Electronic structure of superconducting Cu oxides

On the basis of photoemission results on La_2-xSr_xCuO_4-y and YBa₂Cu₃O_y it is suggested that carriers doped in the otherwise insulating samples are oxygen p holes rather than Cu d holes. The p holes interact with the localized Cu d states through hybridization and may give rise to an anomaly near the Fermi surface as described by a periodic Anderson model with low carrier density.     

A quantitative understanding of pressure dependent conductivity of FeSi1−xGex

A new density of states model, referred to as the Gaussian density of states, is proposed for the quantitative understanding of the electrical conductivity behaviour of FeSi Kondo insulating system. The effects of electron correlation and disorder, responsible for the physical properties of this system, are judiciously incorporated in this model. Within the framework of this model, a detailed quantitative analysis of the temperature and pressure dependent electrical conductivity data of FeSi_1−xGe_x (x=0.0, 0.05 and 0.20) reported by Awadhesh Mani et al. [Phys. Rev. B 63 (2001) 115103] has been carried out. From these analyses the complicated pressure dependence of energy gap seen experimentally in these samples could be satisfactorily rationalized.     

First-principle study of electronic structure and stability of Sn0.5Sb0.5O2

A theoretical study on Sb-doped SnO₂ has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA) level. Stability and conductivity analyses were performed based on the formation energy and electronic structures. The results show that Sn_0.5Sb_0.5O₂ solid solution is stable because the formation energy of Sn_0.5Sb_0.5O₂ is −0.06 eV. The calculated energy band structure and density of states showed that the band gap of SnO₂ narrowed due to the presence of the Sb impurity energy levels in the bottom of the conduction band, namely there is Sb 5s distribution of electronic states from the Fermi level to the bottom of conduction band after the doping of antimony. The studies provide a theoretical basis to the development and application of Sn_1−xSb_xO₂ solid solution electrode.     

Electronic structure and thermodynamic properties of platinum–lead oxides PbPt2O4 and Pb2PtO4 by ab initio methods

We report the electronic structure and thermodynamic properties of platinum–lead oxides PbPt₂O₄ and Pb₂PtO₄. The band structure is calculated by the full-potential, full-relativistic local orbital minimum basis (FPLO) method in the local density approximation, including the generalized gradient approximation (GGA) and GGA+U, respectively. The values of gaps in Pb₂PtO₄ were estimated as 0.47, 1.02, and 0.52 eV in GGA, GGA+U, and the full-relativistic GGA, respectively. Pb₂PtO₄ is a semiconductor, and PbPt₂O₄ is a metallic conductor with the small values of the density of states at the Fermi level. The effect of chemical disorder in Pb_2?xPt_1+xO₄ alloys is studied by the relaxation of the volume, shape, and the positions of atoms in the unit cell, using the Vienna Ab initio Simulation Package (VASP) method. The thermodynamic properties are computed in the quasi-harmonic Debye–Grüneisen model. The dependence of the thermodynamic parameters on the pressure and temperature is also presented.     

Energy bands of LaAg and LaCd and martensitic transformation

Energy bands of the simple cubic intermetallic compounds LaAg and LaCd have been calculated by using both a non-relativistic APW and relativistic KKR method in the muffin-tin-potential approximation. The density of states of LaAg is found to peak sharply just above the Fermi energy and the Fermi surface geometry shows that in this high density-of-states region the conditions are met for the band-Jahn-Teller-effect to drive the martensitic transformation observed in LaAg_{1_x} In _x ,x >0.05, and in LaCd. The case of YZn, which shows no martensitic transformation, is discussed briefly.     

Al和N共掺p型Zn1-xMgxO电子结构的第一性原理计算

采用密度泛函理论下的第一性原理平面波超软赝势方法,对Zn_1-xMg_xO超晶胞和掺杂Al,N后的Zn_1-xMg_xO超晶胞分别进行了优化计算.结合广义梯度近似计算了Al和N共掺杂后Zn_1-xMg_xO的能带结构、电子态密度和Mulliken电荷布居分布.计算表明：掺入N原子的2p态电子为Zn_1-xMg_xO价带顶提供空穴载流子,使Zn_1-xMg_xO价带顶向高能方向移动;掺入Al原子的3p态电子则与N原子的2p态电子在费米能级附近发生轨道杂化,使费米能级处价带能级展宽,Al和N共掺杂可获得p型Zn_1-xMg_xO. 关键词： 密度泛函理论 1-xMg_xO')" href="#">Zn_1-xMg_xO 电子结构 共掺杂     

Observation of a degenerate Fermi gas confined by a standing electromagnetic wave

We have observed a gas of Fermi atoms confined in the antinodes of a standing electromagnetic wave. The standing wave is formed by two counter-propagating beams with the wavelength of 10.6 μm focused on the same spot. Each antinode confines a pancake-shaped cloud of 7500 lithium-6 atoms in two equally populated spin states at the temperature T = 0.1E _F, where E _F is the Fermi energy. The system is in the regime beyond the local density approximation: Only the 3 lowest energy states of the axial motion are populated. The system may become an instrument for the study of 2D Fermi physics and 3D effects beyond the local density approximation.     

Stress-dependent magnetoresistance in NbSe3

We have performed electrical resistance measurements on NbSe₃ in the lower charge density wave (CDW) state under the application of transverse magnetic fields and while the sample is under uniaxial stress. For zero stress, increasing the magnetic field results is an enhancement of the resistance anomaly consistent with previous measurements by Coleman et al. In zero magnetic field, increasing stress also enhances the resistance anomaly consistent with previous measurements by Lear et al. For combined stress and magnetic field, the resistance anomaly is not further enhanced, i.e. the effects are not additive. We examine the possibilities that magnetic fields and stress act on similar portions of the Fermi surface.     

Phase diagram of the relaxor (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 investigated by dielectric and Raman spectroscopies

Lead zinc niobate-lead titanate[(1−x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.     

The Fermi surface of indium and indium-based alloys

The calculation is made of uniaxial stresses applied along [0 0 1] and of hydrostatic pressure inducing topological transitions in indium. The Fermi surface of In_1?xMg_x alloy system is studied at various magnesium concentrations and the character of the change of electronic state density at the Fermi level is analysed.     

Band structure of YBa2Cu3Ox in relation with the oxygen vacancy distribution

The electronic band structure and the density of states of the YBa₂Cu₃O_x superconductor are computed for x = 7 using the extended Hückel scheme. Two ordered arrangements of the oxygen vacancies in the basal Cu-O planes have been considered. It is shown that the oxygen vacancy distribution plays a major role in the electronic structure of this compound and deeply influences the density of states at the Fermi level.     

Charge-density-wave partial gap opening in quasi-2D KMo6O17 purple bronze studied by angle resolved photoemission spectroscopy

Low dimensional (LD) metallic oxides have been a subject of continuous interest in the last two decades, mainly due to the electronic instabilities that they present at low temperatures. In particular, charge density waves (CDW) instabilities associated with a strong electron-phonon interaction have been found in Molybdenum metallic oxides such as KMo₆O₁₇ purple bronze. We report an angle resolved photoemission (ARPES) study from room temperature (RT) to T ∼40 K well below the Peierls transition temperature for this material, with CDW transition temperature T_CDW ∼120 K. We have focused on photoemission spectra along ΓM high symmetry direction as well as photoemission measurements were taken as a function of temperature at one representative k_F point in the Brillouin zone in order to look for the characteristic gap opening after the phase transition. We found out a pseudogap opening and a decrease in the density of states near the Fermi energy, E_F, consistent with the partial removal of the nested portions of the Fermi surface (FS) at temperature below the CDW transition. In order to elucidate possible Fermi liquid (FL) or non-Fermi liquid (NFL) behaviour we have compared the ARPES data with that one reported on quasi-1D K_0.3MoO₃ blue bronze.     

Electronic properties and superconductivity of amorphous Si1-xAux alloys near the metal-insulator transition

We report on specific-heat and resistivity measurements on quench-condensed Si_1-xAu_x films for 0.11 ⩽ x 0.36 in the temperature range 0.35 K ⩽ T ⩽ 6 K. A distinct increase of the specificheat derived electronic density of states at the Fermi level is observed at x_b ≈ 0.2, i.e., in the vicinity of the metal-insulator transition occurring for our samples at x_c = 0.16. This suggests a different type of bonding between Au and Si for x < x_b and x > x_b. While resistive transitions to superconductivity are observed for x⩾0.21, the absence of a specific-heat anomaly at the transition points to filamentary superconductivity except for × = 0.35 where a sizable anomaly is seen. The difference in various electronic properties between differently prepared samples of these metastable alloys, in particular the influence of different preparation and annealing temperatures is emphasized. It is suggested that these differences are caused by incipient phase separation in the room-temperature prepared samples.     

High field Mössbauer study of Zn-Rich Zn x Mg1−x Fe2O4 spinels

The ferrite spinels Zn_xMg_1?xFe₂O₄ (x≥0.8) have been studied at low temperature by high-field Mössbauer spectroscopy. When x increases, the magnetic structure, which is relevant of local canted states, evolves from a randomly canted ferrimagnetic structure towards a randomly canted antiferromagnetic order.     

The valence band structure of V−Mo solid solutions

The electronic structure of V _x Mo_1–x (x=0.2; 0.4; 0.6; 0.75) solid solutions was studied by XPS and UPS. The density of states at the Fermi energy,N(E _F), deduced from these measurements, shows a minimum as a function of the alloy concentration on the Mo rich side. This behaviour can be explained by band structure calculations and is in good agreement with previous NMR measurements. The relation between the electronic structure at the Fermi level and the superconducting properties is discussed. The band structure of the Mo rich alloys can be understood in terms of a rigid band model.     

Elastic and vibrational properties of Mg2Si1−xSnx alloy from first principles calculations

The structural, elastic and phonon properties of Mg₂Si_1?xSn_x alloy are investigated by performing density functional theory and density functional perturbation theory calculations. The calculated lattice parameter increases with the increase of Sn content obeying Vegard’s Law that is in good agreement with available experimental data. Shear modulus, Young’s modulus and sound velocities are determined from the obtained elastic constants. Phonon dispersion curves show a pronounced softening with increasing of Sn content. The softening mechanism has been discussed based upon the element mass and bond strength. Besides, phonon contribution to the Helmholtz free energy, the entropy and the constant-volume heat capacity are calculated within the harmonic approximation based on the calculated phonon density of states. Results show Mg₂Si_1?xSn_x is thermodynamically more stable with higher Sn content.     

The thermodynamic properties of lithium peroxide,Li2O2

Recently, Cota et al. gave an improved crystal structure of lithium peroxide. For the sake of investigation of the thermal properties for this structure of Li₂O₂, the Debye model is adopted in our work. This model combines with the ab initio calculations within local density approximation (LDA) using pseudopotentials and a planewave basis in the framework of density functional theory (DFT), and it takes into account the phononic effects within the quasi-harmonic approximation. We find that our calculated lattice constant using this model is in excellent agreement with the data from Cota et al. Based on the first principles study and the Debye model, the thermal properties including the equation of state, the Debye temperature, the heat capacity and the thermal expansion are obtained in the whole pressure range from 0 to 10 GPa and temperature range from 0 to 500 K.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

Local Pairs in High-Temperature Superconductors: Proof of Existence and Manifestation in Heat Transfer

The comparison of the temperature anomalies of the charge carrier density in high-temperature cuprate superconductor Bi_1.6(Pb_0.4)Sr₂Ca₂Cu₃O_{10 + x} and features of the parameters of the crystal lattice has demonstrated that the concept of pairing of single-particle hole excitations on the Fermi surface into singlet pairs is realistic. Anomalies of the thermal conductivity correlated with the presence of paired quasiparticle states have been revealed. Both types of anomalies indicate the existence of three stages of the temperature evolution of a pseudogap.     

Effects of vitrification suppression on structure morphology, conductivity and dielectric properties of vanadium phosphate glasses

Vitrification suppression in the (V₂O₅)_1−x (P₂O₅)_x glasses where x=0.10, 0.15, 0.20, and 0.25 was controlled by changing the rate of quenching glasses. The structure variations occurring in the glasses were detected by differential thermal analysis and optical microscope. The results implied the separation and growth of V₂O₅ orthorhombic microcrystal in the samples with x=0.10 and 0.15 whereas other samples did not illustrate remarkable changes in their microstructure. However, in temperature range between 300 and 473 K a semiconducting behavior for all samples appears during the study of electrical conductivity-temperature dependence. A decrease in conductivity values accompanied with some variations in activation energies by reducing quenching rate was observed. The conductivity results suggested that the conduction occurs by the phonon assisted hopping of a small polaron between V⁴⁺ and V⁵⁺ states at relatively higher temperature range above θ_D/2. Whereas at relatively low temperatures the conduction may occur by electron jumping between filled and empty states at Fermi level in the disordered matrix besides polaronic conduction. Reasonable values for the density of localized states, carrier concentration and carrier mobility were estimated and discussed. Also, dielectric constant and dielectric loss were studied as a function of frequency at different temperatures confirming the structure variations in the glass system.