RESUMEN
Within the framework of the quantum mechanical approach, the available experimental data are analyzed to identify the electronic structure of the multiferroic FeCr2O4. The relative values of the key contributions to the parameters of even and odd crystal fields acting on the 3delectrons of the Fe2+ion are determined. Data on local lattice distortions are systematized. The parameter of the electron-deformation interaction of the ground term Fe2+(5E) is determined considering lattice distortions, and the parameters of binding of the spins of Fe2+and Cr3+to the electric field are estimated. The calculation results are compared with the available experimental data on the magnetic and structural characteristics of FeCr2O4, the critical temperature of the transition to an orbitally ordered state, optical conductivity data, the Mössbauer effect study, and measurements of spontaneous electric polarization.
RESUMEN
We analyse the spin excitations near the optimal doping of superconducting layered cuprates taking into account both the local and the itinerant spin components self-consistently. The obtained expression allows us to reproduce well the basic features of the resonant inelastic x-ray scattering and neutron scattering data experiments using a reasonable set of tight-binding parameters corresponding to the angle-resolved photoemission spectroscopy data. We also find that the spin excitation branch along the (0,0) - (0,π) symmetry direction in the first Brillouin zone shows a splitting at T < Tc. Possible experiments for verification of that prediction are briefly discussed.
RESUMEN
We present a simple derivation of an expression for the superfluid density n(s) α 1/λ(2) in superconductors with the tight binding energy dispersion. The derived expression is discussed in detail because of its distinction from the known expressions for ordinary superconductors with parabolic energy dispersion. We apply this expression for the experimental data analysis of the isotope effect in London penetration depth parameter λ in the BiSrCuO and YBaCuO family compounds near optimal doping, taking into account the orthorhombic distortion of crystal structure, and estimate the isotopic change of hopping parameters from the experimental data. We point out that 1/λ(2) temperature behaviour is very sensitive to the ratio 2Δ(m)(T = 0)/k(B)T(c) and estimate this quantity for a number of compounds.
RESUMEN
We report on optical measurements of the 1D Heisenberg antiferromagnet KCuF3. The crystal-field excitations of the Cu2+ ions have been observed and their temperature dependence can be understood in terms of magnetic and exchange-induced dipole mechanisms and vibronic interactions. Above TN we observe a new temperature scale TS characterized by the emergence of narrow absorption features that correlate with changes of the orbital ordering as observed by Paolasini et al. [Phys. Rev. Lett. 88, 106403 (2002)]. The appearance of these optical transitions provides evidence for a symmetry change above the Néel temperature that affects the orbital ordering and paves the way for the antiferromagnetic ordering.
RESUMEN
The spin dynamics of the prototypical quasi-one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fluorine ions perpendicular to the crystallographic c axis. This new mechanism allows us to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound.
RESUMEN
The strong line broadening observed in electron spin resonance on NaV2O5 is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost 2 orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K.
