4,572 research outputs found
The two dimensional Antiferromagnetic Heisenberg model with next nearest neighbour Ising exchange
We have considered the antiferromagnetic Heisenberg model in two
dimensions, with an additional Ising \nnn interaction. Antiferromagnetic \nnn
interactions will lead to frustration, and the system responds with flipping
the spins down in the plane. For large next nearest neighbour coupling the
system will order in a striped phase along the z axis, this phase is reached
through a first order transition. We have considered two generalizations of
this model, one with random \nnn interactions, and one with an enlarged unit
cell, where only half of the atoms have \nnn interactions. In both cases the
transition is softened to a second order transition separating two ordered
states. In the latter case we have estimated the quantum critical exponent
. These two cases then represent candidate examples of
deconfined quantum criticality.Comment: Extensive revisions. Two new models with contious quantum phase
transitio
Spin dynamics across the superfluid-insulator transition of spinful bosons
Bosons with non-zero spin exhibit a rich variety of superfluid and insulating
phases. Most phases support coherent spin oscillations, which have been the
focus of numerous recent experiments. These spin oscillations are Rabi
oscillations between discrete levels deep in the insulator, while deep in the
superfluid they can be oscillations in the orientation of a spinful condensate.
We describe the evolution of spin oscillations across the superfluid-insulator
quantum phase transition. For transitions with an order parameter carrying
spin, the damping of such oscillations is determined by the scaling dimension
of the composite spin operator. For transitions with a spinless order parameter
and gapped spin excitations, we demonstrate that the damping is determined by
an associated quantum impurity problem of a localized spin excitation
interacting with the bulk critical modes. We present a renormalization group
analysis of the quantum impurity problem, and discuss the relationship of our
results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion
of fixed points in Section V
Metallic spin glasses
Recent work on the zero temperature phases and phase transitions of strongly
random electronic system is reviewed. The transition between the spin glass and
quantum paramagnet is examined, for both metallic and insulating systems.
Insight gained from the solution of infinite range models leads to a quantum
field theory for the transition between a metallic quantum paramagnetic and a
metallic spin glass. The finite temperature phase diagram is described and
crossover functions are computed in mean field theory. A study of fluctuations
about mean field leads to the formulation of scaling hypotheses.Comment: Contribution to the Proceedings of the ITP Santa Barbara conference
on Non-Fermi liquids, 25 pages, requires IOP style file
Interface ordering and phase competition in a model Mott-insulator--band-insulator heterostructure
The phase diagram of model Mott-insulator--band-insulator heterostructures is
studied using the semiclassical approximation to the dynamical-mean-field
method as a function of thickness, coupling constant, and charge confinement.
An interface-stabilized ferromagnetic phase is found, allow the study of its
competition and possible coexistence with the antiferromagnetic order
characteristic of the bulk Mott insulator.Comment: 5 pages, 3 figures, manuscript revised, results unchange
Engineering correlation and entanglement dynamics in spin systems
We show that the correlation and entanglement dynamics of spin systems can be
understood in terms of propagation of spin waves. This gives a simple, physical
explanation of the behaviour seen in a number of recent works, in which a
localised, low-energy excitation is created and allowed to evolve. But it also
extends to the scenario of translationally invariant systems in states far from
equilibrium, which require less local control to prepare. Spin-wave evolution
is completely determined by the system's dispersion relation, and the latter
typically depends on a small number of external, physical parameters.
Therefore, this new insight into correlation dynamics opens up the possibility
not only of predicting but also of controlling the propagation velocity and
dispersion rate, by manipulating these parameters. We demonstrate this
analytically in a simple, example system.Comment: 4 pages, 4 figures, REVTeX4 forma
Specific heat of the S=1/2 Heisenberg model on the kagome lattice: high-temperature series expansion analysis
We compute specific heat of the antiferromagnetic spin-1/2 Heisenberg model
on the kagome lattice. We use a recently introduced technique to analyze
high-temperature series expansion based on the knowledge of high-temperature
series expansions, the total entropy of the system and the low-temperature
expected behavior of the specific heat as well as the ground-state energy. In
the case of kagome-lattice antiferromagnet, this method predicts a
low-temperature peak at T/J<0.1.Comment: 6 pages, 5 color figures (.eps), Revtex 4. Change in version 3: Fig.
5 has been corrected (it now shows data for 3 different ground-state
energies). The text is unchanged. v4: corrected an error in the temperature
scale of Fig. 5. (text unchanged
Solving the puzzle of an unconventional phase transition for a 2d dimerized quantum Heisenberg model
Motivated by the indication of a new critical theory for the spin-1/2
Heisenberg model with a spatially staggered anisotropy on the square lattice as
suggested in \cite{Wenzel08}, we re-investigate the phase transition of this
model induced by dimerization using first principle Monte Carlo simulations. We
focus on studying the finite-size scaling of and ,
where stands for the spatial box size used in the simulations and
with is the spin-stiffness in the -direction.
Remarkably, while we do observe a large correction to scaling for the
observable as proposed in \cite{Fritz11}, the data for
exhibit a good scaling behavior without any indication of a large
correction. As a consequence, we are able to obtain a numerical value for the
critical exponent which is consistent with the known O(3) result with
moderate computational effort. Specifically, the numerical value of we
determine by fitting the data points of to their expected scaling
form is given by , which agrees quantitatively with the most
accurate known Monte Carlo O(3) result . Finally, while we can
also obtain a result of from the observable second Binder ratio
which is consistent with , the uncertainty of calculated
from is more than twice as large as that of determined from
.Comment: 7 figures, 1 table; brief repor
Numerical evidence for the spin-Peierls state in the frustrated quantum antiferromagnet
We study the spin- Heisenberg antiferromagnet with an
antiferromagnetic (third nearest neighbor) interaction on a square
lattice. We numerically diagonalize this ``-'' model on clusters up
to 32-sites and search for novel ground state properties as the frustration
parameter changes. For ``larger'' we find enhancement of
incommensurate spin order, in agreement with spin-wave, large- expansions,
and other predictions. But for intermediate , the low lying excitation
energy spectrum suggests that this incommensurate order is short-range. In the
same region, the first excited state has the symmetries of the columnar dimer
(spin-Peierls) state. The columnar dimer order parameter suggests the presence
of long-range columnar dimer order. Hence, this spin-Peierls state is the best
candidate for the ground state of the - model in an intermediate
region.Comment: RevTeX file with five postscript figures uuencode
Singular order parameter interaction at nematic quantum critical point in two dimensional electron systems
We analyze the infrared behavior of effective N-point interactions between
order parameter fluctuations for nematic and other quantum critical electron
systems with a scalar order parameter in two dimensions. The interactions
exhibit a singular momentum and energy dependence and thus cannot be
represented by local vertices. They diverge for all N greater or equal 4 in a
collinear static limit, where energy variables scale to zero faster than
momenta, and momenta become increasingly collinear. The degree of divergence is
not reduced by any cancellations and renders all N-point interactions marginal.
A truncation of the order parameter action at quartic or any other finite order
is therefore not justified. The same conclusion can be drawn for the effective
action describing fermions coupled to a U(1) gauge field in two dimensions.Comment: 18 pages, 1 figur
- …