Acoustic-like Plasmon Research Articles

Collective electronic excitations in charge density wave systems: The case of CuTe

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Many-body effects and optical properties of single and double layer α- lattices

An extensive analytical and numerical investigation has been carried out to examine the role played by many-body effects on various α- materials under an off-resonance optical dressing field. Additionally, we explore its dependence on the hopping parameter α as well as the electron–light coupling strength λ0. The obtained dressed states due to mutual interaction between Dirac electrons and incident light are shown to demonstrate rather different electronic and optical properties in comparison with those in the absence of incident light. Specifically, various collective transport and optical properties of these electron dressed states are discussed in detail and compared for both single- and double layer α- lattices. All of these novel properties are due to the presence of a middle flat band and the interband transitions between it and an upper conduction band. Also, coupled plasmon dispersions for interacting double layer α- lattices are calculated, revealing a lower acoustic-like plasmon branch with tunable group velocity determined by both the layer separation and Fermi energy of each layer. Finally, a many-body theory is presented within the random-phase approximation for calculating the optical absorbance of doped multi-layered α- lattices in a linearly-polarized light field. We anticipate that the discoveries reported here could impact the design of the next-generation nano-optical and nano-plasmonic devices.

Close inspection of plasmon excitations in cuprate superconductors

Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${\bf q}_{\parallel}=(0,0)$ for various doping rates in electron-doped cuprates ${\rm La_{2-x}Ce_xCuO_4}$. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered $t$-$J$ model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.

2D materials integrated with metallic nanostructures: fundamentals and optoelectronic applications

Abstract Due to their novel electronic and optical properties, atomically thin layered two-dimensional (2D) materials are becoming promising to realize novel functional optoelectronic devices including photodetectors, modulators, and lasers. However, light–matter interactions in 2D materials are often weak because of the atomic-scale thickness, thus limiting the performances of these devices. Metallic nanostructures supporting surface plasmon polaritons show strong ability to concentrate light within subwavelength region, opening thereby new avenues for strengthening the light–matter interactions and miniaturizing the devices. This review starts to present how to use metallic nanostructures to enhance light–matter interactions in 2D materials, mainly focusing on photoluminescence, Raman scattering, and nonlinearities of 2D materials. In addition, an overview of ultraconfined acoustic-like plasmons in hybrid graphene–metal structures is given, discussing the nonlocal response and quantum mechanical features of the graphene plasmons and metals. Then, the review summarizes the latest development of 2D material–based optoelectronic devices integrated with plasmonic nanostructures. Both off-chip and on-chip devices including modulators and photodetectors are discussed. The potentials of hybrid 2D materials plasmonic optoelectronic devices are finally summarized, giving the future research directions for applications in optical interconnects and optical communications.

Insights on the Excitation Spectrum of Graphene Contacted with a Pt Skin.

The excitation spectrum in the region of the intraband (Dirac plasmon) and interband ( plasmon) plasmons in graphene/Pt-skin terminated PtNi(111) is reproduced by using an ab-initio method and an empirical model. The results of both methods are compared with experimental data. We discover that metallic screening by the Pt layer converts the square-root dispersion of the Dirac plasmon into a linear acoustic-like plasmon dispersion. In the long-wavelength limit, the Pt d electron excitations completely quench the plasmon in graphene at about eV, that is replaced by a broad peak at about 6 eV. Owing to a rather large graphene/Pt-skin separation (≈3.3 Å), the graphene/Pt-skin hybridization becomes weak at larger wave vectors, so that the plasmon is recovered with a dispersion as in a free-standing graphene.

Origin of high-energy charge excitations observed by resonant inelastic X-ray scattering in cuprate superconductors

The recent development of x-ray scattering techniques revealed the charge-excitation spectrum in high-Tc cuprate superconductors. While the presence of a dispersive signal in the high-energy charge-excitation spectrum is well accepted in the electron-doped cuprates, its interpretation and universality are controversial. Since charge fluctuations are observed ubiquitously in cuprate superconductors, the understanding of its origin is a pivotal issue. Here, we employ the layered t − J model with the long-range Coulomb interaction and show that an acoustic-like plasmon mode with a gap at in-plane momentum (0, 0) captures the major features of the high-energy charge excitations. The high-energy charge excitations, therefore, should be a universal feature in cuprate superconductors and are expected also in the hole-doped cuprates. Acoustic-like plasmons in cuprates have not been recognized yet in experiments. We propose several experimental tests to distinguish different interpretations of the high-energy charge excitations.

Controlling plasmon modes and damping in buckled two-dimensional material open systems

Full ranges of both hybrid plasmon-mode dispersions and their damping are studied systematically by our recently developed mean-field theory in open systems involving a conducting substrate and a two-dimensional (2D) material with a buckled honeycomb lattice, such as silicene, germanene, and a group IV dichalcogenide as well. In this hybrid system, the single plasmon mode for a free-standing 2D layer is split into one acoustic-like and one optical-like mode, leading to a dramatic change in the damping of plasmon modes. In comparison with gapped graphene, critical features associated with plasmon modes and damping in silicene and molybdenum disulfide are found with various spin-orbit and lattice asymmetry energy bandgaps, doping types and levels, and coupling strengths between 2D materials and the conducting substrate. The obtained damping dependence on both spin and valley degrees of freedom is expected to facilitate measuring the open-system dielectric property and the spin-orbit coupling strength of individual 2D materials. The unique linear dispersion of the acoustic-like plasmon mode introduces additional damping from the intraband particle-hole modes, which is absent for a free-standing 2D material layer, and the use of molybdenum disulfide with a large bandgap simultaneously suppresses the strong damping from the interband particle-hole modes.

Low-energy dielectric screening in Pd and PdHx systems

Modifications in dielectric properties of palladium upon absorption of hydrogen are investigated theoretically in the low-energy (0–2 eV) region. The calculations were performed with full inclusion of the electronic band structure obtained within a self-consistent pseudopotential approach. In particular, we trace the evolution of the acoustic-like plasmon (AP) found previously in clean Pd with increasing hydrogen concentration. It exists in PdHx up to the hydrogen content x corresponding to the complete filling of the 4d Pd-derived energy bands because of the presence of two kinds of carriers at the Fermi surface. At higher H concentration the AP disappears since only one kind of carrier within the sp-like energy band exists at the Fermi level. Additionally, we investigate the spatial distribution inside the crystal of a potential caused by a time-dependent external perturbation and observe drastic modifications in the screening properties in the PdHx systems with energy and with hydrogen concentration.

Ab initiostudy of low-energy collective electronic excitations in bulk Pb

A theoretical study of the dynamical dielectric response of bulk lead at low energies is presented. The calculations are performed with full inclusion of the electronic band structure calculated by means of a first-principles pseudopotential approach. The effect of inclusion of the spin-orbit interaction in the band structure on the excitation spectra in Pb is analyzed, together with dynamical exchange-correlation and local-field effects. In general, results show significant anisotropy effects on the dielectric response of bulk Pb. At small momentum transfers along three different high-symmetry directions, the calculated excitation spectra present several peaks with strong acousticlike dispersion in the energy range below 2 eV. The analysis shows that only one of such modes existing at momentum transfers along the $\ensuremath{\Gamma}$--$K$ direction can be interpreted as a true acousticlike plasmon, whereas all other modes are originated from the enhanced number of intraband electron-hole excitations at corresponding energies. Comparison with available optical experimental data shows good agreement.

New collective modes in wide quantum wells with in-plane magnetic fields

We theoretically investigate, within the random phase approximation, the collective excitations of symmetric wide quantum wells under in-plane magnetic fields B || . We find that the B || field significantly affects the frequencies, (an-)isotropy and types of the plasmon modes. In particular, a high B || can induce an additional branch of undamped acoustic-like plasmons, which does not exist in single quantum wells without B || .

Production and optical properties of steady-state photoinjected plasmas in quantum wires

We describe the production and the optical properties of the non-equilibrium photoinjected plasma in semiconductor quantum wires under continuous UV-light illumination. The wavenumber-dependent dynamic dielectric function of this system is derived and the Raman scattering cross-section calculated. From the latter we identify the contributions from different types of elementary excitations. They consist of, besides the single-particle excitations, two types of collective oscillations: an upper one, consisting of an intrasubband-like plasmon and a lower one, identified as an acoustic-like plasmon. The dependence of both on the non-equilibrium (dissipative) macroscopic state of the system is evidenced and discussed.