356 research outputs found

### Phase Diagram of the J1, J2, J3 Heisenberg Models on the Honeycomb Lattice: A Series Expansion Study

We study magnetically ordered phases and their phase boundaries in the
$J_1-J_2-J_3$ Heisenberg models on the honeycomb lattice using series
expansions around N\'eel and different colinear and non-colinear magnetic
states. An Ising anisotropy ($\lambda=J_{\perp}/J_z\ne 1$) is introduced and
ground state energy and magnetization order parameter are calculated as a power
seies expansion in $\lambda$. Series extrapolation methods are used to study
properties for the Heisenberg model ($\lambda=1$). We find that at large $J_3$
($>0.6$) there is a first order transition between N\'eel and columnar states,
in agreement with the classical answer. For $J_3=0$, we find that the N\'eel
phase extends beyond the region of classical stability. We also find that
spiral phases are stabilized over large parameter regions, although their
spiral angles can be substantially renormalized with respect to the classical
values. Our study also shows a magnetically disordered region at intermedaite
$J_2/J_1$ and $J_3/J_1$ values.Comment: 6 pages, 9 figure

### Ground state properties, excitation spectra and phase transitions in the $S=1/2$ and $S=3/2$ bilayer Heisenberg models on the honeycomb Lattice

Motivated by the observation of a disordered spin ground state in the $S=3/2$
material Bi$_3$Mn$_4$O$_{12}$NO$_3$, we study the ground state properties and
excitation spectra of the $S=3/2$ (and for comparison $S=1/2$) bilayer
Heisenberg model on the honeycomb lattice, with and without frustrating further
neighbor interactions. We use series expansions around the N\'eel state to
calculate properties of the magnetically ordered phase. Furthermore, series
expansions in $1/\lambda=J_1/J_{\perp}$, where $J_1$ is an in-plane exchange
constant and $J_\perp$ is the exchange constant between the layers are used to
study properties of the spin singlet phase. For the unfrustrated case, our
results for the phase transitions are in very good agreement with recent
Quantum Monte Carlo studies. We also obtain the excitation spectra in the
disordered phase and study the change in the critical $\lambda$ when
frustrating exchange interactions are added to the $S=3/2$ system and find a
rapid suppression of the ordered phase with frustration. Implications for the
material Bi$_3$Mn$_4$O$_{12}$NO$_3$ are discussed.Comment: 5 pages, 6 figure

### Discerning Incompressible and Compressible Phases of Cold Atoms in Optical Lattices

Experiments with cold atoms trapped in optical lattices offer the potential
to realize a variety of novel phases but suffer from severe spatial
inhomogeneity that can obscure signatures of new phases of matter and phase
boundaries. We use a high temperature series expansion to show that
compressibility in the core of a trapped Fermi-Hubbard system is related to
measurements of changes in double occupancy. This core compressibility filters
out edge effects, offering a direct probe of compressibility independent of
inhomogeneity. A comparison with experiments is made

### Universal Finite Temperature Properties of a Three Dimensional Quantum Antiferromagnet in the Vicinity of a Quantum Critical Point

We consider a 3-dimensional quantum antiferromagnet which can be driven
through a quantum critical point (QCP) by varying a tuning parameter g.
Starting from the magnetically ordered phase, the N{\'e}el temperature will
decrease to zero as the QCP is approached. From a generic quantum field theory,
together with numerical results from a specific microscopic Heisenberg spin
model, we demonstrate the existence of universal behaviour near the QCP. We
compare our results with available data for TlCuCl_

### Spin-waves in the $J_{1a}-J_{1b}-J_{2}$ orthorombic square-lattice Heisenberg models: Application to the iron pnictide materials

Motivated by the observation of spatially anisotropic exchange constants in
the iron pnictide materials, we study the spin-wave spectra of the
$J_{1a}-J_{1b}-J_{2}$ Heisenberg models on a square-lattice with nearest
neighbor exchange $J_{1a}$ along x and $J_{1b}$ along y axis and a second
neighbor exchange $J_2$. We focus on the regime, where the spins order at
($\pi,0$), and compute the spectra by systematic expansions around the Ising
limit. We study both spin-half and spin-one Heisenberg models as well as a
range of parameters to cover various cases proposed for the iron pnictide
materials. The low-energy spectra have anisotropic spin-wave velocities and are
renormalized with respect to linear spin-wave theory by up to 20 percent,
depending on parameters. Extreme anisotropy, consisting of a ferromagnetic
$J_{1b}=- |J_F|$, is best distinguished from a weak anisotropy ($J_{1a}\approx
J_{1b}=J_1$, $J_2>J_1/2$) by the nature of the spin-waves near the wavevectors
($0,\pi$) or ($\pi,\pi$). The reported spectra for the pnictide material
CaFe$_2$As$_2$ clearly imply such an extreme anisotropy.Comment: 6 pages, 10 figure

### Thermodynamics of strongly interacting fermions in two-dimensional optical lattices

We study finite-temperature properties of strongly correlated fermions in
two-dimensional optical lattices by means of numerical linked cluster
expansions, a computational technique that allows one to obtain exact results
in the thermodynamic limit. We focus our analysis on the strongly interacting
regime, where the on-site repulsion is of the order of or greater than the band
width. We compute the equation of state, double occupancy, entropy, uniform
susceptibility, and spin correlations for temperatures that are similar to or
below the ones achieved in current optical lattice experiments. We provide a
quantitative analysis of adiabatic cooling of trapped fermions in two
dimensions, by means of both flattening the trapping potential and increasing
the interaction strength.Comment: 7 pages, 7 figure

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