Hot-exciton spectra in TlBr and TlCl

The luminescence excitation spectra of the direct exciton resonance emissions are measured for TlBr and TlCl at 1.8 K. A remarkable oscillation structure with a period of the Γ-point LO phonon energy is observed. Such a hot exciton effect is explained on the basis of the characteristic energy band structure of TlBr and TlCl.     

Transmittance of CsI,AgCl, KRS-5, and KRS-6 Crystals in the Terahertz Range

Optics and Spectroscopy - Spectral dependences of the transmittance of CsI, AgCl, KRS-5 (TlBr–TlI), and KRS-6 (TlCl–TlBr) single crystals in the infrared (IR) and terahertz (THz)...     

Nuclear spin relaxation in stoichiometric and thallium-doped thallous halide melts

Relaxation times for ^203,205Tl, ^79,81Br and ^35,37Cl are reported for molten TlBr, TlCl and TlBr : Tl. For Br and Cl, relaxation is predominantly quadrupolar; for Tl the relaxation indicates contact hyperfine interaction with paramagnetic centres produced thermally and by adding Tl.     

Zur Polymorphie der Thalliumhalogenide in Aufdampfschichten

TlCl, TlBr und TlJ kristallisieren nach Elektroneninterferenz-Untersuchungen in dünnen Aufdampfschichten auf Spaltflächen von Alkalihalogeniden des NaCl-Typs bei Zimmertemperatur nicht wie bisher angenommen im NaCl-Typ, sondern im orthorhombischen TlJ-Typ. Erst von einer gewissen Schichtdicke an tritt bei TlCl und TlBr der normale CsCl-Typ auf. Damit konnte der TlJ-Typ, der bisher nur bei TlJ gefunden worden war, auch bei TlCl und TlBr nachgewiesen werden. In sehr dünnen Schichten wurde bei den drei Tl-Halogeniden eine von der Unterlage und der Schichtdicke abhängige gegenseitige Annäherung bzw. Angleichung der Längen der zur Unterlage parallelena- undb-Achse des TlJ-Typs beobachtet, bei der jedoch der Abstand zwischen benachbarten Ionen verschiedenen Vorzeichens ungeändert bleibt.     

X-ray study of CsI-TlI and RbI-TlI solid solutions

The dependence of the lattice constants of CsI-TlI and RbI-TlI solid solutions on the TII concentration has been studied by means of x-ray powder diffraction. The compressibilities of TlCl, TlBr, and TlI, having the NaCl structure, are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 38–41, June, 1969.The author thank A. Ya. Pae for discussion of these topics.     

High second-harmonic generation of antiferromagnetic/ionic-crystal composite medium with negative refraction

Second harmonic generation (SHG) from a short-period structure composed of alternating antiferromagnetic (AF) and ionic-crystal layers is investigated, where the generated harmonic waves are situated in the far-infrared range and attributed to the magnetically nonlinear interaction in AF layers. The presence of a kind of appropriate ionic-crystal layers in the structure can support negative refraction for the pumping wave and positive refraction for the SH wave, so the SHG is greatly amplified in the vicinity of each AF resonant frequency. For the composite structure FeF₂/TlBr, we found that the SH output is about 8 times higher than that of the FeF₂ bulk in the same frequency range.     

Polaron with the anisotropic mass

Mass correction formulae for the polaron with the anisotropic mass are derived and used to calculate the bare masses from the observed anisotropic polaron masses in TlCl and TlBr.     

Optical properties of antiferromagnetic/ion-crystal superlattices

Transmission, refraction and absorption properties of an antiferromagnetic/ion-crystal superlattice are investigated. The transmission spectra based on FeF₂/TlBr superlattices reveal that there exist two intriguing guided modes in a wide stop band. Additionally, FeF₂/TlBr superlattices possess either the negative refraction or the quasi left-handedness, or even simultaneously hold them at certain frequencies of two guided modes, which require both negative magnetic permeability of antiferromagnetic layers and negative permittivity of ion-crystal layers. Frequency regimes of the guided modes will be dependent on the magnitude of the external magnetic field. Therefore, handedness and refraction properties of the system can be manipulated by modifying the external magnetic field. Absorption spectra exhibit that absorption corresponding to guided modes is noticeable.     

A three-body force shell model calculation of Schottky defect formation energies of ionic crystals with CsCl structure

A three-body force shell model (TSM) for the calculation of Schottky defect formation energies in solids with cesium chloride structure has been developed by incorporating the effects of long-range three-body interactions (TBI) in the shell model. These TBI in the defect lattice arise from the deformation of electron shells when the nearest neighbour ions get relaxed from their equilibrium position. This model has been used to calculate the cation and anion extraction and Schottky defect formation energies of CsCl, CsBr, CsI, TlCl, TlBr and NH₄Cl crystals. The calculated values of these defect properties agree reasonably well with their measured values.     

Ground-state energy of the exciton-phonon system in a magnetic field

Upper bounds for the ground-state energy of the exciton-phonon system in a magnetic field are calculated variationally using a wavefunction whose symmetry is suited both to low and high magnetic fields. The displacement amplitudes for the exciton-phonon interaction are those known from the free field case. The results compare satisfactorily with experimental data for TlCl and TlBr.     

Effect of Hydrostatic Pressure on Optical Properties of TlBr and TlCl

We present first principles calculations of optical properties of binary semiconductor compounds TlBr and TlCl.Dependences of dielectric function,reflectivity and refractive index on hydrostatic pressure are calculated using self-consistent scalar relativistic full potential linear augmented plane wave method(FP-LAPW) within a generalized gradient approximation(GGA).They are compared with previous calculations and experimental measurements.Good agreements are found.     

The effect of experimental geometry and initial conditions on the shape of coherent population trapping resonances on the fine structure levels of thallium atoms

Specific features of the coherent population trapping effect are considered in the generalized Λ system whose lower levels are the magnetic sublevels of the fine structure levels of the thallium atom. Numerical experiments were performed aimed at examination of the coherent population trapping for the case of nontrivial, but feasible, initial populations of the upper metastable fine structure level. Such populations may be obtained, for example, due to the photodissociation of TlBr molecules. The possibility of reducing the number of resonances of the coherent population trapping in a multilevel system, which may be useful for high-resolution spectroscopy, is demonstrated. It is shown that the magnitude and shape of the resonances can be controlled by varying the orientation of the polarization vectors of the light field components with respect to each other and to a magnetic field. In addition, studying the shape of the coherent population trapping resonances for the atoms obtained by photodissociation of molecules may provide information about these molecules.     

h.c.p.-f.c.c. phase transition in solid parahydrogen

The difference of free energies for the h.c.p. and f.c.c. parahydrogen is calculated as a function of molar volume. It is shown that the observed transition in solid parahydrogen¹ can be explained by differences in the structure of the cubic and quartic anharmonic terms.     

Hall effect studies in ytterbium through the f.c.c.-h.c.p. transformation

The Hall coefficient (R_H) of polycrystalline Yb showing the f.c.c.-h.c.p. transformation has been determined at 20 koe in the range 1.8–420°K. Both the structural and the associated magnetic transformation would separately be expected to modify R_H, but the evidence suggests that the observed behaviour through the transition arises primarily from the effects of the change in structure. A more detailed interpretation is presently not possible, however, because of the lack of knowledge of the band structure of h.c.p. Yb.     

Optical and structural properties of thallium halide microcrystals in a porous matrix

Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.     

Cyclotron resonance of electrons and holes in TlBr

Cyclotron resonance measurements on TlBr with circularly polarized waves at 34 GHz show the presence of an electron resonance at (0.52 ± 0.03)m_o and a hole resonance at (0.70 ± 0.03)m_o. Mobilities of both electrons and holes are determined in the temperature range 6.5°K to 14°K.     

Normal and wavelength-modulated absorption of thallium bromide solutions

New results on the absorption spectra of Tl ions in aqueous and alcoholic solutions of TlBr are reported and compared with spectra of Tl ions in single KBr crystals. We study the Tl absorption bands under different conditions and we show that the A-band can be attributed to Tl-Br complexes. The energy parameters of these complexes in alcoholic solutions are given.     

Magnetic properties of CoFeRu alloys in the f.c.c. and h.c.p. phases

Measurements have been made of the magnetization, the Fe⁵⁷ Mössbauer effect, and the crystal structures of Co-rich CoFeRu alloys. The alloys with f.c.c. structure behave in a typical ferromagnetic way, while those with h.c.p. structure show non-saturation of the magnetization in magnetic fields as high as 80 kG, and they show relatively small magnetic moments per atom and a broad single Mössbauer absorption.     

Resonant Raman effect in TlBr: Forbidden one-phonon and intervalley scattering at the direct exciton

Resonant Raman scattering at the direct exciton in TlBr at 1.8K is reported. The cross-section for forbidden 1LO (Γ) scattering shows resonances at both the 1s and 2s exciton. For the first time strongly resonant forbidden 1TO (Γ) scattering is observed for which possible mechanisms are discussed. Several two-phonon Raman processes involving M-point phonons confirm by direct observation the phonon intervalley scattering between non-equivalent X-points.     

Superconductivity and the b.c.c. to A-15 transformation in Nb-Au alloys2

Nb-Au alloys near the composition Nb₃Au can be quenched from the high temperature α solid solution phase field, retaining the b.c.c. structure. We report results on their superconducting, mechanical and physical properties as a function of composition. Short low temperature anneals transform these materials to the equilibrium A-15 structure with T_c up to 11K. We report results on the superconducting and physical properties of the transformed A-15 structure materials and describe how the properties of the transformed materials are affected by annealing temperature and time.