The behavior of f-levels in hcp and bcc rare-earth elements in the ground state and XPS and BIS spectroscopy from density-functional theory.

The electronic structures of rare-earth elements in the hexagonal close-packed structure and Europium in the body-centered cubic structure are calculated using density-functional theory (DFT). X-ray photoemission spectroscopy (XPS) and bremsstrahlung isochromatic spectroscopy (BIS) simulations are made within DFT by implying that the f-electrons are excited by a large photon energy, either by removal from the occupied states in XPS or by addition to the unoccupied f-states in BIS. The results show sizable differences in the apparent position of the f-states compared to the f-band energy of the ground states. This result is fundamentally different from calculations assuming strong on-site correlation, since all the calculations are based on DFT. The spin-orbit coupling and multiplet splittings are not included, and the present simulation accounts for almost half of the difference between the f-level positions in the DFT ground states and the observed f-level positions. The electronic specific-heat at low T is compatible with the DFT ground state, where f-electrons often reside at the Fermi level.

Q-dependence of spin excitations in high-T(C) cuprates from spin-phonon coupling.

An understanding of spin excitations in cuprates is essential since the mechanism of high-T(C) superconductivity might be linked to spin fluctuations. Band calculations for 'one-dimensional' unit cells of La(2)CuO(4) show larger coupling (spin-phonon coupling, SPC) between anti-ferromagnetic spin waves and O-phonons than for Cu- or La-phonons. When this result is applied to a two-dimensional, free-electron like band, it leads to an 'hourglass' shape of the spin excitation spectrum, as in recent experiments. Isotope shifts and doping dependences of the excitations are discussed.

Importance of local band effects for ferromagnetism in hole-doped La2CuO4 cuprate superconductors.

Band calculations for supercells of La((2-x))Ba(x)CuO(4) show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism can appear locally around clusters of high Ba concentration. The clustering is important at large dilution, and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give support to the idea that weak ferromagnetism can be the cause of the destruction of superconductivity at high hole doping.

Metallic state in cubic FeGe beyond its quantum phase transition.

We report on results of electrical resistivity and structural investigations on the cubic modification of FeGe under high pressure. The long-wavelength helical order (T(C) = 280 K) is suppressed at a critical pressure p(c) approximately 19 GPa. An anomaly at T(X)(p) and strong deviations from a Fermi-liquid behavior in a wide pressure range above p(c) suggest that the suppression of T(C) disagrees with the standard notion of a quantum critical phase transition. The metallic ground state persisting at high pressure can be described by band-structure calculations if zero-point motion is included. The shortest FeGe interatomic distance display discontinuous changes in the pressure dependence close to the T(C)(p) phase line.

Direct observation of the multisheet Fermi surface in the strongly correlated transition metal compound ZrZn2.

The existence of flat areas of a Fermi surface (FS), predicted by electronic structure calculations and used in models of both magnetically mediated and phonon-mediated Fulde-Ferrell-Larkin-Ovchinnikov superconducting states, is reported in the paramagnetic phase of the ferromagnetic superconductor ZrZn2 using positron annihilation. The strongly mass-renormalized FS sheet, dominating the Fermi level density of states, is seen for the first time. The delocalization of the magnetization is studied using measured and calculated magnetic Compton profiles.

Longitudinal spin fluctuations and superconductivity in ferromagnetic ZrZn2 from Ab initio calculations.

The recent discovery of superconductivity coexisting with weak itinerant ferromagnetism in the d-electron intermetallic compound ZrZn2 strongly suggests spin-fluctuation mediated superconductivity. Ab initio electronic structure calculations of the Fermi surface and generalized susceptibilities are performed to investigate the viability of longitudinal spin-fluctuation-induced spin-triplet superconductivity in the ferromagnetic state. The critical temperature is estimated to be of the order of 1 K. Additionally, it is shown that in spite of a strong electron-phonon coupling ( lambda(ph) = 0.7), conventional s-wave superconductivity is inhibited by the presence of strong spin fluctuations.

Ferromagnetism below the stoner limit in La-doped SrB6

Spin-polarized band calculations for LaSr7B48 show a weak ferromagnetic state. This is despite a low density of states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin splitting is imposed. A DOS with an impuritylike La band is essential for this effect. It makes a correction to the Stoner factor and provides an explanation of the recently observed weak ferromagnetism in doped hexaborides.