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Materials with strongly correlated electrons often exhibit interesting physical properties. An example of these materials is the layered oxide perovskite Sr2RuO4, which has been intensively investigated due to its unusual properties. Whilst the debate on the symmetry of the superconducting state in Sr2RuO4 is still ongoing, a deeper understanding of the Sr2RuO4 normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of surface magnetism in Sr2RuO4 in its normal state. We detect static weak dipolar fields yet manifesting at an onset temperature higher than 50 K. We ascribe this unconventional magnetism to orbital loop currents forming at the reconstructed Sr2RuO4 surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence electron pairing with broken time reversal symmetry., Strontium Ruthenate, Sr2RuO4, displays a remarkable number of intriguing physical phenomena, from superconductivity, to strain-induced ferromagnetism. Here, using low-energy muon spectroscopy, Fittipaldi et al. demonstrate the existence of unconventional magnetism at the surface of Sr2RuO4 in its normal state and without any applied strain.

Magneto-optical measurements in Bi2Sr2CaCu2O8+d demonstrate enhanced effects of transient disordered vortex states (TDVS) in a prism-shaped sample. The existence of such transient states is indicated by the appearance of a sharp change (‘break’) in the slope of the induction profiles. As the external magnetic field is swept up or down, the break, which signifies the front of the TDVS, moves towards the sample center at a constant velocity determined by the rate of change of the external field. We discuss the results in terms of a competition between injection and annealing processes, the latter being ineffective in the prism due to direct injection of disorder into the bulk through surface barriers distributed along the tilted facets of the prism. � 2007 Elsevier B.V. All rights reserved.

Measurements of the nonlinear flux-flow resistivity $\ensuremath{\rho}$ and the critical vortex velocity ${v}_{\ensuremath{\varphi}}^{*}$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov (Z. Eksp. Teor. Fiz 68, 1915 (1975) [Sov. Phys. JETP 41, 960 (1976)]) are reported along the node and antinode directions of the $d$-wave order parameter in the $a\mathrm{\text{\ensuremath{-}}}b$ plane of epitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. In this pinning-free regime, $\ensuremath{\rho}$ and ${v}_{\ensuremath{\varphi}}^{*}$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $\ensuremath{\rho}$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$.

We observed an oscillatory behavior, both in space and time, of the induction in Bi2Sr2CaCu2O8+delta crystals exposed to a steady magnetic field. This new "flux waves" phenomenon appears near the order-disorder vortex phase transition, under specific conditions of temperature and induction gradient. A theoretical description of this effect is based on two coupled equations: the Landau-Khalatnikov dynamic equation for the order parameter of the vortex phase transition and the diffusion equation for the time evolution of the magnetic induction. A linear stability analysis of these equations predicts an oscillatory instability characterized by a period and wavelength in accordance with the experimental results.

Magneto-optical measurements of transient vortex states in Bi2Sr2CaCu2O8+delta show enhanced effects of metastability in prism-shaped as compared to platelet crystals including a significant shift of the second magnetization peak and qualitatively different dynamics. In contrast to platelets, where dislocations are generated only at the sample edges, we propose that in prism samples the dislocations are generated dynamically in the entire sample due to distributed surface barriers. As a result, a dynamic phase transition from a Bragg glass to a metastable disordered phase may occur well below the thermodynamic transition field.

Measurements of the nonlinear flux-flow resistivity rho and the critical vortex velocity vphi* at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov (Z. Eksp. Teor. Fiz 68, 1915 (1975) [Sov. Phys. JETP 41, 960 (1976)]) are reported along the node and antinode directions of the d-wave order parameter in the a-b plane of epitaxial YBa2Cu3O7-delta films. In this pinning-free regime, rho and vphi* are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of rho is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of YBa2Cu3O7-delta.