NMR/NQR Spectroscopy

The NMR/NQR group has continued its general research plan of studying current problems of condensed matter physics by employing NMR (nuclear magnetic resonance) and NQR (nuclear quadrupole resonance) methods. These spectroscopies allow one to investigate electronic properties in solids at a microscopic level and thus provide an essential contribution for a better understanding of the materials studied. Our main interest is still devoted to (i) high{temperature superconductors, (ii) their antiferromagnetic parent compounds, (iii) superionic glasses, and (iv) high-pressure NMR/NQR using either the diamond-anvil cell or helium as the pressure medium. 9.1 High-temperature superconductors Our research interest during the last year was mainly concerned with the following topics: (i) the nature of the pseudo gap in the normal state of high{temperature superconductors and its relation to other normal state electronic eeects; (ii) calculation of the dynamic spin susceptibility under various circumstances; (iii) further experimental data for the coupling in the CuO 2 bilayers; (iv) electronic phase separation, and (v) the anomaly in the irreversible magnetization hysteresis curve. 9.1.1 Isotope eeect of the spin gap Recently, we have discovered an electronic crossover occurring in YBa 2 Cu 4 O 8 (1{2{4) around T y = 180 K 1]. The crossover, which is not a structural phase transition, involves enhanced charge uctuations in planes and chains accompanied by a charge (hole) transfer from chain to plane. T y is relatively close to another temperature, T , which is characteristic for underdoped high{temperature superconductors and which refers to the opening of a spin gap (as seen by NMR) in the electron spin excitation spectrum. In 1{2{4, T 150 K which lies above T c = 82 K. This spin gap eeect explains the strong temperature dependence of the normal state susceptibility in 1{2{4. In collaboration with M. Eremin et al. from the Kazan State University, we have argued that the spin gap eeect in 1{2{4 is caused by a transition due to a charge density wave (CDW) instability 2]. The proximity of T y and T has been a trigger for these theoretical investigations. The most crucial experimental veriication whether the spin gap is related to or triggered by a CDW or spin density wave (SDW) instability, is probably the measurement of the isotope eeect, i:e: the change, T , of the temperature T resulting from a change, m, of the isotope composition and its dependence on doping level. We have calculated T for …