arXiv subjects

M. O. Nestoklon

Publications and source records attributed to M. O. Nestoklon.

At least 19 recordsLinked to original sources

arXiv2026

Structural phase transitions in double perovskite crystals studied by Brillouin light scattering

Inorganic lead-free double perovskites represent particular interest as non-toxic and stable material platform for optoelectronic applications. Here, we employ Brillouin light scattering spectroscopy to investigate the elastic properties and structural phase transitions in single crystals of Cs2AgBiBr6 and Cs2AgBiCl6. A complete set of elastic constants is determined from the Brillouin scattering measurements performed along three different crystallographic directions. Both materials exhibit similar elastic constants and weak elastic anisotropy in the cubic phase. At low temperatures, the lifting of degeneracy of transverse acoustic phonon modes is attributed to a lowering of crystal symmetry. From the temperature dependence of the acoustic phonon frequencies, we determine the structural phase transition temperature of about 43 K for Cs2AgBiCl6, compared to 122 K for the cubic-to-tetragonal phase transition in Cs2AgBiBr6.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Theory for electron and hole fine structure and Landé g-factors in lead chalcogenide nanowires

Using the atomistic tight-binding method in combination with symmetry analysis and extended effective mass theory we derive a phenomenological model for the fine structure of the ground electron and hole states in $[111]$-grown PbX, X=S,Se hexagonal and cylindrical nanowires. Projection of the tight-binding states to the basis of valley states enables the explicit parametrization of the valley mixing Hamiltonian, which is essential to obtain correct combinations of the valley states for electron (hole) ground levels analytically. The effective Hamiltonian of valley mixing allows to convert the intravalley components of the $g$-factors tensors to the Zeeman splittings of electron and hole levels in nanowires.Tensors of the intravalley electron (hole) Landé $g$-factors, parameters of the valley mixing Hamiltonian as well as the valley and anisotropic splitting energies and velocity matrix elements are also calculated.

cond-mat.mes-hall↗

arXiv2024

Untangling the valley structure of states for intravalley exchange anisotropy in lead chalcogenides quantum dots

We put forward a generalized procedure which allows to restore the bulk-like electron and hole wave functions localized in certain valleys from the wave functions of quantum confined electron/hole states obtained in atomistic calculations of nanostructures. As a demonstration, the procedure is applied to the lead chalcogenide quantum dots to extract the effective intravalley Hamiltonian of the exchange interaction for the ground exciton state PbS and PbSe quantum dots. Renormalization of the anisotropic intravalley matrix elemets of velocity is also calculated. The results demonstrate that the matrix elements of intravalley exchange in PbS quantum dots are much more anisotropic than ones in PbSe.

cond-mat.mes-hall↗

arXiv2023

Tailoring the electron and hole Landé factors in lead halide perovskite nanocrystals by quantum confinement and halide exchange

The tunability of the optical properties of lead halide perovskite nanocrystals makes them highly appealing for applications. Both, halide anion exchange and quantum confinement pave the way for tailoring their band gap energy. For spintronics applications, the Landé g-factors of electrons and hole are of great importance. By means of the empirical tight-binding and $\textbf{k}\cdot\textbf{p}$ methods, we calculate them for nanocrystals of the class of all-inorganic lead halide perovskites CsPb$X_3$ ($X = \text{I},\,\text{Br},\,\text{Cl}$). The hole g-factor as function of the band gap follows the universal dependence found for bulk perovskites, while for the electrons a considerable modification is predicted. Based on the $\textbf{k}\cdot\textbf{p}$ analysis we conclude that this difference arises from the interaction of the bottom conduction band with the spin-orbit split electron states. The model predictions are confirmed by experimental data for the electron and hole g-factors in CsPbI3 nanocrystals placed in a glass matrix, measured by time-resolved Faraday ellipticity in a magnetic field at cryogenic temperatures.

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

Weak localization in p-type heterostructures in the presence of parallel magnetic field

Theory of weak localization is developed for two-dimensional holes in the presence of in-plane magnetic field. The Zeeman splitting even in the hole momentum results in the spin-dependent phase changing the quantum interference. The negative correction to the conductivity is shown to decrease by a factor of two by the in-plane magnetic field. The positive magnetoconductivity in a classically weak perpendicular field caused by the weak localization is calculated for both quadratic and quartic in momentum Zeeman hole splittings. Calculations show that the conductivity corrections are very close to each other in these two cases of low and high hole density.

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↗

arXiv2021

The Landé factors of electrons and holes in lead halide perovskites: universal dependence on the band gap

The Landé or $g$-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI$_3$, MAPb(Br$_{0.5}$Cl$_{0.5}$)$_3$, MAPb(Br$_{0.05}$Cl$_{0.95}$)$_3$, FAPbBr$_3$, FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$) and all-inorganic (CsPbBr$_3$) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10~T at cryogenic temperatures. Further, we use first-principles DFT calculations in combination with tight-binding and $\mathbf k \cdot \mathbf p$ approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2021

Electron density effect on spin-orbit interaction in [001] GaAs quantum wells

The spin-orbit interaction of two-dimensional (2D) electrons in semiconductor quantum wells is usually considered to be determined by the band profile of a heterostructure. In the GaAs/AlGaAs type heterosystems, this interaction consists of the isotropic Bychkov-Rashba term, which is absent in symmetric wells, and the anisotropic Dresselhaus term, reflecting the lattice symmetry. It is well-known that the first term can be controlled by electric fields in the growth direction: external or internal, induced by a charge density of 2D electrons. In this work we reveal that the 2D electron charge can substantially affect also the Dresselhaus interaction in symmetric quantum wells. Within the one-band electron Hamiltonian containing, together with the bulk Dresselhaus interaction, the two contributions to the Dresselhaus term from the quantum well interfaces, we show that the internal electric field from the 2D electron charge density can substantially renormalize the anisotropic spin-orbit interaction of 2D electrons. This effect may be important in quantitative studies of spin-dependent phenomena in quantum wells.

cond-mat.mes-hall↗

arXiv2021

Tight-binding description of inorganic lead halide perovskites in cubic phase

Band structure of inorganic lead halide perovskites is substantially different from the band structure of group IV, III-V and II-VI semiconductors. However, the standard empirical tight-binding model with sp3d5s* basis gives nearly perfect fit of band structure calculated in the density functional theory. The tight-binding calculations of ultrathin CsPbI3 layer show good agreement with the corresponding DFT calculations. The parameters corrected for the experimental data allow for the numerically cheap atomistic calculations of inorganic perovskite nanostructures.

cond-mat.mes-hall↗

arXiv2019

Electronic band structure in $n$-type GaAs/AlGaAs wide quantum wells in tilted magnetic field

Oscillations of the real component of AC conductivity $σ_1$ in a magnetic field were measured in the n-AlGaAs/GaAs structure with a wide (75 nm) quantum well by contactless acoustic methods at $T$=(20-500)~mK. In a wide quantum well, the electronic band structure is associated with the two-subband electron spectrum, namely the symmetric (S) and antisymmetric (AS) subbands formed due to electrostatic repulsion of electrons. A change of the oscillations amplitude in tilted magnetic field observed in the experiments occurs due to crossings of Landau levels of different subbands (S and AS) at the Fermi level. The theory developed in this work shows that these crossings are caused by the difference in the cyclotron energies in the S and AS subbands induced by the in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2019

Tight-binding calculations of SiGe alloy nanocrystals in SiO2 matrix

In the empirical tight-binding approach we study the electronic states in spherical SiGe nanocrystals embedded in SiO2 matrix. The energy and valley structure is obtained as a function of Ge composition and nanocrystal size. The calculations show that the mixing of hot electrons in the nanocrystal with electrons in wide band gap matrix is possible and this mixing strongly depends on the Ge composition in the nanocrystal.

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↗

arXiv2018

Intersubband scattering in n-GaAs/AlGaAs wide quantum wells

Slow magnetooscilations of the conductivity are observed in a 75 nm wide quantum well at heating of the two-dimensional electrons by a high-intensity surface acoustic wave. These magnetooscillations are caused by intersubband elastic scattering between the symmetric and asymmetric subbands formed due to an electrostatic barrier in the center of the quantum well. The tunneling splitting between these subbands as well as the intersubband scattering rate are determined.

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↗