Magnetic properties of spin-orbital polarons in lightly doped cobaltates

We present a numerical treatment of a spin-orbital polaron model for Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intra-layer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.Comment: 4+ pages, 6 figures, accepted for publication in Phys. Rev. Let

Spin, Orbital and Charge Order at the Interface between Correlated Oxides

The collective behavior of correlated electrons in the VO$_2-$interface layer of LaVO$_3$/SrTiO$_3$ heterostructure is studied within a quarter-filled $t_{2g}$-orbital Hubbard model on a square lattice. We argue that the ground state is ferromagnetic driven by the double exchange mechanism, and is orbitally and charge ordered due to a confined geometry and electron correlations. The orbital and charge density waves open gaps on the entire Fermi surfaces of all orbitals. The theory explains the observed insulating behavior of the $p$-type interface between LaVO$_3$ and SrTiO$_3$.Comment: 4 pages, 5 figures; revised, to appear in Phys. Rev. Let

Charge dynamics of t-J model and anomalous bond-stretching phonons in cuprates

The density response of a doped Mott-Hubbard insulator is discussed starting from the t-J model in a slave boson 1/N representation. In leading order O(1) the density fluctuation spectra $N({\bf q},\omega)$ are determined by an undamped collective mode at large momentum transfer, in striking disagreement with results obtained by exact diagonalization, which reveal a very broad dispersive peak, reminescent of strong spin-charge coupling. The 1/N corrections introduce the polaron character of the bosonic holes moving in a uniform RVB background. The resulting $N({\bf q},\omega)$ captures all features observed in diagonalization studies, fulfills the appropriate sum rules, and apart from the broadening of the collective mode shows a new low energy feature at the energy $\chi J+\delta t$ related to the polaron motion in the spinon background. It is further shown that the low energy structure, which is particularly pronounced in $(\pi,0)$ direction, describes the strong renormalization and anomalous damping of the highest bond-stretching phonons in La$_{2-x}$Sr$_x$CuO$_4$.Comment: Presented at the meeting "Highlights in Condensed Matter Physics" in honor of the 60th birthday of Prof. Ferdinando Mancini, May 9-11, 2003, Salerno, Ital

Charge susceptibility in the t-J model

Momentum and doping dependence of the static charge susceptibility \chi(q) in the t-t'-J model is investigated. Correlations lead to a strongly momentum dependent renormalization of \chi(q). The charge susceptibility near (\pi,\pi) region of the Brillouin zone is strongly suppressed as the hole density \delta is decreased. However, contrary to naive expectations, \chi(q) around q = (\pi,0) and (0,\pi) remains large and practically unchanged at \delta \sim 0.1-0.5. This effect is consistent with a tendency towards low-energy charge fluctuations with the wave vectors along the \Gamma-X direction, reported in earlier studies. Our main finding is that the above trends are amplified by J-driven pairing effects, indicating that the pseudogap formation may promote the charge inhomogeneity. The next-nearest hopping t' leads to weakening of the above momentum-selective renormalizations of \chi(q). We analyze the effects of long-range Coulomb interaction, taking into account a layered structure of cuprates. As an application, the results are discussed in the context of bond-stretching phonon softening in hole-doped cuprates. In particular, a peculiar doping and momentum dependence of the electron-phonon coupling constant is found.Comment: 12 pages, 11 figures, references added, explanations provided on the meaning and limitations of our formalis

Magnon softening and damping in the ferromagnetic manganites due to orbital correlations

We present a theory for spin excitations in ferromagnetic metallic manganites and demonstrate that orbital fluctuations have strong effects on the magnon dynamics in the case these compounds are close to a transition to an orbital ordered state. In particular we show that the scattering of the spin excitations by low-lying orbital modes with cubic symmetry causes both the magnon softening and damping observed experimentally.Comment: 2 pages, 2 figures, SCES2003 Roma, to appear in J. Mag. Magn. Ma

Density response of the t-J model and renormalization of breathing and half-breathing phonon modes: A slave-fermion calculation

The density fluctuation spectrum $N({\bf k},\omega)$ is calculated for the t-J model in the low-doping regime using a slave-fermion method for the constrained fermions. The obtained results for $N({\bf k},\omega)$ are in good agreement with diagonalization results. The density response is characterized by incoherent, momentum dependent spectral functions reaching up to energies $\sim 8t$ and a low-energy structure at energy $\sim J$ due to transitions in the quasiparticle band. $N({\bf k},\omega)$ is shown to lead to a strong renormalization of planar bond-streching and breathing phonon modes with a large phonon linewidth at intermediate momenta caused by the low-energy response. Our results are consistent with recent neutron scattering data, showing the peculiar behavior of these modes.Comment: 4 pages, 3 figures, `Materials and Mechanisms of Superconductivity VI' conference, Houston, USA, Febr. 20-25, 200

Electronic excitations in the edge-shared relativistic Mott insulator: Na2IrO3

We have investigated the excitation spectra of j(eff) = 1/2 Mott insulator Na2IrO3. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity sigma(omega) and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical j(eff) = 1/2 system Sr2IrO4. Distinctly from the two-peak structure in Sr2IrO4, sigma(omega) in Na2IrO3 has a broad single peak dominated by interband transitions from j(eff) = 3/2 to 1/2. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in Sr2IrO4. In addition, a small peak near the optical-absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole (e-h) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in Na2IrO3.open1122sciescopu

Orbital order out of spin disorder: How to measure the orbital gap

The interplay between spin and orbital degrees of freedom in the Mott-Hubbard insulator is studied by considering an orbitally degenerate superexchange model. We argue that orbital order and the orbital excitation gap in this model are generated through the order-from-disorder mechanism known previously from frustrated spin models. We propose that the orbital gap should show up indirectly in the dynamical spin structure factor; it can therefore be measured using the conventional inelastic neutron scattering method