[en] Quasiparticle band structures for the cubic trihydrides YH3 and LaH3 have been calculated by evaluating the self-energy in the GW approximation using ab initio pseudopotentials and plane waves. These are the protype metal hydrides that exhibit switchable optical properties. For both materials, the local-density approximation (LDA) yields semimetallic energy bands with a direct overlap of about 1 eV. We find the self-energy correction to the LDA energies opens a gap at Gamma of 0.8-0.9 eV for LaH3 and 0.2-0.3 eV for YH3, where the latter is in sharp contrast to a previous study using linear-muffin-tin orbitals. The quasiparticle band gaps are analyzed as a function of an initial shift in the LDA bands used to evaluate the random-phase approximation screening in constructing the self-energy. We also make a comparison of results obtained by using two different pseudopotentials, each designed to better approximate exchange and correlation between the semicore states and valence states of Y and La

[en] We have probed the nature of magnetism at the surface of (001), (110) and (111)-oriented La_0.7Sr_0.3MnO₃ thin films. The spin polarization of La_0.7Sr_0.3MnO₃ thin films is not intrinsically suppressed at all surfaces and interfaces but is highly sensitive to both the epitaxial strain state as well as the substrate orientation. Through the use of soft x-ray spectroscopy, the magnetic properties of (001), (110) and (111)-oriented La_0.7Sr_0.3MnO₃/SrTiO₃ interfaces have been investigated and compared to bulk magnetometry and resistivity measurements. The magnetization of (110) and (111)-oriented La_0.7Sr_0.3MnO₃/SrTiO₃ interfaces are more bulk-like as a function of thickness whereas the magnetization at the (001)-oriented La_0.7Sr_0.3MnO₃/SrTiO₃ interface is suppressed significantly below a layer thickness of 20 nm. Such findings are correlated with the biaxial strain state of the La_0.7Sr_0.3MnO₃ films; for a given film thickness it is the tetragonal distortion of (001) La_0.7Sr_0.3MnO₃ that severely impacts the magnetization, whereas the trigonal distortion for (111)-oriented films and monoclinic distortion for (110)-oriented films have less of an impact. These observations provide evidence that surface magnetization and thus spin polarization depends strongly on the crystal surface orientation as well as epitaxial strain

[en] Synchrotron infrared measurements were conducted on a self-doped LaxMnO3-? (x?0.8) film. From these measurements we determined the conductivity and the temperature dependence of the effective number of carriers. While the metal-insulator transition temperature (TMI) and the magnetic ordering temperature (TC) approximately coincide, the onset of the change in the free carrier density occurs at a significantly lower temperature (?45 K below). This suggests that local distortions exist below TMI and TC which trap the eg conduction electrons. These regions with local distortions constitute an insulating phase which persists for temperatures significantly below TMI and TC.

[en] The intermediate valence compounds Yb₂M₃Ga₉ (M = Rh, Ir) exhibit an anisotropic magnetic susceptibility. We report measurements of the temperature dependence of the 4f occupation number, n_f(T), for Yb₂M₃Ga₉ as well as the magnetic inelastic neutron scattering spectrum S_mag(ΔE) at 12 and 300 K for Yb₂Rh₃Ga₉. Both n_f(T) and S_mag(ΔE) were calculated for the Anderson impurity model with crystal field terms within an approach based on the non-crossing approximation. These results corroborate the importance of crystal field effects in these materials; they also suggest that Anderson lattice effects are important to the physics of Yb₂M₃Ga₉

[en] Ab initio pseudopotential total-energy calculations on infinite monatomic chains of Au, Al, Ag, Pd, Rh, and Ru were performed within the local-density approximation. We used the frozen phonon approximation to study the stability of these chains as a function of strain. Within a window of strains the au, Al, Ag, Pd, and Rh linear chains are stable with respect to q=pi/a deformations. For large strains all the chains dimerize. All the chains exhibit at least one zero-strain zigzag stable equilibrium configuration, and Au, Al, and Rh zigzag chains exhibit two. The ideal strengths of the different chains were calculated. The stability of the chains is discussed in connection with the electronic structure

[en] We have studied the spin relaxation in diluted spin ice Ho_2-x Y_x Ti₂O₇ by means of neutron spin echo spectroscopy. Remarkably, the geometrical frustration is not relieved by doping with non-magnetic Y, and the dynamics of the freezing is unaltered in the spin echo time window up to x ≅ 1.6. At higher doping with non-magnetic Y (x (ge) 1.6) a new relaxation process at relatively high temperature (up to at least T ≅ 55 K) appears which is more than 10 times faster than the thermally activated main relaxation process. We find evidence that over the whole range of composition all Ho spins participate in the dynamics. These results are compared to a.c. susceptibility measurements of the diluted Ho and Dy spin ice systems. X-ray absorption fine structure (EXAFS) spectra and x-ray diffraction show that the samples are structurally well ordered

[en] We report high pressure dynamic compression experiments of liquid water along a quasi-adiabatic path leading to the formation of ice VII. We observe dynamic features resembling Van der Waals loops and find that liquid water is compacted to a metastable state close to the ice density before the onset of crystallization. By analyzing the characteristic kinetic time scale involved we estimate the nucleation barrier and conclude that liquid water has been compressed to a high pressure state close to its thermodynamic stability limit

No abstract available

[en] We demonstrate a sensitivity to strain of the Mn 3d valence states in the ferromagnetic semiconductors (Ga,Mn)As and (Al,Ga,Mn)As, using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn L_2,3 XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain gives an opposite sign of the uniaxial anisotropy of the Mn L₃ pre-peak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the L₃ pre-peak intensity increases, indicating an increasing 3d character of the hybridized holes

[en] We present measurements of the resistivity and the upper critical field H_c2 of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is comparable to ∼100 T of high T_c cuprates. H_c2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H_c2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4--5 decades. The activation energy has very different field dependencies for H(parallel)ab and H(perpendicular)ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of H_c2, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on H_c2(T), which may significantly reduce H_c2(0) as compared to H_c2(0) ∼200--300 T based on extrapolations of H_c2(T) near T_c down to low temperatures.