Test Experiment for Time-Reversal Symmetry Breaking Superconductivity

A new experiment is proposed to probe the time-reversal symmetry of a superconductor. It is shown that a time-reversal symmetry breaking superconductor can be identified by the observation of a fractional flux in connection with a Josephson junction in a special geometry.Comment: 4 pages, 2 figures available upon request, Revtex, MIT-CMT-OC

Metamagnetism of itinerant electrons in multi-layer ruthenates

The problem of quantum criticality in the context of itinerant ferro- or metamagnetism has received considerable attention [S. A. Grigera et. al., Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It has been proposed that a new kind of quantum criticality is realised in materials such as MnSi or Sr_3Ru_2O_7. We show based on a mean-field theory that the low-temperature behaviour of the n-layer ruthenates Sr_{n+1}Ru_nO_{3n+1} can be understood as a result of a Van Hove singularity (VHS). We consider a single band whose Fermi energy, E_F, is close to the VHS and deduce a complex phase diagram for the magnetism as a function of temperature, magnetic field and E_F. The location of E_F with respect to the VHS depends on the number of layers or can be tuned by pressure. We find that the ferromagnetic quantum phase transition in this case is not of second but of first order, with a metamagnetic quantum critical endpoint at high magnetic field. Despite its simplicity this model describes well the properties of the uniform magnetism in the single, double and triple layer ruthenates. We would like to emphasise that the origin of this behaviour lies in the band structure.Comment: 7 pages, 3 figures, typos corrected and acknowledgement added, to appear in the Europhysics Letter

Fractional vortices on grain boundaries --- the case for broken time reversal symmetry in high temperature superconductors

We discuss the problem of broken time reversal symmetry near grain boundaries in a d-wave superconductor based on a Ginzburg-Landau theory. It is shown that such a state can lead to fractional vortices on the grain boundary. Both analytical and numerical results show the structure of this type of state.Comment: 9 pages, RevTeX, 5 postscript figures include

Dimensional crossover in Sr2_2RuO4_4 within slave-boson mean-field theory

Motivated by the anomalous temperature dependence of the c-axis resistivity of Sr$_2$RuO$_4$, the dimensional crossover from a network of perpendicular one-dimensional chains to a two-dimensional system due to a weak hybridization between the perpendicular chains is studied. The corresponding two-orbital Hubbard model is treated within a slave-boson mean-field theory (SBMFT) to take correlation effects into account such as the spin-charge separation on the one-dimensional chains. Using an RPA-like formulation for the Green's function of collective spinon-holon excitations the emergence of quasiparticles at low-temperatures is examined. The results are used to discuss the evolution of the spectral density and the c-axis transport within a tunneling approach. For the latter a regime change between low- and high-temperature regime is found in qualitative accordance with experimental data