Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V04: Dirac Electron Physics and Nanoscale Scanning Probes of Quantum Dynamics in Graphene: Atomic Defects, Topology and GeometryInvited
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Sponsoring Units: DCMP DMP Chair: Jairo Velasco, University of California, Santa Cruz Room: LACC 151 |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V04.00001: Quantum Critical Transition and Kondo Screening of Magnetic Moments in Graphene Invited Speaker: Eva Andrei In normal metals, the local moment of a magnetic impurity begins to decrease below a characteristic “Kondo temperature” which marks the formation of an entangled state consisting of the moment surrounded by polarized conduction-band electrons that screen it. In contrast, moments embedded in insulators remain unscreened at all temperatures. This raises the question about the fate of magnetic moments in intermediate pseudogap systems, such graphene or high temperature superconductors. We use scanning tunneling microscopy and spectroscopy, and numerical renormalization group calculations to study the screening of local magnetic moments in graphene introduced by creating single-atom vacancies in its honeycomb structure. Identifying Kondo screening of the vacancy spin by its characteristic spectroscopic signature, we detect and map the existence of a quantum phase transition separating magnetic from non-magnetic states. Furthermore, using the unique properties of this phase transition we show that the local magnetic moment can be turned on or off with either a gate voltage or through the local curvature of the graphene membrane. |
Thursday, March 8, 2018 3:06PM - 3:42PM |
V04.00002: Berry phase jumps and giant nonreciprocity in Dirac quantum dots Invited Speaker: Joaquin Rodriguez Nieva Dirac electrons in 2D materials exhibit a Berry phase, an unusual feature induced by the pseudospin which causes the wave function to acquire a measurable phase when system parameters cycle around a closed path. Observing manifestations of the Berry phase via coherent control of quantum state trajectories is experimentally challenging in solid-state systems. In this talk, I will discuss our theoretical proposal to exploit confined Dirac electrons inside circular quantum dots to control and measure the Berry phase of electron orbital states[1]. In particular, we show that weak magnetic fields can be used to control the behavior of electron trajectories and induce a giant non-reciprocal effect driven by the Berry phase. Non-reciprocity is manifest in anomalously large field-induced splittings of quantum dot resonances which are degenerate at B=0 due to time-reversal symmetry. The effect, which is strongest for gapless Dirac particles, overwhelms the magnetic-field-induced orbital and Zeeman splittings. This exotic behavior is unique to quantum dots in Dirac materials and is absent in conventional quantum dots. The non-reciprocity, predicted for a large family of two-dimensional Dirac materials, has recently been observed in scanning tunneling spectroscopy measurements in graphene[2]. The effect is shown to be robust against perturbations of the circular confining potential and, as such, can be exploited in a variety of switchable optoelectronic applications. |
Thursday, March 8, 2018 3:42PM - 4:18PM |
V04.00003: Characterization and Control of Dirac Fermions Within Nanoscale p-n Junctions Invited Speaker: Jairo Velasco Jr. Dirac fermions have coexisting electron-hole states and exhibit angular anisotropy when transmitting through a potential barrier. Because of these attributes, electrostatically confined Dirac fermions are fundamentally different from similarly trapped Schrödinger fermions and are predicted to exhibit new states. Such states have properties that depend on the integrability of their confinement potential and whether they are massless or massive Dirac fermions. Several recent experiments have investigated electrostatically confined states within integrable structures. However, the spatial behavior of states within non-integrable structures and of massive Dirac fermions still remains unexplored. Here, I will discuss two experiments that use scanning tunneling microscopy (STM) to characterize and manipulate Dirac fermions within circular and non-circular pn junctions. These confinement structures were fabricated by using a flexible technique for creating pn junctions on graphene/boron nitride and bilayer graphene/boron nitride heterostructures. First, I will discuss our realization of new gate tunable quantum interference patterns within circular and stadium shaped graphene pn junctions. Secondly, I will discuss our visualization and control of single electron charging events with massive Dirac fermions that are confined within circular bilayer graphene pn junctions. The techniques and findings presented here open the door to highly controlled studies on the ergodicity of confined Dirac fermions—gate tunable Dirac billiards. |
Thursday, March 8, 2018 4:18PM - 4:54PM |
V04.00004: Quantized States, Berry Phases, and Wedding Cakes in Graphene Quantum Dots Invited Speaker: Daniel Walkup The wave nature of quantum mechanics is revealed when the particle’s de Broglie wavelength becomes comparable to the system length scale. Quantum dots (QD) offer an ideal platform for studying the interplay between quantum confinement, caused by spatial constraints or by large magnetic fields via cyclotron motion, and interaction effects. Recently, the ability to apply local nanometer scale gate potentials in graphene heterostructures has enabled the creation of QDs for Dirac particles. Graphene QDs are formed inside circular p-n junction [1,2], where one has detailed control of electron orbits by means of local gate potentials and magnetic fields. In this talk, I describe scanning tunneling spectroscopy measurements of the energy spectrum of graphene QDs as a function of energy, spatial position, and magnetic field. In zero field, the Dirac quasiparticles are confined by Klein scattering at large incident angle at the p-n junction boundary. The confined carriers give rise to an intricate eigenstate spectrum, characterized by radial and angular momentum quantum numbers, creating a multi-electron artificial atom [1]. Applying a weak magnetic field results in a sudden and giant increase in energy for certain angular momentum states of the QD, creating a discontinuity in the energy spectrum as a function of magnetic field [2]. This behavior results from a π-Berry phase, which I show can be turned on and off with magnetic field. With increased applied magnetic field, the QD states are observed to condense into Landau levels, providing a direct visualization of the transition from spatial to magnetic confinement, along with “wedding cake” profiles arising from interaction effects [3]. |
Thursday, March 8, 2018 4:54PM - 5:30PM |
V04.00005: Tunable giant valley splitting in edge-free graphene quantum dots on boron nitride Invited Speaker: Markus Morgenstern Graphene, the first two-dimensional material, provides two extra binary degrees of freedom, sublattice and valley, which are adequately described as pseudospins. These degrees might offer additional possibilities for information processing. Via low-temperature STM measurements, I will demonstrate the control of the valley degree of freedom within an edgeless quantum dot (QD) of graphene on BN. The QD is induced by the potential of the STM tip in combination with an external B field, which provides the required gaps by Landau quantization [1]. As such, the QD can be moved freely across the graphene/BN sample. The laterally changing orientation of the C atoms of graphene with respect to the B and N atoms of hBN changes the valley splitting of the confined state continuously [2]. This eventually leads to a tunable inversion of the valley splitting on nm length scales. Besides, I provide calculations based on density functional theory combined with a tight binding model, which excellently fit the experimental data revealing the possibility to use atomic interactions by adequate stacking of materials to control electronic degrees of freedom. |
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