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We use ^{79}Br nuclear quadrupole resonance (NQR) to demonstrate that ultraslow lattice dynamics set in below the temperature scale set by the Cu-Cu superexchange interaction J (≃160  K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50 K, and ^{79}Br NQR line shapes become twice broader due to increased lattice distortions. Moreover, the frozen lattice exhibits an oscillatory component in the transverse spin echo decay, a typical signature of pairing of nuclear spins by indirect nuclear spin-spin interaction. This indicates that some Br sites form structural dimers via a pair of kagome Cu sites prior to the gradual emergence of spin singlets below ∼30  K. Our findings underscore the significant roles played by subtle structural distortions in determining the nature of the disordered magnetic ground state of the kagome lattice.

We report on the spin dynamics of 13C isotope enriched inner walls in double-wall carbon nanotubes using 13C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly 13C enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the D line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.

We demonstrate that the spin dynamics in underdoped Y(1-z)Ca(z)Ba(2)Cu(3)O(y) for y approximately equal to 6.0 exhibit qualitatively the same behavior to underdoped La(2-x)Sr(x)CuO(4) for an equal amount of hole concentration p = z/2 = x< or =0.11. However, a spin gap appears as more holes are doped into the CuO(2) plane by increasing the oxygen concentration to y approximately equal to 6.5 for a fixed value of Ca concentration z. Our results also suggest that Ca doping causes disorder effects that enhance the low frequency spin fluctuations.

We report experimental evidence for the spatial variation of hole concentration x(hole) in the high T(c) superconductor La2-xSrxCuO4 ( 0.04 < or = x < or = 0.16) via 63Cu NQR using 63Cu isotope enriched samples. We demonstrate that the extent of the spatial variation of the local hole concentration Delta(x)hole is reflected on (63)1/T1 and deduce the temperature dependence. Delta(x)hole increases below 500-600 K, and reaches values as large as Delta(x)hole/x approximately 0.5 below approximately 150 K. We estimate the length scale of the spatial variation in x(hole) to be R(hole) greater, similar 3 nm from an analysis of the NQR spectrum.

We report (13)C NMR characterization of the new superconductor MgCNi(3) [T. He et al., Nature (London) 411, 54 (2001)]. We found that both the uniform spin susceptibility and the spin fluctuations show a strong enhancement with decreasing temperature, and saturate below approximately 50 K and approximately 20 K, respectively. The nuclear spin-lattice relaxation rate 1/(13)T(1) exhibits typical behavior for isotropic s-wave superconductivity with a coherence peak below T(c) = 7.0 K that grows with decreasing magnetic field.