Transport Properties of Bi2Te3 and Proximity Effect with Aluminum Superconductors
Examensarbete för masterexamen
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|Type: ||Examensarbete för masterexamen|
|Title: ||Transport Properties of Bi2Te3 and Proximity Effect with Aluminum Superconductors|
|Authors: ||Ekström, Maria|
|Abstract: ||Topological insulators are materials with an insulating bulk and a gap-less metallic
surface. On the surface the energy dispersion is linear and described by an odd number
of Dirac cones. The interest for these materials was renewed recently when room temperature
topological insulators among the bismuth compounds was discovered. Intensive
research in the last years is focusing on observing the signatures of the topological surface.
However, it is di cult to isolate from the bulk and the e ects observed can have
alternative interpretations. So far the surface states have not been totally distinguished
from the bulk. Therefore topological insulators need further characterization and this
thesis is a part of that research.
The two main focuses were to characterize the transport properties of molecular
beam epitaxy grown Bi2Te3 thin lms and Bi2Se3 single crystal with Hall e ect and
proximity e ect. The Hall measurements of Bi2Te3 were used as feedback to the growers
in collaboration to achieve better quality thin lms. The Bi2Te3 showed negative
charge carriers and the volume carrier concentration was improved from 1x1021 cm3 to
4.4x1018 cm8. The mobility was improved from 150 cm2/Vs to 5500 cm2/Vs. For Bi2Se3
samples the typical values were 1.3 x 1019 cm3 and 5100 cm2/Vs, which was comparable
with the best Bi2Te3 lms.
The properties of the topological insulators Bi2Te3 and Bi2Se3 were also investigated
using proximity induced superconductivity in Josephson junctions and superconducting
quantum interference devices with aluminum contacts at temperatures down to 20mK.
The Josephson coupling was con rmed by the response in microwave radiation and magnetic
eld. The height of the observed steps corresponded well to integer Shapiro steps.
The response of the devices in magnetic eld showed expected Fraunhofer patterns, where
the e ective areas for both the Josephson junctions and the superconducting quantum
interference devices was in good agreement with the design. In addition the temperature
dependence of the junctions was examined and evaluated in the clean and dirty
regimes. The critical current scaled with the temperature, according to simulations of
the resistively shunted junction model.
To further characterize the Bi2Te3 thin lm topography and spectroscopy was measured,
describing the roughness of the lm and indicating a Dirac cone around 200mV.|
|Issue Date: ||2020|
|Publisher: ||Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2)|
|Collection:||Examensarbeten för masterexamen // Master Theses|
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