596 research outputs found
Magnetization Process of the S=1 and 1/2 Uniform and Distorted Kagome Heisenberg Antiferromagnets
The magnetization process of the S=1 and 1/2 kagome Heisenberg
antiferromagnet is studied by means of the numerical exact diagonalization
method. It is found that the magnetization curve at zero temperature has a
plateau at 1/3 of the full magnetization. In the presence of lattice distortion, this plateau is enhanced and eventually the
ferrimagnetic state is realized. There also appear the minor plateaux above the
main plateau. The physical origin of these phenomena is discussed.Comment: 5 pages, 10 figures included, to be published in J. Phys. Soc. Jp
Isotropic Spin Wave Theory of Short-Range Magnetic Order
We present an isotropic spin wave (ISW) theory of short-range order in
Heisenberg magnets, and apply it to square lattice S=1/2 and S=1
antiferromagnets. Our theory has three identical (isotropic) spin wave modes,
whereas the conventional spin wave theory has two transverse and one
longitudinal mode. We calculate temperature dependences of various
thermodynamic observables analytically and find good (several per cent)
agreement with independently obtained numerical results in a broad temperature
range.Comment: 4 pages, REVTeX v3 with 3 embedded PostScript figure
The two-dimensional quantum Heisenberg antiferromagnet: effective Hamiltonian approach to the thermodynamics
In this paper we present an extensive study of the thermodynamic properties
of the two-dimensional quantum Heisenberg antiferromagnet on the square
lattice; the problem is tackled by the pure-quantum self-consistent harmonic
approximation, previously applied to quantum spin systems with easy-plane
anisotropies, modeled to fit the peculiar features of an isotropic system.
Internal energy, specific heat, correlation functions, staggered
susceptibility, and correlation length are shown for different values of the
spin, and compared with the available high-temperature expansion and quantum
Monte Carlo results, as well as with the available experimental data.Comment: 14 pages, 13 Postscript figures embedded by psfig.sty; revisions:
paper shortened, some parts moved in the appendices, 4 figures replaced by 2
only, minor errors correcte
Mott-Superfluid transition in bosonic ladders
We show that in a commensurate bosonic ladder, a quantum phase transition
occurs between a Mott insulator and a superfluid when interchain hopping
increases. We analyse the properties of such a transition as well as the
physical properties of the two phases. We discuss the physical consequences for
experimental systems such as Josephson Junction arrays.Comment: 4 pages, 2 figures, revtex
Dynamical Properties of a Haldane Gap Antiferromagnet
We study the dynamic spin correlation function of a spin one
antiferromagnetic chain with easy-plane single-ion anisotropy. We use exact
diagonalization by the Lancz\H os method for chains of lengths up to N=16
spins. We show that a single-mode approximation is an excellent description of
the dynamical properties. A variational calculation allows us to clarify the
nature of the excitations. The existence of a two-particle continuum near zero
wavevector is clearly seen both in finite-size effects and in the dynamical
structure factor. The recent neutron scattering experiments on the
quasi-one-dimensional antiferromagnet NENP are fully explained by our results.Comment: 14 pages, SphT/92-135 plain tex with Postscript figures included.
Postscipt file available by anonymous ftp at amoco.saclay.cea.fr by get
pubs.spht/92-135.ps local_file (290 kb) or get pubs.spht/92-135.ps.Z
local_file.Z (compressed - 120 kb
Realization of a large J_2 quasi-2D spin-half Heisenberg system: Li2VOSiO4
Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of
in-plane couplings J_1 + J_2 = 9.5 \pm 1.5 K and the inter-plane coupling
J_{perp} \sim 0.2 - 0.3 K agree with recent experimental data, the ratio
J_2/J_1 \sim 12 exceeds the reported value by an order of magnitude. Using
geometrical considerations, high temperature expansions and perturbative mean
field theory, we show that the LDA derived exchange constants lead to a
remarkably accurate description of the properties of these materials including
specific heat, susceptibility, Neel temperature and NMR spectra.Comment: 4 two-column pages, 4 embedded postscript figure
Long range Neel order in the triangular Heisenberg model
We have studied the Heisenberg model on the triangular lattice using several
Quantum Monte Carlo (QMC) techniques (up to 144 sites), and exact
diagonalization (ED) (up to 36 sites). By studying the spin gap as a function
of the system size we have obtained a robust evidence for a gapless spectrum,
confirming the existence of long range Neel order. Our best estimate is that in
the thermodynamic limit the order parameter m= 0.41 +/- 0.02 is reduced by
about 59% from its classical value and the ground state energy per site is
e0=-0.5458 +/- 0.0001 in unit of the exchange coupling. We have identified the
important ground state correlations at short distance.Comment: 4 pages, RevTeX + 4 encapsulated postscript figure
Spinons, Solitons and Magnons in One-dimensional Heisenberg-Ising Antiferromagnets
We calculate the excitation spectra for the one- Heisenberg-Ising
antiferromagnets by expansions around the Ising limit. For , the
calculated expansion coefficients for the spinon-spectra agree term by term
with the solution of Johnson and McCoy. For , the solitons become gapless
before the Heisenberg limit is reached, signalling a transition to the Haldane
phase. By applying a staggered field we calculate the one-magnon spectra for
the Heisenberg chain. For the quantum renormalization of the
spin-wave spectra is calculated to be approximately .Comment: 4 pages, RevTex, 3 postscript figures, Latex file and figures have
been uuencode
Is a Trapped One-Dimensional Bose Gas a Luttinger Liquid?
The low-energy fluctuations of a trapped, interacting quasi one-dimensional
Bose gas are studied. Our considerations apply to experiments with highly
anisotropic traps. We show that under suitable experimental conditions the
system can be described as a Luttinger liquid. This implies that the
correlation function of the bosons decays algebraically preventing
Bose-Einstein condensation. At significantly lower temperatures a finite size
gap destroys the Luttinger liquid picture and Bose-Einstein condensation is
again possible.Comment: 4 pages (revtex), 1 figure (eps file
Spin wave analysis to the spatially-anisotropic Heisenberg antiferromagnet on triangular lattice
We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model
on the triangular lattice with spatially-anisotropic interactions. For values
of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave
theory predicts that quantum fluctuations melt the classical structures, for
S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate
spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where
the ground state is disordered. The existence of such nonmagnetic states
suggests the possibility of spin liquid behavior for intermediate values of the
anisotropy.Comment: Revised version, 4 pages, Latex (twocolumn), 4 figures as eps files.
To appear in PR
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