Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K27: Fractional Quantum Hall Effect III. |
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Sponsoring Units: FIAP Chair: Robert Wilett, Bell Laboratories Room: 290 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K27.00001: Roton Minimum in the Excitation spectrum of the 2$+$1/3 FQHE State Ursula Wurstbauer, Lingjie Du, ANTONIO Levy, ARON PINCZUK, MICHAEL MANFRA, KEN WEST, LOREN PFEIFFER The physics of low-lying excitations in the second Landau level (SLL) is remarkably different from that in the lowest LL. Our work explores the fascinating excitations in the second Landau level by measurements of low-lying neutral excitation modes in the SLL from resonant inelastic light scattering (RILS) experiments [1,2]. We see clear signatures for gapped modes for several filling factors known from transport to be incompressible FQHE states such as $\nu =$2$+$2/5, 2$+$3/8 and 2$+$1/3, where three modes can be excited by RILS. For v$=$2$+$1/3, a band of a dispersive mode in the range of 0.15 meV \textless EDOS \textless 0.35 meV, a mode centered at Egap $\approx $ 0.08meV and a weak, sharp mode at ES $\approx $ 0.1meV occur. The first two modes are interpreted as collective charge excitations in good agreement with a pronounced roton minimum and other critical points in the calculated wave-vector dispersion. The third mode is assigned as excitation with spin reversal [2]. Remarkably, all modes exhibit a striking dependence on the filling factor uncovering incompressible quantum states. [1] U. Wurstbauer, et al. Phys. Rev. B. 92, 241407(R) (2015).[2] U. Wurstbauer, et al. Phys. Rev. Lett. 110, 026801 (2013). [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K27.00002: Positions of the magneto-roton minima in the fractional quantum Hall effect Songyang Pu, Ajit Coimbatore Balram The minima in the dispersion of the neutral excitation, which is a composite fermion exciton, are called “magneto-roton” minima. Golkar \emph{et al.}[1] have predicted the positions of the magneto-roton minima at filling factors $s/(2s+1)$ by treating the excitation as a deformation of the parent composite fermion Fermi sea at $1/2$. We use the composite fermion theory to calculate the exciton dispersion for different filling factors up to $5/11$, and find the positions of the first few magneto-roton minima agree well with Golkar \emph{et al.}’s predictions. Furthermore, we test the prediction that the positions of magneto-roton minima are insensitive to the microscopic form of the interaction by applying two different interactions in our calculation, namely the usual Columb interaction and the effective interaction in the $n=1$ Landau level of graphene. We see the positions of magneto-roton minima are nearly unchanged with these two different interactions.\\ [1] Golkar \emph{et al.} cond-mat arXiv:1602.08499 [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K27.00003: Microscopic study of the composite Fermi liquid Scott Geraedts, Jie Wang, F. D. M. Haldane We use exact diagonalization (ED) to study a many-body generalization of the $k$-space Berry phase for taking a quasiparticle around the Fermi surface of a $\nu=1/2$ composite Fermi liquid (CFL). Composite fermion ``occupations'' are identified by a large overlap between the ED state and an explicit model CFL wavefunction. This model wavefunction is found to be very accurate, and also nearly particle-hole symmetric, when the composite fermion quasiparticles are clustered to form a compact Fermi sea. For a sequence of occupation configurations where excitations stay close to the Fermi surface, we find a net phase of $\pi$, consistent with an anomalous Hall effect with $\sigma^{xy}=1/2 (e^2/h)$ and with a recent Dirac fermion effective theory. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K27.00004: Effect of excitation parameters on the damping of microwave induced magnetoresistance oscillations in GaAs/AlGaAs 2D electron system Rasanga Samaraweera, Binuka Gunawardana, Han-Chun Liu, Christian Reichl, Werner Wegscheider, Ramesh Mani Two-dimensional electron systems (2DES) realized with GaAs/AlGaAs heterostructures, with 2D electron mobilities well above 10$^{\mathrm{7}}$ cm$^{\mathrm{2}}$/Vs, exhibit interesting electrical and physical phenomena including novel microwave induced zero-resistance states and associated radiation-induced magnetoresistance oscillations. Previous studies reveal the effect of different experimental factors such as microwave power, temperature, and linear polarization angle, etc., on the radiation-induced magnetoresistance oscillations of GaAs/AlGaAs 2DES [1-2]. In this experimental study, we present the effect of the excitation current on the microwave-induced magnetoresistance oscillations, and present a quantitative analysis of the results. The aim of the study is to determine the correlation between the applied excitation and the damping of the microwave induced magnetoresistance oscillations. [1].A. N. Ramanayaka, R.G. Mani and W. Wegscheider, Phys. Rev. B.83, 165303 (2011). [2].R. G. Mani, C. Gerl, S. Schmult, W. Wegscheider and V. Umansky, Phys. Rev. B.81, 125320 (2010). [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K27.00005: Cyclotron resonance in the high mobility GaAs/AlGaAs 2D electron system over the microwave, mm-wave, and terahertz- bands Annika Kriisa, R. L. Samarweera, M. S. Heimbeck, H. O. Everitt, W. Wegscheider, R. G. Mani We have developed a measurement technique that allows measuring the power signal of the microwaves reflected back from the photo-excited high mobility GaAs/AlGaAs 2D device, while simultaneously measuring standard magnetotransport, over the wide frequency range from 30 to 330 GHz. We found that the effective mass extracted from the easily distinguishable peaks observed in the reflected power signal was equal to the cyclotron resonance mass 0.067m$_{\mathrm{o}}$ within the experimental error, over the whole examined frequency range (here m$_{\mathrm{o}}$ is free electron mass). As expected, the magnetotransport measurements show Shubnikov de Haas oscillations with amplitude changes at cyclotron resonance over the frequency range of 130 to 330 GHz, and the extracted effective mass again equals to the cyclotron resonance mass 0.067m$_{\mathrm{o}}$. Over the lower frequency range of 30 to 130 GHz, the magnetotransport measurements show microwave radiation-induced magneto-resistance oscillations with a lower effective mass of 0.062m$_{\mathrm{o}}$. We explore possible explanations to the discrepancies between the observed effective masses. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K27.00006: Current bias and temperature effect on the Shubnikov de Haas oscillations in high mobility GaAs/AlGaAs system Chathuranga Munasinghe, Rasanga Samaraweera, Binuka Gunawardana, Zhuo Wang, Christian Reichl, Werner Wegscheider, Ramesh Mani The study of the temperature dependence of Shubnikov de Haas (SdH) oscillations in two dimensional systems serves to investigate electronic properties such as, for example, the carrier effective mass. This study was conducted to determine carrier heating induced by the current bias utilized in typical magnetotransport measurements of the high mobility GaAs/AlGaAs two dimensional electron gas system. Carrier heating due to the current bias is investigated by comparing the observed variation in the amplitude of the SdH oscillations as a function of the current, with the SdH lineshape observed at a vanishing current bias at various temperatures over the temperature range 1.6 $\le $ T $\le $ 4.2K, with the specimen immersed in pumped liquid helium. Thus, carrier heating due to the current bias is determined by quantitatively matching the large current SdH lineshape at base temperature with the vanishing current SdH lineshape at an elevated temperature. For this purpose, lineshape fits of the SdH oscillations to the standard Lifshitz-Kosevich theory have been carried out. The results of the study will be presented here. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K27.00007: Experimental investigation of the mobility/quality dichotomy in ultra-high quality AlGaAs/GaAs two-dimensional electron gases Qi Qian, James Nakamura, Saeed Fallahi, Geoffrey Gardner, John Watson, Silvia Luscher, Joshua Folk, Gabor Csathy, Michael Manfra We detailed the relationship between mobility lifetime ($\tau _{\mathrm{tr}})$, and quantum scattering lifetime ($\tau_{\mathrm{q}})$ measured near zero magnetic field and the excitation gap ($\Delta _{\mathrm{5/2}})$ at $\nu =$5/2 in ultra-high quality AlGaAs/GaAs two-dimensional electron gases. While the lack of correlation between $\tau _{\mathrm{tr}}$ and $\Delta_{\mathrm{5/2\thinspace }}$has been noted previously, we demonstrate that $\tau_{\mathrm{q}}$ is also a poor predictor of $\Delta_{\mathrm{5/2.\thinspace }}$The impact of small density inhomogeneities on determination of $\tau_{\mathrm{q}}$ in the limit of very large $\tau_{\mathrm{q}}$ and low temperatures is discussed. We define, analyze and discuss the utility of a different metric $\rho_{\mathrm{cf}}$, the so-called composite fermion resistivity, as a high temperature (T$=$0.3K) predictor of $\Delta_{\mathrm{5/2.}}$ [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K27.00008: Geometric Resonance of Composite Fermions near Bilayer Quantum Hall States M.A. Mueed, D. Kamburov, M. Shayegan, L.N. Pfeiffer, K.W. West, K.W. Baldwin Via the application of parallel magnetic field, we induce a single-layer to bilayer transition in two-dimensional electron systems confined to wide GaAs quantum wells, and study the geometric resonance of composite fermions (CFs) with a periodic density modulation in our samples. The measurements reveal that CFs exist close to bilayer quantum Hall states, formed at Landau level filling factors $\nu=1$ and 1/2. Near $\nu=1$, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a two-component behavior. In contrast, close to $\nu=1/2$, CFs appear single-layer-like (single-component) as their resonance features correspond to the total density. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K27.00009: Phase diagram of $\nu=1/2+1/2$ quantum Hall bilayers Zheng Zhu, D.N. Sheng, Liang Fu We study $\nu=1/2+1/2$ quantum Hall bilayer systems with different layer distance $d$ by exact diagonalization method. The energy spectrum under periodical and twisted boundary conditions shows the gap opening in spin sector near $d/l_B=1.6$. For the smaller layer distance, we find the the change of low-lying excitations induced by energy level crossing near $d/l_B=1.1$, the goldstone mode or pseudo-spin wave excitation is no longer the lowest excitation. Besides, the drag Hall conductance is finite and non-quantized. For the larger layer distance, we find the intermediate phase with four-fold degeneracy and zero drag Hall conductance, which has distinguished energy spectrum compared with the decoupled CF Fermi liquid states. The transition for different phases has also been discussed. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K27.00010: Measurement of integer and fractional quantum Hall states in electronic Fabry-Perot interferometers. James Nakamura, Qi Qian, Michael Manfra, Robert Willett, Saeed Fallahi, Geoffrey Gardner Here we present measurements of electronic Fabry-Perot interferometers operating in the quantum Hall regime. Fabry-Perot interferometry can be used to extract the effective charge and statistics of quasiparticles at fractional states. The devices are fabricated on high mobility GaAs/AlGaAs heterostructures and are measured in a dilution refrigerator at T\textasciitilde 10mK. We have measured Coulomb blockade oscillations at numerous integer states as well as at the $\nu =$5/3 fractional quantum Hall state; this allowed us to extract an effect charge e* $=$ e/3 at $\nu $ $=$ 5/3. Efforts are underway to fabricate interferometers which can operate in the pure Aharonov-Bohm regime, where quasiparticle statistics can be probed, and to extend these results to fractional states in the 2nd Landau level. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K27.00011: Interplay of Hofstadter and quantum Hall states in bilayer graphene Eric M. Spanton, Alexander A. Zibrov, Haoxin Zhou, Takashi Taniguchi, Kenji Watanabe, Andrea Young Electron interactions in ultraclean systems such as graphene lead to the fractional quantum Hall effect in an applied magnetic field. Long wavelength periodic potentials from a moiré pattern in aligned boron nitride-graphene heterostructures may compete with such interactions and favor spatially ordered states (e.g. Wigner crystals orcharge density waves). To investigate this competition, we studied the bulk phase diagram of asymmetrically moiré-coupled bilayer graphene via multi-terminal magnetocapacitance measurements at ultra-high magnetic fields. Two quantum numbers characterize energy gaps in this regime: $t$, which indexes the Bloch bands, and $s$, which indexes the Landau level. Similar to past experiments, we observe the conventional integer and fractional quantum Hall gaps ($t=0$), integer Hofstadter gaps (integer $s$ and integer $t\neq0$), and fractional Bloch states associated with an expanded superlattice unit cell (fractional $s$ and integer $t$). Additionally, we find states with fractional values for both $s$ and $t$. Measurement of the capacitance matrix shows that these states occur on the layer exposed to the strong periodic potential. We discuss the results in terms of possible fractional quantum hall states unique to periodically modulated systems. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K27.00012: Probing quantum Hall states with single-electron transistors at high magnetic fields Martin Gustafsson, Matthew Yankowitz, Carlos Forsythe, Xiaoyang Zhu, Cory Dean The sequence of fractional quantum Hall states in graphene is not yet fully understood, largely due to disorder-induced limitations of conventional transport studies. Measurements of magnetotransport in other 2D crystals are further complicated by the difficulties in making ohmic contact to the materials. On the other hand, bulk electronic compressibility can provide clear signatures of the integer and fractional quantum Hall effects, does not require ohmic contact, and can be localized to regions of low disorder. The single-electron transistor (SET) is a suitable tool for such experiments due to its small size and high charge sensitivity, which allow electric fields penetrating the 2D electron system to be detected locally and with high fidelity. Here we report studies of exfoliated 2D van der Waals materials fully encapsulated in flakes of hexagonal boron nitride. SETs are fabricated lithographically on top of the encapsulation, yielding a structure which lends itself to experiments at high electric and magnetic fields. We demonstrate the method on monolayer graphene, where we observe fractional quantum Hall states at all filling factors $\nu = n/3$ up to $n=17$ and extract their associated energy gaps for magnetic fields up to 31 tesla. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K27.00013: Geometric Defects in Quantum Hall States Andrey Gromov I will describe a geometric analogue of Laughlin quasiholes in fractional quantum Hall (FQH) states. These ``quasiholes'' are generated by an insertion of quantized fluxes of curvature - which can be modeled by branch points of a certain Riemann surface - and, consequently, are related to genons. Unlike quasiholes, the genons are not excitations, but extrinsic defects. Fusion of genons describes the response of an FQH state to a process that changes (effective) topology of the physical space. These defects are abelian for IQH states and non-abelian for FQH states. I will explain how to calculate an electric charge, geometric spin and adiabatic mutual statistics of the these defects. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K27.00014: Strain-Induced Anisotropic Fermi Contour of 2D Holes and Composite Fermions Insun Jo, K. A. V. Rosales, M. A. Mueed, M. Padmanabhan, L. N. Pfeiffer, K. W. West, K. W. Baldwin, R. Winkler, M. Shayegan We present experimental and theoretical results demonstrating strain-induced Fermi contour anisotropy of two-dimensional (2D) holes and composite fermions (CFs) confined to a (001) GaAs quantum well. We apply a tunable uniaxial strain to a thinned (001) GaAs wafer, glued to a piezoelectric actuator. When the 2D holes are subjected to an in-plane uniaxial strain, their band structure and Fermi contour become anisotropic by about 30{\%} even for a minute amount of strain, on the order of 10$^{\mathrm{-4}}$. Via measurements of commensurability oscillations, we determine the Fermi contour anisotropy for holes near zero magnetic field, and for CFs at high magnetic fields, as a function of uniaxial strain. The measured Fermi contour anisotropy of holes is consistent with the calculation results. The observed CF Fermi contour anisotropy also shows a strong dependence on the applied strain, which we compare quantitatively to that of the low-field holes. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K27.00015: Second quantized approach to zero mode properties of projected Trugman-Kivelson interaction Li Chen, Sumanta Bandyopadhyay, Alexander Seidel We have analyzed general zero mode properties of the parent Hamiltonian of the unprojected Jain 2/5 state. Earlier numerical claims in the literature about ground state uniqueness on the sphere are substantiated on analytic grounds. Preference is given to second quantized methods, where zero mode properties are derived not from given analytic wave functions, but from a “lattice” Hamiltonian and associated zero mode conditions. [Preview Abstract] |
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