It is found that in some metals an intrinsic localized mode may exist with frequency above the top of the phonon spectrum. The necessary condition, requiring sufficiently high ratio of quartic to cubic anharmonicity may be fulfilled because of screening of the interaction between ions by free electrons. Starting from the known literature values of the pair potentials we have found that in Ni and Nb the derived localized mode condition is fulfilled. MD simulations of the nonlinear dynamics of Ni and Nb confirmed that high frequency ILMs may exist in these metals.

The Kotliar-Ruckenstein slave-boson scheme is used to allow for an unrestricted variation of the bosonic and fermionic fields on the saddle-point level. Various inhomogeneous solutions, such as spin polarons and domain walls, are discussed within the two-dimensional Hubbard model and compared with results of unrestricted Hartree-Fock (HF) calculations. We find that the present approach drastically reduces the polarization of these states and leads to increased delocalized wave functions as compared to the HF model. The interaction between two spin-polarons turns out to be attractive over a wide range of the on-site repulsion U. In addition we obtain the crossover from vertical to diagonal domain walls at a higher value of U than predicted by HF. 71.27,71.10.Fd,75.10.Lp,75.60.CH

PACS: 63.20.Pw; 78.20.Ci The optical dephasing, caused by a quadratic interaction with acoustic phonons, is studied on the basis of the nonperturbative theory. The effect of a strong weakening of the elastic springs on the electronic transition is studied. It is found that, if the weakening brings the system close to the dynamical instability, then the usual / T 7 dependence of the homogeneous width of a zero-phonon line (ZPL) is replaced by the / T 3 dependence in a broad temperature interval. An equation is derived which describes the additional broadening of a ZPL caused by the dynamical instability of the final state. The model explains the temperature broadening and shift of the zero-phonon lines of some nitrogen-containing centers in diamond crystals.

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