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 (${\rm V_2O_3}$), 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 ($i$) the switch of orbital occupation and ($ii$) S=1 at each $V^{3+}$ site across the MIT, and ($iii$) 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 ${\rm V_2O_3}$.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 Tl$_2$Mn$_2$O$_7$, 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 Tl$_2$Mn$_2$O$_7$ 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 t−t′t-t^{\prime} ionic-Hubbard chain

We investigate the ground state phase diagram of the half-filled $t-t^{\prime}$ repulsive Hubbard model in the presence of a staggered ionic potential $\Delta$, using the continuum-limit bosonization approach. We find, that with increasing on-site-repulsion $U$, depending on the value of the next-nearest-hopping amplitude $t^{\prime}$, the model shows three different versions of the ground state phase diagram. For $t^{\prime} < t^{\prime}_{\ast}$, the ground state phase diagram consists of the following three insulating phases: Band-Insulator at $U<U_{c}$, Ferroelectric Insulator at $U_{c} U_{c}$. For $t^{\prime} > t^{\prime}_{c}$ there is only one transition from a spin gapped metallic phase at $U U_{c}$. Finally, for intermediate values of the next-nearest-hopping amplitude $t^{\prime}_{\ast} < t^{\prime} < t^{\prime}_{c}$ we find that with increasing on-site repulsion, at $U_{c1}$ the model undergoes a second-order commensurate-incommensurate type transition from a band insulator into a metallic state and at larger $U_{c2}$ 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. $T_m$ 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 LiV2O4LiV_{2}O_{4}

Motivated by the heavy fermion Fermi liquid (HFFL) features observed at low-$T$ in the pyrochlore $LiV_{2}O_{4}$, 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, $T^{*}<<J$, 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 $LiV_{2}O_{4}$, 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