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

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

Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field

Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure.Comment: 6 pages, 5 figure

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

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

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

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

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

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

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

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

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

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

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

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

Magnetic breakdown in double quantum wells

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