We report on the electrical conductivity s of a series of nominally uncompensated neutrontransmutation-doped isotopically enriched 70 Ge:Ga samples with the Ga concentration [Ga] near N c for the metal-insulator transition. s of all insulating samples obeys ln s~2͑T 0 ͞T͒ 1͞2 with T 0~͑ N c -͓Ga͔͒͞N c while the zero temperature conductivity s͑0͒ of the metallic samples is s͑0͕͒ ͓͑Ga͔-N c ͒͞N c ͖ n with the critical exponent n ഠ 0.5. The values of N c obtained from the two independent scalings of T 0 and s͑0͒ are identical, i.e., n ഠ 0.5 is established unambiguously for uncompensated Ge:Ga. [S0031-9007(96)01533-5] PACS numbers: 71.30.+h, 72.80.Cw In experimental studies of the metal-insulator (MI) transition one measures the critical behavior of physical quantities such as conductivity, dielectric constant, heat capacity, etc. The doping induced MI transition in semiconductors is considered to be a model case for the general theory of the critical behaviors of solids. In particular the conductivity extrapolated to zero temperature ͓s͑0͔͒ is evaluated routinely as a function of doping concentration (N) immediately above the MI transition critical concentration ͑N c ͒;where s 0 is the prefactor and n is the critical exponent. The value of n, determined experimentally, is compared with theoretical predictions. Up to now n ഠ 0.5 has been obtained with nominally uncompensated semiconductors (Si:P [1], Si:As [2,3], Ge:As [4], Si:B [5]) while n ഠ 1 has been found with compensated semiconductors (Ge:Sb [6], Si:P,B [7], Ge:Ga,As [8]) and amorphous alloys [9-12]. Exceptions are uncompensated Ge:Sb with n ഠ 1 [13] and Ga x Ar 12x amorphous alloys with n ഠ 0.5 [14]. As we explain below, the value of n ഠ 0.5 obtained with simple systems like uncompensated semiconductors turns out to be inconsistent with theoretical predictions [15]. In his original theory Mott considered only the electron-electron ͑e 2 -e 2 ͒ interaction (Mott transition) and predicted a discontinuous transition of s͑0͒ at N c [16]. Although there is much evidence for the importance of e 2 -e 2 interactions, no experimental observation of such an abrupt transition has been reported. Anderson's idea of MI transitions is based solely on the disordered potential arising from randomly distributed dopants (Anderson transition) [17]. This lead to the development of the well-known "scaling theory" which predicted n ഠ 1 for three dimensional systems [18]. More recently, higher order calculations of the scaling theory (exclusively with disorder and no interactions) predict n ഠ 1.3 [19], and, more importantly, this value is shown to be independent of time reversal invariance [20] and of the strength of spin-orbit interactions [21]. It is therefore clear that the effect of disorder alone cannot explain the experimental results of n ഠ 0.5 or 1. Chayes et al. combined the theories of Mott and Anderson and successfully set the lowest limit n . 2͞3 [22]. This result permits n ഠ 1 obtained with compensated semiconductors and amorphous alloys. However, there still is no th...

70Ge and 74Ge isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity 70Ge and 74Ge single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of very small weight (∼4 g). A 70Ge and a 74Ge crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both 70Ge and 74Ge crystals contain low electrically active chemical net-impurity concentrations of ∼2 × 1012 cm−3, which is two orders of magnitude better than that of 74Ge crystals previously grown by two different groups.1,2 Isotopic enrichment of the 70Ge and the 74Ge crystals is 96.3% and 96.8%, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition, less than 1011 cm−3 gallium, aluminum, and indium were found in the 70Ge crystal.

We have performed a systematic study of the evolution of the superconducting and normal state properties of neutron irradiated MgB2 wire segments as a function of fluence and post exposure annealing temperature and time. All fluences used suppressed the transition temperature, Tc, below 5 K and expanded the unit cell. For each annealing temperature Tc recovers with annealing time and the upper critical field, Hc2(T=0), approximately scales with Tc. By judicious choice of fluence, annealing temperature and time, the Tc of damaged MgB2 can be tuned to virtually any value between 5 and 39 K. For higher annealing temperatures and longer annealing times the recovery of Tc tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters.

Deep level transient spectroscopy and resistivity measurements have been used to characterize defects in as-grown and neutron irradiated epitaxially grown 3C-SiC on Si(100) substrates. The thick epilayers were free of defects; neutron irradiation induced an electron trap with an activation energy of 0.49 eV. The SiC-Si interface has a large density of defects and dislocations. Most of the irradiation defects are confined to the lower two-thirds of the band gap. Ninety percent of these defects can be removed by annealing at 350 °C.

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