arXiv subjects

A. M. Oles

Publications and source records attributed to A. M. Oles.

23 records · Page 2Linked to original sources

arXiv2001

Spin Order due to Orbital Fluctuations: Cubic Vanadates

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of $t_{2g}$ orbitals trigger a {\it novel mechanism of ferromagnetic interactions} between spins S=1 of V$^{3+}$ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange $J_H$, and leads to the C-type antiferromagnetic phase in LaVO$_3$. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C-phase at higher temperatures, as observed in YVO$_3$.

cond-mat.str-el↗

arXiv2000

Magnetic and orbital ordering in cuprates and manganites

The mechanisms of magnetic and orbital interactions due to double exchange (DE) and superexchange (SE) in transition metal oxides with degenerate e_g orbitals are presented. Specifically, we study the effective spin-orbital models derived for the d^9 ions as in KCuF_3, and for the d^4 ions as in LaMnO_3, for spins S=1/2 and S=2, respectively. Such models are characterized by three types of elementary excitations: spin waves, orbital waves, and spin-and-orbital waves. The SE interactions between Cu^{2+} (d^9) ions are inherently frustrated, which leads to a new mechanism of spin liquid which operates in three dimensions. The SE between Mn^{3+} (d^4) ions explains the A-type antiferromagnetic order in LaMnO_3 which coexists with the orbital order. In contrast, the ferromagnetic metallic phase and isotropic spin waves observed in doped manganites are explained by DE for degenerate e_g orbitals. It is shown that although a hole does not couple to spin excitations in ferromagnetic planes of LaMnO_3, the orbital excitations change the energy scale for the coherent hole propagation and cause a large redistribution of spectral weight. Finally, we point out some open problems in the present understanding of doped manganites.

cond-mat.str-el↗

arXiv1999

One-dimensional metallic behavior of the stripe phase in La$_{2-x}$Sr$_x$CuO$_4$

Using an exact diagonalization method within the dynamical mean-field theory we study stripe phases in the two-dimensional Hubbard model. We find a crossover at doping $δ\simeq 0.05$ from diagonal stripes to vertical site-centered stripes with populated domain walls, stable in a broad range of doping, $0.05<δ<0.17$. The calculated chemical potential shift $\propto -δ^2$ and the doping dependence of the magnetic incommensurability are in quantitative agreement with the experimental results for doped La$_{2-x}$Sr$_x$CuO$_4$. The electronic structure shows one-dimensional metallic behavior along the domain walls, and explains the suppression of spectral weight along the Brillouin zone diagonal.

cond-mat.str-el↗

arXiv1998

Classical frustration and quantum disorder in spin-orbital models

The most elementary of all physical spin-orbital models is the Kugel-Khomskii model describing a S=1/2, $e_g$ degenerate Mott-insulator. Recent theoretical work is reviewed revealing that the classical limit is characterized by a point of perfect dynamical frustration. It is suggested that this might give rise to a quantum disordered ground state.

cond-mat.str-el↗

arXiv1998

Dynamical Mean-Field Theory for Doped Antiferromagnets

We have generalized the dynamical mean-field theory to study the doping dependence of the crossover from antiferromagnetic to short-range order modelled by an incommensurate spin density wave in the Hubbard model. The local selfenergy which includes spin fluctuations gives quasiparticle weights and spectral properties in good agreement with quantum Monte Carlo and exact diagonalization data in two dimensions. The spectra at finite doping are characterized by a Mott-Hubbard `gap' accompanied by a pseudogap induced by the local spin order.

cond-mat.str-el↗