Superconductivity and electronic specific heat in V-Mo system

The parameters of superconductivity and specific heat are determined from the low temperature specific heat measurements of V_(1−x)Mo_x (0 ⩽ x ⩽ 1) solid solutions. The coefficient of electronic specific heat γ decreases with increasing Mo concentration and shows a minimum around 80 at. %Mo. Debye temperature θ_D varies slightly over the whole composition range. The superconducting transition temperature T_c also decreases with increasing Mo concentration. The variation of T_c is explained by the variation of γ and discussed in terms of the band structure.     

Metamagnetic transitions of Lu2Cu2O5 studied by submillimeter-wave ESR measurement

Lu₂Cu₂O₅ is a related substance of Y₂Cu₂O₅ which is an unusual copper oxide with two successive metamagnetic transitions. We have performed the antiferromagnetic resonance (AFMR) measurements of Lu₂Cu₂O₅ in the millimeter and submillimeter wave region with the pulsed magnetic field up to 16 T at 1.8 K. Our measurements this time are very detailed frequency dependence measurements compared to the previous measurement and we found new AFMR modes which were not observed previously. From the analysis of the observed AFMR modes, we found that the magnetic phase aboveH _cl = 3.1 T in Lu₂Cu₂O₅ is different from that in Y₂Cu₂O₅.     

Self-consistent calculation of the band structure of copper by the LMTO method

Conclusion In our paper we have presented an effective self-consistent procedure which has been applied to the calculation of the electronic structure of copper. The results attest to the high stability of this procedure. The method of previously attaining atomic self-consistency makes possible a substantial reduction of the volume of band-structure calculations and, therefore, of the total time of the SC procedure. It has thus become possible to carry out calculations with good accuracy for a number of physical properties and the binding energy of metals on medium-sized computers. It must be pointed out that the self-consistent calculation of the band structure with prior attainment of atomic self-consistency is also possible in other methods of constructing the band structure.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 103–107, July, 1983.The authors are grateful to Yu. Kh. Velikov and E. G. Maksimov for their attention and advice.     

Electronic structure of YMn2O5 studied by EELS and first-principles calculations

The electronic structure of multiferroic YMn₂O₅ material has been studied by use of the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U, we performed the GGA+U calculations for 0 < U ? 8 eV, and it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn-O hybridization. The experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O-K edge and Mn-L edges. A theoretical and experimental analysis on the O-K edge suggests that the on-site Coulomb interaction (U) in YMn₂ O₅ could be less than 4 eV. Certain electronic structural features of LaMn₂O₅ have been discussed in comparison with those of YMn₂O₅.     

High-temperature specific heat of Bi2GeO5 and SmBiGeO5 compounds

The SmBiGeO₅ compound is synthesized from Sm₂O₃, Bi₂O₃, and GeO₂ by solid-state synthesis with subsequent annealing at 1003, 1073, 1123, 1143, 1173, and 1223 K. The metastable Bi₂GeO₅ compound is prepared from melt. Temperature dependences of specific heat of Bi₂GeO₅ (350–1000 K) and SmBiGeO₅ (370–1000 K) are measured by differential scanning calorimetry. Basing on the experimental dependences C _P = f(T), the thermodynamic functions of the oxide compounds are calculated.

     

First-principles energy band calculation for undoped and S-doped TiO2 with anatase structure

The electronic structure of S-doped TiO₂ with an optimized anatase structure was calculated within the framework of the density functional theory (DFT). For the calculation we built four kinds of supercells; type-A and B supercells consist of 12 and 48 atoms and a centered Ti atom is substituted for an S atom, while type-C and D supercells consist of 12 and 48 atoms and a centered O atom is substituted for an S atom. The supercells (type-B and D) were employed to adjust the S-concentration in TiO₂ to an experimental value of a few %. The changes of the lattice parameters are not significant in the type-A and B supercells. The phase transition from the tetragonal to the orthorhombic occurs in the type-C and D supercells. In the small supercell (type-A), S-related states are located in the range of −1.6 to 0 eV, and the S-states are band-like. In contrast, in the large supercell (type-B), S-related states appeared at about 0.9 eV above the top of the valence band, and the S-states are atomic-like. The localization of the S-related states is remarkable in the type-B supercell. In the type-D supercell, the S-related states were merged with the top of the valence band, and as a result the band-gap energy is narrowed by 0.7 eV. Despite a low S-concentration (3%) in the type-D supercell, the S-related states are somewhat band-like.     

The electronic structure of polymeric complex Co(thiazole)2Cl2 studied by first-principle calculation

One-dimensional dihalide-bridged polymer Co(thiazole)₂Cl₂ has been studied with the self-consistent full-potential linearized augmented plane wave method (FP_LAPW) based on the density functional theory (DFT). Spin distributions in ferromagnetic and antiferromagnetic states of it have been obtained by the calculation. The electronic structure and magnetic coupling between tow cobalt (II) ions along chain are discussed.     

Superconductivity in LaPd_2Bi_2 with CaBe_2Ge_2-type structure

正Since the discovery of superconductivity in LaFeAsO_(1-x)F_x,the high-T_c iron-based superconductors have been extensively studied from both experimental and theoretical viewpoints [1-8]. However, the mechanism of the unconventional superconductivity is still to be resolved. To     

Field-induced magnetic ordering in the S = 1 singlet ground-state system Ni(C5H5NO)6(ClO4)2 studied by specific heat

Specific-heat data on Ni(C₅H₅NO)₆(ClO₄)₂ are presented for fields 0 ⩽ H ⩽ 60 kOe and temperatures 0.1 ⩽ T ⩽ 1.3 K. In this material the S = 1 ground triplet is split by a trigonal crystal field term into a singlet and a higher lying doublet, such that ordering cannot occur in zero field. The field-induced magnetic phase transitions are studied both experimentally and theoretically, using molecular field theory, high-temperature series expansions and spin-wave theory. We obtain the values J/k = -(0.20 ± 0.01)K and D/k = 6.6 ± 0.2 K for the antiferromagnetic (subcritical) exchange constant and for the crystal-field splitting parameter, respectively.     

Electronic band structure and Fermi surface of CaB6 studied by angle-resolved photoemission spectroscopy

We report high-resolution angle-resolved photoemission spectroscopy (ARPES) on CaB6. The band structure determined by ARPES shows a 1 eV energy gap at the X point between the valence and the conduction bands. We found a small electron pocket at the X point, whose carrier number is estimated to be (4-5) x 10(19) cm(-3), in good agreement with the Hall resistivity measurement with the same crystal. The experimental results are discussed in comparison with band structure calculations and theoretical models for the high-temperature ferromagnetism.     

Energy band structure and thermo-mechanical properties of tungsten and tungsten carbides as studied by the LMTO-ASA method

The calculations of the energy band structures of W (b.c.c. lattice), WC (f.c.c. and h.c.p. lattices) and W₂C have been performed by the LMTO-ASA method. The results of the band structure calculations have been used for the evaluations of the unit cell parameters, bulk moduli, Debye temperatures. The values obtained are in good agreement with the experimental data. It is shown that h.c.p. WC has the highest bulk modulus, while W₂C possess the highest density-of-states at the Fermi level. The calculations reveal a noticeable charge transfer from tungsten to carbon in WC, while the effective charges in W₂C are rather small.     

Complex quasiparticle band structure induced by electron-phonon interaction: band splitting in the 1x1H/W110 surface

We show that the self-consistent solution of the complex Dyson equation for the electron-phonon (EP) problem introduces many body effects which are often observed in photoemission experiments. The formalism is applied to the H covered W(110) surface, using first-principles results for the electronic and vibrational structure. We demonstrate that the measured spin-polarized surface band splitting [Phys. Rev. Lett. 84, 2925 (2000)10.1103/PhysRevLett.84.2925; 89, 216802 (2002)] can be traced back to different quasiparticle (QP) states induced by EP coupling. Despite the breakdown of the single QP picture, the spectral functions are very well represented by the predicted multiple QP structure.     

Mechanical stress effect on the crystallization behavior of Ge2Sb2Te5 films studied by electrical resistance measurement

The mechanical stress effect on the crystallization behaviour of Ge₂Sb₂Te₅(GST) thin films is carefully investigated by electrical resistance measurements. It is found that the crystallization temperature of GST films increases as external compressive stress is applied, while the crystallization temperature decreases under tensile stress. We also find that the uneven distribution of extrinsic stress can widen the span of transition temperature. These results clearly demonstrate that mechanical stress plays an important role during the crystallization process of GST films and may further influence the reliability and storage speed of relevant devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cd-Impurities in the semiconductor in2Te5 studied by the PAC-method

The temperature dependence of the electric field gradient at a Cd acceptor atom in the semiconductor In₂Te₅ was measured with the PAC-method. The results are explained in a bond-switching model.     

Direct measurement of strain-induced changes in the band structure of carbon nanotubes

The effect of uniaxial strain on the optical transition energies of single-walled carbon nanotubes with known chiral indices was measured by Rayleigh scattering spectroscopy. Existing theory accurately predicts the trends in the measured strain-induced shifts, but overestimates their magnitude. Modification of the analysis to account for internal sublattice relaxation results in quantitative agreement with experiment.     

Superconductivity caused by the pairing of plutonium 5f Eelectrons in PuCoGa5

On the basis of electronic structure calculations we identify the superconductivity in the novel, high-temperature superconductor PuCoGa5 to be caused by the pairing of Pu 5f electrons. Assuming delocalized Pu 5f states, we compute theoretical crystallographic constants very near to the experimental ones, and the calculated specific heat coefficient compares reasonably to the measured coefficient. The theoretical Fermi surface is quasi-two-dimensional and the material appears to be close to a magnetic phase instability.     

Charge order in NaV2O5 studied by EPR

We present angular dependent EPR measurements in NaV2O5 at X-band frequencies in the temperature range 4.2/=100 K, is followed by zigzag charge-order fluctuations which become long range and static below T(SP) = 34 K.     

Crystal-field excitations in the heavy-fermion alloys CeCu6−x Au x studied by specific heat and inelastic neutron scattering

The specific heat of CeCu_6–x Au _x withx=0,0.3, and 0.9, and of the corresponding La homologues has been measured between 1.5 K and 150 K. With increasingx we find progressively better-defined Schottky anomalies arising from the crystal-field splitting, which is attributed to the decrease of the Kondo temperature. Both observations are in line with the results of inelastic neutron-scattering data performed on samples withx=0.5. Overall, the crystal-field splitting does not change very much upon alloying with Au. A detailed analysis shows that forx=0.5 the quasielastic line width is smaller and the inelastic crystal-field transitions are better resolved compared to CeCu₆.     

Ab initio electronic band structure calculation of InP in the wurtzite phase

We present ab initio calculations of the InP band structure in the wurtzite phase and compare it with that of the zincblende phase. In both calculations, we use the full potential linearized augmented plane wave method as implemented in the WIEN2k code and the modified Becke-Johnson exchange potential, which provides an improved value of the bandgap. The structural optimization of the wurtizte InP gives , , and an internal parameter u=0.371, showing the existence of a spontaneous polarization along the growth axis. As compared to the ideal wurtzite structure (that with the lattice parameter derived from the zincblende structure calculations), the actual wurtzite structure is compressed (−1.3%) in plane and expanded (0.7%) along the c-direction. The value of the calculated band gaps agrees well with recent optical experiments. The calculations are also consistent with the optical transitions found using polarized light.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.