Showing papers by "L. N. Pfeiffer published in 2008"

Quasi-particle properties from tunneling in the v = 5/2 fractional quantum Hall state.

Abstract: Quasi-particles with fractional charge and statistics, as well as modified Coulomb interactions, exist in a two-dimensional electron system in the fractional quantum Hall (FQH) regime. Theoretical models of the FQH state at filling fraction \(v={^{5}}/{_{2}}\) make the further prediction that the wave function can encode the interchange of two quasi-particles, making this state relevant for topological quantum computing. We show that bias-dependent tunneling across a narrow constriction at \(v={^{5}}/{_{2}}\) exhibits temperature scaling and, from fits to the theoretical scaling form, extract values for the effective charge and the interaction parameter of the quasi-particles. Ranges of values obtained are consistent with those predicted by certain models of the \({^{5}}/{_{2}}\) state.

Observation of a Fractional Quantum Hall State at ν=1/4 in a Wide GaAs Quantum Well

Abstract: We report the observation of an even-denominator fractional quantum Hall state at $\ensuremath{ u}=1/4$ in a high quality, wide GaAs quantum well. The sample has a quantum well width of 50 nm and an electron density of ${n}_{e}=2.55\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$. We have performed transport measurements at $T\ensuremath{\sim}35\text{ }\text{ }\mathrm{mK}$ in magnetic fields up to 45 T. When the sample is perpendicular to the applied magnetic field, the diagonal resistance displays a kink at $\ensuremath{ u}=1/4$. Upon tilting the sample to an angle of $\ensuremath{\theta}=20.3\ifmmode^\circ\else\textdegree\fi{}$ a clear fractional quantum Hall state emerges at $\ensuremath{ u}=1/4$ with a plateau in the Hall resistance and a strong minimum in the diagonal resistance.

Resonant phonon scattering in quantum Hall systems driven by dc electric fields.

Abstract: Using dc excitation to spatially tilt Landau levels, we study resonant acoustic phonon scattering in two-dimensional electron systems. We observe that dc electric field strongly modifies phonon resonances, transforming resistance maxima into minima and back into maxima. Further, phonon resonances are enhanced dramatically in the nonlinear dc response and can be detected even at low temperatures. Most of our observations can be explained in terms of dc-induced (de)tuning of the resonant acoustic phonon scattering and its interplay with inter-Landau level impurity scattering. Finally, we observe a resistance maximum when the electron drift velocity approaches the speed of sound and a dc-induced zero-differential resistance state.

Intrinsic gap of the nu=5/2 fractional quantum Hall state.

Abstract: The fractional quantum Hall effect is observed at low magnetic field where the cyclotron energy is smaller than the Coulomb interaction energy. The nu=5/2 excitation gap at 2.63 T is measured to be 262+/-15 mK, similar to values obtained in samples with twice the electronic density. Examining the role of disorder on the 5/2 state, we find that a large discrepancy remains between theory and experiment for the intrinsic gap extrapolated from the infinite mobility limit. The observation of a 5/2 state in the low-field regime suggests that inclusion of nonperturbative Landau level mixing may be necessary to fully understand the energetics of half-filled fractional quantum Hall liquids.

Anisotropic modification of the effective hole g factor by electrostatic confinement.

Abstract: We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexpectedly, the effective $g$ factor does not depend on the 1D energy level number $N$ for $B\ensuremath{\parallel}[0\overline{1}1]$ while it has strong $N$ dependence for $B\ensuremath{\parallel}[\overline{2}33]$. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions, we also report qualitative differences in the appearance of spin-split plateaus, with nonquantized plateaus observed only for the confinement in the $[0\overline{1}1]$ direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.

Observation of pinning mode of stripe phases of 2D systems in high Landau levels.

Abstract: We study the radio-frequency diagonal conductivities of the anisotropic stripe phases of higher Landau levels near half-integer fillings. In the hard direction, in which larger dc resistivity occurs, the spectrum exhibits a striking resonance, while in the orthogonal, easy direction, no resonance is discernible. The resonance is interpreted as a pinning mode of the stripe phase.

Nonlinear magnetotransport in microwave-illuminated two-dimensional electron systems

Abstract: We study magnetoresistivity oscillations in a high-mobility two-dimensional electron system subject to both microwave and dc electric fields. First, we observe that the oscillation amplitude is a periodic function of the inverse magnetic field and is strongly suppressed at microwave frequencies near half-integers of the cyclotron frequency. Second, we obtain complete conditions for the differential resistivity extrema and saddle points. These findings indicate the importance of scattering without microwave absorption and a special role played by microwave-induced scattering events antiparallel to the electric field.

Quasiparticle Tunneling in the Fractional Quantum Hall State at \nu = 5/2

Abstract: Theory predicts that quasiparticle tunneling between the counter-propagating edges in a fractional quantum Hall state can be used to measure the effective quasiparticle charge e* and dimensionless interaction parameter g, and thereby characterize the many-body wavefunction describing the state. We report measurements of quasiparticle tunneling in a high mobility GaAs two dimensional electron system in the fractional quantum Hall state at nu=5/2 using a gate-defined constriction to bring the edges close together. We find the dc-bias peaks in the tunneling conductance at different temperatures collapse onto a single curve when scaled, in agreement with weak tunneling theory. Various models for the u=5/2 state predict different values for g. Among these models, the non-abelian states with e*=1/4 and g=1/2 are most consistent with the data.

Microwave photoresistance in dc-driven 2D systems at cyclotron resonance subharmonics.

Abstract: We study microwave photoresistivity oscillations in a high-mobility two-dimensional electron system subject to strong dc electric fields. We find that near the second subharmonic of the cyclotron resonance the frequency of the resistivity oscillations with a dc electric field is twice the frequency of the oscillations at the cyclotron resonance, its harmonics, or in the absence of microwave radiation. This observation is discussed in terms of the microwave-induced sidebands in the density of states and the interplay between different scattering processes in the separated Landau level regime.

Area dependence of interlayer tunneling in strongly correlated bilayer two-dimensional electron systems at nuT=1

Abstract: The area and perimeter dependence of the Josephson-like interlayer tunneling signature of the coherent $ u_T=1$ quantum Hall phase in bilayer two-dimensional electron systems is examined. Electrostatic top gates of various sizes and shapes are used to locally define distinct $ u_T=1$ regions in the same sample. Near the phase boundary with the incoherent $ u_T=1$ state at large layer separation, our results demonstrate that the tunneling conductance in the coherent phase is closely proportional to the total area of the tunneling region. This implies that tunneling at $ u_T=1$ is a bulk phenomenon in this regime.

Mechanism of microwave-induced photoluminescence modulation and optically detected resonances due to a two-dimensional electron gas in a heterostructure

Abstract: The effect of pulsed microwave mw irradiation at 36 GHz on the photoluminescence PL of modulationdoped GaAs/AlGaAs quantum wells MDQWs containing a two-dimensional electron gas 2DEG is studied by time-resolved spectroscopy at an ambient temperature of 2 K. The temporal response of the twodimensional electron-hole PL to a short mw pulse reveals remarkable PL intensity kinetics with overshoots and a long decay time 10 ns after the mw pulse terminates. The observed effects are explained by the temporal evolution of the energy redistribution of photoexcited holes. This redistribution is caused by nonequilibrium, long-lived acoustic phonons that are emitted by the mw heated 2DEG and are absorbed by the holes. Optically detected resonances ODRs of the 2DEG are observed in both the two-dimensional electron-hole PL band and in the PL line of excitons that are photoexcited in the undoped superlattice adjoining the MDQW. This provides an experimental proof that the emitted acoustic phonons propagate ballistically throughout the entire heterostructure and induce an effective, long range, indirect interaction between the mw heated 2DEG and the holes as well as with the spatially separated excitons . This interaction constitutes the underlying mechanism of the mw-induced ODRs observed in heterostructures containing a 2DEG.

Characterization of a series of high-quality wide GaAs quantum wells

Abstract: Due to the current interest in quantum Hall states described by the Moore-Read or Pfaffian wavefunction, we have recently begun experiments involving a series of high-quality wide GaAs quantum well in an effort to search for new Pfaffian states. Here we report the characterization of these samples. The quantum well in our series have differing well widths which range from 50 to 80 nm. The self-consistently calculated electronic band edge and electron distribution function are shown for each sample. We present the magnetoresistance up to 18T where the quantum Hall states at v = 1 and 3 are seen to vanish with increasing L. We also discuss the observation of a fractional quantum Hall state at v = 1/2 in the L = 50 nm sample.

Observation of Chiral Heat Transport in the Quantum Hall Regime

Abstract: Heat transport in the quantum Hall regime is investigated using micron-scale heaters and thermometers positioned along the edge of a millimeter-scale two dimensional electron system (2DES). The heaters rely on localized current injection into the 2DES, while the thermometers are based on the thermoelectric effect. In the nu=1 integer quantized Hall state, a thermoelectric signal appears at an edge thermometer only when it is "downstream," in the sense of electronic edge transport, from the heater. When the distance between the heater and the thermometer is increased, the thermoelectric signal is reduced, showing that the electrons cool as they propagate along the edge.

Metamorphosis of quantum Hall bilayer state into a composite fermion metal

Abstract: Abstract Composite fermion (CF) metal states emerge in quantum Hall bilayers at total Landau level filling factor ν T = 1 when the tunneling gap collapses by application of in-plane component of the magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index. The results highlight significant differences of bilayer CF quasiparticles from those in single layers.

Dynamics of microwave-induced processes in photoexcited GaAs/AlGaAs heterostructures

Abstract: Photoluminescence (PL) of high-quality GaAs/AlGaAs–based heterostructures (HS) shows a remarkable time-dependent response to microwave irradiation pulses at temperature 2 K. The mw-frequency was 36 GHz and the modulation pulse widths varied in a wide range. Sharp PL intensity flashes (reaching 100 fold enhancement) at the leading or/and trailing mw-pulse edges are observed. The transient PL response exhibits a complex interplay of various mw-induced physical processes: electron heating, electron and exciton density changes as well as an interaction of electrons, holes and excitons with non-equilibrium acoustic phonons that are emitted by the mw-heated 2DEG. It is noted that in commonly studied cw, mw-induced PL (or resistivity) modulation, the observed (cw) effects are determined by a combination of these processes. We developed a model that considers the instantaneous mw-induced electron heating and following relaxation of the photoexcited electrons, excitons and acoustic phonon flux. Using this model we simulate various types of mw-induced PL transients observed in undoped HS's as well as in HS's that contain a 2DEG. These simulations prove that non-equilibrium acoustic phonons play an important role in the mw-induced PL dynamics. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interferometric measurement of filling factor 5/2 quasiparticle charge

Abstract: A standing problem in low dimensional electron systems is the nature of the 5/2 fractional quantum Hall state: its elementary excitations are a focus for both elucidating the state's properties and as candidates in methods to perform topological quantum computation. Interferometric devices may be employed to manipulate and measure quantum Hall edge excitations. Here we use a small area edge state interferometer designed to observe quasiparticle interference effects. Oscillations consistent in detail with the Aharanov-Bohm effect are observed for integer and fractional quantum Hall states (filling factors 2, 5/3, and 7/3) with periods corresponding to their respective charges and magnetic field positions. With these as charge calibrations, at 5/2 filling factor and at lowest temperatures periodic transmission through the device consistent with quasiparticle charge e/4 is observed. The principal finding of this work is that in addtion to these e/4 oscillations, periodic structures corresponding to e/2 are also observed at 5/2 and at lowest temperatures. Properties of the e/4 and e/2 oscillations are examined with the device sensitivity sufficient to observe temperature evolution of the 5/2 quasiparticle interference. In the model of quasiparticle interference, this presence of an effective e/2 period may empirically reflect an e/2 quasiparticle charge, or may reflect multiple passes of the e/4 quasiparticle around the interferometer. These results are discussed within a picture of e/4 quasiparticle excitations potentially possessing non-Abelian statistics. These studies demonstrate the capacity to perform interferometry on 5/2 excitations and reveal properties important for understanding this state and its excitations.

Intrinsic Gap of the v = 5/2 Fractional Quantum Hall State

Abstract: The fractional quantum Hall effect is observed at low magnetic field where the cyclotron energy is smaller than the Coulomb interaction energy. The v = 5 2 excitation gap at 2.63 T is measured to be 262 ± 15 mK, similar to values obtained in samples with twice the electronic density. Examining the role of disorder on the 5 2 state, we find that a large discrepancy remains between theory and experiment for the intrinsic gap extrapolated from the infinite mobility limit. The observation of a 5 2 state in the low-field regime suggests that inclusion of nonperturbative Landau level mixing may be necessary to fully understand the energetics of half-filled fractional quantum Hall liquids.

Emergence of a reentrant insulating phase around ν=13 in a series of GaAs/AlGaAs quantum wells with varying well widths

Abstract: We report on magneto-transport measurements of a systematic set of five GaAs/AlGaAs quantum wells with well widths ranging from 7.9 to 33.0 nm. We find that as L is decreased into the regime where the zero-field mobility is limited due to surface roughness, a reentrant insulating phase around filling fraction ν=1/3 emerges. As L is decreased further, only a single insulating phase is observed for ν<1.