Analytic Study of the E ? e Jahn–Teller Polaron

A review is given of recent work on the E e Jahn–Teller (JT) polaron, i.e., a mobile e_g electron linearly coupled to the local e_g normal vibrations of a periodic array of octahedral complexes. The main ingredient of our theory is a mapping of the Hamiltonian onto a new Hilbert space, belonging to a fixed angular momentum eigenvalue j > 0. In this representation, the Hamiltonian depends explicitly on j and decomposes into a Holstein term and a residual JT interaction. The ground state of the JT polaron belongs to the sector j = 1/2, whereas the Holstein polaron is obtained for the unphysical value j = 0. The new Hamiltonian is then subjected to a variational treatment, yielding the dispersion relations, effective masses and band widths for both kinds of polarons. The calculated polaron masses are in remarkably good agreement with recent quantum Monte Carlo data.     

XRD and PES Studies of the Bi2Sr2Ca1?xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2?xSnxO8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi₂Sr₂CaCu₂O₈₊ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.     

Anisotropic Properties of Bi2Sr2?xLaxCu1O6+δ Thin-Films Grown on Vicinal SrTiO3 Substrates

Epitaxial Bi₂Sr_2–xLa _x Cu₁O₆₊ thin films were grown on vicinal substrates by magnetron sputtering. The growth mode and the superconductivity of the thin films were studied. The films were deposited on 5–7° misoriented (100) SrTiO₃ (STO). The surface of misoriented substrate is tilted to the 111 azimuth. The topographies of the Bi-2201 films show strip-like shapes. The transmission electron microscopy (TEM) study revealed that the films grew with very good in-plane alignments which were comparable to those of single crystals. Both of the temperature dependence of the a- and c-axis resistivities were studied under magnetic fields. The corresponding anisotropic activation energies and upper critical magnetic fields were investigated.     

Formation of the La0.03Sr0.255Ba0.7Nb2–yTiyO6–y/2 ferroelectric ceramic system

The study of the formation of the Sr_0.255La_0.03Ba_0.7Nb_{2 – y} Ti _y O_{6 – y/2} ferroelectric ceramic system and the influence of the sintering conditions and titanium concentration on the densification process is reported. A single-phase compound is observed for low titanium content (y 0.1) in the XRD spectra, being isostructural with the tetragonal tungsten bronze SBN phase. For high titanium concentrations (y > 0.1) the XRD patterns show, besides the tetragonal phase reflections, several small peaks evidencing an additional phase in the compound. This second phase was identified as isostructural to hexagonal Ba₆Nb₉Ti₇O₄₂. The grain size shows a linear increase with titanium content in the region of monophasic compositions and a possible liquid phase aided sintering process is analyzed.     

Investigations of Sr3Fe2−xMnxO7−δ the n=2 Ruddlesden–Popper phases with d3/d4 interactions

We have prepared a series of Mn substituted strontium ferrates, Sr₃Fe_2−xMn_xO_7−δ where x=2/3, 1 and 4/3, and investigated their properties with X-ray diffraction, X-ray absorption near edge spectroscopy (XANES), Mössbauer spectroscopy, variable-temperature resistivity and magnetic susceptibility. These compounds are metastable and have to be prepared at high temperatures, >1250°C, with solid state techniques followed by quenching in air to room temperature. As a consequence the compounds are oxygen deficient and contain some Fe³⁺. They are insulators and exhibit spin-glass like behavior at low temperatures due to frustrated magnetic interactions between the disordered array of transition metal ions on the B site. Mössbauer and XANES spectra show that the B cations are not fully oxidized and that the Fe⁴⁺ in these compounds are charge-disproportionated into Fe³⁺ and Fe⁵⁺ at low temperature.     

Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2?xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-T _c superconductor Bi₂Sr_2–x La _x CuO₆₊ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-T _c superconductivity.     

Kinks in the a/2?111? screw dislocation in Ta

We study the structure and formation energy of kinks in the 1/2a〈111〉 screw dislocations in metallic Tantalum (Ta) using molecular dynamics with a first principles based many-body interatomic potential. In our study, four a/3〈112〉 kinks are constructed in a quadrupole arrangement in the simulation cell. To the simulation cell, we impose periodic boundary conditions in the directions perpendicular to the [111] direction and fix boundary condition in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a〈111〈 dislocation lead to 8 distinguishable single kinks and 16 kinds of kink pairs. The different mismatches of the core configurations along [111] direction attributed to the differences in the formation energy for various types of kinks. Formation energies for all possible kinds of isolated single kinks and kink pairs have been determined. It was found that 0.730 eV was the lowest energy cost to form a kink pair in the a/2〈111〉: screw dislocation in Ta. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of Y(Gd) and Ni(Fe) on the principal structural characteristics of Al87Y5? x GdxNi8? y Fe ( x = 0?5; y = 0?4) amorphous metallic alloys

The X-ray diffraction method is used to study the short-range ordering of Al₈₇Y₅Ni₈, Al₈₇Gd₅Ni₈, Al₈₇Y₄Gd₁Ni₈, Al₈₇Y₄Gd₁Ni₄Fe₄, and Al₈₇Gd₅Ni₄Fe₄ amorphous alloys. The structural factors are obtained and the pair correlation functions are computed. The principal structural parameters are analyzed. It is shown that the investigated amorphous alloys are characterized by the topological short-range order based on the close-packed structure of aluminum and Al₃Y(Gd)-and Al₃Ni(Fe)-type chemical short-range ordering. It is discovered that the replacement of Y atoms with Gd atoms and Ni atoms with Fe atoms weakly affects the topology of the atomic distribution of the aluminum matrix and leads to changes in the character of chemical ordering. This affects the degree of short-range microhomogeneity of the short-range ordering and the dimensions of structural units. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4, pp. 75–79, July–August, 2008.     

Out-of-Plane Negative Magnetoresistance of Bi2Sr1.6La0.4CuO6+δ Single Crystals in the Underdoped Region

In order to investigate a relationship between the carrier concentration and negative magnetoresistance in the underdoped region, we have performed out-of-plane electrical resistivity measurements for underdoped Bi ₂ Sr _1.6 La _0.4 CuO ₆₊ single crystals. Giant negative magnetoresistance has been observed in the most abundant carrier sample with = 0.12 in our experiment. The negative magnetoresistance reduces with decreasing carrier concentration.     

Confirmation of Superconductivity in Sr2YRu1?uCuuO6

Evidence is reported that Sr₂YRu_1–u Cu _u O₆ superconducts with a transition temperature of at least T _c 30 K for u = 0.1, confirming the reports by Wu et al. that this material superconducts despite having no cuprate planes. (Such planes are widely believed to be necessary for high-temperature superconductivity.) Coexisting with the superconductivity, Sr₂YRu_1–u Cu _u O₆ exhibits magnetic order due to antiferromagnetic Ru and Cu.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Structure and Diamagnetic Properties of (Pb0.6SnyCu0.4 ? y)Sr2(Y1 ? xCax)Cu2Oz

The effect of Sn doping in (Pb_0.6Sn _y Cu_{0.4 – y} )Sr₂(Y_{1 – x} Ca _x )Cu₂O _z with 0 y 0.3 and 0 x 0.7 was investigated. It was established that a nearly pure 1212 phase can be obtained at 0 y 0.1 and 0 x 0.3. The obtained XRD patterns as well as the results of the EDX and ICP-AES analyses showed that Sn substitution is possible in the (Pb,Cu)-1212 phase. Superconductivity was observed at 0.4 x 0.7. The onset of the diamagnetic transitions varied from 10 to 30 K. The influence of the strong Pb deficiency on the superconducting properties of the samples was discussed.     

Effect of heat treatment in sulfur and mercury vapors on the magnetic susceptibility of Hg1 ? x Mn x Te1 ? y S y crystals

We have studied the effect of heat treatment in mercury and sulfur vapors on the magnetic susceptibility of Hg_{1 − x} Mn _x Te_{1 − y} S _y crystals. Measurements were performed by the Faraday method in the temperature range 77–300 K at H= 318 kA/m before and after heat treatments. The results demonstrate that the behavior of the magnetic susceptibility can be understood in terms of Mn-S-Mn-S, Mn-Te-Mn-Te, and (mixed) Mn-Te-Mn-S clusters of different sizes with indirect antiferromagnetic exchange interaction between the Mn atoms through the chalcogen atoms. Heat treatment in sulfur and mercury vapors leads to the formation of new clusters or changes (increases or decreases) the size of already existing clusters. Original Russian Text ? P.D. Maryanchuk, E.V. Maistruk, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 5, pp. 549–554.     

Sr2YRu1?uCuuO6: Evidence for SrO-Layer Superconductivity

Sr₂YRuO₆ doped on its Ru site by Cu superconducts at the below 45 K, although its Ru and Cu are magnetically ordered at 23 K and 86 K, respectively. The SrO layers superconduct. Ba₂GdRuO₆, when doped with Cu, does not superconduct, because L = 0 Gd is not crystal-field split, and so induces Cooper pair-breaking.     

Electrical properties of p -Si1 ? x Ge x Au-Based p - i - n structures and Schottky barriers

We have studied the properties of Si_{1 − x} Ge _x -based p-i-n structures and Schottky barriers in which the i-region had been produced via compensation with gold. The results demonstrate that the use of a guard ring in p-Si_{1 − x} Ge _x 〈Au〉 structures reduces the room-temperature reverse leakage current by two to three orders of magnitude. Such structures have sufficiently small reverse currents and a barrier on the order of 0.75 eV. p-Si_{1 − x} Ge _x 〈Au〉-based guard-ring structures are suitable for the fabrication of IR and nuclear detectors. Original Russian Text ? I.G. Atabaev, N.A. Matchanov, E.N. Bakhranov, M.U. Khazhiev, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 7, pp. 775–780.     

Oxygen Configuration in the Superconductor (Hg0.7Mo0.3)Sr2(Ca1?xRx)Cu2Oz (R = Nd and Pr)

Two series of Hg-based oxides (Hg_0.7Mo_0.3)Sr₂(Ca_1–x R _x )Cu₂O _z (R = Nd and Pr, 0.2 x 0.7) have been synthesized. Electrical-resistivity measurements show that these compounds are superconductors with maximum onset T _c of 107 and 102 K for Nd- and Pr-containing samples, respectively. The neutron powder diffraction experiments on both as-prepared and O₂-annealed samples of R = Nd revealed that the O(3) site at the HgO sheets are fully occupied and shifted towards the Hg/Mo site to form Mo–O bonds.     

In Situ Investigation of Reversible Exsolution/Dissolution of CoFe Alloy Nanoparticles in a Co-Doped Sr2Fe1.5Mo0.5O6−δ Cathode for CO2 Electrolysis

Reversible exsolution and dissolution of metal nanoparticles in perovskite has been investigated as an efficient strategy to improve CO₂ electrolysis performance. However, fundamental understanding with regard to the reversible exsolution and dissolution of metal nanoparticles in perovskite is still scarce. Herein, in situ exsolution and dissolution of CoFe alloy nanoparticles in Co-doped Sr₂Fe_1.5Mo_0.5O_6–δ (SFMC) revealed by in situ X-ray diffraction, scanning transmission electron microscopy, environmental scanning electron microscopy, and density functional theory calculations are reported. Under a reducing atmosphere, facile exsolution of Co promotes reduction of the Fe cation to generate CoFe alloy nanoparticles in SFMC, accompanied by structure transformation from double perovskite to layered perovskite at 800 °C. Under an oxidizing atmosphere, spherical CoFe alloy nanoparticles are first oxidized to flat CoFeO_x nanosheets, and then dissolved into the bulk with structure evolution from layered perovskite back to double perovskite. Electrochemically, CO₂ electrolysis performance can be retrieved during 12 redox cycles due to the regenerative ability of the CoFe alloy nanoparticles. The anchoring of the CoFe alloy nanoparticles in SFMC perovskite via reduction shows enhanced CO₂ electrolysis performance and stability compared with the parent SFMC perovskite.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO₄₊ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7–. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7– samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7–, dominate superconducting properties of bulk HgBa₂CuO₄₊. With respect to YBa₂Cu₃O_7–, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Photocurrent amplification effect of the fet gate p–n junction

A superlinear increase in the light current of the gate with an increase in the exciting integral light intensity (from 100 to 1000 lx) has been revealed experimentally. This superlinear increase is due to the increase in the internal gradient field generated by the minority carriers by the cutoff voltage of the gate leading to an additional interband generation of photocarriers. On the basis of the investigation of the dependence of photocurrent on the cutoff voltage it has been shown that the photocurrent increases linearly, which is explained by the creation of an internal electric field by the majority carriers of the base zone of the gate p–n junction. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 191–198, January–February, 2009.