19,210 research outputs found
Bifurcation analysis and phase diagram of a spin-string model with buckled states
We analyze a one-dimensional spin-string model, in which string oscillators
are linearly coupled to their two nearest neighbors and to Ising spins
representing internal degrees of freedom. String-spin coupling induces a
long-range ferromagnetic interaction among spins that competes with a spin-spin
antiferromagnetic coupling. As a consequence, the complex phase diagram of the
system exhibits different flat rippled and buckled states, with first or second
order transition lines between states. The two-dimensional version of the model
has a similar phase diagram, which has been recently used to explain the
rippled to buckled transition observed in scanning tunnelling microscopy
experiments with suspended graphene sheets. Here we describe in detail the
phase diagram of the simpler one-dimensional model and phase stability using
bifurcation theory. This gives additional insight into the physical mechanisms
underlying the different phases and the behavior observed in experiments.Comment: 15 pages, 7 figure
Spectroscopic Signatures of Electronic Excitations in Raman Scattering in Thin Films of Rhombohedral Graphite
Rhombohedral graphite features peculiar electronic properties, including
persistence of low-energy surface bands of a topological nature. Here, we study
the contribution of electron-hole excitations towards inelastic light
scattering in thin films of rhombohedral graphite. We show that, in contrast to
the featureless electron-hole contribution towards Raman spectrum of graphitic
films with Bernal stacking, the inelastic light scattering accompanied by
electron-hole excitations in crystals with rhombohedral stacking produces
distinct features in the Raman signal which can be used both to identify the
stacking and to determine the number of layers in the film.Comment: 15 pages in preprint format, 4 figures, accepted versio
Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius
We have carried out optical-absorption and reflectance measurements at room
temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr,
and Zn). From the experimental results their band-gap energy has been
determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV
(CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The
results are discussed in terms of the electronic structure of the studied
tungstates. It has been found that those compounds where only the s electron
states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4)
have larger band-gap energies than those where also p, d, and f states of the
A2+ cation contribute to the top of the valence band and the bottom of the
conduction band (e.g. PbWO4). The results are of importance in view of the
large discrepancies existent in prevoiusly published data.Comment: 16 pages, 3 figures, 1 tabl
Electronic Raman Scattering in Twistronic Few-Layer Graphene
We study electronic contribution to the Raman scattering signals of two-,
three- and four-layer graphene with layers at one of the interfaces twisted by
a small angle with respect to each other. We find that the Raman spectra of
these systems feature two peaks produced by van Hove singularities in moir\'{e}
minibands of twistronic graphene, one related to direct hybridization of Dirac
states, and the other resulting from band folding caused by moir\'{e}
superlattice. The positions of both peaks strongly depend on the twist angle,
so that their detection can be used for non-invasive characterization of the
twist, even in hBN-encapsulated structures.Comment: 7 pages (including 4 figures) + 10 pages (3 figures) supplemen
Non-linear response of single-molecule magnets: field-tuned quantum-to-classical crossovers
Quantum nanomagnets can show a field dependence of the relaxation time very
different from their classical counterparts, due to resonant tunneling via
excited states (near the anisotropy barrier top). The relaxation time then
shows minima at the resonant fields H_{n}=n D at which the levels at both sides
of the barrier become degenerate (D is the anisotropy constant). We showed that
in Mn12, near zero field, this yields a contribution to the nonlinear
susceptibility that makes it qualitatively different from the classical curves
[Phys. Rev. B 72, 224433 (2005)]. Here we extend the experimental study to
finite dc fields showing how the bias can trigger the system to display those
quantum nonlinear responses, near the resonant fields, while recovering an
classical-like behaviour for fields between them. The analysis of the
experiments is done with heuristic expressions derived from simple balance
equations and calculations with a Pauli-type quantum master equation.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B, brief report
What drives the helpfulness of online reviews? A deep learning study of sentiment analysis, pictorial content and reviewer expertise for mature destinations.
User-generated content (UGC) is a growing driver of destination choice. Drawing on dual-process theories on how individuals process information, this study focuses on the role of central and peripheral information processing routes in the formation of consumers’ perceptions of the helpfulness of online reviews. We carried out a two-step process to address the perceived helpfulness of user-generated content, a sentiment analysis using advanced machine-learning techniques (deep learning), and a regression analysis. We used a database of 2,023 comments posted on TripAdvisor about two iconic Venetian cultural attractions, St. Mark’s Square (an open, free attraction) and the Doge’s Palace (a museum which charges an entry fee). Following the application of deep-learning techniques, we first identified which factors influenced whether a review received a “helpful” vote by means of logistic regression. Second, we selected those reviews which received at least one helpful vote to identify, through linear regression, the significant determinants of TripAdvisor users’ voting behaviour. The results showed that reviewer expertise is an influential factor in both free and paid-for attractions, although the impact of central cues (sentiment polarity, subjectivity and pictorial content) is different in both attractions. Our study suggests that managers should look beyond individual ratings and focus on the sentiment analysis of online reviews, which are shown to be based on the nature of the attraction (free vs. paid-for)
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