Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P01: Fractional Quantum Hall Effect: New Systems |
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Sponsoring Units: DCMP Chair: Robert Willett, Bell Labs Room: BCEC 106 |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P01.00001: Single photon Chern insulator in superconducting microwave lattices Clai Owens, Brendan Saxberg, Ruichao Ma, Jonathan Simon, David Schuster We present the latest progress in developing a novel architecture for exploration of topological quantum matter. We construct microwave photonic lattices from tunnel-coupled, time-reversal-broken microwave cavities that are both low loss and compatible with Josephson junction-mediated photon-photon interactions, allowing us access to topological phenomena such as the fractional quantum Hall effect. We employ seamless 3D microwave cavities all machined from a single block of niobium, so our meta-material is scalable and directly compatible with the cQED toolbox, as it is composed only of niobium for the cavities, plus Yttrium-Iron-Garnet (YIG) spheres and Neodymium magnets to produce the synthetic magnetic field. After observing topologically protected chiral edge states with microsecond lifetimes circling the superconducting lattice, we now push to couple tunable qubits to the lattice in order to make lattice sites nonlinear and create particle interactions. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P01.00002: Cu[Li_{1/3}Sn_{2/3}]O_{2} and Cu[Na_{1/3}Sn_{2/3}]O_{2}: Crystal Structure and Phonon Dispersion in Quaternary Honeycomb Delafossites Faranak Bahrami, Mykola Abramchuk, Oleg I. Lebedev, Olle Hellman, Natalia E. Mordvinova, Jason W. Krizan, Kenneth Metz, David A Broido, Fazel Fallah Tafti Magnetic materials with spin-1/2 ions on a two dimensional honeycomb lattice are candidates for a Kitaev spin liquid phase. The study of non-magnetic honeycomb delafossites provides a basis to model the physical properties of magnetic honeycomb materials. Here, we report two new quaternary non-magnetic delafossites, Cu[Li_{1/3}Sn_{2/3}]O_{2} and Cu[Na_{1/3}Sn_{2/3}]O_{2}, with honeycomb ordering synthesized via a topotactic cation-exchange reaction. The monoclinic C2/c space group and twinned stacking faults are determined from the Rietveld refinement and TEM analysis. Additionally, the phonon heat capacity for these non-magnetic layered oxides is captured accurately by first principles calculations. These results are an important step toward having a reliable model to calculate phonon contribution to the heat capacity in magnetic delafossites^{1}. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P01.00003: Observation of Chiral Surface Excitons in a Topological Insulator Bi_{2}Se_{3 } Hsiang-Hsi Kung, Adamya P Goyal, Dmitrii Maslov, Xueyuen Wang, Alexander Lee, Alexander Kemper, Sang-Wook Cheong, Girsh E Blumberg Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by collective modes during exciton thermalization. Here on the contrary, we observe almost perfectly polarization-preserving PL peak centered at 2.3 eV from the surface of an archetypical three-dimensional topological insulator (TI), Bi_{2}Se_{3}. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that the observed PL can be semi-quantitatively explained by composite particles – chiral excitons – formed by the Coulomb attraction between massless (Dirac) electrons and massive holes, both subject to strong spin-orbit coupling which locks their spins and momenta into chiral textures. We experimentally demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies between 2.5 to 2.8 eV, and that the orientation remains preserved even at room temperature. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P01.00004: Spin wave transport through electron solids and fractional quantum Hall liquids in graphene Haoxin Zhou, Hryhoriy Polshyn, Takashi Taniguchi, Kenji Watanabe, Andrea Young In monolayer graphene, the interplay of electronic correlations with the internal spin- and valley- degrees of freedom leads to a complex phase diagram of isospin symmetry breaking at high magnetic fields. Recently, Wei et al. (Science (2018)) demonstrated that spin waves can be electrically generated and detected in graphene heterojunctions, allowing direct experiment access to the spin degree of freedom. Here, we apply this technique to high quality graphite-gated graphene devices showing robust fractional quantum Hall phases and isospin phase transitions. We use an edgeless Corbino geometry to eliminate the contributions of edge states to the spin-wave mediated nonlocal voltage, allowing unambiguous identification of spin wave transport signatures. Our data reveal two phases within the ν = 1 plateau. For exactly ν=1, charge is localized but spin waves propagate freely while small carrier doping completely quenches the low-energy spin-wave transport, even as those charges remain localized. We identify this new phase as a spin textured electron solid. We also find that spin-wave transport is modulated by phase transitions in the valley order that preserve spin polarization, suggesting that this technique is sensitive to both spin and valley order. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P01.00005: Bounding the excitation gap of the Laughlin and other model incompressible fractional quantum Hall (FQH) states. Frederick D Haldane The Laughlin and other model FQH states have the property that (a) they are exact eigenstates (and are highest-density kernel or zero-energy eigenstates) of model "pseudopotential" Hamiltonians, and (b) are related to (chiral) conformal blocks of (Euclidean) 2D conformal field theories (cft). These model Hamiltonians have a non-trivial kernel spanned by a basis of quasi-hole states related to a chiral cft. What is still lacking is information about the non-kernel (quasiparticle) eigenstates of these models, and the persistence of the energy gap in the thermodynamic limit.They have analogies to the AKLT spin-chain model, for which such questions have been answered. The question of proving (or disproving) a lower bound to the spectral gap separating kernel and non-kernel states becomes more interesting because certain of these models (e.g., the "Gaffnian") are related to a non-unitary cft which is now believed to mean that they have gapless bulk excitations in the thermodynamic limit, and perhaps represent critical states at continuous transitions between topologically-distinct FQH states. I will discuss these fundamental questions, and review strategies and progress towards answering them. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P01.00006: Formation of helical domain walls in the fractional quantum Hall regime as a step toward realization of high-order non-Abelian excitations Tailung Wu, Zhong Wan, Aleksandr Kazakov, Ying Wang, Leonid Rokhinson It has been proposed that parafermion excitations can emerge in the fractional quantum Hall effect (FQHE) regime if two counter-propagating chiral edge states with fractionally charged excitations and opposite polarization are brought into close proximity in the presence of superconducting coupling.In this work we present formation of conducting channels at boundaries between incompressible polarized and unpolarized$\nu =2/3$states in FQHE regime.We demonstrate that in a triangular quantum well polarizations of a 2DEG can be locally controlled by electrostatic gating at a filling factor$\nu=2/3$.Conductance of channels formed at a boundary of polarized and unpolarized phases does not depend significantly on the magnetic field direction, indicating formation of helical domain walls.Such local control of polarization allows formation of a reconfigurable network of domain walls and can provide a platform for parafermion manipulation and braiding when superconductivity is proximity-induced into these domain walls. |
Wednesday, March 6, 2019 3:42PM - 3:54PM |
P01.00007: Particle-Hole Symmetric Pfaffian Kiryl Pakrouski, Frederick D Haldane, Edward H Rezayi, Jie Wang, Kun Yang A recent experiment of Banerjee et al. [Nature 559, 205 (2018)] measured the thermal Hall conductance to be 5/2 for the ν=5/2 fractional quantum Hall effect. This observation is compatible with the particle-hole symmetric Pfaffian topological order where the chirality of the Majorana edge mode is reversed with respect to that of the Pfaffian. The particle-hole symmetric Pfaffian has not appeared as a viable candidate for the stable gapped phase at ν=5/2 in any numerical calculations for a disorder free system. We construct the particle-hole symmetric Pfaffian wavefunction in sphere and torus geometries for a number of system sizes and study its properties and further investigate the prospect of a particle-hole symmetric Pfaffian topological phase in clean systems. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P01.00008: Parent-Hamiltonians for any Jain composite fermion state Sumanta Bandyopadhyay, Gerardo Ortiz, Zohar Nussinov, Alexander Seidel Jain's composite fermion (CF) picture is a remarkable generalization of Laughlin's wavefunction, explaining a great many observed quantum Hall plateaux and associated low energy spectra in a unified approach. A solvable local Hamiltonian that stabilizes these states, however, is lacking in almost all cases, unlike for Laughlin states and many other seemingly more complicated (non-Abelian) fractional quantum Hall states. Here we resolve this issue by departing from the usual construction scheme of quantum Hall parent Hamiltonians, which emphasizes analytic clustering properties. Based on a recent recursion relation for unprojected CF states, and an associated operator algebra of ``zero mode generators'', we construct a class of universal two-body pseudo-potentials in the presence of Landau level mixing that can be used to construct local parent Hamiltonians uniquely stabilizing Jain states for any filling fraction of the form n/(2 p n+1). The structure of the resulting zero mode spaces is governed by an ``entangled Pauli principle'', and zero mode counting agrees with the expected (multiple branch) free chiral boson edge conformal field theory. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P01.00009: Study of the inelastic length under microwave photo-excitation in the GaAs/AlGaAs 2D electron system Rasanga Samaraweera, Binuka Gunawardana, Tharanga Nanayakkara, C. Rasadi Munasinghe, U. Kushan Wijewardena, Sajith Withanage, Annika Kriisa, Christian Reichl, Werner Wegscheider, Ramesh Mani Magnetotransport measurements in ultra-high mobility GaAs/AlGaAs 2D electron systems (2DES) exhibit a narrow negative magnetoresistance (MR) effect around the null magnetic field, which is similar in appearance to the well-known weak localization (WL) effect in metals, semiconductors, etc. However, the origin of WL-like effect in high mobility GaAs/AlGaAs 2DES specimens is still under debate. In this experimental work, the results exhibit that the incident microwave (MW) significantly modifies the observed narrow negative-MR feature, such that the WL-like peaks disappear at high power regime. This study aims to examine the influence of MW power on the WL-like effect. Thus, the observed negative-MR data were fit using 2D WL model by Hikami et al., to extract the characteristic inelastic length (l_{i}) as a function of MW power [1]. Results suggest that the fit extracted l_{i }is reduced by 50% upon increasing the MW power up to 2 mW. Further analysis of the data suggest that MW induced electron heating is partly responsible for the observed reduction in the l_{i }under photo-excitation. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P01.00010: Interacting topological phases in Fibonacci quasicrystals Pouyan Ghaemi Mohammadi, Moshe Fink, Areg Ghazaryan Recently it was realized that quasicrystals could exhibit non-trivial topological properties and topological equivalence between different types of quasicrystals (such as the Harper model and the Fibonacci quasicrystal). Exciton-polaritons have been recently employed to experimentally verify both band structure and the topological character of the Fibonacci quasicrystal. Given these recent advancements, it is essential to understand the effects of interactions, such as exciton-exciton interactions, on the topological nature of the states corresponding to topological quasicrystals. In this work, we have studied the interacting topological phases in Fibonacci quasicrystals and examined the possible experimental signatures for the effect of interactions in exciton-polariton systems formed in quasicrystal structures. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P01.00011: Evolution of the line shape of radiation-induced magnetoresistance oscillations under bi-chromatic microwave excitation Binuka Gunawardana, C.Rasadi Munasinghe, Rasanga Samaraweera, Tharanga Nanayakkara, Annika Kriisa, U. Kushan Wijewardena, Sajith Withanage, Christian Reichl, Werner Wegscheider, Ramesh Mani We examine the evolution of the line shape of radiation-induced magnetoresistance oscillations under bi-chromatic microwave excitation as the microwave power level at one frequency is held fixed and the power level at the other frequency is varied. In order to overcome signal to noise issues in the limit of small oscillations at very low magnetic fields, we applied a double modulation, double lock-in technique where the magnetic field was modulated sinusoidally while applying a small ac-current to the sample. With this approach, it became possible to measure the magnetic field derivative, dR_{xx} /dB, of the diagonal resistance, and realize more oscillations, especially in the low $B$-field limit. Thus, while using this double lock in measurement technique, the sample was bi-chromatically excited at various bichromatic microwave frequency combinations. Half cycle plots of dR_{xx}/dB reveal that the bichromatic response magneto-transport response follows mostly low frequency response at low magnetic field region and starts following the high frequency response at high magnetic field regions. Here we present the results to address the question of whether or not superposition is followed in the bichromatic microwave magnetoresistance response. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P01.00012: Interferometer parameters that allow observation of non-Abelian e/4 properties. Robert Willett, Kirill Shtengel, Michael Manfra, Loren Pfeiffer, Saeed Fallahi, Geoffrey Gardner, Kenneth West, Kirk Baldwin Fabry-Perot interferometers have been used to examine 5/2 filling factor, and have shown interference consistent with Abelian e/2 charges and non-Abelian e/4 charges. Simple magnetic field sweeps and independently side gate sweeps can each expose even-odd effects indicating non-Abelian e/4. These results have been compiled using a series of different heterostructure types, different illumination procedures, and different interferometer geometries, sizes, and gate charging procedures. In this talk summary results will be presented, but the focus will be these device parameters and procedures employed to reveal the non-Abelian properties. Central to this effort is the use of heterostructures that require illumination to produce high mobilities. The illumination and charging protocols exercised on different fundamental heterostructure types will be described, including the consequences, both good and bad, on 5/2 interference. Given these advantages and constraints on the 2D electron material systems, means to mitigate problems in the measurements by adjusting the interference devices themselves will be discussed. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P01.00013: A Particle-Hole-Symmetric Model for Paired Fractional Quantum Hall States In a Half-filled Landau Level Michael Peterson, William Hutzel, John McCord, Peter Raum, Ben Stern, hao wang, Vito Scarola The fractional quantum Hall effect (FQHE) observed at half filling of the second Landau level is believed to be caused by a BCS-type pairing of composite fermions captured by the Moore-Read Pfaffian wave function. The generating Hamiltonian for the Moore-Read Pfaffian is a purely three-body model that breaks particle-hole symmetry and lacks properties expected from a physical model. We use exact diagonalization to study the low energy states of a more physical two-body generator model derived from the three-body model. We find that the two-body model exhibits the essential features expected from the Moore-Read Pfaffian: pairing, non-Abelian anyon excitations, and a neutral fermion mode. The model also satisfies constraints expected for a physical model of the FQHE at half-filling because it is: short range, spatially decaying, particle-hole symmetric, and has a roton mode with a robust spectral gap in the thermodynamic limit. Hence, this two-body model offers a bridge between exact generator models of paired states and the physical Coulomb interaction and can be used to further explore properties of non-Abelian physics in the FQHE. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P01.00014: Spin-resolved tunneling in the fractional quantum Hall effect regime Heun Mo Yoo, Loren Pfeiffer, Kirk Baldwin, Kenneth West, Raymond Ashoori Strong Coulomb interactions and internal spin degrees of freedom lead to a plethora of correlated electronic phases in quantum Hall (QH) systems. Despite the successes of nuclear magnetic resonance and optical techniques, spin-sensitive measurements have remained challenging, particularly at low carrier density or in higher Landau levels (LLs). Here we introduce a new method, spin-resolved tunneling, that can probe the spin texture of both the ground and excited states of QH systems. We establish a complete phase diagram of the ground-state spin in a wide range of filling factors ν and magnetic fields. Our phase diagrams show the detailed structure of the composite fermion (CF) phases in the lowest LL, in which the changes in the Zeeman energy and ν drive phase transitions between the spin-unpolarized, the spin-polarized, and the topological spin skymion states. On the other hand, the non-Laughlin correlated behavior, such as the absence of spin transitions at ν = 2 + 1/2 and 2 + 2/3 states, is observed in the first excited LL, where the conventional CF picture is no longer valid due to the softened pair interactions. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P01.00015: Symmetry protected Luttinger liquids in Quantum Hall Ferromagnets on the surface of Bi(111). Kartiek Agarwal, Mallika Randeria, Shivaji Sondhi, Ali Yazdani, Siddharth Parameswaran Quantum Hall Ferromagnets are a unique platform to study the confluence of symmetry-broken order parameter and topology. Recent experiments by Feldman et al.[1] observe clear signatures of valley-polarized Quantum Hall Ferromagnets on the surface of Bi(111) in the presence of strong magnetic fields. The tunneling conductance shows a discrete spectrum indicating the formation of Landau levels while individual nematic Landau level orbits pinned to impurities indicate selective occupation of certain valleys. Further recent experiments[2] observe domain wall states between such nematic domains. Most curiously, domain walls between these domains appear to host low energy excitations that appear to be gapped/gapless depending upon the filling fraction of the nematic quantum Hall states. We explain[3] these observations both qualitatively and quantitatively by highlighting the role of interactions and symmetries in engendering such exotic Luttinger liquids. |
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