Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H1: DCMP Prize Session |
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Sponsoring Units: DCMP Chair: Julia Phillips, Sandia National Laboratories Room: Colorado Convention Center Four Seasons 2-3 |
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H1.00001: Beyond the Quantum Hall Effect: New Phases of 2D Electrons at High Magnetic Field Invited Speaker: In this talk I will discuss recent experiments on high mobility single and double layer 2D electron systems in which collective phases lying outside the usual quantum Hall effect paradigm have been detected and studied. For example, in single layer 2D systems near half-filling of highly excited Landau levels new states characterized by a massive anisotropy in the electrical resistivity of the sample are observed at very low temperature. The anisotropy has been widely interpreted as the signature of a new class of correlated electron phases which incorporate a stripe-like charge density modulation. Orientational ordering of small striped domains at low temperatures accounts for the resistive anisotropy and is reminiscent of the isotropic-to-nematic phase transition in classical liquid crystals. \\ \\ Double layer 2D electron systems possess collective phases not present in single layer systems. In particular, when the total number of electrons in the bilayer equals the degeneracy of a single Landau level, an unusual phase appears at small layer separation. This phase possesses a novel broken symmetry, spontaneous interlayer phase coherence, which has a number of dramatic experimental signatures. The interlayer tunneling conductance develops a strong and very sharp resonance around zero bias resembling the dc Josephson effect. At the same time, both the longitudinal and Hall resistances of the sample vanish at low temperatures when currents are driven in opposite directions through the two layers. These, and other observations are broadly consistent with theories in which the broken symmetry phase can equivalently be described as a pseudospin ferromagnet or an (imperfect) excitonic superfluid. \\ \\ This work reflects a collaboration with M.P. Lilly, K.B. Cooper, I.B. Spielman, M. Kellogg, L.A. Tracy, L.N. Pfeiffer, and K.W. West. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H1.00002: Off-Diagonal Long-Range Order and Collective Excitations in the Fractional Quantum Hall Effect Invited Speaker: The experimental discovery of the fractional quantum Hall effect was a stunning surprise which came to be understood in terms of a novel state of matter in which strongly correlated electrons acquire a new and unprecedented type of collective quantum order. The mystery of superconductivity was first understood macroscopically in terms of Ginsburg-Landau effective theory before the microscopic BCS theory was developed. Here the historical order was reversed. Laughlin discovered his essentially exact microscopic wave function and only subsequently did we begin to understand its implications in terms of a new type of off-diagonal long-range order and an effective Chern-Simons field theory for composite particles carrying magnetic flux. In this gauge theory, the fact that Laughlin's quasi-particle excitations carry sharply quantized fractional charge could be understood as analogous to sharp flux quantization in a superconductor. The fact these vortex excitations have finite energy could be understood as the result of magnetic screening by the gauge field. In addition to discussing this macroscopic picture of the FQHE, I will also discuss the microscopic wave function that Allan MacDonald and I developed in collaboration with Phil Platzman which accurately describes the gapped magneto-phonon and magneto-roton collective excitations of the system. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:48AM |
H1.00003: Oliver E. Buckley Prize Talk Invited Speaker: |
Tuesday, March 6, 2007 9:48AM - 10:24AM |
H1.00004: Atom Chains at Surfaces: A Playground for Low-Dimensional Physics Invited Speaker: One-dimensional physics is particularly elegant because of its mathematical transparency. However, it is not easy to realize a one-dimensional system experimentally. Using self-assembly techniques, it has become possible to produce atomic chain structures at silicon surfaces and to control their dimensionality, their band filling, and their magnetic moment [1]. The atoms are locked to the surface, but metallic electrons are de-coupled from the substrate due to the band gap of silicon. In a sense, these are the ultimate nanowires, each consisting of a single chain of orbitals. Angle-resolved photoemission reveals surprising features, such as a fractional band filling [2], a spin-splitting at a non-magnetic surface [3], and the one-dimensional analog of stripes (alternating metallic and semiconducting sections). \newline \newline [1] Crain and Himpsel, Appl. Phys. A \textbf{82}, 431 (2006). \newline [2] Crain et al., Phys. Rev. Lett. \textbf{90}, 176805 (2003). \newline [3] Barke et al. Phys. Rev. Lett., in press (2006). [Preview Abstract] |
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