1,721,024 research outputs found
Noisy fractions
No full textFractional charges are one of the hallmarks of topological matter and the building blocks of various topological devices. Now, there are indications that their fingerprint in terms of electrical noise is less obvious, but more universal, than expected
Bipolar Thermoelectricity in Bilayer-Graphene-Superconductor Tunnel Junctions
No full textWe investigate the thermoelectric properties of a hybrid nanodevice composed of a two-dimensional carbon-based material and a superconductor. This system presents nonlinear bipolar thermoelectricity, as induced by the spontaneous breaking of the particle-hole (PH) symmetry in a tunnel junction between bilayer graphene (BLG) and a Bardeen-Cooper-Schrieffer superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in superconductor-insulator-superconductor' junctions, is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guaran-tees improved thermoelectric performances that reach up to 1 mV/K, regarding the Seebeck coefficient, and a power density of 1 nW/mu m2 for temperature gradients of tens of kelvin. Furthermore, the exter-nally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity, even in the presence of the controlled sup-pression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available nanofabrication techniques
Perfecting the Growth and Transfer of Large Single-Crystal CVD Graphene: A Platform Material for Optoelectronic Applications
No full textIn this work, we demonstrate the synthesis of millimetre-sized single-crystals of graphene, achievable in a commercially available cold-wall CVD reactor, and several different approaches to transfer it from the growth substrate to a target substrate of choice. We confirm the high crystal quality of this material using various characterisation techniques, including optical and scanning electron microscopy as well as Raman spectroscopy. By performing field effect and quantum Hall effect measurements, we demonstrate that the electronic properties of such single crystals are comparable to those of ideal mechanically exfoliated flakes of graphene. Several applications of this high-quality material are also reviewed
Cascaded quantum hall bisection and applications to quantum metrology
No full textWe demonstrate a programmable quantum Hall circuit that implements an iterative voltage bisection scheme and allows any binary fraction (k/2n) of the fundamental resistance quantum RK/2=h/2e2 to be obtained. The circuit requires a number n of bisection stages that only scales logarithmically with the resolution of the set of possible output fractions. The value of k can be set to any integer between 1 and 2n by proper and easily predictable gate configuration. The architecture exploits gate-controlled routing, mixing, and equilibration of edge modes of robust quantum Hall states. The device does not contain internal Ohmic contacts and is thus naturally robust towards stray-resistance effects. Our scheme offers an alternative way to obtain custom quantum Hall resistance standards, and its potential advantages are discussed. The basic viability of the approach is demonstrated in a proof-of-principle two-stage bisection circuit built on a high-mobility GaAs/(Al,Ga)As heterostructure operating at a temperature of 260mK and a magnetic field of 4.1T. Our prototype achieves a relative quantization precision of the order of 10-4, which is limited by the experimental setup rather than by the circuit itself
Electron localization in periodically strained graphene
No full textPseudo-magnetic field (PMF) in deformed graphene has been proposed as a promising and flexible method to quantum-confine electronic states and create gaps in the local density of states. Motivated by this perspective, we numerically analyze various different configurations leading to electronic localization and band flattening in periodically strained graphene. In particular, we highlight the existence of a fine structure in the pseudo-Landau levels confined in large-PMF regions, the emergence of states confined to PMF nodes as well as of snake-like orbits. In our paper, we further analyze the importance of the relative rotation and asymmetry of the strain lattice with respect to the atomic lattice and show how it can be used to modulate the PMF periodicity and to create localized orbits far from the strain points. Possible implementations and applications of the simulated structures are discussed
Mapping the mechanical properties of a graphene drum at the nanoscale
Full text linkThe operation of graphene-based nanoelectromechanical systems (NEMS) crucially depends on the local mechanical characteristics of the graphene drum resonator. In particular, inhomogeneity in the residual strain (pre-strain) of the graphene membrane may affect the vibration dynamics as well as the energy dissipation. Despite its importance, achieving a precise local mapping of the pre-strain of a graphene membrane remains challenging. Here, we correlate scanning-probe force microscopy and Raman spectroscopy to map the local mechanical properties of circular monolayer-graphene drums. At odds with other techniques, we obtain maps of the membrane pre-strain with nanometric resolution and measure the effective Young's modulus in a non invasive way. Moreover, we show that the common topographic artefacts stemming from tip-induced deformations can be precisely corrected using the information derived from force-spectroscopy data. As a result, the local map of the pre-strain can be correlated with the true morphology of the graphene drum. Our analysis demonstrates that graphene resonators can be characterized by a non-flat morphology and a non-uniform pre-strain distribution, as a consequence of complex boundary conditions at the edge of the membrane and in correlation with local material defects. Since these non-ideal features are strictly related to the growth and the fabrication procedures, our method can provide a useful screening tool for the development of 2D materials-based NEMSs
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
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