arXiv subjects

V. V. Belykh

Publications and source records attributed to V. V. Belykh.

At least 19 recordsLinked to original sources

arXiv2022

Mode locking of hole spin coherences in CsPb(Cl,Br)$_3$ perovskite nanocrystals

The spin physics of perovskite nanocrystals with confined electrons or holes is attracting increasing attention, both for fundamental studies and spintronic applications. Here, stable CsPb(Cl$_{0.5}$Br$_{0.5}$)$_3$ lead halide perovskite nanocrystals embedded in a fluorophosphate glass matrix are studied by time-resolved optical spectroscopy to unravel the coherent spin dynamics of holes and their interaction with nuclear spins of the $^{207}$Pb isotope. We demonstrate the spin mode locking effect provided by the synchronization of the Larmor precession of single hole spins in each nanocrystal in the ensemble that are excited periodically by a laser in an external magnetic field. The mode locking is enhanced by nuclei-induced frequency focusing. An ensemble spin dephasing time $T_2^*$ of a nanosecond and a single hole spin coherence time of $T_2=13\,$ns are measured. The developed theoretical model accounting for the mode locking and nuclear focusing for randomly oriented nanocrystals with perovskite band structure describes the experimental data very well.

cond-mat.mes-hall↗

arXiv2022

Selective measurement of the longitudinal electron spin relaxation time $T_1$ of Ce$^{3+}$ ions in a YAG lattice: Resonant spin inertia

Electron spin oriented along an external magnetic field is subject to longitudinal spin relaxation with characteristic time $T_1$. The corresponding decay is nonoscillating, so one cannot readily ascribe $T_1$ to a certain $g$ factor. This becomes a problem when several electronic states with different $g$ factors are present in the system, e.g. electrons and holes. We solve this problem by optically pumping spin polarization and then selectively depolarizing it using a radio frequency (rf) field. By modulating the rf field one can observe the retarded modulation of spin polarization which depends on the relation between the modulation period and $T_1$. Using this selective spin inertia method, we unveil the strong anisotropy of $T_1$ for rare-earth Ce$^{3+}$ ions in a YAG crystal at low temperatures and low magnetic fields. We also show that the spread of Larmor frequencies within the electron ensemble in this system is not static, but results from the fluctuations of internal magnetic fields on a timescale much shorter than $T_1$.

cond-mat.mes-hall↗

arXiv2021

Stimulated resonant spin amplification reveals millisecond electron spin coherence time of rare-earth ions in solids

The inhomogeneity of an electron spin ensemble as well as fluctuating environment acting upon individual spins drastically shorten the spin coherence time $T_2$ and hinder coherent spin manipulation. We show that this problem can be solved by the simultaneous application of a radiofrequency (rf) field, which stimulates coherent spin precession decoupled from an inhomogeneous environment, and periodic optical pulses, which amplify this precession. The resulting resonance, taking place when the rf field frequency approaches the laser pulse repetition frequency, has a width determined by the spin coherence time $T_2$ that is free from the inhomogeneity effects. We measure a 50-Hz-narrow electron spin resonance and milliseconds-long $T_2$ for electrons in the ground state of Ce$^{3+}$ ions in the YAG lattice at low temperatures, while the inhomogeneous spin dephasing time $T_2^*$ is only 25 ns. This study paves the way to coherent optical manipulation in spin systems decoupled from their inhomogeneous environment.

cond-mat.mes-hall↗

arXiv2021

Spatiotemporal dynamics of free and bound excitons in CVD-grown MoS$_2$ monolayer

We study photoluminescence (PL) spectra and exciton dynamics of MoS$_2$ monolayer (ML) grown by the chemical vapor deposition technique. In addition to the usual direct A-exciton line we observe a low-energy line of bound excitons dominating the PL spectra at low temperatures. This line shows unusually strong redshift with increase in the temperature and submicrosecond time dynamics suggesting indirect nature of the corresponding transition. By monitoring temporal dynamics of exciton PL distribution in the ML plane we observe diffusive transport of A-excitons and measure the diffusion coefficient up to $40$~cm$^2$/s at elevated excitation powers. The bound exciton spatial distribution spreads over tens of microns in $\sim 1$ $μ$s. However this spread is subdiffusive, characterized by a significant slowing down with time. The experimental findings are interpreted as a result of the interplay between the diffusion and Auger recombination of excitons.

cond-mat.mes-hall↗

arXiv2020

Nonexponential photoluminescence dynamics in an inhomogeneous ensemble of excitons in WSe$_2$ monolayers

The spectral and spatiotemporal dynamics of photoluminescence in monolayers of transition metal dichalcogenide WSe$_2$ obtained by mechanical exfoliation on a Si/SiO$_2$ substrate is studied over a wide range of temperatures and excitation powers. It is shown that the dynamics is nonexponential and, for times $t$ exceeding $\sim$50 ps after the excitation pulse, is described by a dependence of the form $1/(t+t_0)$. Photoluminescence decay is accelerated with a decrease in temperature, as well as with a decrease in the energy of emitting states. It is shown that the observed dynamics cannot be described by a bimolecular recombination process, such as exciton--exciton annihilation. A model that describes the nonexponential photoluminescence dynamics by taking into account the spread of radiative recombination times of localized exciton states in a random potential gives good agreement with experimental data.

cond-mat.mes-hall↗

arXiv2020

Resonant spin amplification meets electron spin resonance in $n$-GaAs

Periodic excitation of electron spin polarization by consecutive laser pulses in phase with Larmor spin precession about a magnetic field results in resonant spin amplification (RSA). We observe a drastic modification of RSA in $n$-doped bulk GaAs under the influence of external oscillating magnetic field. We find a double-peaked electron spin resonance instead of a single-peaked resonance expected without optical pumping. The frequency splitting increases linearly with amplitude of field oscillations, while the spin deviation increases quadratically. Moreover, we show that the oscillating field can both significantly suppress RSA and induce new conditions for resonance. Using quaternions to describe spin rotations, we develop a theory that simultaneously considers spin precession, decay, and amplification and reproduces the entire set of the experimental data. Using the radio-frequency field allows one to control the conditions of RSA and achieve fine tuning of average spin polarization without modifying the parameters of optical pumping.

cond-mat.mes-hall↗

arXiv2020

Anomalous magnetic suppression of spin relaxation in a two-dimensional electron gas in a GaAs/AlGaAs quantum well

We study the spin dynamics in a high-mobility two-dimensional electron gas confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of the spin relaxation is found: as the magnetic field becomes stronger, the spin relaxation time first increases quadratically but then changes to a linear dependence, before it eventually becomes oscillatory, whereby the longitudinal and transverse times reach maximal values at even and odd filling Landau level factors, respectively. We show that the suppression of spin relaxation is due to the effect of electron gyration on the spin-orbit field, while the oscillations correspond to oscillations of the density of states appearing at low temperatures and high magnetic fields. The transition from quadratic to linear dependence can be related to a transition from classical to Bohm diffusion and reflects an anomalous behavior of the two-dimensional electron gas analogous to that observed in magnetized plasmas.

cond-mat.mes-hall↗

arXiv2020

Optical detection of electron spin dynamics driven by fast variations of a magnetic field: a simple method to measure $T_1$, $T_2$, and $T_2^*$ in semiconductors

We develop a simple method for measuring the electron spin relaxation times $T_1$, $T_2$ and $T_2^*$ in semiconductors and demonstrate its exemplary application to $n$-type GaAs. Using an abrupt variation of the magnetic field acting on electron spins, we detect the spin evolution by measuring the Faraday rotation of a short laser pulse. Depending on the magnetic field orientation, this allows us to measure either the longitudinal spin relaxation time $T_1$ or the inhomogeneous transverse spin dephasing time $T_2^*$. In order to determine the homogeneous spin coherence time $T_2$, we apply a pulse of an oscillating radiofrequency (rf) field resonant with the Larmor frequency and detect the subsequent decay of the spin precession. The amplitude of the rf-driven spin precession is significantly enhanced upon additional optical pumping along the magnetic field.

cond-mat.mes-hall↗

arXiv2019

Radiofrequency driving of coherent electron spin dynamics in $n$-GaAs detected by Faraday rotation

We suggest a new pump-probe method for studying semiconductor spin dynamics based on pumping of carrier spins by a pulse of oscillating radiofrequency (rf) magnetic field and probing by measuring the Faraday rotation of a short laser pulse. We demonstrate this technique on $n$-GaAs and observe the onset and decay of coherent spin precession during and after the course of rf pulse excitation. We show that the rf field resonantly addresses the electron spins with Larmor frequencies close to that of the rf field. This opens the opportunity to determine the homogeneous spin coherence time $T_2$, that is inaccessible directly in standard all-optical pump-probe experiments.

cond-mat.mes-hall↗

arXiv2019

Jitter of condensation time and dynamics of spontaneous symmetry breaking in a gas of microcavity polaritons

We investigate the statistics of microcavity polariton Bose-Einstein condensation by measuring photoluminescence dynamics from a GaAs microcavity excited by single laser excitation pulses. We directly observe fluctuations (jitter) of the polariton condensation onset time and model them using a master equation for the occupancy probabilities. The jitter of the condensation onset time is an inherent property of the condensate formation and its magnitude is approximately equal to the rise time of the condensate density. We investigate temporal correlations between the emission of condensate in opposite circular or linear polarizations by measuring the second-order correlation function $g^{(2)}(t_1,t_2)$. Polariton condensation is accompanied by spontaneous symmetry breaking revealed by the occurrence of random (i.e., varying from pulse to pulse) circular and linear polarizations of the condensate emission. The degree of circular polarization generally changes its sign in the course of condensate decay, in contrast to the degree of linear polarization.

cond-mat.mes-hall↗

arXiv2018

Electron and hole spin relaxation in InP-based self-assembled quantum dots emitting at telecom wavelengths

We investigate the electron and hole spin relaxation in an ensemble of self-assembled InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As/InP quantum dots with emission wavelengths around $1.5$~$μ$m by pump-probe Faraday rotation spectroscopy. Electron spin dephasing due to the randomly oriented nuclear Overhauser fields is observed. At low temperatures we find a sub-microsecond longitudinal electron spin relaxation time $T_1$ which unexpectedly strongly depends on temperature. At high temperatures the electron spin relaxation time is limited by optical phonon scattering through spin-orbit interaction decreasing down to $0.1$~ns at 260~K. We show that the hole spin relaxation is activated much more effectively by a temperature increase compared to the electrons.

cond-mat.mes-hall↗

arXiv2018

Decay and revival of electron spin polarization in an ensemble of (In,Ga)As quantum dots

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the build-up of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes.

cond-mat.mes-hall↗

arXiv2018

Quantum Interference Controls the Electron Spin Dynamics in n-GaAs

Manifestations of quantum interference effects in macroscopic objects are rare. Weak localization is one of the few examples of such effects showing up in the electron transport through solid state. Here we show that weak localization becomes prominent also in optical spectroscopy via detection of the electron spin dynamics. In particular, we find that weak localization controls the free electron spin relaxation in semiconductors at low temperatures and weak magnetic fields by slowing it down by almost a factor of two in $n$-doped GaAs in the metallic phase. The weak localization effect on the spin relaxation is suppressed by moderate magnetic fields of about 1 T, which destroy the interference of electron trajectories, and by increasing the temperature. The weak localization suppression causes an anomalous decrease of the longitudinal electron spin relaxation time $T_1$ with magnetic field, in stark contrast with well-known magnetic field induced increase in $T_1$. This is consistent with transport measurements which show the same variation of resistivity with magnetic field. Our discovery opens a vast playground to explore quantum magneto-transport effects optically in the spin dynamics.

cond-mat.mes-hall↗

arXiv2017

Electron charge and spin delocalization revealed in the optically probed longitudinal and transverse spin dynamics in $n$-GaAs

The evolution of the electron spin dynamics as consequence of carrier delocalization in $n$-type GaAs is investigated by the recently developed extended pump-probe Kerr/Faraday rotation spectroscopy. We find that isolated electrons localized on donors demonstrate a prominent difference between the longitudinal and transverse spin relaxation rates in magnetic field, which is almost absent in the metallic phase. The inhomogeneous transverse dephasing time $T_2^*$ of the spin ensemble strongly increases upon electron delocalization as a result of motional narrowing that can be induced by increasing either the donor concentration or the temperature. An unexpected relation between $T_2^*$ and the longitudinal spin relaxation time $T_1$ is found, namely that their product is about constant, as explained by the magnetic field effect on the spin diffusion. We observe a two-stage longitudinal spin relaxation which suggests the establishment of spin temperature in the system of exchange-coupled donor-bound electrons.

cond-mat.mes-hall↗

arXiv2017

Electron spin dynamics of Ce$^{3+}$ ions in YAG crystals studied by pulse-EPR and pump-probe Faraday rotation

The spin relaxation dynamics of Ce$^{3+}$ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the 4$f$ ground state, while pump-probe Faraday rotation provides also access to the excited 5$d$ state. We measure a millisecond spin-lattice relaxation time $T_1$, a microsecond spin coherence time $T_2$ and a $\sim 10$ ns inhomogeneous spin dephasing time $T_2^*$ for the Ce$^{3+}$ ground state at low temperatures. The spin-lattice relaxation of Ce$^{3+}$ ions is due to modified Raman processes involving the optical phonon mode at 125 cm$^{-1}$. The relaxation at higher temperature goes through a first excited level of the $^{2}$F$_{5/2}$ term at about $\hbar ω\approx 228$ cm$^{-1}$. Effects provided by the hyperfine interaction of the Ce$^{3+}$ with the $^{27}$Al nuclei are observed.

cond-mat.mtrl-sci↗

arXiv2017

Extended pump-probe Faraday rotation spectroscopy of the submicrosecond electron spin dynamics in n-type GaAs

We develop an extended pump-probe Faraday rotation technique to study the submicrosecond electron spin dynamics with picosecond time resolution in a wide range of magnetic fields. The electron spin dephasing time $T_2^*$ and the longitudinal spin relaxation time $T_1$, both approaching $250$ ns in weak fields, are measured thereby in $n$-type bulk GaAs. By tailoring the pump pulse train through increasing the contained number of pulses, the buildup of resonant spin amplification is demonstrated for the electron spin polarization. The spin precession amplitude in high magnetic fields applied in the Voigt geometry shows a non-monotonic dynamics deviating strongly from a mono-exponential decay and revealing slow beatings. The beatings indicate a two spin component behavior with a $g$-factor difference of $Δg \sim 4\times10^{-4}$, much smaller than the $Δg$ expected for free and donor-bound electrons. This $g$-factor variation indicates efficient, but incomplete spin exchange averaging.

cond-mat.mes-hall↗

arXiv2017

Non-equilibrium nuclear spin distribution function in quantum dots subject to periodic pulses

Electron spin dephasing in a singly charged semiconductor quantum dot can partially be suppressed by periodic laser pulsing. We propose a semi-classical approach describing the decoherence of the electron spin polarization governed by the hyperfine interaction with the nuclear spins as well as the probabilistic nature of the photon absorption. We use the steady-state Floquet condition to analytically derive two subclasses of resonance conditions excellently predicting the peak locations in the part of the Overhauser field distribution which is projected in the direction of the external magnetic field. As a consequence of the periodic pulsing, a non-equilibrium distribution develops as a function of time. The numerical simulation of the coupled dynamics reveals the influence of the hyperfine coupling constant distribution onto the evolution of the electron spin polarisation before the next laser pulse. Experimental indications are provided for both subclasses of resonance conditions.

cond-mat.mes-hall↗

arXiv2016

Dynamics of spatial coherence and momentum distribution of polaritons in a semiconductor microcavity under conditions of Bose-Einstein condensation

The dynamics of spatial coherence and momentum distribution of polaritons in the regime of Bose-Einstein condensation are investigated in a GaAs microcavity with embedded quantum wells under nonresonant excitation with picosecond laser pulses. It is shown that the onset of the condensate first order sparial coherence is accompanied by narrowing of the polariton momentum distribution. At the same time, at sufficiently high excitation densities, there is significant qualitative discrepancy between the dynamic behavior of the width of the polariton momentum distribution determined from direct measurements and that calculated from the coherence spatial distribution. This discrepancy is observed at the fast initial stage of the polariton system kinetics and, apparently, results from the strong spatial nonuniformity of the phase of the condensate wave function, which equilibrates on a much longer time scale.

cond-mat.quant-gas↗