V. V. Eremenko

Experimental detection of stacking faults in rare gas clusters

Abstract The theoretical prediction by van de Waal of stacking faults in rare gas clusters has been verified experimentally. It was established that a defect fcc structure has been formed in clusters of mean sizes N ⩾ 1500 atoms/cluster. The stacking fault density is a linear function of N − 1 3 . The number of ‘defect’ planes does not depent on the cluster size and is four.

Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa2Cu3O6+x films

The absorption spectra of single-crystal YBa2Cu3O6+x films with various doping levels in the range from x≈0.35 to x≈0.9 are measured in the energy region 0.3–3 eV. An analysis of the spectral composition of the absorption is made with allowance for intraband and interband transitions and the local dd transitions in the Cu2+ ion. It is concluded that the dd band (the transition dxy→dx2−y2 at 1.5 eV) reflects the enhancement of the covalent bonding (pd hybridization) upon metallization and that the spectral feature at ≈1.8 eV carries information about the contribution of electronic correlations, since it is sensitive to the opening of a spin gap in the insulator and to antiferromagnetic fluctuations in the metal. Although the covalent (≈1.5 eV) and correlation (≈1.8 eV) absorption peaks compete with each other, the coexistence of these bands in the metal supports the validity of a model based on the correlation polaron — a charge carrier which creates a region of covalent bonding in a Hubbard matrix of anti...

Structure and photoluminescence of helium-intercalated fullerite C60

The intercalation of C60 single crystals with helium is studied by powder x-ray diffractometry. It is established that the intercalation is a two-stage process: octahedral cavities are filled first and then tetrahedral ones, the chemical pressure being negative during both stages. The low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C60 are studied for the first time. The presence of helium in lattice voids is shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C60 molecules, the so-called “deep traps” with the 0–0 transition energy close to 1.69 eV. The mechanism of the effect of intercalation with helium on the pair formation in fullerite C60 is discussed.

Light scattering in LiCoPO4 single crystal: analysis of the vibrational spectrum

In a single crystal of LiCoPO4 Raman scattering has been studied in a wide temperature range. Thirty-two of the 36 Raman-active vibrational modes predicted by group-theory analysis were detected. The experimental lines are identified on the basis of their polarization rules, frequency position, and temperature behavior.

Optical evidence for compatibility of antiferromagnetism and superconductivity in YBa2Cu3O6+x

The evolution of the spectral composition of the absorption in the 1.25–2.6 eV region for metallic films of YBa2Cu3O6+x with superconducting transition temperatures of Tc=51 and 74 K is measured as the films are cooled from 180 to 20 K. Particular attention is paid to the temperature effects in two absorption bands: the A band (≃1.8 eV), which reflects the appearance of holes dressed in antiferromagnetic (AFM) fluctuations, and the (A+J) band (≃2.15 eV), which reflects an additional (magnon) excitation of the short-range AFM order. It is found that the changes of these bands begin in the normal phase at T<T* in the temperature region corresponding to the opening of the pseudogap state, and the (A+J) magnon band arises in the pseudogap state even in the case when it is absent at room temperatures. At the superconducting transition the parameters of the bands stop changing, and the (A+J) magnon band is preserved in the superconducting state. The results are interpreted as evidence of a magnetic nature of th...

Vibrational characteristics of niobium diselenide and graphite nanofilms

The phonon spectra of niobium diselenide nanofilms, consisting of several structural elements of this compound, as well as graphite nanofilms starting with bigraphene are analyzed at the microscopic level. The partial contributions of the displacements of atoms along the strong and weak coupling directions (i.e. along and perpendicular to the layers) to the phonon density of states are calculated. The characteristic distinguishing features of the vibrational spectral of these structures are analyzed. The fact that our calculations are practically identical to the data obtained from neutron diffraction, acoustic, and optical experiments confirms that the description of the phonon spectra of the compounds is highly accurate. The temperature dependences of the mean-square shifts for bulk samples and nanofilms along different crystallographic directions, making it possible to evaluate the stability of the graphite and niobium diselenide nanofilms at low temperatures, are calculated for each compound studied.

Magnetostriction in the mixed state of superconducting 2H-NbSe2 single crystals

Magnetostriction measurements on 2H-NbSe2 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 Hc2. 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.

d-f-Exchange resonance in gadolinium: Theory and experiment

Abstract The single-crystal gadolinium dielectric permeability spectrum is investigated experimentally and theoretically in the frequency range 0.5 ⩽ h ω ⩽1 eV. The temperature dependence of d-f exchange resonance spectrum observed in the above range is interpreted. This is done in terms of the concept of intra-atomic light absorption and the magnetic order parameter dependence of the coherence degree of angular parts of quasi-bound 5d-electron wave functions.

Magnetic phase diagram of the system of manganites Nd0.6Ca0.4(Mn1−xCrx)O3

The following sequence of phase transformations is observed in the Nd0.6Ca0.4(Mn1−xCrx)O3 system: antiferromagnet (x=0), mixed magnetic state (0.015⩽x⩽0.04), ferrimagnet (0.04⩽x⩽0.8), and spin glass (x=1). At chromium concentrations near x∼0.5 the Neel temperature increases sharply, and the contribution from the Nd sublattice increases substantially. Since the chromium and manganese ions have a tendency to order, these results can be interpreted in a model of superexchange interactions.

X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.