Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L2: Physics of Interacting Particles in Two Dimensional Electron Systems at HalfFillingInvited

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Sponsoring Units: DCMP Chair: Mansour Shayegan, Princeton University Room: Ballroom II 
Wednesday, March 16, 2016 11:15AM  11:51AM 
L2.00001: Do composite fermions satisfy Luttinger's area rule? Invited Speaker: Jainendra Jain While an ordinary Fermi sea is perturbatively robust to interactions, the paradigmatic compositefermion Fermi sea [1] arises as a nonperturbative consequence of emergent gauge fields in a system where there was no Fermi sea to begin with. A meanfield picture suggests two Fermi seas, of composite fermions made from electrons or holes in the lowest Landau level, which occupy different areas away from half filling and thus appear to represent distinct states. We show [2] that in the microscopic theory of composite fermions, which satisfies particlehole symmetry in the lowest Landau level to an excellent degree, the Fermi wave vectors at filling factors $\nu$ and $1\nu$ are the same, and are generally consistent with the experimental findings of Kamburov {\em et al.} [3]. Our calculations [2] suggest that the area of the CF Fermi sea may slightly violate the Luttinger area rule. We further determine the area of the spin unpolarized compositefermion Fermi sea, for which the Fermi seas at $\nu$ and $1\nu$ are not related by particle hole symmetry. [1] B.I. Halperin, P.A. Lee, N. Read, PRB 47, 7312 (1993). [2] A. C. Balram, C. T\H{o}ke, J. K. Jain, Phys. Rev. Lett. 115, 186805 (2015). [3] D. Kamburov {\em et al.} Phys. Rev. Lett. {\bf 113}, 196801 (2014). [Preview Abstract] 
Wednesday, March 16, 2016 11:51AM  12:27PM 
L2.00002: Spontaneous time reversal symmetry breaking in atomically confined twodimensional impurity bands in silicon and germanium Invited Speaker: Arindam Ghosh Threedimensional bulkdoped semiconductors, in particular phosphorus (P)doped silicon (Si) and germanium (Ge), are among the best studied systems for many fundamental concepts in solid state physics, ranging from the Anderson metalinsulator transition to the manybody Coulomb interaction effects on quantum transport. Recent advances in material engineering have led to vertically confined doping of phosphorus (P) atoms inside bulk crystalline silicon and germanium, where the electron transport occurs through one or very few atomic layers, constituting a new and unique platform to investigate many of these phenomena at reduced dimensions. In this talk I shall present results of extensive quantum transport experiments in deltadoped silicon and germanium epilayers, over a wide range of doping density that allow independent tuning of the onsite Coulomb interaction and hopping energy scales. We find that lowfrequency flicker noise, or the $1/f$ noise, in the electrical conductance of these systems is exceptionally low, and in fact among the lowest when compared with other lowdimensional materials. This is attributed to the physical separation of the conduction electrons, embedded inside the crystalline semiconductor matrix, from the charged fluctuators at the surface. Most importantly, we find a remarkable suppression of weak localization effects, including the quantum correction to conductivity and universal conductance fluctuations, with decreasing doping density or, equivalently, increasing effective onsite Coulomb interaction. Inplane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping although no signatures of long range magnetic order could be identified. We argue that these results indicate a spontaneous breakdown of time reversal symmetry, which is one of the most fundamental and robust symmetries of nonmagnetic quantum systems. While the microscopic origin of this spontaneous time reversal symmetry breaking remains unknown, we believe this indicates a new manybody electronic phase in twodimensionally doped silicon and germanium with a halffilled impurity band. [Preview Abstract] 
Wednesday, March 16, 2016 12:27PM  1:03PM 
L2.00003: The halffilled Landau level and topological insulator surfaces Invited Speaker: T Senthil The metallic state of the halffilled Landau level  described originally in pioneering work by Halperin , Lee, and Read as a liquid of composite fermions  was proposed recently by Son to be described by a particlehole symmetric effective field theory distinct from that in the prior literature. This talk will develop a simple picture of the particlehole symmetric composite fermion through a modification of older pictures as electrically neutral ``dipolar" particles. This picture, and the proposed particlehole symmetric theory, will be further substantiated through a recently developed deep connection between the halffilled Landau level and correlated surface states of certain three dimensional topological insulators. The phenomenology of composite fermi liquids (with or without particlehole symmetry) will be revisited. It will be shown that their heat/electrical transport dramatically violates the conventional WiedemannFranz law but satisfies a modified one. References: 1. Chong Wang and T. Senthil, “Halffilled Landau Level, Topological Insulator Surfaces, and Three Dimensional Quantum Spin Liquids,” condmat arXiv:1507.08290 (2015). [Preview Abstract] 
Wednesday, March 16, 2016 1:03PM  1:39PM 
L2.00004: Experimental Observations of Particlehole Asymmetry for Composite Fermions Invited Speaker: Yang Liu In this talk, I will present our experimental study of the breaking of particlehole symmetry for composite fermions (CFs), quasiparticles formed by attaching two flux quanta to each electron at large perpendicular magnetic fields. We measure the Fermi contour of the spinpolarized CFs near $\nu=1/2$ via commensurability oscillations, and find an asymmetry of the Fermi wave vector for $\nu<1/2$ and $>1/2$. In particular, we find that the deduced wave vector is smaller for $\nu>1/2$ compared to $\nu<1/2$, and on both sides consistent with the density of minority carriers in the lowest Landau level. We also study the spinpolarization transitions of fractional quantum Hall states near $\nu=3/2$ and 1/2; these states are particlehole conjugates of each other and are expected to have the same polarization energies. Our systematic results clearly show the transition energies are about three times larger for states near $\nu=3/2$ compared to those near $\nu=1/2$. Work done in collaboration with D. Kamburov, M. A. Mueed, S. Hasdemir, A. Wojs, J.K. Jain, L.N. Pfeiffer, K.W. West, K.W. Baldwin, and M. Shayegan. [Preview Abstract] 
Wednesday, March 16, 2016 1:39PM  2:15PM 
L2.00005: A model wavefunction for the composite Fermi liquid: its geometry and entanglement. Invited Speaker: F. D. M. Haldane I will describe a model wavefunction for the composite Fermi liquid in a partiallyfilled Landau level, recently formulated in a torus geometry (Shao et al., Phys. Rev. Lett. 114, 206402 (2015)), that allows a manifold of gapless composite Fermiliquid states to be characterized, parametrized by an analog of the ``occupation number'' that defines the Fermi surface in a freeelectron gas. Unlike incompressible FQHE states, which only occur in an inversionsymmetric momentum sector, these CFL states occur in each distinct momentum sector allowed by the periodic boundary condition. The fundamental wavefunction of this type describes a system with $\nu$ = 1/2, but multiplication by (or division by) a Vandermonde factor describes states at $\nu$ = $1/m$. The CFL states are characterized by an ``intrinsic metric" which determines the shape of the Fermi surface, and corresponds to the shape of the ``fluxattachment'' that forms the composite fermion. The wavefunction is wellsuited for MonteCarlo calculations, as it is analogous to a determinant form used by Jain in spherical geometry. The violation of ``arealaw" (perimeterlaw) entanglement found in MonteCarlo calculations will be described. [Preview Abstract] 
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