We have measured the thermal conductivity of seven germanium crystals with different isotopic compositions in the temperature range between 2 K and 300 K. These samples, including one made of highly enriched 70 Ge͑99.99%͒, show intrinsic behavior at room temperature with the exception of a p-type sample with ͉N d-N a ͉Х2ϫ10 16 cm Ϫ3. The ''undoped'' samples exhibit a T 3 dependence at low temperatures, basically determined by boundary scattering. The maximum value of ͑which falls in the range between 13 K and 23 K͒ is found to be a monotonically decreasing function of the isotopic mass variance parameter g. The maximum m measured for the most highly enriched 70 Ge͑99.99%͒ sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the full Callaway theory, modified by treating transverse and longitudinal modes separately, using three free adjustable parameters for each set of modes to represent anharmonic effects plus the calculated contributions from isotopic and boundary scattering. For the isotopically purest 70 Ge͑99.99%͒ sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effect of various surface treatments on the thermal conductivity in the low temperature region. The highest values of are found after polish etching with a SYTON suspension. ͓S0163-1829͑97͒00539-0͔

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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...

The effect of 16 O → 18 O isotope exchange on the electric resistivity was studied for (La1−yPry)0.7Ca0.3MnO3 ceramic samples. Depending on y, this mixed perovskite exhibited different types of low-temperature behavior ranging from ferromagnetic metal (FM) to charge ordered (CO) antiferromagnetic insulator. It was found that at y = 0.75, the substitution of 16 O by 18 O results in the reversible transition from a FM to a CO insulator at zero magnetic field. The applied magnetic field (H ≥ 2 T) transformed the sample with 18 O again to the metallic state and caused the increase in the FM transition temperature TC of the 16 O sample. As a result, the isotope shift of TC at H = 2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal-insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.

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.