Types Of Collective Excitations Research Articles

Collective excitations and universal broadening of cyclotron absorption in Dirac semimetals in a quantizing magnetic field

The spectrum of electromagnetic collective excitations in Dirac semimetals placed in a quantizing magnetic field is considered. We have found the Landau damping regions using the energy and momentum conservation law for allowed transitions between one-particle states of electron excitations. Analysis of dispersion equations for longitudinal and transverse waves near the window boundaries in the Landau damping regions reveals different types of collective excitations. We also indicate the features of universal broadening of cyclotron absorption for a magnetic field variation in systems with linear dispersion of the electron spectrum. The use of the obtained spectrum also allows us to predict a number of oscillation and resonance effects in the field of magneto-optical phenomena.

Spin and charge go their separate ways

Quantum Simulation Strongly interacting chains of fermions are predicted to exhibit two types of collective excitations: spinons, which carry only spin, and holons, which carry only charge. These excitations move at different velocities. Signatures of this so-called spin-charge separation have been observed in solid-state systems, but obtaining direct dynamical evidence is tricky. With this goal in mind, Vijayan et al. perturbed a chain of ultracold interacting fermions housed in a one-dimensional optical lattice by removing one of the atoms. This gave rise to two independent excitations, which the researchers identified as spinons and holons using a quantum gas microscope. Science , this issue p. [186][1] [1]: /lookup/doi/10.1126/science.aay2354

Probing the local nature of excitons and plasmons in few-layer MoS2

Excitons and plasmons are the two most fundamental types of collective electronic excitations occurring in solids. Traditionally, they have been studied separately using bulk techniques that probe their average energetic structure over large spatial regions. However, as the dimensions of materials and devices continue to shrink, it becomes crucial to understand how these excitations depend on local variations in the crystal- and chemical structure on the atomic scale. Here, we use monochromated low-loss scanning-transmission-electron-microscopy electron-energy-loss spectroscopy, providing the best simultaneous energy and spatial resolution achieved to-date to unravel the full set of electronic excitations in few-layer MoS2 nanosheets over a wide energy range. Using first-principles, many-body calculations we confirm the excitonic nature of the peaks at ~ 2 and ~ 3 eV in the experimental electron-energy-loss spectrum and the plasmonic nature of higher energy-loss peaks. We also rationalise the non-trivial dependence of the electron-energy-loss spectrum on beam and sample geometry such as the number of atomic layers and distance to steps and edges. Moreover, we show that the excitonic features are dominated by the long wavelength (q = 0) components of the probing field, while the plasmonic features are sensitive to a much broader range of q-vectors, indicating a qualitative difference in the spatial character of the two types of collective excitations. Our work provides a template protocol for mapping the local nature of electronic excitations that open new possibilities for studying photo-absorption and energy transfer processes on a nanometer scale.

Erratum: "A search for manifestation of two types of collective excitations in dynamic structure of a liquid metal: Ab initio study of collective excitations in liquid Na" [J. Chem. Phys. 144, 194501 (2016)

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A search for manifestation of two types of collective excitations in dynamic structure of a liquid metal: Ab initio study of collective excitations in liquid Na.

Using a combination of ab initio molecular dynamics and several fit models for dynamic structure of liquid metals, we explore an issue of possible manifestation of non-acoustic collective excitations in longitudinal dynamics having liquid Na as a case study. A model with two damped harmonic oscillators (DHOs) in time domain is used for analysis of the density-density time correlation functions. Another similar model with two propagating contributions and three lowest exact sum rules is considered, as well as an extended hydrodynamic model known as thermo-viscoelastic one which permits two types of propagating modes outside the hydrodynamic region to be used for comparison with ab initio obtained time correlation functions and calculations of dispersions of collective excitations. Our results do not support recent suggestions that, even in simple liquid metals, non-hydrodynamics transverse excitations contribute to the longitudinal collective dynamics and can be detected as a DHO-like spectral shape at their transverse frequency. We found that the thermo-viscoelastic dynamic model permits perfect description of the density-density and current-current time correlation functions of the liquid Na in a wide range of wave numbers, which implies that the origin of the non-hydrodynamic collective excitations contributing to longitudinal dynamics can be short-wavelength heat waves.

Novel Metallic Phase Emerging from Quantum Criticality: We Couldn��t Observe Electrons at Low Energies

The emergence of non-Fermi liquids near metallic quantum criticality is an essential feature in the system of strongly correlated electrons, where electron quasiparticles decay into other types of collective excitations. As a result, novel metallic phases appear without electrons, showing various exotic non-Fermi liquid physics beyond the Landau Fermi-liquid theory. In this review, we discuss what electrons disintegrate into, which allows us to classify theories for quantum critical metals into the Hertz-Moriya-Millis, Kondo breakdown, and local quantum criticality scenarios. We discuss both the goodness and the weakness of each theoretical framework, recalling experimental aspects for heavy-fermion quantum criticality.

Excitons in Molecular Aggregates with Lévy-Type Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra

We predict the existence of exchange broadening of optical line shapes in disordered molecular aggregates and a nonuniversal disorder scaling of the localization characteristics of the collective electronic excitations (excitons). These phenomena occur for heavy-tailed Lévy disorder distributions with divergent second moments-distributions that play a role in many branches of physics. Our results sharply contrast with aggregate models commonly analyzed, where the second moment is finite. They bear a relevance for other types of collective excitations as well.

NONLINEAR EXCITATIONS IN STRONGLY-COUPLED PLASMA LATTICES: ENVELOPE SOLITONS, KINKS AND INTRINSIC LOCALIZED MODES

Ensembles of charged particles (plasmas) are a highly complex form of matter, most often modeled as a many-body system characterized by weak inter-particle interactions (electrostatic coupling). However, strongly-coupled plasma configurations have recently been produced in laboratory, either by creating ultra-cold plasmas confined in a trap or by manipulating dusty plasmas in discharge experiments. In this paper, the nonlinear aspects involved in the motion of charged dust grains in a one-dimensional plasma monolayer (crystal) are discussed. Different types of collective excitations are reviewed, and characteristics and conditions for their occurrence in dusty plasma crystals are discussed, in a quasi-continuum approximation. Dust crystals are shown to support nonlinear kink-shaped supersonic solitary longitudinal excitations, as well as modulated envelope localized modes associated with longitudinal and transverse vibrations. Furthermore, the possibility for intrinsic localized modes (ILMs) — Discrete Breathers (DBs) — to occur is investigated, from first principles. The effect of mode-coupling is also briefly considered. The relation to previous results on atomic chains, and also to experimental results on strongly-coupled dust layers in gas discharge plasmas, is briefly discussed.

Collective excitations in quantum dots

We investigate different types of collective excitations in a quantum dot containing finite number of electrons at zero magnetic field. To estimate the excitation energies analytically we follow the energy weighted sum-rule approach. We consider the most general multipole excitation with angular momentum l, and the breathing mode excitation (monopole excitation) of a large quantum dot, for three different types of effective electron–electron interaction. These are the logarithmic interaction, the short-range pseudopotential and the Coulomb interaction. The ground-state density of the many-body system is calculated within Thomas–Fermi approximation. The analytical results for the collective excitation energies and their dependence on the system size and other external parameters are discussed in detail.

Collective Excitations of Alpha Helical Protein Molecules

AbstractA theoretical study is given of collective excitations in long alpha‐helical proteins, including two types of peptide group displacements provided by changes of the helix pitch and its radius. The results provide two types of collective excitations in the continuum approximation called symmetric and asymmetric excited states.

Screened electromagnetic propagators in nonlocal metal optics.

A new type of screened Green's function appropriate for studies of electromagnetic wave propagation in metals in the regime of nonlocal optics is constructed and discussed. The influence on the propagator characteristics of nonlocal screening effects caused by polariton, plasmon, and single-particle excitations is investigated. An eigenvector expansion of the dyadic propagator is used to study the near- and far-field parts of the propagator. In the near field the transverse and longitudinal parts of the propagator each have contributions from the two types of collective excitations. A nonlocal dyadic Green's function associated with adjacent metal-vacuum half-spaces is introduced, and s- and p-polarized screening effects are discussed. The self-field part of the propagator and reflection effects at the boundary are investigated. It is indicated how the present formalism can be useful in studies of (i) selvedge responses in linear and nonlinear metal optics, and (ii) Brillouin scattering and rough-surface diffraction from solid-state plasmas. Finally, it is demonstrated that the present propagator formalism is adequate for the description of reflected and transmitted fields at sharp boundaries.

Selective excitations in actinide nuclei via alpha pickup

The $\ensuremath{\alpha}$-cluster pickup reaction ($d,^{6}\mathrm{Li}$) has been studied at ${E}_{d}=55$ MeV on targets of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$. Members of the ground-state rotational bands in $^{228}\mathrm{Ra}$ and $^{234}\mathrm{Th}$ are excited and absolute reduced $\ensuremath{\alpha}$ widths obtained from finite-range distorted-wave analysis are in good agreement with values deduced from $\ensuremath{\alpha}$ decay. In addition three excited groups of states are very strongly populated in both nuclei with spectroscopic strength per group comparable with those of the respective ground state bands. These groups are apparently excited rotational bands with band heads at ${E}_{x}=700\ifmmode\pm\else\textpm\fi{}40, 1070\ifmmode\pm\else\textpm\fi{}60, \mathrm{and} 1390\ifmmode\pm\else\textpm\fi{}60$ keV in $^{228}\mathrm{Ra}$ and ${E}_{x}=810\ifmmode\pm\else\textpm\fi{}30, 1150\ifmmode\pm\else\textpm\fi{}40, \mathrm{and} 1470\ifmmode\pm\else\textpm\fi{}40$ keV in $^{234}\mathrm{Th}$. The selective and strong excitation in this particular multi-nucleon transfer reaction of several excited bands is not predicted by existing theoretical models. An attempt has been made to describe the systematics of excited ${0}^{+}$ states in the actinide region with the interacting boson model. Excitation energies are reasonably well described but intruder states are present and transfer strengths are not reproduced properly. The observation of strong $\ensuremath{\alpha}$-cluster pickup to excited rotational bands suggests coherent contributions from both neutron and proton pair excitations which can lead to strong four-body correlations and/or to new types of collective excitations which favor quartet structure. It is found that about 25% of the nuclear charge (matter) at $r\ensuremath{\simeq}10.6$ fm must be associated with $\ensuremath{\alpha}$ particles. This high $\ensuremath{\alpha}$-clustering probability indicates $\ensuremath{\alpha}$-particle condensation in low-density nuclear matter.NUCLEAR REACTIONS $^{232}\mathrm{Th}$, $^{238}\mathrm{U}(d,^{6}\mathrm{Li})$, $E=54.8$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$; DWBA analysis; $^{228}\mathrm{Ra}$, $^{234}\mathrm{Th}$ deduced levels, ${S}_{\ensuremath{\alpha}}$, $\ensuremath{\gamma}_{\ensuremath{\alpha}}^{}{}_{}{}^{2}$ (10.5 fm), $\ensuremath{\alpha}$-clustering probabilities.

Relation between many-body and single-particle aspects in nuclei

The process by which certain types of collective excitations in nuclei are built up from single-particle excitations through the nucleon-nucleon interaction is described. Analogies in the methods used to those employed in describing other Fermi systems are shown. Among the types of collective excitations are the dipole state and vibrational motion. A schematic model formulated in the language of the many-body theory and designed to handle the main qualitative features of a finite system is described. It was found that taking ground-state correlations meant doubling the order of the matrix in shell- model calculations. The ground-state correlations enhanced the transition probability for vibrational states where E << epsilon and cut it dow. for plasma oscillations. (M.C.G.)