arXiv subjects

Andrzej M. Oles

Publications and source records attributed to Andrzej M. Oles.

63 records · Page 4Linked to original sources

arXiv2002

Magnetic Order in Transition Metal Oxides with Orbital Degrees of Freedom

We investigate the frustrated magnetic interactions in cubic transition metal oxides with orbital degeneracy. The $e_g$ orbitals order easier and their ordering explains the $A$-type antiferromagnetic phase in KCuF$_3$ and LaMnO$_3$. In $t_{2g}$ systems the magnetic order changes at a transition from an orbital liquid to orbital ordered states. The fluctuations of $t_{2g}$ orbitals play a prominent role in LaVO$_3$ and YVO$_3$, where they compete with the Jahn-Teller effect and trigger the C-type antiferromagnetic order.

cond-mat.str-el↗

arXiv2001

Photoemission spectra of LaMnO3 controlled by orbital excitations

We investigate the spectral function of a hole moving in the orbital-ordered ferromagnetic planes of LaMnO$_3$, and show that it depends critically on the type of orbital ordering. While the hole does not couple to the spin excitations, it interacts strongly with the excitations of $e_g$ orbitals (orbitons), leading to new type of quasiparticles with a dispersion on the orbiton energy scale and with strongly enhanced mass and reduced weight. Therefore we predict a large redistribution of spectral weight with respect to the bands found in local density approximation (LDA) or in LDA+U.

cond-mat.str-el↗

arXiv1999

Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model

We investigate the magnetic instabilities of the nondegenerate (s-band) and a degenerate (d-band) Hubbard model in two dimensions using many-body effects due to the particle-particle diagrams and Hund's rule local correlations. The density of states and the position of Van Hove singularity change depending on the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly reduced in the s-band case, and ferromagnetism survives only if electron density is small, and the band is almost flat at small momenta due to next-nearest neighbor hopping. In contrast, for the d-band case the reduction of the Stoner parameter which follows from particle-particle correlations is much smaller and ferromagnetism survives to a large extent. Inclusion of local spin-spin correlations has a limited destabilizing effect on the magnetic states.

cond-mat.str-el↗

arXiv1999

Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order

We present spectral and optical properties of the Hubbard model on a two-dimensional square lattice using a generalization of dynamical mean-field theory to magnetic states in finite dimension. The self-energy includes the effect of spin fluctuations and screening of the Coulomb interaction due to particle-particle scattering. At half-filling the quasiparticles reduce the width of the Mott-Hubbard `gap' and have dispersions and spectral weights that agree remarkably well with quantum Monte Carlo and exact diagonalization calculations. Away from half-filling we consider incommensurate magnetic order with a varying local spin direction, and derive the photoemission and optical spectra. The incommensurate magnetic order leads to a pseudogap which opens at the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle states survive in the doped systems, but their dispersion is modified with the doping and a rigid band picture does not apply. Spectral weight in the optical conductivity is transferred to lower energies and the Drude weight increases linearly with increasing doping. We show that incommensurate magnetic order leads also to mid-gap states in the optical spectra and to decreased scattering rates in the transport processes, in qualitative agreement with the experimental observations in doped systems. The gradual disappearence of the spiral magnetic order and the vanishing pseudogap with increasing temperature is found to be responsible for the linear resistivity. We discuss the possible reasons why these results may only partially explain the features observed in the optical spectra of high temperature superconductors.

cond-mat.str-el↗

arXiv1999

Quantum melting of magnetic long-range order near orbital degeneracy. Classical phases and Gaussian fluctuations

We study the effective spin-orbital model derived for the d9 ions in a three-dimensional perovskite lattice, as in KCuF_3, where at each site the doubly degenerate eg orbitals contain a single hole. The model describes the superexchange interactions that depend on the pattern of orbitals occupied. We present the ground state properties of this model, depending on the splitting between the eg orbitals E_z, and the Hund's rule coupling in the excited d8 states, J_H. The classical phase diagram consists of six magnetic phases which all have different orbital ordering: two antiferromagnetic (AF) phases with G-AF order and either x2-y2 or 3z2-r2 orbitals occupied, two phases with mixed orbital (MO) patterns and A-AF order, and two other MO phases with either C-AF or G-AF order. All of them become degenerate at the multicritical point M=(E_z,J_H)=(0,0). Using a generalization of linear spin-wave theory we study both the transverse excitations which are spin-waves and spin-and-orbital-waves, as well as the longitudinal (orbital) excitations. The transverse modes couple to each other, and the spin-and-orbital-wave turns into a soft mode near the M point. Therefore, quantum corrections to the long-range-order parameter are drastically increased near the orbital degeneracy, and classical order is suppressed in a crossover regime between the G-AF and A-AF phases in the (E_z,J_H) plane. This behavior is reminiscent of that found in frustrated spin models, and we conclude that orbital degeneracy provides a new and physically realizable mechanism which stabilizes a spin liquid ground state due to inherent frustration of magnetic interactions. We also point out that such a disordered magnetic phase is likely to be realized in LiNiO_2.

cond-mat.str-el↗

arXiv1998

Orbital dynamics in ferromagnetic transition metal oxides

We consider a model of strongly correlated $e_g$ electrons interacting by superexchange orbital interactions in the ferromagnetic phase of LaMnO$_3$. It is found that the classical orbital order with alternating occupied $e_g$ orbitals has a full rotational symmetry at orbital degeneracy, and the excitation spectrum derived using the linear spin-wave theory is gapless. The quantum (fluctuation) corrections to the order parameter and to the ground state energy restore the cubic symmetry of the model. By applying a uniaxial pressure orbital degeneracy is lifted in a tetragonal field and one finds an orbital-flop phase with a gap in the excitation spectrum. In two dimensions the classical order is more robust near the orbital degeneracy point and quantum effects are suppressed. The orbital excitation spectra obtained using finite temperature diagonalization of two-dimensional clusters consist of a quasiparticle accompanied by satellite structures. The orbital waves found within the linear spin-wave theory provide an excellent description of the dominant pole of these spectra.

cond-mat.str-el↗

arXiv1998

Quantum disorder versus order-out-of-disorder in the Kugel-Khomskii model

The Kugel-Khomskii model, the simplest model for orbital degenerate magnetic insulators, exhibits a zero temperature degeneracy in the classical limit which could cause genuine quantum disorder. Khaliullin and Oudovenko [Phys. Rev. B 56, R14 243 (1997)] suggested recently that instead a particular classical state could be stabilized by quantum fluctuations. Here we compare their approach with standard random phase approximation and show that it strongly underestimates the strength of the quantum fluctuations, shedding doubts on the survival of any classical state.

cond-mat.str-el↗

arXiv1998

Electronic origin of magnetic and orbital ordering in insulating LaMnO_3

We derive a spin-orbital model for insulating LaMnO_3 which fulfills the SU(2) symmetry of S=2 spins at Mn^{3+} ions. It includes the complete e_g and t_{2g} superexchange which follows from a realistic Mn^{2+} multiplet structure in cubic site symmetry, and the Jahn-Teller induced orbital interactions. We show that the magnetic ordering observed in LaMnO_3 is stabilized by a purely electronic mechanism due to the e_g-superexchange alone, and provide for the first time a quantitative explanation of the observed transition temperature and the anisotropic exchange interactions.

cond-mat.str-el↗

arXiv1997

Quantum Melting of Magnetic Order due to Orbital Fluctuations

We have studied the phase diagram and excitations of the spin-orbital model derived for a three dimensional perovskite lattice, as in KCuF_3. The results demonstrate that the orbital degeneracy drastically increases quantum fluctuations and suppresses the classical long-range order near the multicritical point in the mean-field phase diagram. This generates a qualitatively new spin liquid, providing the first example of a valence bond ground state in three dimensions.

cond-mat.str-el↗