arXiv subjects

Mikhail O. Nestoklon

Publications and source records attributed to Mikhail O. Nestoklon.

10 recordsLinked to original sources

arXiv2026

Layer-dependent Landé $g$-factors of electrons, holes, and excitons in two-dimensional Ruddlesden-Popper lead halide perovskites

Two-dimensional Ruddlesden-Popper lead halide perovskites provide a valuable platform for tailoring charge and spin properties through quantum confinement and reduced symmetry. While the electron and hole Landé $g$-factors in bulk lead halide perovskites exhibit a universal dependence on the band gap energy, their evolution in two-dimensional perovskites has remained largely unexplored. Here, the Zeeman splittings of electrons and holes in (PEA)$_2$MA$_{n-1}$Pb$_n$I$_{3n+1}$ perovskites with the number of inorganic layers ovarying in the range $n=1,...,8$ are measured by means of the spin-flip Raman scattering and time-resolved Kerr rotation magneto-optical techniques. A systematic evolution of the electron and hole $g$-factors with decreasing layer thickness, which deviates from the universal bulk behavior and reveals confinement-driven trends similar to those observed in perovskite nanocrystals, is found. The experimental results are in good qualitative agreement with empirical tight-binding calculations. The exciton $g$-factors are evaluated from the Zeeman splittings of the exciton resonances in reflectivity measured in pulsed magnetic fields up to 55~T. These results provide comprehensive insight into the spin properties of two-dimensional lead halide perovskites and establish them as a tunable platform for engineering spin-dependent phenomena in quantum-confined semiconductors.

cond-mat.other↗

arXiv2026

Bright-dark exciton splitting in lead halide perovskite crystals accessed via quantum beats in photon echoes

Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization-sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure, which is hidden by inhomogeneous broadening. Exciton quantum beats observed in both Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1$s$ exciton states, enabling determination of the fine structure and bright-dark splittings. Application of this technique to bulk mixed halide perovskite crystals FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ reveals a bright-dark exciton splitting of $Δ_\mathrm{X}=0.46~$meV, along with electron and hole Landé $g$ factors $g_\mathrm{e}=3.38$ and $g_\mathrm{h}=-1.14$, respectively. The quantum beats persist on timescales of 20--50$~$ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2$~$K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright-dark splitting of $\sim0.06~$meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of $\sim0.035~$meV due to anisotropic potentials.

cond-mat.mes-hall↗

arXiv2026

Magnetic switching of exciton lifetime in CrSBr

Exciton dynamics in layered magnetic semiconductors provide a sensitive probe of the interplay between spin order and light-matter interaction. Here, we study thin CrSBr layers using time-resolved photoluminescence spectroscopy in an external magnetic field, revealing a step-like reduction in the exciton lifetime from 11 to 7 ps, during the magnetization flip from the antiferromagnetic to the ferromagnetic phase. The reduction of the exciton lifetime in the ferromagnetic phase persists below the Néel temperature, as evidenced by its strong magnetic-field dependence that disappears in the paramagnetic phase. Ab initio calculations reveal a one-dimensional nature of free excitons accompanied by a pronounced change in the oscillator strength across the magnetic phase transition predicting a shorter radiative lifetime of free excitons in the antiferromagnetic phase of CrSBr contradicting the experimental observations. This discrepancy is explained by strong localization of excitons at low tempature. We show both experimentally and theoretically that the observed magnetic switching of the exciton lifetime is attributed to a larger exciton localization volume leading to a larger oscillator strength in the ferromagnetic phase. The results show that disorder-induced localization effects play a key role in exciton dynamics in CrSBr.

cond-mat.mes-hall↗

arXiv2025

Hyperfine interaction of electrons confined in CsPbI$_3$ nanocrystals with nuclear spin fluctuations

The coherent spin dynamics of electrons are investigated for CsPbI$_3$ perovskite nanocrystals in a glass matrix using time-resolved Faraday ellipticity. In nanocrystals with a diameter of about 11 nm, the Larmor precession frequency has a linear dependence on magnetic field corresponding to the electron Landé $g$-factor of 2.07. We find a finite Larmor precession frequency at zero magnetic field, corresponding to the electron spin splitting of $0.8$ $μ$eV. This splitting is explained by the hyperfine interaction with nuclear spin fluctuations. Our model analysis shows that the hyperfine interaction for the conduction band electrons is contributed both by the $p$-orbitals of the lead atoms and by the $s$-orbitals of the iodine atoms, with the leading contribution to the hyperfine field fluctuations coming from iodine. This fact agrees well with the 9% iodine contribution to the Bloch amplitude of the conduction band, obtained by DFT calculations. From these findings, the atomic hyperfine constant for the $5s$-orbital of iodine is evaluated as 190 $μ$eV.

cond-mat.mes-hall↗

arXiv2025

Confined acoustic phonons in CsPbI3 nanocrystals explored by resonant Raman scattering on excitons

Optical properties of the lead halide perovskites nanocrystals are controlled by confined excitons and rich spectrum of confined acoustic and optical phonons. We study experimentally and theoretically the exciton-phonon interaction in CsPbI3 perovskite nanocrystals embedded in a glass matrix. Energies of phonon modes allowed by selection rules are detected by resonant Raman scattering for nanocrystals with sizes of 4-13 nm, covering exciton energies of 1.72-2.25 eV. While optical phonon energies remain size-independent, the energies of confined acoustic phonons increase in smaller nanocrystals. Acoustic phonons are modeled within the continuum approximation using elastic constants computed by density functional theory. The model provides the energy spectra of confined phonons for nanocrystals of various shapes (cube, sphere, spheroid), crystal symmetries (orthorhombic and tetragonal), and sizes. Exciton confinement restricts efficient coupling to only few phonon modes observable in Raman spectra. By comparing experimental data with model predictions, we conclude that the nanocrystals in our samples predominantly have spherical or spheroidal shapes.

cond-mat.mtrl-sci↗

arXiv2025

Efficient launch of shear phonons in photostrictive halide perovskites

Optical generation of transverse coherent phonons by femtosecond light pulses is appealing for high-speed sub-THz active control of material properties. Lead-free double perovskite semiconductors, such as Cs2AgBiBr6, attract particular interest due to their cubic to tetragonal phase transition below room temperature and strong polaron effects from carrier-lattice coupling. Here, we reveal that the anisotropic photostriction in halide perovskites with tetragonal crystal structure represents an efficient non-thermal tool for generating transverse coherent phonons. In particular, we demonstrate that along with compressive strain, optical generation of photoexcited carriers leads to strong shear strain in Cs2AgBiBr6 below the phase transition temperature of 122 K. Using time-domain Brillouin spectroscopy, we observe coherent transverse and longitudinal acoustic phonons with comparable amplitudes in the tetragonal phase, while in the cubic phase only longitudinal phonons are generated. The polarization of the photoinduced transverse phonons is dictated by the projection of the c-axis on the surface plane, which leads to a prominent anisotropic polarization response in the detection. The generated strain pulses correspond to transverse acoustic soft eigenmodes with a strong temperature dependence of dispersion, which provides an additional degree of freedom for active hypersonic control.

cond-mat.mtrl-sci↗

arXiv2024

Effect of crystal symmetry of lead halide perovskites on the optical orientation of excitons

The great variety of lead halide perovskite semiconductors represents an outstanding platform for studying crystal symmetry effects on the spin-dependent properties. Access to them is granted through optical orientation of exciton and carrier spins by circularly polarized photons. Here, the exciton spin polarization is investigated at $1.6$\,K cryogenic temperature in four lead halide perovskite crystals with different symmetries: (almost) cubic in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ and FAPbBr$_3$, and orthorhombic in MAPbI$_3$ and CsPbBr$_3$. Giant optical orientation of 85\% is found for the excitons in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$, MAPbI$_3$, and CsPbBr$_3$, while it amounts to 20\% in FAPbBr$_3$. For all studied crystals, the optical orientation is robust to detuning of the laser photon energy from the exciton resonance, remaining constant for high energy detunings up to 0.3~eV, above which it continuously decreases to zero for detunings exceeding 1~eV. No acceleration of the spin relaxation for excitons with large kinetic energy is found in the cubic and orthorhombic crystals. This evidences the absence of the Dyakonov-Perel spin relaxation mechanism, which is based on the Rashba-Dresselhaus splitting of spin states at finite $k$-vectors. This indicates that the spatial inversion symmetry is maintained in perovskite crystals, independent of the cubic or orthorhombic phase.

cond-mat.mtrl-sci↗

arXiv2024

Landé $g$-factors of electrons and holes strongly confined in CsPbI$_3$ perovskite nanocrystals in glass

The Landé $g$-factor of charge carriers is a key parameter in spin physics controlling spin polarization and spin dynamics. In turn, it delivers information of the electronic band structure in vicinity of the band gap and its modification in nanocrystals provided by strong carrier confinement. The coherent spin dynamics of electrons and holes are investigated in CsPbI$_3$ perovskite nanocrystals with sizes varied from 4 to 16 nm by means of time-resolved Faraday ellipticity at the temperature of 6 K. The Landé $g$-factors of the charge carriers are evaluated through the Larmor spin precession in magnetic fields up to 430 mT across the spectral range from 1.69 to 2.25 eV, provided by variation of the nanocrystal size. The spectral dependence of the electron $g$-factor follows the model predictions when accounting for the mixing of the electronic bands with increasing confinement resulting from a decrease of the nanocrystal size. The spectral dependence of the hole $g$-factor, changing from $-0.19$ to $+1.69$, is considerably stronger than expected from the model. We analyze several mechanisms and conclude that none of them can be responsible for this difference. The renormalizations of the electron and hole $g$-factors roughly compensate each other, providing spectral independence for the bright exciton $g$-factor with a value of about $+2.2$.

cond-mat.mes-hall↗

arXiv2024

Temperature dependence of the electron and hole Landé g-factors in CsPbI3 nanocrystals in a glass matrix

The coherent spin dynamics of electrons and holes in CsPbI3 perovskite nanocrystals in a glass matrix are studied by the time-resolved Faraday ellipticity technique in magnetic fields up to 430 mT across a temperature range from 6 K up to 120 K. The Landé g-factors and spin dephasing times are evaluated from the observed Larmor precession of electron and hole spins. The nanocrystal size in the three studied samples varies from about 8 to 16 nm, resulting in exciton transition varying from 1.69 to 1.78 eV at the temperature of 6 K, allowing us to study the corresponding energy dependence of the g-factors. The electron g-factor decreases with increasing confinement energy in the NCs as result of NC size reduction, and also with increasing temperature. The hole g-factor shows the opposite trend. A model analysis shows that the variation of g-factors with NC size arises from the transition energy dependence of the g-factors, which becomes strongly renormalized by temperature.

cond-mat.mes-hall↗

arXiv2023

Giant optical orientation of exciton spins in lead halide perovskite crystals

Optical orientation of carrier spins by circularly polarized light is the basis of spin physics in semiconductors. Here, we demonstrate strong optical orientation of 85\%, approaching the ultimate limit of unity, for excitons in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ lead halide perovskite bulk crystals. Time-resolved photoluminescence allows us to distinguish excitons with 60~ps lifetime from electron-hole recombination in the spin dynamics detected via coherent spin quantum beats in magnetic field. We reveal electron-hole spin correlations through linear polarization beats after circularly polarized excitation. Detuning of the excitation energy from the exciton resonance up to 0.5~eV does not reduce the optical orientation, evidencing clean chiral selection rules in agreement with atomistic calculations, and suppressed spin relaxation of electrons and holes even with large kinetic energies.

cond-mat.mes-hall↗