Quantum transport in ballistic graphene devices
Abstract
Graphene is a zero gap 2-D semiconductor having chiral charge carriers described by the massless relativistic Dirac-like Hamiltonian. In this thesis, unique transport properties that emerge from this energy spectrum are studied by using ballistic graphene and coupling its charge carriers with superconducting pair potentials and electrostatic gates. Superconducting correlations can be induced in graphene by bringing it in contact with a superconductor. This superconducting proximity effect (PE) provides a way of exploring phenomena such as pseudo-diffusive dynamics of ballistic carriers, specular Andreev reflections and unconventional quantum Hall effect with Andreev edge states. Hitherto, experimental realizations were limited by diffusive devices coupled to superconductors with low critical fields. In the first part of this work, in order to study these phenomena, we develop ballistic suspended graphene (G)-Niobium type–II superconductor(S) Josephson junctions. Our devices exhibit long mean free paths, small potential fluctuations near the charge neutrality point (CNP) and transparent S-G interfaces that support ballistic super currents. In such a device, when the gate voltage is tuned very close to the CNP, unlike in diffusive junctions, we observe a strong density dependence of the multiple Andreev reflection features and normalized excess current. The observations qualitatively agree with a longstanding theoretical prediction for emergence of evanescent mode mediated…
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