1,350 research outputs found
Critical phenomena and quantum phase transition in long range Heisenberg antiferromagnetic chains
Antiferromagnetic Hamiltonians with short-range, non-frustrating interactions
are well-known to exhibit long range magnetic order in dimensions,
but exhibit only quasi long range order, with power law decay of correlations,
in d=1 (for half-integer spin). On the other hand, non-frustrating long range
interactions can induce long range order in d=1. We study Hamiltonians in which
the long range interactions have an adjustable amplitude lambda, as well as an
adjustable power-law , using a combination of quantum Monte Carlo
and analytic methods: spin-wave, large-N non-linear sigma model, and
renormalization group methods. We map out the phase diagram in the lambda-alpha
plane and study the nature of the critical line separating the phases with long
range and quasi long range order. We find that this corresponds to a novel line
of critical points with continuously varying critical exponents and a dynamical
exponent, z<1.Comment: 27 pages, 12 figures. RG flow added. Final version to appear in JSTA
Response of finite spin-S Heisenberg chains to local perturbations
We consider the properties of finite isotropic antiferromagnetic Heisenberg
chains with S=1/2, 1, 3/2 spins when a weak magnetic field is applied on a few
sites, using White's density matrix renormalization group (DMRG) method. For
the S=1 chain there exists only one length scale in the system which determines
the behavior of the one- and two-point correlation functions both around the
local perturbation and near the free boundary. For the critical,
half-odd-integer spin cases the exponent of the spin-spin correlation function
was found to be , and the exponent of the decay of the site
magnetization around the perturbed site is . Close to a free
boundary, however, the behavior is completely different for S=1/2 and .Comment: 13 pages, 7 figure
S(k) for Haldane Gap Antiferromagnets: Large-scale Numerical Results vs. Field Theory and Experiment
The structure function, S(k), for the s=1, Haldane gap antiferromagnetic
chain, is measured accurately using the recent density matrix renormalization
group method, with chain-length 100. Excellent agreement with the nonlinear
model prediction is obtained, both at where a single
magnon process dominates and at where a two magnon process
dominates. We repeat our calculation with crystal field anisotropy chosen to
model NENP, obtaining good agreement with both field theory predictions and
recent experiments. Correlation lengths, gaps and velocities are determined for
both polarizations.Comment: 11 pages, 3 postscript figures included, REVTEX 3.0, UBCTP-93-02
Impurities in Heisenberg Antiferromagnets
The Heisenberg Antiferromagnet is studied in the presence of two kinds
of local impurities. First, a perturbed antiferromagnetic bond with
at the center of an even-length open chain is considered. Using the density
matrix renormalization group method we find that, for sufficiently strong or
weak , a bound state is localized at the impurity site, giving rise to an
energy level in the Haldane gap. The energy of the bound state is in agreement
with perturbative results, based on chain-end excitations, both in the
weak and strong coupling limit. In a region around the uniform limit, ,
no states are found with energy below the Haldane gap. Secondly, a
impurity at the center of an otherwise even-length open chain is considered.
The coupling to the impurity is varied. Bound states in the Haldane gap
are found {\it only} for sufficiently weak (antiferromagnetic) coupling. For a
impurity coupled with a strong (antiferromagnetic) bond, {\it no}
states are found in the Haldane. Our results are in good qualitative agreement
with recent experiments on doped NENP and YBaNiO.Comment: 29 pages, RevTeX 3.0, 12 uuencoded postscript figures include
Solution of two channel spin-flavor Kondo model
We investigate a model where an impurity couples to both the spin and the
flavor currents of the two channel conduction electrons. This model can be used
as a prototype model of a magnetic impurity tunneling between two sites in a
metal and of some heavy fermion systems where the ground state of the impurity
has a fourfold degeneracy. The system is shown to flow to a doubly degenerate
non fermi-liquid(NFL) fixed point; the thermodynamic quantities show NFL
behaviors, but the transport quantities show fermi liquid (FL) behaviors . A
spin-flavor coupling double tensor term is shown to drive the system to one of
the two singlet FL fixed points. The relation with SU(4) Coqblin-Schrieffer
model is studied. The implications on the possible experiments are given.Comment: 11 pages, REVTEX, no figures. To appear in Phys. Rev. B (Rapid Comm.)
July 1, 199
Critical Nature of Non-Fermi Liquid in Spin 3/2 Multipolar Kondo Model
A multipolar Kondo model of an impurity spin S_I=3/2 interacting with
conduction electrons with spin s_c=3/2 is investigated using boundary conformal
field theory. A two-channel Kondo (2CK) -like non-Fermi liquid (NFL) under the
particle-hole symmetry is derived explicitly using a ``superspin absorption''
in the sector of a hidden symmetry, SO(5). We discuss the difference between
the usual spin-1/2 2CK NFL fixed point and the present one. In particular, we
find that, unlike the usual 2CK model, the low temperature impurity specific
heat is proportional to temperature.Comment: 4 pages, 2 figure
Numerical Study of the Antiferrromagnetic Spin Chain with Bond Alternation
We study the quantum spin chain with bond alternation {\cal H}=\sum _i
(1-(-1)^i\delta)\vect{S}_i\cdot \vect{S}_{i+1} by the density matrix
renormalization group method recently proposed by Steven R. White
(\PRL{69}{3844}{1993}). We find a massless point at .
We also find the edge states in the region under the open
boundary condition, which disappear in the region . At the
massless point, the spin wave velocity is and the central
charge is . Our results indicate that a continuous phase
transition occurs at the massless point accompanying
breaking of the hidden symmetry.Comment: 9 pages and 1 PostScript figure, Revtex 3.0 (Minor corrections in
TEX-file format to remove possible compilatory troubles.
Three-leg Antiferromagnetic Heisenberg Ladder with Frustrated Boundary Condition; Ground State Properties
The antiferromagnetic Heisenberg spin systems on the three-leg ladder are
investigated. Periodic boundary condition is imposed in the rung direction. The
system has an excitation gap for all antiferromagnetic inter-chain coupling
(). The estimated gap for the strong coupling limit
() is 0.28. Although the interaction is
homogeneous and only nearest-neighbor, the ground states of the system are
dimerized and break the translational symmetry in the thermodynamic limit.
Introducing the next-nearest neighbor coupling (), we can see that the
system is solved exactly. The ground state wave function is completely
dimer-ordered. Using density matrix renomalization group algorithm, we show
numerically that the original model () has the same nature with the
exactly solvable model. The ground state properties of the ladder with a higher
odd number of legs are also discussed.Comment: 15 pages, LaTeX, to be published in J.Phys.Soc.Jpn. Vol. 66 No. 1
Chain-Boundary Excitations in the Haldane Phase of 1D Systems
The chain-boundary excitations occurring in the Haldane phaseof
antiferromagnetic spin chains are investigated. The bilinear-biquadratic
hamiltonian is used to study these excitations as a function of the strength of
the biquadratic term, , between . At the AKLT point,
, we show explicitly that these excitations are localized at the
boundaries of the chain on a length scale equal to the correlation length
, and that the on-site magnetization for the first site is
. Applying the density matrixrenormalization group we show that
the chain-boundaryexcitations remain localized at the boundaries for
. As the two critical points are approached the
size of the objects diverges and their amplitude vanishes.Comment: 4 Pages, 4 eps figures. Uses RevTeX 3.0. Submitted to PR
On the Field-Induced Gap in Cu Benzoate and Other S=1/2 Antiferromagnets
Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu
benzoate, discovered an unexpected gap scaling as approximately the 2/3 power
of an applied magnetic field. A theory of this gap, based on an effective
staggered field, orthogonal to the applied uniform field, resulting from a
staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading
to a sine-Gordon quantum field theory, has been developed. Here we discuss many
aspects of this subject in considerable detail, including a review of the S=1/2
chain in a uniform field, a spin-wave theory analysis of the uniform plus
staggered field problem, exact amplitudes for the scaling of gap, staggered
susceptibility and staggered magnetization with field or temperature,
intensities of soliton and breather peaks in the structure function and field
and temperature dependence of the total susceptibility.Comment: 34 pages, 13 postscript figures, Rev Tex. Phys. Rev. B, to appea
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