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We study the response of the one-dimensional charge density wave in K_{0.3}MoO_{3} to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.

High-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD XAS) has been used to probe the geometric and electronic structure of a Cu monolayer model electrocatalyst as a function of applied potential in situ in alkaline electrolyte. In 0.01 M NaOH, a Cu monolayer deposited on an Au(111) single crystal exhibits markedly different redox behavior from Cu multilayers on the same substrate: the Cu monolayer is more stable against oxide formation and, at high potentials (E > 0.5 V vs RHE), undergoes a direct phase transition from Cu0 to CuO rather than forming an intermediate Cu2O phase. The Cu monolayer in its metallic state at low potentials is expanded by 12.5% to match the substrate lattice constant, which can be expected to influence significantly the interaction of Cu surface atoms with intermediates of the electrochemical CO2 reduction. However, we also find that both strained Cu monolayer and thick, structurally relaxed Cu islands are unstable with respect to place-exchange with...

Josephson plasma waves — electromagnetic waves propagating between layered superconductors — lie at the basis of a broad variety of phenomena. Now, parametric amplification of such waves has been shown by tuning the phase between pump and seed waves. Many applications in photonics require all-optical manipulation of plasma waves1, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support Josephson plasma waves2,3, involving oscillatory tunnelling of the superfluid between capacitively coupled planes. Josephson plasma waves are also highly nonlinear4, and exhibit striking phenomena such as cooperative emission of coherent terahertz radiation5,6, superconductor–metal oscillations7 and soliton formation8. Here, we show that terahertz Josephson plasma waves can be parametrically amplified through the cubic tunnelling nonlinearity in a cuprate superconductor. Parametric amplification is sensitive to the relative phase between pump and seed waves, and may be optimized to achieve squeezing of the order-parameter phase fluctuations9 or terahertz single-photon devices.

Scratching beneath the surface: Pt-M(3d)-Pt(111) (M(3d) = Co, Ni) bimetallic subsurface alloys have been designed to show the ligand effect tunes reactivity in oxygen and hydrogen adsorption systems. The platinum-oxygen bond order was investigated by oxygen atom projection in the occupied and unoccupied space using X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS).

Using in situ electrical conductivity and ex situ X-ray photoelectron spectroscopy (XPS) measurements, we have examined how the hydrogen uptake of single-walled carbon nanotubes (SWNTs) is influenced by the addition of Pt nanoparticles. The conductivity of platinum-sputtered single-walled carbon nanotubes (Pt-SWNTs) during molecular hydrogen exposure decreased more rapidly than that of the corresponding pure SWNTs, which supports a hydrogenation mechanism facilitated by "spillover" of dissociated hydrogen from the Pt nanoparticles. C 1s XPS spectra indicate that the Pt-SWNTs store hydrogen by means of chemisorption, that is, covalent C-H bond formation: molecular hydrogen charging at elevated pressure (8.27 bar) and room temperature yielded Pt-SWNTs with up to 16 ± 1.5 at. % sp(3)-hybridized carbon atoms, which corresponds to a hydrogen-storage capacity of 1.2 wt % (excluding the weight of Pt nanoparticles). Pt-SWNTs prepared by the Langmuir-Blodgett (LB) technique exhibited the highest Pt/SWNT ratio and also the best hydrogen uptake.

Motivated by the appearance of optically induced superconductivity in the stripe-ordered La${}_{1.885}$Ba${}_{0.115}$CuO${}_{4}$ cuprate, this group uses time-resolved THz spectroscopy to map out the enhancement of interlayer superconducting coupling as a function of both temperature and time delay, for different pump wavelengths and fluences.

We report on the hydrogen adsorption induced phase transition of a few layer graphene (1 to 4 layers) to a diamondlike structure on Pt(111) based on core level x-ray spectroscopy, temperature programed desorption, infrared spectroscopy, and density functional theory total energy calculations. The surface adsorption of hydrogen induces a hybridization change of carbon from the sp2 to the sp3 bond symmetry, which propagates through the graphene layers, resulting in interlayer carbon bond formation. The structure is stabilized through the termination of interfacial sp3 carbon atoms by the substrate. The structural transformation occurs as a consequence of high adsorption energy.

We use x-ray spectroscopy and density functional theory to investigate the hydrogenation-induced electronic structure changes in graphene on Pt(111). The atom-specific properties of the spectroscopy allow for a direct projection of the band structure onto the carbon atoms; this was compared with the calculated density of states. Instead of the generally expected band opening behavior, we observe states at the Fermi level in the carbon-projected density of states. Hydrogenation is accompanied by pinning of the graphene to the substrate through the formation of local C--Pt bonds which cause the graphene layer to become metallic upon hydrogenation.

The unoccupied and occupied \ensuremath{\sigma} and \ensuremath{\pi} bands of graphene on Pt(111) were measured using near-edge x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the carbon $K$ edge. Elemental specificity and crystal-momentum conservation allows for detection of the dispersive band structure. In addition to features arising from the band structure of free-standing graphene, signatures of symmetry breaking were identified in XAS and in RIXS for excitation around the Fermi level. These additional features are proposed to derive from weak graphene-substrate interactions which manifest as substrate hybridization and surface umklapp processes due to Moir\'e superlattice vectors.