title: MQ NMR dynamics in dipolar ordered state at negative temperature.
abstract: We investigate theoretically the Multiple Quantum (MQ) NMR dynamics at negative absolute temperatures in systems of nuclear spins 1/2 coupled by the dipole-dipole interactions and with the initial conditions determined by the dipolar ordered state. Two different methods of MQ NMR are used. One of them is based on the measurement of the dipolar energy. The other method uses an additional resonance (π/4)y-pulse after the preparation period of the standard MQ NMR experiment. It is shown that at negative temperatures many-spin clusters and spin correlations are created faster, and the intensities of MQ coherences are higher than in the usual MQ NMR experiments. So, the eighth-order MQ coherence in 10-spin system of the cyclopentane molecule appears to be 1.5 times faster and its intensity is four orders higher than at positive temperatures. The proposed MQ NMR methods at negative absolute temperatures can be used for the investigation of many-spin dynamics of nuclear spins in solids.
authors.affiliation: Department of Physics, Ben Gurion University, Beer Sheva 84105, Israel. Electronic address: gregoryf@bgu.ac.il.
authors.forename: G B
authors.initials: GB
authors.lastname: Furman
authors.affiliation: Department of Physics, Ben Gurion University, Beer Sheva 84105, Israel.
authors.forename: S D
authors.initials: SD
authors.lastname: Goren
authors.affiliation: Department of Physics, Ben Gurion University, Beer Sheva 84105, Israel.
authors.forename: V M
authors.initials: VM
authors.lastname: Meerovich
authors.affiliation: Department of Physics, Ben Gurion University, Beer Sheva 84105, Israel.
authors.forename: V L
authors.initials: VL
authors.lastname: Sokolovsky
keywords: Dipolar ordered state; Multiple quantum NMR spin dynamics; Negative temperature