Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Defect equilibria in undoped a -Si:H

Abstract: Experimental and theoretical studies of the thermal equilibrium defect density in undoped a-Si:H are reported. The defect density measured by electron-spin resonance increases with temperature with an activation energy of 0.15\char21{}0.2 eV. The equilibration time is activated with an energy of about 1.5 eV, and the shape of the decay follows a stretched exponential, as in doped a-Si:H. The experiments confirm that defect equilibration occurs over a range of temperatures and sample deposition conditions. The relaxation time depends on the growth conditions, and the thermal defects are shown to anneal more slowly than optically induced defects. The temperature dependence of the thermodynamic equilibrium defect density is calculated, based on the weak-bond\char21{}dangling-bond conversion model. Four specific defect reactions are analyzed, two of which involve the motion of bonded hydrogen. The defect density is sensitive to the details of the model because of entropy effects. The experimental data agree well with the analysis, but do not conclusively distinguish between the different possible defect reactions because of uncertainties in the parameters of the model. The different annealing rates of thermal and optical defects are accounted for by relating the distributions of hydrogen-bonding energies, the defect-formation energies, and the valence-band-tail states. It is proposed that the time dependence of the relaxation is related to the shape of the valence-band-tail distribution.

The Distribution of Relaxation Times in Typical Dielectrics

Abstract: K W Wagner's treatment of the distribution of relaxation times in dielectrics is reviewed; the effect of the density of distribution upon the frequency variation of the dielectric constant and dielectric loss factor is discussed; and a graphical method of evaluating the constants of Wagner's equation from experimental data is described These constants have been evaluated for a number of typical dielectrics, and the frequency variations of the dielectric constant and dielectric loss factor as computed from Wagner's equations and from the simple equations for a single relaxation time are compared with the observed behavior The quantitative correlation obtained between Wagner's equations and the experimental data investigated is viewed as a confirmation of Wagner's theory of a statistical distribution of relaxation times in dielectrics

Ultrafast Vibrational Dynamics at Water Interfaces

Abstract: Time-resolved sum-frequency vibrational spectroscopy permits the study of hitherto neglected ultrafast vibrational dynamics of neat water interfaces. Measurements on interfacial bonded OH stretch modes revealed relaxation behavior on sub-picosecond time scales in close resemblance to that of bulk water. Vibrational excitation is followed by spectral diffusion, vibrational relaxation, and thermalization in the hydrogen-bonding network. Dephasing of the excitation occurs in ≤100 femtoseconds. Population relaxation of the dangling OH stretch was found to have a time constant of 1.3 picoseconds, the same as that for excitation transfer between hydrogen-bonded and unbonded OH stretches of water molecules surrounded by acetone.

Determination of electrical transport properties using a novel magnetic field‐dependent Hall technique

Abstract: A novel technique is presented for interpreting magnetic field‐dependent Hall data at magnetic fields below the range at which Shubnikov–de Haas oscillations occur. The technique generates a ‘‘mobility spectrum’’ in which the maximum carrier density or maximum conductivity is determined as a continuous function of mobility. Examples of the use of the technique with synthetic data as well as data from HgCdTe and GaAs/AlGaAs samples are provided. Other uses of the procedure, including measurement of Fermi surface shapes and direct measurement of the distribution of relaxation times, are discussed.

Nuclear Transfer Effects in Nuclear Magnetic Resonance Pulse Experiments

Abstract: The Bloch equations for the nuclear magnetic resonance of a single nuclear species which is transferred between state environments having different relaxation times (T1 or T2) and different resonance frequencies have been solved for rf pulse experiments. Expressions have been obtained for the free precession signals in two‐pulse experiments. A theoretical study of the signal envelopes has been made for several specific instances. In particular, the effect of the frequency difference between the two states on transverse (T2) relaxation has been investigated; the predicted effect is large in some cases. The longitudinal (T1) relaxation is independent of the frequency separation. A reduction of phase dispersion by the second pulse which is similar to that in molecular diffusion is also predicted.