Bulletin of the American Physical Society
Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020; Virtual
Session J05: Quantum Materials QHE&Graphene |
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Chair: Nikolai Zhitenev, NIST |
Sunday, December 6, 2020 9:00AM - 9:36AM |
J05.00001: Achievement of a new higher electron mobility plateau for GaAs quantum wells Invited Speaker: Loren Pfeiffer Two-dimensional electrons confined to GaAs quantum wells are hallmark platforms for probing electron-electron interaction. Many key observations were made in these systems as sample quality improved over the years. However, progress in quality has been stagnant for over a decade. We present a major breakthrough via source-material purification and innovation in GaAs molecular beam epitaxy vacuum chamber design. Our new samples have a world-record mobility of 44 \textunderscore 106 cm2/Vs at an electron density of 2:0 \textunderscore 1011 /cm2; this is the highest mobility observed in any material. These results imply only \textunderscore 1 residual impurity for every 1010 Ga/As atoms. The impact of such low impurity concentration is extraordinary; several new fractional quantum Hall states emerge, and exotic phases such as the 5$=$2 state, which is widely believed to be non-Abelian and of potential use for topological quantum computing, and stripe/bubble phases are unprecedentedly robust. ~ [Preview Abstract] |
Sunday, December 6, 2020 9:36AM - 9:48AM |
J05.00002: Hall viscosity of the composite-fermion Fermi seas for fermions and bosons Songyang Pu The Hall viscosity has been proposed as a topological property of incompressible fractional quantum Hall states and can be evaluated as Berry curvature. This paper reports on the Hall viscosities of composite-fermion Fermi seas at $\nu=1/m$, where $m$ is even for fermions and odd for bosons. A well-defined value for the Hall viscosity is not obtained by viewing the $1/m$ composite-fermion Fermi seas as the $n\rightarrow \infty$ limit of the Jain $\nu=n/(nm\pm 1)$ states, whose Hall viscosities $(\pm n+m)\hbar \rho/4$ ($\rho$ is the two-dimensional density) approach $\pm \infty$ in the limit $n\rightarrow \infty$. A direct calculation shows that the Hall viscosities of the composite-fermion Fermi sea states are finite and relatively stable with system size variation. However, they are not topologically quantized in the entire $\tau$ space. I find that the $\nu=1/2$ composite-fermion Fermi sea wave function for a square torus yields a Hall viscosity that is expected from particle-hole symmetry and is also consistent with the orbital spin of $1/2$ for Dirac composite fermions. I compare my numerical results with some theoretical conjectures. [Preview Abstract] |
Sunday, December 6, 2020 9:48AM - 10:24AM |
J05.00003: Edge Channels of Quantum Hall States in Graphene probed by Atomic Force Microscopy Invited Speaker: Nikolai Zhitenev The quantum Hall (QH) effect, a topologically non-trivial quantum phase, expanded and brought into focus the concept of topological order in physics. The bulk topology is closely related to the properties of edge states that are of crucial importance to the QH effect. The QH edge states in graphene take on an even richer role as graphene is distinguished by its four-fold nearly-degenerate zero energy Landau level (zLL). The symmetry of zLL is broken by minute lattice-scale potentials enhanced by electron interactions. A particular symmetry of ground state is associated with the distinct dispersion of edge states, however the spatial dispersion has been measured with limited success. In this report, we map the quantum Hall broken-symmetry edge states comprising the graphene zLL at integer filling factors 0 and \textpm 1 across the quantum Hall edge boundary using atomic force microscopy (AFM). Measurements of the chemical potential resolve the energies of the four-fold degenerate zLL as a function of magnetic field. This dependence suggests an interplay of the moir\'{e} superlattice potential of the graphene/boron nitride system and spin/valley symmetry-breaking effects. [Preview Abstract] |
Sunday, December 6, 2020 10:24AM - 11:00AM |
J05.00004: STM-based fabrication and characterization of quantum dots in graphene Invited Speaker: Daniel Walkup When a gate-tunable two-dimensional electron system (2DES) lies atop a flake of hexagonal boron nitride (hBN), charged defects in the hBN can modify the gating locally, creating a potential landscape for the 2DES’s mobile electrons. By pulsing the voltage of a scanning tunneling microscope (STM) tip we create controlled, semi-permanent pockets of these charged defects on the scale of tens of nanometers, inducing corresponding charge puddles in the 2DEG. Here we study monolayer graphene, where Klein tunneling inhibits confinement except in the presence of a Landau-quantization-inducing magnetic field. In such fields the charge pockets are transformed into concentric series of compressible and incompressible rings, corresponding to partially- and fully-filled Landau levels respectively. In this regime the incompressible rings act as tunnel barriers within the graphene, isolating the compressible rings and producing clear Coulomb blockade peaks in dI/dV spectra observed in spectroscopic imaging STM, alongside the usual features due to local density of states. This produces a new and phenomenologically rich quantum dot system in which serial single-electron charging of the dots is tunable via back gate and sample bias voltages, as well as the position of the STM tip. We investigate this system in two basic regimes: (1) where two partially-filled Landau levels compete within a single charge pocket, the tunnel barrier between them remaining intact, and (2) when two adjacent pockets are subject to charging, and the tunnel barrier between two pockets of the same Landau level is tunable, the dots merging when they become sufficiently large. [Preview Abstract] |
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