Superconducting Coherent States for an Extended Hubbard Model

An extended Hubbard model with phonons is considered. q-coherent states relative to the superconducting quantum symmetry of the model are constructed and their properties studied. It is shown that they can have energy expectation lower than eigenstates constructed via conventional processes and that they exhibit ODLRO.Comment: 7 pages, 3 figure

Hot electron cooling by acoustic phonons in graphene

We have investigated the energy loss of hot electrons in metallic graphene by means of GHz noise thermometry at liquid helium temperature. We observe the electronic temperature T / V at low bias in agreement with the heat diffusion to the leads described by the Wiedemann-Franz law. We report on $T\propto\sqrt{V}$ behavior at high bias, which corresponds to a T4 dependence of the cooling power. This is the signature of a 2D acoustic phonon cooling mechanism. From a heat equation analysis of the two regimes we extract accurate values of the electron-acoustic phonon coupling constant $\Sigma$ in monolayer graphene. Our measurements point to an important effect of lattice disorder in the reduction of $\Sigma$, not yet considered by theory. Moreover, our study provides a strong and firm support to the rising field of graphene bolometric detectors.Comment: 5 figure

Nanostructures in suspended mono- and bilayer epitaxial graphene

International audienceSuspended graphene membrane presents a particular structure with fundamental interests and applications in nanomechanics, thermal transport and optoelectronics. Till now, the commonly used geometries are still quite simple and limited to the microscale. We propose here to overcome this problem by making nanostructures in suspended epitaxial bilayer graphene on a large scale and with a large variety of geometries. We also demonstrate a new hybrid thin film of SiC-graphene with an impressive robustness. Since the mechanics and thermal dissipation of a suspended graphene membrane are strongly related to its own geometry, we have in addition focused on thermal transport and strain engineering experiments. Micro-Raman spectroscopy mapping was successfully performed for various geometries with intrinsic properties measurements at the nanoscale. Our engineering of graphene geometry has permitted to reduce the thermal transport, release and modulate the strain in our structures. © 2017 Elsevier Lt

Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures

Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n- or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si: H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si: H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Omega sq(-1) to 1260 Omega sq(-1) for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm(2) V-1 s(-1) indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si: H is a viable route for large scale graphene based solar cells or display applications © 2016 IOP Publishing Ltd1331sciescopu

Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source

Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on thenanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processesrequires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from afree electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparativelyinexpensive alternative. We present measurements of single-shot diffraction patterns from isolatednano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images werereconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots