Low-Temperature Thermopower and Other Transport Properties of Aluminium Containing Dilute Point Defects
We compare new experimental results of the low-temperature thermopower and of the low-field Hall effect (and magnetoresistance) of aluminium containing dilute point defects. The defects were either nonmagnetic impurities (Ge, Mg, Zn, Ga, or Cu) or Frenkel defects (FD, i.e. self-interstitials and vacancies) introduced by reactor irradiation at 4.6 K. Some of the results can be compared with 4 OPW calculations, which were performed using the realistic Al Fermi surface (FS) and tabulated pseudo-potentials and without adjusting any parameter.
The low-field Hall coefficient Ro at 4.6 K was always found to be consistent, even quantitatively, with the generalized Tsuji-formula, see below. The low-temperature thermopower S was shown to agree with the law S = AT+BT3 below about 6–8 K (measurement at 1.3 K<T<13 K, superconducting reference).
The diffusion thermopower coefficient A was essentially independent of the concentration of isolated FD or impurities, but changed drastically during FD agglomeration. This behaviour of A parallels that of Ro and demonstrates that the Al bandstructure always remained sufficiently unchanged by the defects. The different values of A or Ro as observed for different defect types are determined only by the different anisotropy (i.e. k→-dependence) of the relaxation time τk. This is all consistent with the Mott-formula for A. Both Ro and -A depended in about the same way on the defect type, but the “wrong” sign of A shows that A is determined by the energy dependence of vkτk (velocity vk) which overcompensates that of the FS area elements dS. Our 4 OPW calculations gave virtually quantitative agreement for Ro but not for A (many body effects?).
The phonon drag thermopower coefficient B behaved totally different for impurities and for FD. For impurities B was again independent of the defect concentration and determined only by the anisotropy of τk, and both Ro and B depended in about the same way on the defect type. This behaviour of B is in qualitative and even semi-quantitative agreement with the Bailyn-formula, and there is no evidence of “phony phonon drag”. In the FD case, however, B was approaching zero as a function of defect concentration and was independent of the anisotropy of τk. This anomalous behaviour obviously has to do with the exceptionally strong phonon scattering on the FD (resonance vibration modes).
KeywordsPoint Defect Fermi Surface Defect Type Defect Concentration Reactor Irradiation
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