1,721,020 research outputs found
Effective mass and momentum-resolved intrinsic linewidth of image-potential states on Ag(100)
No full textMixed regime of light-matter interaction revealed by phase sensitive measurements of the dynamical Franz-Keldysh effect
No full textSignificant changes of the optical properties of semiconductors can be observed by applying strong
electric fields capable to modify the band structure at equilibrium. This is known as the Franz-Keldysh effect
(FKE). Here we study the FKE in bulk GaAs by combining single cycle THz pumps and broadband optical
probes. The experiments show that the phase content of the selected electromagnetic pulses can be used to
measure the timescales characteristic for the different regimes of matter-light interactions. Furthermore,
the present phase-resolved measurements allow to identify a novel regime of saturation where memory
effects are of relevance
Bypassing the energy-time uncertainty in time-resolved photoemission
Full text linkThe energy-time uncertainty is an intrinsic limit for time-resolved experiments imposing a tradeoff between the duration of the light pulses used in experiments and their frequency content. In standard time-resolved photoemission, this limitation maps directly onto a tradeoff between the time resolution of the experiment and the energy resolution that can be achieved on the electronic spectral function. Here we propose a protocol to disentangle the energy and time resolutions in photoemission. We demonstrate that dynamical information on all time scales can be retrieved from time-resolved photoemission experiments using suitably shaped light pulses of quantum or classical nature. As a paradigmatic example, we study the dynamical buildup of the Kondo peak, a narrow feature in the electronic response function arising from the screening of a magnetic impurity by the conduction electrons. After a quench, the electronic screening builds up on timescales shorter than the inverse width of the Kondo peak and we demonstrate that the proposed experimental scheme could be used to measure the intrinsic time scales of such electronic screening. The proposed approach provides an experimental framework to access the nonequilibrium response of collective electronic properties beyond the spectral uncertainty limit and will enable the direct measurement of phenomena such as excited Higgs modes and, possibly, the retarded interactions in superconducting systems
Effective mass and momentum-resolved intrinsic linewidth of image-potential states on Ag(100)
No full textThe intrinsic linewidth of Ag(100) electronic image-potential states versus the momentum parallel to the surface has been obtained from line-shape analysis of high-resolution angle-resolved two-photon photoemission spectra. For the n=1 state, the measured intrinsic linewidths have been observed to increase from 14 meV at normal emission (k(parallel to)=0) to 70 meV at k(parallel to)=0.18 Angstrom(-1). This finding, when compared to the significantly lower inverse lifetime broadening measured on Cu(100), opens the question of the interplay between collective particle effects and nonlocal mechanisms associated with the image-potential electrons decay dynamics. In addition, the effective electron masses for the n=1 and n=2 states, observed in our experiments along the (&UGamma;) over bar(M) over bar and (&UGamma;) over bar(X) over bar directions of the surface Brillouin zone, improve the agreement between the measured values and the theoretical predictions
Violation of the Electric-Dipole Selection Rules in Indirect Multiphoton Excitation of Image-Potential States on Ag(100)
No full textPhotoemission from image potential states on Ag(100) is investigated using angle resolved multiphoton photoemission induced by 150 fs laser pulses. For the first time we demonstrate that image potential states populated by indirect transitions can be observed with light polarized parallel to the plane of incidence and light polarized normal to the plane of incidence. The latter is a process normally forbidden by the dipole transition selection rules. These findings are related to the creation of a hotelectron population whose properties largely remains to be understood
Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse
Full text linkIn organic bulk heterojunction materials, charge delocalization has been
proposed to play a vital role in the generation of free carriers by reducing
the Coulomb attraction via an interfacial charge transfer exciton (CTX).
Pump-push-probe (PPP) experiments produced evidence that the excess energy
given by a push pulse enhances delocalization, thereby increasing photocurrent.
However, previous studies have employed near-IR push pulses in the range
0.4-0.6 eV which is larger than the binding energy of a typical CTX. This
raises the doubt that the push pulse may directly promote dissociation without
involving delocalized states. Here, we perform PPP experiments with mid-IR push
pulses at energies that are well below the binding energy of a CTX state
(0.12-0.25 eV). We identify three types of CTX: delocalized, localized, and
trapped. The excitation resides over multiple polymer chains in delocalized
CTXs, while is restricted to a single chain (albeit maintaining a degree of
intrachain delocalization) in localized CTXs. Trapped CTXs are instead
completely localized. The pump pulse generates a hot delocalized CTX, which
relaxes to a localized CTX, and eventually to trapped states. We find that
photo-exciting localized CTXs with push pulses resonant to the mid-IR charge
transfer absorption can promote delocalization and contribute to the formation
of long-lived charge separated states. On the other hand, we found that trapped
CTX are non-responsive to the push pulses. We hypothesize that delocalized
states identified in prior studies are only accessible in systems where there
is significant interchain electronic coupling or regioregularity that supports
either interchain or intrachain polaron delocalization. This emphasizes the
importance of engineering the micromorphology and energetics of the
donor-acceptor interface to exploit a full potential of a material for
photovoltaic applications
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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