arXiv subjects

L. N. Pfeiffer

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

At least 451 records · Page 25Linked to original sources

arXiv1999

New insulating phases of two-dimensional electrons in high Landau levels: observation of sharp thresholds to conduction

The intriguing re-entrant integer quantized Hall states recently discovered in high Landau levels of high-mobility 2D electron systems are found to exhibit extremely non-linear transport. At small currents these states reflect insulating behavior of the electrons in the uppermost Landau level. At larger currents, however, a discontinuous and hysteretic transition to a conducting state is observed. These phenomena, found only in very narrow magnetic field ranges, are suggestive of the depinning of a charge density wave state, but other explanations can also be constructed.

cond-mat.mes-hall↗

arXiv1999

Exact Quantization of Even-Denominator Fractional Quantum Hall State at $ν$=5/2 Landau Level Filling Factor

We report ultra-low temperature experiments on the obscure fractional quantum Hall effect (FQHE) at Landau level filling factor $ν$=5/2 in a very high mobility specimen of $μ=1.7 \times 10^7$ cm$^2$/Vs. We achieve an electron temperature as low as $\sim$ 4~mK, where we observe vanishing $R_{xx}$ and, for the first time, a quantized Hall resistance, $R_{xy}=h/(5/2e^2)$ to within 2 ppm. $R_{xy}$ at the neighboring odd-denominator states $ν$=7/3 and 8/3 is also quantized. The temperature dependences of the $R_{xx}$-minima at these fractional fillings yield activation energy gaps $Δ_{5/2}$=0.11K, $Δ_{7/3}$=0.10K, and $Δ_{8/3}$=0.055K.

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arXiv1999

Ultraslow Electron Spin Dynamics in GaAs Quantum Wells Probed by Optically Pumped NMR

Optically pumped nuclear magnetic resonance (OPNMR) measurements were performed in two different electron-doped multiple quantum well samples near the fractional quantum Hall effect ground state nu=1/3. Below 0.5K, the spectra provide evidence that spin-reversed charged excitations of the nu=1/3 ground state are localized over the NMR time scale of ~40 microseconds. Furthermore, by varying NMR pulse parameters, the electron spin temperature (as measured by the Knight shift) could be driven above the lattice temperature, which shows that the value of the electron spin-lattice relaxation time lies between 100 microseconds and 500 milliseconds at nu=1/3.

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arXiv1999

Optically Pumped NMR Studies of Electron Spin Polarization and Dynamics: New Constraints on the Composite Fermion Description of nu = 1/2

The Knight shift and the nuclear spin-lattice relaxation rate 1/T1 of Ga-71 nuclei are measured in GaAs quantum wells at Landau level filling factor nu = 1/2 using optically pumped NMR. The temperature dependences of these data are compared with predictions of a weakly-interacting composite fermion model. Our measurements of the electron spin polarization and spin dynamics near the transition between fully and partially spin-polarized ground states provide new constraints on the theoretical description of nu = 1/2.

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arXiv1999

Tunneling into Ferromagnetic Quantum Hall States: Observation of a Spin Bottleneck

We explore the characteristics of equilibrium tunneling of electrons from a 3D electrode into a high mobility 2D electron system. For most 2D Landau level filling factors, we find that tunneling can be characterized by a single, well-defined tunneling rate. However, for spin-polarized quantum Hall states (nu = 1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to 2 orders of magnitude. The dependence of the two rates on temperature and tunnel barrier thickness suggests that slow in-plane spin relaxation creates a bottleneck for tunneling of electrons.

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arXiv1999

Nonmonotonic Temperature-dependent Resistance in Low Density 2D Hole Gases

The low temperature longitudinal resistance-per-square Rxx(T) in ungated GaAs/AlGaAs quantum wells of high peak hole mobility 1.7x10^6 cm^2/Vs is metallic for 2D hole density p as low as 3.8x10^9 cm-2. The electronic contribution to the resistance, R_{el}(T), is a nonmonotonic function of T, exhibiting thermal activation, R_{el}(T) ~ exp{-E_a/kT}, for kT< EF. The form of R_{el}(T) is independent of density, indicating a fundamental relationship between the low and high T scattering mechanisms in the metallic state.

cond-mat.str-el↗

arXiv1999

Phonon mediated drag in double layer two dimensional electron systems

Experiments studying phonon mediated drag in the double layer two dimensional electron gas system are reported. Detailed measurements of the dependence of drag on temperature, layer spacing, density ratio, and matched density are discussed. Comparisons are made to theoretical results [M. C. Bonsager et al., Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled electron-phonon collective mode. The layer spacing and density dependence at matched densities for samples with layer spacings below 2600 A do not support the existence of this mode, showing behavior expected for independent electron and phonon systems. The magnitude of the drag, however, suggests the alternate limit; one in which electrons and phonons are strongly coupled. The results for still larger layer spacing show significant discrepancies with the behavior expected for either limit.

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arXiv1999

Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field

We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions nu=9/2, 11/2, etc., the in-plane field can lead to a striking interchange of the "hard" and "easy" transport directions. In the second Landau level the normally isotropic resistivity and the weak nu=5/2 quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.

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arXiv1999

Strongly Anisotropic Electronic Transport at Landau Level Filling Factor $ν= 9/2$ and $ν= 5/2$ Under Tilted Magnetic Field

We have investigated the influence of an increasing in-plane magnetic field on the states at half-filling of Landau levels ($ν$ = 11/2, 9/2, 7/2, and 5/2) of a two-dimensional electron system. In the electrically anisotropic phase at $ν$ = 9/2 and 11/2 an in-plane magnetic field of $\sim$ 1-2 T overcomes its initial pinning to the crystal lattice and {\it reorient} this phase. In the initially isotropic phases at $ν$ = 5/2 and 7/2 an in-plane magnetic field {\it induces} a strong electrical anisotropy. In all cases, for high in-plane fields, the high resistance axis is parallel to the direction of the in-plane field.

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arXiv1999

Reply to Simon's Comment on "Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels"

We recently reported [PRL 82, 394 (1999)] large transport anisotropies in a two-dimensional electron gas in high Landau levels. These observations were made utilizing both square and Hall bar sample geometries. Simon recently commented [cond-mat/9903086] that a classical calculation of the current flow in the sample shows a magnification of an underlying anisotropy when using a square sample. In this reply we present more recent data obtained with a very high mobility sample, and reiterate that, with or without magnification, an anisotropic state develops in high Landau levels at very low temperatures.

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arXiv1998

Strongly Anisotropic Transport in Higher Two-Dimensional Landau Levels

Low-temperature, electronic transport in Landau levels N>1 of a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of each such Landau level the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. Such anisotropies are absent in the N=0 and N=1 Landau level, which are dominated by the states of the fractional quantum Hall effect. The transport anisotropies may be indicative of a new many particle state, which forms exclusively in higher Landau levels.

cond-mat.mes-hall↗

arXiv1998

Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels

Magneto-transport experiments on high mobility two-dimensional electron gases in GaAs/AlGaAs heterostructures have revealed striking anomalies near half-filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and non-linearities of the longitudinal resistivity rho_xx which commence only below about 150mK. These phenomena are not seen in the ground or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state.

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arXiv1998

Observation of Collective Excitations of the Dilute 2D Electron System

We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilute 2D electron systems reaching densities less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical points in the spin inter--Landau level mode show a pronounced softening as r_s is increased. Instead of a soft mode instability predicted by Hartree--Fock calculations for r_s ~ 3.3, we find evidence of multiple rotons in the dispersion of the softening spin excitations. Extrapolation of the data indicates the possibility of an instability for r_s >~ 11.

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arXiv1998

Effective Mass and g-Factor of Four Flux Quanta Composite Fermions

We investigate the properties of composite fermions with four attached flux quanta through tilted field experiments near Landau level filling factor nu = 3/4. The observed collapse of fractional quantum Hall gaps in the vicinity of this quarter-filling state can be comprehensively understood in terms of composite fermions with mass and spin. Remarkably, the effective mass and g-factor of these four flux quanta composite fermions around nu = 3/4 are very similar to those of two flux quanta composite fermions around nu = 3/2.

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arXiv1998

Many-body correlations probed by plasmon-enhanced drag measurements in double quantum well structures

Electron drag measurements of electron-electron scattering rates performed close to the Fermi temperature are reported. While evidence of an enhancement due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys. Rev. Lett. 73, 3572 (1994)], is found, important differences with the random-phase approximation based calculations are observed. Although static correlation effects likely account for part of this difference, it is argued that correlation-induced multiparticle excitations must be included to account for the magnitude of the rates and observed density dependences.

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arXiv1998

Negative Electron Drag and Hole-Like behavior in the Integer Quantum Hall Regime

Electron drag between two two-dimensional electron gases in magnetic fields has been observed with a polarity opposite that for zero field. This negative drag requires that the electrons have a hole-like dispersion. Density dependence measurements in the integer quantum Hall regime show that drag is negative only when the upper Landau level of one layer is more than half filled while the other is less than half filled, indicating that hole-like dispersion is present in a half of each Landau level. Negative drag is argued to be a consequence of disorder.

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arXiv1998

Optically Pumped NMR Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factor nu=1/3

The Knight shift of Ga-71 nuclei is measured in two different electron-doped multiple quantum well samples using optically pumped NMR. These data are the first direct measurements of the electron spin polarization, P(nu,T)= /max , near nu=1/3. The P(T) data at nu=1/3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12 Tesla field. The observed depolarization is quite small, consistent with an average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which does not appear to be explicable by the current theoretical understanding of the FQHE near nu=1/3.

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arXiv1998

Ballistic electron transport exceeding 160 microns in an undoped GaAs/AlGaAs FET

We report measurements of GaAs/AlGaAs undoped field-effect transistors in which two-dimensional electron gases (2DEGs) of exceptional quality and versatility are induced without modulation doping. Electron mobilities at T=4.2 K and density 3 E11 /cm^2 exceed 4 E6 cm^2/Vs. At lower temperatures, there is an unusually large drop in scattering, such that the mobility becomes too high to measure in 100 micron samples. Below T=2.5 K, clear signatures of ballistic travel over path lengths in excess of 160 microns are observed in magnetic focusing experiments. Multiple reflections at the edges of the 2DEG indicate a high degree of specularity.

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