Dynamics of supercooled liquids and glassy solids

https://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1123&context=physics_pubs

Spectral densities and nuclear spin relaxation in solids

Rotational Tunneling and Neutron Spectroscopy: A Compilation.

Dynamic nuclear polarization (DNP) at 3.35 T and 1.2 K is characterized for [1-13C]pyruvic acid and the paramagnetic agent OX063Me. This paramagnetic agent belongs to a class of organic radicals (trityls) with unique properties for DNP. [1-13C]Pyruvic acid is a molecule of particular relevance for in vivo metabolic studies, and was chosen for that reason. The studies in this work support the conclusion that DNP is through the thermal mixing effect. Critical parameters for DNP are studied: the electron spin resonance spectrum and relaxation times, the nuclear relaxation time and the microwave frequency dependence of the DNP enhancement. In particular, it is reported that the addition of small quantities of Gd3+ affects the DNP favorably. Of the studied parameters only the trityl electron longitudinal relaxation time is significantly affected by the presence of Gd3+. The current models for DNP by the thermal mixing effect, the high-temperature Provotorov equations and the low-temperature Borghini model, are unable to quantitatively explain the observed DNP.

Dynamic Nuclear Polarization with Trityls at 1.2 K

Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla.

Rotational tunneling effects are analyzed in 2H‐NMR spectra of deuterated ammonium ions in powders. The spectra are generally composed of a strong central line from A symmetry species and weaker T sidebands. At a high magnetic field (∼6.5 T) the central line contains well‐resolved doublets. Their separations supply a direct measure of the rotational tunneling frequency νt in the range 0.1<νt<6 MHz. At low magnetic fields the spectra depend both on νt and the Larmor frequency ν0. Level crossing related effects may be detected in a ±0.75 or ±0.35 MHz range around that Larmor frequency which is equal to νt or 1/2 νt, respectively. Generally, only the fitting of the theoretical spectra to those measured at different ν0 allows an estimation of the tunneling frequency. The measurement of νt up to about 20 MHz becomes thus possible. The theoretical predictions are verified by results on a (ND4)2SnCl6 powder sample. High and low field spectra lead to slightly different values for the tunneling frequency at 4.2 K ...

Rotational tunneling effects in 2H‐NMR spectra of polycrystalline (ND4)2SnCl6

Spin-lattice relaxation of deuterons in an (ND4)2SnCl6 single crystal, with B0|[100], was studied at the resonance frequencies 5.7, 14.5 and 43.9 MHz as a function of temperature. A reorientation-related minimum appears in the relaxation time T1 near 60 K with activation energy 5.9 kJ/mol. A model is presented for the corresponding relaxation rate in the presence of librational tunneling. Below 30 K relaxation is clearly nonexponential and an additional minimum is observed near 15 K. This minimum is probably related to the fact that the equilibrium orientations of the ND bonds are not parallel to [111] and equivalent directions but they make a small angle with these directions. The ND4 ion moves then as an entity amongst such orientations keeping always, at low temperatures when tunneling is ignored, each ND bond near a fixed [111] or equivalent direction. An expression is derived for the relaxation rate through this low-temperature motion. Results are compared with neutron diffraction experiments.

Deuteron spin-lattice relaxation in (ND4)2SnCl6 below 60 K

Theory of2H-NMR spectra is developed for a ND4+ ion. Quadrupole and dipole-dipole interactions for four deuterons of the tetrahedral ion as well as the rotational tunneling are taken into account. Features in the single crystal spectra at the high magnetic field of 6.7 T are analyzed. Their central doublets (±5 kHz range around the Larmor frequency) are attributed to theA symmetry species and exhibit sensitivity to theA−T tunneling frequencyvt. This allows the measurement ofvt in a wide rangevt <10 MHz. The shape of sidebands (±140 kHz range) is related to the T levels structure. Theoretical predictions are verified on a (ND4)2SnCl6 single crystal at 44 MHz. The tunneling frequency at temperatures below 10 K equals 7.5 MHz, which is about 100 times smaller than for (NH4)2SnCl6.vt was measured up to 40 K. At still higher temperatures motional narrowing effects were observed for theA spectral components preventing the determination ofvt.

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: I. Tunneling frequency measurements

We have measured the temperature dependence of the proton spin-lattice relaxation time ${\mathit{T}}_{1}$ in glycerol in the temperature range 5--300 K. The lattice dynamics responsible for the relaxation in the range 5T160 K is calculated using solitons with a formation energy \ensuremath{\Delta}/k=1 K. In the range 220T300 K the dynamics of the lattice includes solitons with a formation energy \ensuremath{\Delta}/k=4200 K and these are used to calculate the relaxation of the protons. In both temperature ranges the heat capacity, thermal expansion, and heat conduction are calculated using the same parameters for the soliton representation as were used to calculate the temperature dependence of the proton spin-lattice relaxation time. Discontinuities in the thermal properties at the glass transition temperature ${\mathit{T}}_{\mathit{g}}$ are attributed to an effective-mass change of the low-energy solitons. This work was conducted to test, with the use of nuclear magnetic resonance, some recent ideas in the dynamics of amorphous materials.

E. Koivula

Papers

Rotational tunneling effects in 2H‐NMR spectra of polycrystalline (ND4)2SnCl6

Deuteron spin-lattice relaxation in (ND4)2SnCl6 below 60 K

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: I. Tunneling frequency measurements

Dynamics of proton spin-lattice relaxation in glycerol.

Deuteron NMR spectrum of ND4VO3 and some other ammonium compounds