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The effect of a magnetic field on the magnetic susceptibility χ(T) and the temperature of the three magnetic phase transitions in single-crystal β-TeVO4 is studied at low temperatures. The H-T phase diagram is obtained for H||b and H ⊥ b. A tricritical point with coordinates H* = 3.2 ± 0.1 T and T* = 3.0 ± 0.1 K is found for H||b.

Magnetic properties of single crystal β-TeVO4 with a zigzag chain of magnetic centers (V4+ ions, S = 1/2) are analyzed in the framework of the J1–J2 model. The best agreement with experimental data is obtained for the J1–J2 model with parameters J1/kB = −38.33 K and J2/kB = 29.48 K (gb = 2.07, ga,c = 1.99). In contrast to the previously used model (a uniform antiferromagnetic spin chain system with J1/kB = 21.4 K (gb = 2.28, ga,c = 2.18)), this improved result allows one to consider the object under study as a zigzag chain system with a dominant ferromagnetic exchange between the nearest neighbors and an antiferromagnetic interaction between the next-nearest neighbors.

We present a comparative study of the low-dimensional compounds α- and β-TeVO4. Our data clearly show that the change in the local coordination geometry of V4+ ions between α- and β-TeVO4 leads to drastic differences in their magnetic properties. Despite sharing the same crystal structure, the two compounds manifest different magnetic exchange topologies. Both compounds exhibit a transition from ferro- to anti-ferromagnetic correlations with decreasing temperature. This effect, however, is driven by different mechanisms in the two compounds. In addition, dimensional crossover is found in β-TeVO4.

The IR reflectance spectra of the cubic noncentrosymmetric ferrimagnet Cu2OSeO3 (TC ≈ 60 K) were investigated in the temperature range 10–80 K. Detailed analysis of the symmetry and composition of vibrational modes was conducted on the basis of Raman scattering data and data obtained in this investigation. 68 out of 69 modes consistent with the crystal lattice symmetry were revealed. Some spectral features found in the magnetically ordered phase were studied. The bands at 280 and 421 cm−1 related to the spectrum of two-magnon light scattering were observed. Dynamic magnetoelectric coupling was found to be responsible for the intensity redistribution in the Raman and IR spectra for the threefold degenerate modes of F-symmetry (modes at 84, 205, and 269 cm−1). No change associated with magnetic ordering was observed in the symmetry of the crystal structure.

The temperature and magnetic field dependent magnetization of the two-dimensional quantum spin system Ni5(TeO3)4Cl2 has been investigated using single crystals in temperature range 1.8–100 K and in magnetic fields up to 14 T. The magnetization below the magnetic phase transition demonstrates an unusual temperature behavior with considerable anisotropy. Combined magnetization and specific heat data allowed to determine the critical temperature of the magnetic phase transition, Tc = 28.4 K. Magnetic fields shift this temperature toward lower temperatures.

We present a compact nuclear magnetic resonance (NMR) probe which is compatible with a magnet of a commercial superconducting quantum interference device magnetometer and demonstrate its application to the study of a quantum magnet. We employ trimmer chip capacitors to construct an NMR tank circuit for low temperature measurements. Using a magnetic insulator MoOPO4 with S = 1/2 (Mo5+) as an example, we show that the T-dependence of the circuit is weak enough to allow the ligand-ion NMR study of magnetic systems. Our 31P NMR results are compatible with previous bulk susceptibility and neutron scattering experiments and furthermore reveal unconventional spin dynamics.

We studied the temperature dependence of heat capacity CP(T) of a quasi-one-dimensional magnetic system β-TeVO4, in a zero magnetic field and a temperature region of 0.1 K ≤ T ≤ 300 K. The CP(T) dependence shows phase transitions at temperatures of 4.65, 3.28, and 2.32 K. Is it established that at a temperature of TN = 4.65 K there is a phase transition from a paramagnetic to an antiferromagnetic state. We provide a theoretical description of the heat capacity's temperature dependence.

We studied the deposition of Au, In, Ag, and Al overlayers onto clean-cleaved GaTe(001) surfaces at room temperature by x-ray photoelectron spectroscopy. Gold and In overlayers did not produce evidence of chemical reactions, and neither with cation nor anion outdiffusion. Gold overlayers exhibited no evidence of island formation and a layer-by-layer overlayer coverage. The deposition of In revealed the presence of clustering for a metal thickness higher than 6 A. On the other hand, the deposition of Al lead to an exchange reaction with a metalliclike Ga phase separation. Core level analysis of the Ag–GaTe interface revealed no noticeable change of the shape of the peaks. The dependence of the peak intensities on overlayer thickness showed an initial layer-by-layer coverage until 1.8 A, followed by a clustering growth mode. From about 3.7 A Ag thickness, significant Te outdiffusion was observed.

The pressure dependence of electrical resistance of single-crystal magnetite (Fe3O4) was measured under quasi-hydrostatic conditions to 100 GPa using low-temperature, megabar diamond-anvil cell techniques in order to gain insight into the anomalous behavior of this material that has been reported over the years in different high-pressure experiments. The measurements under nearly hydrostatic pressure conditions allowed us to detect the clear Verwey transition and the high-pressure structural phase. The appearance of a metallic ground state after the suppression of the Verwey transition around 20 GPa and the concomitant enhancement of the electrical resistance caused by the structural transformation to the high-pressure phase form reentrant semiconducting-metallic-semiconducting behavior, although the appearance of the metallic phase is highly sensitive to stress conditions and details of the measurement technique.

Core-level photoemission peaks taken on several insulators with spatial resolution of 30-300 mu m rigidly shift in energy from place to place, most likely because of local charging effects. Such phenomena can affect the linewidth and the general lineshape of conventional, spatially integrated spectra even when a flood gun is used, which is one of the standard remedies against charging. This may require a critical revision of photoemission linewidths and lineshapes for many past experiments on insulators.

Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals. In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap. As-grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 0.8 eV, which is attributed to Ti3+ (3d) defect states. Annealing the crystals at high temperatures in O2 or subsequent thermal reduction in an Ar-H2 Mixture (95%-5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti3+ States. In addition, some redistribution of the spectral weight is observed in the VB spectra.

The substitution site of Y and Pb in the cuprate-type high temperature superconductor Bi2Sr2Ca1Cu2O8+δ is determined in a very direct and unambiguous way by means of angle-scanned x-ray photoelectron diffraction (XPD). Using XPD as a fingerprinting tool, we conclude that Y occupies the Ca sites and Pb the Bi sites, respectively. Furthermore, low-energy electron diffraction data unequivocally show the presence of the incommensurate lattice modulation which is known for pure Bi2212, but not for sufficiently Pb doped Bi2212. We can, therefore, attribute the reappearance of the modulation directly to the Y doping.

Scanning photoelectron spectromicroscopy experiments on the edge of patterned Si overlayers on GaSe revealed the presence of an inhomogeneous reaction with a metallic-like Ga phase separation. The Si-Se chemically shifted components show lateral variations and a thickness-related SiSe2/SiSex (x = 0.5, 1, and 1.5) ratio on a micrometer scale. The dependence of the peak intensities on the overlayer thickness suggests an initial layer-by-layer coverage until approximate to 2 Angstrom, followed by a clustering growth mode. (C) 1999 American Institute of Physics. [S0021-8979(99)07111-X].

A method for a controlled change of the doping level of high-temperature superconductors with ultraviolet radiation is presented. With photoemission it is shown that the exposure of Bi2Sr2CaCu2O8+δ samples to the light of a He gas-discharge lamp causes oxygen desorption. From measurements of the Fermi surface, it is found that the oxygen desorption causes a decrease of the doping level of the superconductors. From the desorption cross sections that strongly depend on the photon energy, two different oxygen desorption channels are inferred. This procedure for decreasing the doping level has the advantage that the crystallinity of the sample is not altered and that the doping level can be simultaneously measured by photoelectron spectroscopy.

Photoemission spectromicroscopy data on Si–GaSe with 0.2 μm lateral resolution revealed laterally inhomogeneous interface Se–Si chemical reactions. The local Si 2p fits agrees with interface Si selenide parameters; the Se 3d peaks indicate a Si–selenide component; the Ga 3d peaks exhibit a metallic component whose intensity increases with Si coverage. This local interface reaction is related to lateral changes in the band lineup.

A microscopic chemical analysis of BiSrCaCuO (BSCCO) 2212 single crystals was performed with an unprecedented combination of high (submicron) lateral resolution and high energy resolution (100 meV or better). These performances, achieved with imaging synchrotron spectromicroscopy, enabled us to detect in a very small fraction of the investigated specimens, morphological and chemical inhomogeneities. Specifically, we found deviations from stoichiometry in the surface Sr content which have a subtle effect on the chemical environment of at least one other element, Ca. The different chemical environment was found to influence surface chemical reactions.

The switching of second‐harmonic generation (SHG) at a C60 single‐crystal surface has been observed in a pump‐and‐probe experiment. The SHG signal from a picosecond 1.17 eV laser pulse is suppressed by one order of magnitude upon illuminating the crystal surface with a 3.49 eV pump pulse. The nonlinear optical response is faster than 45 ps and persists for longer than 20 ns. SHG suppression to 1/e occurs at pump densities as low as 2.8 μJ/cm2. We suggest that nonlocalized excited electronic states determine the change in the nonlinear optical behavior.

Resistivity, thermopower, and Hall‐effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n‐type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.

We find that even very low Ni doping levels of high-quality Bi2Sr2Ca1Cu2O8 single crystals strongly affect the transition temperature T(c). We also observed that T(c) is not related to the total Ni concentration, but only to that of Ni engaged in NiO-type bonds. By controlling the temperature during crystal growth, one can modify the relative weight of Ni in NiO-type bonds with respect to other configurations-and therefore T(c).

Time- and angle-resolved photoemission spectroscopy is a powerful technique to study ultrafast electronic dynamics in solids. Here, an innovative optical setup based on a 100-kHz Yb laser source is presented. Exploiting non-collinear optical parametric amplification and sum-frequency generation, ultrashort pump (h nu = 1.82 eV) and ultraviolet probe (h nu = 6.05 eV) pulses are generated. Overall temporal and instrumental energy resolutions of, respectively, 85 fs and 50 meV are obtained. Time- and angle-resolved measurements on BiTeI semiconductor are presented to show the capabilities of the setup. (C) 2014 AIP Publishing LLC.

Millimeter-sized cobalt doped rutile crystals with a bi-pyramidal shape are obtained by chemical vapour transport using scroll-type H2Ti3O7 nanotubes as a precursor in which Co2+ ions are introduced by a simple ion exchange method prior to the growth. Despite the low concentration of Co2+ dopants (5 x 10(19) cm(-3)), the resistivity of the single crystal shows a metallic behaviour above 50 K and the Seebeck coefficient has the signatures of polaronic quasiparticles. The magnetic properties of the material show a weak anti-ferromagnetic interaction between the spins on Co atoms below 50 K. This synthesis method could be beneficial for the growth of a large variety of doped TiO2 single crystals. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x = 0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs.

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function., Comment: 13 pages, 4 figures

Raman spectra of optimally doped magnetite (Fe3O4) single crystals reveal broad electronic background extending up to 900 wave numbers (∼110meV). Redistribution of this background is observed when sample is cooled below the Verwey transition temperature (TV=123K). In particular, spectra of the low temperature phase show diminished background below 300 wave numbers followed by an enhancement of the electronic background between 300 and 400 wave numbers with subsequent decrease of the background below 400 wave numbers. Such redistribution may be assigned to an opening of the charge gap at about 350±80 wave numbers (∼43±10meV). The value of the gap is within the range of recent photoemission data on freshly fractured magnetite sample.

We report the results of a low-temperature (300 K-15 K) high-pressure (up to 22 GPa) Raman study of the Verwey transition in magnetite (Fe3O4). We use additional Raman modes observed below the Verwey transition to determine how the transition temperature changes with the quasihydrostatic pressure. Increase of the pressure results in the linear decrease of the Verwey transition temperature, with no discontinuity. The corresponding pressure coefficient dT(v)/dP is found to be -5.16 +/- 1.19 K/GPa. Such a decrease is substantially larger than the one predicted by the mean-field Coulomb interaction model of the transition. (c) 2005 American Institute of Physics.