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Register a new userApproaching two-dimensional superconductivity in ultrathin DyBa$_2$Cu$_3$O$_{7-delta}$
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The temperature dependence of the superfluid density $rho_s(T)$ has been measured for a series of ultrathin MBE-grown DyBa$_2$Cu$_3$O$_{7-delta}$ superconducting (SC) films by sub-mm wave interferometry combined with time-domain THz spectroscopy and IR ellipsometry. We find that all films 10 u.c. and thicker show the same universal temperature dependence of $rho_s(T)$, which follows the critical behavior characteristic of single crystal YBa$_2$Cu$_3$O$_{7-delta}$ as $T$ approaches $T_c$. In 7 u.c. thick films, $rho_s(T)$ declines steeply upon approaching $T_c$, as expected for the Berezinskii-Kosterlitz-Thouless vortex unbinding transition. Our analysis provides evidence for a sharply defined 4 u.c. non-SC interfacial layer, leaving a quasi-2D SC layer on top. We propose that the SC state in this interfacial layer is suppressed by competing (possibly charge) order.
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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.
We examine coherent phonons in a strongly driven sample of optimally-doped high temperature superconductor YBa$_2$Cu$_3$O$_{7-delta}$. We observe a non-linear lattice response of the 4.5,THz copper-oxygen vibrational mode at high excitation densities, evidenced by the observation of the phonon third harmonic and indicating the mode is strongly anharmonic. In addition, we observe how high-amplitude phonon vibrations modify the position of the electronic charge transfer resonance. Both of these results have important implications for possible phonon-driven non-equilibrium superconductivity.
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.
We report here the results of scanning tunneling spectroscopic experiments performed on hole doped NdBa$_2$Cu$_3$O$_{7-delta}$ single crystals of $T_c$ values of 76 K, 93.5 K and 95.5 K. The energy gaps are observed to be increasing with decreasing $T_c$ values. The coherence peaks are asymmetric with the peaks at the filled states being larger than those at the empty ones. The asymmetry increases with decreasing $T_c$. The observed asymmetry and its $T_c$ dependence can be explained by considering the Mott insulating nature of the material at the undoped 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.
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