The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I+1)γI2/NSS(S+1)γS2, or ∼ 103 for S = 13C, I = 1H in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High‐frequency resolution of the S ‐spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.

Proton‐enhanced NMR of dilute spins in solids

Publisher Summary: This chapter discusses two major developments that have taken place over the past decade. First is the enormous wealth of energy band calculations that have had tremendous success in explaining the properties of specific solids, but in which the connection with first principles is not always apparent. Second is the spectacular progress of many-body theory applied to the solid state that has given a number of new results, although often of a rather general and formal nature, such as to provide the justification and a formal basis for a one-electron theory. The electron gas problem is treated in some detail here. The problem of the crystal potential is given due attention. It discusses the potential from the ion cores as well as from the valence electrons, and suggests schemes that incorporate essential exchange and correlation effects for the valence electrons. An energy band calculation that properly includes the effects of exchange and correlation describes the elementary excitations called quasi particles. Quasi-particle properties are usually discussed using the remarkable Landau theory of the Fermi liquid. This chapter gives a brief presentation of the theory and reviews the present status of calculations of the Fermi liquid parameters and how they are determined from experiments. (Less)

Effects of Electron-Electron and Electron-Phonon Interactions on the One-Electron States of Solids

Field-Cycling NMR Relaxometry

NMR field-cycling spectroscopy: principles and a]lications

The results of an electron spin resonance (ESR) study of trans‐polyacetylene, before and after doping with AsF5, are reported. The undoped polymer exhibits a Lorentzian line (g=2.00263) indicative of motional narrowing from a mobile unpaired electron species. Measurement of the intensity indicates a Curie law susceptibility with approximately one unpaired electron per 3×103 carbon atoms in agreement with earlier studies. From these results and comparison with earlier data on the cis polymer, it is suggested that the ESR signal arises from a mobile defect in the π system. Utilizing an apparatus which allows in situ AsF5 doping of a polyacetylene sample in the microwave cavity, we have monitored the ESR signal during the doping process. The spin resonance line narrows and increases in intensity, with no detectable g shift, as the doping proceeds. At high doping levels the line becomes asymmetric with a characteristic Dysonian line shape consistent with metallic behavior.

Electron spin resonance of polyacetylene and AsF5‐doped polyacetylene

The paramagnetic susceptibility ${{\ensuremath{\chi}}_{p}}^{e}$ of conduction electron spins is isolated experimentally from the total magnetic susceptibility in metallic lithium and sodium by studying the intensity of the conduction-electron spin resonances. The absolute intensity of absorption is calibrated by comparison with the nuclear resonance of the metal nuclei in the same sample and at the same frequency, the two resonances being observed merely by changing the static magnetic field. In this manner ${{\ensuremath{\chi}}_{p}}^{e}$ is measured in terms of the nuclear static susceptibility, ${{\ensuremath{\chi}}_{p}}^{n}$, which in turn can be calculated accurately from the Langevin-Debye formula. A narrow band modulation technique gives improved signal to noise over our earlier work. The values of ${{\ensuremath{\chi}}_{p}}^{e}$ are (2.08\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ cgs volume units for lithium at 300\ifmmode^\circ\else\textdegree\fi{}K and (0.95\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ cgs volume units for sodium at 79\ifmmode^\circ\else\textdegree\fi{}K, in rather good agreement with the theory of Pines and Bohm, but in substantial disagreement with the simple Pauli model, or the results of Sampson and Seitz. Experimental precision does not permit conclusions to be drawn about the diamagnetism of conduction electrons.

Robert T. Schumacher

Papers

Electron Spin Paramagnetism of Lithium and Sodium

Dynamics of Interacting Spin Systems