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

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

Mott Transitions and d-wave Superconductivity in Half-Filled-Band Hubbard Model on Square Lattice with Geometric Frustration

Mechanisms of Mott transitions and dx2-y2-wave superconductivity (SC) are studied in the half-filled-band Hubbard model on square lattices with a diagonal hopping term (t'), using an optimization (or correlated) variational Monte Carlo method. In the trial wave functions, a doublon-holon binding effect is introduced in addition to the onsite Gutzwiller projection. We mainly treat a d-wave singlet state and a projected Fermi sea. In both wave functions, first-order Mott transitions without direct relevance to magnetic orders take place at U=Uc approximately of the bandwidth for arbitrary t'/t. These transitions originate in the binding or unbinding of a doublon to a holon. d-wave SC appears in a narrow range immediately below Uc. The robust d-wave superconducting correlation are necessarily accompanied by enhanced antiferromagnetic correlation; the strength of SC becomes weak, as t'/t increases.Comment: 18 pages, 30 figure

Lepton asymmetry in the primordial gravitational wave spectrum

Effects of neutrino free streaming is evaluated on the primordial spectrum of gravitational radiation taking both neutrino chemical potential and masses into account. The former or the lepton asymmetry induces two competitive effects, namely, to increase anisotropic pressure, which damps the gravitational wave more, and to delay the matter-radiation equality time, which reduces the damping. The latter effect is more prominent and a large lepton asymmetry would reduce the damping. We may thereby be able to measure the magnitude of lepton asymmetry from the primordial gravitational wave spectrum.Comment: 14 pages, 5 figure

Effects of Long-Range Correlations on Nonmagnetic Mott Transitions in Hubbard model on Square Lattice

The mechanism of Mott transition in the Hubbard model on the square lattice is studied without explicit introduction of magnetic and superconducting correlations, using a variational Monte Carlo method. In the trial wave functions, we consider various types of binding factors between a doubly-occupied site (doublon, D) and an empty site (holon, H), like a long-range type as well as a conventional nearest-neighbor type, and add independent long-range D-D (H-H) factors. It is found that a wide choice of D-H binding factor leads to Mott transitions at critical values near the band width. We renew the D-H binding picture of Mott transitions by introducing two characteristic length scales, the D-H binding length l_{DH} and the minimum D-D distance l_{DD}, which we appropriately estimate. A Mott transition takes place at l_{DH}=l_{DD}. In the metallic regime (l_{DH}>l_{DD}), the domains of D-H pairs overlap with one another, thereby doublons and holons can move independently by exchanging the partners one after another. In contrast, the D-D factors give only a minor contribution to the Mott transition.Comment: 13 pages, 18 figures, submitted to J. Phys. Soc. Jp

Density fluctuations in one-field inflation

Any one-field inflation is actually realized in a multifield configuration because the inflaton must have couplings with other fields to reheat the universe and is coupled to all other fields at least gravitationally. In all single inflaton models, it is explicitly or implicitly assumed that the heavier fields are stuck to their potential minima during inflation, which are time-dependent in general. We present a formalism to calculate curvature perturbations in such a time-dependent background and show that the proper expression can be obtained using a single-field analysis with a reduced potential in which all these heavy fields are situated at their respective, time-dependent minima. Our results provide a firm ground on the conventional calculation.Comment: 9 pages, to appear in Phys. Rev.

What happens when the inflaton stops during inflation

The spectrum of adiabatic density perturbation generated during inflation is studied in the case the time derivative of an inflation-driving scalar field (inflaton) vanishes at some time during inflation. It is shown that the nondecaying mode of perturbation has a finite value even in this case and that its amplitude is given by the standard formula with the time derivation of the scalar field replaced by the potential gradient using the slow-roll equation

A scanning microcavity for in-situ control of single-molecule emission

We report on the fabrication and characterization of a scannable Fabry-Perot microcavity, consisting of a curved micromirror at the end of an optical fiber and a planar distributed Bragg reflector. Furthermore, we demonstrate the coupling of single organic molecules embedded in a thin film to well-defined resonator modes. We discuss the choice of cavity parameters that will allow sufficiently high Purcell factors for enhancing the zero-phonon transition between the vibrational ground levels of the electronic excited and ground states.Comment: 8 page