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Novel magnetic-field-induced minigap and transport in coupled double quantum wells

Description: A review is given of recent theoretical and experimental work on in-plane electron transport in strongly coupled double quantum wells (QWs) in the presence of an in-plane magnetic field B{sub {parallel}} {parallel} x. This system displays unusual electronic and transport properties arising from a partial minigap ({approximately} a few meV) formed in the transverse in-plane direction k{sub y} {perpendicular} B{sub {parallel}} in k-space due to the anticrossing of the two QW dispersion curves displaced relative to each other by {Delta}k{sub y} {proportional_to} B{sub {parallel}}. Sweeping B{sub {parallel}} moves the minigap through the Fermi level ({mu}), deforming the Fermi surface from a two-component surface (with one orbit inside the other) to a single-orbit surface, and then back to a two-separated-orbit structure, accordingly as {mu} lies above, inside, and below the gap, respectively. The authors show that the density of states develops a sharp van Hove singularity at the lower gap edge, while transport properties such as the in-plane conductance and the cyclotron mass show sharp B{sub {parallel}}-dependent structures as {mu} passes through the gap edges.
Date: August 1, 1995
Creator: Lyo, S.K.; Simmons, J.A.; Harff, N.E. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Electron transport in coupled double quantum wells and wires

Description: Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.
Date: April 1997
Creator: Harff, N. E.; Simmons, J. A. & Lyo, S. K.
Partner: UNT Libraries Government Documents Department

Magnetoresistance and cyclotron mass in extremely-coupled double quantum wells under in-plane magnetic fields

Description: The authors experimentally investigate the transport properties of an extremely-coupled AlGaAs/GaAs double quantum well, subject to in-plane magnetic fields (B{sub {parallel}}). The coupling of the double quantum well is sufficiently strong that the symmetric-antisymmetric energy gap ({Delta}{sub SAS}) is larger than the Fermi energy (E{sub F}). Thus for all B{sub {parallel}} only the lower energy branch of the dispersion curve is occupied. In contrast to systems with weaker coupling such that {Delta}{sub SAS} < E{sub F} the authors find: (1) only a single feature, a maximum, in the in-plane magnetoresistance, (2) a monotonic increase with B{sub {parallel}} in the cyclotron mass up to 2.2 times the bulk GaAs mass, and (3) an increasing Fermi surface orbit area with B{sub {parallel}}, in good agreement with theoretical predictions.
Date: December 1, 1997
Creator: Blount, M.A.; Simmons, J.A.; Lyo, S.K.; Harff, N.E. & Weckwerth, M.V.
Partner: UNT Libraries Government Documents Department

Tuning a double quantum well Fermi surface with in-plane magnetic fields

Description: A double quantum well (QW) subject to in-plane magnetic fields B{sub {parallel}} has the dispersion curves of its two QWs shifted in k-space. When the QWs are strongly coupled, an anticrossing and partial energy gap occur, yielding a tunable multi-component Fermi surface. We report measurements of the resultant features in the conductance, capacitive density of states, and giant deviations in cyclotron effective masses.
Date: July 1, 1995
Creator: Simmons, J.A.; Harff, N.E.; Eiles, T.M.; Lyo, S.K. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Magnetic modulations of optical and transport properties of N-doped coupled double quantum wells

Description: Magnetoquantum resistance (MR) in a perpendicular magnetic field (B{sub {perpendicular}}) and photoluminescence (PL) spectra are shown to be sensitively modulated by an in-plane field (B{sub {parallel}}) due to the B{sub {parallel}}-induced anticrossing of the energy-dispersion curves of the two quantum wells (QWs). Using a self-consistent density functional theory, they find very different B{sub {parallel}}-evolutions of the PL spectra for symmetric and asymmetric double QWs consistent with recent data. The MR is calculated using a linear response theory. The results consist of a superposition of two series of MR oscillations represented by ridges running nearly perpendicular to each other in the B = (B{sub {parallel}}, B{sub {perpendicular}}) plane. The data from GaAs/AlGaAs double QWs agree with this behavior.
Date: August 1, 1998
Creator: Lyo, S.K.; Simmons, J.A.; Huang, D. & Harff, N.E.
Partner: UNT Libraries Government Documents Department

Magnetic-field-induced tunneling and minigap transport in double quantum wells

Description: We review recent theoretical and experimental results on low- temperature tunneling and in-plane transport properties in double quantum wells (DQWs) in an in-plane magnetic field B{parallel}. These properties arise from combined effect of B{parallel}-induced relative displacement of the wave vectors in the two QWs and the interwell tunneling. In weakly coupled DQWs, the tunneling conductance has two sharp maxima as a function of B{parallel}. In strongly coupled DQWs, a partial minigap is formed due to anticrossing of the two QW dispersion curves, yielding sharp B{parallel}-dependent structures in the density of states and in- plane transport properties. Excellent agreement is obtained between theory and data from GaAs/AlGaAs DQWs.
Date: August 1, 1995
Creator: Lyo, S.K.; Simmons, J.A.; Harff, N.E.; Eiles, T.M. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Magnetic breakdown in double quantum wells

Description: The authors find that a sufficiently large perpendicular magnetic field (B{sub {perpendicular}}) causes magnetic breakdown (MB) in coupled double quantum wells (QWs) that are subject to an in-plane magnetic field (B{sub {parallel}}). B{sub {parallel}} shifts one QW dispersion curve with respect to that of the other QW, resulting in an anticrossing and an energy gap. When the gap is below the Fermi level the resulting Fermi surface (FS) consists of two components, a lens-shaped inner orbit and an hour-glass shaped outer orbit. B{sub {perpendicular}} causes Landau level formation and Shubnikov-de Haas (SdH) oscillations for each component of the FS. MB occurs when the magnetic forces from B{sub {perpendicular}} become dominant and the electrons move on free-electron circular orbits rather than on the lens and hour-glass orbits. MB is observed by identifying the peaks present in the Fourier power spectrum of the longitudinal resistance vs. 1/B{sub {perpendicular}} at constant B{sub {parallel}}, an arrangement achieved with an in-situ tilting sample holder. Results are presented for two strongly coupled GaAs/AlGaAs DQW samples.
Date: August 1, 1996
Creator: Harff, N.E.; Simmons, J.A.; Klem, J.F.; Boebinger, G.S.; Pfeiffer, L.N. & West, K.W.
Partner: UNT Libraries Government Documents Department

Magnetic breakdown and Landau level spectra of a tunable double-quantum-well Fermi surface

Description: By measuring longitudinal resistance, the authors map the Landau level spectra of double quantum wells as a function of both parallel (B{sub {parallel}}) and perpendicular (B{sub {perpendicular}}) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B{sub {parallel}}. This causes the two corresponding ladders of Landau levels formed at finite B{sub {perpendicular}} to exhibit multiple crossings. They also observe a third set of landau levels, independent of B{sub {parallel}}, which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full quantum mechanical calculations show good agreement with the data.
Date: December 31, 1997
Creator: Simmons, J.A.; Harff, N.E.; Lyo, S.K.; Klem, J.F.; Boebinger, G.S.; Pfeiffer, L.N. et al.
Partner: UNT Libraries Government Documents Department

Composite fermions in 2 x 10{sup 6} cm{sup 2}/Vs mobility A1GaAs/GaAs heterostructures grown by MOCVD

Description: Recent growth by MOCVD (metalorganic chemical vapor deposition) of 2.0x10{sup 6} cm{sup 2}/Vs mobility heterostructures are reported. These mobilities, the highest reported to date, are attributed to use of tertiarybutylarsine as the arsenic precursor. Measurements in tilted magnetic fields of the fractional quantum Hall effect states near filling factor 3/2 are consistent with a spin-split composite fermion (CF) model proposed earlier. Extracted values of the product of the CF g-factor and CF effective mass agree with values previously obtained for MBE samples.
Date: December 31, 1996
Creator: Simmons, J.A., Chui, H.C., Harff, N.E., Hammons, B.E. & Du, R.R., Zudov, M.A.
Partner: UNT Libraries Government Documents Department

Composite fermions in 2 {times} 10{sup 6} cm{sup 2}/Vs mobility AlGaAs/GaAs heterostructures grown by MOCVD

Description: The authors report on the recent growth by MOCVD of 2.0 {times} 106 cm2/Vs mobility heterostructures. These mobilities, the highest reported to date, are attributed to the use of tertiarybutylarsine as the arsenic precursor. Measurements in tilted magnetic fields of the fractional quantum Hall effect (FQHE) states near filling factor 3/2 are consistent with a spin-split composite fermion (CF) model proposed earlier. The extracted values of the product of the CF g-factor and CF effective mass agree with values previously obtained for MBE samples.
Date: August 1, 1996
Creator: Simmons, J.A.; Chui, H.C.; Harff, N.E.; Hammons, B.E.; Du, R.R. & Zudov, M.A.
Partner: UNT Libraries Government Documents Department

Electron phase coherent effects in nanostructures and coupled 2D systems

Description: This report describes the research accomplishments achieved under the LDRD Project ``Electron Phase Coherent Effects in Nanostructures and Coupled 2D Systems.`` The goal of this project was to discover and characterize novel quantum transport phenomena in small semiconductor structures at low temperatures. Included is a description of the purpose of the research, the various approaches used, and a detailed qualitative description of the numerous new results obtained. The first appendix gives a detailed listing of publications, presentations, patent applications, awards received, and various other measures of the LDRD project success. Subsequent appendices consist of reprinted versions of several specific,`` scientific journal publications resulting from this LDRD project.
Date: May 1995
Creator: Simmons, J. A.; Lyo, S. K.; Klem, J. F.; Sherwin, M. E.; Harff, N. E.; Eiles, T. M. et al.
Partner: UNT Libraries Government Documents Department