Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H23: Stripe and Bubble Phases in a Two-dimensional Electron Gas: Recent DevelopmentsInvited
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Sponsoring Units: DCMP Chair: Lloyd Engel, NHMFL Room: New Orleans Theater B |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H23.00001: Observation of a pressure-driven quantum phase transition from a fractional quantum Hall state to an electronic stripe state at $\nu=5/2$ Invited Speaker: G\'abor Cs\'athy The nature of the ground state of a two-dimensional electron gas at half-filled Landau level has been a remarkably interesting problem. Depending on the number of filled Landau levels, there are three fundamentally distinct ground states: a composite Fermi liquid in the lowest Landau level, a fractional quantum Hall state in the second Landau level, or an electronic stripe (or nematic) state in high Landau levels. The theory of the half-filled Landau level has recently been reexamined in the limit of exact particle-hole symmetry. This talk will present evidence of a stripe state in the second Landau level at filling factor $\nu=5/2$ in a two-dimensional electron gas under hydrostatic pressure. We also observe a pressure driven phase transition at $\nu=5/2$ from a fractional quantum Hall state to a stripe state. Since the former is a topological phase and the latter a traditional broken symmetry phase, the observed transition is an interesting example of a phase transition from a topologically ordered to a broken symmetry phase. Remarkably, the stripe state develops in the absence of any externally applied symmetry breaking fields. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:42PM |
H23.00002: Reorientation of the Stripe Phase of 2D Electrons by a Minute Density Modulation Invited Speaker: M. A. Mueed One of the most exotic manifestations of many-body interaction in nature is the formation of stripe phases. In such phases, instead of a homogeneous arrangement, electrons prefer to cluster and orient themselves in stripe-like structure and create a density modulation. Stripe phases are observed in strongly correlated systems which include the high-Tc superconductors, strontium ruthenate materials, and very clean two dimensional electron systems (2DESs). Amongst these, the quantum Hall stripe phase, observed in 2DESs, continues to be enigmatic despite years of research. Many of its fundamental properties, such as what determines the stripe orientation, and modulation period and strength are yet to be fully understood. Recently, there has been a surge of experimental studies focusing on the quantum Hall stripe phase. In relevance to this current interest, we report a novel method to probe the structure and energetics of the stripes by imposing a minute density modulation on the 2DES. The topic of our study is the parallel field ($B_{||}$)-induced quantum Hall stripe phase which typically orients itself perpendicular to the $B_{||}$'s direction. When a very small, periodic density modulation is introduced parallel to $B_{||}$, we find that the stripes also reorient parallel to $B_{||}$. The reorientation becomes most pronounced when the periods of the stripe phase and the external modulation are comparable. This phenomenon is remarkable since the external modulation amplitude is expected to be much smaller than that of the stripe phase and yet is sufficient to cause a reorientation. Our data thus suggest that the parallel and perpendicular orientations of the $B_{||}$-induced stripes must be energetically very close. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 4:18PM |
H23.00003: The observation of negative permittivity in stripe and bubble phases Invited Speaker: Jurgen Smet The physics of itinerant two-dimensional electrons is by and large governed by repulsive Coulomb forces. However, cases exist where the interplay of attractive and repulsive interaction components may instigate spontaneous symmetry lowering and clustering of charges in geometric patterns such as bubbles and stripes, provided these interactions act on different length scales. The existence of these phases in higher Landau levels has so far been concluded from transport behavior. Here, we report surface acoustic wave experiments. They probe the permittivity at small wave vector. This technique offers true directionality, whereas in transport the current distribution is complex and strongly affected by the inhomogeneous density pattern. Outside the charge density wave regime, the measured permittivity is always positive. However, negative permittivity is observed in the bubble phase irrespective of the propagation direction. For the stripe phase the permittivity takes on both positive as well as negative values depending on the propagation direction. This confirms the stripe phase to be a strongly anisotropic medium. The observation of negative permittivity is considered an immediate consequence of the exchange related attractive interaction. It makes charge clustering favorable in higher Landau levels where the repulsive direct Coulomb interaction acts on a longer length scale and is responsible for a negative compressibility of the electronic system. This work has been carried out with B. Friess, K. von Klitzing (MPI-FKF), Y. Peng, F. von Oppen (FU Berlin), B. Rosenow (Uni Leipzig) and V. Umansky (Weizmann Institute of Science). [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:54PM |
H23.00004: Orientation of quantum Hall stripes under in-plane magnetic fields Invited Speaker: Qianhui Shi One longstanding mystery about quantum Hall stripes is the origin their native orientation, which is found to be along the $\left < 110 \right >$ crystal axis of GaAs in most cases. An in-plane magnetic field $B_\parallel$ can modify the orientation, and a ``standard picture" was established that stripes perpendicular to $B_\parallel$ are favored in a single-subband quantum well. This talk will discuss recent experiments showing that this ``standard picture" is incomplete. First, a regime for stripes parallel to $B_\parallel$ being the ground state has been established. Second, the orientation of stripes with respect to $B_\parallel$ is shown to depend on the carrier density and the filling factor. Qualitative examination of these findings suggests that screening plays an important role in determining the orientation of stripes. In addition, we find that the reorientations by $B_\parallel$ are sensitive to the partial filling of a given Landau level. The implications on the native orienting mechanism will also be discussed. This work was done in collaboration with M. Zudov, J. Watson, G. Gardner, Q. Qian and M. Manfra. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:30PM |
H23.00005: Nematic Order in Two-Dimensional Electronic Fluids in magnetic Fields Invited Speaker: Eduardo Fradkin I will review the experimental evidence and theoretical underpinnings of nematic order in two-dimensional electronic fluids in magnetic fields where there is strong evidence both in the compressible and in the fractional regimes. I will also comment on the role of nematicity in other systems such as high Tc superconductors and in layered ruthenates. [Preview Abstract] |
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