University of Twente Student Theses


Towards controlling the fermi energy in topological materials

Slöetjes, Sam (2014) Towards controlling the fermi energy in topological materials.

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Abstract:P-wave superconductivity can arise in topological insulator/s-wave superconductor hybrid systems. In this thesis it is theoretically investigated if this can also happen if the topological insulator is replaced by a metal with spin-orbit coupling. The analysis is done assuming a fully transparant barrier between the two materials, using techniques from BCS theory. It is found that the presence of the spin-orbit coupling term in the Hamiltonian gives an order parameter that has a full p-wave pairing symmetry. Experimentally, transport measurements have been done on 50 and 100 nm thick flakes of the topological insulator material Bi1.5Sb0.5Te1.7Se1.3. A back gate voltage was applied to observe if the Fermi energy can be controlled. From ρxx(T) measurements it is found that the surface contribution to conductivity is influenced by the back-gate. Measurements of ρxx as a function of applied gate voltage show an increasing resistance with a decreasing gate voltage, but no sign of the Dirac point. The Hall resistivity was measured for gate voltages of -21, 0 and 21 V and the approximate carrier density and mobility were found. The carriers are found to be n-type and the carrier density increases with increasing gate voltage. The mobility decreased for both positive and negative gate voltages, indicating that the gate voltage dependence in the resistivity measurements is not only the result of a modulated Fermi energy.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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