arXiv subjects

Yu. G. Kusrayev

Publications and source records attributed to Yu. G. Kusrayev.

At least 19 recordsLinked to original sources

arXiv2026

Cascade spin dynamics of excitons localized in indirect-band-gap (In,Al)As/AlAs quantum dots with type-I band alignment

We investigate the spin dynamics of excitons localized in type I (In,Al)As/AlAs quantum dots with an indirect in momentum space band structure. Polarized selective photoluminescence spectroscopy, i.e. fluorescence line narrowing, under magnetic fields up to 5 T applied in the Faraday geometry is employed. The experiment reveals a cascade spin evolution process of excitons in the indirect band-gap quantum dots: an initial short term spin dynamics associated with excited direct exciton states possessing a large oscillator strength is followed by electron relaxation into the X valley of the Brillouin zone and subsequent long term spin dynamics of indirect excitons. The two step mechanism manifests itself in the distinct features of the magnetic field dependences of photoluminescence: two component recovery of optical orientation, two component linear to circular polarization conversion and the presence of the linear polarization plane rotation. At the same time, suppression of the optical alignment shows one-component behavior governed by the spin dynamics of the indirect exciton states. Within the pseudospin formalism, we derive analytical expressions that quantitatively describe the observed dependences and yield estimates for the anisotropic exchange splitting: 210 {\mu}eV for direct excitons and 1.3 {\mu}eV for indirect excitons. Further analysis using the density matrix formalism agrees well with the pseudospin model calculations and shows that the finite optical orientation at zero magnetic field is due to comparable magnitudes of the anisotropic splitting of the indirect exciton states and the splitting of the X-valley electron states caused by the hyperfine interaction with nuclei.

cond-mat.mes-hall↗

arXiv2025

Kerr effect induced by exchange interaction of electrons separated by a tunnel barrier in a double quantum well

In a structure with two tunnel-coupled quantum wells of different widths, the spin dynamics resulting from resonant pulsed optical pumping of the narrow-well exciton includes the wide-well electron magnetization dynamics. Our analysis shows that the effect is driven by electron exchange between narrow-well excitons and spin-polarized electrons in the wide well. A theoretical model of the spin Kerr effect has been developed accounting for the interwell electron spin exchange. In the studied double-well structure with CdTe and Cd$_{0.98}$Mn$_{0.02}$Te quantum wells and a well-separating barrier thickness of 5 monolayers (1.6 nm), the model accurately describes the experimental results and allows us to estimate the interwell electron exchange constant as $\delta_{e} \approx 0.9\times10^{-15}~\textrm{eV}~\textrm{cm}^{2}$.

cond-mat.mes-hall↗

arXiv2024

Optical Orientation of Mn$^{2+}$ Spins in Bulk (Zn, Mn)Se Induced by Magnetic Field

The optical orientation of Mn$^{2+}$ spins in the first excited state $^4$T$_1$ was experimentally observed in bulk (Zn, Mn)Se ($x_\mathrm{Mn}=0.01$) in the an external magnetic field of up to $6\,$T in Faraday geometry. This occurred during quasi-resonant continuous wave circularly polarized photoexcitation of the intracenter d-d transitions. A non-monotonic dependence of the thermal circular polarization of the intracenter photoluminescence on the magnetic field was observed. A theoretical model is proposed to describe the selection rules for resonant optical d-d transitions of an isolated Mn$^{2+}$ ion in a ZnSe cubic crystal. These rules are based on the analysis of the total angular momentum symmetry for the ground ($^6$A$_1$) and first excited ($^4$T$_1$) states of the Mn$^{2+}$ ion. This discussion neglects the specific mechanism for spin-flip processes in a d-shell of the ion during optical excitation. The analysis is founded on the rotational symmetry of the effective total angular momenta and parity for each state as a whole. Additionally, the Jahn-Teller coupling of the excited state orbital parts with tetragonal ($e$-type) local distortions of the crystal lattice is considered. This coupling results in the segregation of cubic axes and spin projections on these axes due to weak spin-orbit and spin-spin coupling in the excited state. This leads to energy splitting for spin states with their projections of $\pm 1/2$ and $\pm 3/2$ on each axis distinguished by specific Jahn-Teller distortion in the corresponding atomic potential minimum. By introducing two different times of relaxation to reach thermodynamic equilibrium for $\pm 1/2$ and $\pm 3/2$ states in each Jahn-Teller configuration, an angle dependent optical orientation contribution in photoluminescence polarization arises in the presence of a magnetic field.

cond-mat.mes-hall↗

arXiv2023

Influence of Gamma-X mixing on optical orientation and alignment of excitons in (In,Al)As/AlAs quantum dots

The effect of Gamma-X mixing on the energy levels fine structure of indirect in k-space excitons in an ensemble of (In,Al)As/AlAs quantum dots with type I band alignment was experimentally studied. Using the methods of optical spin orientation and optical alignment in a magnetic field, an increase in the anisotropic exchange splitting of excitonic levels (from approximately 0.6 to 5 ueV) due to the Gamma-X mixing was revealed. The extent of direct electronic states admixing to indirect ones depends on the size of the quantum dot. On the other hand, the optical and spin properties of excitons change radically with increasing of the Gamma states admixture to the X states: in the absence of a magnetic field, the optical orientation of excitons decreases from 18 to 3%, while the alignment of excitons is restored from 6 to 53%.

cond-mat.mes-hall↗

arXiv2023

Electrical resistance associated with the scattering of optically oriented electrons in n-GaAs

In a bulk GaAs crystal, an unusual magnetoresistance effect, which takes place when a spin-polarized current flows through the sample, was detected. Under conditions of optical pumping of electron spins, an external magnetic field directed along the electric current and perpendicular to the oriented spins decreases the resistance of the material. The phenomenon is due to the spin-dependent scattering of electrons by neutral donors. It was found that the sign of the magnetoresistance does not depend on the sign of the exciting light circular polarization, the effect is even with respect to the sign of the spin polarization of the carriers, which indicates a correlation between the spins of optically oriented free electrons and electrons localized on donors.

cond-mat.mes-hall↗

arXiv2023

Optical orientation of excitons in a longitudinal magnetic field in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment are studied. The negligible (less than $0.2~μ$eV) value of the anisotropic exchange interaction in these QDs prevents a mixing of the excitonic basis states with pure spin and allows for the formation of spin polarized bright excitons for quasi-resonant circularly polarized excitation. In a longitudinal magnetic field, the recombination and spin dynamics of the excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons is observed in magnetic field eliminating the impact of the nuclear spin fluctuations. A kinetic equation model, which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics, has been developed, which allows for a quantitative description of the experimental data.

cond-mat.mes-hall↗

arXiv2022

Optical alignment and orientation of excitons in ensemble of core/shell CdSe/CdS colloidal nanoplatelets

We report on the experimental and theoretical studies of optical alignment and optical orientation effects in an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. The dependences of three Stokes parameters on the magnetic field applied in the Faraday geometry are measured under continuous wave resonant excitation of the exciton photoluminescence. Theoretical model is developed to take into account both bright and dark exciton states in the case of strong electron and hole exchange interaction and random in-plane orientation of the nanoplatelets in ensemble. The data analysis allows us to estimate the time and energy parameters of the bright and dark excitons. The optical alignment effect enables identification of the exciton and trion contributions to the photoluminescence spectrum even in the absence of a clear spectral line resolution.

cond-mat.mes-hall↗

arXiv2022

Trion magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

A trion magnetic polaron formed by the exchange interaction of a positively charged exciton (trion) with localized spins of Mn$^{2+}$ ions is found experimentally in a 4\,nm wide Cd$_{0.98}$Mn$_{0.02}$Te/Cd$_{0.78}$Mn$_{0.02}$Mg$_{0.2}$Te quantum well containing resident holes. The experiment is performed at a temperature of 1.6 K using resonant excitation of the trion with circularly polarized light. The trion is formed from a resident hole, which is in a hole magnetic polaron state, and a photogenerated electron-hole pair. The dynamical evolution from the hole magnetic polaron to the trion magnetic polaron is accompanied by a spin-flip of the electron, which results in negative circular polarization of the photoluminescence. The degree of circular polarization reaches $-8\%$ at zero magnetic field and strongly decreases in transverse magnetic fields exceeding 0.2 T. Our model considerations show that different localization sizes of the resident and photogenerated holes and the resulting difference in their exchange interaction with the Mn$^{2+}$ spins maintains Mn spin polarization. The resulting exchange field of Mn acting on the electron provides a robust spin polarization of the trion magnetic polaron. We evaluate the electron exchange energy in the T$^+$MP to be 0.19~meV, and the T$^+$MP binding energy to be about 0.5 - 1 meV.

cond-mat.mes-hall↗

arXiv2020

Magneto-optics of excitons interacting with magnetic ions in CdSe/CdMnS colloidal nanoplatelets

Excitons in diluted magnetic semiconductors represent excellent probes for studying the magnetic properties of these materials. Various magneto-optical effects, which depend sensitively on the exchange interaction of the excitons with the localized spins of the magnetic ions can be used for probing. Here, we study core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets hosting diluted magnetic semiconductor layers. The inclusion of the magnetic Mn$^{2+}$ ions is evidenced by three magneto-optical techniques using high magnetic fields up to 15 T: polarized photoluminescence, optically detected magnetic resonance, and spin-flip Raman scattering. In particular, information on the Mn$^{2+}$ concentration in the CdS shell layers can be obtained from the spin-lattice relaxation dynamics of the Mn$^{2+}$ spin system.

cond-mat.mes-hall↗

arXiv2020

Effect of electric current on optical orientation of electrons in AlGaAs/GaAs heterostructure

The effect of a lateral electric current on the photoluminescence H-band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H-band are studied by means of continuous wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H-band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H-band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of the lateral electric field of ~ 100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ~ 100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (> 40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons.

cond-mat.mes-hall↗

arXiv2019

Spin beats in the photoluminescence polarization dynamics of charged excitons in InP/(In,Ga)P quantum dots in presence of nuclear quadrupole interaction

The spin dynamics of positively (X$^{+}$) and negatively (X$^{-}$) charged excitons in InP/In$_{0.48}$Ga$_{0.52}$P quantum dots subject to a magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e. for X$^{-}$ trion photoexcitation. The presence of X$^{-}$ and X$^{+}$ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization was fitted by a form which describes the electron spin relaxation in "frozen" nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved.

cond-mat.mes-hall↗

arXiv2019

Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $Γ$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

cond-mat.mes-hall↗

arXiv2018

Voltage control of the long-range p-d exchange coupling in a ferromagnet-semiconductor quantum well hybrid structure

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices. Here a key challenge is to implement and combine low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are attractive candidates in that respect as such systems bring together the properties of the isolated constituents: They are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1 V bias across the heterostructure. The mechanism of this electric field control is essentially different from the conventional concept, in which wavefunctions are spatially redistributed to vary the exchange interaction, requiring high field strengths. Here we address instead control of the novel exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet, i.e. the phononic ac Stark effect. An essential parameter of this coupling is the splitting between heavy and light hole states in the quantum well which can be varied by the electric field induced band bending. Thereby the splitting can be tuned with respect to the magnon-phonon resonance energy in the ferromagnet, leading to maximum coupling for flat band conditions. Our results demonstrate the feasibility of electrically controlled exchange coupling in hybrid semiconductor nanostructures at quite moderate electric field strengths.

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

Long-range p-d exchange interaction in a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered as prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range $p-d$ exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10~nm thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10~nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift $Δ_S$ on the external magnetic field $B$. We show that in strong magnetic field $Δ_S$ is a linear function of $B$ with an offset of $Δ_{pd} = 50-100~μ$eV at zero field from the FM induced effective exchange field. On the other hand, the $s-d$ exchange interaction between conduction band electrons and FM, as well as the $p-d$ contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure.

cond-mat.mtrl-sci↗

arXiv2017

Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: the effect of self-localization

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells using time-resolved photoluminescence (PL). The magnetic field and temperature dependencies of this dynamics allow us to separate the non-magnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wavefunction. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and temperature.

cond-mat.mtrl-sci↗

arXiv2016

Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and Manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy-hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.

cond-mat.mes-hall↗

arXiv2015

Förster energy transfer of dark excitons enhanced by a magnetic field in an ensemble of CdTe colloidal nanocrystals

We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely-packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2-3 in a field of 15T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field.

cond-mat.mes-hall↗