V. Hizhnyakov

Prediction of high-frequency intrinsic localized modes in Ni and Nb

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 a 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. Molecular-dynamics simulations of the nonlinear dynamics of Ni and Nb confirmed that high-frequency intrinsic localized modes may exist in these metals.

Theory and molecular dynamics simulations of intrinsic localized modes and defect formation in solids

Molecular dynamics (MD) simulations of recoil processes following the scattering of x-rays or neutrons have been performed in ionic crystals and metals. At small energies (< 10 eV) the recoil can induce intrinsic localized modes (ILMs) and linear local modes associated with these modes. As a rule, the frequencies of such modes are located in the gaps of the phonon spectrum. However, in metallic Ni, Nb and Fe, due to the renormalization of atomic interactions by free electrons, the frequencies mentioned are found to be positioned above the phonon spectrum. It has been shown that these ILMs are highly mobile and can efficiently transfer a concentrated vibrational energy over large distances along crystallographic directions. If the recoil energy exceeds tens of eVs, vacancies and interstitials can be formed, which are strongly dependent on the direction of the recoil momentum. In NaCl-type lattices the recoil in (110) direction can produce a vacancy and a crowdion, while in the case of a recoil in (100) and in (111) directions a bi-vacancy and a crowdion can be formed.

On the percolation induced conductivity in high-Tc superconducting materials

Abstract Following the ideas of a previously proposed model, in which the transition from the antiferromagnetic insulating to the conducting or superconducting state is of a percolative nature, critical cluster sizes N c in the systems La 2− x (Ba, Sr) x CuO 4 and YBa 2 Cu 3 O 6+ x , corresponding to doping concentrations x c at which the systems become conducting, are calculated. The calculations are based only on experimental values of x c and some very general properties of fractal percolation clusters and lead to values of N c which are in agreement with values estimated independent of Hizhnyakov and Sigmund (Physica C 156 (1988) 655). Finally the stability of the clusters is discussed.

Zero‐Phonon Lines: The Effect of a Strong Softening of Elastic Springs in the Excited State

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 ∝ T7 dependence of the homogeneous width of a zero-phonon line (ZPL) is replaced by the ∝ T3 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.

UNRESTRICTED SLAVE-BOSON MEAN-FIELD APPROXIMATION FOR THE TWO-DIMENSIONAL HUBBARD MODEL

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

Quantum properties of intrinsic localized modes

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Transverse intrinsic localized modes in monatomic chain and in graphene

. In addition we obtain the crossover from vertical to diagonal domain walls at a higher value of

Strong Jahn–Teller effect in the excited state: Anomalous temperature dependence of the zero-phonon line

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Anisotropic pairing caused by unscreened long-range interactions.

than predicted by HF.

Optical dephasing in defect-rich crystals

The quantum properties of intrinsic localized modes in perfect anharmonic lattices are considered. A method is developed which allows one to calculate the quantum levels of these modes. Calculations are presented for the lifetime of intrinsic localized gap modes in NaI and KI due to their two-phonon quantum decay.