Transference of Fermi Contour Anisotropy to Composite Fermions.

There has been a surge of recent interest in the role of anisotropy in interaction-induced phenomena in two-dimensional (2D) charged carrier systems. A fundamental question is how an anisotropy in the energy-band structure of the carriers at zero magnetic field affects the properties of the interacting particles at high fields, in particular of the composite fermions (CFs) and the fractional quantum Hall states (FQHSs). We demonstrate here tunable anisotropy for holes and hole-flux CFs confined to GaAs quantum wells, via applying in situ in-plane strain and measuring their Fermi wave vector anisotropy through commensurability oscillations. For strains on the order of 10^{-4} we observe significant deformations of the shapes of the Fermi contours for both holes and CFs. The measured Fermi contour anisotropy for CFs at high magnetic field (α_{CF}) is less than the anisotropy of their low-field hole (fermion) counterparts (α_{F}), and closely follows the relation α_{CF}=sqrt[α_{F}]. The energy gap measured for the ν=2/3 FQHS, on the other hand, is nearly unaffected by the Fermi contour anisotropy up to α_{F}∼3.3, the highest anisotropy achieved in our experiments.

Graphene-MoS2 hybrid structures for multifunctional photoresponsive memory devices.

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.

Ferromagnetic fractional quantum Hall states in a valley-degenerate two-dimensional electron system.

We study a two-dimensional electron system where the electrons occupy two conduction band valleys with anisotropic Fermi contours and strain-tunable occupation. We observe persistent quantum Hall states at filling factors nu=1/3 and 5/3 even at zero strain when the two valleys are degenerate. This is reminiscent of the quantum Hall ferromagnet formed at nu=1 in the same system at zero strain. In the absence of a theory for a system with anisotropic valleys, we compare the energy gaps measured at nu=1/3 and 5/3 to the available theory developed for single-valley, two-spin systems, and find that the gaps and their rates of rise with strain are much smaller than predicted.

Dependence of effective mass on spin and valley degrees of freedom.

We measure the effective mass (m) of interacting two-dimensional electrons confined to an AlAs quantum well while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively. Compared to its band value, m is enhanced unless the electrons are fully valley- and spin-polarized. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior.

Effective mass suppression in dilute, spin-polarized two-dimensional electron systems.

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 spin-polarized, 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.