Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G45: Fractional Quantum Hall Effect: Second Landau Level |
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Sponsoring Units: FIAP Chair: Gabor Csathy, Purdue University Room: Mile High Ballroom 4D |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G45.00001: Low-Energy Excitations in the Second LL: Fundamental Insights from Inelastic Light Scattering Ursula Wurstbauer, Aron Pinczuk, Antonio L. Levy, John Watson, Sumit Mondal, Michael J. Manfra, Ken West, Loren Pfeiffer The competition between quantum phases that dictates the physics in the second Landau level (SLL) results in striking phenomena. Our work explores this fascinating interaction physics by measurements of low-lying neutral excitation modes in the SLL from resonant inelastic light scattering experiments. We focus here on the marked differences of the low-lying collective excitation spectra of the even-denominator state at $\nu$=5/2 with those in the range 5/2>$\nu$>2. Filling factor 5/2 is characterized by the presence of gapped modes, a spin mode exactly at $E_{Z}$ and the absence of a continuum of low-lying excitations. In contrast, a continuum of low-lying excitations and gapped modes are coexistent at $\nu$=2+1/3, 2+3/8 and 2+2/5 and the spin-modes appear significantly below $E_{Z}$. All observed modes weakens with smallest variations in filling factor substantiating the transition from an incompressible quantum Hall fluid to compressible states. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G45.00002: Optical Emission from quantum phases of the second Landau level Antonio Levy, Ursula Wurstbauer, Aron Pinczuk, John Watson, Sumit Mondal, Michael Manfra, Ken West, Loren Pfeiffer Optical emission across the host semiconductor bandgap has proven a powerful tool in examining the properties fractional quantum Hall sates (fqhs). While the luminescence of fqhs in the first (N$=$0) Landau Level has been extensively studied, there are significantly fewer studies of the optical emission in the N$=$1 Landau Level. We report studies of luminescence in the filling factor range 4\textgreater nu\textgreater 2 N$=$1 Landau level. The marked dependence on filling factor suggests that optical emission is here linked to competing quantum phases. A comparison of luminescence in a range about $\nu =$7/3 with extensively studied optical emission near $\nu =$1/3 creates venues to explore the competing quantum phases of the second Landau levels. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G45.00003: Disorder matters in tilt magnetic field induced anisotropy in the $\nu =5/2$ fractional quantum Hall effect W. Pan, Xiaoyan Shi, K.W. Baldwin, K.W. West, L.N. Pfeiffer, D.C. Tsui It is known that under a moderate in-plane magnetic field ($B_{\parallel}$) the even-denominator fractional quantum Hall effect (FQHE) at the Landau level filling $\nu=5/2$ is destroyed and becomes anisotropic. However, in recent two reports, it was observed that this tilt magnetic field induced anisotropy depends on GaAs crystallographic directions. Electronic transport becomes anisotropic when $B_{\parallel}$ is parallel to [$\bar{1}10$] but remains isotropic if $B_{\parallel}$ parallel to [110]. In this talk, we report a systematic tilt-magnetic field study of the $\nu=5/2$ FQHE in a series of high quality GaAs/Al$_x$Ga$_{1-x}$As heterostructure samples, in which the level of disorder is varied continuously by changing the setback distance (\textit{d}) between the modulation doping layers and the GaAs quantum well. We observed that in highly disordered samples electronic transport is anisotropic in one crystallographic direction but remains more or less isotropic in the other direction, consistent with the recent two reports. In contrast, in less-disordered samples, where \textit{d} is large, electronic transport is anisotropic in both crystallographic directions. Our results clearly show that disorder matters in tilt magnetic field induced anisotropy in the 5/2 FQHE. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G45.00004: The $\nu=5/2$ Fractional Quantum Hall State in presence of Alloy Disorder Nianpei Deng, Geoffrey Gardner, Sumit Mondal, Ethan Kleinbaum, Michael Manfra, Gabor Csathy Disorder plays a prominent role in the formation and the strength of all fractional quantum Hall states, including the one forming at filling factor $\nu=5/2$. Many aspects of the disorder are, however, poorly understood. At the root of this lack of understanding one often finds our inability to control and characterize disorder. We have investigated the effect of a specific type of disorder, alloy disorder, on the $\nu=5/2$ state. The alloy disorder is controllably introduced into the electron channel by growing a series of Al$_{0.24}$Ga$_{0.76}$As/Al$_x$Ga$_{1-x}$As/Al$_{0.24}$Ga$_{0.76}$As quantum well samples with different aluminum molar fraction $x$ using Molecular Beam Epitaxy techniques. We find a suppression of the energy gap of $\nu=5/2$ state with increasing $x$. To our surprise, we observe a fully quantized $\nu=5/2$ state in an extremely low mobility regime in which, based on existing data, we did not expect the $\nu=5/2$ state to develop. Such a result indicates that $\nu=5/2$ state is unusually robust to the short-ranged alloy disorder. This work was supported by the DOE BES contract no. DE-SC0006671. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G45.00005: Development of high quality, density-tunable two-dimensional electron gases for the study of the quantum Hall effect in the 2$^{\mathrm{nd}}$ Landau level John Watson, Sumit Mondal, Michael Manfra We report on progress in state-of-the-art high mobility two-dimensional electron gases (2DEGs) in 30 nm GaAs/AlGaAs quantum wells in which the density is modulated by an in-situ grown back-gate. Such in-situ gates can be grown close to the 2DEG ($\sim$ 1 $\mu$m) and without doping layers between the 2DEG and gate, resulting in non-hysteretic gating with a very uniform electric field and large gate capacitance. We discuss heterostructure design parameters and device processing conditions leading to low gate leakage currents, low ohmic contact resistances, high electron mobilities (17 x 10$^{6}$ cm$^{2}$/Vs), and large fractional quantum Hall energy gaps in the second Landau level. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G45.00006: Proliferation of upstream neutral modes in the fractional quantum Hall regime Hiroyuki Inoue, Anna Grivnin, Yuval Ronen, Moty Heiblum, Vladimir Umansky, Diana Mahalu The fractional quantum Hall effects (FQHE) are canonical examples of topological phases, resulting from correlations among planar electrons under strong perpendicular magnetic field. Chargeless energy transport, in the form of upstream (anti-chiral) neutral edge modes, were recently observed in the \textit{hole-conjugate} FQHEs (filling $\nu $ of the $n^{\mathrm{th}}$ Landau level in the range $n+$1/2\textgreater $\nu $\textit{\textgreater n}$+$1, with $n=$0, 1, 2) as well as $\nu =$5/2. These modes had been predicted to appear due to edge reconstructions by Coulomb interaction and random tunneling among multiple channels. Here we report highly sensitive shot noise measurements that reveal, unexpected theoretically, the presence of such upstream neutral modes in \textit{electron-like} FQHEs such as $\nu =$1/3, 2/5, etc$.$ Furthermore, we also found neutral bulk modes that propagate through the incompressible bulk; though weaker than the edge modes. The proliferation of such neutral modes detected only in FQHEs drastically changes the accepted picture of the transport therein. Moreover, our observation may shed new light on a source of decoherence, which prevented thus far a definite observation of quantum interference of fractional quasiparticles. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G45.00007: Anomalous Insulating States in Landau Levels $N \geq 1$ Talbot Knighton, Jian Huang, Zhe Wu, Loren Pfeiffer, Ken West Quantum Hall measurements are performed for a rectangular two-dimensional (2D) hole system confined to a 20 nm quantum well in $\langle 100 \rangle$ GaAs. Quantum oscillations reveal a density of $4.3 \cdot 10^{10}$ cm$^{-2}$ with mobility $\mu = 1.9\cdot10^{6}$ cm$^2$/V$\cdot$s. For temperatures less than $\sim$150 mK, anomalous insulating peaks are observed near integer fillings 1,2, and 3 for which both in-phase and out-of-phase signals rise substantially to be near or well above the quantum resistance. They differ from usual re-entrant insulating phases such as that observed before $\nu = 1/3$ where the out-of-phase signal remains less than 3\% of the in-phase signal. The relationship between in-phase and out-of-phase signals of the magnetoresistances resembles that of the orthogonal components $\rho_{xx}$ and $\rho_{yy}$ previously observed for collective anisotropic states in $\langle 100 \rangle$ and $\langle 311 \rangle$ GaAs 2D systems. These non-monotonic phase shifts will be discussed in relation to possible stripe phases. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G45.00008: Flip Chip: Keep pristine materials clean Arjan Beukman, Fanming Qu, Leo Kouwenhoven We introduce the Flip Chip setup, a new platform which allows nanoscale electrical gating of a material without exposing it to invasive nanoprocessing. The conventional fabrication of metallic gate structures on pristine materials degrades their interesting properties, e.g., ultra-high mobility in GaAs heterostructures. This research takes a new approach to keep the material unaffected. The gate structure is fabricated on a separate chip which is flipped and brought close (\textless 100 nm) to the sample under research. A vacuum gap between the gates and material, acting as an insulating layer, solves the problem of leakage and trapped charges in traditional gate-dielectrics. In addition, this approach allows more freedom in fabrication methods, as the `dirty' processing is done on a separate chip. With the Flip Chip setup we intend to study the 5/2 FQH state in GaAs heterostructures which is expected to have non-Abelian statistics. I will present measurement results for an interferometer at integer quantum Hall states using the Flip Chip technique. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G45.00009: Theoretical Investigation of Edge Reconstruction in $\nu=5/2$ and $7/3$ Fractional Quantum Hall States Yuhe Zhang, Yinghai Wu, Jimmy Hutasoit, Jainendra K. Jain We study the possibility of edge reconstruction for the $\nu=5/2$ fractional quantum Hall (FQH) state in a realistic geometry which includes the background positive charge at a distance $d$ and the lower filled Landau level. For this purpose, we diagonalize the second Landau level Coulomb interaction within the Pfaffian basis of edge excitations, and find the range of setback distance d where edge reconstruction occurs. We also study the edge of the $\nu= 7/3$ FQH system with composite fermion diagonalization method and find the edge reconstruction occurs more easily at $\nu= 7/3$ than at $\nu= 1/3$. We also ask how edge reconstruction affects the exponent associated with tunneling into the edge. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G45.00010: Study of Quasiholes in FQHE for different Landau Levels R.N. Bhatt, Sonika Johri, Zlatko Papic, Peter Schmitteckert We study the sizes of elementary quasiholes in $\nu=1/3$ and $\nu=7/3$ quantum Hall states. Calculations are carried out by exact diagonalization and density matrix renormalization group method for spherical and cylindrical geometries. We use both short- and long-range pinning potentials to localize the quasihole [Johri et. al arXiv:1310.2263]. The size of the quasihole in the model Laughlin state is estimated to be around $\approx 2.5\ell_B$, where $\ell_B$ is the magnetic length . In contrast, the size of the quasihole in the Coulomb ground state at filling factor $\nu=1/3$ is $\approx 4\ell_B$, while that at $\nu=7/3$ is $\approx 7\ell_B$. Our results support the earlier findings by Balram et. al [PRL {\bf 110}, 186801 (2013)] that the $e/3$ quasihole in the first excited Landau level is significantly larger than in the lowest Landau level. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G45.00011: Probing non-unitarity in fractional quantum Hall states Nicolas Regnault, Benoit Estienne, Andrei Bernevig Recent developments[1,2,3] have proposed an exact matrix product state representation of a large series of fractional quantum Hall states. The model states include all paired $k=2$ Jack polynomials, such as the Moore-Read and Gaffnian states, as well as the Read-Rezayi $k=3$ state. We will discuss how this approach provides some new insight on the pathological features on the non-unitarity states such as the Gaffnian. [1] M.P. Zaletel and R.S. K. Mong, Phys. Rev. B 86, 245305 (2012). [2] B. Estienne, Z. Papic, N. Regnault, B. A. Bernevig, Phys. Rev. B 87, 161112(R) (2013). [3] B. Estienne, N. Regnault, B. A. Bernevig, arXiv:1311.2936. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G45.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G45.00013: New topological excitations in quantum Hall systems Yuli Lyanda-Geller, Tsuging Lin, George Simion, John D. Watson, Michael J. Manfra, Gabor Csathy, Leonid Rokhinson We discover new topological excitations of two dimensional electrons in the quantum Hall regime. The strain dependence of resistivity observed experimentally is shown to change sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the known symmetry of electron bubbles thought to be responsible for the RIQHE. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Calculations confirm that textures lower the energy of excitations. In the two-electron bubble crystal these textures form two-dimensional hedgehogs around defects having one extra electron, and vortices around defects lacking one electron. Strain affects vortices and hedgehogs differently, explaining striking strain-dependent resistivity. The sharp transition from insulating RIQHE state to conducting state is caused by melting of Abrikosov crystal comprised of the defects. The proposed physical mechanism of conductivity due to topological defects is shown to lead to an unusually large magnitude of the strain effect on resistivity in the range of RIQHE filling factors, in agreement with experiment. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G45.00014: New melting transition in Quantum Hall systems George Simion, Tsuging Lin, John D. Watson, Michael J. Manfra, Gabor Csathy, Leonid Rokhinson, Yuli Lyanda-Geller We discover a new melting transition caused by topological excitations of two dimensional electrons in the quantum Hall regime. Experimentally, strain dependence of resistivity changes sign upon crossing filling-factor-specified boundaries of reentrant integer quantum Hall effect (RIQHE) states. This observation violates the symmetry of electron bubble crystal, whose melting was thought to be responsible for insulator to metal transition in the range of RIQHE filling factors. We demonstrate theoretically that electron bubbles become elongated in the vicinity of charge defects and form textures of finite size. Textures lower the energy of excitations. In the two-electron bubble crystal these textures form hedgehogs (vortices) around defects having (lacking) one extra electron. At low density these textures form an insulating Abrikosov lattice. At densities sufficient to cause the textures to overlap, their interactions are described by the XY-model and the defect lattice melts. This explains the sharp metal-insulator transition observed in finite temperature conductivity measurements. In this regime, melting is a function of several variables and forms a continuous phase boundary in the field-temperature ($B-T$) plane. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G45.00015: Superconducting analogue of the parafermion fractional quantum Hall states Abolhassan Vaezi Read and Rezayi $Z_k$ parafermion wavefunctions describe $\nu= 2 + k/(kM+2)$ fractional quantum Hall (FQH) states. These states support non-Abelian excitations from which protected quantum gates can be designed. However, there is no experimental evidence for these non-Abelian anyons to date. In this talk, we discuss the $\nu=2/k$ FQH-superconductor heterostructure and through analytical and numerical calculations we argue that it can yield the superconducting analogue of the $Z_k$ parafermion FQH state. The resulting topological state has a gapless chiral edge state with $Z_k$ parafermion conformal field theory description. For instance, we find that a $\nu= 2/3$ FQH in proximity to a superconductor produces a $Z_3$ parafermion superconducting state. This state can host Fibonacci anyons capable of performing universal quantum computation through braiding operations. We finally discuss our experimental proposal for realizing parafermion superconductors. Reference: arXiv:1307.8069 [Preview Abstract] |
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