Friday, December 17, 2010

Phys. Rev. B.

NMR and NQR study of the tetrahedral frustrated quantum spin system Cu_{2} Te_{2} O_{5} Br_{2} in its paramagnetic phase

Author(s): Arnaud Comment, Hadrien Mayaffre, Vesna Mitrović, Mladen Horvatić, Claude Berthier, Béatrice Grenier, and Patrice Millet
The quantum antiferromagnet Cu2Te2O5Br2 was investigated by NMR and nuclear quadrupole resonance (NQR). The 125Te NMR investigation showed that there is a magnetic transition around 10.5 K at 9 T, in agreement with previous studies. From the divergence of the spin-lattice relaxation rate, we ruled out the possibility that the transition could be governed by a one-dimensional divergence of the spin-spin correlation function. The observed anisotropy of the 125Te shift was shown to be due to a spin polarization of the 5s2 “E” doublet of the[TeO3E] tetrahedra, highlighting the importance of tellurium in the exchange paths. In the paramagnetic state, Br NQR and NMR measurements led to the determination of the Br hyperfine coupling and the electric field gradient tensor, and to the spin polarization of Br porbitals. The results demonstrate the crucial role of bromine in the interaction paths between Cu spins.

NMR evidence for the partially gapped state in CeOs_{2} Al_{10}

Author(s): C. S. Lue, S. H. Yang, T. H. Su, and Ben-Li Young
We report the results of a 27Al nuclear magnetic resonance (NMR) study of CeOs2Al10 at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature To≃29 K. Five 27Al NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below To, indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.

Spin order and lattice frustration in optimally doped manganites: A high-temperature NMR study

Author(s): N. Panopoulos, D. Koumoulis, G. Diamantopoulos, M. Belesi, M. Fardis, M. Pissas, and G. Papavassiliou

Understanding the complex glassy phenomena, which accompany polaron formation in optimally doped manganites (ODMs) is a cumbersome issue with many unexplained perspectives. Here, on the basis of 139La and 55Mn nuclear magnetic resonance (NMR) measurements, performed in the temperature range 80–900 K we show that glass freezing, observed in the paramagnetic (PM) phase of ODM La0.67Ca0.33MnO3, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which forT<Tc consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed onLa1−xCaxMnO3 systems for 0.0≤x≤0.41 and ODM La0.70Sr0.30MnO3, demonstrates the key role played by the local lattice distortions, which control (i) the stability of the spin-glass phase component and (ii) the kind (first or second order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond-random field Ising model, where consideration of a strain field induced by lattice distortions is shown to invoke at Tc a discontinuous first-orderlike change in both the FM and the “glassy” Edwards-Anderson order parameters.

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