1,540 research outputs found
Exact low-temperature properties of a class of highly frustrated Hubbard models
We study the repulsive Hubbard model both analytically and numerically on a
family of highly frustrated lattices which have one-electron states localized
on isolated trapping cells. We construct and count exact many-electron ground
states for a wide range of electron densities and obtain closed-form
expressions for the low-temperature thermodynamic quantities. Furthermore, we
find that saturated ferromagnetism is obtained only for sufficiently high
electron densities and large Hubbard repulsion while there is no finite
average moment in the ground states at lower densities.Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev.
Lanczos Study of the S=1/2 Frustrated Square-Lattice Antiferromagnet in a Magnetic Field
We study the zero-temperature phase diagram of the frustrated square-lattice
S=1/2 antiferromagnet in an external magnetic field numerically with the
Lanczos algorithm. For strong frustration, we find disordered phases at high
(and low) magnetic fields. Between these two disordered phases we find a
plateau in the magnetization curve at half of the saturation magnetization
which corresponds to a state with up-up-up-down (uuud) spin order. This and
other considerations [cond-mat/0003343] suggest an unusual ordering scenario:
There are an ordered phase with a spin gap (the plateau) and disordered
magnetically gapless phases above and below.
The transition to saturation is studied in further detail and problematic
conclusions in earlier investigations of this region are pointed out.Comment: 4 pages REVTeX, 5 PostScript figures included using psfig.sty;
submitted to the proceedings of the conference Highly Frustrated Magnetism
2000, Waterloo, June 11-15, 2000 (to appear in Canadian Journal of Physics
Orbifolds versus smooth heterotic compactifications
Following the recent exploration of smooth heterotic compactifications with
unitary bundles, orbifold compactifications in six dimensions can be shown to
correspond in the blow-up to compactifications with U(1) gauge backgrounds. A
powerful tool is the comparison of anomaly polynomials. The presentation here
focuses on heterotic SO(32) compactifications in six dimensions including
five-branes. Four dimensional and E8 x E8 models are briefly commented on.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTe
Towards exact field theory results for the Standard Model on fractional D6-branes
Fractional D6-branes on toroidal orbifold backgrounds are known to be able to
accommodate the particle spectrum and gauge group of the Standard Model, but up
to now exact results for their low-energy effective action are missing. Here we
discuss how the conceptual ansatz of matching the string theoretic gauge
couplings at one-loop with the supergravity expressions is generalised from the
six-torus to orbifold backgrounds on which the Standard Model spectrum can be
realised on fractional D6-branes. The Kaehler metrics and perturbatively exact
holomorphic gauge kinetic functions can be classified in terms of the vanishing
of some intersection angle and the related beta function coefficients, which
potentially opens the possibility to extrapolate to smooth Calabi-Yau
backgrounds.Comment: 1+3 pages, 2 tables; proceedings of the EPS-HEP 2011 conference in
Grenoble 21-27 July 2011; v2: introduction and conclusion extended, published
versio
Magnetocaloric effect in two-dimensional spin-1/2 antiferromagnets
The magnetocaloric effect is studied at the transition to saturation in the
antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional
lattices, namely the square and the triangular lattice. Numerical results are
presented for the entropy which are consistent with identical universal
properties. However, the absolute values of the entropy are bigger on the
geometrically frustrated triangular lattice than on the non-frustrated square
lattice, indicating that frustration improves the magnetocaloric properties.Comment: 2 pages, 2 figures included, to appear in Physica B (proceedings of
SCES'05
Planar pyrochlore: a strong-coupling analysis
Recent investigations of the two-dimensional spin-1/2 checkerboard lattice
favor a valence bond crystal with long range quadrumer order [J.-B. Fouet et
al., preprint cond-mat/0108070]. Starting from the limit of isolated
quadrumers, we perform a complementary analysis of the evolution of the
spectrum as a function of the inter quadrumer coupling j using both, exact
diagonalization (ED) and series expansion (SE) by continuous unitary
transformation. We compute (i) the ground state energy, (ii) the elementary
triplet excitations, (iii) singlet excitations on finite systems and find very
good agreement between SE and ED. In the thermodynamic limit we find a ground
state energy substantially lower than documented in the literature. The
elementary triplet excitation is shown to be gapped and almost dispersionless,
whereas the singlet sector contains strongly dispersive modes. Evidence is
presented for the low energy singlet excitations in the spin gap in the
vicinity of j=1 to result from a large downward renormalization of local
high-energy states.Comment: 4 pages REVTeX4 including 3 figures: published version (minor
changes
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