Investigation of the processes of heat propagation in W/FeSb<Subscript>2</Subscript>/W detection pixel of the single photon thermoelectric detector

The results are presented of the computer modeling of the processes of heat propagation in the single layer detection pixel of the thermoelectric single photon detector after the absorption of photon with the energy of 1–1000 eV. The different geometries of the detection pixel consisting of the tungsten absorber and thermoelectric sensor from the strongly correlated FeSb₂ semiconductor are considered. The differences of temporal dependence of the signal appearing on the sensor for various size of the sensitive pixel elements are studied in detail. The energy resolution and the count rate of the detector are estimated. It is shown that the FeSb₂ sensor of thermoelectric detector provides the generation of significantly higher signal as compared with the CeB₆ sensor and has the perspectives of application in the single photon detectors with the high energy resolution.     

Simulation of Heat Propagation Processes in the Detection Pixel with Superconducting Layers of Single-Photon Thermoelectric Detector

The results of computer simulation of heat propagation processes in the three-layer detection pixel with the superconducting layers of thermoelectric detector after the absorption of single photons energy of 1–1000 eV are presented. We consider the different geometries of the detection pixel consisting of CeB₆ or (La,Ce)B₆ thermoelectric sensor, absorber and heat sink of Nb, Pb or YBCO superconductors. The computations based on the heat conduction equation from the limited volume are carried out by the three-dimensional matrix method for differential equations. It is shown that by changing the materials and dimension of the detection pixel elements, as well as the operating temperature of the detector enables one to obtain the detector to register the photons within the given spectral range, required energy resolution, and counting rate. Such a detector has a number of advantages that allow one to consider the thermoelectric detector as a real alternative to the most promising single photon detectors.     

Hexaborides of rare earths as a sensor material for thermoelectric single-photon detectors

We consider the prospects of crystals of cerium hexaboride CeB₆ and lanthanum-cerium hexaborides (La,Ce)B₆ as a sensitive element in the so-called QVD thermoelectric single-photon detectors, operating at cryogenic temperatures. We have collected and analyzed the values of thermoelectric parameters of CeB₆ and (La,Ce)B₆ available in the literature. On this basis we calculated the energy resolution and photon count rate of the future thermoelectric QVD detectors. We conclude that the detectors on CeB₆ can register a single X-ray photon, and detectors on (La,Ce)B₆ — a single UV photon.     

Single-Layer Detection Pixel of Single-Photon Thermoelectric Detector Based on Rare-Earth Hexaborides

The results of computer simulation of the heat propagation processes in the single-layer detection pixel of single-photon thermoelectric detector after absorption of photons with the energy of 0.8 eV are presented. The various geometries of detection pixel made from rare-earth hexaborides are considered. As the material of absorber, the lanthanum hexaboride (LaB₆) is chosen, and as the materials of thermoelectric sensor, the hexaborides of cerium (CeB₆), and lanthanum–cerium (La_0.99 Ce_0.01)B₆ are used. The choice of LaB6 as an absorber material had the goal to ensure a high system efficiency of photons detection in the near IR region. The computer modeling was carried out based on the equation of heat propagation from a limited volume, using the three-dimensional matrix method for differential equations. It is shown that the single-photon thermoelectric detector with the single-layer detection pixel made only of hexaborides will have the count rates of GHz and the higher detection efficiency as compared with the sensitive element with the heavy metal as an absorber. In addition, such a sensitive element is more stable mechanically when it is cooled to the operating temperatures of 0.5 and 9 K.     

Preparation and Investigation of the Properties of W/CeB<Subscript>6</Subscript>/W Heterostructure as a Sensitive Element of Single-Photon Thermoelectric Detector

The possibility of W/CeB₆/W heterostructure preparation on Al₂O₃, AlN, Si, and W substrates by electron-beam evaporation method was investigated. The conditions for preparation of W thin films on dielectric substrates and CeB₆ films, as well as of stoichiometric CeB₆ films on W films, dielectric and tungsten substrates are determined. The reflection spectra of W films, the results of X-ray diffractometry, X-ray microanalysis, and electron microscopy of W and CeB₆ films are presented. W/CeB₆/W heterostructures of various configurations and sizes are produced. It was shown by means of computer simulation that at the detection of 6–50 eV photons, a detector with W/CeB₆/W heterostructure-based sensitive element may provide microvolt level signal at terahertz count rate. The obtained results serve as the basis for creation of a prototype of a sensitive element of single-photon thermoelectric detector.     

Three-Layer Detection Pixel of Single-Photon Thermoelectric Detector Based on Rare-Earth Hexaborides

The results of computer simulation of heat propagation processes in the three-layer detection pixel of single-photon thermoelectric detector after the absorption of single photons with the energies 0.5–4.13 eV are presented. The various geometries of the detection pixel consisting of rareearth hexaborides are considered. The lanthanum hexaboride (LaB₆) is chosen as the absorber material, and for the materials of thermoelectric sensor the cerium (CeB₆) and lanthanum–cerium (La_0.99Ce_0.01) B6 hexaborides are chosen. The problem is solved to achieve the high system efficiency of thermoelectric detector for the detection of photons in the wavelength range from the UV to the near IR. The computer modeling was carried out based on the equation of heat propagation from the limited volume with the use of three-dimensional matrix method for differential equations. It is shown that a single-photon thermoelectric detector with a three-layer detection pixel made only of hexaborides will have the gigahertz count rate, high-energy resolution, and detection efficiency exceeding 90%. Taking into account the advantages of the three-layer detection pixel compared to the single-layer it can be argued that the three-layer detection pixel of the thermoelectric detector has the great prospects to solve a number of single-photon detection tasks.     

Investigation of processes of heat propagation in multilayer sensor of thermoelectric single-photon detector

The results of computer simulation of the processes of heat propagation inside the multilayer sensor of thermoelectric detector after absorbing the single photons with the energies 1 keV and 100 eV are presented. The variants are considered for different geometries of the sensor which consists of the tungsten absorber and of the thermoelectric layer of the cerium hexaboride. The differences of temporal dependence of the signal arising in the thermoelectric layer when the photon is absorbed in the various areas of absorber are studied in details. The energy resolution and the count rate of the sensor are estimated. It is shown that the multilayer sensors have a number of advantages as compared to the single-layered, and their use is promising in the various fields of science and technology.     

Specific heat of La(Fe<Subscript>0.873</Subscript>Co<Subscript>0.007</Subscript>Al<Subscript>0.12</Subscript>)<Subscript>13</Subscript> compound in antiferromagnetic and ferromagnetic states

The low-temperature specific heat C _p of La(Fe_0.873Co_0.007Al_0.12)₁₃ compound has been measured in two states: (i) antiferromagnetic (AFM) with a Néel temperature of T _N = 192 K and (ii) ferromagnetic (FM). The FM order appears at T = 4.2 K in a sample exposed to an external magnetic field with induction B _C ≥ 2.5 T and is retained for a long time in a zero field at temperatures up to T*_C = 23 K. The coefficient γ_FM in the low-temperature specific heat C = γT + βT ³ in the FM state differs quite insignificantly from that (γ_AFM) in the AFM state. Contributions to the low-temperature specific heat, which are related to a change in the elastic and magnetoelastic energy caused by magnetostrictive deformations, are considered.     

Stratification of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Single Crystals and Sintering of the Obtained Micro- and Nanoscale Plates

The use of surface active liquids facilitates intense stratification of mechanically strained Bi_0.5Sb_1.5Te₃ crystallites. A Bi_0.5Sb_1.5Te₃ heat element with specified thickness and structure is formed by layer-by-layer deposition of “thermoelectric ink” on its free surface. A heat treatment of the formed thermoelectric element in argon at a temperature of 800 K makes it possible to minimize radically the resistance of the grain boundaries introduced into its bulk.     

Magnetic structure and magnetic excitations in the two-dimensional spin system of the Cu<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript> compound

This paper reports on the results of measurements of the magnetic susceptibility, heat capacity, neutron scattering, muon spin relaxation, and electron paramagnetic resonance in Cu₃B₂O₆ for the study of the ground state of the spin system of this compound. The results obtained suggest that, at a temperature of 10 K, the spin subsystem of the crystal, which consists of single spins and clusters of pairs and fours of spins interacting with one another, undergoes a transition to a state representing a superposition of the singlet (for clusters) and magnetically ordered (for single spins) states.     

Thermoelectric Properties of the TlBiS<Subscript>2</Subscript>-PbS Alloys

High efficiency of thermoelectric conversion can be achieved by using materials with a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Mass-difference-scattering of the phonons is one of the most effective way for reducing the thermal conductivity in bulk thermoelectric materials. Investigations of transport phenomena in (TlBiS₂)_1-x (2PbS)_x alloys system have shown that in solid solutions of the (A³B⁵C ₂ ⁶ )_1-x (2A⁴B⁶)_x type at cation substitution according to scheme 2A⁴⁽⁺²⁾ A ³⁽⁺¹⁾ + B⁵⁽⁺³⁾ occurs a strong decrease of the lattice thermal conductivity. In the vicinity of x = 0. 50 the lattice part of thermal conductivity of (TlBiS₂)_1-x (2PbS)_x alloys decreases down to 0. 26 W/mK, i. e., it approaches the theoretical minimum. As a result, the thermoelectric figure of merit for these alloys ( 25%) exceeds the respective value for lead sulfide at room temperature.     

Luminescence of excitons and antisite defects in Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce³⁺ ion) in phosphors based on Lu₃Al₅O₁₂:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu _Al ³⁺ type and vacancy-type defects. The luminescence band with λ_max = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λ_max = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λ_max = 253.5 nm occurs predominantly near Lu _Al ³⁺ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λ_max = 325 nm (τ = 540 ns), which is excited in the band with λ_max = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu _Al ³⁺ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce³⁺ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce³⁺ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce³⁺ ions with λ_max = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce³⁺ ions is significantly smaller due to a low concentration of these types of defects.     

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe<Subscript>2</Subscript>MnAl,Fe<Subscript>2</Subscript>MnSi,and Co<Subscript>2</Subscript>MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe₂MnAl, Fe₂MnSi, and Co₂MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L2₁ structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.     

Magnetic properties of the Nd<Subscript>0.5</Subscript>Gd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

The magnetic properties of the Nd_0.5Gd_0.5Fe₃(BO₃)₄ single crystal have been studied in principal crystallographic directions in magnetic fields to 90 kG in the temperature range 2–300 K; in addition, the heat capacity has been measured in the range 2–300 K. It has been found that, below the Néel temperature T _N = 32 K down to 2 K, the single crystal exhibits an easy-plane antiferromagnetic structure. A hysteresis has been detected during magnetization of the crystal in the easy plane in fields of 1.0–3.5 kG, and a singularity has been found in the temperature dependence of the magnetic susceptibility in the easy plane at a temperature of 11 K in fields B < 1 kG. It has been shown that the singularity is due to appearance of the hysteresis. The origin of the magnetic properties of the crystal near the hysteresis has been discussed.     

Optical absorption in layered MnGaInS<Subscript>4</Subscript> single crystals

Optical absorption in MnGaInS₄ single crystals has been studied. Direct and indirect optical transitions are found to occur in the range of photon energies of 2.37–2.74 eV and in the temperature range of 83–270 K. The temperature dependence of the band gap has been determined; its temperature coefficients E _gd and E _gi are −5.06 × 10⁻⁴ and −5.35 × 10⁻⁴ eV/K, respectively. MnGaInS4 single crystals exhibit anisotropy in polarized light at the absorption edge; the nature of this anisotropy is explained.     

Edge absorption spectra of crystalline and glassy PbGeS<Subscript>3</Subscript>

Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS₃) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization E ∥ c is described by the Urbach rule. For the crystal in the polarization E ∥ b, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ₀, related to the electron-phonon coupling constant, and the energy ?ω_ph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS₃ are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS₃, as well as the topological disorder of glassy PbGeS₃, have been estimated.     

Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.     

Exciton absorption spectrum of thin CsPbI<Subscript>3</Subscript> and Cs<Subscript>4</Subscript>PbI<Subscript>6</Subscript> films

A method for preparing thin films of CsPbI₃ and Cs₄PbI₆ complex compounds has been developed. Their absorption spectrum is investigated in the energy range of 2–6 eV at temperatures from 90 to 500 K. It is found that the CsPbI₃ compound is unstable and passes to the Cs₄PbI₆ phase upon heating at T ≥ 400 K.     

Thermionic emission properties of hexaborides

Thermionic emission properties of the single crystal hexaborides LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, (La, Sr)B₆, (La, Ba)B₆, (La, Ce)B₆, (La, Pr)B₆, (La, Sm)B₆, and (La, Dy)B₆ are measured in the temperature range between 1250 and 1700°C. Of these, LaB₆ is shown to have the highest emission current density in the temperature range investigated. The LaB₆-based mixed hexaborides, (La, M)B₆, show current densities similar to LaB₆, but a little lower. Analyses by Auger electron spectroscopy indicate that the surface composition of (La, M)B₆ approaches that of LaB₆ at elevated temperatures and that the thickness of the surface layer whose composition is different from that of the bulk is typically several atomic layers. The formation of the surface layer is considered to be caused by a relatively slow evaporation rate of La compared to that of the other metal.     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.