${}^{63}$Cu and ${}^{199}$Hg nuclear magnetic resonance shifts for an optimally doped and underdoped HgBa${}_2$CuO${}_{4+\delta }$ single crystal are reported, and the temperature dependence dictates a two-component description of the uniform spin susceptibility. The first component, associated with the pseudogap phenomenon in the NMR shifts, decreases at room temperature and continues to drop as the temperature is lowered, without a drastic change at the transition temperature into the superconducting state. The second component is temperature independent above the superconducting transition temperature and vanishes rapidly below it. It is a substantial part of the total $T$-dependent susceptibility measured at both nuclei.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffusivities of guest molecules. Spatial resolution, i.e., the minimum diffusion path length experimentally observable, is limited by the time interval over which the pulsed field gradients may be applied. In “conventional” PFG NMR measurements, this time interval is determined by a characteristic quantity of the host-guest system under study, the so-called transverse nuclear magnetic relaxation time. This leads, notably when considering systems with low molecular mobilities, to severe restrictions in the applicability of PFG NMR. These restrictions may partially be released by performing PFG NMR measurements in combination with “magic-angle spinning” (MAS) of the NMR sample tube. The present review introduces the fundamentals of this technique and illustrates, via a number of recent cases, the gain in information thus attainable. Examples include diffusion measurements with nanoporous host-guest systems of low intrinsic mobility and selective diffusion measurement in multicomponent systems.

A new analysis of Cu-63 and O-17 NMR shift data on La1.85Sr0.15CuO4 is reported that supports earlier work arguing for a two-component description of La1.85Sr0.15CuO4, but conflicts with the widely held view that the cuprates are a one-component system. The data are analyzed in terms of two components A and B with susceptibilities chi(AA), chi(AB)(= chi(BA)) and chi(BB). We find that above T-c, chi(AB) and chi(BB) are independent of temperature and obtain for the first time the temperature dependence of all three susceptibilities above Tc as well as the complete temperature dependence of chi(AA) + chi(AB) and chi(AB) + chi(BB) below T-c. The form of the results agrees with that recently proposed by Barzykin and Pines.

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature ( T _c ). But the family-specific maximum T _c can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO ₂ plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances T _c above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum T _c is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum T _c on the Cu–O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum T _c points to the same mechanism.