7,415 research outputs found
A Maximum Mass-to-Size Ratio in Scalar-Tensor Theories of Gravity
We derive a modified Buchdahl inequality for scalar-tensor theories of
gravity. In general relativity, Buchdahl has shown that the maximum value of
the mass-to-size ratio, , is 8/9 for static and spherically symmetric
stars under some physically reasonable assumptions. We formally apply
Buchdahl's method to scalar-tensor theories and obtain theory-independent
inequalities. After discussing the mass definition in scalar-tensor theories,
these inequalities are related to a theory-dependent maximum mass-to-size
ratio. We show that its value can exceed not only Buchdahl's limit, 8/9, but
also unity, which we call {\it the black hole limit}, in contrast to general
relativity. Next, we numerically examine the validity of the assumptions made
in deriving the inequalities and the applicability of our analytic results. We
find that the assumptions are mostly satisfied and that the mass-to-size ratio
exceeds both Buchdahl's limit and the black hole limit. However, we also find
that this ratio never exceeds Buchdahl's limit when we impose the further
condition, , on the density, , and pressure, , of the
matter.Comment: 23 pages, 13 figures and 1 tabl
Composite Fermions in Quantum Dots
We demonstrate the formation of composite fermions in two-dimensional quantum
dots under high magnetic fields. The composite fermion interpretation provides
a simple way to understand several qualitative and quantitative features of the
numerical results obtained earlier in exact diagonalization studies. In
particular, the ground states are recognized as compactly filled quasi-Landau
levels of composite fermions.Comment: Revtex. Postscript files of figures are appended the tex
Ordering of the Heisenberg Spin Glass in High Dimensions
Ordering of the Heisenberg spin glass with the nearest-neighbor Gaussian
coupling is investigated by equilibrium Monte Carlo simulations in four and
five dimensions. Ordering of the mean-field Heisenberg spin-glass is also
studied for comparison. Particular attention is paid to the nature of the
spin-glass and the chiral-glass orderings. Our numerical data suggest that, in
five dimensions, the model exhibits a single spin-glass transition at a finite
temperature, where the spin-glass order accompanying the simultaneous
chiral-glass order sets in. In four dimensions, by contrast, the model exhibits
a chiral-glass transition at a finite temperature, not accompanying the
standard spin-glass order. The critical region associated with the chiral-glass
transition, however, is very narrow, suggesting that dimension four is close to
the marginal dimensionality.Comment: 18 pages, 12 figure
Chirality scenario of the spin-glass ordering
Detailed account is given of the chirality scenario of experimental
spin-glass transitions. In this scenario, the spin glass order of weakly
anisotropic Heisenberg-like spin-glass magnets including canonical spin glasses
are essentially chirality driven. Recent numerical and experimental results are
discussed in conjunction with this scenario.Comment: Submitted to J. Phys. Soc. Japan "Special Issue on Frustration
Monte Carlo Studies of the Ordering of the Three-Dimensional Isotropic Heisenberg Spin Glass in Magnetic Fields
Spin and chirality orderings of the three-dimensional Heisenberg spin glass
under magnetic fields are studied by large-scale equilibrium Monte Carlo
simulations. It is found that the chiral-glass transition and the chiral-glass
ordered state, which are essentially of the same character as their zero-field
counterparts, occur under magnetic fields. The chiral-glass ordered state
exhibits a one-step-like peculiar replica-symmetry breaking in the chiral
sector, while it does not accompany the spin-glass order perpendicular to the
applied field. Critical perperties of the chiral-glass transition are different
from those of the standard Ising spin glass. Magnetic phase diagram of the
model is constructed, which reveals that the chiral-glass state is quite robust
against magnetic fields. The chiral-glass transition line has a character of
the Gabay-Toulouse line of the mean-field model, yet its physical origin being
entirely different. These numerical results are discussed in light of the
recently developed spin-chirality decoupling-recoupling scenario. Implication
to experimental phase diagram is also discussed.Comment: 23 pages, 23 figure
Tricritical behavior of the frustrated XY antiferromagnet
Extensive histogram Monte-Carlo simulations of the XY antiferromagnet on a
stacked triangular lattice reveal exponent estimates which strongly favor a
scenario of mean-field tricritical behavior for the spin-order transition. The
corresponding chiral-order transition occurs at the same temperature but
appears to be decoupled from the spin-order. These results are relevant to a
wide class of frustrated systems with planar-type order and serve to resolve a
long-standing controversy regarding their criticality.Comment: J1K 2R1 4 pages (RevTex 3.0), 4 figures available upon request,
Report# CRPS-94-0
Ground State Phase Diagram of Frustrated S=1 XXZ chains : Chiral Ordered Phases
The ground-state phase diagram of frustrated S=1 XXZ spin chains with the
competing nearest- and next-nearest-neighbor antiferromagnetic couplings is
studied using the infinite-system density-matrix renormalization-group method.
We find six different phases, namely, the Haldane, gapped chiral, gapless
chiral, double Haldane, N\'{e}el, and double N\'{e}el (uudd) phases. The gapped
and gapless chiral phases are characterized by the spontaneous breaking of
parity, in which the long-range order parameter is a chirality, \kappa_l =
S_l^xS_{l+1}^y-S_l^yS_{l+1}^x, whereas the spin correlation decays either
exponentially or algebraically. These chiral ordered phases appear in a broad
region in the phase diagram for \Delta < 0.95, where \Delta is an
exchange-anisotropy parameter. The critical properties of phase transitions are
also studied.Comment: 13 pages, 9 figures, to appear in J. Phys. Soc. Jp
Anomalous U(1) D-term Contribution in Type I String Models
We study the -term contribution for anomalous U(1) symmetries in type I
string models and derive general formula for the -term contribution,
assuming that the dominant source of SUSY breaking is given by -terms of the
dilaton, (overall) moduli or twisted moduli fields. On the basis of the
formula, we also point out that there are several different features from the
case in heterotic string models. The differences originate from the different
forms of K\"ahler potential between twisted moduli fields in type I string
models and the dilaton field in heterotic string models.Comment: 16 pages, latex, no figur
- …