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

The appeal of ultra-compact spintronics drives intense research on magnetism in low-dimensional materials. Recent years have witnessed remarkable progress in engineering two-dimensional (2D) magnetism via defects, edges, adatoms, and magnetic proximity. However, intrinsic 2D ferromagnetism remained elusive until recent discovery of out-of-plane magneto-optical response in Cr-based layers, stimulating the search for 2D magnets with tunable and diverse properties. Here we employ a bottom-up approach to produce layered structures of silicene (a Si counterpart of graphene) functionalized by rare-earth atoms, ranging from the bulk down to one monolayer. We track the evolution from the antiferromagnetism of the bulk to intrinsic 2D in-plane ferromagnetism of ultrathin layers, with its characteristic dependence of the transition temperature on low magnetic fields. The emerging ferromagnetism manifests itself in the electron transport. The discovery of a class of robust 2D magnets, compatible with the mature Si technology, is instrumental for engineering new devices and understanding spin phenomena.

A class of intrinsic 2D ferromagnets – layered metalloxenes – is established by coupling graphene-like honeycomb networks of silicene and germanene with 2D lanthanide layers.

The thermal conductivity κ(T ) of single crystals of silicon with two different isotopic compositions: natural and 99.983% enriched 28 Si, was investigated in the temperature range from 0.5 K to 300 K. The enriched 28 Si sample has very high thermal conductivity maximum of 290 W cm −1 K −1 at Tmax = 26.5 K, about 7.5 times higher relative to the conductivity of nat Si with natural isotope abundance. The isotope effect decreases with temperature increase, being 10 ± 2% at room temperature. The data are discussed briefly within the Ambegaocar's theory of isotope effect.

No abstract