Search

The influence of fast-electron irradiation on the properties of the hexagonal modification of niobium diselenide, 2H–NbSe2, is investigated by the methods of x-ray and electron diffractometry. The observed changes in the diffractograms are explained by a redistribution of the electron density under irradiation, viz., an increase of the electron density in the planes with high packing density—the basal and prismatic planes {110}, {210}. The appearance of diffuse scattering of x rays and electrons at high doses of irradiation is attributed to growth of the electron density of the Fermi liquid. [ABSTRACT FROM AUTHOR]

The influence of fast-electron irradiation on the properties of the hexagonal modification of niobium diselenide, 2H–NbSe2, is investigated by the methods of x-ray and electron diffractometry. The observed changes in the diffractograms are explained by a redistribution of the electron density under irradiation, viz., an increase of the electron density in the planes with high packing density—the basal and prismatic planes {110}, {210}. The appearance of diffuse scattering of x rays and electrons at high doses of irradiation is attributed to growth of the electron density of the Fermi liquid. [ABSTRACT FROM AUTHOR]

The Raman scattering spectrum in a KTb(WO4)2 crystal is investigated in the frequency range 3–950 cm-1 at temperatures 5, 80, and 300 K. The symmetry of all 36 vibrational excitations predicted by group-theoretical analysis is determined. The temperature behavior of the scattering spectra indicates that there is no phase transition in the temperature interval studied. Low-energy electronic transitions are found between the levels of the ground-state multiplet 7F6 of Tb3+ split in a crystal field with symmetry C2. It is determined that the first excited electronic quasidoublet consists of two singlet levels with different symmetry and is separated by a quite high (∼75 cm-1) energy from the ground-state quasidoublet. The behavior of all experimentally observed levels in external magnetic fields H⊥C2 and H|C2 is investigated. The results confirm that a virtually pure case of Ising anisotropy is realized at low temperatures and the magnet studied in the present work can be regarded as a system of two-level magnetic ions. [ABSTRACT FROM AUTHOR]

The Raman scattering spectrum in a KTb(WO4)2 crystal is investigated in the frequency range 3–950 cm-1 at temperatures 5, 80, and 300 K. The symmetry of all 36 vibrational excitations predicted by group-theoretical analysis is determined. The temperature behavior of the scattering spectra indicates that there is no phase transition in the temperature interval studied. Low-energy electronic transitions are found between the levels of the ground-state multiplet 7F6 of Tb3+ split in a crystal field with symmetry C2. It is determined that the first excited electronic quasidoublet consists of two singlet levels with different symmetry and is separated by a quite high (∼75 cm-1) energy from the ground-state quasidoublet. The behavior of all experimentally observed levels in external magnetic fields H⊥C2 and H|C2 is investigated. The results confirm that a virtually pure case of Ising anisotropy is realized at low temperatures and the magnet studied in the present work can be regarded as a system of two-level magnetic ions. [ABSTRACT FROM AUTHOR]

An x-ray diffraction analysis of the compound EuBa2Cu3O7-δ is carried out in the temperature range 20–300 K. At all temperatures of measurement the structure is described in the orthorhombic system. Nonmonotonic growth of the crystal lattice parameter c with decreasing temperature is observed in the interval 150–20 K, and substantial features of the temperature dependence of the coefficient of linear thermal expansion appear below the Debye temperature. The results of the experiment are in complete agreement with the conclusions of a microscopic treatment of the vibrations of the atoms of the BaO and europium layers and also the Cu–O chains. [ABSTRACT FROM AUTHOR]

X-ray diffraction and magnetic studies of the Pr0.9MnOx system are carried out for different oxygen concentrations in the interval 2.850.9MnOx. The crystal structure and magnetic properties of the most highly reduced sample Pr0.9MnO2.85 are close to those of the stoichiometric manganite PrMnO3. Pr0.9MnO2.90 is a ferromagnetic insulator with Tc=120 K. In this sample a transition from an orbitally ordered phase to an orbitally disordered phase occurs over a wide range of temperatures. The properties of the samples are explained by the hypothesis that a portion of the praseodymium ions (up to 5%) can be substituted by trivalent manganese ions without forming oxygen vacancies. Upon oxidation of the samples, vacancies of the praseodymium cations appear, while the oxygen skeleton is preserved. [ABSTRACT FROM AUTHOR]

It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced charges with anisotropic structure, which retains a memory of the pumping light polarization for a long time, while photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure. The charges with anisotropic structure are indentified as two-center oxygen holes. The formation of an oxygen hole is preceded by the excitation of a charge-transfer state with an electron partially transferred to a cation C (V5+ for NaCa2Mn2V3O12 garnet) from an adjacent oxygen anion. To turn this excited state into a free hole state requires a time τhole during which the hole axis can reorient, resulting in a diminution of dichroism. The time τhole decreases with increasing ionization potential of the cation C (very high for V5+). Such a mechanism explains qualitatively a number of unusual experimental facts, specifically, very strong dichroism observed only in NaCa2Mn2V3O12 garnet, where after the irradiation is switched off the photoinduced changes in all optical properties disappear much more quickly than in the other garnets studied. [ABSTRACT FROM AUTHOR]

Measurements of the spectral distribution of the light absorption coefficient in the garnet NaCa2Mn2V3O12 are performed in a wide range of wavelengths. The influence of unpolarized illumination of a crystal on the light absorption spectrum is investigated. A substantial long-lived photoinduced effect is observed at low temperatures. This effect is manifested as a change in the spectral distribution of the light absorption coefficient and reflects a lowering of the valence of the vanadium ions under the influence of photoillumination. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurements of the spectral distribution of the light absorption coefficient in the garnet NaCa2Mn2V3O12 are performed in a wide range of wavelengths. The influence of unpolarized illumination of a crystal on the light absorption spectrum is investigated. A substantial long-lived photoinduced effect is observed at low temperatures. This effect is manifested as a change in the spectral distribution of the light absorption coefficient and reflects a lowering of the valence of the vanadium ions under the influence of photoillumination. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mössbauer spectroscopy is used to study electron self-localization in the superconducting cuprate paramagnetic compound Y-Ba-Cu-O. It is shown that the electrons in chains with selflocalization form a phase with local magnetic ordering and relaxation time greater than 10[sup -7] s. Analysis of the magnitude of the accompanying charge redistribution suggests that when an electron undergoes self-localization a pair of exchange-coupled electrons localized predominantly on the oxygen is formed. Such a quasiparticle, consisting of a pair of antiferromagnetically oriented atomic moments and a pair of self-localized electrons, possesses integral spin and can form a Bose condensate. [ABSTRACT FROM AUTHOR]

The static magnetization M[sub N] and dynamic magnetic susceptibility χ[sub ac] of polycrystalline manganite La[sub 0.66]Ba[sub 0.34]MnO[sub 3] are investigated in the temperature region 5 K

Magnetostriction measurements in the mixed state of superconducting 2H-NbSe[SUB2] single crystals under in-plane magnetic fields 0 -- 12 T have revealed a peak on the magnetostriction versus magnetic field dependences in the vicinity of the upper critical field H[SUBc2]. The peak value of the longitudinal magnetostriction is higher by more than an order of magnitude in comparison with that of the transverse magnetostriction when measured along the hexagonal axis. Analysis of the measured field dependences of the magnetostriction and magnetization of 2H-NbSe allows one to relate the observed peculiarities of magnetostriction with the loss of order in the lattice of Abrikosov vortices, which occurs by a first-order phase transition. [ABSTRACT FROM AUTHOR]

The doublet structure of the electron–bimagnon absorption band with maxima at 2.145 and 2.28 eV is observed for the first time for the metallic phase of YBa[sub 2]Cu[sub 3]O[sub 6+x] with x=0.85 (T[sub c]=88 K). As the YBCO film cools down, the doublet band arises quite sharply near the pseudogap state at T<130 K and remains in the superconducting phase at T

Magnetostriction measurements on 2H-NbSe[sub 2] single crystals in the temperature range 1.5-8 K in a magnetic field up to 14 T are reported. A peak and oscillations in the measured field dependences of magnetostriction are observed near H[sub c2]. The reversible and irreversible components are separated and analyzed in the region of peak. The scaling parameters are defined, and the contribution of the elastic constants dependence on magnetic field is demonstrated. The oscillatory component is discussed in connection with Landau quantization of electronic spectrum. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurements of photoinduced light absorption in Ca[sub 3]Mn[sub 2]Ge[sub 3]O[sub 12] revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers associated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the ground state. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbidden optical transitions. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of random-field magnitudes gives rise to a very broad range of hole hopping rates. There is also a much faster annihilation process immediately conditioned by light pumping. The simultaneous action of these relaxation channels, depending on the number of active charges, pumping intensity, and temperature, explains the entire experimental picture qualitatively and in part quantitatively. Photoinduced dichroism as well as birefringence, observed under polarized pumping, are caused by an anisotropic distribution of photoproduced holes over polarization directions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate that anti-Stokes excitation of Mn[sup 2+] ions in the CsMnCl[sub 3]·2H[sub 2]O crystal can occur through multiphonon addition to the energy of the photons of exciting light. The deficit dE between the energy of the Mn[sup 2+] [sup 4]T[sub 1] exciton and the photon energy of the He-Ne laser used as the source of illumination is compensated by simultaneous annihilation of several thermally activated lattice phonons. The absence of two-photon activation of the fluorescence was verified by measurement of the dependence of the fluorescence intensity on the intensity of the exciting light, which appears linear at a fixed energy mismatch dE. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents results of investigations of polarized Raman spectra in hexahydrates of nickel and zinc fluorosilicates. Orientational ordering of the crystals at low temperatures; Description of orientational disordering using a model in which the rotational motion of octahedral ions is determined by a two-well asymmetric potential function.