A. W. Hunt

Cu-63 NQR Measurement of Stripe Order Parameter in La(2-x)Sr(x)CuO(4)

We demonstrate that one can measure the charge-stripe order parameter in the hole-doped CuO(2) planes of La(1.875)Ba(0.125)CuO(4), La(1.48)Nd(0.4)Sr(0.12)CuO(4) and La(1.68)Eu(0.2)Sr(0.12)CuO(4) utilizing the wipeout effects of Cu-63 NQR. Application of the same approach to La(2-x)Sr(x)CuO(4) reveals the presence of similar stripe order for the entire underdoped superconducting regime 1/16 < x < 1/8.

63Cu NQR evidence for spatial variation of hole concentration in La2-xSrxCuO4.

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.

Glassy slowing of stripe modulation in ( L a , E u , N d ) 2 − x ( S r , B a ) x CuO 4 : A 63 Cu and 139 La NQR study down to 350 mK

We report a comprehensive Cu and La NQR study down to 350mK in the stripe phase of the hole-doped high temperature superconductors with hole concentration approximately 1/8. We observe the hyperfine broadened NQR lineshape only below 8-20K for x=1/8, indicating that the ordered moments continue to fluctuate slowly even below the temperature range where they appear static in other measurements. Detailed numerical simulations of the Zeeman perturbed Cu NQR lineshape establish that widely accepted models of periodic modulation of spin density waves with maximum moment approxiamtely 0.3mu_B can not account for the NQR lineshape. These results suggest that significant disorder exists at NQR time scales. By fitting a single model to several sets of experimental data, we deduce the spatial distribution of the spin fluctuation frequency scale and its temperature dependence. Our results indicate that as soon as charge dynamics slow down, the spin fluctuationsbegin to slow dramatically with spin stiffness approximately 200K. However, the motional narrowing feature observed in the Zeeman perturbed NQR measurements suggest that the slowing of the spin fluctuations levels off at frequencies somewhat below the NQR frequency. By extending the analysis to other hole concentrations, we find a qualitative difference above and below x=1/8.

Systematic63CuNQR study of the stripe phase inLa1.6−xNd0.4SrxCuO4for0.07<~x<~0.25

We demonstrate that the integrated intensity of Cu-63 nuclear quadrupole resonance (NQR) in La(1.6-x)Nd(0.4)Sr(x)CuO(4) decreases dramatically below the charge-stripe ordering temperature T(charge). Comparison with neutron and X-ray scattering indicates that the wipeout fraction F(T) (i.e. the missing fraction of the integrated intensity of the NQR signal) represents the charge-stripe order parameter. The systematic study reveals bulk charge-stripe order throughout the superconducting region 0.07 <= x <= 0.25. As a function of the reduced temperature t = T/T(charge), the temperature dependence of F(t) is sharpest for the hole concentration x=1/8, indicating that x=1/8 is the optimum concentration for stripe formation.

17O NMR Study of q = 0 Spin Excitations in a Nearly Ideal S = (1/2) 1D Heisenberg Antiferromagnet, Sr2CuO3, up to 800 K

We used 17O NMR to probe the uniform (wave vector q = 0) electron spin excitations up to 800 K in Sr2CuO3 and separate the q = 0 from the q = +/-pi / a staggered components. Our results support the logarithmic decrease of the uniform spin susceptibility below T approximately 0.015J, where J = 2200 K. From measurement of the dynamical spin susceptibility for q = 0 by the spin-lattice relaxation rate 1/T(1), we demonstrate that the q = 0 mode of spin transport is ballistic at the T = 0 limit, but has a diffusion-like contribution at finite temperatures even for T<<J.

What has NMR taught us about stripes and inhomogeneity

Abstract The purpose of this brief invited paper is to summarize what we have (not) learned from NMR on stripes and inhomogeneity in La 2− x Sr x CuO 4 . We explain that the reality is far more complicated than generally accepted.

17O and 101Ru NMR study of orbital dependent ferromagnetic correlations in Sr2RuO4

Abstract Utilizing 17O and 101Ru NMR we probe the Ru 4dxy and 4dyz,zx orbitals separately in the anomalous normal state of Sr2RuO4 and find orbital dependent (S≠1) behavior. From the 101Ru and 17O nuclear spin–lattice relaxation rates 17,1011/T1 we demonstrate that it is the ferromagnetic component of the spin fluctuations that grows with decreasing temperature.

Contrasting 63Cu and 139La NQR behavior in Eu- and Zn-doped La2−xSrxCuO4

Abstract We carried out 63 Cu and 139 La NQR in Eu and Zn co-doped high Tc superconductor La2−xSrxCuO4 to compare the properties of the stripes induced by Eu and the spin-glass-like disordered magnetism induced by Zn. We demonstrate that magnetic slowing down of stripes below charge ordering temperature Tcharge in La1.8−xEu0.2SrxCuO4 takes place more suddenly than progressive spin-glass-like freezing in La2−xSrxCu0.96Zn0.04O4.