Computer simulation of heat distribution processes in W/(La,Ce)B<Subscript>6</Subscript>/W sensor of thermoelectric detector

The results are presented of the computer simulation of heat distribution in the W/(La,Ce)B₆/W multi-layer sensor of the thermoelectric detector after the absorption of single photons with the energies 1–100 eV. The influence of the choice of computer simulation parameters on the revealing of the peculiarities of heat transmission processes arising in the sensor of detector depending on the photon energy, the sensor geometry, the absorption area of the absorber surface and the depth of photon thermalization is investigated in details. The energy resolution and the count rate of sensor are evaluated. It is shown that the multi-layer sensor with the thermoelectric (La,Ce)B₆ is capable to register a single photon in a wide range of the electromagnetic spectrum at 0.5 K; it has an advantages as compared with the sensor based on the CeB₆ with the operating temperature 9 K, and has perspectives to be used in the science and technology.     

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.     

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.     

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.     

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.     

Neutron diffraction study of multipole order in light rare-earth hexaborides

Multipole interactions are known to play a central role in the unconventional properties of light rare-earth hexaborides and especially of CeB₆. Substituting Pr at the Ce sites has the effect of enhancing exchange interactions and changing the symmetry of the local 4f charge distribution, while suppressing the octupole moment. The (T, H) magnetic phase diagrams of the Ce _x Pr_{1 − x} B₆ compounds display a large variety of ordered phases involving magnetic and/or charge degrees of freedom. Here we focus on the compound Ce_0.7Pr_0.3B₆, which is located slightly beyond the Pr concentration where the antiferroquadrupolar phase of pure CeB₆ is suppressed in zero field. The different magnetic structures have been characterized by neutron diffraction and their origin is discussed in connection with recent non-resonant X-ray results by Tanaka et al.      

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.     

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.     

Enhancement of band magnetism and features of the magnetically ordered state in the CeB<Subscript>6</Subscript> compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB₆ single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T ^?1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T ^?0.8 is obtained for magnetic susceptibility of CeB₆ in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB₆ and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB₆ is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB₆ matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at T _Q ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB₆, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.     

Thermoelectric Generator on CeB<Subscript>6</Subscript> Crystal: a Study of its Characteristics

Experimental results that demonstrate the electric power generation by using thermoelectric generator on CeB₆ crystal at temperatures below 10 K are presented. The dependence of output power, electric current and efficiency of the generator at various external heating power, loads and ambient temperature are studied. The features of the obtained current-voltage characteristics of the superconductor, fed by the electric current of thermoelectric generator are discussed.     

Magnetoelastic effects in La1−xCexB6

The elastic constants of La_1−xCe_xB₆ (x = 0.00, 0.01, 0.03) single crystals were determined by ultrasonic measurements. The temperature dependence of the elastic symmetry modes c₄₄ and (c₁₁ − c₁₂)/2 is explained on the basis of the crystalline-electric-field level scheme of CeB₆ by taking the magnetoelastic interaction into account. An influence of the Kondo effect on the elastic constants could not be detected.     

Superconducting and magnetic properties of new ternary borides with the CeCo3B2-type structure

Four new ternary boride systems with the CeCo₃B₂-type structure are reported with the general formulae: MRu₃B₂ (M = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Th or U), MRh₃B₂ (M = La, Ce, Pr, Nd, Sm, Eu or Gd), MOs₃B₂ (M = Lu or U) and MIr₃B₂ (M = La, Th or U). Most members of these systems were found to become either superconducting or magnetically ordered. The structure and properties of these materials are discussed in relation to those of other ternary systems previously reported to exhibit superconductivity and/or magnetic order.     

Lu2SiO5:Ce and Y2SiO5:Ce single crystals and single crystalline film scintillators: Comparison of the luminescent and scintillation properties

The paper is dedicated to development of scintillators based on the single crystalline films of Ce³⁺ doped Lu₂SiO₅ (LSO:Ce) and Y₂SiO₅ (YSO:Ce) orthosilicates grown by Liquid Phase Epitaxy method onto YSO substrates from melt-solutions based on the PbO–B₂O₃ flux. We also compare the luminescent and scintillation properties of Ce doped LSO:Ce and YSO:Ce single crystalline films with those of their single crystal counterparts, grown by the Czochralski method.     

Theoretical study of magnetism in RM12B6 compounds (R=Y,La or Ce; M=Fe,Co)

Electronic structure study for RM₁₂B₆ intermetallics (R=Y, La or Ce; M=Fe, Co) was performed. Fixed spin moment calculations for different volumes of unit cell were used to find low and high moment states in LaFe₁₂B₆. Obtained results are in agreement with previously obtained experimental and theoretical results. Total magnetic moments obtained for YCo₁₂B₆ and LaCo₁₂B₆ are in fair agreement with experimental values. In CeCo₁₂B₆ discrepancy between theory and experiment seems to be more pronounced. It seems that the calculated Co magnetic moments could be overestimated in the studied RCo₁₂B₆ compounds. Present calculations indicate that Fe (Co) atoms prefer 18(h) (18(g)) atomic positions what is in agreement with experiment.     

Studying the energy dependence of intrinsic conversion efficiency of single crystal scintillators under X-ray excitation

Single crystal scintilators are used in various radiation detectors applications. The efficiency of the crystal can be determined by the Detector Optical Gain (DOG) defined as the ratio of the emitted optical photon flux over the incident radiation photons flux. A parameter affecting DOG is the intrinsic conversion efficiency (n _C ) giving the percentage of the X-ray photon power converted to optical photon power. n _C is considered a constant value for X-ray energies in the order of keV although a non-proportional behavior has been reported. In this work an analytical model, has been utilized to single crystals scintillators GSO:Ce, LSO:Ce and LYSO:Ce to examine whether the intrinsic conversion efficiency shows non proportional behavior under X-ray excitation. DOG was theoretically calculated as a function of the incident X-ray spectrum, the X-ray absorption efficiency, the energy of the produced optical photons and the light transmission efficiency. The theoretical DOG values were compared with experimental data obtained by irradiating the crystals with X-rays at tube voltages from 50 to 140 kV and by measuring the light energy flux emitted from the irradiated screen. An initial value for n _C (calculated from literature data) was assumed for the X-ray tube voltage of 50 kV. For higher X-ray tube voltages the optical photon propagation phenomena was assumed constant and any deviations between experimental and theoretical data were associated with changes in the intrinsic conversion efficiency. The experimental errors were below 7% for each experimental setup. The behavior of n _C values for LSO:Ce and LYSO:Ce were found very similar, i.e., ranging with values from 0.089 at 50 kV to 0.015 at 140 kV, while for GSO:Ce, n _C demonstrated a peak at 80 kV.     

Scintillation timing characteristics of (La,Gd)2Si2O7:Ce and Gd2SiO5:Ce single crystal scintillators: A comparative study

The scintillation timing characteristics of (La,Gd)₂Si₂O₇:Ce (GPSLa23.5%:Ce) single crystal were studied and compared with Gd₂SiO₅:Ce (GSO:Ce) single crystal. The photoelectron yield, scintillation decay times and coincidence time resolution were measured. At 511 keV γ-rays, the photoelectron yield of 10,770 ± 500 phe MeV⁻¹ and energy resolution of 5.4 ± 0.2% obtained for GPSLa23.5%:Ce are much better than those of 3350 ± 160 phe MeV⁻¹ and 7.8 ± 0.3% obtained for GSO:Ce. The scintillation decay time profile was measured by the time-correlated single photon counting technique using a fast-slow coincidence setup. In both materials the comparable rise times of several nanoseconds are present. The fast component decay time of 56 ns with relative intensity of 49% obtained for GPSLa23.5%:Ce is inferior to that of 32 ns(88%) obtained for GSO:Ce. Consequently, the coincidence time resolution of GPSLa23.5%:Ce is slightly worse than that of GSO:Ce. The normalized time resolution was also discussed in terms of a number of photoelectrons and decay time of the scintillation pulse.     

Gamma-ray emission from 134Ce and levels in 134La

The decay of ¹³⁴Ce to doubly odd ¹³⁴La has been studied using Ge(Li) and Si(Li) photon detectors. Samples of the ¹³⁴Ce(75.9 h)- ¹³⁴La(6.67 min) have been produced by ≈ 800 MeV proton bombardment of Pr foils in LAMPF, followed by chemical and mass separation. To increase the sensitivity (by up to a factor ≈ 50) for detection of ¹³⁴Ce γ-rays in the presence of the interfering ¹³⁴La activity, the ¹³⁴Ce-¹³⁴La samples were loaded onto a HDEHP column and the ¹³⁴La eluted off continuously. Gamma-ray singles and γ-γ coincidence measurements were made using the ¹³⁴Ce activity remaining on the column as the source. Some 32 γ-rays, ranging in energy up to 355 keV, are observed to be associated with this decay. Approximately 30 of these have been fitted into a level scheme for ¹³⁴La with excited states at energies of 31, 54, 93, 136 or 148, 162, 187, 150 or 205, 252, 294 and 355 keV. Only limits can be set for the spin and parity values of each of these states. The observation of γ-rays with energies up to 355 keV indicates that the previously reported values of ≈ 0.1 MeV for the ¹³⁴Ce electron-capture decay energy, inferred from the K-electron capture probability, are incorrect.     

Raman scattering in CeB6

Raman spectra of phonons in CeB₆ were measured. The observed three peaks were assigned to the A_1g, E_g and F_2g phonon modes. The frequencies of the modes suggest that the Ce ion is in the trivalent state.     

Electronic structures and thermoelectric properties of La or Ce-doped Bi2Te3 alloys from first principles calculations

Thermoelectric properties of La or Ce-doped Bi₂Te₃ alloys were systematically investigated by ab initio calculations of electronic structures and Boltzmann transport equations. The Seebeck coefficient of p-type LaBi₇Te₁₂ and La₂Bi₆Te₁₂ was larger than that of Bi₂Te₃, because La doping increased the effective mass of carriers. On the other hand, the electrical conductivity of LaBi₇Te₁₂ and La₂Bi₆Te₁₂ decreased, which caused a reduction of power factor of these La-doped Bi₂Te₃ alloys in comparison with Bi₂Te₃. The influence of Ce doping on the band structure and thermoelectric properties of Bi₂Te₃ was similar to that of La doping. The theoretical calculation provided an insight into the transport properties of La or Ce-doped Bi₂Te₃-based thermoelectric materials.     

Magnetic properties of R3Co29Si4B10 (R=La, Ce, Pr, Nd, Sm, Gd and Dy) borides

The crystal structure and magnetic properties of quaternary rare-earth intermetallic borides R₃Co₂₉Si₄B₁₀ with R=La, Ce, Pr, Nd, Sm, Gd and Dy have been studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in a tetragonal crystal structure with the space group P4/nmm. Compounds with R=La, Ce, Pr, Nd and Sm are ferromagnets, while ferrimagnetic behavior is observed for R=Gd and Dy. The Curie temperatures vary between 149 K and 210 K. The Curie temperatures in R₃Co₂₉Si₄B₁₀ (R=Ce, Pr, Nd, Sm, Gd, Dy) compounds are roughly proportional to the de Gennes factors.