Electronic transport in tungsten and iron-doped tungsten below 1 K

The electrical resistivity and the thermoelectric ratio G have been measured for zone-refined single crystals of both tungsten and iron-doped tungsten from 5 K down to 40 mK. The samples had residual resistance ratios RRR ranging from 1750 to 90,000. The observed behavior is conveniently divided into two classes, normal and anomalous. Completely normal behavior was displayed by only three W samples with high RRRs. The Fe-doped W and the remaining W samples contained one or more anomalies. Normal behavior is that which would be expected for W containing impurities with no internal degrees of freedom. In normal behavior decreased monotonically with decreasing temperature and was consistent with the equation = _o + AT² below about 1.5 K. This form for is taken as evidence that electron-electron scattering dominates electron-phonon scattering in for W at such low temperatures. For the range of sample purities studied, A increased slightly with increasing ₀, but did not vary systematically with either the sample diameter d or with the ratio (273 K)/d(T). In normal behavior, G was positive and constant below about 0.5 K, increased in magnitude as T rose to 4 or 5 K, and then began to decrease, becoming negative above about 7 K. An explanation is provided for this behavior. Those samples that fell into the anomalous class displayed at least one of three anomalies: (1) a minimum in the electrical resistivity, with an approximately logarithmic variation with T at temperatures below the minimum; (2) a positive contribution to G which increased in magnitude with decreasing temperature approximately as T^–1/2from about 4 K down to at least 0.5 K; and (3) a negative contribution to G which set in at about 0.5 K, varied approximately as log T, and dominated G at the lowest temperatures. These anomalies are presumably due to one or more impurities dissolved in the W, possibily including Fe. However, chemical and spectroscopic analyses of pieces from several samples, including the Fe-doped W, failed to establish a clear link between any specific impurities and the observed anomalies. At the moment we neither know the source nor understand the nature of these anomalies.Supported in part by NSF Grants DMR-77-04680 and DMR-78-07892.Part of this work was done while the author was at the Physics Department, University of Lancaster, Lancaster, England, during a sabbatical.Part of this work was done while the author was at the Physics Department, Catholic University, Nijmegen, The Netherlands, during a sabbatical.