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Search for the proximity effect in MgB_2 epitaxial films on Mg(0001)

Zwier, O. (2008) Search for the proximity effect in MgB_2 epitaxial films on Mg(0001). Bachelor's Thesis, Physics.

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Magnesium diboride was discovered in 2001 to superconduct at temperatures below 39 K, a very high critical temperature for a phonon-assisted superconductor, which MgB2 has been determined to be. This research tries to determine the relation between the thickness of a sample of MgB2 and its superconductive properties, a relation due to the proximity effect. To produce thin MgB2 films, we have deposited boron, or magnesium and boron simultaneously, on the Mg(0001) single crystal surface by means of molecular beam epitaxy (MBE). The growth took place in an ultra high vacuum (UHV) environment with a base pressure of below 2 x 10-10 mbar. The Mg(0001) substrate was thoroughly cleaned and ordered beforehand by several cycles of sputtering and annealing. In our conditions, the temperature of the substrate during growth must be kept between 498 and 513 K, a temperature range which allows Mg to react with boron and which is sufficiently low not to allow a massive Mg sublimation, which may hinder the MgB2 formation (Mg vapour pressure in UHV:~ 10-9 mbar at 530K). Due to the experimental set-up, we were not able to measure directly the substrate temperature during annealing. As a consequence, during growth, the substrate temperature was increased up to when we observed the appearance of Mg mass in the residual gas analyser. Analysis of the surface lattice geometry was done by Low Energy Electron Diffraction (LEED), growth rate and sample composition were characterized by Auger Electron Spectroscopy (AES) the surface was analyzed by Scanning Tunneling Microscopy (STM) and scanning electron spectroscopy (STS), at room temperature as well as at 4.2 K. The results obtained by this method show that while the stoichiometry of our MgB2 film is correct, our growth procedure is not optimized to reliably prepare large regions of MgB2. Scanning Tunneling Spectroscopy (STS) was also performed to search for the superconductive gap. These measurements, when they were not rendered unusable by noise, did however not show an adequate superconductive gap. Several improvements are suggested to grow MgB2 more successfully. A better control over the temperature of the sample during deposition is required, since the temperature window for epitaxial growth is very narrow. For the measurements by STS it is found imperative that the noise in the signal is minimized before reliable results are possible.

Item Type: Thesis (Bachelor's Thesis)
Degree programme: Physics
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 15 Feb 2018 07:45
Last Modified: 15 Feb 2018 07:45

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