The electrical conductivity o (extrapolated to T=0) of uncompensated Si:P indicates a crossover as a function of P concentration TV at TV cr slightly above the metal-insulator transition at TV C . For TV > TV cr the exponent of a-(N-N c )' i is // ~ 0.64, while n « 1.3 for N c < TV < TV cr . At TV cr do/dT changes sign from negative for TV > TV cr to positive for TV < TV cr . o in a magnetic field also yields pi ~ 1. The apparent discrepancy between uncompensated and compensated semiconductors is traced back to a difference in the (nonuniversal) width of the critical region.

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The thermoelectric power S of uncompensated Si:P with P concentration N near the metal-insulator transition occurring at N c has been measured at very low temperatures (0.04 < T< 3 K). For Ny>N c , S is negative and shows the linear T dependence of a metal, whereas close to N c an anomalous behavior with a sign change of S at low T is observed. The strong dependence of S on magnetic fields up to 6 T relates the anomaly to magnetic scattering, thus giving the first experimental evidence for localized moments near the metal-insulator transition in a transport property. PACS numbers: 71.30.+h, 72.15.Jf, 72.15.Qm, 72.20.Pa The metal-insulator (MI) transition in disordered systems is one of the central themes in condensed matter physics. Besides disorder, electron-electron interactions are thought to play a major role in this transition. Recent theoretical work emphasizes the role of local moments near the MI transition, possibly even leading to a non-Fermi-liquid behavior with the magnetic susceptibility x an< 3 the linear specific-heat coefficient y diverging for T 7 -• 0 [1-3]. Doped semiconductors like Si:P where the disorder stems from the random distribution of donors constitute a convenient system for studying the MI transition, which occurs as a function of the donor concentration N at a critical concentration N c . In this material, the existence of local moments in the metallic state has been observed in magnetic resonance [4] and static % measurements [5]. Their density as a function of TV has been mapped out in detail with specific-heat measurements [6], The possible influence of local moments on the electrical conductivity a [2,7] is difficult to estimate because of the large T and B dependence of a due to localization effects and electron-electron interactions [8].In this situation, more specific information on transport in disordered materials close to the MI transition is highly desirable.A particularly sensitive transport property is the thermoelectric power S. In view of the giant thermopower observed in Kondo systems, S should be susceptible to local moments near the MI transition. However, generally an accurate analysis of S is difficult, because (in a Fermi-liquid description) the explicit dependence of the scattering time r and density of states at the Fermi level D(Ef) on energy has to be taken into account. Furthermore, at moderate and high temperatures (T^&D, the Debye temperature) S is often dominated by the phonon-drag contribution due to the electron-phonon interaction. Therefore one has to work below 1 K to circumvent this difficultly. Approaching the transition from the metallic side a divergent coefficient S/T is predicted for T-0 [9,10], S/T -(N-N C )~M, where the exponent JA should depend on the significance of the electronelectron interactions or whether symmetry-breaking fields such as an external magnetic field, magnetic impurities, or spin-orbit coupling are present. On the insulating side where a is due to variable-range hopping [11], 5* will also be influenced by correlations....

The specific heat C of uncompensated phosphorus-doped silicon with P concentration TV between (0.34 and 7.3)xlO 18 cm" 3 , i.e., in the vicinity of the metal-insulator transition, has been measured over a large range of temperatures (0.04 K< T< 3 K), allowing the unambiguous detection of an anomalous contribution AC~r a , with a becoming negative for small TV. This is attributed to exchange-coupled clusters. The magnetic field dependence of C (up to 5.7 T) allows us to deduce the relative contributions of localized and delocalized electrons and gives evidence for interactions and correlations between these.PACS numbers: 71.30.4-h, 65.40.Em, 71.55.Ht ] or, for fixed donor density TV slightly below N Cy with increasing uniaxial stress. 2 In a grossly simplified picture, the transition is due to the increasing overlap of the Pderived donor electronic wave functions until an impurity band is formed. An interesting and hitherto unexplained feature is the fact that for uncompensated Si:P the exponent v of the electrical conductivity a-(TV -N c ) v is y, 1,2 while for virtually all other materials, including compensated semiconductors, it is close to 1, in agreement with scaling predictions. 3 The metal-insulator (MI) transition as driven by the combined effects of disorder and electronic correlations, the Anderson-Mott transition, is one of the major fields of research in solid-state physics. The terms metal and insulator are used to denote a finite or vanishing electrical conductivity a for T-• 0, respectively. A welldefined experimental realization of such a transition is found in phosphorus-doped silicon (Si:P), where the MI transition occurs at a critical density of donors TV C ,

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