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S. V. Goupalov

Publications and source records attributed to S. V. Goupalov.

13 recordsLinked to original sources
arXiv2025

Application of perturbation theory to finding vibrational frequencies of a spheroid

We apply perturbation theory of boundary conditions, originally developed by A.B. Migdal and independently by S.A. Moszkowski for deformed atomic nuclei, to finding eigenfrequencies of Raman-active spheroidal modes of a spheroid from these of a sphere and compare the outcomes with the results of numerical calculations for CdSe and silver nanoparticles. The modes are characterized by the total angular momentum $j=2$ and are five-fold degenerate for a sphere but split into three distinct modes, characterized by the absolute value of the total angular momentum projection onto the spheroidal axis, in case of a spheroid. The perturbation method works well in case of the rigid boundary conditions, with the displacement field set to zero at the boundary, and accurately predicts the splittings when the spheroidal shape is close to a sphere, but fails in case of the stress-free boundary conditions.

cond-mat.mtrl-sci↗
arXiv2025

Quantum beats of exciton-polarons in CsPbI3 perovskite nanocrystals

Exciton-phonon interactions govern the energy level spectrum and thus the optical response in semiconductors. In this respect, lead-halide perovskite nanocrystals represent a unique system, for which the interaction with optical phonons is particularly strong, giving rise to a ladder of multiple exciton states which can be optically excited with femtosecond pulses. We establish a new regime of coherent exciton-polaron dynamics with exceptionally long coherence times (T2 ~300 ps) in an ensemble of CsPbI3 nanocrystals embedded in a glass matrix. Using transient two-pulse photon echo at 2 K temperature, we observe quantum beats between the exciton-polaron states. Within a four-level model, we directly quantify the exciton-phonon coupling strength through the Huang-Rhys factors of 0.05-0.1 and 0.02-0.04 for low-energy optical phonons with energies of 3.2 and 5.1 meV, respectively. The pronounced size dependence of both coupling strengths and phonon lifetimes offers a path to tune the optical transitions between polaron states and to tailor the coherent optical dynamics in perovskite semiconductors for solid-state quantum technologies.

cond-mat.mes-hall↗
arXiv2022

Effective Landé factors of electrons and holes in lead chalcogenide nanocrystals

The Landé or g-factors of charge carriers in solid state systems provide invaluable information about response of quantum states to external magnetic fields and are key ingredients in description of spin-dependent phenomena in nanostructures. We report on the comprehensive theoretical analysis of electron and hole g-factors in lead chalcogenide nanocrystals. By combining symmetry analysis, atomistic calculations, and extended k.p theory, we relate calculated linear-in-magnetic field energy splittings of confined electron states in nanocrystals to the intravalley g-factors of the multi-valley bulk materials, renormalized due to the quantum confinement. We demonstrate that this renormalization is correctly reproduced by analytical expressions derived in the framework of the extended k.p model.

cond-mat.mes-hall↗
arXiv2022

Exciton interaction with acoustic phonons in PbS nanocrystals

The interaction between exciton and acoustic phonons via deformation potential in PbS nanocrystals is calculated in the ${\bf k} \cdot {\bf p}$ model. The size-dependence of the Huang-Rhys factors for the fundamental breathing mode and its overtones as well as for spheroidal vibrations with the total angular momentum $j=2$ are evaluated.

cond-mat.mes-hall↗
arXiv2022

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

cond-mat.mes-hall↗
arXiv2018

Optical orientation and alignment of excitons in ensembles of inorganic perovskite nanocrystals

We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI$_3$ perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120μeV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals.

cond-mat.mes-hall↗
arXiv2017

Valley and spin splittings in PbSe nanowires

We use an empirical tight-binding approach to calculate electron and hole states in [111]-grown PbSe nanowires. We show that the valley-orbit and spin-orbit splittings are very sensitive to the atomic arrangement within the nanowire elementary cell and differ for [111]-nanowires with microscopic $D_{3d}$, $C_{2h}$ and $D_{3}$ symmetries. For the nanowire diameter below 4 nm the valley-orbit splittings become comparable with the confinement energies and the $\boldsymbol{k}\cdot\boldsymbol{p}$ method is inapplicable. Nanowires with the $D_{3}$ point symmetry having no inversion center exhibit giant spin splitting $E = αk_z$, linear in one-dimensional wave vector $k_z$, with the constant $α$ up to 1 eV$\cdot$Å.

cond-mat.mes-hall↗
arXiv2013

Finite-height effect on electron energy structure of lead salts nanorods

The effect of the finite height of a cylindrical lead salt nanorod on its electronic structure is studied within the effective mass formalism. It is demonstrated that for most practical purposes it can be accounted for by sampling the subband dispersion dependencies of the infinite cylindrical quantum wire of the same radius at wave numbers $k_z=πn_z/H$, where $H$ is the nanorod height and $n_z$ is an integer. However, under certain conditions the otherwise degenerate electron energy levels will repel. A detailed account of these conditions is provided.

cond-mat.mes-hall↗
arXiv2011

Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals without Inversion Center

Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has T_d symmetry, the splitting is strongly suppressed as compared to the more symmetric nanocrystals with O_h symmetry, having an inversion center.

cond-mat.mes-hall↗
arXiv2008

Electron-phonon Interaction in Non-polar Quantum Dots Induced by the Amorphous Polar Environment

We propose a mechanism of energy relaxation for carriers confined in a non-polar quantum dot surrounded by an amorphous polar environment. The carrier transitions are due to their interaction with the oscillating electric field induced by the local vibrations in the surrounding amorphous medium. We demonstrate that this mechanism controls energy relaxation for electrons in Si nanocrystals embedded in a SiO$_2$ matrix, where conventional mechanisms of electron-phonon interaction are not efficient.

cond-mat.mes-hall↗
arXiv2008

Carrier relaxation in Si/SiO$_2$ quantum dots

Carrier relaxation due to both optical and nonradiative intraband transitions in silicon quantum dots in SiO$_2$ has been considered. Interaction of confined holes with optical phonons has been studied. The Huang-Rhys factor is calculated for such transitions. The probability of intraband transition of a confined hole emitting several optical phonons is estimated.

cond-mat.mtrl-sci↗
arXiv2007

Coherent defect-assisted multiphonon intraband carrier relaxation in semiconductor quantum dots

new defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.

cond-mat.mes-hall↗