Combining the electronic properties of graphene and molybdenum disulphide (MoS2) in hybrid heterostructures offers the possibility to create devices with various functionalities. Electronic logic and memory devices have already been constructed from graphene-MoS2 hybrids, but they do not make use of the photosensitivity of MoS2, which arises from its optical-range bandgap. Here, we demonstrate that graphene-on-MoS2 binary heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 1 × 10(10) A W(-1) at 130 K and 5 × 10(8) A W(-1) at room temperature, making them the most sensitive graphene-based photodetectors. When subjected to time-dependent photoillumination, the hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. These effects can be quantitatively explained by gate-tunable charge exchange between the graphene and MoS2 layers, and may lead to new graphene-based optoelectronic devices that are naturally scalable for large-area applications at room temperature.

We report magnetotransport measurements of fractional quantum Hall states in an AlAs quantum well around a Landau level filling factor nu=3/2, demonstrating that the quasiparticles are composite fermions (CFs) with a valley degree of freedom. By monitoring the valley level crossings for these states as a function of applied symmetry-breaking strain, we determine the CF valley susceptibility and polarization. The data can be explained well by a simple Landau level fan diagram for CFs, and are in nearly quantitative agreement with the results reported for CF spin polarization.

When interacting two-dimensional electrons are placed in a large perpendicular magnetic field, to minimize their energy, they capture an even number of flux quanta and create new particles called composite fermions (CFs). These complex electron-flux-bound states offer an elegant explanation for the fractional quantum Hall effect. Furthermore, thanks to the flux attachment, the effective field vanishes at a half-filled Landau level and CFs exhibit Fermi-liquid-like properties, similar to their zero-field electron counterparts. However, being solely influenced by interactions, CFs should possess no memory whatever of the electron parameters. Here we address a fundamental question: Does an anisotropy of the electron effective mass and Fermi surface (FS) survive composite fermionization? We measure the resistance of CFs in AlAs quantum wells where electrons occupy an elliptical FS with large eccentricity and anisotropic effective mass. Similar to their electron counterparts, CFs also exhibit anisotropic transport, suggesting an anisotropy of CF effective mass and FS.

We report effective mass (m ) measurements, via analyzing the temperature dependence of the Shubnikov-de Haas oscillations, for dilute, interacting, two-dimensional electron systems (2DESs) occupying a single conduction-band valley in AlAs quantum wells. When the 2DES is partially spinpolarized, m is larger than its band value, consistent with previous results on various 2DESs. However, as we fully spin-polarize the 2DES by subjecting it to strong parallel magnetic fields, m is unexpectedly suppressed and falls even below the band mass.In a crystalline solid, electrons moving in the periodic potential of ions are described as quasiparticles with a band effective mass, m b , which is inversely proportional to the curvature of the energy versus wave vector (band) dispersion. In the presence of electron-electron interaction, in the Fermi liquid description, the electrons can still be treated as quasiparticles but with a further renormalized effective mass, m . Numerous studies, both experimental and theoretical, have indeed reported that for low-disorder, dilute, two-dimensional electron systems (2DESs), m is typically larger than m b at very low densities (n), and increases as n is reduced and the system is made more interacting [1][2][3][4][5][6][7][8][9][10][11][12]. Note that the parameter r s , defined as the ratio of the Coulomb to kinetic (Fermi) energy, increases as the 2DES is made more dilute [13]. Here, we report m measurements in dilute 2DESs confined to AlAs quantum wells. Our main finding, shown in Fig. 1, is that m depends not only on n but also on the spin-polarization of the 2DES. When the 2DES is partially spin-polarized, the measured m is larger than m b , consistent with most previous reports on other 2DESs. When we fully spinpolarize the 2DES by applying a parallel magnetic field, however, m is strongly suppressed to values below m b .Our AlAs quantum well samples were grown, using molecular beam epitaxy, on semi-insulating (001) GaAs substrates. The AlAs wells in these samples have widths of 11 nm (sample A), 12 nm (sample B), or 15 nm (samples C and D). They are bounded by AlGaAs barriers and are modulation-doped with Si [14]. In our samples, thanks to a combination of residual and applied uniaxial in-plane strain [14], the electrons occupy one conduction band minimum (valley) with an anisotropic (elliptical) Fermi contour, characterized by transverse and longitudinal band effective masses, m t 0:205m e and m l 1:05m e , where m e is the free electron mass. This means that the relevant (density-of-states) band effective mass in our 2DES is m b m t m l p 0:46m e ; this is the value to which we normalize and report all our measured masses.The samples were Hall bar or van der Pauw mesas, fitted with back and front gates to tune n. For the density range 0:55 ÿ 4:8 10 11 cm ÿ2 , the low-temperature mobilities for the samples are between 0.9 and 6 m 2 =V s when current is passed along the low-mobility (longitudinal) direction of the occupied valley; for current along the high-mobility (transverse) d...

Using symmetry breaking strain to tune the valley occupation of a two-dimensional (2D) electron system in an AlAs quantum well, together with an applied in-plane magnetic field to tune the spin polarization, we independently control the system's valley and spin degrees of freedom and map out a spin-valley phase diagram for the 2D metal-insulator transition. The insulating phase occurs in the quadrant where the system is both spin-and valley-polarized. This observation establishes the equivalent roles of spin and valley degrees of freedom in the 2D metal-insulator transition.PACS numbers: 71.30.+h, 73.43.Qt, 73.50.Dn, The scaling theory of localization in two dimensions [1], which predicts an insulating phase for two-dimensional electron systems (2DESs) with arbitrarily weak disorder, was challenged by the observation of a metallic temperature dependence (dρ/dT > 0) of the resistivity, ρ, in low-disorder Si metal-oxide-semiconductor field-effect transistors (Si-MOSFETs) [2]. The associated metal-toinsulator transition (MIT) has subsequently become the subject of intense interest and controversy [3]. While behavior similar to that of Ref.[2] has now been reported for a wide variety of 2D carrier systems such as n-AlAs, 10], and p-Si/SiGe [11,12], the origin of the metallic state and its transition into the insulating phase remain major puzzles in solid state physics.Several experiments have demonstrated the important role of the spin degree of freedom in the MIT problem, either in systems with a strong spin-orbit interaction [10,13,14], or via the application of an external magnetic field to spin polarize the carriers [15,16,17,18,19]. The latter experiments have shown that a magnetic field applied parallel to the 2DES plane suppresses the metallic temperature dependence, ultimately driving the 2DES into the insulating regime as the 2DES is spin polarized. The relevance of multiple conduction-band valleys, on the other hand, is less known. Although it has been discussed theoretically that the occupation of multiple valleys may also be important [20,21], there has been no direct experimental demonstration. Here we show that the electrons' valley degree of freedom indeed plays a crucial role, analogous to that of spin. We study a 2DES, confined to an AlAs quantum well, in which we can independently tune both the spin and valley degrees of freedom. By studying the temperature dependence of ρ at various degrees of spin and valley polarization, we map out the metal-insulator phase diagram in this system at a constant density. The 2DES exhibits a metallic behavior when either the valley or spin are left fully unpolarized, and a minimum amount of both spin and valley polarization is required to enter the insulating phase.We performed experiments on 2DESs confined to modulation-doped, AlAs quantum wells of width 11 nm and 15 nm [22]. In these systems, the electrons occupy two conduction-band valleys of AlAs centered at the edges of the Brillouin zone along the [100] and [010] directions. We denote these valleys as X and Y...