Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering

We have performed elastic neutron scattering experiments under uniaxial stress sigma applied along the tetragonal [100], [110] and [001] directions for the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with sigma along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment mu_o continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four times larger than that for the hydrostatic pressure (partial mu_o/partial P sim 0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa, indicating that the development of the AF state highly depends on the direction of sigma. We have also found a clear hysteresis loop in the isothermal mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.Comment: 9 pages, 7 figures, to appear in Physical Review

Moduli decay in the hot early Universe

We consider moduli fields interacting with thermalized relativistic matter. We determine the temperature dependence of their damping rate and find it is dominated by thermal effects in the high temperature regime, i.e. for temperatures larger than their mass. For a simple scalar model the damping rate is expressed through the known matter bulk viscosity. The high temperature damping rate is always smaller than the Hubble rate, so that thermal effects are not sufficient for solving the cosmological moduli problem.Comment: Numerical error in the final result for the damping rate corrected, conclusions of the paper are not affecte

Role of strong correlation in the recent ARPES experiments for cuprate superconductors

Motivated by recent photoemission experiments on cuprates, the low-lying excitations of a strongly correlated superconducting state are studied numerically. It is observed that along the nodal direction these low-lying one-particle excitations show a linear momentum dependence for a wide range of excitation energies and, thus, they do not present a kink-like structure. The nodal Fermi velocity $v_{\rm F}$, as well as other observables, are systematically evaluated directly from the calculated dispersions, and they are found to compare well with experiments. It is argued that the parameter dependence of $v_{\rm F}$ is quantitatively explained by a simple picture of a renormalized Fermi velocity.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let

Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model

Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of the strong-coupling expansion are considered in the wave functions beyond the Gutzwiller factor. Superconductivity of d_x^2-y^2-wave is widely stable, and exhibits a crossover around U=U_co\sim 12t from a BCS type to a new type. For U\gsim U_co (U\lsim U_co), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and \propto exp(-t/J) [tiny] on the strong [weak] coupling side of U_co. Cuprates belong to the strong-coupling regime.Comment: 4 pages, 6 figure

The moduli problem at the perturbative level

Moduli fields generically produce strong dark matter -- radiation and baryon -- radiation isocurvature perturbations through their decay if they remain light during inflation. We show that existing upper bounds on the magnitude of such fluctuations can thus be translated into stringent constraints on the moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum expectation value at the end of inflation). These constraints are complementary to previously existing bounds so that the moduli problem becomes worse at the perturbative level. In particular, if the inflationary scale H_{inf}~10^{13} GeV, particle physics scenarios which predict high moduli masses m_\sigma > 10-100 TeV are plagued by the perturbative moduli problem, even though they evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the amplitude/transfer of isocurvature modes at the end of inflation, typos corrected, references added, basic result unchange