Mott transition and suppression of orbital fluctuations in orthorhombic 3d1 perovskites

Using t(2g) Wannier functions, a low-energy Hamiltonian is derived for orthorhombic 3d(1) transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for noncubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO3-type distortions) is found to suppress orbital fluctuations in LaTiO3 and even more in YTiO3, and to favor the transition to the insulating state.     

Domain formation and orbital ordering transition in a doped Jahn-Teller insulator

The ground state of a double-exchange model for orbitally degenerate e(g) electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real-space Monte Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low-doping phase diagram of La1 - xCaxMnO3.     

Persistence of Jahn-Teller distortion up to the insulator to metal transition in LaMnO3

High pressure, low temperature Raman measurements performed on LaMnO3 up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that LaMnO3 is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that LaMnO3 becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.     

Quantum versus Jahn-Teller orbital physics in YVO3 and LaVO3

We argue that the large Jahn-Teller (JT) distortions in YVO3 and LaVO3 should suppress the quantum orbital fluctuation. The unusual magnetic properties can be well explained based on local density approximation + Hubbard U calculations using experimental structures, in terms of the JT orbital. The observed splitting of the spin-wave dispersions for YVO3 in a C-type antiferromagnetic state is attributed to the inequivalent VO2 layers in the crystal structure, instead of the "orbital-Peierls state." Alternative stacking of ab-plane exchange couplings produces the c-axis spin-wave splitting; thus, the spin system is highly three dimensional rather than quasi-one-dimensional. Similar splitting is also predicted for LaVO3, although it is weak.     

Orbital fluctuations and orbital flipping in RVO3 perovskites

The effect of the average R-site ionic radius IR and variance on the orbital and magnetic order in R3+-doped YVO3 was studied in Y1-xLaxVO3 and Y1-x(La0.2337Lu0.7663)xVO3 with fixed IR. The orbital flipping temperature T{CG} increases nonlinearly with increasing R-site variance, indicating that the V-O-V bond angle is not the primary driving force stabilizing the C-type orbitally ordered phase. The suppressed thermal conductivity in the G-type orbitally ordered phase signals some remaining orbital randomness that is enhanced by t{2} and et hybridization in {3}T{1g} site symmetry.     

Ferroelectricity in the magnetic E-phase of orthorhombic perovskites

We show that the symmetry of the spin zigzag chain E phase of the orthorhombic perovskite manganites and nickelates allows for the existence of a finite ferroelectric polarization. The proposed microscopic mechanism is independent of spin-orbit coupling. We predict that the polarization induced by the E-type magnetic order can potentially be enhanced by up to 2 orders of magnitude with respect to that in the spiral magnetic phases of TbMnO3 and similar multiferroic compounds.     

Transition from a two-dimensional superfluid to a one-dimensional Mott insulator

A two-dimensional system of atoms in an anisotropic optical lattice is studied theoretically. If the system is finite in one direction, it is shown to exhibit a transition between a two-dimensional superfluid and a one-dimensional Mott insulating chain of superfluid tubes. Monte Carlo simulations are consistent with the expectation that the phase transition is of Kosterlitz-Thouless type. The effect of the transition on experimental time-of-flight images is discussed.     

Transition to deformed shapes as a nuclear Jahn-Teller effect

The Jahn-Teller effect is well known in molecular physics as an interplay between degenerate electronic states and a molecular vibration, giving rise to a spontaneous breaking of a molecular symmetry. We translate the concept of the Jahn-Teller effect to nuclear physics and discuss nuclear collective motion and deformation from that point of view. It becomes transparent that the well-known surface quadrupole motion, with its tendency to stable deformations, can be understood as a nuclear Jahn-Teller mode emerging from the interaction between degenerate nucleon states and giant resonance vibrations. The axial symmetry may break down further to non-axial or to reflection asymmetry shapes by the same mechanism.     

Transition to a gapless peierls insulator in heavily-doped polyacetylene

A high doping concentrations, polyacetylene undergoes a transformation into a state characterized by high d.c. conductivity, by a large Pauli susceptibility and by the absence of the interband transition of the updoped polymer. The optical spectrum shows a pseudogap at ≈0.2 eV and vibrational modes which imply that the carbon-carbon bond length is not uniform. We conclude that this state is best described as a gapless incommensurate Peierls insulator.     

Orbital liquid in three-dimensional mott insulator: LaTiO3

We present a theory of spin and orbital states in Mott insulator LaTiO3. The spin-orbital superexchange interaction between d(1)(t(2g)) ions in cubic crystal suffers from a pathological degeneracy of orbital states at the classical level. Quantum effects remove this degeneracy and result in the formation of the coherent ground state, in which the orbital moment of t(2g) level is fully quenched. We find a finite gap for orbital excitations. Such a disordered state of local degrees of freedom on unfrustrated, simple cubic lattice is highly unusual. Orbital liquid state naturally explains observed anomalies of LaTiO3.     

Effect of orbital ordering on the magnetic-structure formation in the LaMnO3 Jahn-Teller magnet

A study is reported on the relation between orbital ordering and the magnetic structure of an LaMnO₃ crystal. The dependence of the exchange parameters on the orbital-structure angle Φ has been determined. Inclusion into the spin Hamiltonian of isotropic exchange interaction and single-ion anisotropy, which depends on the angle Φ and rotational distortions, results in a four-sublattice structure (A _X , F _Y , G _Z ), with the sublattice magnetic moments oriented close to the long axis of the orthorhombic cell in the basal plane of the crystal . The effect of the rare-earth-ion size in RMnO₃ manganites on the orbital and magnetic structures is considered.     

Transition from a strongly interacting 1d superfluid to a Mott insulator

We study 1D trapped Bose gases in the strongly interacting regime. The systems are created in an optical lattice and are subject to a longitudinal periodic potential. Bragg spectroscopy enables us to investigate the excitation spectrum in different regimes. In the superfluid phase a broad continuum of excitations is observed which calls for an interpretation beyond the Bogoliubov spectrum taking into account the effect of strong interactions. In the Mott insulating phase a discrete spectrum is measured. Both phases are compared to the 3D situation and to the crossover regime from 1D to 3D. The coherence length and coherent fraction of the gas are measured in all configurations. We observe signatures for increased fluctuations characteristic for 1D systems. Moreover, the collective oscillations cease near the transition to the Mott insulator phase.     

Impurities in noncubic crystals: stabilization mechanisms for Jahn-Teller ions in layered perovskites

Mechanisms responsible for the local geometry around Jahn-Teller impurities in K2NiF4 type lattices are shown to be different from those generating the warping in cubic crystals. The present density functional theory calculations reveal that the elastic anisotropy of the host lattice (visible for closed shell impurities) and the electric field created by the rest of lattice ions upon active electrons make it possible to have d(9) ions in an elongated geometry but with a A(1g) ground state. The puzzling difference between equilibrium geometries for Cu2+ and Ni+ in layered perovskites can reasonably be understood.     

Suppression of Jahn-Teller Distortion and Insulator-To-Metal Transition in LaMnO 3 at High Pressures

Structural, vibrational and electronic properties of LaMnO 3 under pressures up to 38 GPa have been studied by synchrotron X-ray powder diffraction, Raman spectroscopy, optical reflectivity, and transport measurements. The cooperative Jahn-Teller distortion of the MnO 6 octahedra of the perovskite-type structure is continuously suppressed with increasing pressure, a process which appears completed at ～20 GPa. The system remains insulating to 32 GPa, where an insulator-metal transition is observed. This transition is attributed to strengthened Mn--O--Mn interactions due to the increasing overlap of atomic orbitals.     

Hole dynamics in spin and orbital ordered vanadium perovskites

A theory of doped perovskite vanadates with spin and orbital orders is presented. Mobile holes are strongly renormalized by spin excitations (magnons) in the spin G-type and orbital C-type (SG-OC) order, and orbital excitations (orbitons) in the spin C-type and orbital G-type (SC-OG) one. Hole dynamics in a staggered t(2g) orbital array is distinguished from that in the antiferromagnetic order and the e(g) orbital one. The fragile character of the (SG-OC) order in Y1-xCaxVO3 is attributed to the orbiton softening induced by a reduction of the spin order parameter.