W. M. Walsh

Electron Spin Resonance of Irradiated DNA

The electron spin resonance (ESR) spectra of free radicals formed in-the sodium salt of desoxyribonucleic acid (DNA) by 1-Mev electrons were examined. Dry samples were irradiated at approximates 200°K and examined at 77°K at microwave power levels below approximates 0.1 mw. At levels below 1 Mrad only a single, rather broad absorption line was seen. At around this dosage, an additional spectrum containing several resolved hyperfine satellites was found. A free radical fragment compatible with the results was - C (CH 3)CH 2--. This free radical postulated must come from thymine which contains the only methyl group in DNA.

A systematic study of an isomorphous series of organic solid state conductors based on tetrathiafulvalene

The solid state physical properties of an isomorphous series of TTF salts (TTF11 (SCN)6, TTF11 (SeCN)6, and TTF7I5) were examined. While there was no noticeable trend in the conductivity (as a function of temperature and anion), the effective Fermi energy (?F) and the magnetic susceptibility transition temperature exhibited a definite trend as a function of anion. There is a correlation between the above properties and subtle variations in solid state structure of each of the above salts.

Band structure of (TMTSF)2X

The electronic structure of the organic conductors (TMTSF)2X has been explored in terms of the tight binding band structures calculated for a sheet of TSF molecules. The Se 4d-orbitals appear to be critical in enhancing the interstack Se…Se interaction to the point that (TMTSF)2X becomes pseudo two-dimensional. Based upon the present band structure study, it is discussed whether a normal metallic state or a spin density wave state provides the closed Fermi surface responsible for the Shubnikov-de Haas oscillations observed in (TMTSF)2PF6.

A comparison of electron-spin resonance spectra in some organic charge-transfer salts

Abstract Studies of multiline ESR spectra in ten organic charge-transfer salts give g-tensors which are characteristic of individual molecular ions containing sulphur, nitrogen and smaller atoms. Such nearly universal g-tensors are not found in three salts with molecules containing selenium. These conclusions are supported by some results at variable temperature and hydrostatic pressure.

On the resistivity of the magnetically ordered ground state of the intermediate valence compound TmSe

Abstract Large reductions in the low-temperature resistivity of TmSe as increasing magnetic fields change the magnetic structure from multidomain to two-domain antiferromagnetism and, finally, to ferromagnetic alignment are consistent with a homogeneous intermediate valence state of the Tm ions.

(TMTSF) 2F2PO2: An unusual member of the (TMTSF) 2X family of organic metals

Abstract Resistivity measurements were carried out as a function of temperature and pressure in (TMTSF) 2F2PO2. A strong metal-insulator transition is observed at ∼137K which is only incompletely suppressed by high pressures. The nature of the resistivity curves obtained at high pressure suggest the onset of a Fermi surface instability; however, the metallic-like transport at lower temperatures suggests that the energy gap covers only part of the Fermi surface. The surprising increase in ϱ (T) at temperatures below 20K (under pressure) may result from disorder induced localization arising from the strong electronic scattering from the randomly oriented dipole moments.

Electron spin resonance indication of the ferromagnet-spin glass transition in amorphous FeMn alloys

We find that ESR linewidth measurements of amorphous FeMn alloys indicate the ferromagnet-spin glass transition temperature without a scaling analysis. Comparison with neutron scattering data shows the importance of studying the long-wavelength limit.

Tetramethyldithiadiselenafulvalene (TMDTDSF): Properties of its hexafluorophosphate salt and alloys with tetramethyltetraselenafulvalene

Preparation and some properties of tetramethyldithiadiselenafulvalene (TMDTDSF) are described. It forms the expected (donor)2 X salts. Properties of the PF−6 salt are described as well as properties of alloys of the title compound with tetramethyltetraselenafulvalene (TMTSF). (TMDTDSF)2PF6 is a relatively poor conductor even though it is isomorphous with the high‐pressure superconductor (TMTSF)2PF6. The profound difference in properties between these two solids is ascribed to disorder in the arrangement of sulfur and selenium atoms along the donor stacks in (TMDTDSF)2PF6. Alloying very small amounts of TMDTDSF into (TMTSF)2PF6 causes the metal‐to‐insulator transition temperature (TMI) to move up smoothly from 11 K for pure (TMTSF)2PF6 to ∼200 K for pure (TMDTDSF)2PF6 due to increasing disorder as TMDTDSF is incorporated.

Fermi-energy scaling of the metal-insulator transition temperature in quasi-one-dimensional systems

Abstract Several physical properties of four quasi-one-dimensional organic conductors are compared. Assuming that the intermolecular spacing along the conducting stacks and the magnitude of the susceptibility serve as indicators of the degree of electronic overlap, the data indicate that an increase in this overlap enhances the stability of the insulating ground state of these systems. As a result, the metal—insulator transition temperature is found to scale roughly linearly with an effective Fermi energy.

Magnetic resonance of Gd IN LaNi5 and LaNi5 hydride

The influence of hydrogen uptake on the magnetic properties of LaNi5 has been investigated by observing the electron spin resonance of small amounts of Gd substituted for La. Hydrogenation causes a large exchange induced g‐shift to decrease very strongly in magnitude and to reverse sign. Only ESR signals corresponding to the unhydrided or fully states are observed, indicating very little range of solid solutions. These results are similar to the case of ESR of Gd aND Mn in the Pd, H system but some distinction are drawn. In particular appreciable microwave loss in fully hydrided LaNi5 shows it to be only weakly conducting rather tham metallic. Repeated hydrogenation cycles produce an increasingly intense ferromagnetic resonance signal suggesting precipitation of metallic Ni.