We used a micromechanical torsional oscillator to measure the magnetic response of a twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ 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.

We used a micromechanical torsional oscillator to measure the magnetic response of a twinned YBaBa2Cu3O7-δ 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.

We used a micromechanical torsional oscillator to measure the magnetic response of a twinned YBa2Cu3O7-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.

We present measurements of the electrical resistivity and Hall coefficient, ρ and RH, in Cr films of different thicknesses grown on MgO (100) substrates, as a function of temperature T and applied magnetic field H. The results show a low temperature minimum in ρ(T), which is thickness dependent. From 40 K to 2 K, the Hall coefficient is a monotonous increasing function as T is reduced with no particular signature at the temperature Tmin where the minimum develops. We explain the resistivity minimum assuming an imperfect nesting of the Fermi surface leading to small electron and hole pockets. We introduce a phenomenological model which supports this simple physical picture.

We present measurements of the electrical resistivity rho in epitaxial Cr films of different thicknesses grown on MgO (100) substrates, as a function of temperature T. The rho(T) curves display hysteretic behavior in a certain temperature range, which is film thickness dependent. The hysteresis are related to the confinement of quantized incommensurate spin density waves (ISDW) in the film thickness. Our important finding is to experimentally show that the temperature T-mid where the ISDW changes from N to N + 1 nodes decreases as the film thickness increases. Identifying T-mid with a first-order transition between ISDW states with N and N + 1 nodes, and using a Landau approach to the free energy of the ISDW together with Monte Carlo simulations, we show that the system at high temperatures explores all available modes for the ISDW, freezing out in one particular mode at a transition temperature that indeed decreases with film thickness L. The detailed dependence of T-mid(L) seems to depend rather strongly on the boundary conditions at the Cr film interfaces.

We present an experimental study of the changes generated on the electrical resistance R(T) of epitaxial Cr thin films by the transformation of quantized spin density wave domains as the temperature is changed. A characteristic resistance noise appears only within the same temperature region where a cooling-warming cycle in R(T) displays hysteretic behavior. We propose an analysis based on an analogy with the Barkhausen noise seen in ferromagnets. There fluctuations in the magnetization M(H) occur when the magnetic field H is swept. By mapping M -> psi(0) and H -> T, where psi(0) corresponds to the order parameter of the spin density wave, we generalize the Preisach model in terms of a random distribution of resistive hysterons to explain our results. These hysterons are related to distributions of quantized spin density wave domains with different sizes, local energies and number of nodes. Copyright (C) EPLA, 2012

Electrical resistivity measurements of the superconductor La1.80sr0.20CuO4-δ over a Wide range of temperatures and oxygen concentration, together with the analysis of superconducting properties of the material, indicate that the ceramic superconductors are high < materials in the clean limit. The granular superconducting behavior is determined by the oxygen content.

We describe our latest results in the separation dependence of the Casimir interaction in the sphere-plane geometry for two Au-coated surfaces. All results are obtained by measuring the change in the resonant frequency of a sensitive microelectromechanical torsional oscillator as the separation between the sphere and the plane is changed. By means of the proximity force approximation, the change in resonant frequency yields the Casimir pressure between two parallel plates at the same separation. We present results for a new sample at room temperature, where the dielectric function has been measured in the 190-825 nm range. We show that the results of the Casimir force in this sample and in previous samples are virtually indistinguishable. Furthermore, the observed differences between measured and tabulated optical properties data do not show any effect on the calculation of the Casimir interaction. We also present results of the measurement of the Casimir force between a sphere and a plane at 300, 77, 4.2 and 2.1 K. While low temperature results are noisier than room" temperature ones, precluding a direct exclusion of either the Drude or the plasma model, the average of the measurements coincide at all temperatures.

Josephson junctions were photogenerated in underdoped thin films of the YBa2Cu3O6+x family using a near-field scanning optical microscope. The observation of the Josephson effect for separations as large as 100 nm between two wires indicates the existence of an anomalously large proximity effect and show that the underdoped insulating material in the gap of the junction is readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson relationship. This result constrains the applicability of SO(5) theory to explain the phase diagram of high critical temperature superconductors. 74.50.+r, 74.80.Fp, 74.72.Bk, 74.76.Bz Typeset using REVTEX

We have grown a-axis oriented EuBa2Cu3O7 thin films on SrTiO3 (001) substrates using a magnetron sputtering technique. In these films the CuO2 planes are aligned parallel to the substrates. The film microstructure shows microdomains. The microdomains are grains with a 90° rotation of the c-axis. We have done transport measurements in applied magnetic fields up to 16T. We have found a reduced quasi-2D dimensionality of the vortex glass transition probably resulting of vortex coherence being limited by disorder at domain boundaries. The activation energy for vortex motion in the liquid state displays a 1/H0.5 field dependence characteristic of strongly anisotropic superconductor.

We measure the local harmonic generation from superconducting thin films at microwave frequencies to investigate the intrinsic nonlinear Meissner effect near T-c in the zero magnetic field. Both second and third harmonic generation are measured to identify time-reversal symmetry breaking (TRSB) and time-reversal symmetric (TRS) nonlinearities. We perform a systematic doping-dependent study of the nonlinear response and find that the TRS characteristic nonlinearity current density scale follows the doping dependence of the depairing critical current density. We also extract a spontaneous TRSB characteristic current density scale that onsets at T-c, grows with decreasing temperature, and systematically decreases in magnitude (at fixed T/T-c) with underdoping. The origin of this current scale could be Josephson circulating currents or the spontaneous magnetization associated with a TRSB order parameter.

The infrared (900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in the normal state of underdoped High T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples, which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in density wave models.

We experimentally study the dynamical response of the superconducting vortex system near the solid-liquid transition by applying forces parallel and perpendicular to a planar random defect array. Our results show that for magnetic fields above a certain critical field Hcr≈4 T the solid glassy vortex phase in YBa2Cu3O7−δ crystals with oriented twin planes does not correspond to the so-called Bose-glass phase.