Electronic compressibility of layer polarized bilayer graphene

We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a 1-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while D < 0 the peak appears only for holes. This asymmetry that can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization

Hanbury Brown and Twiss interferometry at a free-electron laser

We present measurements of second- and higher-order intensity correlation functions (so-called Hanbury Brown and Twiss experiment) performed at the free-electron laser (FEL) FLASH in the non-linear regime of its operation. We demonstrate the high transverse coherence properties of the FEL beam with a degree of transverse coherence of about 80% and degeneracy parameter of the order 10^9 that makes it similar to laser sources. Intensity correlation measurements in spatial and frequency domain gave an estimate of the FEL average pulse duration of 50 fs. Our measurements of the higher-order correlation functions indicate that FEL radiation obeys Gaussian statistics, which is characteristic to chaotic sources.Comment: 19 pages, 6 figures, 1 table, 40 reference

Brønsted Acid Catalysis—Structural Preferences and Mobility in Imine/Phosphoric Acid Complexes

Despite the huge success of enantioselective Bronsted acid catalysis, experimental data about structures and activation modes of substrate/catalyst complexes in solution are very rare. Here, for the first time, detailed insights into the structures of imine/Bronsted acid catalyst complexes are presented on the basis of NMR data and underpinned by theoretical calculations. The chiral Bronsted acid catalyst R-TRIP (3,3'-bis(2,4,6-triisopropylphenyl)-1,1'-binaphthyl-2,2'-diyl hydrogen phosphate) was investigated together with six aromatic imines. For each investigated system, an E-imine/R-TRIP complex and a Z-imine/R-TRIP complex were observed. Each of these complexes consists of two structures, which are in fast exchange on the NMR time scale; i.e., overall four structures were found. Both identified E-imine/R-TRIP structures feature a strong hydrogen bond but differ in the orientation of the imine relative to the catalyst. The exchange occurs by tilting the imine inside the complex and thereby switching the oxygen that constitutes the hydrogen bond. A similar situation is observed for all investigated Z-imine/R-TRIP complexes. Here, an additional exchange pathway is opened via rotation of the imine. For all investigated imine/R-TRIP complexes, the four core structures are highly preserved. Thus, these core structures are independent of electron density and substituent modulations of the aromatic imines. Overall, this study reveals that the absolute structural space of binary imine/TRIP complexes is large and the variations of the four core structures are small. The high mobility is supposed to promote reactivity, while the preservation of the core structures in conjunction with extensive pi-pi and CH-pi interactions leads to high enantioselectivities and tolerance of different substrates

Selective ultrafast probing of transient hot chemisorbed and precursor states of CO on Ru(0001)

We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell’Angela et al. Science 339 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2  ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process

Boron nitride substrates for high-quality graphene electronics

Graphene devices on standard SiO2 substrates are highly disordered, exhibiting characteristics far inferior to the expected intrinsic properties of graphene[1-12]. While suspending graphene above the substrate yields substantial improvement in device quality[13,14], this geometry imposes severe limitations on device architecture and functionality. Realization of suspended-like sample quality in a substrate supported geometry is essential to the future progress of graphene technology. In this Letter, we report the fabrication and characterization of high quality exfoliated mono- and bilayer graphene (MLG and BLG) devices on single crystal hexagonal boron nitride (h-BN) substrates, by a mechanical transfer process. Variable-temperature magnetotransport measurements demonstrate that graphene devices on h-BN exhibit enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping in comparison with SiO2-supported devices. The ability to assemble crystalline layered materials in a controlled way sets the stage for new advancements in graphene electronics and enables realization of more complex graphene heterostructres.Comment: 20 pages (includes supplementary info), 7 figure

The soft x-ray instrument for materials studies at the linac coherent light source x-ray free-electron laser

This content may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This material originally appeared in Review of Scientific Instruments 83, 043107 (2012) and may be found at https://doi.org/10.1063/1.3698294.The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480–2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser