arXiv subjects

P. A. Alekseev

Publications and source records attributed to P. A. Alekseev.

16 recordsLinked to original sources

arXiv2026

Purcell enhancement in layered InSe on the Mie-resonant silicon nitride waveguide

Layered van der Waals semiconductors are promising active materials for nanoscale photonic and optoelectronic devices because their excitonic emission can be integrated with heterogeneous photonic architectures. For on-chip applications, this emission must be efficiently coupled to guided modes, while its recombination dynamics should be controlled by the local photonic environment. Although dielectric waveguides enable such integration, substantial control over radiative recombination generally requires resonant engineering of the local photonic density of states. Resonant dielectric nanostructures provide such control by modifying the photonic environment while preserving compatibility with guided-wave photonic architectures. Here, we demonstrate Purcell-enhanced excitonic emission from a thin InSe flake integrated with a Mie-resonant Si$_3$N$_4$ waveguide. The structure incorporates a resonant nanoparticle array with a resonance that overlaps the InSe PL band, thereby enhancing excitonic coupling to the guided mode. Optical spectroscopy confirms the designed resonance, while micro-photoluminescence measurements reveal enhanced and spectrally selective waveguide-coupled emission. Time-resolved photoluminescence measurements show a threefold shortening of the excitonic decay time relative to planar InSe. Analysis of the decay dynamics using a simple rate-equation model yields an effective Purcell factor of approximately 3 for the dominant out-of-plane excitonic emission channel. These results establish Mie-resonant dielectric waveguides as a compact platform for on-chip control of excitonic recombination in layered semiconductors.

cond-mat.mes-hall↗

arXiv2026

Fractional-Monolayer 2D-GaN/AlN Structures: Growth Kinetics and UVC-emitter Applications

The paper reports on fundamental properties of the GaN/AlN quantum wells (QWs) with nominal subcritical thicknesses of 0.75-2 monolayers (MLs). They are grown by plasma-activated molecular beam epitaxy, varying either the nominal thickness or the gallium-to-nitrogen flux ratio. In situ monitoring reveals difference in 2D nucleation and step-flow growth modes of the QWs. The emission charactestics of QWs with integer thicknesses of 1 and 2 MLs depend weakly on the growth mechanism. In contrast, the intensity and spectral position luminescence of QWs with fractional-ML thicknesses are determined by the growth mechanism. Using ab initio calculations, a phenomenological model is proposed that describes fractional-ML QWs either as arrays of 2D quantum disks or as arrays of 2D quantum ribbons, in cases where 2D nucleation or step-flow growth mechanisms predominate, respectively. This model is generally consistent with experimental data on photo- and cathodoluminescence of heterostructures with multiple (250) GaN/AlN QWs. These heterostructures, when pumped by electrom beam at an energy 12.5 keV with a maximum pulse current of 2 A, exhibit linear current dependences of optical peak powers up to 1 and 37 W for wavelengths of 228 and 256 nm, respectively, making them promising for use as powerfull ultraviolet-C emitters.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic switching of self-hybridized exciton-polaritons in CrSBr photonic crystal slabs

Layered van der Waals antiferromagnet CrSBr supports strong light--matter coupling and formation of magnetically tunable exciton-polaritons, yet active magnetic control over polariton propagation direction has remained elusive. Here, we investigate self-hybridized exciton-polaritons in photonic crystal slabs fabricated from CrSBr flakes and their evolution across the antiferromagnetic-to-ferromagnetic spin-flip transition induced by moderate in-plane magnetic fields. Using angle-resolved reflectance and photoluminescence spectroscopy supported by modeling, we show that the polariton energy continuously tracks the layer-by-layer magnetization switching, revealing a gradual redistribution of oscillator strength from antiferromagnetic to ferromagnetic excitons near the critical field. Most notably, we demonstrate that the sign of the polariton group velocity can be reversed by a small change in the external magnetic field of only 40 mT, resulting in complete switching of the polariton propagation direction. Our results establish CrSBr photonic crystal slabs as a platform for magnetically controlled polariton transport, opening opportunities for active integrated photonic and polaritonic devices.

physics.optics↗

arXiv2025

Photoluminescence Features of Few-Layer Hexagonal $α$-In$_2$Se$_3$

Indium (III) selenide is currently one of the most actively studied materials in the two-dimensional family due to its remarkable ferroelectric and optical properties. This study focuses on the luminescent properties of few-layer In$_2$Se$_3$ flakes with thicknesses ranging from 7 to 100 monolayers. To explore the photoluminescence features and correlate them with changes in crystal symmetry and surface potential, we employed a combination of techniques, including temperature-dependent micro-photoluminescence, time-resolved photoluminescence, Raman spectroscopy, atomic force microscopy, and Kelvin probe force microscopy. X-ray diffraction and Raman spectroscopy confirmed that the samples studied possess the $α$-polytype structure. The micro-photoluminescence spectrum consists of two bands, A and B, with band B almost completely disappearing at room temperature. Temperature-dependent photoluminescence and time-resolved measurements helped us to elucidate the nature of the observed bands. We find that peak A is associated with emission from interband transitions in In$_2$Se$_3$, while peak B is attributed to defect-related emission. Additionally, the photoluminescence decay times of In$_2$Se$_3$ flakes with varying thicknesses were determined. No significant changes were observed in the decay components as the thickness increased from 7 to 100 monolayers, suggesting that there are no qualitative changes in the band structure.

cond-mat.mtrl-sci↗

arXiv2024

Engineering whispering gallery modes in MoSe$_2$/WS$_2$ double heterostructure nanocavities: Towards developing all-TMDC light sources

Transition metal dichalcogenides (TMDCs) have emerged as highly promising materials for nanophotonics and optoelectronics due to their exceptionally high refractive indices, strong excitonic photoluminescence (PL) in monolayer configurations, and the versatility to engineer van der Waals (vdW) heterostructures. In this work, we exploit the intense excitonic PL of a MoSe$_2$ monolayer combined with the high refractive index of bulk WS$_2$ to fabricate microdisk cavities with tunable light emission characteristics. These microdisks are created from a 50-nm-thick WS$_2$/MoSe$_2$/WS$_2$ double heterostructure using frictional mechanical scanning probe lithography. The resulting cavities achieve a 4-10-fold enhancement in excitonic PL from the MoSe$_2$ monolayer at wavelengths near 800 nm. The excitonic PL peak is modulated by sharp spectral features, which correspond to whispering gallery modes (WGMs) supported by the cavity. A microdisk with a diameter of 2.35 $μ$m demonstrates WGMs with a quality factor of up to 700, significantly surpassing theoretical predictions and suggesting strong potential for lasing applications. The spectral positions of the WGMs can be finely tuned by adjusting the microdisk's diameter and thickness, as confirmed by theoretical calculations. This approach offers a novel route for developing ultra-compact, all-TMDC double heterostructure light sources with record-small size.

physics.optics↗

arXiv2020

Effect of Nd and Rh substitution on the spin dynamics of Kondo insulator CeFe2Al10

The dynamic magnetic properties of the Kondo-insulator state in CeFe2Al10 (spin gap, resonance mode) have been investigated using polarized neutrons on a single crystal of pure CeFe2Al10. The results indicate that the magnetic excitations are polarized mainly along the orthorhombic a axis and their dispersion along the orthorhombic c direction could be determined. Polycrystalline samples of Nd- and Rh-doped CeFe2Al0 were also studied by the time-of-flight technique, with the aim of finding out how the low-energy magnetic excitation spectra change upon isoelectronic substitution of the rare-earth (Nd) on the magnetic Ce site or electron doping (Rh) on the transition-element Fe sublattice. The introduction of magnetic Nd impurities strongly modifies the spin gap in the Ce dynamic magnetic response and causes the appearance of a quasielastic signal. The crystal-field excitations of Nd, studied in both LaFe2Al10 and CeFe2Al10, also reveal a significant influence of f-electron hybridization (largest in the case of Ce) on the crystal-field potential. As a function of the Rh concentration, a gradual change is observed from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting the decrease in the hybridization energy.

cond-mat.str-el↗

arXiv2019

Ab initio study of lattice dynamics of dodecaborides ZrB12 and LuB12

We performed ab initio lattice-dynamics calculations of frame-cluster dodecaborides ZrB12 and LuB12. As a whole, our calculated phonon frequencies and atom-projected density of states are consistent with the results of available first-principles calculations and experimental measurements. So we conclude that the ab initio DFT approach is quite appropriate to study the sufficiently subtle physics of these compounds. Our experiment-independent calculations provide an explicit quantitative confirmation of mixing the eigenvectors of boron and metal vibrations, which was previously observed in experiments.

physics.chem-ph↗

arXiv2019

Measurement of local optomechanical properties of a direct bandgap 2D semiconductor

Strain engineering is a powerful tool for tuning physical properties of 2D materials, including monolayer transition metal dichalcogenides (TMD) -- direct bandgap semiconductors with strong excitonic response. Here, we demonstrate an approach for local characterization of strain-induced modification of excitonic photoluminescence in TMD-based materials. We reversibly stress a monolayer of MoSe$_2$ with an AFM tip and perform spatio-spectral mapping of the excitonic photoluminescence in the vicinity of the indentation point. To fully reproduce the experimental data, we introduce the linear dependence of the exciton energy and corresponding photoluminescence intensity on the induced strain. Careful account for the optical resolution allows extracting these quantities with good agreement with the previous measurements, which involved macroscopic sample deformation. Our approach is a powerful tool for the study of local optomechanical properties of 2D direct bandgap semiconductors with strong excitonic response.

cond-mat.mes-hall↗

arXiv2017

Unified mechanism of the surface Fermi level pinning in III-As nanowires

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As (0$\le$x$\le$1) alloys is pinned at the same position of 4.8$\pm$0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As nanowires leads to the accumulation of an excess arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.

cond-mat.mes-hall↗

arXiv2017

Observing visible-range photoluminescence in GaAs nanowires modified by laser irradiation

We study structural and chemical transformations induced by focused laser beam in GaAs nanowires with axial zinc-blende/wurtzite (ZB/WZ) heterostucture. The experiments are performed using a combination of transmission electron microscopy, energy-dispersive X-ray spectroscopy, Raman scattering, and photoluminescence spectroscopy. For the both components of heterostructure, laser irradiation under atmospheric air is found to produce a double surface layer which is composed of crystalline arsenic and of amorphous GaO$_{x}$. The latter compound is responsible for appearance of a peak at 1.76 eV in photoluminescence spectra of GaAs nanowires. Under increased laser power density, due to sample heating, evaporation of the surface crystalline arsenic and formation of $β$-Ga$_{2}$O$_{3}$ nanocrystals proceed on surface of the zinc-blende part of nanowire. The formed nanocrystals reveal a photoluminescence band in visible range of 1.7-2.4 eV. At the same power density for wurtzite part of the nanowire, total amorphization with formation of $β$-Ga$_{2}$O$_{3}$ nanocrystals occurs. Observed transformation of WZ-GaAs to $β$-Ga$_{2}$O$_{3}$ nanocrystals presents an available way for creation of axial and radial heterostuctures ZB-GaAs/$β$-Ga$_{2}$O$_{3}$ for optoelectronic and photonic applications.

cond-mat.mes-hall↗

arXiv2016

First evidence for a Sm$^{3+}$-type contribution to the magnetic form factor in the quasielastic spectral response of intermediate valence SmB$_{6}$

The momentum transfer dependence of the magnetic form factor associated with the quasielastic spectral component in the dynamic magnetic response of intermediate valence SmB$_{6}$ has been measured using inelastic neutron scattering on a double-isotope ($^{154}$Sm, $^{11}$B) single crystal. The experimental dependence differs qualitatively from those obtained earlier for the inelastic signals, as well as from the field-induced magnetic form factor of SmB$_{6}$ obtained by polarized neutron diffraction. This observation is interpreted by specifically considering the Curie-type contributions to the dynamic susceptibility, which arise from the mixing of 4f$^{5}$ and 4f$^{6}$ J-multiplets into the intermediate valence state wave function.

cond-mat.str-el↗

arXiv2015

Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6

Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. We find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.

cond-mat.str-el↗

arXiv2006

Magnetic spectral response and lattice properties in mixed-valence Sm1-x Yx S solid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering

Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases of Sm1-x Yx S have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the B phase. On the other hand, pronounced temperature anomalies in the thermal expansion α(T), as well as in the Sm mean-square displacements denote the onset of the B-G transition for the compositions x = 0.33 and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the Sm2+ spin-orbit component close to 36 meV. A strongly overdamped Sm3+ contribution appears only for x >= 0.33 near room-temperature. The quasielastic signal is strongly suppressed below 70 K, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around 36 meV is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the "exciton" peak reported previously for SmB6 .

cond-mat.str-el↗

arXiv2005

Evidence for short-range antiferromagnetic fluctuations in Kondo-insulating YbB12

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at E = 14.5 and approximately 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are in overall agreement with those expected for a "spin exciton'' branch in an indirect hybridization gap semiconductor.

cond-mat.str-el↗

arXiv2004

Yb-Yb correlations and crystal-field effects in the Kondo insulator YbB12 and its solid solutions

We have studied the effect of Lu substitution on the spin dynamics of the Kondo insulator YbB12 to clarify the origin of the spin-gap response previously observed at low temperature in this material. Inelastic neutron spectra have been measured in Yb1-xLuxB12 compounds for four Lu concentrations x = 0, 0.25, 0.90 and 1.0. The data indicate that the disruption of coherence on the Yb sublattice primarily affects the narrow peak structure occurring near 15-20 meV in pure YbB12, whereas the spin gap and the broad magnetic signal around 38 meV remain almost unaffected. It is inferred that the latter features reflect mainly local, single-site processes, and may be reminiscent of the inelastic magnetic response reported for mixed-valence intermetallic compounds. On the other hand, the lower component at 15 meV is most likely due to dynamic short-range magnetic correlations. The crystal-field splitting in YbB12 estimated from the Er3+ transitions measured in a Yb0.9Er0.1B12 sample, has the same order of magnitude as other relevant energy scales of the system and is thus likely to play a role in the form of the magnetic spectral response.

cond-mat.str-el↗

arXiv2001

Collective magnetic excitations in mixed-valence Sm0.83Y0.17S

The magnetic spectral response of black-phase mixed-valence Sm0.83Y0.17S has been measured by inelastic neutron scattering on a single crystal. Two magnetic peaks are observed in the energy range of the Sm2+ spin-orbit transition (25-40 meV). Both of them exhibit significant dispersion along the three main symmetry directions, reminiscent of the spin-orbit exciton branch found in pure divalent SmS. The results can be reproduced by a simple phenomenological model accounting for the existence of sizeable Sm-Sm exchange interactions, and a microscopic mechanism is proposed on the basis of the "local-bound-state" theory developed previously for SmB6.

cond-mat.str-el↗