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Correlated electron phenomena in 2D materials
Dissertation   Open access

Correlated electron phenomena in 2D materials

Joseph G. Lambert
Doctor of Philosophy (Ph.D.), Drexel University
Feb 2016
DOI:
https://doi.org/10.17918/etd-6679
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Abstract

Semiconductors--Characterization Nanostructures Quasiparticles (Physics) Physics
In this thesis, I present experimental results on coherent electron phenomena in layered two-dimensional materials: single layer graphene and van der Waals coupled 2D TiSe₂. Graphene is a two-dimensional single-atom thick sheet of carbon atoms first derived from bulk graphite by the mechanical exfolia- tion technique in 2004. Low-energy charge carriers in graphene behave like massless Dirac fermions, and their density can be easily tuned between electron-rich and hole-rich quasiparticles with electrostatic gating techniques. The sharp interfaces between regions of different carrier densities form barriers with selective transmission, making them behave as partially reflecting mirrors. When two of these interfaces are set at a separation distance within the phase coherence length of the carriers, they form an electronic version of a Fabry-Perot cavity. I present measurements and analysis of multiple Fabry-Perot modes in graphene with parallel electrodes spaced a few hundred nanometers apart. Transition metal dichalcogenide (TMD) TiSe₂ is part of the family of materials that coined the term "materials beyond graphene". It contains van der Waals coupled trilayer stacks of Se-Ti-Se. Many TMD materials exhibit a host of interesting correlated electronic phases. In particular, TiSe₂ exhibits chiral charge density waves (CDW) below TCDW ~ 200 K. Upon doping with copper, the CDW state gets suppressed with Cu concentration, and CuxTiSe₂ becomes superconducting with critical temperature of Tc = 4.15 K. There is still much debate over the mechanisms governing the coexistence of the two correlated electronic phases - CDW and superconductivity. I will present some of the first conductance spectroscopy measurements of proximity coupled superconductor- CDW systems. Measurements reveal a proximity-induced critical current at the Nb-TiSe₂ interfaces, suggesting pair correlations in the pure TiSe₂. The results indicate that superconducting order is present concurrently with CDW in pure TiSe₂, contradicting the notion of a competition between two correlated electron states.

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