Browsing by Subject "EXCITATIONS"

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  • Müller, Eric; Büchner, Bernd; Habenicht, Carsten; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth; Huotari, Simo (2016)
    We report the behavior of the charge carrier plasmon of 2H-transition metal dichalcogenides (TMDs) as a function of intercalation with alkali metals. Intercalation and concurrent doping of the TMD layers have a substantial impact on plasmon energy and dispersion. While the plasmon energy shifts are related to the intercalation level as expected within a simple homogeneous electron gas picture, the plasmon dispersion changes in a peculiar manner independent of the intercalant and the TMD materials. Starting from a negative dispersion, the slope of the plasmon dispersion changes sign and grows monotonously upon doping. Quantitatively, the increase of this slope depends on the orbital character (4d or 5d) of the conduction bands, which indicates a decisive role of band structure effects on the plasmon behavior.
  • Paredes-Mellone, O. A.; Stutz, G. E.; Ceppi, S. A.; Arneodo Larochette, P.; Huotari, S.; Gilmore, K. (2019)
    The Li 1s core excitation spectra in LiH was studied by means of x-ray Raman scattering (XRS) spectroscopy in a wide range of momentum transfers q. The analysis of the near-edge region of the measured spectra in combination with q-dependent ab initio calculations of XRS spectra based on the Bethe-Salpeter equation (BSE) reveals that the prominent peak at the excitation onset arises from two main contributions, namely a pre-edge peak associated to a p-type core exciton and strong transitions to empty states near the bottom of the conduction band, which is in contrast to previous experimental studies that attributed that feature to a single excitonic peak. The p-like angular symmetry of the core exciton is supported by BSE calculations of the relative contributions to the XRS spectra from monopole and dipole transitions and by the observed decrease of its normalised intensity for increasing momentum transfers. Higher energy spectral features in the measured XRS spectra are well reproduced by BSE, as well as by real-space multiple-scattering calculations.
  • Wang, Kai; Kortelainen, M.; Pei, J. C. (2017)
    To explore the nature of collective modes in weakly bound nuclei, we have investigated deformation effects and surface flow patterns of isovector dipole modes in a shape-coexisting nucleus, Mg-40. The calculations were done in a fully self-consistent continuum finite-amplitude quasiparticle random phase approximation in a large deformed spatial mesh. An unexpected result of pygmy and giant dipole modes having disproportionate deformation splittings in strength functions was obtained. Furthermore, the transition current densities demonstrate that the long-sought core-halo oscillation in pygmy resonances is collective and compressional, corresponding to the lowest excitation energy and the simplest quantum flow topology. Our calculations show that surface flow patterns become more complicated as excitation energies increase.
  • Fei, Na; Pei, J. C.; Wang, K.; Kortelainen, M. (2019)
    We have investigated collective breathing modes of a unitary Fermi gas in deformed harmonic traps. The ground state is studied by the superfluid local density approximation (SLDA) and small-amplitude collective modes are studied by the iterative quasiparticle random phase approximation (QRPA). The results illustrate the evolutions of collective modes of a small system in traps from spherical to elongated or pancake-shaped deformations. For small spherical systems, the influences of different SLDA parameters are significant, and, in particular, a large pairing strength can shift up the oscillation frequency of collective modes. The transition currents from QRPA show that the compressional flow patterns are nontrivial and dependent on the deformation. Finally, the finite-size effects are demonstrated to be reasonable when progressing towards larger systems. The hydrodynamical results of collective frequencies can be reproduced by SLDA-QRPA with reduced pairing strengths. Our studies indicate that experiments on small and medium systems are valuable for understanding effective interactions in systems with varying sizes and trap deformations.
  • Jokela, Niko; Lifschytz, Gilad; Lippert, Matthew (2017)
    The choice of statistics for a quantum particle is almost always a discrete one: either bosonic or fermionic. Anyons are the exceptional case for which the statistics can take a range of intermediate values. Holography provides an opportunity to address the question of how the behavior of interacting anyons depends on the choice of statistics. In this paper, we analyze the spectrum of a strongly coupled, gapless fluid of anyons described holographically by the D3-D7' model with alternative boundary conditions. We investigate how these alternative boundary conditions impact the instability of the gapless homogeneous phase toward the formation of spatial order. In addition, we also show that for a particular, limiting choice of the alternative boundary conditions, this holographic system can be interpreted as describing strongly coupled (2 + 1)-dimensional QED. In this case, the instability leads to a spontaneous, spatially modulated magnetic field.