Spin fluctuation spectrum of electron-doped high-Tc superconductors

The magnetic excitation spectrum of electron-doped copper oxide superconductors is calculated using the Hubbard model on a square lattice. First, the on-site repulsion is treated with the random phase approximation. The spectrum of electron-doped systems in the superconducting state is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot (an intersection of the Fermi surface and the magnetic Brillouin zone boundary) is investigated. As compared with the hole-doped systems, the resonance condition is difficult to be satisfied in the electron-doped systems because of the small density of states around the hot spot. Moreover, the correlation effect in the Hubbard model is treated by the fluctuation-exchange approximation (FLEX), and the spin fluctuation spectra in the superconducting state in a wide region of the wave vector and frequency are calculated. We have found that the intensity of the magnetic spectrum at incommensurate wave vectors obtained with the FLEX is considerably weaker than that obtained with the RPA. The validity of the Fermi-liquid approach is also discussed.     

Many-body interactions in hole-doped high-Tc cuprates studied by high-resolution ARPES

We have performed high-resolution angle-resolved photoemission spectroscopy on hole-doped high-T_c cuprate superconductors (HTSCs) to study many-body interaction and the universality of low-energy excitation gap. In Bi₂Sr₂CaCu₂O₈ (Bi2212), we observed a kink in the dispersion in the off-nodal region in the superconducting state, which remarkably weakens on impurity substitution. We also find that the appearance of the kink in the off-nodal region is a common feature of Bi2212 and La_1.85Sr_0.15CuO₄ (LSCO), while the energy scale is remarkably different between two compounds (70 and 20 meV). We discuss universality of the kink in dispersion in the hole-doped HTSCs in terms of the coupling of electrons with spin fluctuations.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Constraints on the mechanism of superconductivity for MgB2 from Tc and the total isotope effect

We examine the possible electron-phonon spectra that produce both T_c=39 K and an isotope coefficient β=0.32±0.01, with Eliashberg theory. We assess the viability of the conventional electron-phonon mechanism in light of these results, compared with ab initio calculations of the electron-phonon spectrum. Comparisons are made with similar considerations for low T_c materials.     

Charge dynamics in electron-underdoped high-Tc cuprates

We investigate charge dynamics in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for an electron-doped t-t′-J model. When AF correlation develops with decreasing temperature, a gap-like behavior emerges in the optical conductivity accompanied by the enhancement of the coherent motion of carriers due to the same sublattice hoppings. This is a remarkable contrast to the behavior of a hole-doped t-t′-J model.     

Crystallization study of sol-gel un-doped and Pd-doped TiO2 materials

Sol-gel nanostructured titania materials have been reported to have applications in areas ranging from optics via solar energy to gas sensors. In order to enhance the photocatalytic activity, there are many studies regarding the doping of titanium dioxide (TiO₂) material with either non-metals (S, C, N, P) or metals (Ag, Pt, Nd, Fe). The present work has studied some un-doped and Pd-doped sol-gel TiO₂ materials (films and gels), with various surface morphologies and structures, obtained by simultaneous gelation of both precursors Ti(OEt)₄ and Pd(acac)₂. Their structural evaluation and crystallization behavior with thermal treatment were followed by DTA/TG analysis, infrared (IR) spectroscopy, Fourier transform infrared (FTIR), spectroellipsometry (SE), X-ray diffraction (XRD) and atomic force microscope (AFM). The influence of Pd on TiO₂ crystallization for both supported and un-supported materials was studied (lattice parameters, crystallite sizes, internal microstrains). The changes in the optical properties of the TiO₂-based vitreous materials were correlated with the changes of the structure. The hydrophilic properties of the films were also connected with their structure, composition and surface morphology.     

Effect of Bi substitution on the magnetic and dielectric properties of epitaxially grown BaFe0.3Zr0.7O3−δ thin films on SrTiO3 substrates

The structural, dielectric and magnetic properties of single crystalline Ba_1−xBi_xFe_0.3Zr_0.7O_3−δ (x=0.0-0.29) thin films have been studied. The pseudotetragonal epitaxial thin films were obtained by pulsed laser-beam deposition (PLD) on (0 0 1) SrTiO₃ (STO) single-crystal substrates. The Bi substitution for the Ba ions up to an extent of x=0.18 caused a slight improvement in the leakage current properties, as well as an enhancement of the apparent dielectric constant. The saturation magnetization of the films was significantly decreased following Bi substitution. These changes were thought to be related to the increase in oxygen deficiencies in the films. The effect of the Bi substitution on the dielectric and magnetic properties was analyzed in conjunction with the change in valence value of the Fe ions.     

Nanoindentation analysis of the pop-in events on SiGe single layer

The growth of metastable silicon germanium (Si_0.8Ge_0.2) thin film on Si(1 0 0) by ultrahigh-vacuum chemical vapor deposition has been subjected to residual indentation studies. A nanoindentation system has been applied to analyze SiGe film after different annealing treatments. A number of phenomena have been found for the heteroepitaxial growth of SiGe film at the critical thickness of 350 nm, including single discontinuity (the so-called “pop-in” event) as well as the elastic/plastic contact translation. Atomic force microscopy is employed to investigate the surface impression. Pop-in events in the load-indentation depth curves of 400 and 500 °C and no nano-cracks in the vicinity regions are found. The values of H ranging from 13.13±0.9, 21.66±1.3, 18.52±1.1, 14.47±0.7 GPa and the values of E ranging from 221.8±5.3, 230.7±6.4, 223.5±4.6, 156.7±3.8 GPa, are obtained. The elastic/plastic contact translation of the SiGe film occurs at different annealing conditions, with h_f/h_max values in the range of 0.501, 0.392, 0.424, and 0.535 for samples are treated at RT, 400, 500, and 600 °C, respectively. The mechanism responsible for the pop-in event in such crystal structure is due to the interaction of the indenter tip with the pre-existing threading dislocations, since the release of the indentation load is bound to be reflected in the directly compressed volume.     

Piezoelectric non-linearity in PbSc0.5Ta0.5O3 thin films

Epitaxial (001)-oriented PbSc_0.5Ta_0.5O₃ (PST) thin films were deposited by pulsed laser deposition. Local piezoelectric investigations performed by piezoelectric force microscopy show a dual slope for the piezoelectric coefficient. A piezoelectric coefficient of 3 pm/V was observed at voltages up to 0.8 V. However, at voltages above 0.8 V, there is a steep increase in piezoelectric coefficient mounting to 23.2 pm/V. This nonlinear piezoelectric response was observed to be irreversible in nature. In order to better understand this nonlinear behavior, voltage dependent dielectric constant measurements were performed. These confirmed that the piezoelectric non-linearity is indeed a manifestation of a dielectric non-linearity. In contrast to classical ferroelectric systems, the observed dielectric non-linearity in this relaxor material cannot be explained by the Rayleigh model. Thus the dielectric non-linearity in the PST films is tentatively explained as a manifestation of a percolation of the polar nano regions.     

Superconducting properties in Rh17S15 under magnetic field and pressure

We have studied superconducting properties by measuring the electrical resistivity and magnetization for a single crystal of Rh₁₇S₁₅ with a superconducting transition temperature T_c=5.4 K. The upper critical field H_c2(0) and the lower critical field H_c1(0) were obtained as 20.5 and 0.0033 T, respectively. Correspondingly, the coherence length and the penetration depth were estimated to be 40 and 4900 Å, respectively, indicating that Rh₁₇S₁₅ is a typical type-II superconductor with strong correlations of conduction electrons with a 4d-electron character of Rh atoms. The present electron correlations are formed to be enhanced with increasing pressure.     

P/Si-TiO2 transparent films with high anatase stability and photocatalytic activity

The anatase-TiO₂ transparent films, containing 3 mol% of Si and P elements (as dopants), were synthesized using a process combining the sol-gel method and spin-coating technique. Effects of relative ratio of dopants and calcination temperature on phase transformation, grain growth, surface morphology, light transmittance, band-gap energy and photocatalytic activity of the P/Si-TiO₂ films were examined and their results were compared with those of the undoped-TiO₂ and Si-TiO₂ films. The P/Si-TiO₂ films calcined at temperature between 600 and 900 °C adhered strongly to the surface of fused-silica substrate and were composed of anatase-TiO₂ monophase. The photocatalytic activities of the films were measured and represented using a characteristic time constant (τ) for the methylene blue (MB) photodegradation. The small τ stands for high photocatalytic ability of the film. The P/Si-TiO₂ film, containing equalmolar Si and P dopants, calcined at 800 °C gave the best performance in photocatalysis; this film had τ=5.7 h and decomposed about 90 mole% of MB in the water after 12 h of the 365-nm UV light irradiation.     

Preparation of REFeAsO1−xFx (RE=Sm and Gd) superconductors at a relatively low temperature

A series of the SmFeAsO_1−xF_x and GdFeAsO_1−xF_x (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF₃ as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO_1−xF_x and GdFeAsO_1−xF_x, respectively. These temperatures are at least 50-60° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in T_c proved that this low temperature process was more effective to introduce fluorine into R_EFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO_1−xF_x with x=0.05 and at 22 K for GdFeAsO_1−xF_x with x=0.1. The highest T_c was detected to be 54 K in SmFeAsO_0.8F_0.2 and 40.2 K in GdFeAsO_0.75F_0.25. The use of the nano-scaled ReF₃ compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.     

Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction

Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism of superconductivity for underdoped cuprates. The gluing force of the superconducting mechanism is an attraction between spin vortices on two different Néel sublattices, centered around the empty sites described in terms of fermionic holons. The spin fluctuations are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Through gauge interaction the spin vortex attraction induces the formation of spin-singlet (RVB) spinon pairs with a lowering of the spinon gap. Lowering the temperature, the approach exhibits two crossover temperatures: at the higher crossover a finite density of incoherent holon pairs are formed leading to a reduction of the hole spectral weight, while at the lower crossover a finite density of incoherent spinon RVB pairs are also formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic vortices appear in the plasma phase. Finally, at a even lower temperature the hole pairs become coherent, the magnetic vortices becoming dilute and superconductivity appears. The superconducting mechanism is not of BCS-type since it involves a gain in kinetic energy (for spinons) coming from the spin interactions.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Effects of an external electromagnetic field on rutile Tio2: A molecular dynamics study

Non-equilibrium molecular dynamics simulations of the bulk rutile phase of TiO₂ have been carried out in an intense external electromagnetic field with frequency in the microwave to far-infrared range. Simulations were performed in constant-volume ensembles with and without a thermostat coupled to the atomic degrees of freedom, at 298 K and from 298 K to well above the crystal melting temperature, respectively. Fields were applied along the a and c crystallographic directions. Both the Lekner and Ewald techniques were used to handle long-range electrostatics and the impact of different choices of Ewald parameters on the results has been evaluated. It was found that the ions respond rapidly to the field. The peaks and troughs in the Ti-Ti radial distribution functions were sharpened relative to the zero-field case at the instants of maximum electric field intensity, but little evidence of this was found for the Ti-O or O-O distributions. For pure Newtonian dynamics, the 500 GHz field excited the vibrational modes to the greatest extent, raising the system temperature at the fastest rate. For temperatures above 2000-2500 K, the crystal structure was found to melt. In the canonical ensemble, the 50 GHz field led to the greatest enhancement of ionic translational mobility.     

Reaction kinetics of α-CuInSe2 formation from an In2Se3/CuSe bilayer precursor film

The reaction pathway and kinetics of α-CuInSe₂ formation from a glass/In₂Se₃/CuSe polycrystalline bilayer precursor film were investigated using time-resolved, in situ high-temperature X-ray diffraction. Bilayer glass/In₂Se₃/CuSe precursor films were deposited on thin glass substrates in a migration enhanced molecular beam epitaxial deposition system. These films were then temperature ramp annealed or isothermally soaked while monitoring the phase evolution. The initial In₂Se₃ and CuSe reactant phases were directly transformed to α-CuInSe₂ without any detectable intermediate phase. Kinetic parameters were estimated using the Avrami and parabolic diffusion controlled reaction models. The parabolic reaction model fitted the experimental data better than the Avrami model over the entire temperature range (230-290 °C) of the set of isothermal experiments, with an estimated activation energy of 162 (±5) kJ/mol.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Review of superconductivity in BiS2-based layered materials

In 2012, a new layered superconductor where BiS₂ layer is the superconducting layer was discovered. So far, seven types of BiS₂-based superconductors and two related superconductors have been discovered. In this article, the diversity of the crystal structure and the physical properties of the BiS₂-based superconductors are reviewed. Furthermore, notable characteristics of superconductivity in the BiS₂ family are introduced. The prospects for raising T_c in this family are proposed on the basis of experimental and theoretical studies.     

Schottky defects in cubic lattice of SrTiO3

Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO₃ crystal are investigated by means of a quantum-chemical simulation based on the Hartree-Fock methodology. The occurrence of Sr partial Schottky defect (V_Sr+V_O) and two types of Ti partial Schottky defects (V_Ti+2V_O) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies.     

Defect formation and water incorporation in Y2O3

The molecular statics method is used to study the formation of defects and water incorporation in Y₂O₃. The crystal structure, the isothermal compressibility, and the formation enthalpy of Y₂O₃ calculated with the chosen interaction potentials are in good agreement with the experimental data. The formation energies of intrinsic and impurity defects are evaluated. The binding energy of protons and oxygen vacancies with an acceptor impurity at different distances is calculated. Various water incorporation reactions in the oxide are examined, including the mechanisms involving oxygen interstitial sites and oxygen vacancies produced by the acceptor doping. It is shown that the water incorporation in pure Y₂O₃ is energetically less favorable than in the acceptor doped oxide.