18 research outputs found
Critical Behavior of the Antiferromagnetic Heisenberg Model on a Stacked Triangular Lattice
We estimate, using a large-scale Monte Carlo simulation, the critical
exponents of the antiferromagnetic Heisenberg model on a stacked triangular
lattice. We obtain the following estimates: ,
. These results contradict a perturbative
Renormalization Group calculation that points to Wilson-Fisher O(4) behaviour.
While these results may be coherent with results from
Landau-Ginzburg analysis, they show the existence of an unexpectedly rich
structure of the Renormalization Group flow as a function of the dimensionality
and the number of components of the order parameter.Comment: Latex file, 10 pages, 1 PostScript figure. Was posted with a wrong
Title !
Fermions out of Dipolar Bosons in the lowest Landau level
In the limit of very fast rotation atomic Bose-Einstein condensates may
reside entirely in the lowest two-dimensional Landau level (LLL). For small
enough filling factor of the LLL, one may have formation of fractional quantum
Hall states. We investigate the case of bosons with dipolar interactions as may
be realized with Chromium-52 atoms. We show that at filling factor equal to
unity the ground state is a Moore-Read (a.k.a Pfaffian) paired state as is the
case of bosons with purely s-wave scattering interactions. This Pfaffian state
is destabilized when the interaction in the s-wave channel is small enough and
the ground state is a stripe phase with unidimensional density modulation. For
filling factor 1/3, we show that there is formation of a Fermi sea of
``composite fermions''. These composites are made of one boson bound with three
vortices. This phase has a wide range of stability and the effective mass of
the fermions depends essentially only of the scattering amplitude in momentum
channels larger or equal to 2. The formation of such a Fermi sea opens up a new
possible route to detection of the quantum Hall correlations.Comment: 12 pages, 5 figures, published versio
Exchange couplings in the magnetic molecular cluster Mn12Ac
The magnetic properties of the molecular cluster Mn12Ac are due to the four
Mn3+ ions which have spins S=3/2 and the eight Mn4+ ions with spins S=2. These
spins are coupled by superexchange mechanism. We determine the four exchange
couplings assuming a Heisenberg-type interaction between the ions. We use exact
diagonalization of the spin Hamiltonian by a Lanczos algorithm and we adjust
the couplings to reproduce the magnetization curve of Mn12Ac. We also impose
the constraint of reproducing a gap of 35K between a S=10 ground state and a
first excited state with S=9. We predict that there is an excited level with
S=8 at 37K above the ground state, only slightly above the S=9 excited state
which lies at 35K and the next excited state is a S=9 multiplet at 67K above
the S=10 ground state.Comment: 15 pages, 6 figures, submitted to Phys Rev B, corrected a misTeX:
values of J1, J2 have changed, refs update
Percolation Transition in the random antiferromagnetic spin-1 chain
We give a physical description in terms of percolation theory of the phase
transition that occurs when the disorder increases in the random
antiferromagnetic spin-1 chain between a gapless phase with topological order
and a random singlet phase. We study the statistical properties of the
percolation clusters by numerical simulations, and we compute exact exponents
characterizing the transition by a real-space renormalization group
calculation.Comment: 9 pages, 4 encapsulated Postscript figures, REVTeX 3.
Edge Logarithmic Corrections probed by Impurity NMR
Semi-infinite quantum spin chains display spin autocorrelations near the
boundary with power-law exponents that are given by boundary conformal field
theories. We show that NMR measurements on spinless impurities that break a
quantum spin chain lead to a spin-lattice relaxation rate 1/T_1^edge that has a
temperature dependence which is a direct probe of the anomalous boundary
exponents. For the antiferromagnetic S=1/2 spin chain, we show that 1/T_1^edge
behaves as T (log T)^2 instead of (log T)^1/2 for a bulk measurement. We show
that, in the case of a one-dimensional conductor described by a Luttinger
liquid, a similar measurement leads to a relaxation rate 1/T_1^{edge} behaving
as T, independent of the anomalous exponent K_rho.Comment: 4 pages, 1 encapsulated figure, corrected typo
Nuclear Spin Relaxation in Hole Doped Two-Leg Ladders
The nuclear spin-lattice relaxation rate () has been measured in the
single crystals of hole doped two-leg ladder compounds
SrCaCuO and in the undoped parent material
LaCaCuO. Comparison of at the Cu and the two
distinct oxygen sites revealed that the major spectral weight of low frequency
spin fluctuations is located near for most of the
temperature and doping ranges investigated. Remarkable difference in the
temperature dependence of for the two oxygen sites in the heavily doped
=12 sample revealed reduction of singlet correlations between two legs in
place of growing antiferromagnetic correlations along the leg direction with
increasing temperature. Such behavior is most likely caused by the dissociation
of bound hole pairs.Comment: 4 pages. to appear in J. Phys. Soc. Jpn. Vol. 6
Experimental Evidence of a Haldane Gap in an S = 2 Quasi-linear Chain Antiferromagnet
The magnetic susceptibility of the quasi-linear chain Heisenberg
antiferromagnet (2,-bipyridine)trichloromanganese(III), MnCl_{3}(bipy), has
been measured from 1.8 to 300 K with the magnetic field, H, parallel and
perpendicular to the chains. The analyzed data yield and K. The magnetization, M, has been studied at 30 mK and 1.4 K in H up to 16
T. No evidence of long-range order is observed. Depending on crystal
orientation, at 30 mK until a critical field is achieved ( and $H_{c\bot} = 1.8\pm 0.2 T), where M increases continuously
as H is increased. These results are interpreted as evidence of a Haldane gap.Comment: 11 pages, 4 figure
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
Spin-Wave Description of Haldane-gap antiferromagnets
Modifying the conventional antiferromagnetic spin-wave theory which is
plagued by the difficulty of the zero-field sublattice magnetizations diverging
in one dimension, we describe magnetic properties of Haldane-gap
antiferromagnets. The modified spin waves, constituting a grand canonical
bosonic ensemble so as to recover the sublattice symmetry, not only depict well
the ground-state correlations but also give useful information on the
finite-temperature properties.Comment: to be published in J. Phys. Soc. Jpn. Vol. 72, No. 4 (2003
Self-consistent interaction of magnons in a frustrated quantum antiferromagnet
We study the magnon interactions by means of a self-consistent calculation in a frustrated quantum Heisenberg antiferromagnet with first and second neighbor exchange couplings. We have used the Hartree-Fock-Bogolyubov scheme at zero temperature. The effect of frustration on the shape of the spin-wave spectrum is investigated: we find a strong decrease of the spin-wave velocity. A clear deviation from spin wave theory is found when comparing with finite lattice results