14 research outputs found
Spin stiffness in the frustrated Heisenberg antiferromagnet
We calculate the spin stiffness of the S= frustrated Heisenberg antiferromagnet directly from a general formula which is evaluated in the Schwinger-boson mean-field approximation. Both Néel and collinear ordering are considered. For collinear ordering, we take the anisotropy of this phase into account, unlike previous approaches. For Néel ordering, a detailed study is made of the finite-size scaling behavior of the two terms that make up the spin stiffness. The exponents of the scaling with the system size of the two terms comprising the spin stiffness turn out to be identical to those of the unfrustrated case.Theoretical Physic
Quantum coherent dynamics of molecules: A simple scenario for ultrafast photoisomerization
Theoretical Physic
Phase Diagram of the BCC S=1/2 Heisenberg Antiferromagnet with First and Second Neighbor Exchange
We use linked-cluster series expansions, both at T=0 and high temperature, to
analyse the phase structure of the spin-\half Heisenberg antiferromagnet with
competing first and second-neighbor interactions on the 3-dimensional
body-centred-cubic lattice. At zero temperature we find a first-order quantum
phase transition at between AF (Ne\'el)
and AF ordered phases. The high temperature series yield quite accurate
estimates of the bounding critical line for the AF phase, and an apparent
critical line for the AF phase, with a bicritical point at , . The possibility that this latter transition is
first-order cannot be excluded.Comment: 10 pages, 4 figure
Incorporation of Density Matrix Wavefunctions in Monte Carlo Simulations: Application to the Frustrated Heisenberg Model
We combine the Density Matrix Technique (DMRG) with Green Function Monte
Carlo (GFMC) simulations. The DMRG is most successful in 1-dimensional systems
and can only be extended to 2-dimensional systems for strips of limited width.
GFMC is not restricted to low dimensions but is limited by the efficiency of
the sampling. This limitation is crucial when the system exhibits a so-called
sign problem, which on the other hand is not a particular obstacle for the
DMRG. We show how to combine the virtues of both methods by using a DMRG
wavefunction as guiding wave function for the GFMC. This requires a special
representation of the DMRG wavefunction to make the simulations possible within
reasonable computational time. As a test case we apply the method to the
2-dimensional frustrated Heisenberg antiferromagnet. By supplementing the
branching in GFMC with Stochastic Reconfiguration (SR) we get a stable
simulation with a small variance also in the region where the fluctuations due
to minus sign problem are maximal. The sensitivity of the results to the choice
of the guiding wavefunction is extensively investigated. We analyse the model
as a function of the ratio of the next-nearest to nearest neighbor coupling
strength. We observe in the frustrated regime a pattern of the spin
correlations which is in-between dimerlike and plaquette type ordering, states
that have recently been suggested. It is a state with strong dimerization in
one direction and weaker dimerization in the perpendicular direction.Comment: slightly revised version with added reference
Phase transition in the transverse Ising model using the extended coupled-cluster method
The phase transition present in the linear-chain and square-lattice cases of
the transverse Ising model is examined. The extended coupled cluster method
(ECCM) can describe both sides of the phase transition with a unified approach.
The correlation length and the excitation energy are determined. We demonstrate
the ability of the ECCM to use both the weak- and the strong-coupling starting
state in a unified approach for the study of critical behavior.Comment: 10 pages, 7 eps-figure
New quantum phase transitions in the two-dimensional J1-J2 model
We analyze the phase diagram of the frustrated Heisenberg antiferromagnet,
the J1-J2 model, in two dimensions. Two quantum phase transitions in the model
are already known: the second order transition from the Neel state to the spin
liquid state at (J_2/J_1)_{c2}=0.38, and the first order transition from the
spin liquid state to the collinear state at (J_2/J_1)_{c4}=0.60. We have found
evidence for two new second order phase transitions: the transition from the
spin columnar dimerized state to the state with plaquette type modulation at
(J_2/J_1)_{c3}=0.50(2), and the transition from the simple Neel state to the
Neel state with spin columnar dimerization at (J_2/J_1)_{c1}=0.34(4). We also
present an independent calculation of (J_2/J_1)_{c2}=0.38 using a new approach.Comment: 3 pages, 5 figures; added referenc
Quantum disorder in the two-dimensional pyrochlore Heisenberg antiferromagnet
We present the results of an exact diagonalization study of the spin-1/2
Heisenberg antiferromagnet on a two-dimensional version of the pyrochlore
lattice, also known as the square lattice with crossings or the checkerboard
lattice. Examining the low energy spectra for systems of up to 24 spins, we
find that all clusters studied have non-degenerate ground states with total
spin zero, and big energy gaps to states with higher total spin. We also find a
large number of non-magnetic excitations at energies within this spin gap.
Spin-spin and spin-Peierls correlation functions appear to be short-ranged, and
we suggest that the ground state is a spin liquid.Comment: 7 pages, 11 figures, RevTeX minor changes made, Figure 6 correcte
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
Low-energy fixed points of random Heisenberg models
The effect of quenched disorder on the low-energy and low-temperature
properties of various two- and three-dimensional Heisenberg models is studied
by a numerical strong disorder renormalization group method. For strong enough
disorder we have identified two relevant fixed points, in which the gap
exponent, omega, describing the low-energy tail of the gap distribution,
P(Delta) ~ Delta^omega is independent of disorder, the strength of couplings
and the value of the spin. The dynamical behavior of non-frustrated random
antiferromagnetic models is controlled by a singlet-like fixed point, whereas
for frustrated models the fixed point corresponds to a large spin formation and
the gap exponent is given by omega ~ 0. Another type of universality classes is
observed at quantum critical points and in dimerized phases but no infinite
randomness behavior is found, in contrast to one-dimensional models.Comment: 11 pages RevTeX, eps-figs included, language revise