) vortex creep. The disappearance of the $S(T)$ plateau in the high-$H$ domain $(H\ensuremath{\ge}20\text{ }\text{kOe})$ is not completely understood. We show that in the case of high-temperature superconductors with significant quenched disorder the $S(T)$ plateau is directly related to a crossover in the vortex-creep process…

Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxation measurements

We investigated the relaxation of the irreversible magnetization in a series of 200?nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T?plastic vortex creep at high T, generated by the …

Improvement of the critical current density of spark plasma sintered MgB2by C60addition

We investigated the influence of fullerene (C60) addition on the superconducting parameters of MgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5?wt% C60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70?kOe at temperature T = 10?K. This indicates substitution of B by C in C60 added MgB2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T…

On the determination of vortex creep parameters in superconductors using standard magnetization relaxation data

The relaxation of the irreversible magnetic moment m(t) in YBa2Cu3O7 (YBCO) films was investigated as a function of temperature T and the external magnetic field H along the c axis applied in zero-field cooling conditions, for the determination of vortex creep parameters. The data analysis was performed using the T and current density dependence of the normalized vortex creep activation energy, or by the fit of the m(t) data with the well known interpolation formula in the framework of the general vortex creep equation. It was found that (i) even for specimens with strong static pinning the characteristic pinning energy remains small in the low-T range, where the vortex creep appearing in s…