63Cu NQR Study of the Inhomogeneous Electronic State in La2-xSrxCuO4

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We report detailed systematic measurements of the spatial variation in electronic states in the high Tc superconductor La2-xSrxCuO4 (0.04<= x <= 0.16) using 63Cu NQR for 63Cu isotope enriched poly-crystalline samples. We demonstrate that the spatial variation in local hole concentration 63xlocal given by 63xlocal = x +/- 63Dxlocal, where x is the nominal hole concentration and 63Dxlocal is defined as the amplitude (or extent) of the spatial variation, is reflected in the frequency dependence of the spin-lattice relaxation rate 631/T1 across the inhomogeneous linebroadening of the 63Cu NQR spectrum. By using high precision measurements of the temperature dependence of 631/T₁ at various positions across the 63Cu NQR lineshape, we demonstrate that 63Dxlocal increases below 500 - 600 K and reaches values as large as 63Dxlocal / x 0.5 in the temperature region > 150 K. By incorporating the random positioning of +2Sr donor ions in the lattice in a novel approach, a lower bound to the length scale of the spatial variation 63Rpatch is deduced by fitting the entire 63Cu NQR spectrum (including the ``B'' -line) using a patch-by-patch distribution of the spatial variation 63xlocal with the patch radius 63Rpatch > 3.0 nm as the only free parameter. A corresponding upper bound to the amplitude of the spatial variation 63Dxpatch (1/63Rpatch) is deduced within the model, and consistent results are found with 63Dxlocal. We also deduce the onset temperature TQ (> 400 K) for local orthorhombic lattice distortions which, in the region x > 0.04, is found to be larger than the onset temperature of long range structural order.