13,640 research outputs found
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 , 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 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
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