1,720,983 research outputs found
Single-electron dynamics in topologically protected systems
No full textElectron quantum optics is a fast growing research field which aims at preparing, controlling and coherently manipulating single- to few-electrons states in mesoscopic condensed matter systems, in the same way as single photons can be manipulated in conventional quantum optics. Recently developed coherent single-electron sources are used to generate few-electron excitations in ballistic conductors, where their propagation is not affected by backscattering and phase coherence is preserved. Among several interesting problems related to electron quantum optics, an important question is whether and how interaction effects can influence the evolution of single-electron excitations generated by coherent sources. This will be the main topic of this thesis, where we investigate the properties of excitations created by applying a voltage pulse to a quantum conductor. The thesis can be conceptually divided into two main blocks, depending on which kind of interactions are taken into account. At first we consider a couple of conduction channels coupled by repulsive electron-electron interactions, focusing on two scenarios. Initially, co-propagating edge channels in the integer quantum Hall effect are considered, followed by counterpropagating channels emerging at the edge of a quantum spin Hall insulators. In both systems, electronic interactions induce a fractionalization process causing the initially generated excitations to split into smaller ones, carrying only a fraction of the injected charge. These fractionalized excitations are carefully analyzed both in the time domain as well as in energy and momentum space, which allows to access their particle-hole content. The analysis is based on an analytic approach relying on Luttinger liquid theory and bosonization techniques and applies to any voltage drive. Moreover, specializing to the relevant case of excitations created by quantized Lorentzian voltage pulses, known as Levitons, we show that the noise generated when they are partitioned at a scatterer is minimal, regardless of interactions. Further on, a completely different kind of interaction is addressed, namely superconducting correlations. In particular, we investigate the transport properties of a superconducting tunnel junction under the effect of an arbitrary periodic drive, showing that Levitons do minimize the low frequency noise in this kind of device too
Impact of Channel Mixing on the Visibility of Two-particle Interferometry in Quantum Hall Edge States
Full text linkWe consider a two-particle interferometer, where voltage sources applied to
ohmic contacts inject electronic excitations into a pair of copropagating edge
channels. We analyze the impact of channel mixing due to inter-edge tunneling
on the current noise measured at the output of the interferometer. Due to this
mixing, the noise suppression typically expected for synchronized injecting
sources is incomplete, thereby reducing the visibility of the interference. We
investigate to which extent the impact of mixing on the noise visibility
depends on different shapes of the voltage drives. Furthermore, we compare a
simple model involving a single mixing point between the sources and the
quantum point contact to the more realistic case of a continuous distribution
of weak mixing points.Comment: 7 pages, 3 figures; accepted for publication in the proceedings of
the LT29 Conference (Sapporo, Japan
Spectral features of voltage pulses in interacting helical channels
Full text linkWe investigate the interplay of voltage-driven excitations and electron-electron interactions in a pair of counterpropagating helical channels capacitively coupled to a time-dependent gate. By focusing on the non-equilibrium spectral properties of the system, we show how the spectral function is modified by external drives with different time profile in presence of Coulomb interactions. In particular, we focus on a Lorentzian drive and a square single pulse. In presence of strong enough electron-electron interactions, we find that both drives can result in minimal excitations, i.e. characterized by an excess spectral function with a definite sign. This is in contrast with what happens in the non-interacting case, where only properly quantized Lorentzian pulses are able to produce minimal excitations
Time-resolved energy dynamics after single electron injection into an interacting helical liquid
Full text linkpeer reviewedThe possibility of injecting a single electron into ballistic conductors is at the basis of the new field of electron quantum optics. Here, we consider a single electron injection into the helical edge channels of a topological insulator. Their counterpropagating nature and the unavoidable presence of electron-electron interactions dramatically affect the time evolution of the single wave packet. Modeling the injection process from a mesoscopic capacitor in the presence of nonlocal tunneling, we focus on the time-resolved charge and energy packet dynamics. Both quantities split up into counterpropagating contributions whose profiles are strongly affected by the interaction strength. In addition, stronger signatures are found for the injected energy, which is also affected by the finite width of the tunneling region, in contrast to what happens for the charge. Indeed, the energy flow can be controlled by tuning the injection parameters, and we demonstrate that, in the presence of nonlocal tunneling, it is possible to achieve a situation in which charge and energy flow in opposite directions
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
