Interplay between disorder and intersubband collective excitations in the two-dimensional electron gas

Intersubband absorption in modulation-doped quantum wells is usually appropriately described as a collective excitation of the confined two-dimensional electron gas. At sufficiently low electron density and low temperatures, however, the in-plane disorder potential is able to damp the collective modes by mixing the intersubband charge-density excitation with single-particle localized modes. Here we show experimental evidence of this transition. The results are analyzed within the framework of the density functional theory and highlight the impact of the interplay between disorder and the collective response of the two-dimensional electron gas in semiconductor heterostructures.Comment: 5 pages, 4 figures, RevTeX. Accepted for publication in Phys. Rev. B (Rapid. Comm.

First-Order Phase Transition in a Quantum Hall Ferromagnet

The single-particle energy spectrum of a two-dimensional electron gas in a perpendicular magnetic field consists of equally-spaced spin-split Landau levels, whose degeneracy is proportional to the magnetic field strength. At integer and particular fractional ratios between the number of electrons and the degeneracy of a Landau level (filling factors n) quantum Hall effects occur, characterised by a vanishingly small longitudinal resistance and quantised Hall voltage. The quantum Hall regime offers unique possibilities for the study of cooperative phenomena in many-particle systems under well-controlled conditions. Among the fields that benefit from quantum-Hall studies is magnetism, which remains poorly understood in conventional material. Both isotropic and anisotropic ferromagnetic ground states have been predicted and few of them have been experimentally studied in quantum Hall samples with different geometries and filling factors. Here we present evidence of first-order phase transitions in n = 2 and 4 quantum Hall states confined to a wide gallium arsenide quantum well. The observed hysteretic behaviour and anomalous temperature dependence in the longitudinal resistivity indicate the occurrence of a transition between the two distinct ground states of an Ising quantum-Hall ferromagnet. Detailed many-body calculations allowed the identification of the microscopic origin of the anisotropy field

Inter-edge strong-to-weak scattering evolution at a constriction in the fractional quantum Hall regime

Gate-voltage control of inter-edge tunneling at a split-gate constriction in the fractional quantum Hall regime is reported. Quantitative agreement with the behavior predicted for out-of-equilibrium quasiparticle transport between chiral Luttinger liquids is shown at low temperatures at specific values of the backscattering strength. When the latter is lowered by changing the gate voltage the zero-bias peak of the tunneling conductance evolves into a minimum and a non-linear quasihole-like characteristic emerges. Our analysis emphasizes the role of the local filling factor in the split-gate constriction region.Comment: 4 pages, 4 figure

La mediazione c.d. obbligatoria. Evoluzione normativa.

Nell'elaborato si ripercorre l'evoluzione storica dell'istituto della conciliazione giudiziale e stragiudiziale con particolare riferimento alla mediazione obbligatoria così come disciplinata dalla normativa europea e italiana. Si traccia una comparazione con i modelli stranieri e si analizano i profili più controversi della istituto nel nostro ordinamento

Particle-hole symmetric Luttinger liquids in a quantum Hall circuit

We report current transmission data through a split-gate constriction fabricated onto a two-dimensional electron system in the integer quantum Hall (QH) regime. Split-gate biasing drives inter-edge backscattering and is shown to lead to suppressed or enhanced transmission, in marked contrast with the expected linear Fermi-liquid behavior. This evolution is described in terms of particle-hole symmetry and allows us to conclude that an unexpected class of gate-controlled particle-hole-symmetric chiral Luttinger Liquids (CLLs) can exist at the edges of our QH circuit. These results highlight the role of particle-hole symmetry on the properties of CLL edge states.Comment: 4 pages, 4 figure

Tuning non-linear charge transport between integer and fractional quantum Hall states

Controllable point junctions between different quantum Hall phases are a necessary building block for the development of mesoscopic circuits based on fractionally-charged quasiparticles. We demonstrate how particle-hole duality can be exploited to realize such point-contact junctions. We show an implementation for the case filling factors $\nu=1$ and $\nu^*\le1$ in which both the fractional filling $\nu^*$ and the coupling strength can be finely and independently tuned. A peculiar crossover from insulating to conducting behavior as $\nu^*$ goes from 1/3 to 1 is observed. These results highlight the key role played on inter-edge tunneling by local charge depletion at the point contact.Comment: 4 pages, 3 figures, suppl.ma

The Influence of Graphene Curvature on Hydrogen Adsorption: Towards Hydrogen Storage Devices

The ability of atomic hydrogen to chemisorb on graphene makes the latter a promising material for hydrogen storage. Based on scanning tunneling microscopy techniques, we report on site-selective adsorption of atomic hydrogen on convexly curved regions of monolayer graphene grown on SiC(0001). This system exhibits an intrinsic curvature owing to the interaction with the substrate. We show that at low coverage hydrogen is found on convex areas of the graphene lattice. No hydrogen is detected on concave regions. These findings are in agreement with theoretical models which suggest that both binding energy and adsorption barrier can be tuned by controlling the local curvature of the graphene lattice. This curvature-dependence combined with the known graphene flexibility may be exploited for storage and controlled release of hydrogen at room temperature making it a valuable candidate for the implementation of hydrogen-storage devices

Self-assembly and electron-beam-induced direct etching of suspended graphene nanostructures

We report on suspended single-layer graphene deposition by a transfer-printing approach based on polydimethylsiloxane stamps. The transfer printing method allows the exfoliation of graphite flakes from a bulk graphite sample and their residue-free deposition on a silicon dioxide substrate. This deposition system creates a blistered graphene surface due to strain induced by the transfer process itself. Single-layer-graphene deposition and its "blistering" on the substrate are demonstrated by a combination of Raman spectroscopy, scanning electron microscopy and atomic-force microscopy measurements. Finally, we demonstrate that blister-like suspended graphene are self-supporting single-layer structures and can be flattened by employing a spatially-resolved direct-lithography technique based on electron-beam induced etching.Comment: 17 pages, 5 figure

Spacecraft Proximity Navigation and Autonomous Assembly based on Augmented State Estimation: Analysis and Experiments

AIAA Guidance, Navigation, and Control Conference 2 - 5 August 2010, Toronto, Ontario CanadaThis paper presents a spacecraft relative navigation scheme based on a tracking technique. The augmented state estimation technique is a variable-dimension filtering approach, originally introduced by Bar-Shalom and Birmiwal [1]. In this technique, the state model for a target spacecraft is augmented by introducing, as extra state components, the target's control inputs. The maneuver, modeled as accelerations, is estimated recursively along with the other states associated with position and velocity, while a target maneuvers. By using the proposed navigation method, a chaser spacecraft can estimate the relative position, the attitude and the control inputs of a target spacecraft, flying in its proximity. It is assumed that the chaser spacecraft is equipped with on-board sensors able to measure the relative position and relative attitude of the target spacecraft. The available sensors would provide a measurement update sample time of the order of one second and be subject to random measurement interruption longer than one second. As preliminary analysis, this work introduces the technique applied to the planar, three-degree-of-freedom, spacecraft relative motion. The proposed approach is validated via hardware-in-the-loop experimentation, using four autonomous three-degree-of-freedom robotic spacecraft simulators, floating on a flat floor. The proposed navigation method is proved to be more robust than a standard Kalman Filter estimating relative position and attitude only