arXiv subjects

A. Shelkan

Publications and source records attributed to A. Shelkan.

8 recordsLinked to original sources

arXiv2020

Phase separation and pairing fluctuations in oxide materials

We investigate the microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations. It is demonstrated that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2016

Transverse intrinsic localized modes in monatomic chain and in graphene

In this paper an analytical and numerical study of anharmonic vibrations of monatomic chain and graphene in transverse (perpendicular) with respect to the chain/plane direction is presented. Due to the lack of odd anharmonicities and presence of hard quartic anharmonicity for displacements in this direction, there may exist localized anharmonic transverse modes with the frequencies above the spectrum of the corresponding phonons. Although these frequencies are in resonance with longitudinal (chain) or in-plane (graphene) phonons, the modes can decay only due to a weak anharmonic process. Therefore the lifetime of these vibrations may be very long. E.g. in the chain, according to our theoretical and numerical calculations it may exceed 10^10 periods. We call these vibrations as transverse intrinsic localized modes.

nlin.PS↗

arXiv2013

Theory and MD simulations of intrinsic localized modes and defect formation in solids

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 them. 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 to large distances along crystallographic directions. If the recoil energy exceeds tens of eVs, vacancies and interstitials can be formed, being 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.

cond-mat.mtrl-sci↗

arXiv2013

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 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.

cond-mat.mtrl-sci↗

arXiv2011

Linear local modes induced by intrinsic localized modes in a monatomic chain

A theory is developed to describe the effect of an intrinsic localized mode (ILM) on small vibrations in a monatomic chain with hard quartic anharmonicity. One prediction is the appearance in the chain of linear local modes nearby the ILM. To check this result, MD calculations of vibrations under strong local excitation are carried through with high precision. The results fully confirm the prediction.

nlin.PS↗

arXiv2011

Pressure dependence of two-level systems in disordered atomic chain

The dependence of two-level systems in disordered atomic chain on pressure, both positive and negative was studied numerically. The disorder was produced through the use of interatomic pair potentials having more than one energy minimum. It was found that there exists a correlation between the energy separation of the minima of two-level systems Delta and the variation of this separation with pressure. The correlation may have either positive or negative sign, implying that the asymmetry of two-level systems may in average increase or decrease with pressure depending on the interplay of different interactions between atoms in disordered state. The values of Delta depend on the sign of pressure.

cond-mat.mtrl-sci↗

arXiv2011

Self-consistent theory of intrinsic localized modes: application to monatomic chain

A theory of intrinsic localized modes (ILMs) in anharmonic lattices is developed, which allows one to reduce the original nonlinear problem to a linear problem of small variations of the mode. This enables us to apply the Lifshitz method of the perturbed phonon dynamics for the calculations of ILMs. In order to check the theory, the ILMs in monatomic chain are considered. A comparison of the results with the corresponding molecular dynamics calculations shows an excellent agreement.

nlin.PS↗

arXiv1998

Self-consistent calculation of the autolocalization barrier for quasiparticles in anisotropic crystal

The energy of the electron wave packet interacting with lattice distortion, is considered in anisotropic crystal. Anisotropy of the electron and phonon spectra as well as of the electron-phonon interaction are taken into account. The height of the barrier between free and self-trapped states is calculated in dependence on the anisotropy parameters. The calculation is done numerically, using continual aproximation. The analytical solution is obtained for some cases of quasi-two and quasi-one-dimensional spectra. Key words: anisotropy; polarons; barrier.

cond-mat.soft↗