Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V43: Pairing and Nematicity in Half-filled Landau LevelsInvited
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Sponsoring Units: DCMP Chair: Raymond Ashoori, Massachusetts Institute of Technology Room: BCEC 210B |
Thursday, March 7, 2019 2:30PM - 3:06PM |
V43.00001: The role of electron-electron interactions in the paired-to-nematic transition Invited Speaker: Gabor Csathy The problem of the ground state of a two-dimensional electron gas at half-filled Landau levels has been a remarkably interesting one. Depending on the number of filled Landau levels, besides the Fermi liquid ground state there are two well-known ordered phases: paired fractional quantum Hall states in the second Landau level and the electronic nematic (or stripe) phases in high Landau levels. Nonetheless, the presence of both of these ordered phases and therefore a transition between them did not seem possible in the absence of an externally applied symmetry breaking field favoring the nematic. This state of affairs has changed recently with the observation a pressure-driven quantum phase transition between the ν=5/2 fractional quantum Hall state, a paired state, and the nematic phase. Here we provide evidence of a similar quantum phase transition at filling factor ν=7/2 and discuss the role of the hydrostatic pressure in driving the transition. Our work highlights universal aspects of the competition of pairing and nematicity present in various condensed matter systems as well as in very clean two-dimensional electron gases. Work done in collaboration with Katherine Schreiber, Nodar Samkharadze, Y. Lyanda-Geller, G. Gardner, M.J. Manfra, L.N. Pfeiffer, and K.W. West. |
Thursday, March 7, 2019 3:06PM - 3:42PM |
V43.00002: Even-denominator fractional quantum Hall states in bilayer graphene Invited Speaker: Cory Dean TBD |
Thursday, March 7, 2019 3:42PM - 4:18PM |
V43.00003: Competing correlated phases in orbitally mixed half-filled Landau levels Invited Speaker: Joseph Falson An astounding array of correlated phases have been experimentally realized in high quality two-dimensional electron systems (2DES). By exploiting the quantizing effects of a magnetic field, these systems form an ideal platform for studying the competition between compressible, incompressible and nematic liquids owing to a tunable Landau level index (N) which influences the nature of the ground state. Here, I will present data taken on state-of-the-art MgZnO/ZnO heterostructures that offer the ability to tune N while remaining at a constant Landau level filling. By rotating the sample within a magnetic field, it is possible to selectively decouple the Zeeman and cyclotron energy terms and induce a level crossing between opposing spin branches of the neighboring N=1 and 0 levels. In contrast to the naive expectation of a first-order spin flop transition, an unexpectedly complex cascade of phases is resolved as we incrementally shift the spin and orbital polarization of carriers between levels while remaining at ν=5/2 filling. Transport signatures associated with two instances of incompressibility are observed, in addition to two compressible phases and an unanticipated anisotropic phase which breaks rotational symmetry. Our experiments indicate that the depolarization process is gradual and complex. In addition to level polarized states, the unexpected incompressible and anisotropic phases observed in the orbitally mixed regime open questions concerning the possibility of novel interlevel coherence occurring at fractional fillings in systems with strongly mixed levels. |
Thursday, March 7, 2019 4:18PM - 4:54PM |
V43.00004: Competing charge density waves probed by non-linear transport and noise in the second and third Landau levels Invited Speaker: Guillaume Gervais Competing quantum ground states are ubiquitous in strongly-interacting many-body systems. One prime example is the competition between Wigner crystallization and fractional quantum Hall (FQH) liquids that occurs deep in the first Landau level (LL) of the two-dimensional electron gas (2DEG). In higher LLs, the situation is much more complex, where charge density waves (CDWs) in the form of stripe and bubble phases are also competing with incompressible FQH liquids. In this talk, I will present our results [1] concerning the competition of CDWs and FQH in the second and third LL of an ultra-high mobility GaAs/AlGaAs 2DEG, as probed by both non-linear electronic transport and noise measurements. These measurements were performed in a Corbino geometry to ensure that only bulk properties of the 2DEG were probed, thereby precluding any contribution coming from edge states. Sliding transport of CDWs was revealed by narrow-band noise in re-entrant quantum Hall states of the second LL identified by the Csathy group [2], as well as in pinned CDWs of the third LL. The combining of noise data with maps of conductivity versus magnetic field and bias voltage revealed a complex competition occurring between various quantum phases in the form of stripes, pinned CDWs and fractional quantum Hall liquids, and is consistent with recent results from the Manfra group for the stripe phase suggesting a transition between a smectic and nematic phase at finite temperatures [3]. |
Thursday, March 7, 2019 4:54PM - 5:30PM |
V43.00005: Energy, Momentum, and Spin-resolved tunneling spectra of quantum Hall systems Invited Speaker: Heun Mo Yoo Tunneling spectroscopy has an unique power in probing strong electronic correlations of many-body states in a solid. Here, we introduce a novel contactless pulsed tunneling technique that can visualize the energy, momentum, and spin-resolved electronic structure of a quantum Hall effect system. Unlike conventional planar tunneling that requires in-plane conductivity of the system, the pulsed tunneling method functions on strongly insulating systems at exact integer or fractional quantum Hall states. Furthermore, through use of pulses that drive tunneling in the extremely short time intervals, the technique eliminates perturbations such as heating effects or photo-excited defects that commonly occur in other methods. Using the pulsed tunneling technique, we visualize the evolution of discrete quantization of energy levels as well as the effect of electron-optic phonon interactions in energy-momentum space [1]. In addition to momentum and energy resolution, we performed spin-resolved tunneling that probes the ground-state spin polarization of the fractional quantum Hall states in a wide range of magnetic fields and filling factors. Moreover, we can detect the spin-dependent high energy states arising from the strong pair interactions. From these high energy features, we measure Haldane’s pseudopotentials that enabled us to directly determine the stability of the composite Fermi sea in a half-filled Landau level. These results illustrate the potentially broad applicability of the pulsed tunneling technique for studying the correlated electronic phases in a variety of two-dimensional materials. |
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