Technique for anisotropic extension of organic crystals: application to temperature dependence of electrical resistance

We have developed a technique for the anisotropic extension of fragile molecular crystals. The pressure medium and the instrument, which extends the pressure medium, are both made from epoxy resin. Since the thermal contraction of our instrument is identical to that of the pressure medium, the strain applied to the pressure medium has no temperature dependence down to 2 K. Therefore, the degree of extension applied to the single crystal at low temperatures is uniquely determined from the degree of extension in the pressure medium and thermal contractions of the epoxy resin and the single crystal at ambient pressure. Using this novel instrument, we have measured the temperature dependence of the electrical resistance of metallic, superconducting, and insulating materials. The experimental results are discussed from the viewpoint of the extension (compression) of the lattice constants along the parallel (perpendicular) direction.     

Network formation in poly(ethylene terephthalate) by thermooxidative degradation

Gelation of poly(ethylene terephthalate) by heating at 263°–300°C was investigated. Under nitrogen flow, crosslinks were scarcely formed. However in air, degradation and crosslinking were common, and these were accelerated by purging gaseous and sublimable degradation products out of the system with a stream of air. The main component of the sublimate was terephthalic acid. Infusible and insoluble gel was treated with methanol at 260°C, and then the methanolysis products were separated into two parts. The methanol-insoluble part exhibited a polyene structure with ester groups, and the methanol-soluble part contained dimethyl terephthalate, ethylene glycol, and some 1,2,4-butanetriol. To clarify the relation between the crosslinking and the formation of vinyl ester groups, the degradation of vinyl methyl terephthalate was studied. Thermoxidative degradation of linear polyesters other than poly(ethylene terephthalate) was also studied. Poly(ethylene isophthalate) and poly(ethylene sebacate) were easily gelated. However, poly(trimethylene terephthalate) and poly(neopentyl terephthalate) were scarcely gelated. The primary reaction leading to crosslinking is assumed as follows. At first, the random scission of polyester chain may take place forming carboxylic acids, vinyl esters, aldehydes, etc. After accumulation of vinyl esters to some extent, vinyl polymerization of the esters takes place and network structures are formed.     

Microtribological applications of probe microscopy

This paper introduces the new field of microtribology; it gives a general overview and then presents some typical research results. Micromachines use very lightweight sliding parts, and their wear is primarily due to surface interaction forces rather than load or weight. The ultimate goal of microtribology is to create practical zero-wear devices. Microtribological evaluations of surfaces have started using new tools, such as the scanning probe microscope. Quasi-static indentation tests, impact indentation tests, line scratch tests, and scanning scratch tests have been performed using an atomic force microscope with a diamond tip. Frictional force distributions and adhesive force distributions have also been obtained using atomic force microscopes. Water clusters adsorbed onto solid surfaces have been observed using a scanning tunnelling microscope. The configuration, adsorption, and mobility of lubricant molecules have also been evaluated using a scanning tunnelling microscope.     

Some recent progress in microtribology in Japan

The micro in the word of microtribology is defined as science and engineering supported by modern physics and chemistry. In the engineering area, the ultimate goal of microtribology is to create practical zero-wear devices. The importance of microtribology was recognized by Japanese scientists and engineers in the latter half of the 1980s, and much progress has been made in microtribological studies of mechanisms of friction, lubrication with very thin liquid films, and microwear processes.     

Effects of humidity on nanometer scale wear of a carbon film

Influence of environmental humidity on scanning-scratched wear tests of a carbon film deposited by electron cyclotron resonance plasma sputtering are evaluated in both air and nitrogen with an atomic force microscope and a diamond tip. Wear depth is larger in higher humidity and also larger in air than in nitrogen. Effect of humidity on scratch load dependence and repeated scanning-scratch tests are larger in nitrogen than in air. Effect of humidity on wear is clearer in the repeated scanning-scratch test than in the scratch load dependence test especially in air, and also clearer at larger scratch load. In scratch velocity dependence test, there is a tendency that wear depth decreases with increase in logarithm of scratch velocity both in air and nitrogen. Wear models to explain the experimental results are discussed.     

Determination of the π-Charge Distribution of the DMe-DCNQI Molecule in (DMe-DCNQI) 2 M, M=Li, Ag, and Cu

Solid state high-resolution NMR of ¹H and ¹³C along with ¹⁵N is analyzed to investigate the electronic states of the charge transfer salts (DMe-DCNQI)₂M, (M=Li, Ag, and Cu). We determined the spin/charge distribution in a DMe-DCNQI molecule of the Li-salt from the Knight shifts at each atom on the molecule. It is found that the obtained charge distribution is similar to the theoretical prediction. The charge density on the DCNQI molecules of the Ag-salt is found to be smaller by 20% than the Li-salt, which could be an origin of differences from the Li-salt. This result is consistent with the first principle calculations (Miyazaki and Terakura, Phys. Rev. B 54, 10452, 1996).     

Fluctuation of charge density wave of TTF–TCNQ under extreme pressure

We have presented that the charge density wave (CDW) in TTF–TCNQ is suppressed by extremely high hydrostatic pressure (8 GPa). To understand the nature of CDW under pressure, we shed light on the fluctuation of CDW under pressure, by examining the resistance anisotropy and their temperature dependence along the b-axis (1D) and a-axis. It turned that the fluctuation of CDW above T_CDW continues to be present in the whole pressure range where CDW is present, but is gradually suppressed as CDW is suppressed. Also remarkable is that this behavior is only observed in the resistance behavior along the b-axis (1D) but not along the a-axis.     

An Interlayer Magnetoresistance Peak Effect below the Superconducting Transition Temperature in Et2Me2P[Pd(dmit)2]2

We studied the normal state properties of an organic superconductor, κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, through the measurement of the Hall effect as functions of temperature and pressure. At low pressure, the Hall coefficent shows the peak structure. From the coincidence of the temperature and its pressure evolution with those of anomalies observed in other physical properties, the peak temperature is considered to be T*. At high pressure, we observed the drastic decrease and sign change of the Hall coefficient. From the incompatibility with the behavior of a similar compound, κ-(BEDT-TTF)₂Cu(NCS)₂, we concluded that this behavior orignate from the singularity of the Fermi surface of this salt.