A transport study of emerging phenomena in bilayer graphene nanostructures

Restricted (Penn State Only)
Author:
Li, Jing
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 07, 2017
Committee Members:
  • Jun Zhu, Dissertation Advisor
  • Jun Zhu, Committee Chair
  • Chaoxing Liu, Committee Member
  • Nitin Samarth, Committee Member
  • Saptarshi Das, Outside Member
Keywords:
  • bilayer graphene
  • valleytronics
  • quantum Hall effect
  • quantum valley Hall
  • kink state
  • effective mass
  • quantum Hall edge tunneling
Abstract:
Since the advent of graphene, it has attracted lots of attention in the scientific community because of its outstanding mechanical, optical, thermal and electrical transport properties. Its remarkably high carrier mobility in a wide range of temperature makes it sui generis, to date the mean free path in h-BN encapsulated graphene devices can be as large as 10 μm. In addition, Dirac fermions in graphene possess extra pseudospin and valley degree of freedom, this makes graphene unique compared with conventional semiconductor 2DEGs. Charge carriers in bilayer graphene are viewed as massive Dirac fermions, they inherit novel properties, e.g. high mobility, pseudospin and valley degree of freedom from graphene. Moreover, bilayer graphene supports an up to 250 meV band gap tunable by a perpendicular electric field. Besides, the electric field serves as an efficient knob to manipulate the valley degree of freedom, and this makes bilayer graphene an ideal platform to implement new types of valley-based electronics (valleytronics). This motivates the experimental studies done in this dissertation. In this dissertation, six electrical transport studies in bilayer graphene are summarized in three chapter. (1) The electric field tunable band gap in bilayer graphene are obtained using thermal activation measurements with high precision and in a large field range. This provides precise energy scales in other bilayer graphene studies, e.g. calculating Landau level (LL) energies described in Chapter 5 (in Chapter 2). (2) We carefully measured the electrons and holes effective mass (m*) in bilayer graphene using temperature-dependent Shubnikov-de Haas oscillations, and observed a strong suppression in holes m* at low carrier density. This study reveals a surprising and unusual effect of disorder on m* that is unique to gapless 2D materials (in Chapter 2). (3) We experimentally demonstrated the theory predicted valley-momentum locked conducting channels (kink states) in the line junction of two oppositely gated bilayer graphene using a dual-split-gated device structure (in Chapter 4). (4) We obtained ballistic kink states (4 e2/h conductance) with improved sample quality, and further demonstrated a few valleytronics operations, e.g. valley valve, waveguide and electron beam splitter, by constructing multiple kink states with controllable helicity in a dual-quad-split-gated bilayer graphene device. This is the first experimental realization of a valleytronic device with valley based operational capabilities (in Chapter 4). (5) We built an empirical LL diagram for the E = 0 octet in bilayer graphene in the presence of both perpendicular electric field and magnetic field, using measured band gap values and coincident-points D field for the ν = 0 state up to 31 T. This study provides a unified and intuitive framework which can interpret many experimental observations in literature and offers a good base for future experimental and calculation studies (in Chapter 5). (6) We demonstrated gate-controlled transmission of quantum Hall edge states in bilayer graphene, where perfect transmission and sequential pinch-off of edge states were observed by controlling the tunnel junction potential with a gate. This study is the first demonstration of controllable transmission of quantum Hall edge states in graphene systems, and is a starting point for designing more sophisticated structures, e.g. interferometers, to study exotic quasiparticle statistics in the novel even denominator fractional quantum Hall states in bilayer graphene (in Chapter 5). This dissertation also present device fabrication details, e.g. efforts on making clean h-BN encapsulated graphene stacks, fabricating sub-100 nm nanostructures on h-BN substrate and precise alignment of top and bottom gates. We also discuss how COMSOL simulations can guide the designing of gates in nanostructures (in Chapter 3).