Quantum Transport Property of Twisted/Bernal Bilayer Graphene

Abstract

The graphene which is one-monolayer of graphite has novel electronic properties such as linear energy dispersion, klein tunneling and half integer quantum Hall effect. In addition, it is transparent and stronger than iron and chemically inert. Thus many physicists and engineers pay attention to graphene research. Moreover double-layer graphene systems have interesting properties. The most important property is tunability of the low energy band structure which is extremely rare example in condensed matter physics. In bernal bilayer graphene, their energy band is tuned by perpendicular electric field and in twisted bilayer graphene, twist angle can change band structure significantly. This thesis consists of two main parts; one is interlayer and intralayer electrical properties on twisted bilayer graphene and another is fractional quantum Hall effect in bernal bilayer graphene. At part 1, we investigate electronic transport property by twisted bilayer graphene. In chapter 3, Interlayer conduction in twisted bilayer graphene is mainly discussed. Using transfer method, we make the graphene cross junction and measure the interlayer resistivity as varying twist angle, gate voltage and temperatures. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite, and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene. In chapter 4, we focus on the multiple degeneracy in twisted bilayer graphene which provides an access to rich quantumHall states (QHS) with broken symmetry, arising from electron-electron interactions and Zeeman splitting. We present quantum Hall effect in high-quality twisted bilayergraphene. At high density regime, we found several QH plateaus are suppressed or emerged with magnetic fields, indicating transitions between different QH states. We ascribe this to imperfect screening of twisted bilayer, which results in different Landau levels formation on each layer and their mixings. As low density regime, odd integer QHS are observed, suggesting an important role of the interlayer tunneling for stabilizing broken symmetry QHS. Moreover, we investigate its twist angle dependence. At the high angle device, simple sum of 2xmonolayer's quantum Hall effect. Whereas at low angle device, their lattice constant is big enough, it can feel the magnetic length. This device leads to observe the fractal quantum Hall effect in twisted bilayer graphene firstly. At part 2, we perform the transconductance fluctuation measurements on bernal bilayer graphene. Transport measurements normally probe macroscopic only averaged properties of the sample. However, recent transconductance fluctuation measurements in a single layer graphene demonstrate that these fluctuations reflect the properties of the localized states in a nano-scale which allow to access the fragile quantum hall states. We report the evidence of the fractional quantum hall states in high-quality bilayer graphene on h-BN using transconductance fluctuation method. The gate dependence at zero magnetic field shows that carrier inhomogeneity is ~ 1x1010 cm-2 both electron and hole. The quantum hall effect developed around 1.5 T and the broken symmetry state appeared at 3 T. Transconductance fluctuation shows rich filling factor slope lines including v = -11/3, -10/3, -4/3, -2/3, -1/5, 1/5, 1/3, 2/5, 2/3, 4/3, 10/3 and the even-denominator state at v = ±1/2. These observations clearly demonstrate that electron interaction in bilayer graphene depends also on magnetic field and the electric field between the layers more significant than any other 2DEG including graphene. Moreover valley degeneracy is will modify the Landau level spectrum and the role of long-range Coulomb interactions, affecting the gap of the fractional quantum Hall states.