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Sep 22, 2013
09/13

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R. Deutscher; H. U. Everts

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We study the ground state properties of the S=$\frac{1}{2}$ Heisenberg antiferromagnet (HAF) on the triangular lattice with nearest-neighbour ($J$) and next-nearest neighbour ($\alpha J$) couplings. Classically, this system is known to be ordered in a $120^\circ$ N\'eel type state for values $-\infty

Source: http://arxiv.org/abs/cond-mat/9306044v1

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Sep 19, 2013
09/13

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M. Sandhoff; H. Pfnuer; H. U. Everts

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We investigate the phase diagram and the critical properties of the adsorbate system sulphur/ruthenium(0001) in the coverage region $\frac{1}{4} < \Theta < \frac{1}{3}$ by means of Monte-Carlo simulations of a simple lattice gas model on a triangular lattice. The model contains only repulsive nearest and next-nearest neighbor interactions. Combining results obtained by using both Glauber and Kawasaki kinetics in the simulations we identify two tricritical points, three coexistence regions...

Source: http://arxiv.org/abs/cond-mat/9307015v1

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Sep 20, 2013
09/13

by
M. Wintel; H. U. Everts; W. Apel

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We study the topological defects in the classical Heisenberg antiferromagnet in two dimensions on a triangular lattice (HAFT). While the topological analysis of the order parameter space indicates that the defects are of $Z_2$ type, consideration of the energy leads us to a description of the low--energy stationary points of the action in terms of $\pm$ vortices, as in the planar XY model. Starting with the continuum description of the HAFT, we show analytically that its partition function can...

Source: http://arxiv.org/abs/cond-mat/9402060v1

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Sep 21, 2013
09/13

by
M. Wintel; H. U. Everts; W. Apel

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We have simulated the classical Heisenberg antiferromagnet on a triangular lattice using a local Monte Carlo algorithm. The behavior of the correlation length $\xi$, the susceptibility at the ordering wavevector $\chi(\bf Q)$, and the spin stiffness $\rho$ clearly reflects the existence of two temperature regimes -- a high temperature regime $T > T_{th}$, in which the disordering effect of vortices is dominant, and a low temperature regime $T < T_{th}$, where correlations are controlled...

Source: http://arxiv.org/abs/cond-mat/9508128v1

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Sep 20, 2013
09/13

by
W. Apel; T. Yavors'kii; H. -U. Everts

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In the search for spin-1/2 kagome antiferromagnets, the mineral volborthite has recently been the subject of experimental studies [Hiroi et al.,2001]. It has been suggested that the magnetic properties of this material are described by a spin-1/2 Heisenberg model on the kagome lattice with spatially anisotropic exchange couplings. We report on investigations of the Sp(N) symmetric generalisation of this model in the large N limit. We obtain a detailed description of the dependence of possible...

Source: http://arxiv.org/abs/cond-mat/0608393v1

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Sep 18, 2013
09/13

by
T. Yavors'kii; W. Apel; H. -U. Everts

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We study the properties of the Heisenberg antiferromagnet with spatially anisotropic nearest-neighbour exchange couplings on the kagome net, i.e. with coupling J in one lattice direction and couplings J' along the other two directions. For J/J' > 1, this model is believed to describe the magnetic properties of the mineral volborthite. In the classical limit, it exhibits two kinds of ground states: a ferrimagnetic state for J/J' < 1/2 and a large manifold of canted spin states for J/J'...

Source: http://arxiv.org/abs/0704.1441v3

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Sep 18, 2013
09/13

by
W. Apel; H. -U. Everts; U. Koerner

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A coherent state representation for the electrons of ordered antiferromagnets is used to derive effective Hamiltonians for the dynamics of holes in such systems. By an appropriate choice of these states, the constraint of forbidden double occupancy can be implemented rigorously. Using these coherent states, one arrives at a path integral representation of the partition function of the systems, from which the effective Hamiltonians can be read off. We apply this method to the t-J model on the...

Source: http://arxiv.org/abs/cond-mat/9803048v1

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Sep 19, 2013
09/13

by
W. Apel; H. -U. Everts; U. Koerner

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The quasi-particle weight of a single hole in an antiferromagnetic background is studied in the semiclassical approximation. We start from the t-J model, generalize it to arbitrary spin S by employing an appropriate coherent state representation for the hole, and derive an effective action for the dynamics in the long-wavelength low-energy limit. In the same limit, we find an expression for the single-hole Green's function which we evaluate in an 1/S expansion. Our approach has the advantage of...

Source: http://arxiv.org/abs/cond-mat/0208554v1

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Sep 18, 2013
09/13

by
Igor F. Lyuksyutov; H. -U. Everts; H. Pfnuer

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We study the collective diffusion in chain structures on anisotropic substrates like (112) bcc and (110) fcc surfaces with deep troughs in the substrate potential corrugation. These chain structures are aligned normal to the troughs and can move only along the troughs. In a combination of theoretical arguments and of numerical simulations, we study the mass transport in these anisotropic systems. We find that a mechanism similar to soliton diffusion, instead of single particle diffusion, is...

Source: http://arxiv.org/abs/cond-mat/0101067v1

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Jul 20, 2013
07/13

by
A. Honecker; D. C. Cabra; H. -U. Everts; P. Pujol; F. Stauffer

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We investigate the classical counterpart of an effective Hamiltonian for a strongly trimerized kagome lattice. Although the Hamiltonian only has a discrete symmetry, the classical groundstate manifold has a continuous global rotational symmetry. Two cases should be distinguished for the sign of the exchange constant. In one case, the groundstate has a 120^\circ spin structure. To determine the transition temperature, we perform Monte-Carlo simulations and measure specific heat, the order...

Source: http://arxiv.org/abs/cond-mat/0609312v1

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Sep 21, 2013
09/13

by
A. Honecker; D. C. Cabra; H. -U. Everts; P. Pujol; F. Stauffer

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Frustration has proved to give rise to an extremely rich phenomenology in both quantum and classical systems. The leading behavior of the system can often be described by an effective model, where only the lowest-energy degrees of freedom are considered. In this paper we study a system corresponding to the strong trimerization limit of the spin 1/2 kagome antiferromagnet in a magnetic field. It has been suggested that this system can be realized experimentally by a gas of spinless fermions in...

Source: http://arxiv.org/abs/1108.5268v2

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Sep 22, 2013
09/13

by
K. Budde; I. F. Lyuksyutov; H. Pfnuer; G. Godzik; H. U. Everts

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The first order phase transitions between a two-dimensional (2d) gas and the 2d solid of the first monolayer have been studied for the noble gases Ar, Kr and Xe on a NaCl(100) surface in quasi-equilibrium with the three-dimensional gas phase. Using linear temperature ramps, we show that the widths of the hysteresis loops of these transitions as a function of the heating rate, r, scales with a power law r^alpha with alpha between 0.4 and 0.5 depending on the system. The hysteresis loops for...

Source: http://arxiv.org/abs/cond-mat/9807225v1

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Sep 20, 2013
09/13

by
B. Damski; H. Fehrmann; H. -U. Everts; M. Baranov; L. Santos; M. Lewenstein

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We study low temperature properties of atomic gases in trimerized optical kagom\'{e} lattices. The laser arrangements that can be used to create these lattices are briefly described. We also present explicit results for the coupling constants of the generalized Hubbard models that can be realized in such lattices. In the case of a single component Bose gas the existence of a Mott insulator phase with fractional numbers of particles per trimer is verified in a mean field approach. The main...

Source: http://arxiv.org/abs/cond-mat/0506580v3

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Sep 24, 2013
09/13

by
B. Damski; H. -U. Everts; A. Honecker; H. Fehrmann; L. Santos; M. Lewenstein

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We study low temperature properties of an atomic spinless interacting Fermi gas in the trimerized Kagom\'e lattice for the case of two fermions per trimer. The system is described by a quantum spin 1/2 model on the triangular lattice with couplings depending on bonds directions. Using exact diagonalizations we show that the system exhibits non-standard properties of a {\it quantum spin-liquid crystal}, combining a planar antiferromagnetic order with an exceptionally large number of low energy...

Source: http://arxiv.org/abs/cond-mat/0412133v3

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Sep 18, 2013
09/13

by
L. Santos; M. A. Baranov; J. I. Cirac; H. -U. Everts; H. Fehrmann; M. Lewenstein

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We demonstrate the possibility of creating and controlling an ideal and \textit{trimerized} optical Kagom\'e lattice, and study the low temperature physics of various atomic gases in such lattices. In the trimerized Kagom\'e lattice, a Bose gas exhibits a Mott transition with fractional filling factors, whereas a spinless interacting Fermi gas at 2/3 filling behaves as a quantum magnet on a triangular lattice. Finally, a Fermi-Fermi mixture at half filling for both components represents a...

Source: http://arxiv.org/abs/cond-mat/0401502v2

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Sep 20, 2013
09/13

by
Ch. Waldtmann; H. -U. Everts; B. Bernu; P. Sindzingre; C. Lhuillier; P. Lecheminant; L. Pierre

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We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagom\'e lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis of N\'eel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather unusual. The magnetic ($\Delta S=1$) excitations are separated from the ground state by a gap. However, this gap...

Source: http://arxiv.org/abs/cond-mat/9802168v1