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Register a new userNovel Vortex Solid State in YBa$_2$Cu$_3$O$_{7 - delta}$ Twinned Crystals
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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.
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We used a micromechanical torsional oscillator to measure the magnetic response of a twinned YBa$_2$Cu$_3$O$_{7-delta}$ single crystal disk near the Bose glass transition. We observe an anomaly in the temperature dependence of the magnetization consistent with the appearance of a magnetic shielding perpendicular to the correlated pinning of the twin boundaries. This effect is related to the thermodynamic transition from the vortex liquid phase to a Bose glass state.
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 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.
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.
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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