5 research outputs found
The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory
We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.Comment: 10 pages, RevTeX + epsf, 5 figures Replaced with published version.
Comparison to series expansions energies include
Phase diagram for a class of spin-half Heisenberg models interpolating between the square-lattice, the triangular-lattice and the linear chain limits
We study the spin-half Heisenberg models on an anisotropic two-dimensional
lattice which interpolates between the square-lattice at one end, a set of
decoupled spin-chains on the other end, and the triangular-lattice Heisenberg
model in between. By series expansions around two different dimer ground states
and around various commensurate and incommensurate magnetically ordered states,
we establish the phase diagram for this model of a frustrated antiferromagnet.
We find a particularly rich phase diagram due to the interplay of magnetic
frustration, quantum fluctuations and varying dimensionality. There is a large
region of the usual 2-sublattice Ne\'el phase, a 3-sublattice phase for the
triangular-lattice model, a region of incommensurate magnetic order around the
triangular-lattice model, and regions in parameter space where there is no
magnetic order. We find that the incommensurate ordering wavevector is in
general altered from its classical value by quantum fluctuations. The regime of
weakly coupled chains is particularly interesting and appears to be nearly
critical.Comment: RevTeX, 15 figure
Cooperative Jahn-Teller effect in a 2D mesoscopic C
Fullerene molecules adsorbed on surfaces often show macroscopic average distortions. As
charged ions C60n- are known to be Jahn-Teller (JT)
active, it is suggested that these distortions could be a manifestation of cooperative JT
effects (CJTE) due to interactions between neighbouring fullerene ions. In order to
understand the distortion properties it is necessary to take correlations between
different distortions into account. However, this canât easily be done in the mean field
approximation usually used to describe the CJTE. We therefore propose an alternative
procedure to describe 2D mesoscopic islands of C60 ions in which a pseudo
vector spin \hbox{} is evoked to represent degenerate JT-distorted states when the quadratic
JT coupling is considered. This approach is analogous to methods used for 2D magnetic
systems. We then use the differential operator technique in effective field theory within
the Ising approach. We include the effects of weak surface interactions and dynamic motion
between equivalent distortions via terms equivalent to anisotropy and a transverse field
in magnetism respectively. For distortions to
D5d symmetry, we determine single site
correlations as a function of temperature, the macroscopic average distortion describing a
structural phase transition, and the isothermal response function. Phase diagrams are
presented for relevant cases of the system parameters