Atsuhiro Fujii

Indirect-Forbidden Transitions in TlCl and TlBr

The optical absorption spectra of purified TlCl and TlBr have been investigated near the band edge. It was found that the absorption rises as a three half power of the energy near the edge and as a cube of the energy in the high energy region. It is concluded that the edge is due to the creation of the is indirect-forbidden exctions of X 6 + ×R 6 - and the lowest minimum in the conduction band is at the R-point. Theroretical calculation of an indirect-forbidden exciton transition is made. The result is differnt from what has been proposed by the previous works, but it describes well the expermental results. The binding energy of the indirect-forbidden exction in TlBr is determined as 23±5 meV.

Edge Absorption Stimulated by Disorder in Mixed Crystals of Thallous Halides

Low level absorption spectra at the edges of TlCl(I), TlBr(I), TlCl(Br) and TlBr(Cl) mixed crystals are measured. At the low concentration of minority halogen, the absorption peak orginated in the localized level due to iodine is found in TlCl(I) while the dicorder-assisted zerophonon X 6 + ×R 6 - exciton absorptions are observed in TlBr(I),TlCl(Br) and TlBr(Cl). Above the critical concentration of the minority halogen, a long tail associated with the bound states due to the cluster formation appears. The absorption is analyzed and the growth of the long tail is discussed.

Neutron Scattering Study of Phonon Dispersion Relations along [110] in TlCl

The phonon dispersion relations of TlCl along the [110] direction have been studied by inelastic neutron scattering at 80 K, partly at 293 K. The energies of four zone-boundary phonons at the \(M(\frac{1}{2}\), \(\frac{1}{2}\), 0) point associated with the indirect band-gap transition between the X (0, 0, \(\frac{1}{2})\) and \(R(\frac{1}{2}\), \(\frac{1}{2}\), \(\frac{1}{2})\) points were determined. The softening of the TA 2 branch ( e //[001]) was found around the zone-boundary. The zone-boundary phonon softens from 1.75 meV (293 K) to 1.28 meV (4.5 K). The mechanism of this phenomenon is discussed in connection with the CsCl–TlI or CsCl–NaCl type transitions which can be accomplished by a shear of the (110) planes corresponding to the TA 2 mode in the CsCl structure.

Relaxed states of indirect excitons in mixed crystals of thallous halides

By measurements of luminescence and the Urbach tail of indirect absorption, exciton states were investigated in mixed crystals of thallous halides for a wide range of compositions. From the estimation of the steepness coefficients of the Urbach tail in TlCl x Br 1- x mixed crystals, the relaxed exciton state was found to change from the self-trapped state to the free state at x =0.7-0.6, consistent with the results of the emission studied before. On the other hand, the indirect excitons in TlBr x I 1- x (0.3< x <1) were found, upon analysis of the emission band and the steepness coefficient of the Urbach tail, to relax always to the self-trapped states, which become shallow with increasing iodine concentration. Exciton states in mixed thallous halides are discussed together with a theoretical study on a binary mixed crystal and in relation to percolation of minority-halogen ion clusters.

Phonon Strucutures of Forbidden-Indirect Transitions in TlCl and TlBr

Optical absorptions due to the phonon-forbidden indirect X + 6 × R - 6 exciton in TlCl and TlBr have been studied near the fundamental absorption edge by using a wavelength-moduration technique to resolve their fine structures. The experimental spectral band shape of wavelength-derivative absorptions is well described by a theoretical curve expected for a forbidden indirect exciton transition. It is found that four kinds of momentum conserving phonons at the M -point are associated with this transition, and their energies are in good agreement with the values obtained by the neutron scattering studies. The binding energy of the X + 6 × R - 6 exciton is determined as 23±3 meV in TlCl and 19±1 meV in TlBr. Phonon modes which are associated with edge emissions observed in these substances are assigned.

Magneto-Optical Absorption in Thallous Iodide of CsCl Structure

Magneto-optical absorption due to exciton is investigated for thallous iodide of CsCl structure at low temperatures. By the application of magnetic field appear the absorptions due to 2s and 3s excitons which are not observed in zero magnetic field. One-phonon replicas of zero-phonon excitons are found to grow and exciton polaron pinning effect is observed in high magnetic field. The exciton binding energy is determined as 11±1 meV and its reduced mass as (0.25±0.05) m 0 . The dielectric constant effective to the exciton binding and the longitudinal optical phonon energy are estimated to be 17±2 and 11±2 meV, respectively.

Resonant Polaron Coupling and Excitons in TlBr under Magnetic Fields

Absorption and reflection spectra due to direct interband transitions are measured in TIBr in magnetic fields up to 15 tesla using a highly sensitive detection technique. Both the upper and lower branches appearing in the resonant polaron coupling are calculated by a Brillouin-Wigner perturbation method. Using this result, the pinning observed in the interband transition is discussed.

The Optical Absorption in Cubic PbF2 Crystal Induced with γ-Ray Irradiation at Liquid-Helium Temperature

A new absorption band at 4.26 eV (291 nm) induced in cubic PbF2 crystal by γ-irradiation at liquid-helium temperature was studied. From the growth curves of this and 443-nm bands it has been found that the concentrations of the centers responsible for these two absorption bands are proportional to each other during the low temperature irradiation. The experimental result of the thermal bleaching of the 291-nm band has shown that this band begins to bleach thermally at around 60 K and almost disappears around 90 K. The half-width of this band is 0.58 eV at LHeT.

Optical Absorption of TlBr–TlI Mixed Crystals of CsCl Structure

The electronic states in TlBr-TlI mixed crystals of CsCl structure are investigated by means of the optical absorption measurements in the mixed crystals with different compositions. The lowest direct exciton peak shifts continuously with the composition; its energy in pure TlI of CsCl structure, which is unstable at room temperature and below, is estimated to be 2.81±0.02 eV at 13 K. The second and third absorption peaks of TlBr disappear by adding small amount of TlI. The fourth peak is found to shift continuously as the iodine concentration increases in the bromine-rich mixed crystals but fades away in the iodine-rich one. These results are discussed in terms of calculated energy band structures of TlBr and TlI of CsCl structures and a theory of electronic states in mixed crystals.

Electroabsorption of Indirect Forbidden Exciton in TlBr

Optical absorption due to the phonon forbidden indirect X 6 + ×R 6 - exciton in TlBr has been studied near the fundamental absorption edge by using an electric field modulation method. The electroabsorption (EA) spectrum is analyzed under the approximation of sufficiently weak electric fields. The experimental spectral band shape of EA is explained by the quadratic Stark variation of exciton state for 1s and 2s excitons. It is shown that for the case of 1s exciton the onset of shoulder in the EA spectrum corresponds to the threshold energy of the exciton whereas for 2s exciton the peak of the spectrum corresponds to its threshold energy. Finally, a shift of the 1s exciton state under the applied electric field is estimated to be about 2×10 -3 meV(cm/kV) 2 by the intensity ratio between the EA spectrum and the wavelength derivative absorption one.