S. Cojocaru

Modification of the Bloch law in ferromagnetic nanostructures

The temperature dependence of magnetization in ferromagnetic nanostructures (e.g., nanoparticles or nanoclusters) is usually analyzed by means of an empirical extension of the Bloch law sufficiently flexible for a good fitting to the observed data and indicates a strong softening of magnetic coupling compared to the bulk material. We analytically derive a microscopic generalization of the Bloch law for the Heisenberg spin model which takes into account the effects of size, shape and various surface boundary conditions. The result establishes explicit connection to the microscopic parameters and differs significantly from the existing description. In particular, we show with a specific example that the latter may be misleading and grossly overestimates magnetic softening in nanoparticles. It becomes clear why the usual dependence appears to be valid in some nanostructures, while large deviations are a general rule. We demonstrate that the combination of geometrical characteristics and coupling to environment can be used to efficiently control magnetization and, in particular, to reach a magnetization higher than in the bulk material.

Incoherent midinfrared charge excitation and the high-energy anomaly in the photoemission spectra of cuprates

On the basis of a semiphenomenological model, it is argued that the high-energy anomaly observed in recent photoemission experiments on cuprates is caused by interaction with an overdamped bosonic mode in the midinfrared region of the spectrum. Analysis of optical conductivity allows us to connect this excitation to the incoherent charge response reported for the majority of high-Tc materials and some other perovskites. We show that its large damping is an essential feature responsible for the “waterfall” dispersion and linewidth of the spectral weight. The “high-energy anomaly” HEA in the angle-resolved photoemission spectra of hole- and electron-doped cuprates has been reported recently in a number of works. 1–6 The anomaly is characterized by the presence of high-energy kink in the dispersion derived from angle-resolved photoemission spectroscopy ARPES at about 0.2– 0.4 eV below the Fermi surface followed by a puzzling “waterfall” structure at larger energies. The latter is highly incoherent and follows an almost vertical dispersion around the point, extending to 0.9 eV. When measured along the main symmetry directions of the Brillouin zone, this broad linewidth remains roughly constant along the waterfall. 1,6 The momentum distribution of the spectral weight at a fixed energy above the kink resembles a diamond shape e.g., for E = 0.4 in Bi 2 Sr 2 CaCu 2 O 8+ BISCO 2212 with the corner at K

Metallic ferromagnetism in the presence of orbital degeneracy

The effect of orbital degeneracy on metallic ferromagnetism is considered in the strong coupling limit of the Hubbard model within a generalized Roth approximation. For concentrations of less than one electron per lattice site and forbidden double occupancy we find that orbital degeneracy favours magnetic polarization on different lattice structures. The effect is strong enough to induce a ferromagnetic ordering even on frustrated lattices, where polarization is excluded in the absence of degeneracy. We discuss the physical origin of the new results in terms of competition between tendencies towards localization and delocalization.

Anomalous bond stretching phonons as a probe of charge fluctuations in perovskites

Important information on momentum resolved low energy charge response can be extracted from anomalous properties of bond stretching in plane phonons observed in inelastic neutron and X-ray scattering in cuprates and some other perovskites. We discuss a semiphenomenological model based on coupling of phonons to a single charge mode. The phonon dispersion and linewidth allow to locate the energy of the charge excitation in the mid infrared part of the spectrum and to determine some of its characteristics. New experiments on oxygen isotope substitution could allow to achieve a more detailed description. Corresponding relations following from the model can be used for the interpretation of experiments and as test of the model.