1,720,964 research outputs found
Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB
Full text linkMultiband superconductors can host collective excitations with marked
differences with respect to their single-band counterpart. We first study the
spectrum of collective amplitude fluctuations in a clean two-bands
superconductor, showing that the spectral weight of the Higgs mode rapidly
deviates from the naive extension of the single band case as the interband
coupling is turned on. These results are then used to critically analyze the
non-linear optical response in MgB, providing an explanation for the
apparently contradictory results of recent experiments, pointing towards a
selective relevance either of the Leggett mode or of the amplitude fluctuations
at twice the lower gap. By using exact numerical simulations and realistic
estimate of disorder we compute the relative contribution of the quasiparticle,
amplitude and phase fluctuations to the non-linear optical response. We show
that at low pumping frequency only the resonance at twice the smaller gap
emerges, as due to the BCS response, while the Leggett mode dominates only in a
narrow range of higher pumping frequencies matching its low-temperature value.
Our findings provide a fresh perspective on the potential of non-linear THz
spectroscopy to detect collective modes in other multiband systems, as e.g.
iron-based superconductors
Manipulating Plasma Excitations with Terahertz Light Pulses in Superconducting Cuprates
Full text linkLayered cuprates offer a preferential playground for optical non-linearity
thanks to the emergence, below Tc, of soft out-of-plane Josephson plasmons. The
hallmark of such a non-linearity is the observation of Third Harmonic
Generation, that has been theoretically understood as a sum-frequency process
involving a two-plasmon excitation. However, recent experiments in cuprates
with two planes per unit cell challenge this interpretation, due to the lack of
resonant response at the temperature where the driving frequency matches the
plasma energy scale, as observed instead in single-layer cuprates. Here we show
that such an apparent discrepancy in bilayer systems can be resolved by taking
into account the combined effect of light polarization and Josephson-coupling
anisotropy on setting the energy range where three-dimensional layered plasma
modes can be resonantly excited. Our results offer a novel perspective on the
possibility to tune on demand high-harmonic generation by artificially
designing Josephson heterostructures
Charge instabilities and electron-phonon interaction in the Hubbard-Holstein model
No full textWe consider the Hubbard-Holstein model in the adiabatic limit to investigate the effects of electron-electron interactions on the electron-phonon coupling. To this aim we compute at any momentum and filling the static charge susceptibility of the Hubbard model within the Gutzwiller approximation, and we find that electron-electron correlations effectively screen the electron coupling to the lattice. This screening is more effective at large momenta, and as a consequence, the charge-density-wave phase due to the usual Peierls instability of the Fermi-surface momenta is replaced by a phase-separation instability when the correlations are sizable
Fermi surface dichotomy in systems with fluctuating order
No full textWe investigate the effect of a dynamical collective mode coupled with quasiparticles at specific wave vectors only. This coupling describes the incipient tendency to order and produces shadow spectral features at high energies while leaving essentially untouched the low-energy quasiparticles. This allows interpretation of seemingly contradictory experiments on underdoped cuprates, where many converging evidences indicate the presence of charge (stripe or checkerboard) order, which remains instead elusive in the Fermi surface obtained from angle-resolved photoemission experiments. © 2009 The American Physical Society
Charge inhomogeneity coexisting with large Fermi surfaces
No full textWe discuss how stripes in cuprates can be compatible with a Fermi-liquid-like Fermi surface and, at the same time, they give rise to a one-dimensional-like pseudo Fermi surface in the momentum distribution function. (c) 2007 Elsevier B.V. All rights reserved
THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode
Full text linkRecent experiments with strong THz fields in unconventional cuprates
superconductors have clearly evidenced an increase of the non-linear optical
response below the superconducting critical temperature Tc. As in the case of
conventional superconductors, a theoretical estimate of the various effects
contributing to the non-linear response is needed in order to interpret the
experimental findings. Here we report a detailed quantitative analysis of the
non-linear THz optical kernel in cuprates within a realistic model, accounting
for the band structure and disorder level appropriate for these systems. We
show that the BCS quasiparticle response is the dominant contribution for
cuprates, and its polarization dependence accounts very well for the
third-harmonic generation measurements. On the other hand, the polarization
dependence of the THz Kerr effect is only partly captured by our calculations,
suggesting the presence of additional effects when the system is probed using
light pulses with different central frequencies.Comment: Paper selected for "From optical to THz control of materials Faraday
Discussion" (23 - 25 May 2022, London, United Kingdom). Link:
https://www.rsc.org/events/detail/40608/from-optical-to-thz-control-of-materials-faraday-discussio
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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