The electronic properties and charge states of atoms in four hafnium-based compounds, with general formulae HfIrxBy, namely HfIr3, Hf2Ir5B2, HfIr3Bx, and HfB2, have been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and calculations based on density functional theory (DFT). It was stated that Hf atoms have a partial positive charge, whereas Ir and B atoms have a partial negative charge in these compounds. Both theoretical and experimental analyses of the electronic structure near the Fermi level showed that all compounds exhibit metallic behavior. The experimental and theoretical work function of the HfIrxBy compounds was determined to lie in the range 4-5 eV.

The topological insulator GeBi4Te7 belongs to a family of compounds consisting of alternating quintuple and septuple layers (QLs and SLs), and its surface electronic structure is determined by whether the crystal terminates with a QL or SL layer block. Using spatially and momentum-resolved photoelectron spectroscopy and STM/STS in the field emission resonance spectroscopy regime as well, we clearly revealed the charge redistribution between the QL and SL terminations, as evidenced by band bending. We attribute the charge redistribution to cation antisite defects in the SLs, which are electrically active and lead to an uncompensated surface charge. Our findings and the ability to manipulate the electronic properties of the surface have profound implications for the entire family of compounds, including the magnetic topological insulator Mn(Bi,Sb)4Te7. Topological insulators like GeBi4Te7 exhibit unique surface electronic structures influenced by their layer terminations, posing challenges in understanding charge distribution. Here, the authors use advanced spectroscopy techniques to uncover charge redistribution due to cation antisite defects, offering insights that could enhance electronic property manipulation across related compounds, including magnetic topological insulators.

Topological insulators (TIs) of the bismuth telluride type are the most investigated among TIs. However, the studies were focused solely on the natural cleavage surface (111) which exhibits a slightly anisotropic Dirac state. Being a strong TI, Bi2Te3 should support topological surface states (TSSs) on any surface. Up to now, direct ARPES measurements of such surfaces have not been performed due to the complexity of preparing the surfaces different from (111). The few early density functional theory (DFT) studies were done for arbitrary side surfaces. In this study, we succeeded in preparing a side surface of a satisfactory quality for ARPES measurements. Also, using the DFT calculations, we explicitly showed the cause of the giant anisotropy of the TSS and its dependence on the projection of the bulk states onto the selected crystallographic plane. Calculations of the surface spectra for several planes that can form facets on the prepared side surface made it possible to identify the (101) surface, the spectrum of which is in perfect agreement with the ARPES measurements.

The structural, magneto-optical, and magnetotransport properties as well as the electronic band structure of the bulk crystalline topological insulator (TI) Sb2Te2Se, grown by the vertical Bridgman technique, were studied. The high structural quality of the grown crystals was established by x-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy revealed a single Dirac cone with the Dirac point, away from the valence as well as conduction bands, 0.22 eV above the Fermi level. The magnetotransport data exhibited a distinct single-frequency Shubnikov-de Haas oscillation in the magnetic fields above B = 10 T. The Lifshitz-Kosevich analysis of the data suggests that this oscillation originates from the Dirac-type states. A sharp fundamental absorption edge in the mid-infrared transmission spectra measured at 4.2 K demonstrated a direct band gap of 0.377 eV located at a momentum of 0.1 & Aring;-1 along the P- K directions of the Brillouin zone. A two-band model developed for massive Dirac electrons in the bulk of topological insulators with the direct band gap at a non-& Gcy; point suggests hyperbolic dispersion relations for the conduction and valence bands displaying the full electron-hole symmetry. Equal magnitude of the electron and hole effective masses me mh 0.21m0 and g factors ge gh 10 were estimated.

Magnetic topological materials (MTIs) including MnBi_2Te_4 are of scientific interest due to the possibility to reveal the interplay between topological and magnetic orders. For these materials preparation issues are still challenging. In our paper we report new synthetic protocol using two-phase source in Bridgman-like growth procedure. We applied it to grow a number of TIs of GeAs_2Te_4 structural type including individual compounds MnBi_2Te_4, MnSb_2Te_4, GeBi_2Te_4, SnBi_2Te_4, PbBi_2Te_4 and several mixed crystals of whole range of compositions: Mn(Bi,Sb)_2Te_4, (Ge,Mn)Bi_2Te_4, new materials Mn(Bi,In)_2Te_4. The method developed can be used to obtain single crystals of any incongruently melted compounds with small region of primary crystallization from melt in the corresponding equilibrium phase diagrams.

Preparation of atomically clean surfaces of lead and tin chalcogenides is an important task for the development of lasers, photodetectors, thermoelectric elements, as well as a prerequisite for the study of electronic properties of the surface, including the properties of topological insulators. We proposed an alternative way to passivate the PbSnTe surface by creating a 3 nm thick tellurium layer by wet chemical treatment and investigated the oxidation process of the Te-capped PbSnTe film by X-ray photoelectron spectroscopy and ellipsometry. Two-stage oxidation with the initial formation of tin oxide and a subsequent formation of lead and tellurium oxides confirmed the mechanism of oxygen diffusion through the tellurium layer with the diffusion coefficient equal to 210(-17) cm(2)s(-1). The protection time of the 3 nm thick tellurium layer was >5 min, allowing samples to be loaded without the use of an inert gas glove box. In addition, further vacuum annealing yields an atomically clean and structurally ordered (111) PbSnTe surface, which makes it possible to study the surface electronic structure by ARPES.

Phonon occupation is temperature (T) dependent; it may participate in scattering with electronic states of topological insulators (TIs). Longitudinal optical (LO) phonons A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ in Bi2 - xSbxTe3 - ySey (BSTS) was resonantly excited by a photon energy (Ep) 2.33 eV due to electronic transition of unoccupied conduction band. The intensity of these modes was enhanced in Bi2Te3 film at Ep 1.87 eV, which is close to the electronic transition of unoccupied Dirac states. Fr & ouml;hlich coupling strength was the main mechanism for higher intensity of A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ modes. At 300 K, the intensity of the A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ mode was significantly decayed in both the BSTS and Bi2Te3 at Ep 2.33 and 1.87 eV due to the anharmonic coupling. However, at similar value of T, spectral profile of A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and Eg2$$ {\mathrm{E}}_g<^>2 $$ modes was not affected because of the lower probability of decay rate of these phonons. In resonant condition, well-resolved Raman forbidden surface modes were observed at T of 50 K. At 300 K, more asymmetric Fano profile of surface phonon was observed due to the anharmonic coupling. The study indicated that at high T, A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ mode and anharmonic coupling may become primary cause for scattering with electronic states of the TIs. However, at low T, primarily, both the LO phonons participated in the scattering.

Optical and photoemission measurements were performed on alkali antimonide Na2KSb and Na2KSb/CsxSb photocathodes in order to determine their energy-band diagrams, elucidate the photoemission pathways, and explore the options for interface engineering in order to reach high quantum efficiencies of the photocathodes. This study is motivated by the recent discovery of optical orientation in Na2KSb and emission of spin-polarized electrons from Na2KSb/C(x)sSb photocathodes [V.S. Rusetsky et al., Phys. Rev. Lett. 129, 166802 (2022)]. We have shown that the band gap E-g of Na2KSb at =295 K lies within the range of 1.40-1.44 eV. The Na2KSb surface activation by the deposition of Cs and Sb results in effective electron affinity decrease by approximately 0.37 eV, and in an increase of the quantum efficiency up to 0.2 electrons per incident photon. The analysis of longitudinal energy distribution curves (EDCs) proves that the surface of activated Na2KSb/CsxSb photocathodes have negative effective electron affinity of approximately -0.1 and -0.25 eV at =295 and 80 K, respectively. EDC measurements under increasing photon energy demonstrate the transition of photoemission pathway from the surface states' photoionization at (h) over barw < E-g to the emission from the conduction-band bottom at approximate to and from the states with high kinetic energy in the conduction band at <(h)over bar>w > E-g. EDCs measured at 80 K reveal a highly directional photoelectron emission from the Na2KSb/CsSb photocathode, as compared to the p-GaAs(Cs,O) photocathode. This fact, along with the observed significant, by an order of magnitude, increase in the photoluminescence intensity under the Na2KSb surface activation by Cs and Sb, indicates relatively weak diffuse scattering in the "quasiepitaxial" CsxSb activation layer of a Na2KSb/CsSb photocathode, compared to strong scattering in the amorphous (Cs,O) activation layer of a p-GaAs(Cs,O) photocathode.

This study investigates methods for controlling the physical properties of the intrinsic magnetic topological insulator MnBi_2Te_4 (MBT) by substituting Mn with Pb in Mn_1-xPb_xBi_2Te_4 (MPBT) solid solutions. This substitution enables tunable magnetic and electronic properties. Using various angle-resolved photoemission spectroscopy (ARPES) techniques, including spin-resolved and circular dichroism (CD) measurements, we analyzed the evolution of the electronic structure across different Pb concentrations, with a focus on topological phase transitions (TPT) near x = 50 TPT include the presence or absence of topological surface states (TSS) and bulk band gap closure. The results show a gradual decrease of the bulk band gap in the electronic structure of MPBT up to x = 40 followed by a constant gap value between 40 - 60 a PbBi_2Te_4-like electronic structure. TSS were observed at x less than 30 were absent near x = 55 semi-metallic or a trivial insulator with a narrow gap phase. These findings demonstrate the tunability of the electronic structure of MPBT, making it a promising candidate for topological and spintronic applications.

An analog of the optical polarizer/analyzer for electrons, a spin filter based on freestanding ferromagnetic (FM) nanomembrane covering the entrance of the microchannel plate (MCP) was applied for efficient spin filtering and electron amplification in the 2D field of view. To study the spin dependent transmission, we constructed a spin-triode device (spintron), which consists of a compact proximity focused vacuum tube with the Na2KSb spin-polarized electron source, the FM-MCP and phosphor screen placed to run parallel to each other. Here, we demonstrate the fabrication of FM nanomembranes consisting of a Co/Pt superlattice deposited on a freestanding 3 nm SiO2 layer with a total thickness of 10 nm. The FM-MCP has 10e6 channels with a single-channel Sherman function S=0.6 and a transmission of 1.5x10e-3 in the low electron energy range. The FM-MCP-based device provides a compact optical method for measuring the spin polarization of free electron beams in the imaging mode and is well suited for photoemission spectroscopy and microscopy methods.

Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), an experimental and theoretical study of changes in the electronic structure (dispersion dependencies) and corresponding modification of the energy band gap at the Dirac point (DP) for topological insulator (TI) $\mathrm{Mn}_{1-x} \mathrm{Ge}_x \mathrm{Bi}_2 \mathrm{Te}_4$ have been carried out with gradual replacement of magnetic Mn atoms by non-magnetic Ge atoms when concentration of the latter was varied from 10$\%$ to 75$\%$. It was shown that when Ge concentration increases then the bulk band gap decreases and reaches zero plateau in the concentration range of 45$\%$-60$\%$ while non-topological surface states (TSS) are present and exhibit an energy splitting of 100 and 70 meV in different types of measurements. It was also shown that TSS disappear from the measured band dispersions at a Ge concentration of about 40$\%$. DFT calculations of $\mathrm{Mn}_{1-x} \mathrm{Ge}_x \mathrm{Bi}_2 \mathrm{Te}_4$ band structure were carried out to identify the nature of observed band dispersion features and to analyze a possibility of magnetic Weyl semimetal state formation in this system. These calculations were performed for both antiferromagnetic (AFM) and ferromagnetic (FM) ordering types while the spin-orbit coupling (SOC) strength was varied or a strain (compression or tension) along the $c$-axis was applied. Calculations show that two different series of topological phase transitions (TPTs) may be implemented in this system depending on the magnetic ordering. At AFM ordering transition between TI and trivial insulator phase goes through the Dirac semimetal state, whereas for FM phase such route admits three intermediate states instead of one (TI - Dirac semimetal - Weyl semimetal - Dirac semimetal - trivial insulator).

It is assumed that the reliability and functionality of nonvolatile memory elements based on MgO are determined by the charge transport in MgO. In the present study, the type of MgO conductivity is established using experiments on the injection of minority charge-carriers in the n(p)-Si/MgO/Mg structures. It is shown that electrons and holes contribute to the MgO conductivity, causing bipolar charge transport. Using ab initio simulations, it was found that native point defects in MgO can provide both electron and hole conductivity.

In this work the effect of indium on the morphology of $\mathrm{Pb}_{1-\mathrm{x}} \mathrm{Sn}_{\mathrm{x}} \mathrm{Te}: \operatorname{In}(x \approx 0.26)$ epitaxial films grown by molecular-beam epitaxy is discussed depending on its concentration and film growth rate. It is shown with atomic force microscopy that under optimal technological conditions (substrate temperature and growth rate) layer-by-layer epitaxy of $\mathrm{Pb}_{1-\mathrm{x}} \mathrm{Sn}_{\mathrm{x}} \mathrm{Te}$ solid solution on BaF 2 (111) substrates is observed. The length of the terraces is up to 250 nm. Moreover, it is shown that the addition of indium in the amount less than one atomic percentage decreases the surface diffusion length of the main components, which is expressed in a decrease in the width of terraces during layer growth. Further increase in indium content (up to 6.3 atomic percentage) completely modifies the growth situation: excess metal precipitation on the surface in the form of inclusions up to several tens of nanometers in height is observed, as well as three-dimensional island growth of the compound itself.

Interaction of phonons with Dirac-like electronic states sets the fundamental limit of electron transport in topological insulators (TIs). Polarization-resolved and resonant Raman scattering of bulk and surface electronic excitation and vibrational modes in Bi2Te3 and Bi2-xSbxTe3-ySey (BSTS) thin films was investigated. At photon energies (Ep) of 1.57 and 2.54 eV, A11g and A21g (LO) modes in Bi2Te3 and BSTS were resonantly excited owing to interband optical excitations of the surface Dirac state (DS) and bulk conduction band (CB), respectively. At room temperature, the resonance of the surface phonon of Raman and IR active modes E1u (LO) and A11u (LO) was observed in Bi2Te3 because of interband excitation of bulk CB, and interband transition of DS resonantly excited the A21u (LO) surface phonon in BSTS. A Fano line-shape suggested interference in the presence of electron-phonon coupling of the surface states.

The spin-resolved dispersion dependencies for the topological insulator Mn_1-xGe_xBi_2Te_4 in the K̅Γ̅K̅' path of the Brillouin zone were studied by spin- and angle-resolved photoemission spectroscopy using laser radiation (Laser Spin-ARPES) with variation of the concentration of substitutional Ge atoms (x from 0.1 to 0.8) for in-plane (s_x) and out-of-plane (s_z) spin orientation. The formation of Rashba-like states is shown, which shift to lower energies with increasing Ge concentration. In the region of Ge concentrations from 50 contribution of these states to the formed spin-dependent dispersions becomes predominant. A pronounced in-plane (s_y) spin polarization, asymmetric for opposite ± k_∥ directions, is revealed for the Dirac cone states, while the Rashba-like states exhibit a pronounced asymmetry in both in-plane (s_y) and out-of-plane (s_z) spin polarizations. Theoretical calculations confirmed the asymmetric polarization of the Rashba-like states formed in the K̅Γ̅K̅' path of the BZ, simultaneously for in-plane and out-of-plane spin orientation. Constant energy maps for Rashba-like states show a pronounced s_z spin component along the Γ̅K̅ direction, with a sign change as the contour crosses the Γ̅M̅ direction. The observed spin polarization can influence the development of spin devices based on magnetic topological insulators.

Organic-inorganic lead halide perovskites have emerged in recent years as semiconductor materials for various optoelectronic and photovoltaic applications. The exciton binding energy is an important parameter because the formation of excitons can potentially hamper charge separation in solar cells. However, it remains a challenge to experimentally determine the exciton binding energy in hybrid bromine-based perovskites: the obtained values have a tens meV spread. Here we present in detail the near-band edge photoluminescence study in CH3NH3PbBr3 single crystals under different photoluminescence excitation densities with a 405 nm laser diode. We show that by using high laser excitation intensities it is possible to make a direct measurement of the exciton binding energy, which we find to be only 12 meV at low temperatures, lower than has been previously determined. In the low temperature orthorhombic phase, besides the free exciton emission at 2.25 eV, we observed the broad yellow-light emission at 2.16 eV, which exhibited a red shift with the increasing temperature and a blue shift with increasing laser excitation intensity. Based on the excitation power density and temperature dependence of the photoluminescence spectra, we interpreted yellow-light emission as a recombination of bound excitons and a donor-acceptor pair transition. The results obtained are essential for a better understanding of the electronic properties of these materials and provide a guideline for their further applications with improved performance.

BSTS epitaxial thin film topological insulators were grown using the MBE technique on two different types of substrates i.e., Si (111) and SiC/graphene with Bi0.7Sb1.6Te1.8Se0.9 and Bi0.9Sb1.5Te1.8Se1.1, respectively. The crystallographic properties of BSTS films were investigated via X-ray diffraction, which showed the strongest reflections from the (0 0 l) facets corresponding to the rhombohedral phase. Superior epitaxial growth, homogeneous thickness, smooth surfaces, and larger unit cell parameters were observed for the films grown on the Si substrate. Polarization dependent Raman spectroscopy showed a weak appearance of the A(g) mode in cross--polarized geometry. In contrast, a strong E-g mode was observed in both parallel and cross-polarized geometries which correspond to the rhombohedral crystal symmetry of BSTS films. A redshift of A(g) and E-g modes was observed in the Raman spectra of BSTS films grown on the Si substrate, compared to those on SiC/graphene, which was directly associated with the unit cell parameter and composition of the films. Raman spectra showed four fundamental modes with asymmetric line shape, and deconvolution of the peaks resulted in additional modes in both the BSTS thin films. The sum of relative ratios of linewidths of fundamental modes (A(g) and E-g) of BSTS films grown on Si substrate was lower, indicating a more ordered structure with lower contribution of defects as compared to BSTS film grown on SiC/graphene substrate.

Floquet-Bloch states can form in solids under periodic driving of electrons. We capture their ultrafast build-up with angle-resolved photoemission spectroscopy and sub-cycle time resolution. For this, we drive the Dirac surface state of a topological insulator with intense mid-infrared fields. Starting with strong intraband currents, Floquet sidebands build up in a single optical cycle. Intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. A full quantum theory describes the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our results shed new light on the dynamics of Floquet states and suggest that optical band-structure engineering is possible within a single oscillation cycle of the carrier field of light.

The magnetoresistance and the Hall effect in transistor structures fabricated on films of the three-dimensional topological insulator (Bi,Sb)2(Te,Se)3 are studied. It is shown that the negative magnetoresistance at low magnetic field is described in terms of quantum corrections to the conductivity. The magnitude of these corrections depends on the gate voltage and increases when approaching the charge neutrality point. The Hall coefficient RH is nonlinear at low magnetic fields for any gate voltage, and the RH nonlinearity is the most pronounced at high negative gate voltages. At high fields, the slope of the magnetic field dependence of the Hall coefficient changes its sign at some gate voltage.