Plasmon dispersion and damping at the surface of a semimetal

Excitonic Effects in Energy-Loss Spectra of Freestanding Graphene

In this work, we perform electron energy-loss spectroscopy (EELS) of freestanding graphene with high energy and momentum resolution to disentangle the quasielastic scattering from the excitation gap of Dirac electrons close to the optical limit. We show the importance of many-body effects on electronic excitations at finite transferred momentum by comparing measured EELS to ab initio calculations at increasing levels of theory. Quasi-particle corrections and excitonic effects are addressed within the GW approximation and the Bethe-Salpeter equation, respectively. Both effects are essential in the description of the EEL spectra to obtain a quantitative agreement with experiments, with the position, dispersion, and shape of both the excitation gap and the π plasmon being significantly affected by excitonic effects.

Fluctuation-induced quantum friction in nanoscale water flows

The flow of water in carbon nanochannels has defied understanding thus far¹, with accumulating experimental evidence for ultra-low friction, exceptionally high water flow rates and curvature-dependent hydrodynamic slippage^2-5. In particular, the mechanism of water-carbon friction remains unknown⁶, with neither current theories⁷ nor classical^8,9 or ab initio molecular dynamics simulations¹⁰ providing satisfactory rationalization for its singular behaviour. Here we develop a quantum theory of the solid-liquid interface, which reveals a new contribution to friction, due to the coupling of charge fluctuations in the liquid to electronic excitations in the solid. We expect that this quantum friction, which is absent in Born-Oppenheimer molecular dynamics, is the dominant friction mechanism for water on carbon-based materials. As a key result, we demonstrate a marked difference in quantum friction between the water-graphene and water-graphite interface, due to the coupling of water Debye collective modes with a thermally excited plasmon specific to graphite. This suggests an explanation for the radius-dependent slippage of water in carbon nanotubes⁴, in terms of the electronic excitations of the nanotubes. Our findings open the way for quantum engineering of hydrodynamic flows through the electronic properties of the confining wall.

Observation of Nodal-Line Plasmons in ZrSiS

Nodal-line semimetals (NLSMs), a large family of new topological phases of matter with continuous linear band crossing points in the momentum space, attract considerable attention. Here, we report the direct observation of plasmons originating from topological nodal-line states in a prototypical NLSM ZrSiS by high-resolution electron energy loss spectroscopy. There exist three temperature-independent plasmons with energies ranging from the near- to the mid-infrared frequencies. With first-principles calculations of a slab model, these plasmons can be ascribed to the correlations of electrons in the bulk nodal lines and their projected surface states, dubbed nodal-line plasmons. An anomalous surface plasmon has higher excitation energy than the bulk plasmon due to the larger contribution from the nodal-line projected surface states. This work reveals the novel plasmons related to the unique nodal-line states in a NLSM.

Ultra-broadband light trapping using nanotextured decoupled graphene multilayers

The ability to engineer a thin two-dimensional surface for light trapping across an ultra-broad spectral range is central for an increasing number of applications including energy, optoelectronics, and spectroscopy. Although broadband light trapping has been obtained in tall structures of carbon nanotubes with millimeter-tall dimensions, obtaining such broadband light-trapping behavior from nanometer-scale absorbers remains elusive. We report a method for trapping the optical field coincident with few-layer decoupled graphene using field localization within a disordered distribution of subwavelength-sized nanotexturing metal particles. We show that the combination of the broadband light-coupling effect from the disordered nanotexture combined with the natural thinness and remarkably high and wavelength-independent absorption of graphene results in an ultrathin (15 nm thin) yet ultra-broadband blackbody absorber, featuring 99% absorption spanning from the mid-infrared to the ultraviolet. We demonstrate the utility of our approach to produce the blackbody absorber on delicate opto-microelectromechanical infrared emitters, using a low-temperature, noncontact fabrication method, which is also large-area compatible. This development may pave a way to new fabrication methodologies for optical devices requiring light management at the nanoscale.

Electric field dependence of excitation spectra in AB-stacked bilayer graphene

The single- and many-particle Coulomb excitation spectra in Bernal bilayer graphene can be modulated by a uniform perpendicular electric field. The field-induced oscillatory parabolic bands possess saddle points and local extrema, which, respectively, lead to logarithmically divergent peaks and discontinuous steps in the bare response functions. Such special structures are associated with the plasmon peaks in the screened loss spectra. Their main characteristics, such as their existence, frequency, and strength, vary strongly with the field strength and transferred momentum. The predicted results could be further examined by inelastic light scattering spectroscopy and electron-energy-loss spectroscopy.

Acoustic plasmon on the Au(111) surface

We report an acoustic surface plasmon mode on the Au(111) surface, which disperses into the visible region, as measured by high resolution electron energy loss spectroscopy. The new mode is assigned to an acoustic surface plasmon arising from the Shockley-type surface state electrons and coexists with the conventional surface plasmon. This low energy collective excitation disperses linearly up to ∼2.2  eV, i.e., into the visible region. The divergence from theoretical prediction appears to emphasize the importance of band structure effects upon the dielectric function of the surface region.

Interface matching method for solving surface plasmon modes with damping in plasmonic crystals

The author proposes an interface matching method for solving surface plasmon modes with damping in plasmonic crystals. The damping constant is considered a crucial parameter instead of a small perturbation to the undamped system. The damping effect is manifest on the complex nature of the eigenfrequency as well as on the eigenfield. For periodic layered structures, the decay factors of the two fundamental modes asymptotically approach gamma/2 in the large-wave-number limit. For two-dimensional plasmonic crystals, the decay factors of surface plasmon modes are gathered around and bounded by gamma/2 .

3D spectrum imaging of multi-wall carbon nanotube coupled pi-surface modes utilising electron energy-loss spectra acquired using a STEM/Enfina system

Numerous studies have utilised electron energy-loss (EEL) spectra acquired in the plasmon (2-10 eV) regime in order to probe delocalised pi-electronic states of multi-wall carbon nanotubes (MWCNTs). Interpretation of electron energy loss (EEL) spectra of MWCNTs in the 2-10 eV regime. Carbon (accepted for publication); Blank et al. J. Appl. Phys. 91 (2002) 1657). In the present contribution, EEL spectra were acquired from a 2D raster defined on a bottle-shaped MWCNT, using a Gatan UHV Enfina system attached to a dedicated scanning transmission electron microscope (STEM). The technique utilised to isolate and sequentially filter each of the volume and surface resonances is described in detail. Utilising a scale for the intensity of a filtered mode enables one to 'see' the distribution of each resonance in the raster. This enables striking 3D resonance-filtered spectrum images (SIs) of pi-collective modes to be observed. Red-shift of the lower energy split pi-surface resonance provides explicit evidence of pi-surface mode coupling predicted for thin graphitic films (Lucas et al. Phys. Rev. B 49 (1994) 2888). Resonance-filtered SIs are also compared to non-filtered SIs with suppressed surface contributions, acquired utilising a displaced collector aperture. The present filtering technique is seen to isolate surface contributions more effectively, and without the significant loss of statistics, associated with the displaced collector aperture mode. Isolation of collective modes utilising 3D resonance-filtered spectrum imaging, demonstrates a valuable method for 'pinpointing' the location of discrete modes in irregularly shaped nanostructures.

Gapless spin-1 neutral collective mode branch for graphite

Using the standard tight binding model of 2D graphite with short range electron repulsion, we predict a gapless spin-1, neutral collective mode branch below the particle-hole continuum with energy vanishing linearly with momenta at the Gamma and K points in the Brillouin zone. This spin-1 mode has a wide energy dispersion, 0 to approximately 2 eV, and is not Landau damped. The "Dirac cone spectrum" of electrons at the chemical potential of graphite generates our collective mode, so we call this "spin-1 zero sound" of the "Dirac sea." Epithermal neutron scattering experiments and spin polarized electron energy loss spectroscopy can be used to confirm and study our collective mode.

Adsorbate-induced pinning of a charge-density wave in a quasi-1D metallic chains: Na on the In/Si(111)-(4x1) surface

We find that foreign adsorbates acting as local impurities can induce a metal-insulator transition by pinning a charge-density wave (CDW) on the quasi-1D metallic In/Si(111)-(4x1) chain system. Our scanning tunneling microscopy image clearly reveals the presence of a new local 4x2 structure nucleated by Na adatoms at room temperature, which turns out to be insulating with a doubled periodicity along the chains. We directly determine a CDW gap energy Delta = 105+/-8 meV by identifying a characteristic loss peak in our high-resolution electron-energy-loss spectra. We thus report the first observation of a local impurity-derived Peierls-like reconstruction of a quasi-1D system.

Observation of disorder-induced 2D mott-hubbard states of the alkali-earth metal (Mg,Ba)-adsorbed Si(111) surface

We report evidence of a disorder-driven Mott-Hubbard-type localization on the alkali-earth metal (AEM) (Mg,Ba)-adsorbed Si(111)-(7x7) surface. The clean metallic Si(111) surface is found to undergo a two-dimensional (2D) metal-insulator transition as randomly distributed AEM adsorbates cause disorder on the surface. A well-defined electron-energy-loss peak unique to the insulating phase is attributed to an interband excitation between the split Hubbard bands originated from a metallic surface band at Fermi energy. A quantitative analysis of the loss peak reveals that the AEM-induced insulating surfaces are of a Mott-Hubbard type driven essentially by disorder.