# Ultra-low electron temperatures in nanostructured samples

Casparis, Lucas. Ultra-low electron temperatures in nanostructured samples. 2015, PhD Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_11137

After optimizing the chip socket and improving the filtering in the system, an electron temperature of 5.2 mK $\pm$ 0.3 mK in a CBT is measured after demagnetization. By measuring the temperature dependent I-V curves of a normal metal/insulator/superconductor (NIS) tunnel junction, we implement yet another thermometer, which we employ as both primary and secondary thermometer. On top of that, we demonstrate with the help of reentrant features in the fractional quantum Hall regime, cooling of electrons in a high mobility GaAs two-dimensional electron gas (2DEG) below the base temperature of our dilution refrigerator.
Using our low electron temperatures, we investigate high mobility GaAs 2DEG devices in large magnetic fields. In our samples the typical signature of the quantum Hall effect is dramatically altered, resulting in a quantized longitudinal resistance. We can show that this quantization, which occurs only at the lowest temperatures, is due to a large electron density gradient in the 2DEG. As we show subsequently for the $\nu$=5/2 fractional quantum Hall state, the electron density gradient heavily influences the extraction of the energy gap between the ground and excited state. Being a candidate for one of the above mentioned topologically non-trivial ground states, our findings could have important consequences for the fabrication of $\nu=5/2$ fractional quantum Hall state samples.
Additionally, we measure the electrical resistance anisotropy in both natural graphite and highly ordered pyrolytic graphite (HOPG), comparing macroscopic samples, with exfoliated, nanofabricated specimens of nanometer thickness. In nanoscale samples, independent on the graphite type, we find a very large c-axis resistivity $\rho_c$ -- much larger than expected from simple band theory -- and non-monotonic temperature dependence. This is similar to macroscopic HOPG, but in stark contrast to macroscopic natural graphite. A recent model of disorder-induced delocalization is consistent with our transport data. Furthermore, Micro-Raman spectroscopy reveals clearly reduced disorder in exfoliated samples and HOPG, as expected within the model.