1,175 research outputs found
Dynamics of the quantum dimer model on the triangular lattice: Soft modes and local resonating valence-bond correlations
We report on an exhaustive investigation of the dynamical dimer-dimer
correlations in imaginary time for the quantum dimer model on the triangular
lattice using the Green's function Monte Carlo method. We show in particular
that soft modes develop upon reducing the dimer-dimer repulsion, indicating the
presence of a second-order phase transition into an ordered phase with broken
translational symmetry. We further investigate the nature of this ordered
phase, for which a 12-site unit cell has been previously proposed, with the
surprising result that significant Bragg peaks are only present at two of the
three high-symmetry points consistent with this unit cell. We attribute the
absence of a detectable peak to its small magnitude due to the nearly uniform
internal structure of the 12-site crystal cell.Comment: 6 pages, 8 figure
The quadrupolar phases of the S=1 bilinear-biquadratic Heisenberg model on the triangular lattice
Using mean-field theory, exact diagonalizations and SU(3) flavour theory, we
have precisely mapped out the phase diagram of the S=1 bilinear-biquadratic
Heisenberg model on the triangular lattice in a magnetic field, with emphasis
on the quadrupolar phases and their excitations. In particular, we show that
ferroquadrupolar order can coexist with short-range helical magnetic order, and
that the antiferroquadrupolar phase is characterized by a remarkable 2/3
magnetization plateau, in which one site per triangle retains quadrupolar order
while the other two are polarized along the field. Implications for actual S=1
magnets are discussed.Comment: 4 pages, 5 figures, published versio
New possibility of the ground state of quarter-filled one-dimensional strongly correlated electronic system interacting with localized spins
We study numerically the ground state properties of the one-dimensional
quarter-filled strongly correlated electronic system interacting
antiferromagnetically with localized spins. It is shown that the
charge-ordered state is significantly stabilized by the introduction of
relatively small coupling with the localized spins. When the coupling becomes
large the spin and charge degrees of freedom behave quite independently and the
ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with
charge order is seen under strong electronic interaction. Our results suggest
that such charge order can be easily controlled by the magnetic field, which
possibly give rise to the giant negative magnetoresistance, and its relation to
phthalocyanine compounds is discussed.Comment: 5pages, 4figure
Dzyaloshinskii-Moriya anisotropy and non-magnetic impurities in the kagome system ZnCu_3(OH)_6Cl_2
Motivated by recent nuclear magnetic resonance experiments on
ZnCu(OH)Cl, we present an exact-diagonalization study of the
combined effects of non-magnetic impurities and Dzyaloshinskii-Moriya (DM)
interactions in the kagome antiferromagnet. The local response to an
applied field and correlation-matrix data reveal that the dimer freezing which
occurs around each impurity for persists at least up to , where and denote respectively the exchange and DM interaction
energies. The phase transition to the () semiclassical, 120
state favored at large takes place at . However, the dimers
next to the impurity sites remain strong up to values , far above
this critical point, and thus do not participate fully in the ordered state. We
discuss the implications of our results for experiments on
ZnCu(OH)Cl.Comment: 11 pages, submitted to PR
Three-sublattice ordering of the SU(3) Heisenberg model of three-flavor fermions on the square and cubic lattices
Combining a semi-classical analysis with exact diagonalizations, we show that
the ground state of the SU(3) Heisenberg model on the square lattice develops
three-sublattice long-range order. This surprising pattern for a bipartite
lattice with only nearest-neighbor interactions is shown to be the consequence
of a subtle quantum order-by-disorder mechanism. By contrast, thermal
fluctuations favor two-sublattice configurations via entropic selection. These
results are shown to extend to the cubic lattice, and experimental implications
for the Mott-insulating states of three-flavor fermionic atoms in optical
lattices are discussed.Comment: 4 pages, 3 figures, minor changes, references adde
Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2
Low energy Raman scattering of the s=1/2 spin tetrahedra system
Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an
energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated
temperatures this mode shows a soft mode-like decrease in energy pointing to an
instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2
with a presumably larger inter-tetrahedra coupling does not show such a low
energy mode. Instead its excitation spectrum and thermodynamic properties are
compatible with long range Neel-ordering. We discuss the observed effects in
the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated
Electron
Dynamical structure factors and excitation modes of the bilayer Heisenberg model
Using quantum Monte Carlo simulations along with higher-order spin-wave
theory, bond-operator and strong-coupling expansions, we analyse the dynamical
spin structure factor of the spin-half Heisenberg model on the square-lattice
bilayer. We identify distinct contributions from the low-energy Goldstone modes
in the magnetically ordered phase and the gapped triplon modes in the quantum
disordered phase. In the antisymmetric (with respect to layer inversion)
channel, the dynamical spin structure factor exhibits a continuous evolution of
spectral features across the quantum phase transition, connecting the two types
of modes. Instead, in the symmetric channel we find a depletion of the spectral
weight when moving from the ordered to the disordered phase. While the
dynamical spin structure factor does not exhibit a well-defined distinct
contribution from the amplitude (or Higgs) mode in the ordered phase, we
identify an only marginally-damped amplitude mode in the dynamical singlet
structure factor, obtained from interlayer bond correlations, in the vicinity
of the quantum critical point. These findings provide quantitative information
in direct relation to possible neutron or light scattering experiments in a
fundamental two-dimensional quantum-critical spin system.Comment: 19 pages, 15 figure
Neutron spectrometer for fast nuclear reactors
In this paper we describe the development and first tests of a neutron
spectrometer designed for high flux environments, such as the ones found in
fast nuclear reactors. The spectrometer is based on the conversion of neutrons
impinging on Li into and whose total energy comprises the
initial neutron energy and the reaction -value. The LiF layer is
sandwiched between two CVD diamond detectors, which measure the two reaction
products in coincidence. The spectrometer was calibrated at two neutron
energies in well known thermal and 3 MeV neutron fluxes. The measured neutron
detection efficiency varies from 4.2 to 3.5 for
thermal and 3 MeV neutrons, respectively. These values are in agreement with
Geant4 simulations and close to simple estimates based on the knowledge of the
Li(n,) cross section. The energy resolution of the spectrometer
was found to be better than 100 keV when using 5 m cables between the detector
and the preamplifiers.Comment: submitted to NI
Doping quantum dimer models on the square lattice
A family of models is proposed to describe the motion of holes in a
fluctuating quantum dimer background on the square lattice. Following
Castelnovo et al. [Ann. Phys. (NY) 318, 316 (2005)], a generalized
Rokhsar-Kivelson Hamiltonian at **finite doping** which can be mapped on a
**doped** interacting classical dimer model is constructed. A simple physical
extension of this model is also considered. Using numerical computations and
simple considerations based on the above exact mapping, we determine the phase
diagram of the model showing a number of quantum phases typical of a doped Mott
insulator. The two-hole correlation function generically exhibits short-range
or long-range algebraic correlations in the solid (columnar) and liquid
(critical) phases of the model, respectively. Evidence for an extended region
of a doped VBS phase exhibiting holon pairing but **no** phase separation is
given. In contrast, we show that hole deconfinement occurs in the staggered
dimer phase.Comment: 5 page
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