3,847 research outputs found
The spin-wave spectrum of the Jahn-Teller system LaTiO3
We present an analytical calculation of the spin-wave spectrum of the
Jahn-Teller system LaTiO3. The calculation includes all superexchange couplings
between nearest-neighbor Ti ions allowed by the space-group symmetries: The
isotropic Heisenberg couplings and the antisymmetric (Dzyaloshinskii-Moriya)
and symmetric anisotropies. The calculated spin-wave dispersion has four
branches, two nearly degenerate branches with small zone-center gaps and two
practically indistinguishable high-energy branches having large zone-center
gaps. The two lower-energy modes are found to be in satisfying agreement with
neutron-scattering experiments. In particular, the experimentally detected
approximate isotropy in the Brillouin zone and the small zone-center gap are
well reproduced by the calculations. The higher-energy branches have not been
detected yet by neutron scattering but their zone-center gaps are in satisfying
agreement with recent Raman data.Comment: 13 pages, 5 figure
Orbital selective insulator-metal transition in V2O3 under external pressure
We present a detailed account of the physics of Vanadium sesquioxide (), a benchmark system for studying correlation induced metal-insulator
transition(s). Based on a detailed perusal of a wide range of experimental
data, we stress the importance of multi-orbital Coulomb interactions in concert
with first-principles LDA bandstructure for a consistent understanding of the
PI-PM MIT under pressure. Using LDA+DMFT, we show how the MIT is of the orbital
selective type, driven by large changes in dynamical spectral weight in
response to small changes in trigonal field splitting under pressure. Very good
quantitative agreement with () the switch of orbital occupation and ()
S=1 at each site across the MIT, and () carrier effective mass in
the PM phase, is obtained. Finally, using the LDA+DMFT solution, we have
estimated screening induced renormalisation of the local, multi-orbital Coulomb
interactions. Computation of the one-particle spectral function using these
screened values is shown to be in excellent quantitative agreement with very
recent experimental (PES and XAS) results. These findings provide strong
support for an orbital-selective Mott transition in paramagnetic .Comment: 12 pages, 7 figure
Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores
We present a correlated {\it ab initio} description of the paramagnetic phase
of TlMnO, employing a combined local density approximation (LDA)
with multiorbital dynamical mean field theory (DMFT) treatment. We show that
the insulating state observed in this colossal magnetoresistance (CMR)
pyrochlore is determined by strong Mn intra- and inter-orbital local
electron-electron interactions. Hybridization effects are reinforced by the
correlation-induced spectral weight transfer. Our result coincides with optical
conductivity measurements, whose low energy features are remarkably accounted
for by our theory. Based on this agreement, we study the disorder-driven
insulator-metal transition of doped compounds, showing the proximity of
TlMnO to quantum phase transitions, in agreement with recent
measurements.Comment: 4 pages, 4 figure
Bound States for a Magnetic Impurity in a Superconductor
We discuss a solvable model describing an Anderson like impurity in a BCS
superconductor. The model can be mapped onto an Ising field theory in a
boundary magnetic field, with the Ising fermions being the quasi-particles of
the Bogoliubov transformation in BCS theory. The reflection S-matrix exhibits
Andreev scattering, and the existence of bound states of the quasi-particles
with the impurity lying inside the superconducting gap.Comment: 7 pages, Plain Te
Hole dynamics in generalized spin backgrounds in infinite dimensions
We calculate the dynamical behaviour of a hole in various spin backgrounds in
infinite dimensions, where it can be determined exactly. We consider hypercubic
lattices with two different types of spin backgrounds. On one hand we study an
ensemble of spin configurations with an arbitrary spin probability on each
sublattice. This model corresponds to a thermal average over all spin
configurations in the presence of staggered or uniform magnetic fields. On the
other hand we consider a definite spin state characterized by the angle between
the spins on different sublattices, i.e a classical spin system in an external
magnetic field. When spin fluctuations are considered, this model describes the
physics of unpaired particles in strong coupling superconductors.Comment: Accepted in Phys. Rev. B. 18 pages of text (1 fig. included) in Latex
+ 2 figures in uuencoded form containing the 2 postscripts (mailed
separately
Transition from overscreening to underscreening in the multichannel Kondo model: exact solution at large N
A novel large-N limit of the multichannel Kondo model is introduced, for
representations of the impurity spin described by Schwinger bosons. Three cases
are found, associated with underscreening, overscreening and exact Kondo
screening of the impurity. The saddle-point equations derived in this limit are
reminiscent of the ``non-crossing approximation'', but preserve the
Fermi-liquid nature of the model in the exactly screened case. Several physical
quantities are computed, both numerically, and analytically in the low-\omega,T
limit, and compared to other approaches.Comment: 4 pages, RevTeX3.0, 2 EPS figures. Published versio
Band-Insulator-Metal-Mott-Insulator transition in the half--filled ionic-Hubbard chain
We investigate the ground state phase diagram of the half-filled
repulsive Hubbard model in the presence of a staggered ionic
potential , using the continuum-limit bosonization approach. We find,
that with increasing on-site-repulsion , depending on the value of the
next-nearest-hopping amplitude , the model shows three different
versions of the ground state phase diagram. For , the ground state phase diagram consists of the following
three insulating phases: Band-Insulator at , Ferroelectric Insulator
at . For
there is only one transition from a spin gapped
metallic phase at .
Finally, for intermediate values of the next-nearest-hopping amplitude
we find that with increasing
on-site repulsion, at the model undergoes a second-order
commensurate-incommensurate type transition from a band insulator into a
metallic state and at larger there is a Kosterlitz-Thouless type
transition from a metal into a ferroelectric insulator.Comment: 9 pages 3 figure
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
On the heavy-fermion behavior of the pyrochlore transition-metal oxide
Motivated by the heavy fermion Fermi liquid (HFFL) features observed at
low- in the pyrochlore , we consider a material-specific model
that includes aspects of the local quantum chemistry, the geometrically
frustrated lattice structure, and strong correlations in a {\it single}
approach. In particular, we show how geometrical frustration (GF) gives rise to
a crossover scale, , the intersite (AF) exchange, below which the
metallic system shows HFFL features. Our scenario is a specific realization of
the importance of GF effects in driving HFFL behavior in , and
provides a natural understanding of various puzzling features observed
experimentally.Comment: 4 pages, 3 figure
Low-energy excitations in the three-dimensional random-field Ising model
The random-field Ising model (RFIM), one of the basic models for quenched
disorder, can be studied numerically with the help of efficient ground-state
algorithms. In this study, we extend these algorithm by various methods in
order to analyze low-energy excitations for the three-dimensional RFIM with
Gaussian distributed disorder that appear in the form of clusters of connected
spins. We analyze several properties of these clusters. Our results support the
validity of the droplet-model description for the RFIM.Comment: 10 pages, 9 figure
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