28 research outputs found
Strong two-dimensional plasmon in Li-intercalated hexagonal boron-nitride film with low damping
The field of plasmonics seeks to find materials with an intensive plasmon
(large plasmon pole weight) with low Landau, phonon and other losses (small
decay width). In this paper we propose a new class of materials that show
exceptionally good plasmonic properties. These materials consist of van der
Waals stacked 'plasmon active' layers (atomically thin metallic layers) and
'supporting' layers (atomically thin wide band gap insulating layers). One such
material that can be experimentally realized - lithium intercalated hexagonal
boron-nitride is studied in detail. We show that its 2D plasmon intensity is
superior to intensity of well studied Dirac plasmon in heavy doped graphene
which is hard to achieve. We also propose the method for computationally very
cheap, but accurate analysis of plasmon spectra in such materials, based on one
band tight-binding approach and effective background dielectric function
Extraordinary low-energy charge excitations in high- cuprate superconductors
Despite decades of intensive experimental and theoretical efforts, the
physics of cuprate high-temperature superconductors in general and, in
particular, the nature of the normal state, is still under debate. Numerous
collective excitations arising from the proximity to two other phases, magnetic
and charge density waves, make it difficult to elucidate their origin.
Here, we report our investigation of low-energy charge excitations in the
normal state. We find that the peculiarities of the electronic band structure
at low energies have a profound impact on the nature of the intraband
collective modes. We show that it gives rise to a new kind of modes with huge
intensity and non-Lorentzian spectral function in addition to well-known
collective excitations like conventional plasmons and spin-fluctuation. We
predict the existence of two such modes with maximal spectral weight in the
nodal and antinodal directions. Additionally, we found a long-living
quasi-one-dimensional plasmon becoming an intense soft mode over an extended
momentum range along the antinodal direction. These modes might explain some of
the RIXS data and might also contribute to the strong renormalization of
quasiparticles in high- cuprates in these regions.Comment: 9 pages, 5 figure
Image potential states as quantum probe of graphene interfaces
Image potential states (IPSs) are electronic states localized in front of a
surface in a potential well formed by the surface projected bulk band gap on
one side and the image potential barrier on the other. In the limit of a
two-dimensional solid a double Rydberg series of IPSs has been predicted which
is in contrast to a single series present in three-dimensional solids. Here, we
confirm this prediction experimentally for mono- and bilayer graphene. The IPSs
of epitaxial graphene on SiC are measured by scanning tunnelling spectroscopy
and the results are compared to ab-initio band structure calculations. Despite
the presence of the substrate, both calculations and experimental measurements
show that the first pair of the double series of IPSs survives, and eventually
evolves into a single series for graphite. Thus, IPSs provide an elegant
quantum probe of the interfacial coupling in graphene systems.Comment: Accepted for publication in New Journal of Physic
Electronic temperature and two-electron processes in overbias plasmonic emission from tunnel junctions
The accurate determination of electronic temperatures in metallic
nanostructures is essential for many technological applications, like
plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this
Letter we demonstrate that the electronic temperature can be accurately
measured by the shape of the tunnel electroluminescence emission edge in tunnel
plasmonic nanocavities, which follows a universal thermal distribution with the
bias voltage as the chemical potential of the photon population. A significant
deviation between electronic and lattice temperatures is found below 30 K for
tunnel currents larger than 15 nA. This deviation is rationalized as the result
of a two-electron process in which the second electron excites plasmon modes
with an energy distribution that reflects the higher temperature following the
first tunneling event. These results dispel a long-standing controversy on the
nature of overbias emission in tunnel junctions and adds a new method for the
determination of electronic temperatures and quasiparticle dynamics
Direct observation of many-body charge density oscillations in a two-dimensional electron gas
Quantum interference is a striking manifestation of one of the basic concepts of quantum mechanics: the particle-wave duality. A spectacular visualization of this effect is the standing wave pattern produced by elastic scattering of surface electrons around defects, which corresponds to a modulation of the electronic local density of states and can be imaged using a scanning tunnelling microscope. To date, quantum-interference measurements were mainly interpreted in terms of interfering electrons or holes of the underlying band-structure description. Here, by imaging energy-dependent standing-wave patterns at noble metal surfaces, we reveal, in addition to the conventional surface-state band, the existence of an ‘anomalous’ energy band with a well-defined dispersion. Its origin is explained by the presence of a satellite in the structure of the many-body spectral function, which is related to the acoustic surface plasmon. Visualizing the corresponding charge oscillations provides thus direct access to many-body interactions at the atomic scale
Probing the role of grain boundaries in single Cu nanoparticle oxidation by in situ plasmonic scattering
Grain boundaries determine physical properties of bulk materials including ductility, diffusivity, and electrical conductivity. However, the role of grain boundaries in nanostructures and nanoparticles is much less understood, despite the wide application of nanoparticles in nanophotonics, nanoelectronics, and heterogeneous catalysis. Here, we investigate the role of high-angle grain boundaries in the oxidation of Cu nanoparticles, using a combination of in situ single particle plasmonic nanoimaging and postmortem transmission electron microscopy image analysis, together with ab initio and classical electromagnetic calculations. We find an initial growth of a 5-nm-thick Cu2O shell on all nanoparticles, irrespective of different grain morphologies. This insensitivity of the Cu2O shell on the grain morphology is rationalized by extraction of Cu atoms from the metal lattice being the rate limiting step, as proposed by density functional theory calculations. Furthermore, we find that the change in optical scattering intensity measured from the individual particles can be deconvoluted into one contribution from the oxide layer growth and one contribution that is directly proportional to the grain boundary density. The latter contribution signals accumulation of Cu vacancies at the grain boundaries, which, as corroborated by calculations of the optical scattering, leads to increased absorption losses and thus a decrease of the scattering, thereby manifesting the role of grain boundaries as vacancy sinks and nuclei for Kirkendall void formation at a later stage of the oxidation process