Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R22: Unification of Topological Insulators and the Halffilled Landau LevelInvited

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Sponsoring Units: DCMP Chair: Olexei Motrunich, California Institute of Technology Room: New Orleans Theater A 
Thursday, March 16, 2017 8:00AM  8:36AM 
R22.00001: Electricmagnetic duality of topological insulators Invited Speaker: Max Metlitski Recently, we have seen unexpected connections emerge between three seemingly unrelated problems: i) Strongly correlated surface states of 3d topological insulators, ii) Quantum spinliquids with emergent photon excitations, iii) Quantum Hall (QH) fluid at filling factor $\nu = 1/2$. These connections have lead to a dual description of the single Dirac cone on the topological insulator surface, given by quantum electrodynamics (QED3) with a single Dirac cone coupled to a fluctuating gauge field. The dual Dirac fermion has the interpretation of a double vortex in the electronic fluid, and the duality is akin to the celebrated particlevortex duality of bosons. A striking consequence of the duality is a novel theory of the composite fermion liquid state of the QHfluid at $\nu$ = 1/2, where the composite fermion is a Dirac fermion. This theory resolves a 20 year old puzzle regarding the fate of particlehole symmetry at $\nu = 1/2$ and is strongly supported by recent DMRG simulations. [Preview Abstract] 
Thursday, March 16, 2017 8:36AM  9:12AM 
R22.00002: Particlehole symmetry and the Dirac composite fermion Invited Speaker: Dam Son Composite fermion is a central notion in quantum Hall physics. For a long time, field theories of the composite fermion suffers from the lack of particlehole symmetry, an exact symmetry of the lowest Landau level. I will describe how recent theoretical ideas have lead to a new understanding of the physics of the halffilled Landau level. According to the new picture, the composite fermion at half filling is a Dirac particle with a nontrivial pi Berry phase around the Fermi surface. Consequences of the new proposal are outlined. [Preview Abstract] 
Thursday, March 16, 2017 9:12AM  9:48AM 
R22.00003: Dirac composite fermions in the halffilled Landau level Invited Speaker: Roger Mong Recent works have speculated on a possible particlehole symmetric state in the lowest Landau level at halffilling. Here, we use infinite cylinder DMRG to provide compelling numerical evidence in favour for the theory. We demonstrate the existence of a Fermi surface of composite fermions for realistic interactions between electrons at halffilling. Using both theoretical and numerical arguments, we find that the composite fermions are massless Dirac particles, at finite density, similar to the surface state of a 3D topological insulator. Exploiting this analogy we devise a numerical test and successfully observe the suppression of $2k_F$ backscattering characteristic of Dirac particles. [Preview Abstract] 
Thursday, March 16, 2017 9:48AM  10:24AM 
R22.00004: Measurements of Composite Fermion Geometric Resonance Invited Speaker: Mansour Shayegan There has been a surge of recent interest in the physics of interacting, twodimensional (2D) electrons in a large perpendicular magnetic field when they occupy a halffilled Landau level. The long ago proposed composite fermion (CF) picture, in which two magnetic flux quanta are bound to each electron to form a CF, explains many properties of the system. These include the compressible (metallic) behavior of the 2D system at filling factor $\nu =$ 1/2, the existence of a Fermi contour with a welldefined Fermi wave vector, and the presence of fractional quantum Hall states as the filling deviates from $\nu =$ 1/2 In this talk, I will highlight the results of several recent experiments that probe the presence and properties of CFs via measuring the geometric resonance of CFs' cyclotron orbit diameter with the period of an imposed, unidirectional density modulation. The data reveal several important aspects: (1) An unexpected \textit{asymmetry} of the CFs' Fermi wave vector for filling factors smaller and larger than $\nu =$ 1/2, suggesting a subtle breaking of particlehole symmetry. (2) \textit{Anisotropic} Fermi contours for CFs that can be tuned by applying inplane magnetic field or inplane strain. (3) The intriguing presence and behavior of CFs on the \textit{flanks of bilayer quantum Hall states} at filling factors $\nu =$ 1 and 1/2, presumably the $\Psi_{\mathrm{111}}$ and $\Psi _{\mathrm{333}}$ states. Surprisingly, near $\nu =$ 1, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a twocomponent behavior. In contrast, close to $\nu =$ 1/2, CFs appear singlelayerlike (singlecomponent) as their resonance features correspond to the total density. (Work done in collaboration with M.A. Mueed, D. Kamburov, I. Jo, Yang Liu, H. Deng, Md. Shafayat Hossain, L.N. Pfeiffer, K.W. West, and K.W. Baldwin. [Preview Abstract] 
Thursday, March 16, 2017 10:24AM  11:00AM 
R22.00005: A Duality Web in 2 $+$ 1 Dimensions and the Unity of Physics \newline Invited Speaker: Nathan Seiberg A combination of ideas originating from Condensed Matter physics, Supersymmetric Field Theory, and AdS/CFT has led to a detailed web of conjectured dualities.~ These relate the long distance behavior of different short distance theories.~ These dualities clarify a large number of confusing and controversial issues in Condensed Matter physics and in the study of 2$+$1 dimensional quantum field theory. [Preview Abstract] 
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