Monday, August 18, 2008

Physical Review B - Up to vol 78 no 4

Multinuclear NMR studies of mixed Ca1−xSrxF2 crystals
R. E. Youngman and C. M. Smith
The local environments of fluorine and calcium ions in Ca1−xSrxF2 single crystals have been studied using 19F and 43Ca magic-angle spinning (MAS) NMR spectroscopies. The fluorine speciation is found to depend on both the distribution of cations (Ca and Sr) around each fluorine site in the fluorite lattice, as well as fluctuations in the ionic character of bonding between fluorine and these cations. The natural abundance 43Ca MAS NMR results show a clear relation between 43Ca chemical shielding and bond distances in halides, with increasing shielding as the fluorite lattice expands and Ca-F distances lengthen. When taken together, the results from 19F and 43Ca NMR are consistent with a random substitution of Sr for Ca in the mixed fluoride system.

Similar glassy features in the 139La NMR response of pure and disordered La1.88Sr0.12CuO4
V. F. Mitrović, M.-H. Julien, C. de Vaulx, M. Horvatić, C. Berthier, T. Suzuki, and K. Yamada
High Tc superconductivity in La2−xSrxCuO4 coexists with (striped and glassy) magnetic order. Here, we report NMR measurements of the 139La spin-lattice relaxation, which displays a stretched-exponential time dependence, in both pure and disordered x=0.12 single crystals. An analysis in terms of a distribution of relaxation rates 139T indicates that: (i) the spin-freezing temperature is spatially inhomogeneous with an onset at T=20 K for the pristine samples and (ii) the width of the T distribution in the vicinity of T is insensitive to an ~1% level of atomic disorder in CuO2 planes. This suggests that the stretched-exponential 139La relaxation, considered as a manifestation of the system's glassiness, may not arise from quenched disorder.

77Se NMR investigation of the field-induced spin-density-wave transitions in (TMTSF)2ClO4
L. L. Lumata, J. S. Brooks, P. L. Kuhns, A. P. Reyes, S. E. Brown, H. B. Cui, and R. C. Haddon
Complementary 77Se nuclear magnetic resonance (NMR) and electrical transport have been used to correlate the spin-density dynamics with the subphases of the field-induced spin-density wave (FISDW) ground state in (TMTSF)2ClO4. We have found that the peaks in the spin-lattice relaxation rate 1/T1 appear within the metal-FISDW phase boundary and/or at first-order subphase transitions. In the quantum limit above 25 T, the NMR data have given an insight into the FISDW electronic structure.

Comparative NMR investigation of the Re-based borides
C. S. Lue, Y. F. Tao, and T. H. Su
We report a systematic study of the rhenium-based borides, ReB2, Re7B3, and Re3B, by means of the 11B nuclear magnetic resonance (NMR) spectroscopy. While Re7B3 and Re3B are superconductors, ReB2 exhibits no superconducting signature but is of current interest due to its superhard mechanical property. Since the major focus of this investigation is their electronic characteristics in the normal states, we performed the measurements at temperatures between 77 and 295 K. For Re7B3 and Re3B, s-character electrons were found to be responsible for the observed 11B NMR Knight shift and spin-lattice relaxation rate (1/T1). From T1 analysis, we thus deduce the partial B s Fermi-level density of states (DOS) of both borides. On the other hand, the relaxation rate of ReB2 is mainly associated with p electrons, similar to the cases of OsB2 and RuB2. In addition, the extracted B 2p Fermi-level DOS is in good agreement with the theoretical prediction from band-structure calculations.

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