arXiv subjects

A. G. Yashenkin

Publications and source records attributed to A. G. Yashenkin.

17 recordsLinked to original sources

arXiv2024

Raman peak shift and broadening in crystalline nanoparticles with lattice impurities

The effect of point-like lattice impurities on nanoparticle Raman spectra (RS) is studied using both numerical and analytical methods. Particular cases of replacement atoms of various masses, vacancies, and disorder in interatomic bonds are considered. It is shown that the disorder leads not only to the broadening of optical phonon lines but also to the shift of the corresponding Raman peak. The latter can be either positive (i.e., blueshift) or negative (redshift) depending on the type of impurities. Thus there is an additional contribution to the well-known redshift that occurs due to the size-quantization (confinement) effect. Considering nanometer-sized diamond particles as a representative example, we show that the broadening and the shift are, as a rule, of the same order of magnitude. The results are discussed in the framework of the self-consistent T-matrix approach. It is argued that both effects should be considered for accurate treatment of experimental Raman spectra. Simple recipes to do so are formulated for several important cases including NV centers in nanodiamonds.

cond-mat.mes-hall↗

arXiv2023

Localized and extended collective optical phonon modes in regular and random arrays of contacting nanoparticles: escape from phonon confinement

In the present paper, we utilize the coupled-oscillator model describing the hybridization of optical phonons in touching and/or overlapping particles in order to study the Raman spectra of nanoparticles organized into various types of regular and random arrays including nanosolids, porous media, and agglomerates with tightly bonded particles. For the nanocrystal solids, we demonstrate that the ratio of the size variance to the coupling strength allows us to judge the character (localized or propagating) of the optical phonon modes which left the particles of their origin and spread throughout an array. The relation between the shift and the broadening of the Raman peak and the coupling strength and the disorder is established for nanocrystal solids, agglomerates, and porous media providing us with information about the array structure, the structure of its constituents, and the properties of optical phonons.

cond-mat.mes-hall↗

arXiv2022

Coupled oscillators model for hybridized optical phonon modes in contacting nanosized particles and quantum dot molecules

Modification of optical phonon spectra in contacting nanoparticles as compared to the single ones is studied. Optical phonons in dielectric and semiconducting particles obey the Euclidean metric Klein-Fock-Gordon equation with Dirichlet boundary conditions. The latter is supposed to be solved numerically for manifolds of interpenetrating spheres. It is proposed to replace this problem with the simpler-to-solve coupled oscillators model (COM), where an oscillator is attributed to each phonon mode of a particle and the particles overlap leads to appearance of additional couplings for these oscillators with the magnitude proportional to the overlapped volume. For not too big overlaps this model describes solutions of the original eigenvalue problem on a good level of accuracy. In particular, it works beyond isotropic s modes, which has been demonstrated for p modes in dimer and also for tetramer. It is proposed to apply COM for the description of recently manufactured dimer nanoparticles and quantum dots. The obtained results are in agreement with the dynamical matrix method for optical phonons in nanodiamonds. The latter is used to demonstrate that the van der Waals contacts between faceted particles lead to very small modifications of the optical phonon spectra, which therefore could be neglected when discussing the propagation of vibrational excitations via a nanopowder. The possibility to distinguish between dimerized and size-distributed single particles from their Raman spectra is also considered.

cond-mat.mes-hall↗

arXiv2018

Raman spectra of nanoparticles: elasticity theory-like approach for optical phonons

A simple way to investigate theoretically the Raman spectra (RS) of nonpolar nanoparticles is proposed. For this aim we substitute the original lattice optical phonon eigenproblem by the continuous Klein-Fock-Gordon-like equation with Dirichlet boundary conditions. This approach provides the basis for the continuous description of optical phonons in the same manner how the elasticity theory describes the longwavelength acoustic phonons. Together with continuous reformulation of the bond polarization model it allows to calculate the RS of nanoparticles without referring to their atomistic structure. It ensures the powerful tool for interpreting the experimental data, studying the effects of particle shape and their size distribution. We successfully fit recent experimental data on very small diamond and silicon particles, for which the commonly used phonon confinement model fails. The predictions of our theory are compared with recent results obtained within the dynamical matrix method - bond polarization model (DMM-BPM) approach and an excellent agreement between them is found. The advantages of the present theory are its simplicity and the rapidity of calculations. We analyze how the RS are affected by the nanoparticle faceting and propose a simple power law for Raman peak position dependence on the facets number. The method of powder RS calculations is formulated and the limitations on the accuracy of our analysis are discussed.

cond-mat.mes-hall↗

arXiv2018

Novel approach to Raman spectra of nanoparticles

In crystalline nanoparticles the Raman peak is downshifted with respect to the bulk material and has asymmetric broadening. These effects are straightly related to the finite size of nanoparticles, giving the perspective to use the Raman spectroscopy as the size probe. By combining the dynamical matrix method (DMM) and the bond polarization model (BPM), we develop a new (DMM-BPM) approach to the description of Raman spectra for random arrays of nanoparticles. The numerical variant of this approach is suitable for the description of small particles, whereas its simplier to implement analytical version allows to obtain the Raman spectra of arbitrary sized particles. Focusing on nanodiamond powders, the DMM-BPM theory is shown to fit the most recent experimental data much better than the commonly used phonon confinement model (PCM), especially for small enough nanoparticles.

cond-mat.mes-hall↗

arXiv2015

Self-consistent T-matrix approach to Bose-glass in one dimension

Based on self-consistent T-matrix approximation (SCTMA), the Mott insulator - Bose-glass phase transition of one-dimensional noninteracting bosons subject to binary disorder is considered. The results obtained differ essentially from the conventional case of box distribution of the disorder. The Mott insulator - Bose-glass transition is found to exist at arbitrary strength of the impurities. The single particle density of states is calculated within the frame of SCTMA, numerically, and (for infinite disorder strength) analytically. A good agreement is reported between all three methods. We speculate that certain types of the interaction may lead to the Bose-glass - superfluid transition absent in our theory.

cond-mat.str-el↗

arXiv2015

Low-temperature spin Coulomb drag in a two-dimensional electron gas

The phenomenon of low-temperature spin Coulomb drag in a two-dimensional electron gas is investigated. The spin transresistivity coefficient is essentially enhanced in the diffusive regime, as compared to conventional predictions. The origin of this enhancement is the quantum coherence of spin-up and spin-down electrons propagating in the same random impurity potential and coupled via the Coulomb interaction. A comprehensive analysis of spin and interlayer Coulomb drag effects is presented.

cond-mat.str-el↗

arXiv2008

Quantum interference and spin-charge separation in a disordered Luttinger liquid

We study the influence of spin on the quantum interference of interacting electrons in a single-channel disordered quantum wire within the framework of the Luttinger liquid (LL) model. The nature of the electron interference in a spinful LL is particularly nontrivial because the elementary bosonic excitations that carry charge and spin propagate with different velocities. We extend the functional bosonization approach to treat the fermionic and bosonic degrees of freedom in a disordered spinful LL on an equal footing. We analyze the effect of spin-charge separation at finite temperature both on the spectral properties of single-particle fermionic excitations and on the conductivity of a disordered quantum wire. We demonstrate that the notion of weak localization, related to the interference of multiple-scattered electron waves and their decoherence due to electron-electron scattering, remains applicable to the spin-charge separated system. The relevant dephasing length, governed by the interplay of electron-electron interaction and spin-charge separation, is found to be parametrically shorter than in a spinless LL. We calculate both the quantum (weak localization) and classical (memory effect) corrections to the conductivity of a disordered spinful LL. The classical correction is shown to dominate in the limit of high temperature.

cond-mat.str-el↗

arXiv2003

Planar Dirac fermions in long-range-correlated random vector potential

We study the behavior of two-dimensional Dirac fermions in the presence of a static long-range-correlated random vector potential. By applying an exact path integral representation for the propagator of a spinor particle we obtain asymptotics of the gauge invariant spectral function and the correlation function of the local density of states, both in the ballistic regime of sufficiently high energies. We also discuss localization properties of the random Dirac wave functions in the complementary zero energy limit and the putative localization scenario.

cond-mat↗

arXiv2003

Two different quasiparticle scattering rates in vortex line liquid phase of layered d-wave superconductors

We carry out a quantum mechanical analysis of the behavior of nodal quasiparticles in the vortex line liquid phase of planar d-wave superconductors. Applying a novel path integral technique we calculate a number of experimentally relevant observables and demonstrate that in the low-field regime the quasiparticle scattering rates deduced from photoemission and thermal transport data can be markedly different from that extracted from tunneling, specific heat, superfluid stiffness or spin-lattice relaxation time.

cond-mat↗

arXiv2002

Interacting random Dirac fermions in superconducting cuprates

We study the effects of quasiparticle interactions on disorder-induced localization of Dirac-like nodal excitations in superconducting high-$T_c$ cuprates. As suggested by the experimental ARPES and terahertz conductivity data in $Bi_2Sr_2CaCu_2O_{8+δ}$, we focus on the interactions mediated by the order parameter fluctuations near an incipient second pairing transition $d\to d+is$. We find interaction corrections to the density of states, specific heat, and conductivity as well as phase and energy relaxation rates and assess the applicability of the recent localization scenarios for non-interacting random Dirac fermions to the cuprates.

cond-mat↗

arXiv2001

Nesting symmetries and diffusion in disordered d-wave superconductors

The low-energy density of states (DOS) of disordered 2D d-wave superconductors is extremely sensitive to details of both the disorder model and the electronic band structure. Using diagrammatic methods and numerical solutions of the Bogoliubov-de Gennes equations, we show that the physical origin of this sensitivity is the existence of a novel diffusive mode with momentum close to $(π,π)$ which is gapless only in systems with a global nesting symmetry. We find that in generic situations, the DOS vanishes at the Fermi level. However, proximity to the highly symmetric case may nevertheless lead to observable non-monotonic behavior of the DOS in the cuprates.

cond-mat↗

arXiv1999

Coulomb Drag in Double Layers with Correlated Disorder

We study the effect of correlations between impurity potentials in different layers on the Coulomb drag in a double-layer electron system. It is found that for strongly correlated potentials the drag in the diffusive regime is considerably enhanced as compared to conventional predictions. The appropriate experimental conditions are discussed, and the new experiments are suggested.

cond-mat.mes-hall↗

arXiv1996

Comment on ``Impurity states and the absence of quasiparticle localization in disordered d-wave superconductors''

In a recent Letter Balatsky and Salkola made their conclusion about the absence of localization in disordered d-wave superconductors exploiting the idea that strongly overlapping impurity states formed a network capable to provide the transport. We present here the correct estimate of hopping matrix elements between two impurities which were the basic quantities for the subsequent consideration in the abovementioned Letter. We conclude, that the picture of the impurity-impurity potential acting only within the slowly broadening tails claimed by Balatsky and Salkola as well as all the physical consequences of this picture appear to be incorrect.

cond-mat.str-el↗

arXiv1996

RKKY interaction in Layered Superconductors with Anisotropic Pairing

The RKKY interaction between rare-earth (RE) ions in high-$T_c$ superconductors is considered at $T\ll T_c$. It is shown that this interaction consists of two terms: conventional oscillating one and the positive term, which is proportional to the gap function and decreases in the $2D$ case inversely proportional to the distance. In the antiferromagnetic state of the RE subsystem this positive interaction gives rise for frustrations which diminishes the Neel temperature. In the case of strongly anisotropic gap function this frustration produces two different values of the effective nearest neighbor exchange coupling between RE ions along the $a$ and $b$. This anisotropy has been established experimentally in Ref.\cite{6,7,8}.

cond-mat↗

arXiv1995

Quasiparticle damping in two-dimensional superconductors with unconventional pairing.

We calculate the damping of excitations due to four-fermionic interaction in the case of two-dimensional superconductor with nodes in the spectrum. At zero temperature and low frequencies it reveals gapless $ω^3$ behavior at the nodal points. With the frequency increasing the crossover to the normal-state regimes appears. At high frequencies the damping strongly depends on details of a normal-state spectrum parametrization. Two important particular cases such as the models of almost free and tight-binding electrons are studied explicitly and the characteristic scales are expressed through the model-free parameters of the spectrum at the nodal points. The possibility of crossover in temperature dependence of damping in the superconducting phase is discussed.

cond-mat↗

arXiv1994

NMR in ``underdoped'' and ``overdoped'' YBaCuO compounds: fermi-liquid approach

We examine the NMR experimental data for the normal state of YBa$_2$Cu$_3$O$_{7-δ}$ within the Fermi-liquid approach. We show that the observed temperature dependence of the Knight shift and $^{17}$O ($^{89}$Y) relaxation rate in ``underdoped'' ( $T_c=60\,$K ) compound can be interpreted as resulting from a peak in the density of states near the Fermi level. The possible origin of such peak is the quasi-one-dimensional van Hove singularity in the fermionic spectrum, conjectured by Abrikosov, Gofron and Campuzano. The proposed spectrum allows us to account for the qualitatively different NMR data for ``underdoped'' and ``overdoped''( $T_c=90\,$K ) compounds by varying the value of chemical potential only.

cond-mat↗