We report direct measurements of the valley susceptibility, the change of valley population in response to applied symmetry-breaking strain, in an AlAs two-dimensional electron system. As the two-dimensional density is reduced, the valley susceptibility dramatically increases relative to its band value, reflecting the system's strong electron-electron interaction. The increase has a remarkable resemblance to the enhancement of the spin susceptibility and establishes the analogy between the spin and valley degrees of freedom.PACS numbers: 71.70.Fk , 73.43.Qt Currently, there is considerable interest in controlled manipulation of electron spin in semiconductors. This interest partly stems from the technological potential of spintronics, namely, the use of carrier spin to realize novel electronic devices. More important, successful manipulation of spins could also impact the more exotic field of quantum computing since many of the current proposals envision spin as the quantum bit (qubit) of information [1,2,3]. Here we describe measurements of another property of electrons, namely their valley degree of freedom, in a semiconductor where they occupy multiple conduction band minima (valleys) [ Fig. 1(a)]. Specifically, for a two-valley, two-dimensional electron system (2DES) in an AlAs quantum well, we have determined the "valley susceptibility", χ v , i.e., how the valley populations respond to the application of symmetry-breaking strain. This is directly analogous to the spin susceptibility, χ s , which specifies how the spin populations respond to an applied magnetic field [Figs. 1(d)]. Our data show that χ v and χ s have strikingly similar behaviors, including an interaction-induced enhancement at low electron densities. The results establish the general analogy between the spin and valley degrees of freedom, implying the potential use of valleys in applications such as quantum computing. We also discuss the implications of our results for the controversial metal-insulator transition problem in 2D carrier systems.It is instructive to describe at the outset the expressions for the band values of spin and valley susceptibilities χ s,b and χ v,b [4]. The spin susceptibility is defined as χ s,b = d∆n/dB = g b µ B ρ/2, where ∆n is the net spin imbalance, B is the applied magnetic field, g b is the band Landé g-factor, and ρ is the density of states at the Fermi level. Inserting the expression ρ = m b /πh 2 for 2D electrons, we have χ s,b = (µ B /2πh 2 )g b m b , where m b is the band effective mass. In analogy to spin, we can define valley susceptibility as χ v,b = d∆n/dǫ = ρE 2,b = (1/πh 2 )m b E 2,b , where ∆n is the difference between the populations of the majority and minority valleys, ǫ is strain, and E 2,b is the conduction band deformation potential [5]. In a Fermi liquid picture, the interparticle interaction results in replacement of the parameters m b , g b , and E 2,b [6] by their normalized values m * , g * , and E * 2 . Note that χ s ∝ m * g * and χ v ∝ m * E * 2 . Our experiments were performed on...

Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the Xpoints of the Brillouin zone. These valleys have large effective mass and g-factor compared to the standard GaAs electrons, and are also highly anisotropic. With proper choice of well width and by applying symmetry-breaking strain in the plane, one can control the occupation of different valleys thus rendering a system with tuneable effective mass, g-factor, Fermi contour anisotropy, and valley degeneracy. Here we review some of the rich physics that this system has allowed us to explore.

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...

We demonstrate the use of a piezoelectric actuator to apply, at low-temperatures, uniaxial stress in the plane of a two-dimensional electron system confined to a modulation-doped AlAs quantum well. Via the application of stress, which can be tuned in-situ and continuously, we control the energies and occupations of the conduction-band minima and the electronic properties of the electron system. We also report measurements of the longitudinal and transverse strain versus bias for the actuator at 300, 77, and 4.2K. A pronounced hysteresis is observed at 300 and 77K, while at 4.2K strain is nearly linear and shows very little hysteresis with the applied bias.

We report phenomena manifesting nonequilibrium superconductivity induced by spin-polarized quasiparticles in perovskite ferromagnet-insulator-superconductor ͑F-I-S͒ heterostructures. Measurements of the critical current J c , using a pulsed current technique to minimize Joule heating, reveal a monotonic increase with increasing insulator thickness; and for F-IS with thin insulating barriers, a slight increase in J c is observed under small injection currents I m from the ferromagnet, followed by a strong suppression of J c under large I m. In contrast, no effect of injection on J c can be detected in the N-IS control sample ͑N: nonmagnetic metal͒. The spin diffusion time and length in cuprate superconductors are also estimated and compared with our experimental results. ͓S0163-1829͑99͒05638-6͔