摘要:

We present some characteristic properties of the ground state of a two-level system linearly coupled to a phonon field. Such properties are valid over the whole range of the parameters appearing in the Hamiltonian of the system. The knowledge of these exact results provides a physical basis for an accurate variational determination of the ground state. A physical picture of the self-trapping transition is discussed in terms of particle-field correlations. Our treatment is based on operator methods combining symmetry considerations with some general properties of the lowest energy state.

ISSN:1042-0150    

DOI:10.1080/10420159508227215

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Single-crystal fibers of Bi12SiO20and Bi12TiO20up to 90 mm in length with diameters of 200-1200 pm have been pulled along the <001> and <011> crystallographic axis by the LHPG method. The powder X-ray diffraction and the electron-probe microanalysis have been used for characterization of the as-grown fibers. Microstructure of the sillenite fibres has been studied by the chemical etching method. The dislocations, inclusions and striations have been found to be the main imperfections in the as-grown fibers. The dislocation density has been estimated on the ground of the etching pits using optical and electronic microscopy and compared with that evaluated by the Tsivinskii's relation. The conditions of formation of microdefects are discussed in relation with the growth conditions, morphology and chemical composition of the as-grown fibers.

ISSN:1042-0150    

DOI:10.1080/10420159508227216

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

High resolution (0.05 cm−1) FTIR spectroscopy in the temperature range 9-320 K is applied to study the OH and OD stretching modes in BSO, BGO, BTO and mixed BSO-BGO sillenite crystals. A model is proposed for the OH-defect responsible for the main line. The anharmonic effects are analyzed in the framework of the Morse model and compared to those reported for OH-defects in insulating materials. The temperature dependence of the line position and width fits to a weak phonon coupling model and gives the frequency of the coupled phonons.

ISSN:1042-0150    

DOI:10.1080/10420159508227217

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

K-edge EXAFS spectra of 5 molar% Cr or Fe have been observed in LiNbO3and also in Mg co-doped samples. The results in congruent LiNbO3can be interpreted in terms of a relaxed Li substitution model, however, in the case of Cr, the formation of pairs occupying Li and Nb sites seems also to be important. In stoichiometric or Mg-containing samples Fe is partly displaced to the Nb site while for Cr the dominating process seems to be the formation of aggregates with the participation of Mg.

ISSN:1042-0150    

DOI:10.1080/10420159508227218

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Conduction electrons released from oxygen vacancies in gallium oxide exhibit an electron spin resonance (CESR) line with pronounced hysteresis when the magnetic field is swept upward and backward. This memory effect is due to the existence of a bistable nuclear field induced by dynamic nuclear polarization of gallium nuclei. It is shown that this bistability is the consequence of the one-dimensional (1D) character of the band structure of gallium oxide, with the conduction band formed almost exclusively from 4s orbitals of gallium in octahedral sites, forming isolated double chains aligned along the b-axis.

ISSN:1042-0150    

DOI:10.1080/10420159508227219

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In the incipient ferroelectric KTaO3, small substitutional Li-ions are known to induce dipolar microgregions due to their off-center dipole moment. A sensitive method to observe the appearance of non-inversion symmetric regions in K1−XLiXTaO3, is based upon monitoring the second harmonic intensity (SHG) of Nd: YAG laser light. At low temperatures a considerable SHG intensity (calibrated with respect to LiNbO3) is observed with a characteristic temperature-, field- and Li-concentration dependence. The time and temperature dependence of the electric field induced SHG intensity change shows an Arrhenius-like behavior with fast and slow relaxation processes. For concentrations larger that x ≊ 0.02 the slow relaxation process with an activation energy of EA≊86 meV has been interpreted as a reorientation of mainly antiparallel ferroelectric domains. Measurements of the thermally stimulated depolarization current (TSDC), performed after removing the bias field at LHe temperature, yield for a x = 0.028 sample a broad current peak with a maximum at ≊60 K. A good fit to the experimental data can be reached for a distribution of relaxing dipoles according to a Havriliak-Negami mode with a mean activation energy Δ A = 178 meV. The difference in activation energies will be discussed in terms of different reorientation processes.

ISSN:1042-0150    

DOI:10.1080/10420159508227220

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The influence of yttrium doping on the alumina microstructure is correlated to the Y amount and to the oxygen pressure. A low oxygen pressure promotes the solubility of yttrium in the bulk, its segregation in grain boundaries and increases the alumina toughness. Moreover alumina grain growth is limited either by garnet precipitation or by yttrium segregation along boundaries.

ISSN:1042-0150    

DOI:10.1080/10420159508227221

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The experimental data on photoconductivity, X-ray luminescence, thermoluminescence, and optical absorption of corundum single crystals doped with chromium testify to the existence of gigantic ions of chromium in corundum lattice.

ISSN:1042-0150    

DOI:10.1080/10420159508227222

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159508227223

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The Ionic Thermocurrent (ITC) technique probes a relaxation mechanism in both polycrystalline and monocrystal dolomite, at 189 K. Dielectric characterization shows that it is dipolar and is probably related to the defect structure of the calcium sublattice.

ISSN:1042-0150    

DOI:10.1080/10420159508227224

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor