The Wiedemann-Franz (WF) ratio compares the thermal and electrical conductivities in a metal. We describe a new way to determine its value, based on the thermal Hall conductivity. The technique is applied to copper and to untwinned YBaCuO. In the latter, we uncover a T-linear dependence and suppression of the Hall-channel WF ratio. We discuss the implications of this suppression. The general suppression of the WF ratio in systems with predominant electron-electron scattering is discussed.

We have investigated the electrical conductivity of λ-DNA using DNA covalently bonded to Au electrodes. Thiol-modified dTTP was incorporated into the 'sticky' ends of bacteriophage λ-DNA using DNA polymerase. Two-probe measurements on such molecules provide a hard lower bound for the resistivity ρ > 10 6 Ωcm at bias potentials up to 20 volts, in conflict with recent claims of moderate to high conductivity. By direct imaging, we show that the molecules are present after the measurements. We stress the importance of eliminating salt residues in these measurements.The question whether DNA is electrically conducting has generated broad interest. The initial spurt of interest arose in photoexcitation experiments which were interpreted in terms of long-range electron transfer [1]. In the past few years, there have been upwards of 20 papers reporting the results of more direct electrical measurements ranging from contactless meaurements at microwave frequencies to DC measurements. A distressingly wide range of conductivity values -from ρ < 10 −4 Ωcm to ρ > 10 6 Ωcm -has been reported [2,3,4,5,6]. Proximity-induced superconductivity in DNA has also been claimed [7]. Recently, local polarization measurements by 'electrostatic force microscopy' have been used to show that λ-DNA is insulating [8,9]. We note, however, that the force-microscopy experiments probe conductivity at relatively weak bias potentials.In many of the DC measurements, contact with the metal electrodes (usually Au) was achieved by laying down the molecules directly on the electrodes. Although expedient, this approach raises several concerns. It is very difficult to prove that the DNA molecule is in direct physical contact with the electrodes. Even if contact is attained, the weak physical adhesion between DNA and Au may produce an insulating contact and possibly account for the wide variation in reported resistivities [10]. A recent experiment on octanedithiol [11] has shown that deliberate chemical bonding between organic molecules and metal electrodes is a pre-requisite for achieving reproducible conductivity results. Thus a better approach would be to achieve direct chemical binding between the open ends of λ-DNA and Au. The bonds should be strong enough to withstand shear forces in a flow, and should survive the measurement process. A second concern is the shunting effect of buffer residue. Because of its finite conductance, the buffer salts which coat the electrodes and substrate produce a spurious conductance signal. Hence adequate salt removal is important. We report the results of experiments performed along these lines. Our results show that λ-DNA is a good insulator up to bias potentials of 20 volts.Chemical binding between organic molecules and Au is usually achieved by the Au-thiol (SH) chemical bond [12]. Commercially available oligonucleotides modified to incorporate the thiol group usually have carbon-chain spacers (C3 or C6) between the thiol group and DNA [3,6,13], which may present barriers to electron transfer. To avoid the spacer pro...

In high-purity YBa(2)Cu(3)O(7), the (weak-field) thermal Hall conductivity kappa(xy) is observed to increase a thousand-fold between 90 and 30 K. The inferred quasiparticle lifetime tau increases a hundred-fold starting below 90 K, in disagreement with a recent photoemission experiment. We show that kappa(xy) exhibits a specific scaling behavior below approximately 30 K. This scaling may bear on the issue of whether Landau quantization of the quasiparticle states occurs.

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We present a way to extract the quasiparticle thermal conductivity k e and mean free path in YBa 2 Cu 3 O x using the thermal Hall effect and the field dependence of k e . The results compare well with heat capacity experiments. Moreover, we find a simple relation between the thermal Hall angle u Q and the H dependence of k e , as well as numerical equality between u Q and the electrical Hall angle. The results also reveal an anomalously anisotropic scattering process in the normal state.[S0031-9007 (99)09469-7] PACS numbers: 74.25.Fy, 72.15.Gd, 74.60.Ge, 74.72.Bk 0031-9007͞99͞82(25)͞5108(4)$15.00