arXiv subjects

T. S. Shamirzaev

Publications and source records attributed to T. S. Shamirzaev.

14 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 μeV for direct excitons and 1.3 μeV 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↗

arXiv2026

Scaling laws of electron and hole spin relaxation in indirect band gap (In,Al)As/AlAs quantum dots

We investigate the electron and heavy hole spin dynamics as a function of magnetic field in ensembles of indirect band gap (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment. Employing a comprehensive model that accounts for both the exciton level quartet and the magnetic-field-driven redistribution of excitons between these states via spin relaxation processes, we extract the electron ($τ_{se}$) and heavy hole ($τ_{sh}$) spin relaxation times as a function of magnetic field for QDs of varying sizes. Our analysis reveals that both $τ_{se}(B)$ and $τ_{sh}(B)$ exhibit power-law scaling behavior, yet the scaling exponents for electrons and heavy holes show markedly different evolution with QD size. For QDs with a diameter of about 9 nm, we find $τ_{se}(B)\propto B^{-5}$ and $τ_{sh}(B)\propto B^{-3}$. Remarkably, increasing the QD diameter to about 16 nm results in a drastic change of the scaling laws, with both $τ_{se}(B)$ and $τ_{sh}(B)$ following a $\propto B^{-9}$ dependence. We discuss the underlying mechanisms responsible for this size-dependent transformation of the magnetic field scaling behavior of carrier spin relaxation.

cond-mat.mes-hall↗

arXiv2025

Magnetic field induced exciton spin dynamics in indirect band gap (In,Al)As/AlAs quantum dots

The exciton recombination and spin dynamics are investigated both experimentally and theoretically in an ensemble of indirect band gap (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment. The magnetic-field-induced circular polarization of the time-integrated photoluminescence changes sign across the emission spectrum with a width reflecting the QD size. It is negative on the low energy side, i.e. for emission from large QDs, but positive on the high energy side, i.e. for emission from small QDs. However, the exciton g factor, measured by spin-flip Raman scattering, is positive across the whole QD ensemble. The magnetic-field-induced circular polarization of the photoluminescence dynamics is studied as function of the magnetic field strength and direction. The dynamics are non-monotonic over a time range up to milliseconds. The time dependence of the photoluminescence circular polarization degree and sign strongly depends on the emission energy and changes with magnetic field orientation. The observed nonmonotonic behavior is provided by the interplay of bright and dark exciton states, contributing to the emission. The experiment is interpreted using a kinetic theory, which accounts for the dynamics of the spin states in the exciton level quartet in longitudinal and tilted magnetic fields, the radiative recombination processes, and the redistribution of the excitons between these states as result of spin relaxation. The model allows us to evaluate the electron and heavy hole spin relaxation times in QDs with different sizes.

cond-mat.mes-hall↗

arXiv2025

The effect of doping layers position on the heterojunction sharpness in (In,Al)As/AlAs quantum dots

Effect of doped layer placed in structures with indirect band-gap (In,Al)As/AlAs quantum dots (QDs) on heterointerface sharpness is investigated. We demonstrate that growth of n (p) doped layer below QDs sheet leads to pronounced deceleration (acceleration) for dynamics of exciton recombination (which is very sensitive to heterointeface structure in these QDs) in compare with the undoped structure. Opposite, the placing of the same doped layers above the QDs sheet does not effect on the exciton recombination dynamic at all. The experimental data are explained by increase (decrease) charged vacancy formation rate in the cation sublattice, that result in QD/matrix interface bluring (sharping), with the increases in the electron (hole) concentration at this heterointerface formation. The thicknesses of the diffuse layer on QD/matrix heterointerface estimated is in range from 0 up to 5 in the lattice constant depending on doped layer placing.

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

Spin light emitting diode based on exciton fine structure tuning in quantum dots

We propose a concept of quantum dot based light emitting diode that produces circularly polarized light due to the tuning of the exciton fine structure by magnetic field and electron nuclear hyperfine interaction. The device operates under injection of electrons and holes from nonmagnetic contacts in a small field of the order of milliteslas. Its size can be parametrically smaller than the light wavelength, and circular polarization degree of electroluminescence can reach 100%. The proposed concept is compatible with the micropillar cavities, which allows for the deterministic electrical generation of single circularly polarized photons.

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

New spin-polarized electron source based on alkali-antimonide photocathode

New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the Na$_2$KSb/Cs$_3$Sb heterostructure is designed as a spin-polarized electron source in combination with the Al$_{0.11}$Ga$_{0.89}$As target as a spin-detector with spatial resolution. In the Na$_2$KSb/Cs$_3$Sb photocathode, spin-dependent photoemission properties were established through detection of high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, that can exclude the construction of the photocathode growth chambers for photoinjectors.

cond-mat.mtrl-sci↗

arXiv2021

Dynamic polarization of electron spins in indirect band gap (In,Al)As/AlAs quantum dots in weak magnetic field: experiment and theory

A novel spin orientation mechanism - dynamic electron spin polarization has been recently suggested in Phys. Rev. Lett. $\mathbf{125}$, 156801 (2020). It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting.

cond-mat.mes-hall↗

arXiv2020

Dynamic polarization of electron spins interacting with nuclei in semiconductor nanostructures

We suggest a new spin orientation mechanism for localized electrons: $dynamic~electron~spin~polarization~provided~by~nuclear~spin~fluctuations$. The angular momentum for the electrons is gained from the nuclear spin system via the hyperfine interaction in a weak magnetic field. For this the sample is illuminated by an unpolarized light, which directly polarizes neither the electrons nor the nuclei. We predict, that for the electrons bound in localized excitons 100% spin polarization can be reached in longitudinal magnetic fields of a few millitesla. The proof of principle experiment is performed on momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic field of 17 mT the electron spin polarization of 30% is measured.

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↗

arXiv2017

Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $τ_{se} = 33~μ$s, and hole, $τ_{sh} = 3~μ$s, bound in the exciton.

cond-mat.mes-hall↗

arXiv2014

Spin-flip Raman scattering of the $Γ$-X mixed exciton in indirect band-gap (In,Al)As/AlAs quantum dots

The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the $Γ$- and X-conduction band valleys, from which control of their spin states can be gained. This access is used here for studying the exciton spin-level structure by resonant spin-flip Raman scattering, allowing us to accurately measure the anisotropic hole and isotropic electron $g$ factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of $Γ$-X-valley mixing, as evidenced by both experiment and theory.

cond-mat.mes-hall↗

arXiv2011

Influence of the heterointerface sharpness on exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap

The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons which are indirect in momentum-space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape and composition. The different exciton lifetimes, which result in photoluminescence with non-exponential decay obeying a power-law function, can be described by a phenomenological distribution function, which allows one to explain the photoluminescence decay with one fitting parameter only.

cond-mat.mes-hall↗