University of Minnesota
School of Physics & Astronomy

Academic Calendar

Friday, July 14th 2017
2:30 pm:
Thesis Defense in 110 PAN
Speaker: Han Fu, University of Minnesota
Subject: Electron transport in SrTiO3 accumulation layers and semiconductor nanocrystal films
This is the public portion of Ms. Fu's Thesis Defense. Her advisor is Boris Shklovskii

We start from the low temperature conductivity of electron accumulation layers induced by the very strong electric fi eld at the surface of STO sample. Due to the strongly nonlinear lattice dielectric response, the three-dimensional density of electrons n(z) in such a layer decays with the distance from the surface z very slowly as n(z) ~1/z^{12/7}. We show that when the mobility is limited by the surface scattering, the contribution of such a tail to the conductivity diverges at large z because of growing time electrons need to reach the surface. We explore truncation of this divergence by the fi nite sample width, by the bulk scattering rate, by the back gate voltage, or by the crossover to the bulk linear dielectric response with the dielectric constant \kappa. As a result we arrive at the anomalously large mobility, which depends not only on the rate of the surface scattering, but also on the physics of truncation. Similar anomalous behavior is found for the Hall factor, the magnetoresistance, and the thermopower.

Finally, we switch to the study of NC films. We focus on the variable-range hopping of electrons in semiconductor NC fi lms below the critical doping concentration nc at which fi lms become metallic. The hopping conductivity is then described by the Efros-Shklovskii law which depends on the localization length of electrons. We study how the localization length grows with the doping concentration n in the film of touching NCs. For that we calculate the electron transfer matrix element t(n) between neighboring NCs when NCs touch by small facets. We use the ratio of t(n) to the disorder-induced NC level dispersion to fi nd the localization length of electrons due to the multi-step elastic co-tunneling process.

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