Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A3: Experimental Studies of 5/2 Fractional Quantum Hall Effect |
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Sponsoring Units: DCMP Chair: James Eisenstein, California Institute of Technology Room: Ballroom A3 |
Monday, March 21, 2011 8:00AM - 8:36AM |
A3.00001: Observation of neutral modes via shot noise measurements Invited Speaker: Current propagates in the quantum Hall regime along the edges of a two-dimensional-electron gas via chiral edge modes, with chirality dictated by the applied magnetic field. In the fractional regime, for some fractional states - the so called ``holes-conjugate'' states -- e.g., between filling factor 1/2 and 1 - early predictions suggested the presence of counter propagating edge modes: a ``downstream'' mode with the expected chirality and an ``upstream'' mode with an opposite chirality. Since experiments in the ubiquitous 2/3 state did not find upstream propagating edge modes, it had suggested that in the presence of interactions and disorder edge reconstruction may take place with a resultant downstream charge mode accompanied by upstream neutral mode - with the latter carrying only energy - thus explaining why the upstream modes were not detected thus far. Moreover, a neutral upstream Majorana mode is also expected for selected wavefunctions proposed for the even denominator state 5/2. I will review some of our observations of neutral modes in selected quantum Hall states. Neutral mode detection was performed by allowing a chiral mode to impinge on a quantum point contact (QPC) constriction. The partitioning of the neutral mode led to current fluctuations propagating in the downstream chirality. The main following effects that were observed were: (a) Current noise, being proportional to the applied voltage on the injecting contact, without net current; (b) Similarly, partitioning charge current in a QPC led to generation of an upstream neutral mode; (c) The neutral mode decays fast with length and temperature; (d) Having a neutral mode impinge simultaneously with a charge mode affects strongly the Fano factor and the temperature of the partitioned charged quasiparticles; (e) For the 5/2 fractional state, our observation of an upstream neutral mode is likely to single out the proposed reconstructed Pfafian or anti-Pfafian wavefunctions for non-abelian quasiparticles. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 9:12AM |
A3.00002: Competing Phases of 2D Electrons at $\nu$ = 5/2 and 7/3 Invited Speaker: The N=1 Landau level (LL) exhibits collective electronic phenomena characteristic of both fractional quantum Hall (FQHE) states seen in the lowest LL and anisotropic nematic states in the higher LLs. A modest in-plane magnetic field $B_{||}$ is sufficient to destroy the fractional quantized Hall states at $\nu = 5/2$ (and 7/2) and replace them with anisotropic compressible nematic phases, revealing the close competition between the two. We find that at larger $B_{||}$ these anisotropic phases $\nu = 5/2$ can themselves be replaced by a new isotropic state, dubbed re-entrant isotropic compressible (RIC) phase. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands in the confinement potential. In addition, we find that with $B_{||}$, the normally isotropic $\nu = 7/3$ FQHE state can transform into an anisotropic phase with an accurately quantized Hall plateau but an anisotropic longitudinal resistivities. As temperature is lowered towards zero, $\rho_{xx}$ diminishes while $\rho_{yy}$ tends to diverge, reminiscent of the anisotropic nematic states, while surprisingly $\rho_{xy}$ and $\rho_{yx}$ remain quantized at $3h/7e^{2}$, indicating a completely new quantum phase. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:48AM |
A3.00003: Luminescence experiments measuring spin at 5/2 Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:24AM |
A3.00004: Electrostatic Measurements of Fractional Charge in the Second Landau Level Invited Speaker: The fractional quantum Hall state at filling factor 5/2 is predicted to result from a BCS pairing instability in a Fermi sea of composite fermions. The resulting p-wave paired state would have the lucrative property of supporting non-Abelian braiding statistics which can be leveraged for decoherence-free quantum computation. A robust prediction of any theory involving pairing at half-integer filling in the quantum Hall regime is that quasiparticles should have charge e/4. Local compressibility measurements allow us to compare how quasiparticles charge disorder puddles at 7/3 and 5/2. From this comparison, we can extract the ratio of quasiparticle charges for these states. The value we obtain, 4/3, suggests a local charge of e/4 at 5/2 (assuming e/3 at 7/3). We additionally show that these e/4 quasiparticles can be pinned by disorder, a prerequisite for the interferometry measurements that may demonstrate non-Abelian braiding statistics. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 11:00AM |
A3.00005: ``Looking'' at Competing Quantum Phases of the Second Landau Level Invited Speaker: Partially populated higher Landau levels of 2D electron systems support striking collective states in which quantum Hall phases overlap and compete with alternate phases emergent from remarkable interplays between fundamental interactions and quantization of 2D states in a magnetic field. Optical studies by light scattering methods are revealing previously unexpected roles of the spin degree of freedom in quantum phases of the second (N=1) Landau level [1,2]. Inelastic light scattering experiments uncover the collapse of the long wavelength ferromagnetic spin wave for filling factors that are below nu=3. This discovery, interpreted as loss of spin polarization in the N=1 Landau level, is made more intriguing by findings that a sharp spin wave does not recover at filling factors nu=8/3 and 5/2 that support well-known fractional quantum Hall states. Simultaneous resonant elastic (Rayleigh) scattering measurements indicate that below nu=3 the collective states of quasiparticles in the partially populated N=1 Landau level break into sub-micron size domains of fluid that seem to lack full spin polarization and that persist to temperatures that are above 1K. The determination of spin polarization in the quantum Hall state at nu=5/2 requires further consideration. While coexisting with spin unpolarized domains, quasiparticle condensation at nu=5/2 may still result in an incompressible fluid that has spin polarization. This work is in collaboration with T. D. Rhone, U. Wurstbauer, Y. Gallais, J. Yan, L.N. Pfeiffer and K.W. West. \\[4pt] [1] T.D. Rhone et al BAPS.2010.MAR.Y2.3 \\[0pt] [2] T.D. Rhone et al., submitted for publication. [Preview Abstract] |
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