Realization of an Interacting Two-Valley AlAs Bilayer System

Abstract

By using different widths for two AlAs quantum wells comprising a bilayer system, we force the X-point conduction-band electrons in the two layers to occupy valleys with different Fermi contours, electron effective masses, and g-factors. Since the occupied valleys are at different X-points of the Brillouin zone, the interlayer tunneling is negligibly small despite the close electron layer spacing. We demonstrate the realization of this system via magneto-transport measurements and the observation of a phase-coherent, bilayer ν=1 quantum Hall state flanked by a reentrant insulating phase.PACS numbers: 71.18.+y, 73.21.Fg, 73.43.Qt Two-dimensional (2D) electron systems subjected to large perpendicular magnetic fields exhibit a wealth of phenomena, such as the fractional quantum Hall effect, that are associated with electron-electron interactions. When two 2D electron systems are brought in close proximity, the additional, interlayer interaction can lead to new many-body states that have no analogue in the single-layer case. Examples include quantum Hall states (QHSs) at even-denominator fillings ν=1/2 and 3/2 [1, 2] and a special, bilayer ν=1 QHS with interlayer phase coherence [3] (ν is the Landau level filling factor of the bilayer system). Such states form when the interlayer distance is on the order of or smaller than the magnetic length. It is also often desirable to have as little interlayer tunneling as possible so that, e.g., independent contacts can be made to the two layers [4]; moreover, negligible tunneling makes the theoretical treatment of the phenomena in these systems easier.We report here the fabrication of a novel bilayer system comprised of two AlAs quantum wells (QWs) with different widths, wherein the electrons in the two layers occupy different conduction-band valleys. The key to the fabrication of our sample is the following. Bulk AlAs has an indirect band-gap with the conduction band minima at the X-points of the Brillouin zone. The constant energy ellipsoids (or valleys) formed at these minima are anisotropic with two characteristic effective masses (measured in units of the free electron mass): m t =0.2 for the two transverse directions, and m l =1 for the longitudinal direction. This is somewhat similar to Si, except that in Si there are six ellipsoids centered around six equivalent points along the ∆-lines of the Brillouin zone, while in AlAs we have three (six half-) ellipsoids at the Xpoints. When electrons are confined along the growth (z) direction in an AlAs QW, one might expect that only the out-of-plane (X Z ) valley would be occupied because the larger electron mass along the confinement direction should lower the energy of this valley. This is indeed the case in Si MOSFETs and QWs. However, in AlAs QWs grown on GaAs substrates, the strain induced by the lattice mismatch between AlAs and GaAs causes the inplane valleys to be occupied, unless the QW is narrower than a threshold value of approximately 55Å [5,6,7,8]. By growing a modulation-doped, double QW sample with well width...

Two‐dimensional electrons occupying multiple valleys in AlAs

“…We have also succeeded in fabricating 2DESs confined to narrow (< 5 nm-wide) AlAs QWs where the electrons occupy the Z valley. 9 The lowtemperature mobility in this case is limited by interface roughness scattering; 13 nevertheless, we have achieved the highest mobilities yet reported, ~ 5 × 10 4 cm 2 /Vs. 9,13 This is more than an order of magnitude larger than the highest previously reported mobility values for similar types of samples 6 and comparable to the highest mobilities reported in Si-MOSFET 2DESs.…”

“…9 The lowtemperature mobility in this case is limited by interface roughness scattering; 13 nevertheless, we have achieved the highest mobilities yet reported, ~ 5 × 10 4 cm 2 /Vs. 9,13 This is more than an order of magnitude larger than the highest previously reported mobility values for similar types of samples 6 and comparable to the highest mobilities reported in Si-MOSFET 2DESs. 14 And again, the samples reveal novel behavior, e.g., an interaction-induced enhancement of the spin-susceptibility at low densities that is the largest among all 2DESs and is remarkably close to that of an ideal, interacting 2DES.…”

“…The samples exhibit a pronounced field-effect persistent photo-conductivity, 11 which we utilize to populate the AlAs QW with electrons. We have made breakthroughs by producing both wide 7 and narrow 9 AlAs QWs with record-high low-temperature mobilities. In wide QWs, where the in-plane valleys (X and Y) are occupied, the mobilities of our best samples exceed 3 × 10 5 cm 2 /Vs.…”

“…It turns out, however, that in a properly designed structure, containing a pure AlAs layer and selectively-doped AlGaAs barriers, one can confine electrons to the AlAs layer. [2][3][4][5][6][7][8][9] In this case, the electrons occupy the AlAs X-point conduction band valleys (Fig. 1b) and have properties that are quite distinct from GaAs 2DESs.…”

Effective mass and spin susceptibility of dilute two-dimensional holes in GaAs

“…In the highly interacting, dilute regime (r s > ∼ 3), χ * and m * are typically enhanced compared to the band values and increase with increasing r s , as confirmed both theoretically [1][2][3][4][5][6][7][8] and experimentally. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Besides r s , the spin and/or valley degrees of freedom also play an important role in the re-normalization of m * and χ * since they modify the exchange interaction. 5,6,17,[24][25][26][27] In particular, it was recently demonstrated that when a 2D electron system is fully spin and valley polarized, m * is suppressed compared to its band value.…”