402 research outputs found
Path Integral Calculations of exchange in solid 4He
Recently there have been experimental indications that solid 4He might be a
supersolid. We discuss the relation of supersolid behavior to ring exchange.
The tunnelling frequencies for ring exchanges in quantum solids are calculated
using Path Integral Monte Carlo by finding the free energy for making a path
that begins with the atoms in one configuration and ends with a permutation of
those positions. We find that the exchange frequencies in solid 4He are
described by a simple lattice model which does not show supersolid behavior.
Thus, the PIMC calculations constrain the mechanism for the supersolid
behavior. We also look at the characteristics of very long exchanges needed for
macroscopic mass transport
quantum Heisenberg antiferromagnet on the triangular lattice: a group symmetry analysis of order by disorder
On the triangular lattice, for between and , the classical
Heisenberg model with first and second neighbor interactions presents
four-sublattice ordered ground-states. Spin-wave calculations of Chubukov and
Jolicoeur\cite{cj92} and Korshunov\cite{k93} suggest that quantum fluctuations
select amongst these states a colinear two-sublattice order. From theoretical
requirements, we develop the full symmetry analysis of the low lying levels of
the spin-1/2 Hamiltonian in the hypotheses of either a four or a two-sublattice
order. We show on the exact spectra of periodic samples ( and )
how quantum fluctuations select the colinear order from the four-sublattice
order.Comment: 15 pages, 4 figures (available upon request), Revte
Many-body wavefunctions for normal liquid He
We present new trial wave-functions which include 3-body correlations into
the backflow coordinates and a 4-body symmetric potential. We show that our
wavefunctions lower the energy enough to stabilize the ground state energies of
normal liquid He in the unpolarized state at all pressures in agreement
with experiment; however, quantitative discrepancies remain. Further, we
include strong spin coupling into the Fermi liquid by adapting pairing wave
functions. We demonstrate a new, numerically stable method to evaluate pairing
functions which is also useful for Path Integrals calculations at low, but
non-zero temperatures.Comment: 5 page
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
The Magnetic Ordering of the 3d Wigner Crystal
Using Path Integral Monte Carlo, we have calculated exchange frequencies as
electrons undergo ring exchanges of 2, 3 and 4 electrons in a ``clean'' 3d
Wigner crystal (bcc lattice) as a function of density. We find pair exchange
dominates and estimate the critical temperature for the transition to
antiferromagnetic ordering to be roughly Ry at melting. In
contrast to the situation in 2d, the 3d Wigner crystal is different from the
solid bcc 3He in that the pair exchange dominates because of the softer
interparticle potential. We discuss implications for the magnetic phase diagram
of the electron gas
Spin Liquid in the Multiple-Spin Exchange model on the Triangular lattice: 3He on graphite
Using exact diagonalizations, we investigate the T=0 phase diagram of the
Multi-Spin Exchange (MSE) model on the triangular lattice: we find a transition
separating a ferromagnetic phase from a non-magnetic gapped Spin Liquid phase.
Systems far enough from the ferromagnetic transition have a metamagnetic
behavior with magnetization plateaus at m/m_sat=0 and 1/2. The MSE has been
proposed to describe solid 3He films adsorbed onto graphite, thus we compute
the MSE heat capacity for parameters in the low density range of the 2nd layer
and find a double-peak structure.Comment: Revtex, 4 pages, 4 figures. Accepted to Phys. Rev. Let
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