13,482 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
Vacuum selection by inflation as the origin of the dark energy
I propose a new mechanism to account for the observed tiny but finite dark
energy in terms of a non-Abelian Higgs theory, which has infinitely many
perturbative vacua characterized by a winding number, in the framework of
inflationary cosmology. Inflation homogenizes field configuration and
practically realizes a perturbative vacuum with vanishing winding number, which
is expressed by a superposition of eigenstates of the Hamiltonian with
different vacuum energy density. As a result, we naturally find a nonvanishing
vacuum energy density with fairly large probability, under the assumption that
the cosmological constant vanishes in some vacuum state. Since the predicted
magnitude of dark energy is exponentially suppressed by the instanton action,
we can fit observation without introducing any tiny parameters.Comment: Honorable Mention, 2002 Gravity Research Foundation Essay
Increasing d-wave superconductivity by on site repulsion
We study by Variational Monte Carlo an extended Hubbard model away from half
filled band density which contains two competing nearest-neighbor interactions:
a superexchange favoring d-wave superconductivity and a repulsion
opposing against it. We find that the on-site repulsion effectively
enhances the strength of meanwhile suppressing that of , thus favoring
superconductivity. This result shows that attractions which do not involve
charge fluctuations are very well equipped against strong electron-electron
repulsion so much to get advantage from it.Comment: 4 pages, 3 figure
Controllable spin transport in ferromagnetic graphene junctions
We study spin transport in normal/ferromagnetic/normal graphene junctions
where a gate electrode is attached to the ferromagnetic graphene. We find that
due to the exchange field of the ferromagnetic graphene, spin current through
the junctions has an oscillatory behavior with respect to the chemical
potential in the ferromagnetic graphene, which can be tuned by the gate
voltage. Especially, we obtain a controllable spin current reversal by the gate
voltage. Our prediction of high controllability of spin transport in
ferromagnetic graphene junction may contribute to the development of the
spintronics.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
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