Ken Ichiro Gondaira

The spin-polarised electronic band structure of chromium spinels. II. CuCr2Se4 and CuCr2Te4

For pt.I see ibid., vol.15, p.2613 (1982). The electronic band structures of ferromagnetic chromium spinels CuCr2Se4 and CuCr2Te4 are calculated self-consistently by using the discrete variational X alpha method. The main features of the band structure are almost common among the three compounds CuCr2X4 for X=S, Se and Te: relatively narrow valence bands constructed mainly from the anion p orbitals lie in a lower energy region. Very narrow bands having mainly d character range over the intermediate energy region; a few d bands cross the Fermi level for both spin bands of the selenide and telluride while d bands with the majority spin only cross the Fermi level for the sulphide. Wide conduction bands composed of Cu and Cr 4s, 4p orbitals lie in the higher energy region. The valency of each ion obtained from the self-consistent calculation are Cu1.25+(Cr1.41+)2(Se1.02-)4 and Cu0.85+(Cr1.03+)2(Te0.73-)4, which are compared with Cu1.30+(Cr1.55+)2(S1.11-)4. The atomic spin-polarisations of Cr, Cu and X are +3.54, -0.26 and -0.36 respectively for CuCr2Se4, and +3.74, -0.19 and -0.48 for CuCr2Te4. (They were +3.20, -0.32 and -0.27 for CuCr2S4 by way of comparison.).

Superconducting properties of (Y1−xPrx)Ba2Cu4O8 compounds

Abstract (Y 1- x Pr x )Ba 2 Cu 4 O 8 compounds with x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were synthesized using a high-pressure oxygen technique. The solubility limit x s in this compound system was found to be 0.7. In this composition range, (Y 1- x Pr x )Ba 2 Cu 4 O 8 compounds have the YBa 2 Cu 4 O 8 (124) structure and the lattice parameters a , b and c increase with increasing Pr content x . The superconducting transition temperature T c decreases monotonically with increasing x , and the critical content x c , where superconductivity disappears, has been estimated to be 0.8. There is an obvious effect of Pr substitution on superconductivity, although the effect is smaller in the 124 system than that in the 123 system. X-ray diffraction results suggest that Pr is almost 3+ in this system. A model for the Pr substitution is suggested to make holes immobile to suppress superconductivity as the result of the hybridization of O 2p π with Pr 4f orbitals.

Effect of Pr substitution on superconductivity in (PrxY1−x)Ba2Cu4O8

Abstract The compounds of (Pr x Y 1−x )Ba 2 Cu 4 O 8 (x=0, 0.1 and 0.6) were prepared in the oxygen atmosphere of 33 atm using a electrical furnace, where the materials were annealed at 930°C for 48 h. Magnetic and electrical measurements were performed: Transition temperatures for compounds of x=0, 0.1 and 0.6 were 80 K, 75 K and 44 K, respectively. The critical content X c , at which superconductivity disappears, was estimated to be 0.9. Effective moment and the Pauli paramagnetic component of Pr 0.6 Y 0.4 Ba 2 Cu 4 O 8 were 3.05±0.15 and (1.3±0.2) x 10 -6 cm 3 /g, respectively.

Electronic structure and spin polarisation of the transition metal thin film V(100)

The electronic structure and spin-polarisation of five-layer vanadium (100) thin films have been calculated using the self-consistent-charge spin-polarised discrete variational X alpha method. The calculations were made for the three thin-film spacings between the surface and the inner adjacent layers at the bulk value, at a 10% contracted value and at a 10% expanded value. The bandstructures the layer density of states and the charge and spin densities are presented. It is shown that the spin polarisation appears on the surface layer and that the magnetic moment is more than 0.2 mu B for each atom on the surface. The surface relaxation affects the distribution of the spin polarisation on the inner layers as well as on the surface layer.

The spin polarised electronic band structure of chromium spinels: I. CuCr2S4

The electronic band structure of a ferromagnetic chromium spinel CuCr2S4 is calculated self-consistently by using the discrete variational X alpha method. The main features of the band structure are: relatively narrow valence bands constructed mainly from the S 3p orbitals lie in the lower energy region; very narrow bands having mainly d character range over the intermediate energy region and a few d bands with only majority spin cross the Fermi level, which corresponds to the p-type metallic conduction observed; true conduction bands composed of Cu and Cr 4s, 4p orbitals lie in the higher energy region. The valency of each ion obtained from the self-consistent calculation is Cu1.3+(Cr1.55+)2(S1.11-)4 and the atomic spin-polarisation is 3.2, -0.32 and -0.27 for Cr, Cu and S, respectively.

Electronic band structures of semiconducting ferromagnetic spinels CdCr2S4 and CdCr2Se4

The electronic band structures of the paramagnetic and ferromagnetic phases of chromium spinels CdCr2S4 and CdCr2Se4 are calculated by the use of the extended Huckel method and assignments of the optical spectra are given.

Electronic States of Fe in I-III-VI_2 Compounds

The electronic states of a tetrahedral cluster of a Fe ion and four S ions are calculated in order to obtain the electronic structure around a Fe ion in I-III-VI 2 compounds. The LCAO-MO scheme is used with account of configuration interactions (CI). The ground state is found to have 6 A 1 symmetry and two 6 T 2 states, to which allowed transitions are expected, are obtained in an energy region below the fundamental absorption edge, in agreement with experiments. It is remarked that the ground 6 A 1 state arises by no means from the pure atomic d 5 configuration: mixing of charge transferred configurations is quite large. The energies of quartet states ( 4 T 1 , 4 T 2 ) are lower than those of the corresponding sextet states ( 6 T 1 , 6 T 2 ) in harmony with the observed optical spectra. Account of CI is essential to the findings.

Electronic Band Structures of I-III-VI2 Compounds

Electronic band structures of CuGaS2 and CuAlS2 are selfconsistently calculated by using the discrete variational Xα method and the results are compared with the optical data. The contents of the valence and core bands are reasonably coincident with the XPS mesurements. The top of the valence bands is an admixture state of Cu3d and S3 p orbitals. The crystal-field splitting of the uppermost valence band is about 0.2 eV. The fundamental energy gaps are direct transitions from \varGamma4 to \varGamma1.

Electronic Band Structure of Magnetic Semiconductors with Spinel Structure

The electronic band structures of ferromagnetic semiconductors CdCr2S4, CdCr2Se4, and HgCr2Se4 are calculated self-consistently by the DV-Xα method. The general features of the band structures are quite similar for three substances except for the relative positions of the 3d bands: Each structure consists of relatively narrow valence bands, fairly wide conduction bands, and very narrow d bands. The de and dγ bands for up-spin lie in the energy region near the top of the valence bands and around the bottom of the conduction bands, respectively, and both d bands for down-spin fall in the conduction bands. The fundamental energy gap at the Γ point is 2.6, 2.3 and 1.8 eV for CdCr2S4, CdCr2Se4, and HgCr2Se4, respectively. The spin polarization of the Cr-3d orbitals is about 3.5, while the spin polarization of the valence p orbitals of a chalcogen ion has the opposite sign.

The discontinuous volume change in transition-metal compounds due to the pressure-induced spin-state transition

Several transition-metal compounds change their volume abruptly under high pressure through their electronic spin-state transition. The pressure-induced spin-state transitions in transition-metal compounds are studied on the basis of the ligand-field theory by using the model in which the coupling of transition-metal ions with crystal distortions is taken into account. The various types of transitions occur due to the crystal distortion induced by pressure, that is, the continuous and discontinuous high-spin (HS) to low-spin (LS) transitions with increasing pressure in the weak-crystal-field compounds, the continuous and discontinuous LS to HS transitions and the multiple LS to HS to LS to HS transition in the strong-crystal-field compounds. The pressure against volume curve shows various features of changes due to the crystal distortion associated with the spin-state transitions. The observed pressure against volume curve of haematite Fe2O3 is well explained by the discontinuous HS to LS transition of Fe3+ ions.