1,337 research outputs found
A Theorem on the origin of Phase Transitions
For physical systems described by smooth, finite-range and confining
microscopic interaction potentials V with continuously varying coordinates, we
announce and outline the proof of a theorem that establishes that unless the
equipotential hypersurfaces of configuration space \Sigma_v ={(q_1,...,q_N)\in
R^N | V(q_1,...,q_N) = v}, v \in R, change topology at some v_c in a given
interval [v_0, v_1] of values v of V, the Helmoltz free energy must be at least
twice differentiable in the corresponding interval of inverse temperature
(\beta(v_0), \beta(v_1)) also in the N -> \infty and the
{\Sigma_v}_{v > v_c}, which is the consequence of the existence of critical
points of V on \Sigma_{v=v_c}, that is points where \nabla V=0.Comment: 10 pages, Statistical Mechanics, Phase Transitions, General Theory.
Phys. Rev. Lett., in pres
Superfrustration of charge degrees of freedom
We review recent results, obtained with P. Fendley, on frustration of quantum
charges in lattice models for itinerant fermions with strong repulsive
interactions. A judicious tuning of kinetic and interaction terms leads to
models possessing supersymmetry. In such models frustration takes the form of
what we call superfrustration: an extensive degeneracy of supersymmetric ground
states. We present a gallery of examples of superfrustration on a variety of 2D
lattices.Comment: 8 pages, 5 figures, contribution to the proceedings of the XXIII
IUPAP International Conference on Statistical Physics (2007) in Genova, Ital
Effective Hamiltonian for Excitons with Spin Degrees of Freedom
Starting from the conventional electron-hole Hamiltonian , we
derive an effective Hamiltonian for excitons with
spin degrees of freedom. The Hamiltonian describes optical processes close to
the exciton resonance for the case of weak excitation. We show that
straightforward bosonization of does not give the correct form
of , which we obtain by a projection onto the subspace
spanned by the excitons. The resulting relaxation and renormalization
terms generate an interaction between excitons with opposite spin. Moreover,
exciton-exciton repulsive interaction is greatly reduced by the
renormalization. The agreement of the present theory with the experiment
supports the validity of the description of a fermionic system by bosonic
fields in two dimensions.Comment: 12 pages, no figures, RevTe
Mixed biexcitons in single quantum wells
Biexcitonic complexes in a ZnSe single quantum well are investigated by spectrally resolved four-wave mixing (FWM). The formation of heavy-heavy-hole XXh and of mixed heavy-light-hole XXm biexcitons showing binding energies of Δh=4.8meV and Δm=2.8meV is identified by polarization selection rules. The coherent dynamics of the FWM response and the observed FWM intensity ratio between the XXh and XXm biexciton-induced nonlinear signals are in agreement with the solution of an extended optical Bloch equation
On the integral cohomology of smooth toric varieties
Let be a smooth, not necessarily compact toric variety. We show
that a certain complex, defined in terms of the fan , computes the
integral cohomology of , including the module structure over the
homology of the torus. In some cases we can also give the product. As a
corollary we obtain that the cycle map from Chow groups to integral Borel-Moore
homology is split injective for smooth toric varieties. Another result is that
the differential algebra of singular cochains on the Borel construction of
is formal.Comment: 10 page
Theory of Adsorption and Surfactant Effect of Sb on Ag (111)
We present first-principles studies of the adsorption of Sb and Ag on clean
and Sb-covered Ag (111). For Sb, the {\it substitutional} adsorption site is
found to be greatly favored with respect to on-surface fcc sites and to
subsurface sites, so that a segregating surface alloy layer is formed. Adsorbed
silver adatoms are more strongly bound on clean Ag(111) than on Sb-covered Ag.
We propose that the experimentally reported surfactant effect of Sb is due to
Sb adsorbates reducing the Ag adatom mobility. This gives rise to a high
density of Ag islands which coalesce into regular layers.Comment: RevTeX 3.0, 11 pages, 0 figures] 13 July 199
Renormalized Bosonic Interaction of Excitons
An effective bosonic Hamiltonian of excitons with ``spin'' degrees of
freedom in two dimension is obtained through a projection procedure, starting
from a conventional electron-hole Hamiltonian . We first
demonstrate that a straightforward transformation of into a
Hamiltonian of bosonic excitons does not give the two-body interaction between
an ``up-spin'' exciton and a ``down-spin'' exciton, which are created by the
left- and right-circularly polarized light beams, respectively. We then show
that this interaction is generated through a projection procedure onto the
subspace spanned by excitons, as a renormalization effect coming from
higher exciton states. The projection also renormalizes the interaction between
excitons with the same spins by a large amount. These renormalization
effects are crucial for the polarization dependence of the optical responses
from semiconductors. The present theory gives the microscopic foundation of the
phenomenology that was successfully applied to the analysis of four-wave mixing
experiments in GaAs quantum wells strongly coupled to the radiation field in a
high-Q micro cavity.Comment: 2 figure
``Smoke Rings'' in Ferromagnets
It is shown that bulk ferromagnets support propagating non-linear modes that
are analogous to the vortex rings, or ``smoke rings'', of fluid dynamics. These
are circular loops of {\it magnetic} vorticity which travel at constant
velocity parallel to their axis of symmetry. The topological structure of the
continuum theory has important consequences for the properties of these
magnetic vortex rings. One finds that there exists a sequence of magnetic
vortex rings that are distinguished by a topological invariant (the Hopf
invariant). We present analytical and numerical results for the energies,
velocities and structures of propagating magnetic vortex rings in ferromagnetic
materials.Comment: 4 pages, 3 eps-figures, revtex with epsf.tex and multicol.sty. To
appear in Physical Review Letters. (Postscript problem fixed.
The Seven-sphere and its Kac-Moody Algebra
We investigate the seven-sphere as a group-like manifold and its extension to
a Kac-Moody-like algebra. Covariance properties and tensorial composition of
spinors under are defined. The relation to Malcev algebras is
established. The consequences for octonionic projective spaces are examined.
Current algebras are formulated and their anomalies are derived, and shown to
be unique (even regarding numerical coefficients) up to redefinitions of the
currents. Nilpotency of the BRST operator is consistent with one particular
expression in the class of (field-dependent) anomalies. A Sugawara construction
is given.Comment: 22 pages. Macropackages used: phyzzx, epsf. Three epsf figure files
appende
Relaxation and reconstruction on (111) surfaces of Au, Pt, and Cu
We have theoretically studied the stability and reconstruction of (111)
surfaces of Au, Pt, and Cu. We have calculated the surface energy, surface
stress, interatomic force constants, and other relevant quantities by ab initio
electronic structure calculations using the density functional theory (DFT), in
a slab geometry with periodic boundary conditions. We have estimated the
stability towards a quasi-one-dimensional reconstruction by using the
calculated quantities as parameters in a one-dimensional Frenkel-Kontorova
model. On all surfaces we have found an intrinsic tensile stress. This stress
is large enough on Au and Pt surfaces to lead to a reconstruction in which a
denser surface layer is formed, in agreement with experiment. The
experimentally observed differences between the dense reconstruction pattern on
Au(111) and a sparse structure of stripes on Pt(111) are attributed to the
details of the interaction potential between the first layer of atoms and the
substrate.Comment: 8 pages, 3 figures, submitted to Physical Review
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