Friday, March 28, 2008

Physical Review B – March 2008

Phys. Rev. B 77, 092103 (2008)
Ab initio investigation of intermolecular interactions in solid benzene
O. Bludský, M. Rubeš and P. Soldán

A computational strategy for the evaluation of the crystal lattice constants and cohesive energy of the weakly bound molecular solids is proposed. The strategy is based on the high level ab initio coupled-cluster determination of the pairwise additive contribution to the interaction energy. The zero-point-energy correction and nonadditive contributions to the interaction energy are treated using density functional methods. The experimental crystal lattice constants of the solid benzene are reproduced, and the value of 480 meV/molecule is calculated for its cohesive energy.

Phys. Rev. B 77, 094124 (2008)
Nuclear quadrupole interaction at 44Sc in the anatase and rutile modifications of TiO2: Time-differential perturbed-angular-correlation measurements and ab initio calculations
Seung-baek Ryu, Satyendra K. Das, Tilman Butz, Werner Schmitz, Christian Spiel, Peter Blaha, and Karlheinz Schwarz

The nuclear quadrupole interaction of the first excited I=1 state of 44Sc in the anatase and rutile modifications of bulk TiO2 was determined by time-differential perturbed-angular correlation of gamma rays. New LaBr3(Ce) scintillators with excellent energy resolution allowed us to separate the 68 and 78 keV lines of the gamma-gamma cascade. For anatase, an almost axially symmetric electric field gradient (EFG) with an asymmetry parameter of eta=0.10(1) and a nuclear quadrupole frequency of omega-Q=9.29(3) Mrad/s was obtained. For rutile, an almost antiaxial EFG with eta=0.94(1) and omega-Q=16.14(6) Mrad/s was obtained, which contradicts literature values. Ab initio calculations of the EFG using the WIEN2K code were performed for anatase and rutile for pure systems as well as for relevant impurities, such as Sc and Cd on Ti sites. The nuclear quadrupole moment of the I=1 state in 44Sc is redetermined with a sevenfold improved accuracy to be Q=±0.214(3) b.

Phys. Rev. B 77, 104301 (2008)
6Li and 7Li NMR line-shape and stimulated-echo studies of lithium ionic hopping in LiPO3 glass
Sandra Faske, Hellmut Eckert, and Michael Vogel

6Li and 7Li NMRs are used to investigate the lithium ion dynamics in LiPO3 glass. In particular, 6Li NMR stimulated-echo experiments are used to provide straightforward access to two-time correlation functions characterizing the lithium ionic hopping motion in the millisecond regime in a glassy ion conductor. Temperature-dependent measurements serve to separate the spin diffusion contribution and the dynamic contribution to the stimulated-echo decays. The 6Li NMR correlation functions of LiPO3 glass describing the lithium ionic motion show pronounced nonexponential decays, which can be well described by a stretched exponential function with a temperature-independent small stretching parameter beta=0.27, indicating the complex nature of the lithium dynamics. The temperature dependence of the mean correlation times tau resulting from these stimulated-echo experiments is described by an activation energy Ea=0.66 eV. The values of tau are in good agreement with time constants from previous electrical and mechanical relaxation studies. At appropriate temperatures, the 6Li and 7Li NMR spectra are superpositions of a broad and a narrow spectral component, which result from slow and fast lithium ions, respectively, on the NMR time scale. A detailed analysis of the temperature dependence of these line shapes provides information about the distribution of correlation times.

Phys. Rev. B 77, 104433 (2008)
Origin of the magnetic-field dependence of the nuclear spin-lattice relaxation in iron
G. Seewald, E. Zech, H.-J. Körner, D. Borgmann, and M. Dietrich

The magnetic-field dependence of the nuclear spin-lattice relaxation at Ir impurities in Fe was measured for fields between 0 and 2 T parallel to the [100] direction. The reliability of the applied technique of nuclear magnetic resonance on oriented nuclei was demonstrated by measurements at different radio-frequency (rf) field strengths. The interpretation of the relaxation curves, which used transition rates to describe the excitation of the nuclear spins by a frequency-modulated rf field, was confirmed by model calculations. The magnetic-field dependence of the so-called enhancement factor for rf fields, which is closely related to the magnetic-field dependence of the spin-lattice relaxation, was also measured. For several magnetic-field-dependent relaxation mechanisms, the form and the magnitude of the field dependence were derived. Only the relaxation via eddy-current damping and Gilbert damping could explain the observed field dependence. Using reasonable values of the damping parameters, the field dependence could perfectly be described. This relaxation mechanism is, therefore, identified as the origin of the magnetic-field dependence of the spin-lattice relaxation in Fe. The detailed theory, as well as an approximate expression, is derived, and the dependences on the wave vector, the resonance frequency, the conductivity, the temperature, and the surface conditions are discussed. The theory is related to previous attempts to understand the field dependence of the relaxation, and it is used to reinterpret previous relaxation experiments in Fe. Moreover, it is predicted that the field dependences of the relaxation in Fe and Co, on one side, and in Ni, on the other side, differ substantially, and it is suggested that the literature values of the high-field limits of the relaxation constants in Fe are slightly too large.

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