Heat Pulse Attenuation and Magnetothermal Resistance in Li-doped Si
Lithium impurities behave as shallow donors in Si, they are believed to occupy an interstitial site. The six-fold valley degeneracy and the tetrahedral symmetry give rise to the usual ground state levels 2A1, 2E and 2T2. These levels are inverted with respect to those of a substitutional group V ion. The lower levels are then the approximately degenerate 2E+2T2 and the excited level is 2A1 at Δ = 440GHz . Theoretically a phonon-induced T2→ A1 transition is symmetry forbidden; the deformation Hamiltonian transforms as A1 +E , the resonant scattering should therefore be from E→A1 and only scatter phonons producing E type distortions. We have tested this prediction using heat pulses. The orbital degeneracy, E+T2 should also lead to scattering from within the ground state which is not the case in the group V systems. This scattering is likely to be very sensitive to strain and it now seems clear that some of the early experiments were significantly affected by random strains probably caused by the relatively high concentrations of C and 0 present in Si available at that time. This can be seen in the strong scattering observed in the thermal conductivity below 1K which has been attributed to ground state splittings of 50GHz in about 20% of the Li ions . A further interesting aspect is the likelihood of Jahn-Teller effects, which could significantly modify the phonon scattering .
KeywordsSulphide Lithium Attenuation Cadmium Shrinkage
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