228 research outputs found
Elementary structural building blocks encountered in silicon surface reconstructions
Driven by the reduction of dangling bonds and the minimization of surface
stress, reconstruction of silicon surfaces leads to a striking diversity of
outcomes. Despite this variety even very elaborate structures are generally
comprised of a small number of structural building blocks. We here identify
important elementary building blocks and discuss their integration into the
structural models as well as their impact on the electronic structure of the
surface
Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach
Recently strong evidence has been found in favor of a BCS-like condensation
of excitons in 1\textit{T}-TiSe. Theoretical photoemission intensity maps
have been generated by the spectral function calculated within the excitonic
condensate phase model and set against experimental angle-resolved
photoemission spectroscopy data. Here, the calculations in the framework of
this model are presented in detail. They represent an extension of the original
excitonic insulator phase model of J\'erome \textit{et al.} [Phys. Rev. {\bf
158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A
detailed analysis of its properties and further comparison with experiment are
also discussedComment: Submitted to PRB, 11 pages, 7 figure
Temperature dependence of the excitonic insulator phase model in 1T-TiSe2
Recently, detailed calculations of the excitonic insulator phase model
adapted to the case of 1\textit{T}-TiSe have been presented. Through the
spectral function theoretical photoemission intensity maps can be generated
which are in very good agreement with experiment [Phys. Rev. Lett. {\bf 99},
(2007) 146403]. In this model, excitons condensate in a BCS-like manner and
give rise to a charge density wave, characterized by an order parameter. Here,
we assume an analytical form of the order parameter, allowing to perform
temperature dependent calculations. The influence of this order parameter on
the electronic spectral function, to be observed in photoemission spectra, is
discussed. The resulting chemical potential shift and an estimation of the
resistivity are also shown.Comment: 4 pages, 3 figures, paper submitted at the Strongly Correlated
Electron System conference, Brazil, 200
Temperature dependent photoemission on 1T-TiSe2: Interpretation within the exciton condensate phase model
The charge density wave phase transition of 1T-TiSe2 is studied by
angle-resolved photoemission over a wide temperature range. An important
chemical potential shift which strongly evolves with temperature is evidenced.
In the framework of the exciton condensate phase, the detailed temperature
dependence of the associated order parameter is extracted. Having a
mean-field-like behaviour at low temperature, it exhibits a non-zero value
above the transition, interpreted as the signature of strong excitonic
fluctuations, reminiscent of the pseudo-gap phase of high temperature
superconductors. Integrated intensity around the Fermi level is found to
display a trend similar to the measured resistivity and is discussed within the
model.Comment: 8 pages, 6 figure
Influence of elastic scattering on the measurement of core-level binding energy dispersion in X-ray photoemission spectroscopy
We explore the interplay between the elastic scattering of photoelectrons and the surface core level shifts with regard to the determination of core level binding energies in Au(111) and Cu3Au(100). We find that an artificial shift is created in the binding energies of the Au 4f core levels, that exhibits a dependence on the emission angle, as well as on the spectral intensity of the core level emission itself. Using a simple model, we are able to reproduce the angular dependence of the shift and relate it to the anisotropy in the electron emission from the bulk layers. Our results demonstrate that interpretation of variation of the binding energy of core-levels should be conducted with great care and must take into account the possible influence of artificial shifts induced by elastic scatterin
An Alternative Interpretation of Recent ARPES Measurements on TiSe2
Recently there has been a renewed interest in the charge density wave
transition of TiSe2, fuelled by the possibility that this transition may be
driven by the formation of an excitonic insulator or even an excitonic
condensate. We show here that the recent ARPES measurements on TiSe2 can also
be interpreted in terms of an alternative scenario, in which the transition is
due to a combination of Jahn-Teller effects and exciton formation. The hybrid
exciton-phonons which cause the CDW formation interpolate between a purely
structural and a purely electronic type of transition. Above the transition
temperature, the electron-phonon coupling becomes ineffective but a finite
mean-field density of excitons remains and gives rise to the observed diffuse
ARPES signals.Comment: 4 pages, 2 figure
Why do dogs (Canis familiaris) select the empty container in an observational learning task?
Many argue that dogs show unique susceptibility to human communicative signals that make them suitable for being engaged in complex co-operation with humans. It has also been revealed that socially provided information is particularly effective in influencing the behaviour of dogs even when the human’s action demonstration conveys inefficient or mistaken solution of task. It is unclear, however, how the communicative nature of the demonstration context and the presence of the human demonstrator affect the dogs’ object-choice behaviour in observational learning situations. In order to unfold the effects of these factors, 76 adult pet dogs could observe a communicative or a non-communicative demonstration in which the human retrieved a tennis ball from under an opaque container while manipulating another distant and obviously empty (transparent) one. Subjects were then allowed to choose either in the presence of the demonstrator or after she left the room. Results showed a significant main effect of the demonstration context (presence or absence of the human’s communicative signals), and we also found some evidence for the response-modifying effect of the presence of the human demonstrator during the dogs’ choice. That is, dogs predominantly chose the baited container, but if the demonstration context was communicative and the human was present during the dogs’ choice, subjects’ tendency to select the baited container has been reduced. In agreement with the studies showing sensitivity to human’s communicative signals in dogs, these findings point to a special form of social influence in observational learning situations when it comes to learning about causally opaque and less efficient (compared to what comes natural to the dog) action demonstrations
Scanning tunneling microscopy at multiple voltage biases of stable "ring-like" Ag clusters on Si(111)-(77)
Since more than twenty years it is known that deposition of Ag onto
Si(111)-(7\times7) leads under certain conditions to the formation of so-called
"ring-like" clusters, that are particularly stable among small clusters. In
order to resolve their still unknown atomic structure, we performed voltage
dependent scanning tunneling microscopy (STM) measurements providing
interesting information about the electronic properties of clusters which are
linked with their atomic structure. Based on a structural model of Au cluster
on Si(111)-(7\times7) and our STM images, we propose an atomic arrangement for
the two most stable Ag "ring-like" clusters.Comment: 9 pages and 5 figure
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