68 research outputs found
The two dimensional antiferromagnetic Heisenberg model in the presence of an external field
We present numerical results on the zero temperature magnetization curve and
the static structure factors of the two dimensional antiferromagnetic
Heisenberg model in the presence of an external field. The impact of
frustration is also studied.Comment: 6 pages, 16 figures, REVTE
Formation of clusters in the two dimensional t-J model: The mechanism for phase separation
The emergence of phase separation is investigated in the framework of a 2D
t-J model by means of a variational product ansatz, which covers the infinite
lattice by two types of L x L clusters. Clusters of the first type are
completely occupied with electrons, i.e. they carry maximal charge Q_e=L^2 and
total spin 0, and thereby form the antiferromagnetic background. Holes occur in
the second type of clusters -- called ``hole clusters''. They carry a charge
Q_h<L^2. The charge Q_h and the number N(Q_h) of hole clusters is fixed by
minimizing the total energy at given hole density and spin exchange coupling
\alpha=J/t. For \alpha not too small (\alpha>0.5) it turns out that hole
clusters are occupied with an even number Q_h<L^2 of electrons and carry a
total spin 0. For increasing \alpha the charge Q_h(\alpha) of the hole clusters
decreases.Some points on the boundary curve can be extracted from Q_h(\alpha).Comment: 11 pages and 3 figure
The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory
The vacuum effective potential of three-dimensional SU(2) lattice gauge
theory in an applied color-magnetic field is computed over a wide range of
field strengths. The background field is induced by an external current, as in
continuum field theory. Scaling and finite volume effects are analyzed
systematically. The first evidence from lattice simulations is obtained of the
existence of a nontrivial minimum in the effective potential. This supports a
``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the
basis of a one-loop calculation in (3+1)-dimensional QCD.Comment: 9pp (REVTEX manuscript). Postscript figures appende
Instability of isolated triplet excitations on the Shastry-Sutherland lattice (SSL)
Configurations of singlets and triplets on the SSL have been proposed in the
literature as variational ground states of the Shastry-Sutherland model at
fixed magnetization M. We prove, that isolated triplet excitations on the SSL
are unstable if the coupling alpha falls below a critical value alpha_c=2.0
(approx.). The instability should be visible in the compound SrCu_2(BO_3)_2
where a coupling alpha^*=1.48 is realized.Comment: 4 pages, 4 figures, RevTe
Formation of clusters in the ground state of the model on a two leg ladder
We investigate the ground state properties of the model on a two leg
ladder with anisotropic couplings () along rungs and
() along legs. We have implemented a cluster approach based
on 4-site plaqettes. In the strong asymmetric cases and
the ground state energy is well described by plaquette
clusters with charges . The interaction between the clusters favours the
condensation of plaquettes with maximal charge -- a signal for phase
separation. The dominance of Q=2 plaquettes explains the emergence of tightly
bound hole pairs. We have presented the numerical results of exact
diagonalization to support our cluster approach.Comment: 11 pages, 9 figures, RevTex
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