Recombination centers in TlBr

Luminescence from TIBr single crystals at low temperature has been investigated using continuous X-ray and UV excitation as well as pulsed-electron-beam excitation. Four emission bands were observed consistently at 495, 504, 560 and 650 nm, with relative intensities dependent on sample preparation, mode of excitation and measurement temperature. The 560 nm band is shown to be associated with excess thallium at the sample surface. Deformation of the crystal at low temperature (e.g. 9°K) produces recombination centers giving rise to the 504 nm and 650 nm luminescence. Subsequent annealing at temperatures of the order of 120°K causes a decrease in the 504 nm luminescence and the simultaneous appearance of the 495 nm band. These observations are interpreted in terms of the thermal release of recombination centers from the vicinities of the dislocations where they are generated.     

V centers in KBr

One of the major products of radiation damage in the range 120 K < T < 200 K is the D₃ center. Many effects previously associated with the V₄ center are more properly due to the D₃ center in conjunction with a center peaking at 280 nm. All results observed are consistent with the model of the D₃ center proposed by Itoh and Ikeya[7], an X₃⁻² trihalide molecule ion oriented in a 〈100〉 direction.     

Muonium defect centers in semiconductors

After a brief summary of the properties of the muonium defect centers observed in the elemental group IV semiconductors, the status of studies of muonium centers in semiconductors at the time of the last μSR conference in 1983 will be compared with what is currently known. With the introduction of new experimental techniques, such as high-transverse-field μSR and level-crossing spectroscopy, many new results are or soon will be available on muonium centers. These, combined with new theoretical studies, should lead to rapidly increased insight into a subject which has been both puzzling and resistant to clarification.     

Phase-slip centers in superconducting aluminium

Measurements on aluminium strips confirm that the length of phase-slip centers is determined by inelastic scattering of quasiparticles. The relaxation time is found to be about 10^?7 s independent of temperature.     

Paramagnetic centers in tumor cells

Free radical properties of different types of tumor cells were compared. Electron paramagnetic resonance (EPR) studies were performed for human (BM, IGR and SK) and mouse (B16 and S91) melanoma cells. In contrast to melanotic melanoma IGR, BM and B16 cells, amelanotic S91 cells contained only a trace amount of melanin. No EPR signals were detected for Caco2 cells and only a very weak EPR line was measured for fibroblast cells. Melanin does not exist in these cells. The aim of this work was the application of EPR spectroscopy to the determination of the kind of melanin (eu- or pheomelanin) in melanotic tumor cells. Microwave saturation of EPR spectra of tumor cells with high and low melanin content was compared. Eumelanin was identified in human BM, IGR, SK, and B16 melanoma cells. Single asymmetrical EPR lines were detected for these samples. The EPR spectra of human BM melanoma cells had the highest intensity. Paramagnetic centers in amelanotic S91 melanoma cells were also found. Trace amounts of eumelanin free radicals and the other free radicals in cells were responsible for their very weak EPR lines. The obtained results indicate that EPR spectroscopy is a very useful technique for the identification of melanin in tumor cells. Strong differences of microwave saturation of EPR lines for cells with high and low melanin content were observed. EPR lines of tumor cells with a low melanin content did not saturate at the used range of microwave power. Saturation was observed for melanotic BM melanoma cells.     

Thermodynamics of phonon-modulated tunneling centers

In recent years tunneling centers have frequently been used to explain the unusual thermodynamic properties of disordered materials; in these approaches, however, the effect of the tunneling-phonon interaction is neglected. The present study considers the archetype model of phonon-assisted tunneling, which is well known from other areas of tunneling physics (quantum diffusion, etc.). It is shown that the full thermodynamic information can be rigorously extracted from a single Green function. An extended factorization procedure beyond Hartree-Fock is introduced, which is checked by sum rules as well as by exact Goldberger-Adams expansions. The phonon-modulated internal energy and specific heat are calculated for different power-law coupling setups.     

Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

Jahn-Teller centers and pseudospin effects

The characteristics of the phenomenon of polar centers exhibiting Jahn-Teller behavior in copper oxides are investigated by the pseudospin approach. The emergence of relaxation pseudospin (dipole-quadrupole) modes and pseudospin-phonon interaction produces a number of anomalies in inelastic neutron scattering. The Jahn-Teller effect in polar centers brings about a change in the character of the tetra-ortho transition in the system La_2−x M_x-CuO₄ as the concentration x is increased. The possibility of the onset of fluctuation domain nanostructures is suggested. Fiz. Tverd. Tela (St. Petersburg) 39, 1948–1955 (November 1997)     

Scattering from two non-overlapping centers

The scattering of a particle by two non-overlapping fixed centers is studied in the limit where the individual scattering take place in one partial wave only. The resulting scattering amplidtude and its generalized phaseshifts are obtained in closed analytical form. Numerical calculations are presented in the case of d-waves.     

Vibrational fluorescence of CN− centers in KCl by electronic excitation of neighbouring Tl+ centers

Abstract

Infrared emission from higher vibrational states of CN^? in KCl has been achieved by optically exciting nearby Tl⁺ ions with an excimer laser. The electronic vibrational energy transfer, already known from F_H(CN^?) in alkali halides, leads to population of vibrational states up to at least the tenth level.     

Multivortex states in large pinning centers

We have studied the vortex pinning in the large centers, i.e. in the spatial regions with the characteristic size a comparable with the London lenght λ. It is shown that the type of configuration and the number of vortices in the cluster are dependent on the ration a/λ and change nonmonotonically with the temperature. The influence of such vortex clusters on the decay of magnetization and the current-voltage characteristics are discussed. The important role of the potential barrier for the penetration of vortices into the pinning center is shown. The new state of vortex cluster, “vortex polaron”, is predicted. The stability of the multivortex state is discussed.     

Luminescence centers in cubic boron nitride

Complex and multiband photoluminescence spectra for GB and HBN centers in single crystals of cubic boron nitride (cBN) were recorded in the wavelength ranges 385–400 nm and 365–395 nm and the nature of these centers was studied. The use of models involving resonance vibrations and strongly shifted configuration diagrams for the electronic ground state and excited state made it possible to associate formation of the GB-1 center with the presence of tungsten impurity in cBN. It was established that the HBN band in the 300–350 nm range of the cathodoluminescence spectra of cBN polycrystals, single crystals, and micropowders is associated with luminescence centers present in microinclusions of graphite-like boron nitride (hBN). The nature of the hBN band is tentatively interpreted within the model of recombination of donor and acceptor defects in hBN: respectively nitrogen vacancies and carbon atoms in positions substituting for nitrogen. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 241–246, March–April, 2007.