J. Jo

High quality electron system with variable electron layer thickness in a parabolic quantum well

We report the realization of a high quality electron system with variable areal density (ns ) in a selectively doped, parabolic Ax Ga1−x As well. For each ns, quantum oscillations in the magnetoresistance are analyzed to obtain the electron densities of the electric subbands. These densities are in good agreement with the predictions of self‐consistent calculations of the subband structure. The data reveal that with increasing ns , the width of the electron system increases so that the effective three‐dimensional density and the Fermi energy remain essentially constant. The dependence of the low‐temperature electron mobility on ns is also reported.

Novel superlattice in a selectively doped wide parabolic quantum well with a modulated potential

We report the realization of a novel superlattice which contains a high mobility (≂ 1.1×105 cm2/V s at 4 K) degenerate electron system. The structure consists of a wide, undoped AlxGa1−xAs well bounded by undoped (spacer) and doped layers of AlyGa1−yAs (y≳x) on both sides. The alloy composition in the well (x) is graded in such a way as to result in a parabolic potential with an additional sinusoidal modulation superimposed on it. Once transferred into this well, the electrons screen the parabolic potential and an electron system with a modulated charge density is obtained. We present self‐consistent quantum mechanical calculations of the electronic system, and report our characterization of the structure by secondary‐ion mass spectrometry and magnetotransport measurements.

Magnetotransport in a wide parabolic quantum well

Measurements of the magnetoresistance of a high-quality electron system realized in a selectively doped, parabolic AlxGa1−xAs well are reported. Quantum oscillations in the magnetoresistance are analyzed to obtain the electron densities of the electric subbands. These densities are in agreement with the predictions of a self-consistent calculation of the subband structure for this system. With the magnetic field B slightly tilted away from the sample plane, we observe a dramatic manifestation of the subband-Landau-level coupling. At low B, the magnetic depopulation of the hybrid (electric-magnetic) subbands occurs, while at higher fields, Shubnikov-de Haas-like oscillations (periodic in 1B) are observed as the Fermi level crosses the quantized energy levels associated with the lowest hybrid subband.

Fabrication of quasi-three-dimensional electron systems and superlattices in wide parabolic wells

Abstract We report the realization of quasi-three-dimensional electron systems in selectively-doped wide parabolic quantum wells, focusing on a novel superlattice which contains a high-mobility ( ≈ 1.1×10 5 cm 2 /V⋯s at 4 K) degenerate electron system. This molecular beam epitaxy grown structure is a wide undoped Al x Ga 1−x As well bounded by undoped (spacer) and doped layers of Al y Ga 1−y As(y > x) on both sides. The alloy composition in the well is graded in a way that results in a parabolic potential with a sinusoidal modulation superimposed on it. Once transferred into this well, the electrons screen the parabolic potential and an electron system with a modulated charge density profile is obtained. We present self-consistent quantum mechanical calculations of the electronic system and our characterization of the structure by secondary ion mass spectrometry and magnetotransport measurements.

Magnetotransport properties of double-layer systems in wide single quantum wells

Abstract We report magnetotransport measurements in coupled double-layer electron systems realized in wide quantum wells. In these systems the distance and the coupling between the layers can be changed continuously by varying the electron density in the well. When the coupling between the two electron layers is sufficiently weak, we observe the absence of quantum Hall states at odd filling factors. Our results are qualitatively consistent with a recent theoretical model proposed for the magnetic-field-driven destruction of the quatum Hall effect in double quantum wells.

Magnetotransport of a low-disorder triple-layer electron system in perpendicular or parallel magnetic fields

We report magnetotransport measurements on a coupled, triple-layer electron system subjected to either perpendicular (B⊥) or parallel (B∥) magnetic fields. The B⊥ data reveal an abrupt collapse of the v = 1 quantum Hall state as we increase the density in the side-wells. The results suggest the system makes a triple- to a double-layer transition at high B⊥ which is driven by interlayer electron-electron interactions. In B∥ data, we observe two types of resistance oscillations as we decrease front-gate bias: One corresponds to the passage of the Fermi level through the partial energy gaps arising from the B∥-induced level anticrossings, the other is due to the enhanced Coulomb scattering as the system makes layer transitions.

Fractional quantum Hall effect in multilayer electron systems

Abstract We present several novel quasi-two-dimensional electron systems with an additional degree of freedom. These include systems which contain two or more layers of electrons in close proximity so that interlayer Coulomb interactions are strong, and thick-layer electron systems in wide parabolic wells. The magneto-transport data in these systems exhibit new phenomena: examples include the observation of a fractional quantum Hall state at the even-denominator v= 1 2 filling and an insulating phase reentrant around the v=1/3 fractional state in a double-layer electron system in a wide quantum well.

Dependence of the Fractional Quantum Hall Effect Energy Gap on Electron Layer Thickness

A fundamental characteristic of the fractional quantum Hall effect (FQHE) is the existence of an energy gap (Δ) separating the ground state from its elementary quasi-particle and quasi-hole excitations [1], In this paper we report an experimental study of the dependence of Δ for the FQHE states at Landau-level filling factors v = 1/3 and 2/3 on the thickness of the electron layer. We have determined Δ from the temperature dependence of the magnetoresistance minima at v = 1/3 and 2/3 in a variable-width electron system realized in a selectively-doped, parabolic AlGaAs quantum well. We observe a dramatic decrease in the measured Δ for the FQHE states at these filling factors with increasing electron layer thickness. This dramatic decrease of Δ may manifest the collapse of FQHE electron correlations expected in the theory of the FQHE as either the electron layer thickness is increased or the subband separation is decreased.