3,074 research outputs found
Long range order in the classical kagome antiferromagnet: effective Hamiltonian approach
Following Huse and Rutenberg [Phys. Rev. B 45, 7536 (1992)], I argue the
classical Heisenberg antiferromagnet on the kagom\'e lattice has long-range
spin order of the type (modulo gradual orientation
fluctuations of the spins' plane). I start from the effective quartic
Hamiltonian for the soft (out of plane) spin fluctuation modes, and treat as a
perturbation those terms which depend on the discrete coplanar state. Soft mode
correlations, which become the coefficients of a discrete effective
Hamiltonian, are estimated analytically.Comment: 4pp, no figures. Converted to PRB format, extensive revisions/some
reorderings to improve clarity; some cut
Probing the wind-wind collision in Gamma Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization
We present a new analysis of an archived Chandra HETGS X-ray spectrum of the
WR+O colliding wind binary Gamma Velorum. The spectrum is dominated by emission
lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a
combination of broad-band spectral analysis and an analysis of line flux ratios
we infer a wide range of temperatures in the X-ray emitting plasma (~4-40 MK).
As in the previously published analysis, we find the X-ray emission lines are
essentially unshifted, with a mean FWHM of 1240 +/- 30 km/s. Calculations of
line profiles based on hydrodynamical simulations of the wind-wind collision
predict lines that are blueshifted by a few hundred km/s. The lack of any
observed shift in the lines may be evidence of a large shock-cone opening
half-angle (> 85 degrees), and we suggest this may be evidence of sudden
radiative braking. From the R and G ratios measured from He-like
forbidden-intercombination-resonance triplets we find evidence that the Mg XI
emission originates from hotter gas closer to the O star than the Si XIII
emission, which suggests that non-equilibrium ionization may be present.Comment: 22 pages, 14 figures. Accepted for publication in MNRA
Properties of Resonating-Valence-Bond Spin Liquids and Critical Dimer Models
We use Monte Carlo simulations to study properties of Anderson's
resonating-valence-bond (RVB) spin-liquid state on the square lattice (i.e.,
the equal superposition of all pairing of spins into nearest-neighbor singlet
pairs) and compare with the classical dimer model (CDM). The latter system also
corresponds to the ground state of the Rokhsar-Kivelson quantum dimer model at
its critical point. We find that although spin-spin correlations decay
exponentially in the RVB, four-spin valence-bond-solid (VBS) correlations are
critical, qualitatively like the well-known dimer-dimer correlations of the
CDM, but decaying more slowly (as with , compared with
for the CDM). We also compute the distribution of monomer (defect) pair
separations, which decay by a larger exponent in the RVB than in the CDM. We
further study both models in their different winding number sectors and
evaluate the relative weights of different sectors. Like the CDM, all the
observed RVB behaviors can be understood in the framework of a mapping to a
"height" model characterized by a gradient-squared stiffness constant . Four
independent measurements consistently show a value , with the same kinds of numerical evaluations of give
results in agreement with the rigorously known value . The
background of a nonzero winding number gradient introduces spatial
anisotropies and an increase in the effective K, both of which can be
understood as a consequence of anharmonic terms in the height-model free
energy, which are of relevance to the recently proposed scenario of "Cantor
deconfinement" in extended quantum dimer models. We also study ensembles in
which fourth-neighbor (bipartite) bonds are allowed, at a density controlled by
a tunable fugacity, resulting (as expected) in a smooth reduction of K.Comment: 26 pages, 21 figures. v3: final versio
The "Coulomb phase" in frustrated systems
The "Coulomb phase" is an emergent state for lattice models (particularly
highly frustrated antiferromagnets) which have local constraints that can be
mapped to a divergence-free "flux". The coarse-grained version of this flux or
polarization behave analogously to electric or magnetic fields; in particular,
defects at which the local constraint is violated behave as effective charges
with Coulomb interactions. I survey the derivation of the characteristic
power-law correlation functions and the pinch-points in reciprocal space plots
of diffuse scattering, as well as applications to magnetic relaxation,
quantum-mechanical generalizations, phase transitions to long-range-ordered
states, and the effects of disorder.Comment: 30 pp, 5 figures (Sub. to Annual Reviews of Condensed Matter Physics
Compact Electrodynamics in 2+1 dimensions: Confinement with gapless modes
We consider 2+1 dimensional compact U(1) gauge theory at the Lifshitz point
with dynamical critical exponent . As in the usual theory, monopoles
proliferate the vacuum for any value of the coupling, generating a mass scale.
The theory of the dilute monopole gas is written in terms a non-relativistic
Sine-Gordon model with two real fields. While monopoles remove some of the
massless poles of the perturbative field strength propagator, a gapless mode
representing the incomplete screening of monopoles remains, and is protected by
a shift invariance of the original theory. Timelike Wilson loops still obey
area laws, implying that minimal charges are confined, but the action of
spacelike Wilson loops of linear size L goes instead as .Comment: 4 pages, RevTeX. Some equations simplified. Version to appear in
Physical Review Letter
Models at a Lifshitz Point
We consider models in dimensions around Lifshitz fixed
points with dynamical critical exponent , in the large-N expansion. It is
shown that these models are asymptotically free and dynamically generate a mass
for the fields for all . We demonstrate that, for , the
initially nondynamical gauge field acquires kinetic terms in a way similar to
usual models in 1+1 dimensions. Lorentz invariance emerges
generically in the low-energy electrodynamics, with a nontrivial dielectric
constant given by the inverse mass gap and a magnetic permeability which has a
logarithmic dependence on scale. At a special multicritical point, the
low-energy electrodynamics also has , and an essentially singular
dependence of the effective action on .Comment: LaTeX 13 pages; added a comment about constant field effective
action. version published in Physical Review
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