arXiv subjects

L. N. Pfeiffer

Publications and source records attributed to L. N. Pfeiffer.

At least 217 records · Page 12Linked to original sources

arXiv2013

Evidence for a $ν=5/2$ Fractional Quantum Hall Nematic State in Parallel Magnetic Fields

We report magneto-transport measurements for the fractional quantum Hall state at filling factor $ν=$ 5/2 as a function of applied parallel magnetic field ($B_{||}$). As $B_{||}$ is increased, the 5/2 state becomes increasingly anisotropic, with the in-plane resistance along the direction of $B_{||}$ becoming more than 30 times larger than in the perpendicular direction. Remarkably, the resistance anisotropy ratio remains constant over a relatively large temperature range, yielding an energy gap which is the same for both directions. Our data are qualitatively consistent with a fractional quantum Hall \textit{nematic} phase.

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arXiv2013

Magnetic field-tuned Aharonov-Bohm oscillations and evidence for non-Abelian anyons at v=5/2

We show that the resistance of the v=5/2 quantum Hall state, confined to an interferometer, oscillates with magnetic field consistent with an Ising-type non-Abelian state. In three quantum Hall interferometers of different sizes, resistance oscillations at v=7/3 and integer filling factors have the magnetic field period expected if the number of quasiparticles contained within the interferometer changes so as to keep the area and the total charge within the interferometer constant. Under these conditions, an Abelian state such as the (3,3,1) state would show oscillations with the same period as at an integer quantum Hall state. However, in an Ising-type non-Abelian state there would be a rapid oscillation associated with the "even-odd effect" and a slower one associated with the accumulated Abelian phase due to both the Aharonov-Bohm effect and the Abelian part of the quasiparticle braiding statistics. Our measurements at v=5/2 are consistent with the latter.

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arXiv2013

Aharonov-Bohm effect and coherence length of charge e/4 quasiparticles at 5/2 filling factor measured in multiple small Fabry-Perot interferometers

Design of a Fabry-Perot (double point contact) interferometer to measure fractional quantum Hall effect quasiparticle charge properties, and in particular the 5/2 excitations, poses an important trade-off: the device size should be minimized to allow two path interference, since the coherence length of the quasiparticles in the correlated states are expected be limited, yet a small device promotes the dominance of Coulomb charging effects which would overwhelm the Aharonov-Bohm interference effect. In this study a series of small but different size interferometers from the same high density heterostructure wafer are examined for the presence of Coulomb effects versus Aharonov-Bohm (A-B) interference effect when operated in gate configurations that support the 5/2, 7/3, and 8/3 fractional quantum Hall effects. The device sizes vary by more than a factor of three, and over this range explicitly show specific properties of A-B interference, but not Coulomb dominated effects. Given these A-B interference results, the coherence length of the charge e/4 interference is extracted. The coherence length of non-Abelian e/4 quasiparticles is an important parameter for design and development of complex interference devices used to study and apply this exotic excitation. As in prior observations of e/4 excitations, A-B e/4 and e/2 oscillations in alternation are observed in these multiple devices. The amplitudes of the e/4 oscillations are observed to be dramatically reduced for larger area interferometers. Path-length limits are derived from interferometer areas determined directly by A-B measurements, and the attenuation lengths of the e/4 oscillations are shown to be micron to sub-micron scale. This coherence length is consistent with that of the 7/3 excitations measured here, and consistent with theoretical models.

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arXiv2012

Microwave-induced resistance oscillations in tilted magnetic fields

We have studied the effect of an in-plane magnetic field on microwave-induced resistance oscillations in a high mobility two-dimensional electron system. We have found that the oscillation amplitude decays exponentially with an in-plane component of the magnetic field $B_\parallel$. While these findings cannot be accounted for by existing theories, our analysis suggests that the decay can be explained by a $B_\parallel$-induced correction to the quantum scattering rate, which is quadratic in $B_\parallel$.

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arXiv2012

Thermoelectric response of fractional quantized Hall and re-entrant insulating states in the N=1 Landau level

Detailed measurements of the longitudinal thermopower of two-dimensional electrons in the first excited Landau level are reported. Clear signatures of numerous fractional quantized Hall states, including those at $ν= 5/2$ and 7/3, are observed in the magnetic field and temperature dependence of the thermopower. An abrupt collapse of the thermopower is observed below about $T= 40$ mK at those filling factors where re-entrant insulating electronic states have been observed in conventional resistive transport studies. The thermopower observed at $ν= 5/2$ is discussed in the context of recent theories which incorporate non-abelian quasiparticle exchange statistics.

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arXiv2012

Anisotropic Fermi Contour of (001) GaAs Holes in Parallel Magnetic Fields

We report a severe, spin-dependent, Fermi contour anisotropy induced by parallel magnetic field in a high-mobility (001) GaAs two-dimensional hole system. Employing commensurability oscillations created by a unidirectional, surface-strain-induced, periodic potential modulation, we directly probe the anisotropy of the two spin subband Fermi contours. Their areas are obtained from the Fourier transform of the Shubnikov-de Haas oscillations. Our findings are in semi-quantitative agreement with the results of parameter-free calculations of the energy bands.

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arXiv2012

Exciton Transport in a Bilayer Quantum Hall Superfluid

Bilayer quantum Hall systems at ν=1 support an excitonic ground state. In addition to the usual charged quasiparticles, this system possesses a condensate degree of freedom: exciton transport. Detection of this neutral transport mode is facilitated by the use of the Corbino multiply-connected geometry in which charge transport is suppressed. We here summarize our recent experiments on Corbino devices which directly demonstrate exciton transport across the bulk of the incompressible ν=1 quantum Hall state.

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arXiv2012

Nonlinear response in overlapping and separated Landau levels of GaAs quantum wells

We have studied magnetotransport properties of a high-mobility two-dimensional electron system subject to weak electric fields. At low magnetic field $B$, the differential resistivity acquires a correction $δr \propto -λ^2 j^2/B^2$, where $λ$ is the Dingle factor and $j$ is the current density, in agreement with theoretical predictions. At higher magnetic fields, however, $δr$ becomes $B$-independent, $δr \propto -j^2$. While the observed change in behavior can be attributed to a crossover from overlapping to separated Landau levels, full understanding of this behavior remains a subject of future theories.

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arXiv2012

Ballistic transport of (001) GaAs 2D holes through a strain-induced lateral superlattice

We report the observation of ballistic commensurability oscillations and positive magnetoresistance in a high-mobility, (001) GaAs two-dimensional hole system with a unidirectional, surface-strain-induced, periodic potential modulation. The positions of the resistivity minima agree well with the electrostatic commensurability condition. From an analysis of the amplitude of the oscillations we deduce a ballistic scattering time and an effective magnitude for the induced periodic potential seen by the two-dimensional holes.

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arXiv2012

Hole-flux Composite Fermion Commensurability Oscillations

We report the observation of commensurability oscillations of hole-flux composite fermions near filling factor $ν=1/2$ in a high-mobility two-dimensional hole system confined to a GaAs quantum well, and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The oscillations, which are consistent with ballistic transport of fully spin-polarized composite fermions in a weak periodic effective magnetic field, are surprisingly strong and exhibit up to third-order minima. We extract a ballistic mean-free-path of about 0.2 $μ$m for the hole-flux composite fermions.

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arXiv2012

Spin and charge distribution symmetry dependence of stripe phases in two-dimensional electron systems confined to wide quantum wells

Measurements in clean two-dimensional electron systems confined to wide GaAs quantum wells in which two electric subbands are occupied reveal an unexpected rotation of the orientation of the stripe phase observed at a half-filled Landau level. Remarkably, the reorientation is sensitive to the spin of the half-filled Landau level and the symmetry of the charge distribution in the quantum well.

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arXiv2012

Breakdown of the integer and fractional quantum Hall states in a quantum point contact

The integer and fractional quantum Hall states are known to break down at high dc bias, exhibiting deviation from the ideal incompressible behavior. We measure breakdown of the ν= 2, 3, 4, 5 integer and the ν= 4/3 and 5/3 fractional states in a quantum point contact (QPC) of lithographic width ~600 nm. Dependence of the critical current on magnetic field, QPC gate voltage, and QPC width are presented. Of particular interest, the critical current of the 4/3 and 5/3 fractional states shows the opposite dependence on QPC width compared to the integer states. This previously unobserved result is not explained by current theories of breakdown.

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arXiv2012

Measurements of quasi-particle tunneling in the nu = 5/2 fractional quantum Hall state

Some models of the 5/2 fractional quantum Hall state predict that the quasi-particles, which carry the charge, have non-Abelian statistics: exchange of two quasi-particles changes the wave function more dramatically than just the usual change of phase factor. Such non-Abelian statistics would make the system less sensitive to decoherence, making it a candidate for implementation of topological quantum computation. We measure quasi-particle tunneling as a function of temperature and DC bias between counter-propagating edge states. Fits to theory give e*, the quasi-particle effective charge, close to the expected value of e/4 and g, the strength of the interaction between quasi-particles, close to 3/8. Fits corresponding to the various proposed wave functions, along with qualitative features of the data, strongly favor the Abelian 331 state.

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arXiv2012

Shubnikov-de Haas oscillations in GaAs quantum wells in tilted magnetic fields

We report on quantum magneto-oscillations in an ultra-high mobility GaAs/AlGaAs quantum well at very high tilt angles. Unlike previous studies, we find that the spin and cyclotron splittings become equal over a continuous range of angles, but only near certain, angle-dependent filling factors. At high enough tilt angles, Shubnikov-de Haas oscillations reveal a prominent beating pattern, indicative of consecutive level crossings, all occurring at the same angle. We explain these unusual observations by an in-plane field-induced increase of the carrier mass, which leads to accelerated, filling factor-driven crossings of spin sublevels in tilted magnetic fields.

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arXiv2012

Imaging fractional incompressible stripes in integer quantum Hall systems

Transport experiments provide conflicting evidence on the possible existence of fractional order within integer quantum Hall systems. In fact integer edge states sometimes behave as monolithic objects with no inner structure, while other experiments clearly highlight the role of fractional substructures. Recently developed low-temperature scanning probe techniques offer today an opportunity for a deeper-than-ever investigation of spatial features of such edge systems. Here we use scanning gate microscopy and demonstrate that fractional features were unambiguously observed in every integer quantum Hall constriction studied. We present also an experimental estimate of the width of the fractional incompressible stripes corresponding to filling factors 1/3, 2/5, 3/5, and 2/3. Our results compare well with predictions of the edge-reconstruction theory.

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arXiv2012

Experimental evidence of low-lying gapped excitations in the quantum fluid at nu=5/2

The low-lying neutral excitation spectrum of the incompressible quantum Hall fluid at $ν=5/2$ is investigated by inelastic light scattering. Gapped modes are observable only in a very narrow filling factor range centered at 5/2 at energies that overlap estimates from transport activation gaps. The modes are interpreted as critical points in the wave-vector dispersion of excitations that preserve spin orientation. For very small changes $|δν|\lesssim 0.01$ the gapped modes disappear and a continuum of low-lying excitations takes over indicating the transition from an incompressible fluid at 5/2 to a compressible state. Observations of spin wave modes indicate spin polarization of the 5/2 and 2+1/3 quantum Hall fluids.

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arXiv2012

Magnetic field induced resistance properties at filling factor 5/2 consistent with non-Abelian e/4 quasiparticles in multiple sized interferometers

Non-Abelian e/4 quasiparticles at 5/2 filling factor in a correlated two-dimensional electron gas have a proposed specific property in an interference measurement of their edge propagation: encircling an even number of localized e/4 quasiparticles allows expression of e/4 Aharonov-Bohm (A-B) oscillations, but suppression of these oscillations will occur with an odd number encircled. This picture is tested explicitly here in multiple interferometers of different areas. The encircled localized e/4 quasiparticle number near 5/2 filling factor is changed by sweeping B-field, and oscillations are observed in resistance near 5/2 of period specific to that device area for each interferometer. The product of the measured interferometric area of each device and the respective 5/2 resistance oscillation period is found to agree with the expected flux quanta addition needed for parity change in the localized e/4 number. This result shows a highly specific non-Abelian property of the quasiparticle excitations at 5/2 filling expressed in multiple interferometers.

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arXiv2012

Spin Transition in the ν=8/3 Fractional Quantum Hall Effect

We present here the results from a density dependent study of the activation energy gaps of the fractional quantum Hall effect states at Landau level fillings ν=8/3 and 7/3 in a series of high quality quantum wells. In the density range from 0.5 x 10^{11} to 3 x 10^{11} cm^{-2}, the 7/3 energy gap increases monotonically with increasing density, supporting its ground state being spin polarized. For the 8/3 state, however, its energy gap first decreases with increasing density, almost vanishes at n ~ 0.8 x 10^{11} cm^{-2}, and then turns around and increases with increasing density, clearly demonstrating a spin transition.

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