Charge ordering in half-doped Pr(Nd)0.5Ca0.5MnO3 under a magnetic field

Recent experiments in Pr(Nd)$_{0.5}$Ca$_{0.5}$MnO$_3$ thin films exhibited (multiple) reentrant charge ordering (CO) transitions with change of {\it temperature} T under fixed magnetic field H, which are in contrast to the results for the corresponding bulk materials. To explain the experimental findings, a model including the double-exchange mechanism, intersite Coulomb interaction and electron-phonon coupling is proposed, for which the reentrant CO is naturally obtained due to the temperature dependence of the band-type coherent polaron hopping. Various results for the CO in the (H, T) plane are extensively discussed. The theory is considered to be valid for both thin films and bulk materials.Comment: 6 pages, 4 eps figure

Cylic spin exchange in cuprate spin ladders

We investigate the influence of a cyclic spin exchange J_{cyc} on the one- and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by J_{cyc} due to a repulsion between triplets on neighboring rungs. In (La,Ca)_{14}Cu_{24}O_{41} a consistent description of both the spin gap and the S=0 bound state requires $J_{cyc}/J_\perp \approx 0.20-0.27$ and $J/J_{\perp} \approx 1.25-1.35$. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.Comment: 4 pages, 3 figure

Half-h/2e critical current-oscillations in SQUIDs

The current-voltage characteristics of Superconducting Quantum Interference Devices (SQUIDs) are known to modulate as a function of applied magnetic field with a period of one flux quantum $\Phi_0=h$/2e. Here we report on the fabrication and properties of SQUIDs modulating with a fundamental period of $1/2\times\Phi_0$. The characteristics of these bicrystal SQUIDs are consistent with either a strong sin(2φ) component of the current-phase relation of the Josephson current, or with an interaction between the Cooper pairs, causing an admixture of quartets to the condensate