Biomedical subjects

L N Pfeiffer

Publications and source records attributed to L N Pfeiffer.

At least 37 records · Page 2Linked to original sources

PubMed2005

Measurements of the density-dependent many-body electron mass in two dimensional GaAs/AlGaAs heterostructures.

We determine the density-dependent electron mass m(*) in two-dimensional electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov-de Haas measurements. Using very high-quality transistors with tunable electron densities we measure m(*) in single, high mobility specimens over a wide range of r(s) (6 to 0.8). Toward low densities we observe a rapid increase of m(*) by as much as 40%. For 2>r(s)>0.8 the mass values fall approximately 10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail.

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PubMed2004

Competition between a fractional quantum hall liquid and bubble and Wigner crystal phases in the third Landau level.

Magnetotransport measurements were performed in an ultrahigh mobility GaAs/AlGaAs quantum well of density approximately 3.0 x 10(11) cm(-2). The temperature dependence of the magnetoresistance Rxx was studied in detail in the vicinity of nu=9/2. In particular, we discovered new minima in Rxx at a filling factor nu approximately 41/5 and 44/5, but only at intermediate temperatures 80 approximately less than T approximately less than 120 mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0.

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PubMed2004

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime.

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.

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PubMed2004

Evidence for two different solid phases of two-dimensional electrons in high magnetic fields.

We have observed two different rf resonances in the frequency dependent real diagonal conductivity of very high quality two-dimensional electron systems in the high magnetic field insulating phase and interpret them as coming from two different pinned electron solid phases (labeled as "A" and "B"). The "A" resonance is observable for Landau level filling nu<2/9 [reentrant around the nu=1/5 fractional quantum Hall effect (FQHE)] and then crosses over to the different "B" resonance which dominates at sufficiently low nu. Moreover, the "A" resonance is found to show dispersion with respect to the size of the transmission line, indicating that the "A" phase has a large correlation length. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.

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PubMed2004

Evidence of a first-order phase transition between Wigner-crystal and bubble phases of 2D electrons in higher Landau Levels.

For filling factors nu in the range between 4.16 and 4.28, we simultaneously detect two resonances in the real diagonal microwave conductivity of a two-dimensional electron system (2DES) at low temperature T approximately 35 mK. We attribute the resonance to Wigner-crystal and Bubble phases of the 2DES in higher Landau Levels. For nu below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first-order phase transition. We estimate the transition point as nu=4.22.

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PubMed2004

Electron correlation in the second Landau level: a competition between many nearly degenerate quantum phases.

At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed nu=2+2/5 state separated from the even-denominator nu=2+1/2 state by an insulating phase and a nu=2+2/7 and nu=2+1/5 state surrounded by such phases. A developing plateau at nu=2+3/8 points to the existence of other even-denominator states.

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PubMed2004

Spin separation in cyclotron motion.

Charged carriers with different spin states are spatially separated in a two-dimensional hole gas. Because of strong spin-orbit interaction, holes at the Fermi energy in GaAs have different momenta for two possible spin states traveling in the same direction, and, correspondingly, different cyclotron orbits in a weak magnetic field. Two point contacts, acting as a monochromatic source of ballistic holes and a narrow detector arranged in the magnetic focusing geometry are demonstrated to work as a tunable spin filter.

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PubMed2004

Vanishing Hall resistance at high magnetic field in a double-layer two-dimensional electron system.

At total Landau level filling factor nu(tot)=1 a double-layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence. This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers. In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low-temperature limit. This finding supports the prediction that the ground state at nu(tot)=1 is an excitonic superfluid.

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PubMed2004

Evidence for current-flow anomalies in the irradiated 2D electron system at small magnetic fields.

We report experimental transport results in 2D electron systems exposed to dipole radiation up to 20 GHz. Magnetoresistance oscillations occur as seen with higher frequency radiation; however, minima here can be seen to extend to negative biases, and zeros are not observed persistently around sample perimeters. Under radiation, voltages are generated from internal to external contacts in the absence of applied driving currents. These findings may be consistent with theoretical pictures of current instabilities due to local negative resistivities.

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PubMed2004

Possible observation of phase coexistence of the nu=1/3 fractional quantum hall liquid and a solid.

We have measured the magnetoresistance of a very low density and extremely high quality two-dimensional hole system. With increasing magnetic field applied perpendicularly to the sample we observe the sequence of insulating, nu=1/3 fractional quantum Hall liquid, and insulating phases. In both of the insulating phases in the vicinity of the nu=1/3 filling the magnetoresistance has an unexpected oscillatory behavior with the magnetic field. These oscillations are not of the Shubnikov-de Haas type and cannot be explained by spin effects. They are most likely the consequence of the formation of a new electronic phase which is intermediate between the correlated Hall liquid and a disorder pinned solid.

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PubMed2004

Ultrafast dynamic control of spin and charge density oscillations in a GaAs quantum well.

We use subpicosecond laser pulses to generate and monitor in real time collective oscillations of electrons in a modulation-doped GaAs quantum well. The observed frequencies match those of intersubband spin- and charge-density excitations. Light couples to coherent density fluctuations through resonant stimulated Raman scattering. Because the spin- and charge-related modes obey different selection rules and resonant behavior, the amplitudes of the corresponding oscillations can be independently controlled by using shaped pulses of the proper polarization.

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PubMed2004

Cascade of quantum phase transitions in tunnel-coupled edge states.

We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields B(c) is a function of a single scaling argument /B-B(c)/T(-kappa), where the exponent kappa=0.42. This puzzling resemblance to a quantum Hall-insulator transition points to the importance of interedge correlation between the coupled edge states.

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PubMed2004

Nonlinear response of a clean one-dimensional wire.

We study nonlinear transport in a clean one-dimensional wire fabricated by cleaved edge overgrowth in molecular beam epitaxy. At low electron densities, and with a large applied bias, we observe a feature in the differential conductance similar to the so-called "0.7 structure," found in quantum point contact devices. Using a simple model we suggest a link between this phenomenon, charge neutrality, and unidirectional dynamics in the wire.

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PubMed2004

Metastable resistance-anisotropy orientation of two-dimensional electrons in high Landau levels.

In half-filled high Landau levels, two-dimensional electron systems possess collective phases which exhibit a strongly anisotropic resistivity tensor. A weak, but as yet unknown, rotational symmetry-breaking potential native to the host semiconductor structure is necessary to orient these phases in macroscopic samples. Making use of the known external symmetry-breaking effect of an in-plane magnetic field, we find that the native potential can have two orthogonal local minima. It is possible to initialize the system in the higher minimum and then observe its relaxation toward equilibrium.

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PubMed2003

Crossover and coexistence of quasiparticle excitations in the fractional quantum hall regime at nu < or =1/3.

New low-lying excitations are observed by inelastic light scattering at filling factors nu=p/(phip+/-1) of the fractional quantum Hall regime with phi=4. Coexisting with these modes throughout the range nu < or =1/3 are phi=2 excitations seen at 1/3. Both phi=2 and phi=4 excitations have distinct behaviors with temperature and filling factor. The abrupt first appearance of the new modes in the low-energy excitation spectrum at nu > or near 1/3 suggests a marked change in the quantum ground state on crossing the phi=2-->phi=4 boundary at nu=1/3.

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PubMed2003

Layer ordering and faulting in (GaAs)n/(AlAs)n ultrashort-period superlattices.

We report studies of (GaAs)(n)/(AlAs)(n) ultrashort-period superlattices using synchrotron x-ray scattering. In particular, we demonstrate that interfaces of these superlattices contain features on two different length scales: namely, random atomic mixture and ordered mesoscopic domains. Both features are asymmetric on the two interfaces (AlAs-on-GaAs and GaAs-on-AlAs) for n>2. Periodic compositional stacking faults, arising from the intrinsic nature of molecular-beam epitaxy, are found in the superlattices. In addition, the effect of growth interruption on the interfacial structure is discussed. The relevant scattering theory is developed to give excellent fits to the data.

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PubMed2003

Observation of microwave-induced zero-conductance state in Corbino rings of a two-dimensional electron system.

Using Corbino samples we have observed oscillatory dc conductance in a high-mobility two-dimensional electron system when it is subjected to crossed microwave and magnetic fields. At the strongest of the oscillation minima the conductance is found to be vanishingly small, indicating a macroscopic insulating state associated with this minimum. With increasing voltage bias, a crossover from Ohmic to electron-heating regime is observed.

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PubMed2003

Microwave resonance of the 2D Wigner crystal around integer Landau fillings.

We have observed a resonance in the real part of the finite frequency diagonal conductivity using microwave absorption measurements in high quality 2D electron systems near integer fillings. The resonance exists in some neighborhood of filling factor around corresponding integers and is qualitatively similar to previously observed resonance of weakly pinned Wigner crystal in high B and very small filling factor regime. Data measured around both nu=1 and nu=2 are presented. We interpret the resonance as the signature of the Wigner crystal state around integer Landau levels.

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