Electrically confined one-dimensional conducting channel in bilayer graphene: Difference between revisions

Latest revision as of 07:14, 21 January 2016

Speaker: Janghee Lee (POSTECH)
Date: Thursday February 21, 2016 17:00
Place: Jeongho Seminar Room

The band theory and the Landau-Ginzburg theory have been successful in classifying various quantum states of matter in condensed matter physics. However, the quantum Hall effect, which was discovered in 1980 by Klitzing and collaborators [1], could not be classified in terms of Bloch’s theorem and symmetry breaking. Before long, it was proved that the quantized Hall conductivity corresponds to a Chern number (TKNN number) [2]. After that, the topology has been studied widely in condensed matter physics and become another criterion for classifying materials in terms of topological invariant. Recently, it was suggested that another type of topological state can emerge in bilayer graphene (BLG) [3]. If the band gaps in two adjacent regions in BLG are produced by opposite-directional electric fields, topological one-dimensional (1-D) conducting channel emerges with a chiral nature at the boundary between the two gapped regions in the BLG. In this talk, after brief introduction to topology in condensed matter physics, we explain how the topological zero energy mode emerges in BLG. Then, we present the device fabrication procedure, which consists of novel stacking and transfer methods with sophisticated patterning sequence. Lastly, we show our recent results on transport measurements in BLG, which indicate the formation of a 1-D conducting channel between the two regions in gapped BLG.

@@ Line 1: / Line 1: @@
 * Speaker: [[Lee, Janghee|Janghee Lee]] (POSTECH)
-* Date: Thursday March 31, 2016 17:00
+* Date: Thursday February 21, 2016 17:00
 * Place: Jeongho Seminar Room
-Graphene nanoribbons, a one-dimensional (1D) graphene system, with zigzag edges are predicted to exhibit interesting electronic properties stemming from its Dirac band structure. However, to date, investigation of the properties is highly limited because of the defects and the roughness at the edges, which mix different valleys in graphene. Recent progress in preparing a high-quality graphene layer enables one to investigate the intrinsic carrier transport nature in the material. Here, we report the signature of conservation of valley symmetry in two types of quasi-1D ballistic transport devices; one is a quantum point contact (QPC) and another is an Aharonov-Bohm (AB) interferometer. Devices were fabricated on monolayer graphene with high carrier mobility, where the graphene was encapsulated between two thin hexagonal boron nitride (h-BN) layers. In measurements, charge carriers were confined in a potential well formed by the dual operation of the bottom and top gates and the four-terminal magnetoconductance (MC) was measured with varying the charge carrier density, dc bias, and temperature. Graphene in the device was in the ballistic regime, exhibiting the conductance quantization in steps of  delta G = 4e^2/h starting from G = (2, 6), 10 e^2/h in a constricted conducting channel of QPC-type devices. This behavior is similar to the one observed in zigzag graphene nanoribbons having edge localized channels. Our tight-binding calculation shows that quasi-1D charge flow on a graphene plane acts a zigzag-type nanoribbon, unless it is perfectly aligned along the armchair direction. In the AB interferometry, we observed h/e periodic modulation of MC and the zero-field conductance minimum with a negative MC background. All these results strongly suggest that qausi-1D channels built in our devices preserve the intrinsic Dirac transport nature of carriers with its valley symmetry, which would be conveniently utilized for valleytronics in graphene.
+The band theory and the Landau-Ginzburg theory have been successful in classifying various quantum states of matter in condensed matter physics. However, the quantum Hall effect, which was discovered in 1980 by Klitzing and collaborators [1], could not be classified in terms of Bloch’s theorem and symmetry breaking. Before long, it was proved that the quantized Hall conductivity corresponds to a Chern number (TKNN number) [2]. After that, the topology has been studied widely in condensed matter physics and become another criterion for classifying materials in terms of topological invariant. Recently, it was suggested that another type of topological state can emerge in bilayer graphene (BLG) [3]. If the band gaps in two adjacent regions in BLG are produced by opposite-directional electric fields, topological one-dimensional (1-D) conducting channel emerges with a chiral nature at the boundary between the two gapped regions in the BLG. In this talk, after brief introduction to topology in condensed matter physics, we explain how the topological zero energy mode emerges in BLG. Then, we present the device fabrication procedure, which consists of novel stacking and transfer methods with sophisticated patterning sequence. Lastly, we show our recent results on transport measurements in BLG, which indicate the formation of a 1-D conducting channel between the two regions in gapped BLG.
 [[Category: Seminars]]
 [[Category: Young Scientists Seminars]]