16 research outputs found
Phase separation in the 2D Hubbard model: a fixed-node quantum Monte Carlo study
Fixed-node Green's function Monte Carlo calculations have been performed for
very large 16x6 2D Hubbard lattices, large interaction strengths U=10,20, and
40, and many (15-20) densities between empty and half filling. The nodes were
fixed by a simple Slater-Gutzwiller trial wavefunction. For each value of U we
obtained a sequence of ground-state energies which is consistent with the
possibility of a phase separation close to half-filling, with a hole density in
the hole-rich phase which is a decreasing function of U. The energies suffer,
however, from a fixed-node bias: more accurate nodes are needed to confirm this
picture. Our extensive numerical results and their test against size, shell,
shape and boundary condition effects also suggest that phase separation is
quite a delicate issue, on which simulations based on smaller lattices than
considered here are unlikely to give reliable predictions.Comment: 4 pages, 1 figure; revised version, more data point
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
We consider a system of coupled classical harmonic oscillators with spatially
fluctuating nearest-neighbor force constants on a simple cubic lattice. The
model is solved both by numerically diagonalizing the Hamiltonian and by
applying the single-bond coherent potential approximation. The results for the
density of states are in excellent agreement with each other. As
the degree of disorder is increased the system becomes unstable due to the
presence of negative force constants. If the system is near the borderline of
stability a low-frequency peak appears in the reduced density of states
as a precursor of the instability. We argue that this peak
is the analogon of the "boson peak", observed in structural glasses. By means
of the level distance statistics we show that the peak is not associated with
localized states
DYNAMICS OF RANDOMLY DILUTED ANTIFERROMAGNETS
The fracton scaling model, and the efective-medium-approximation are used to describe the excitation spectrum of randomly diluted antiferromagnets. The calculated scattering structure factor exhibits a crossover from extended magnons to localized fractons. These calculations are in agreement with experimental data of inelastic-neutron-scattering off spin-waves in MnxZn1-xF2
Effect of the crystal electric field on the Kondo-type fluctuations in YbAuCu<sub>4</sub>
High energy resolution inelastic neutron scattering experiment on magnetic fracton dispersion in near-percolating three-dimensional Heisenberg antiferromagnet, RbMn<sub>0.4</sub>Mg<sub>0.6</sub>F<sub>3</sub>
Magnetic state of Yb in Kondo-lattice YbNi<sub>2</sub>B<sub>2</sub>C
We report neutron scattering experiments performed to investigate the dynamic
magnetic properties of the Kondo-lattice compound YbNi2B2C. The spectrum of
magnetic excitations is found to be broad, extending up to at least 150 meV,
and contains inelastic peaks centred near 18 meV and 43 meV. At low energies we
observe quasielastic scattering with a width Gamma = 2.1 meV. The results
suggest a Yb3+ ground state with predominantly localized 4f electrons subject
to (i) a crystalline electric field (CEF) potential, and (ii) a Kondo
interaction, which at low temperatures is about an order of magnitude smaller
than the CEF interaction. From an analysis of the dynamic magnetic response we
conclude that the crystalline electric field acting on the Yb ions has a
similar anisotropy to that in other RNi2B2C compounds, but is uniformly
enhanced by almost a factor of 2. The static and dynamic magnetic properties of
YbNi2B2C are found to be reconciled quite well by means of an approximation
scheme to the Anderson impurity model, and this procedure also indicates that
the effective Kondo interaction varies with temperature due to the crystal
field splitting. We discuss the nature of the correlated-electron ground state
of YbNi2B2C based on these and other experimental results, and suggest that
this compound might be close to a quantum critical point on the non-magnetic
side.Comment: Revised version to be published in Phys Rev