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Register a new userReversible magnetization and critical fluctuations in systematically doped YBa$_2$Cu$_3$O$_{7-delta}$ single crystals
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The temperature and field dependence of reversible magnetization have been measured on a YBa$_2$Cu$_3$O$_{7-delta}$ single crystal at six different doping concentrations. It is found that the data above 2 T can be described by the scaling law based on the GL-LLL (lowest Landau level approach based on Ginzburg-Landau theory) critical fluctuation theory yielding the values of the slope of upper critical field $-mathrm{d}H_{mathrm{c2}}(T)/mathrm{d}T$ near $T_mathrm{c}$. This set of values is self-consistent with that obtained in doing the universal scaling for the six samples. Based on a simple Ginzburg-Landau approach, we determined the doping dependence of the coherence length $xi$ which behaves in a similar way as that determined from $xi= hbar v_mathrm{F}/E_mathrm{sc}$ with $E_mathrm{sc}$ the superconducting energy scale. Our results may suggest a growing coherence length towards more underdoping.
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We have studied the normal-to-superconducting phase transition in optimally-doped YBa$_2$Cu$_3$O$_{7-delta}$ in zero external magnetic field using a variety of different samples and techniques. Using DC transport measurements, we find that the dynamical critical exponent $z=1.54pm0.14$, and the static critical exponent $ u=0.66pm0.10$ for both films (when finite-thickness effects are included in the data analysis) and single crystals (where finite-thickness effects are unimportant). We also measured thin films at different microwave frequencies and at different powers, which allowed us to systematically probe different length scales to avoid finite-thickness effects. DC transport measurements were also performed on the films used in the microwave experiments to provide a further consistency check. These microwave and DC measurements yielded a value of z consistent with the other results, $z=1.55pm0.15$. The neglect of finite-thickness, finite-current, and finite-frequency effects may account for the wide ranges of values for $ u$ and $z$ previously reported in the literature.
Most measurements of critical current densities in YBa$_2$Cu$_3$O$_{7-delta}$ thin films to date have been performed on films where the textit{c}-axis is grown normal to the film surface. With such films, the analysis of the dependence of $j_c$ on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of $j_c$, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of $j_c$ in YBa$_2$Cu$_3$O$_{7-delta}$ are reviewed. Measurements on YBa$_2$Cu$_3$O$_{7-delta}$ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa$_2$Cu$_3$O$_{7-delta}$. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa$_2$Cu$_3$O$_{7-delta}$. The experimentally obtained $j_c(theta)$ behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the textit{a-b} planes.
We report on the scaling of transport properties around the vortex melting in YBa$_2$Cu$_3$O$_{7- delta}$ oriented-twin single crystals in applied magnetic fields between 1T and 18T. We find that for all the measured field range the linear resistivity scales as $rho (t,theta) sim t^{sy} {cal F}_{pm} (sin(theta)t^{-sx})$, with $t=|T-T_{BG}|$ and $theta$ the angle between de planar defects and the magnetic field. The scaling is valid only for angles where the transition temperature $T_{BG} (theta)$ shows a cusp. The critical exponents $sx$ and $sy$ are in agreement with the values predicted by Lidmar and Wallin only at magnetic fields below 4T. A change in the value of $sx$ from $sx = 1 pm 0.2$ to $sx = 3 pm 0.2$ at around $H^{cr} approx $ 4T when the magnetic field is increased, is responsible for changes in the shape of the $T_{BG} (theta)$ curve and in the dependence of the linear dissipation on temperature and angle. The results strongly suggest the existence of a different vortex glassy phase in twinned crystals compared to the Bose-glass state found in samples with linear defects.
Combined action of weak and strong pinning centers on the vortex lattice complicates magnetic behavior of a superconductor since temperature and magnetic field differently affect weak and strong pinning. In this paper we show that contributions of weak and strong pinning into magnetization of the layered superconductor YBa$_2$Cu$_3$O$_{7-delta}$ can be separated and analyzed individually. We performed a careful analysis of temperature behavior of the relaxed superconducting current $J$ in YBa$_2$Cu$_3$O$_{7-delta}$ films which revealed two components of the current $J = J_1 +J_2$. A simple method of separation of the components and their temperature dependence in low magnetic fields are discussed. We found that $J_1$ is produced by weak collective pinning on the oxygen vacancies in CuO$_2$ planes while $J_2$ is caused by strong pinning on the Y$_2$O$_3$ precipitates. $J_1$ component weakly changes with field and quasi-exponentially decays with temperature, disappearing at $T simeq 30$--40~K. Rapid relaxation of $J_1$ causes formation of the normalized relaxation rate peak at $T simeq 20$~K. $J_2$ component is suppressed by field as $J_2propto B^{-0.54}$ and decays with temperature following to the power law $J_2propto(1 - T/T_mathrm{dp} )^alpha$ where $T_mathrm{dp}$ is the depinning temperature. Detailed comparison of the experimental data with pinning theories is presented.
Superconductivity of YBa$_2$Cu$_3$O$_{7-delta}$ single crystals was investigated in small magnetic fields. In magnetic measurements the superconducting transition for $textbf{H} | c$ appears 0.4 K higher than for $textbf{H} bot c$. In this temperature range superconductivity is two-dimensional and the total thickness of superconducting layers is about 0.83 of the sample thickness, which is a consequence of the occurrence of the quasi-insulating plane in the unit cell of the crystal structure. Resistivity in the textit{ab}-plane and along the textit{c}-axis was measured simultaneously. In these measurements two-dimensional superconductivity was observed in a temperature range of 0.6-0.8 K with the clear signs of the Berezinskii-Kosterlitz-Touless (BKT) transition which occurs approximately 0.15 K below $T_c$, the mean-field transition temperature.
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