ICT2014 Abstracts

When a crystal is doped with considerable amount of impurities, its electronic structure can change, especially if impurities are of resonant character, strongly affecting host's density of states. However, due to the lack of periodicity of the doped system, the electronic bands E(k) are in principle not well-defined and thus cannot be directly calculated using first-principles methods. The more general quantity, which can describe dispersion relation in a disordered system, is a Bloch spectral function (BSF). 
In this work we apply the KKR-CPA method to calculate BSF for a representative thermoelectric material, PbTe, doped with thallium and titanium. Thallium atom is known to be a resonant impurity in PbTe, which results in enhancement of thermopower. On the other hand, titanium is a resonant impurity as well however no enhancement in thermopower in Ti:PbTe against other impurities was observed. Those results are clearly explained by analyzing the spectral functions in both systems. We show that Tl does not create impurity band near the Fermi level in PbTe, but the existence of resonant level strongly affects the host band structure, leading to disappearance of sharp energy bands near valence band edge. In return, additional electronic states, being of hybridized impurity and host character, appear between L and Sigma points in Brillouin zone, creating a 'cloud' of states around original PbTe band and modifying its Fermi surface. This suggests comparing the effect of resonance to the effect of increase in band degeneracy and offers the alternative way of understanding the positive role of resonant level in enhancing the thermopower. As a counter-example, we show that for Titanium doped PbTe spectral function resemble flat impurity band, which is not favorable for thermoelectric performance of the system. The differences between those two systems help to identify the resonant impurities beneficial for the thermoelectric performance.