Identifying the odd-frequency superconducting state by a field-induced Josephson effect

Superconducting order parameters that are odd under exchange of time-coordinates of the electrons constituting a Cooper-pair, are potentially of great importance both conceptually and technologically. Recent experiments report that such an odd-frequency superconducting {\it bulk} state may be realized in certain heavy-fermion compounds. While the Josephson current normally only flows between superconductors with the same symmetries with respect to frequency, we demonstrate that an exchange field may induce a current between diffusive even- and odd-frequency superconductors. This suggests a way to identify the possible existence of bulk odd-frequency superconductors.Comment: 10 pages, 7 figures. To appear in Physical Review

Spin-Current Relaxation Time in Spin-Polarized Heisenberg Paramagnets

We study the spatial Fourier transform of the spin correlation function G_q(t) in paramagnetic quantum crystals by direct simulation of a 1d lattice of atoms interacting via a nearest-neighbor Heisenberg exchange Hamiltonian. Since it is not practical to diagonalize the s=1/2 exchange Hamiltonian for a lattice which is of sufficient size to study long-wavelength (hydrodynamic) fluctuations, we instead study the s -> infinity limit and treat each spin as a vector with a classical equation of motion. The simulations give a detailed picture of the correlation function G_q(t) and its time derivatives. At high polarization, there seems to be a hierarchy of frequency scales: the local exchange frequency, a wavelength-independent relaxation rate 1/tau that vanishes at large polarization P ->1, and a wavelength-dependent spin-wave frequency proportional to q^2. This suggests a form for the correlation function which modifies the spin diffusion coefficients obtained in a moments calculation by Cowan and Mullin, who used a standard Gaussian ansatz for the second derivative of the correlation function.Comment: 6 pages, 3 figure

Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films

Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow magnetic damping are relevant for magnon-based spintronics and low-power wave-like computing. The excitation frequency of spin waves in YIG is rather low in weak external magnetic fields because of its small saturation magnetization, which limits the potential of YIG films for high-frequency applications. Here, we demonstrate how exchange-coupling to a CoFeB film enables efficient excitation of high-frequency perpendicular standing spin waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense PSSW modes up to 10th order. Strong hybridization between the PSSW modes and the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing behavior in broadband spin-wave spectra. A dynamic exchange torque at the YIG/CoFeB interface explains the excitation of PSSWs. The localized torque originates from exchange coupling between two dissimilar magnetization precessions in the YIG and CoFeB layers. As a consequence, spin waves are emitted from the YIG/CoFeB interface and PSSWs form when their wave vector matches the perpendicular confinement condition. PSSWs are not excited when the exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer. Micromagnetic simulations confirm the exchange-torque mechanism.Comment: 9 pages, 6 figure

Monte-Carlo simulation of supercooled liquids using a self-consistent local temperature

We combine Creutz energy conservation with Kawasaki spin exchange to simulate the microcanonical dynamics of a system of interacting particles. Relaxation occurs via Glauber spin-flip activation using a self-consistent temperature. Heterogeneity in the dynamics comes from finite-size constraints on the spin exchange that yield a distribution of correlated regions. The simulation produces a high-frequency response that can be identified with the boson peak, and a lower-frequency peak that contains non-Debye relaxation and non-Arrhenius activation, similar to the primary response of supercooled liquids.Comment: 16 pages, 4 figure

Activity in Global Foreign Exchange Markets

According to a recent BIS survey, turnover in foreign exchange markets continued to increase between April 2007 and April 2010. Growth was slower than in earlier years, consistent with a slowdown in the underlying demand for foreign exchange owing to the impact of the global financial crisis on international trade and investment. In an exception to this trend, spot turnover increased significantly, driven in part by the ongoing expansion of high-frequency trading.Foreign Exchange Market Turnover; High-frequency Trading; BIS Triennial Survey