72 research outputs found
Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr
We have performed time-of-flight neutron scattering measurements on powder
samples of the unconventional spin-Peierls compound TiOBr using the
fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These
measurements reveal two branches of magnetic excitations within the
commensurate and incommensurate spin-Peierls phases, which we associate with n
= 1 and n = 2 triplet excitations out of the singlet ground state. These
measurements represent the first direct measure of the singlet-triplet energy
gap in TiOBr, which is determined to be Eg = 21.2 +/- 1.0 meV.Comment: 5 pages, 4 figures, submitted for publicatio
Zero-Point Motion of Liquid and Solid Hydrogen
We present an inelastic neutron scattering study of liquid and solid hydrogen
carried out using the wide Angular Range Chopper Spectrometer at Oak Ridge
National Laboratory. From the observed dynamic structure factor, we obtained
empirical estimates of the molecular mean-squared displacement and average
translational kinetic energy. We find that the former quantity increases with
temperature, indicating that a combination of thermal and quantum effects is
important near the liquid-solid phase transition, contrary to previous
measurements. We also find that the kinetic energy drops dramatically upon
melting of the crystals, a consequence of the large increase in molar volume
together with the Heisenberg indeterminacy principle. Our results are compared
with quantum Monte Carlo simulations based on different model potentials. In
general, there is good agreement between our findings and theoretical
predictions based on the Silvera-Goldman and Buck potentials.Comment: 20 pages, 10 figures in color, submitted to Phys. Rev.
High-Order Coupled Cluster Method Study of Frustrated and Unfrustrated Quantum Magnets in External Magnetic Fields
We apply the coupled cluster method (CCM) in order to study the ground-state
properties of the (unfrustrated) square-lattice and (frustrated)
triangular-lattice spin-half Heisenberg antiferromagnets in the presence of
external magnetic fields. Here we determine and solve the basic CCM equations
by using the localised approximation scheme commonly referred to as the
`LSUB' approximation scheme and we carry out high-order calculations by
using intensive computational methods. We calculate the ground-state energy,
the uniform susceptibility, the total (lattice) magnetisation and the local
(sublattice) magnetisations as a function of the magnetic field strength. Our
results for the lattice magnetisation of the square-lattice case compare well
to those results of QMC for all values of the applied external magnetic field.
We find a value for magnetic susceptibility of for the
square-lattice antiferromagnet, which is also in agreement with the results of
other approximate methods (e.g., via QMC). Our estimate for the
range of the extent of the () magnetisation plateau for the
triangular-lattice antiferromagnet is , which is in good
agreement with results of spin-wave theory () and
exact diagonalisations (). The CCM value for the
in-plane magnetic susceptibility per site is , which is below the
result of the spin-wave theory (evaluated to order 1/S) of .Comment: 30 pages, 13 figures, 1 Tabl
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