Lattice thermal conduction in ultra-thin nanocomposites

This is the final version of the article. Available from the publisher via the DOI in this record.We have studied the lattice thermal conductivity of Si/Ge periodic nanocomposites (superlattice, nanowire, and nanodot structures) of sample sizes in the range of 30 nm-30 μm, periodicities 1.1 nm and 2.2 nm, with reasonably dirty interfaces, and n-type doping concentration in the range of 1023-1026m-3. Our calculations employ a judicious combination of ab initio and physically sound semi-empirical methods for detailed calculations of estimates of phonon scattering rates due to anharmonicity and interface formation. Based upon our results we conclude that the formation of ultra-thin nanocomposites in any of the three structures is capable of reducing the conductivity below the alloy limit. This can be explained as a result of combination of the sample length dependence, the on-set of mini-Umklapp three-phonon processes, mass mixing at the interfaces between Si and Ge regions, and the sample doping level.We are grateful to the EPSRC (UK) for supporting this project via the Grant Award No.r EP/H046690/1. Quantum Espresso calculations were performed on the Intel Nehalem (i7) cluster (ceres) at the University of Exeter. I.O.T. also acknowledges support from the John Templeton Foundation as part of the Durham Emergence Project during the final stages of this work

Shell model description of Ge isotopes

A shell model study of the low energy region of the spectra in Ge isotopes for $38\leq N\leq 50$ is presented, analyzing the excitation energies, quadrupole moments, $B(E2)$ values and occupation numbers. The theoretical results have been compared with the available experimental data. The shell model calculations have been performed employing three different effective interactions and valence spaces.We have used two effective shell model interactions, JUN45 and jj44b, for the valence space $f_{5/2} \, p \,g_{9/2}$ without truncation. To include the proton subshell $f_{7/2}$ in valence space we have employed the $fpg$ effective interaction due to Sorlin {\it et al.}, with $^{48}$Ca as a core and a truncation in the number of excited particles.Comment: 10 pages, 10 figures, Proc. of the XXXV Nuclear Physics Symposium, January 3-6 2012, Cocoyoc, Morelos, Mexico. IOP Journal of Physics: Conference Series (in press