The ubiquitous 1100 charge ordering in organic charge-transfer solids

Charge and spin-orderings in the 1/4-filled organic CT solids are of strong interest, especially in view of their possible relations to organic superconductivity. We show that the charge order (CO) in both 1D and 2D CT solids is of the ...1100... type, in contradiction to mean field prediction of >...1010... CO. We present detailed computations for metal-insulator and magnetic insulator-insulator transitions in the theta-ET materials. Complete agreement with experiments in several theta systems is found. Similar comparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET materials prove the ubiquity of this phenomenon.Comment: 3 pages, 4 eps figures; ICSM 200

Oscillating magnetoresistance due to fragile spin structure in metallic GdPd3_3

Studies on the phenomenon of magnetoresistance (MR) have produced intriguing and application-oriented outcomes for decades--colossal MR, giant MR and recently discovered extremely large MR of millions of percents in semimetals can be taken as examples. We report here the investigation of oscillating MR in a cubic intermetallic compound GdPd$_3$, which is the only compound that exhibits MR oscillations between positive and negative values. Our study shows that a very strong correlation between magnetic, electrical and magnetotransport properties is present in this compound. The magnetic structure in GdPd$_3$ is highly fragile since applied magnetic fields of moderate strength significantly alter the spin arrangement within the system--a behavior that manifests itself in the oscillating MR. Intriguing magnetotransport characteristics of GdPd$_3$ are appealing for field-sensitive device applications, especially if the MR oscillation could materialize at higher temperature by manipulating the magnetic interaction through perturbations caused by chemical substitutions.Comment: 10 pages, 7 figures. A slightly modified version is published in Scientific Report

Temporal evolution of mesoscopic structure of some non-Euclidean systems using a Monte Carlo model

A Monte Carlo based computer model is presented to comprehend the contrasting observations of Mazumder et al. [Phys. Rev. Lett. 93, 255704 (2004) and Phys. Rev. B 72, 224208 (2005)], based on neutron-scattering measurements, on temporal evolution of effective fractal dimension and characteristic length for hydration of cement with light and heavy water. In this context, a theoretical model is also proposed to elucidate the same.Comment: 31 Pages, 13 Figure

Identifying the curvaton within MSSM

We consider inflaton couplings to MSSM flat directions and the thermalization of the inflaton decay products, taking into account gauge symmetry breaking due to flat direction condensates. We then search for a suitable curvaton candidate among the flat directions, requiring an early thermally induced start for the flat direction oscillations to facilitate the necessary curvaton energy density dominance. We demonstrate that the supersymmetry breaking $A$-term is crucial for achieving a successful curvaton scenario. Among the many possible candidates, we identify the ${\bf u_1dd}$ flat direction as a viable MSSM curvaton.Comment: 9 pages. Discussion on the evaporation of condensate added, final version published in JCA

A-term inflation and the smallness of the neutrino masses

The smallness of the neutrino masses may be related to inflation. The minimal supersymmetric Standard Model (MSSM) with small Dirac neutrino masses already has all the necessary ingredients for a successful inflation. In this model the inflaton is a gauge-invariant combination of the right-handed sneutrino, the slepton, and the Higgs field, which generate a flat direction suitable for inflation if the Yukawa coupling is small enough. In a class of models, the observed microwave background anisotropy and the tilted power spectrum are related to the neutrino masses.Comment: 13 pages, 1 figure, uses JHEP3.cls, minor modifications, final version accepted for publication in JCA

Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals

It has been suggested that the metal-insulator transitions in a number of spinel materials with partially-filled t_2g d-orbitals can be explained as orbitally-driven Peierls instabilities. Motivated by these suggestions, we examine theoretically the possibility of formation of such orbitally-driven states within a simplified theoretical model, a two-dimensional checkerboard lattice with two directional metal orbitals per atomic site. We include orbital ordering and inter-atom electron-phonon interactions self-consistently within a semi-classical approximation, and onsite intra- and inter-orbital electron-electron interactions at the Hartree-Fock level. We find a stable, orbitally-induced Peierls bond-dimerized state for carrier concentration of one electron per atom. The Peierls bond distortion pattern continues to be period 2 bond-dimerization even when the charge density in the orbitals forming the one-dimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within one-electron theory as well as mean-field theory of electron-electron interactions, even for nearly complete orbital ordering. We discuss the implications of our results in relation to complex charge, bond, and orbital-ordering found in spinels.Comment: 8 pages, 5 figures; revised versio

Seed perturbations for primordial magnetic fields from MSSM flat directions

We demonstrate that the MSSM flat directions can naturally account for the seed magnetic fields in the early Universe. The non-zero vacuum expectation value of an MSSM flat direction condensate provides masses to the gauge fields and thereby breaks conformal invariance. During inflation the condensate receives spatial perturbations and $SU(2) x U(1)_Y$ gauge currents are generated together with (hyper)magnetic fields. When these long wavelength vector perturbations reenter our horizon they give rise to $U(1)_{em}$ magnetic fields with an amplitude of $10^{-30}$ Gauss, as required by the dynamo mechanism.Comment: 4 pages, RevTeX