Article

# Lattice vibrations in KCuF3

Institute of Solid State Physics, Vienna University of Technology, 1040 Vienna, Austria; Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chişinău, Republic of Moldova

Annalen der Physik (Impact Factor: 1.51). 09/2011; 523(8‐9):645 - 651. DOI: 10.1002/andp.201100026 - Citations (25)
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**ABSTRACT:**We present a computational scheme for ab initio total-energy calculations of materials with strongly interacting electrons using a plane-wave basis set. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The present approach allows us to investigate complex materials with strongly interacting electrons and is able to treat atomic displacements, and hence structural transformations, caused by electronic correlations. Here it is employed to investigate two prototypical Jahn-Teller materials, KCuF3 and LaMnO3, in their paramagnetic phases. The computed equilibrium Jahn-Teller distortion and antiferro-orbital order agree well with experiment, and the structural optimization performed for paramagnetic KCuF3 yields the correct lattice constant, equilibrium Jahn-Teller distortion and tetragonal compression of the unit cell. Most importantly, the present approach is able to determine correlation-induced structural transformations, equilibrium atomic positions, and lattice structure in both strongly and weakly correlated solids in their paramagnetic phases as well as in phases with long-range magnetic order.Physical Review B 02/2010; 81(7). · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The temperature dependence and anisotropy of optical spectral weights associated with different multiplet transitions is determined by the spin and orbital correlations. To provide a systematic basis to exploit this close relationship between magnetism and optical spectra, we present and analyze the spin-orbital superexchange models for a series of representative orbital-degenerate transition metal oxides with different multiplet structure. For each case we derive the magnetic exchange constants, which determine the spin wave dispersions, as well as the partial optical sum rules. The magnetic and optical properties of early transition metal oxides with degenerate $t_{2g}$ orbitals (titanates and vanadates with perovskite structure) are shown to depend only on two parameters, viz. the superexchange energy $J$ and the ratio $\eta$ of Hund's exchange to the intraorbital Coulomb interaction, and on the actual orbital state. In $e_g$ systems important corrections follow from charge transfer excitations, and we show that KCuF$_3$ can be classified as a charge transfer insulator, while LaMnO$_3$ is a Mott insulator with moderate charge transfer contributions. In some cases orbital fluctuations are quenched and decoupling of spin and orbital degrees of freedom with static orbital order gives satisfactory results for the optical weights. On the example of cubic vanadates we describe a case where the full quantum spin-orbital physics must be considered. Thus information on optical excitations, their energies, temperature dependence and anisotropy, combined with the results of magnetic neutron scattering experiments, provides an important consistency test of the spin-orbital models, and indicates whether orbital and/or spin fluctuations are important in a given compound.Physical Review B 12/2005; 72:214431. · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Quantum effects dominate the behaviour of many diverse materials. Of particular current interest are those systems in the vicinity of a quantum critical point (QCP). Their physical properties are predicted to reflect those of the nearby QCP with universal features independent of the microscopic details. The prototypical QCP is the Luttinger liquid (LL), which is of relevance to many quasi-one-dimensional materials. The magnetic material KCuF3 realizes an array of weakly coupled spin chains (or LLs) and thus lies close to but not exactly at the LL quantum critical point. By using inelastic neutron scattering we have collected a complete data set of the magnetic correlations of KCuF3 as a function of momentum, energy and temperature. The LL description is found to be valid over an extensive range of these parameters, and departures from this behaviour at high and low energies and temperatures are identified and explained.Nature Material 05/2005; 4(4):329-34. · 35.75 Impact Factor

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