Elektronischer Transport in Wismutkontakten atomarer Größe

Abstract

The electronic properties of Bismuth are the most and longest investigated but least understood in solid state physics. In 1900 Eichhorn studied the resistance behavior of Bismuth in a changing magnetic field. Pyotr Leonidovich Kapitza dicovered the orbital magneto resistance in Bismuth at room temperature in 1928. The story of detected or investigated effects in Bismuth and the involved researchers seems to be the "who is who" of solid state physics of the beginning of the 20th century.Because of its extremely small Fermi surface, the big Fermi wavelength of carriers and the enormeous mean free path it is possible to investigate effects in Bismuth which have been seen years after in normal metals. For example the Fermi surface of Bismuth was the first ever measured by Shoenberg via de-Haas-van-Alphen-effect. The investigations of material and electronic properties of metals in micro, nano or atomic scales in resent years revealed new interesting results in Bismuth. In macroscopic samples the behavior of the semimetal Bismuth is between metal and semiconductor. On the nanoscale its properties may be shifted in the one or the other direction depending whether it is an epitaxial or ^grained film or the transport is investigated in mono or polycristalline nanowires you will see superconducting, semiconducting or metallic behavior. This could be simply modified by dimensions of the investigated structure. Onereason for this behavior is the big Fermi wavelength of about 50 nm. Still it is not clear what the transport properties of contacts smaller than this length are. Of big interest is the conductance of an atomic sized contact and its behavior with changing magnetic field, bias voltage and temperature. Two measurements by Rodrigo et al. and Costa-Krämer et al. are contradictory in their results, both using the same STM technique. Investigation of Krans and van Ruitenbeek on Antimony contacts; a very similar material to Bismuth; approved Rodrigos results using an other technique: mechanically controlled breakjunctions (MCB) used in this work, too.The first goal of this work was to investigate the electronic transport properties of atomic sized Bismuth contacts using MCB at low temperatures. The second was using the methode shown by Scheer et al. to measure the number and the transmission of the transport channels of an atomic Bismuth contact. Therefore the conductance behavior of Aluminum-Bismuth and pure Bismuth samples depending on magnetic field, temperature and bias voltage was investigated. Thereby the results of Rodrigo et al. could be confirmed.This thesis is structred as followes: Chapter 1 introduces the expected effects and their theoretical explanation, the important parameters and the known results on Bismuth nanocontacts. Chapter 2 presents the properties of bulk Bismuth. Chapter 3 will present the measurement setup, the sample preparation and the other techniques used. Chapter 4 will introduce the measured samples and the measurement results which will be concluded in the last chapter of this thesis.