Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A04: Quantum Hall States at EvenDenominator FillingInvited

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Sponsoring Units: DCMP Chair: Ravindra Bhatt, Princeton University Room: LACC 151 
Monday, March 5, 2018 8:00AM  8:36AM 
A04.00001: Effect of band anisotropy on the Fermi contour of composite fermions at half filling Invited Speaker: Matteo Ippoliti Fractional quantum Hall states possess a geometric degree of freedom arising from the competition between the geometry of the band structure and that of Coulomb interactions, which in general can be different. In the composite Fermi liquid (CFL) state at filling nu=½, this degree of freedom determines the shape of the Fermi sea of composite fermions (CFs). 
Monday, March 5, 2018 8:36AM  9:12AM 
A04.00002: Quantization of Heat Flow in Fractional Quantum Hall States Invited Speaker: Mitali Banerjee Quantum mechanics sets an upper bound on the amount of charge flow as well as on the amount of heat flow in ballistic onedimensional channels. The two relevant upper bounds, that combine only fundamental constants, are the quantum of the electrical conductance, G_{e}=e^{2}/h, and the quantum of the thermal conductance, G_{th}=π^{2}k_{B}^{2}T/3h. Remarkably, the latter does not depend on particles charge; particles exchange statistics; and even the interaction strength among the particles. 
Monday, March 5, 2018 9:12AM  9:48AM 
A04.00003: Landau Level Mixing and the Ground State of the ν=5/2 Quantum Hall Effect Invited Speaker: Edward Rezayi One of the most intriguing fractional Hall states was discovered at the halffilled first excited Landau level at total filling of 5/2. It is the strongest evendenominator gapped topological phase that has been observed in a single layer system. Numerical studies consistently indicate that the MooreRead (Pfaffian) state is the underlying phase of the 5/2effect. As a result, the interest in the 5/2effect was intensified due to the exotic property of nonAbelian exchange statistics of the lowenergy excitations. This talk will review these studies that include transitions to other phases at 5/2 filling, which lack topological order. In most of the initial numerical studies the interLandaulevel excitations were ignored. Otherwise the particlehole symmetry is spoiled. Under these circumstances there are two equally likely yet topologically distinct phases of matter for the 5/2effect. The two competing states are the Pfaffian and its particlehole conjugate the socalled antiPfaffian. The ubiquity of the interLandaulevel transitions (or mixing) in experiment breaks particlehole symmetry of an isolated halffilled Landau level and will favor one or the other. More recent studies that addressed mixing effects yielded, depending on the method of approach, opposite predictions. The discrepancy, however, will be shown to disappear if sufficient numbers of 3body pseudopotentials are included in the effective Hamiltonian. 
Monday, March 5, 2018 9:48AM  10:24AM 
A04.00004: Even denominator fractional quantum Hall effects in high quality bilayer graphene Invited Speaker: Andrea Young I will discuss experiments probing the ground states of bilayer graphene in graphite and hexagonal boron nitride encapsulated devices, focusing on even denominator filling factors within the eightfold quasidegenerate lowest Landau level. We observe robust fractional quantum Hall states at half filling whenever the ground state is fully polarized within the first excited Landau (N=1) orbital of conventional quantum Hall systems, and compressible composite fermi liquid states when the ground state is polarized within the N=0 orbital. Thermodynamic and thermal activation measurements show that the energy gap for incompressible 1/2 states are large, reaching several Kelvin, and numerical simulations taking into account details of the bilayer graphene band structure support a Pfaffian ground state. We use electric fields to drive an orbital polarization transition between N=0 and N=1 orbitals at half filling. Direct measurement of the valley polarizability shows the existence of an unexpected intermediate phase with a charge gap but gapless neutral excitations. 
Monday, March 5, 2018 10:24AM  11:00AM 
A04.00005: Possible Nematic to Smectic Phase Transition in a TwoDimensional Electron System at HalfFilling Invited Speaker: Michael Manfra Liquid crystal phases can exist in twodimensional electron systems (2DES) subjected to a perpendicular magnetic field in a halffilled Landau level in the quantum Hall regime. Theory predicts the existence of a lowsymmetry liquid crystal smectic phase that breaks both rotational and translational symmetries. However, previous experiments in 2DES are most consistent with the presence of an anisotropic nematic phase that breaks rotational symmetry but preserves longrange translational symmetry. We report three novel transport phenomena at half filling that indicate a change in symmetry at lowest temperatures in ultralow disorder 2DES: a nonmonotonic temperature dependence of the sample resistance R_{xx}, dramatic onset of large timedependent fluctuations in R_{xx}, and a sharp feature in the differential resistance reminiscent of depinning observed in other crystalline electronic states. Taken together, these data suggest that a sequence of symmetrybreaking phase transitions occurs as temperature is lowered at half filling: first a transition from an isotropic liquid to a nematic phase and finally to a liquid crystal smectic phase. 
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