Charge transport in highly iron-doped oxidized lithium niobate single crystals

The photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt. %. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe²⁺ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beam-coupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport. PACS 72.20.Jv; 72.40.+w; 81.40.Tv     

Electrical transport properties of CuS single crystals

Electrical resistivity, transverse magnetoresistance and thermoelectric power measurements were performed on CuS high quality single crystals in the range 1.2-300 K and under fields of up to 16 T. The zero field resistivity data are well described below 55 K by a quasi-2D model, consistent with a carrier confinement at lower temperatures, before the transition to the superconducting state. The transverse magnetoresistance develops mainly below 30 K and attains values as large as 470% for a 16 T field at 5 K, this behaviour being ascribed to a band effect mechanism, with a possible magnetic field induced DOS change at the Fermi level. The transverse magnetoresistance shows no signs of saturation, following a power law with field Δρ/ρ(0) ∝ H(1.4), suggesting the existence of open orbits for carriers at the Fermi surface. The thermoelectric power shows an unusual temperature dependence, probably as a result of the complex band structure of CuS.     

Thermodynamic study of lithium insertion in V6O13 and Li1+xV3O8

The enthalpies of Li⁺ insertion into two V oxides of interest as cathodes for secondary Li batteries, i.e. V₆O₁₃ and Li_1+xV₃O₈ have been directly measured by solution calorimetry. By comparing the integral molar enthalpies ΔH⁰/x with the corresponding ΔG⁰/x values, ΔS⁰/x higher than expected have been found for Li_1+xV₃O₈. This has been correlated to a reorganization of its distorted structure induced by the initial Li⁺ insertion. Two other cathode materials, having the same variation of the potential with Li⁺ content, i.e. MoO₃ and (Mo_0.3V_0.7)₂O₅, show even higher ΔS⁰/x values. The entropic values stemming from structural reorganizations add to the configurational entropies of inserted ions in determining the initial profile of E as a function of Li⁺.     

Charge transport properties of undoped congruent lithium niobate crystals

Time-resolved light-induced birefringence measurements based on a phase compensation technique are used to determine the charge transport properties of congruent, nominally-undoped lithium niobate crystals (LiNbO₃). Some of the crystals are conventionally oxidized. The steady-state space-charge field E _pv, the bulk photovoltaic coefficient β, and the photoconductivity σ _ph are determined in the intensity range (30–30 000) W/cm². The photovoltaic coefficient β increases by one order of magnitude over this intensity range, the space-charge field E _pv even by two orders of magnitude. We discuss the results in the context of the known one- and two-center charge transport models for LiNbO₃. The experimental findings presented here are of relevance for the long-standing problem of optical damage in such crystals, which inhibits their use in high-intensity applications like nonlinear optics.     

Electron-phonon scattering in transport properties of InSe single crystals

Summary The electron-phonon scattering in indium selenide single crystals has been investigated by Hall-mobility and Raman-spectroscopy measurements. The experimental data have been interpreted according to the Fivaz and Schmid model for homopolar optical scattering. The best fit of the experimental results is obtained by assuming the bidimensional behaviour of the interaction involving anA ₁ phonon with energy 14.5 meV. The bidimensional character is probably due to the presence of a large number of planar defects, confirmed by electron microscope observations, which strongly localize the carriers within the layers.

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Riassunto In questo lavoro è riportato uno studio dell'interazione elettrone-fonone in monocristalli di seleniuro di indio, condotto mediante misure di effetto Hall e spettroscopia Raman. I dati sperimentali sono stati interpretati secondo il modello di Fivaz e Schmid, per lo scattering da fononi ottici omopolari. L'approssimazione migliore delle curve sperimentali è ottenuta assumendo un carattere bidimensionale per l'interazione tra gli elettroni ed i fononi, nel modoA ₁, con energia 14.5 meV. Il carattere bidimensionale delle proprietà di trasporto è probabilmente dovuto alla presenza di difetti planari che localizzano i portatori di carica all'interno degli strati.

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Резюме Исследуется электрон-фононное рассеяние в монокристаллах InSe, используя измерения подвижности Холла и спектроскопии комбинационного рассеяния. Экспериментальые данные интерпретируются в соответствии с моделью Фиваца и Шмидта для гомополярного оптического рассеяния. Наилучшая подгонка экспериментальных результав получается в предположении двумерного характера взаимодействия между электронами и фононами, модыA ₁ , с энергией 14.5 мэВ. Двумерный характер, по-видимому, обусуовлен наличием большого числа плоских дефектов, которые сильно локализуют носители внутри слоев, что подтверждается наблюдениями электронной микроскопии.

     

Synthesis and magnetic properties of Cu3B2O6 single crystals

The temperature dependence of the magnetic susceptibility of Cu₃B₂O₆ single crystals grown by spontaneous crystallization from a melt consisting of a mixture of CuO and B₂O₃ and the behavior of their magnetization are investigated in magnetic fields up to 55 kOe. A broad susceptibility maximum is observed near 39 K, and a sharp drop in susceptibility is observed at T<10 K. The paramagnetic Néel temperatures for all orientations of the magnetic field in the crystal investigated are negative, attesting to the predominantly antiferromagnetic character of the exchange interactions. The effective magnetic moment of the Cu²⁺ ion is anisotropic and lies in the range from 1.054μ _B to 1.545μ _B. The magnetization depends linearly on magnetic field at T>10 K, whereas at temperatures below 10 K a discontinuity is observed at fields of the order of 40 kOe. At room temperature, electron magnetic resonance characterized by an almost isotropic g factor (g=2.165) is detected at 36.22 GHz. The exchange interactions in Cu₃B₂O₆ are analyzed on the basis of the Goodenough-Kanamori rules. The possibility of the establishment of a singlet magnetic state in the crystal is analyzed. Fiz. Tverd. Tela (St. Petersburg) 41, 677–679 (April 1999)     

Thickness-dependent transport properties of Sr4Fe6O13 epitaxial thin films

High-quality epitaxial thin films of Sr₄Fe₆O₁₃ have been deposited on NdGaO₃(001) substrates by pulsed laser deposition. The transport properties have been characterized by impedance spectroscopy. The temperature dependence of conductivity suggests a mechanism of adiabatic hopping by small polarons, in agreement with previous results in bulk samples. The transport properties show a clear dependence on the film thickness with the thinner films (10 nm) presenting conductivity values in oxygen one order of magnitude higher than the thicker ones (313 nm). We correlate this behaviour with the thickness dependence of the epitaxial strain.     

An optical spectroscopy study of defects in lithium tantalate single crystals

The congruent, stoichiometric, and Mg doped stoichiometric LiTaO₃ single crystals have been successfully grown by the Czochralski technique. The evolution of defect structures caused by varying composition and post-growth processing has been evaluated from the optical absorption and photoluminescence measurements. Optical absorption studies showed that the UV absorption edge is very sensitive to the composition of LiTaO₃ crystals. Photoluminescence of various LiTaO₃ single crystals at room temperature was observed. The emission bands centered at 360, 430, and 530 nm were assigned to different defects, which can well show the defect information in LiTaO₃ crystals.     

Magnetic and transport properties of transition-metal implanted ZnO single crystals

ZnO single crystals were implanted with Mn, Co and Ni with fluences between 1 × 10¹⁶ cm^-2 and 1 × 10¹⁷ cm^-2 and energy of 200 keV. Results indicate that aggregation of transition metal ions in the as implanted state occurs only in the case of Ni. After an annealing stage to recover the ZnO structure aggregation occurs for the higher fluences of all implanted species. For lower concentrations paramagnetic behaviour with magnetic moments close to those of individual ions is observed. No polarised impurity band is formed as a result of the presence of transition metal ions and all samples show electrical conduction by carriers in extended states of ZnO. Significant values of magnetoresistance are measured at low temperatures, where electrical transport is described by hopping mechanisms between localized states. The sign of the magnetoresistance is dependent of the doping ion and is correlated with the observed aggregation.     

Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)