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Tunneling spectroscopy of graphene nanodevices coupled to large-gap superconductors
No full textWe performed tunneling spectroscopy measurements of graphene coupled to niobium/niobium-nitride superconducting electrodes. Due to the proximity effect, the graphene density of states depends on the phase difference between the superconductors and exhibits a hard induced gap at zero phase, consistent with a continuum of Andreev bound states. At energies larger than the superconducting gap, we observed phase-dependent energy levels displaying the Coulomb blockade effect, which are interpreted as arising from spurious quantum dots, presumably embedded in the heterostructures and coupled to the proximitized graphene.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0001819)Gordon and Betty Moore Foundation (Grant GBMF4541
Líquids iònics com a dielèctrics de porta en dispositius encapsulats de alta qualitat hBN/MoS2/hBN.
Get PDFThe ability to produce few-atoms-thick two-dimensional materials of high quality such as graphene, transition-metal dichalcogenides and hexagonal boron nitride is a major improvement in condensed-matter physics and in nanoelectronics. When thinned down to the sub-nanometric scale, many layered van der Waals materials exhibit an interesting evolution of their physical properties and clearly show that multilayers of different thickness truly represent distinct electronic systems. The transition metal dichalcogenide semiconductor MoS has attracted particular interest because of its distinctive electronic and optical properties. For instance, in the bulk form, MoS crystals are indirect-gap semiconductors with a band gap of 1.29 eV, but its monolayer version has a direct band gap of 1.8eV. Because of the relatively weak interactions between the different layers and the strong intralayer interactions, the formation of ultrathin crystals of MoS by the micromechanical cleavage technique is actually possible. Early pioneering studies aimed at probing superconductivity in individual layers of superconducting transition metal dichalcogenides (TMDs) date back to the 1970s, but only over the last few years the experimental control necessary to unambiguously identify the thickness of atomically thin layers and the nanofabrication techniques required to manipulate such flakes have been developed. Recently, gate-induced superconductivity at the surface of MoS and other TMDs has been demonstrated by the mean of a field-effect transistor structure with a liquid gate. The relatively high critical temperature and the possibility to obtain chemically stable monolayers by simple exfoliation techniques make it an ideal choice to investigate the gate-induced superconductivity in such systems. This breakthrough work was performed on thick exfoliated layers, therefore behaving as bulk samples and led to the observation of critical temperature values up to 12 K following the accumulation of electron surface densities on the order of . Such unprecedented values were achieved by using ionic liquids, a novel technique (not fully yet understood), which favors the formation of an an electric double layer at the interface between the gate and the channel. Theoretically, superconductivity in MoS monolayers has been predicted and atomically thin crystals have been demonstrated to possess very peculiar and attractive superconducting characteristics, uncommon to other more conventional materials. MoS in its monolayer form is believed to become for instance an unconventional 2D Ising superconductor and is thereby very robust against external magnetic fields. The project herein described has been motivated by such lasts discoveries and aims to report for the first time superconductivity in MoS monolayers. However, the fabrication of high quality samples is far from being trivial. In order to perform multi-terminal transport measurements of MoS, we employed a van der Waals heterostructure device platform. Potential sources of disorder and scattering include defects such as sulfur vacancies in the MoS itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, the MoS ultra-thin layers are fully encapsulated within hexagonal boron nitride. The bottom one would serve as a clean and flat surface, whereas the top one would be as thin as possible and would protect the channel from the ionic liquid gate while preserving its characteristics. As will be mentioned in this report, various procedures and structures were attempted to approach our goal. To investigate the occurrence of superconductivity, we biased the ionic liquid field effect transistor (FET) by applying a gate voltage V to the ionic liquid before cooling it down slowly to a temperature around 4K (or lower, depending on the setup employed). Although we did not transitioned from the metallic state to the superconducting phase in none of our samples, we report an induced charge carrier density in our devices on the order of . Sheet resistance was minimized to below 50 Ohm/sq and record mobility values greater than 2000 were obtained in our samples. We equally report an on/off ratio on the order of 10 in our MoS atomically thin transistors and an ambipolar behavior, which means both electron and hole transport.El control de la densidad de portadores de carga es una pieza clave en el estudio de los semiconductores en 2-D. Por otro lado, el uso de líquidos iónicos como dieléctricos de puerta da lugar a la formación de capas dobles de alta capacitancia eléctrica dobles (en inglés, electric double layer, EDL) que permiten la exploración de regímenes de densidad de portadores de carga (n2D ≈ 10^15 cm-2) muy superiores a los que se obtienen con puertas de estado sólido más clásicas como son las de SiO2 o la de HfO2. Esto posibilita el estudio de estados altamente correlacionados, como la superconductividad, inducidos por efecto campo. A pesar de que ya existen antecedentes de trabajos pioneros realizados con dicalcogenuros de metales de transición en los que se demuestra que la utilización de la técnica EDL es capaz de inducir superconductividad en muestra policristalinas o en la superficie de capas gruesas de algunos sistemas semiconducotores, estos no se han llegado a conseguir en capas de espesor atómico. En este sentido, es importante tener en cuenta que el empleo de la técnica de EDL no está exenta de problemas técnicos que empobrecen la calidad de los dispositivos en los que se usa tal tipo de puertas. Citando algunas de las características nocivas de los líquidos iónicos: presentan una reactividad notable tanto con los dicalcogenuros metálicos como con los metales empleados para realizar los contactos eléctricos; como líquido que son establecen tensiones sobre la superficies de los dispositivos al ser enfriados por debajo de su temperatura de congelación; y el desorden inherente a su estado líquido induce un desorden electrónico en la superficie de contacto con el canal de transferencia electrónica (los denominados 'electron puddles’) que contribuye a la degradación de la movilidad de las cargas y de los estados altamente correlacionados. En este proyecto se propone el empleo de una monocapa de h-BN como medio de protección del canal de transferencia electrónica pero que a la vez y dado su grosor subnanométrico no perturbe la capacitancia de la EDL preservando la alta eficiencia de los líquidos como inductores de densidad de carga. En primer lugar se lleva a cabo la nanofabricación de las heteroestructuras encapsuladas de monocapa de MoS2 y se integran en dispositivos electrónicos. Partiendo de cristales masivos, se realiza la exfoliación y el apilado dirigido de los distintos elementos de la heterostructura unos encima de los otros, finalizando el dispositivo con un proceso estándar de nano-litografía. La segunda etapa del proyecto consiste en emplear un líquido iónico para la inducción de una alta densidad de carga en el MoS2 a través una monocapa de h-BN superior que separa líquido y dicalcogenuro metálico. El proceso de medida conlleva el uso de montajes de medida criogénicos para monitorización a muy baja temperaturas con y sin presencia de campos magnéticos. Afín de obtener superconductividad dos dimensional inducida en el MoS2, se realizan dos tipos de dispositivos con distintas estructuras: “bottom contacts” y “top contats” (via túnel a través de la monocapa de hBN). Se reporta un comportamiento metálico de la monocapa de MoS2 intrínsecamente semiconductura tras la aplicación de voltaje al líquido iónico. Pese a no observar superconductividad a bajas temperaturas en dichos sistemas, se observa una densidad de portadores de carga del orden de 10^{15}cm^{-2}cm^2 V^{-1} s^{-1}^510^{15}cm^{-2}cm^2 V^{-1} s^{-1}^5$. Mitjançant la aplicació de voltatges de porta tant positius com negatius s’obté un comportament ambipolar del canal, és a dir conducció tant d’electrons (voltatge positiu) com de forats (voltatge negatiu)
Líquids iònics com a dielèctrics de porta en dispositius encapsulats de alta qualitat hBN/MoS2/hBN.
No full textThe ability to produce few-atoms-thick two-dimensional materials of high quality such as graphene, transition-metal dichalcogenides and hexagonal boron nitride is a major improvement in condensed-matter physics and in nanoelectronics. When thinned down to the sub-nanometric scale, many layered van der Waals materials exhibit an interesting evolution of their physical properties and clearly show that multilayers of different thickness truly represent distinct electronic systems. The transition metal dichalcogenide semiconductor MoS has attracted particular interest because of its distinctive electronic and optical properties. For instance, in the bulk form, MoS crystals are indirect-gap semiconductors with a band gap of 1.29 eV, but its monolayer version has a direct band gap of 1.8eV. Because of the relatively weak interactions between the different layers and the strong intralayer interactions, the formation of ultrathin crystals of MoS by the micromechanical cleavage technique is actually possible. Early pioneering studies aimed at probing superconductivity in individual layers of superconducting transition metal dichalcogenides (TMDs) date back to the 1970s, but only over the last few years the experimental control necessary to unambiguously identify the thickness of atomically thin layers and the nanofabrication techniques required to manipulate such flakes have been developed. Recently, gate-induced superconductivity at the surface of MoS and other TMDs has been demonstrated by the mean of a field-effect transistor structure with a liquid gate. The relatively high critical temperature and the possibility to obtain chemically stable monolayers by simple exfoliation techniques make it an ideal choice to investigate the gate-induced superconductivity in such systems. This breakthrough work was performed on thick exfoliated layers, therefore behaving as bulk samples and led to the observation of critical temperature values up to 12 K following the accumulation of electron surface densities on the order of . Such unprecedented values were achieved by using ionic liquids, a novel technique (not fully yet understood), which favors the formation of an an electric double layer at the interface between the gate and the channel. Theoretically, superconductivity in MoS monolayers has been predicted and atomically thin crystals have been demonstrated to possess very peculiar and attractive superconducting characteristics, uncommon to other more conventional materials. MoS in its monolayer form is believed to become for instance an unconventional 2D Ising superconductor and is thereby very robust against external magnetic fields. The project herein described has been motivated by such lasts discoveries and aims to report for the first time superconductivity in MoS monolayers. However, the fabrication of high quality samples is far from being trivial. In order to perform multi-terminal transport measurements of MoS, we employed a van der Waals heterostructure device platform. Potential sources of disorder and scattering include defects such as sulfur vacancies in the MoS itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, the MoS ultra-thin layers are fully encapsulated within hexagonal boron nitride. The bottom one would serve as a clean and flat surface, whereas the top one would be as thin as possible and would protect the channel from the ionic liquid gate while preserving its characteristics. As will be mentioned in this report, various procedures and structures were attempted to approach our goal. To investigate the occurrence of superconductivity, we biased the ionic liquid field effect transistor (FET) by applying a gate voltage V to the ionic liquid before cooling it down slowly to a temperature around 4K (or lower, depending on the setup employed). Although we did not transitioned from the metallic state to the superconducting phase in none of our samples, we report an induced charge carrier density in our devices on the order of . Sheet resistance was minimized to below 50 Ohm/sq and record mobility values greater than 2000 were obtained in our samples. We equally report an on/off ratio on the order of 10 in our MoS atomically thin transistors and an ambipolar behavior, which means both electron and hole transport.El control de la densidad de portadores de carga es una pieza clave en el estudio de los semiconductores en 2-D. Por otro lado, el uso de líquidos iónicos como dieléctricos de puerta da lugar a la formación de capas dobles de alta capacitancia eléctrica dobles (en inglés, electric double layer, EDL) que permiten la exploración de regímenes de densidad de portadores de carga (n2D ≈ 10^15 cm-2) muy superiores a los que se obtienen con puertas de estado sólido más clásicas como son las de SiO2 o la de HfO2. Esto posibilita el estudio de estados altamente correlacionados, como la superconductividad, inducidos por efecto campo. A pesar de que ya existen antecedentes de trabajos pioneros realizados con dicalcogenuros de metales de transición en los que se demuestra que la utilización de la técnica EDL es capaz de inducir superconductividad en muestra policristalinas o en la superficie de capas gruesas de algunos sistemas semiconducotores, estos no se han llegado a conseguir en capas de espesor atómico. En este sentido, es importante tener en cuenta que el empleo de la técnica de EDL no está exenta de problemas técnicos que empobrecen la calidad de los dispositivos en los que se usa tal tipo de puertas. Citando algunas de las características nocivas de los líquidos iónicos: presentan una reactividad notable tanto con los dicalcogenuros metálicos como con los metales empleados para realizar los contactos eléctricos; como líquido que son establecen tensiones sobre la superficies de los dispositivos al ser enfriados por debajo de su temperatura de congelación; y el desorden inherente a su estado líquido induce un desorden electrónico en la superficie de contacto con el canal de transferencia electrónica (los denominados 'electron puddles’) que contribuye a la degradación de la movilidad de las cargas y de los estados altamente correlacionados. En este proyecto se propone el empleo de una monocapa de h-BN como medio de protección del canal de transferencia electrónica pero que a la vez y dado su grosor subnanométrico no perturbe la capacitancia de la EDL preservando la alta eficiencia de los líquidos como inductores de densidad de carga. En primer lugar se lleva a cabo la nanofabricación de las heteroestructuras encapsuladas de monocapa de MoS2 y se integran en dispositivos electrónicos. Partiendo de cristales masivos, se realiza la exfoliación y el apilado dirigido de los distintos elementos de la heterostructura unos encima de los otros, finalizando el dispositivo con un proceso estándar de nano-litografía. La segunda etapa del proyecto consiste en emplear un líquido iónico para la inducción de una alta densidad de carga en el MoS2 a través una monocapa de h-BN superior que separa líquido y dicalcogenuro metálico. El proceso de medida conlleva el uso de montajes de medida criogénicos para monitorización a muy baja temperaturas con y sin presencia de campos magnéticos. Afín de obtener superconductividad dos dimensional inducida en el MoS2, se realizan dos tipos de dispositivos con distintas estructuras: “bottom contacts” y “top contats” (via túnel a través de la monocapa de hBN). Se reporta un comportamiento metálico de la monocapa de MoS2 intrínsecamente semiconductura tras la aplicación de voltaje al líquido iónico. Pese a no observar superconductividad a bajas temperaturas en dichos sistemas, se observa una densidad de portadores de carga del orden de 10^{15}cm^{-2}cm^2 V^{-1} s^{-1}^510^{15}cm^{-2}cm^2 V^{-1} s^{-1}^5$. Mitjançant la aplicació de voltatges de porta tant positius com negatius s’obté un comportament ambipolar del canal, és a dir conducció tant d’electrons (voltatge positiu) com de forats (voltatge negatiu)
Líquids iònics com a dielèctrics de porta en dispositius encapsulats de alta qualitat hBN/MoS2/hBN.
No full textThe ability to produce few-atoms-thick two-dimensional materials of high quality such as graphene, transition-metal dichalcogenides and hexagonal boron nitride is a major improvement in condensed-matter physics and in nanoelectronics. When thinned down to the sub-nanometric scale, many layered van der Waals materials exhibit an interesting evolution of their physical properties and clearly show that multilayers of different thickness truly represent distinct electronic systems. The transition metal dichalcogenide semiconductor MoS has attracted particular interest because of its distinctive electronic and optical properties. For instance, in the bulk form, MoS crystals are indirect-gap semiconductors with a band gap of 1.29 eV, but its monolayer version has a direct band gap of 1.8eV. Because of the relatively weak interactions between the different layers and the strong intralayer interactions, the formation of ultrathin crystals of MoS by the micromechanical cleavage technique is actually possible. Early pioneering studies aimed at probing superconductivity in individual layers of superconducting transition metal dichalcogenides (TMDs) date back to the 1970s, but only over the last few years the experimental control necessary to unambiguously identify the thickness of atomically thin layers and the nanofabrication techniques required to manipulate such flakes have been developed. Recently, gate-induced superconductivity at the surface of MoS and other TMDs has been demonstrated by the mean of a field-effect transistor structure with a liquid gate. The relatively high critical temperature and the possibility to obtain chemically stable monolayers by simple exfoliation techniques make it an ideal choice to investigate the gate-induced superconductivity in such systems. This breakthrough work was performed on thick exfoliated layers, therefore behaving as bulk samples and led to the observation of critical temperature values up to 12 K following the accumulation of electron surface densities on the order of . Such unprecedented values were achieved by using ionic liquids, a novel technique (not fully yet understood), which favors the formation of an an electric double layer at the interface between the gate and the channel. Theoretically, superconductivity in MoS monolayers has been predicted and atomically thin crystals have been demonstrated to possess very peculiar and attractive superconducting characteristics, uncommon to other more conventional materials. MoS in its monolayer form is believed to become for instance an unconventional 2D Ising superconductor and is thereby very robust against external magnetic fields. The project herein described has been motivated by such lasts discoveries and aims to report for the first time superconductivity in MoS monolayers. However, the fabrication of high quality samples is far from being trivial. In order to perform multi-terminal transport measurements of MoS, we employed a van der Waals heterostructure device platform. Potential sources of disorder and scattering include defects such as sulfur vacancies in the MoS itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, the MoS ultra-thin layers are fully encapsulated within hexagonal boron nitride. The bottom one would serve as a clean and flat surface, whereas the top one would be as thin as possible and would protect the channel from the ionic liquid gate while preserving its characteristics. As will be mentioned in this report, various procedures and structures were attempted to approach our goal. To investigate the occurrence of superconductivity, we biased the ionic liquid field effect transistor (FET) by applying a gate voltage V to the ionic liquid before cooling it down slowly to a temperature around 4K (or lower, depending on the setup employed). Although we did not transitioned from the metallic state to the superconducting phase in none of our samples, we report an induced charge carrier density in our devices on the order of . Sheet resistance was minimized to below 50 Ohm/sq and record mobility values greater than 2000 were obtained in our samples. We equally report an on/off ratio on the order of 10 in our MoS atomically thin transistors and an ambipolar behavior, which means both electron and hole transport.El control de la densidad de portadores de carga es una pieza clave en el estudio de los semiconductores en 2-D. Por otro lado, el uso de líquidos iónicos como dieléctricos de puerta da lugar a la formación de capas dobles de alta capacitancia eléctrica dobles (en inglés, electric double layer, EDL) que permiten la exploración de regímenes de densidad de portadores de carga (n2D ≈ 10^15 cm-2) muy superiores a los que se obtienen con puertas de estado sólido más clásicas como son las de SiO2 o la de HfO2. Esto posibilita el estudio de estados altamente correlacionados, como la superconductividad, inducidos por efecto campo. A pesar de que ya existen antecedentes de trabajos pioneros realizados con dicalcogenuros de metales de transición en los que se demuestra que la utilización de la técnica EDL es capaz de inducir superconductividad en muestra policristalinas o en la superficie de capas gruesas de algunos sistemas semiconducotores, estos no se han llegado a conseguir en capas de espesor atómico. En este sentido, es importante tener en cuenta que el empleo de la técnica de EDL no está exenta de problemas técnicos que empobrecen la calidad de los dispositivos en los que se usa tal tipo de puertas. Citando algunas de las características nocivas de los líquidos iónicos: presentan una reactividad notable tanto con los dicalcogenuros metálicos como con los metales empleados para realizar los contactos eléctricos; como líquido que son establecen tensiones sobre la superficies de los dispositivos al ser enfriados por debajo de su temperatura de congelación; y el desorden inherente a su estado líquido induce un desorden electrónico en la superficie de contacto con el canal de transferencia electrónica (los denominados 'electron puddles’) que contribuye a la degradación de la movilidad de las cargas y de los estados altamente correlacionados. En este proyecto se propone el empleo de una monocapa de h-BN como medio de protección del canal de transferencia electrónica pero que a la vez y dado su grosor subnanométrico no perturbe la capacitancia de la EDL preservando la alta eficiencia de los líquidos como inductores de densidad de carga. En primer lugar se lleva a cabo la nanofabricación de las heteroestructuras encapsuladas de monocapa de MoS2 y se integran en dispositivos electrónicos. Partiendo de cristales masivos, se realiza la exfoliación y el apilado dirigido de los distintos elementos de la heterostructura unos encima de los otros, finalizando el dispositivo con un proceso estándar de nano-litografía. La segunda etapa del proyecto consiste en emplear un líquido iónico para la inducción de una alta densidad de carga en el MoS2 a través una monocapa de h-BN superior que separa líquido y dicalcogenuro metálico. El proceso de medida conlleva el uso de montajes de medida criogénicos para monitorización a muy baja temperaturas con y sin presencia de campos magnéticos. Afín de obtener superconductividad dos dimensional inducida en el MoS2, se realizan dos tipos de dispositivos con distintas estructuras: “bottom contacts” y “top contats” (via túnel a través de la monocapa de hBN). Se reporta un comportamiento metálico de la monocapa de MoS2 intrínsecamente semiconductura tras la aplicación de voltaje al líquido iónico. Pese a no observar superconductividad a bajas temperaturas en dichos sistemas, se observa una densidad de portadores de carga del orden de 10^{15}cm^{-2}cm^2 V^{-1} s^{-1}^510^{15}cm^{-2}cm^2 V^{-1} s^{-1}^5$. Mitjançant la aplicació de voltatges de porta tant positius com negatius s’obté un comportament ambipolar del canal, és a dir conducció tant d’electrons (voltatge positiu) com de forats (voltatge negatiu)
Graphene-Based Nanodevices in the Superconducting and Strongly Correlated Regimes
No full textThe ability to isolate and manipulate high-quality few-atoms-thick materials represents a major advance in condensed matter physics. Assembling these ultra-thin materials into van der Waals heterostructures, i.e., artificial meta-materials with atomically sharp interfaces, markedly increases the rich variety of physical properties accessible in 2D systems.
Graphene, a carbon-based 2D hexagonal lattice, possesses exceptional properties that since its discovery in 2004 have attracted wide attention from the scientific and engineering communities. In this work, I present a series of experiments via two different approaches, i.e., proximity effect and twist angle design, to induce superconductivity and strong correlations in graphene-based systems—two phenomena that do not intrinsically occur in this material.
In the first part of this thesis, graphene is flanked by two superconductors and inherits their superconducting properties by proximity effect. Initially, the underlying microscopic mechanism of this phenomenon is investigated using planar tunneling spectroscopy. Then, a superconductor-graphene-superconductor junction is coupled to a superconducting circuit to create and manipulate the first graphene-based transmon qubit.
In the second part of this dissertation, the electronic properties of graphene-based systems are engineered by controlling the relative twist angle between the atomic planes. In particular, when two graphene sheets are stacked on top of each other near the “magic angle,” θ ≈ 1.1°, nearly flat bands develop, featuring superconductivity and correlated insulating states. I begin by showing that local electrostatic control over the different electronic phases of magic-angle twisted bilayer graphene (MATBG) enables the creation of versatile hyper-tunable quantum devices. I also present low-temperature transport experiments to demonstrate the emergence of two exotic electronic phases in MATBG previously observed in other strongly correlated systems: nematicity and strange metal behavior. Next, I discuss local electronic compressibility measurements, evidencing that the low-temperature correlated phases originate from a high-energy state with an unusual band population sequence. Then, I describe nano-optics studies, probing plasmonic collective excitations in MATBG. Last, the study of a novel 2D moiré system beyond MATBG, i.e., twisted bilayer-bilayer graphene, is discussed.
The contributions of this thesis to the field pertain to graphene-based superconducting devices and the MATBG rich phase diagram and may find applications in next-generation superconducting electronics.Ph.D
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Graphene-based superconducting qubits
No full textAquest treball de fi de grau (TFG) explora la sinergia entre els camps dels qubits superconductors i els materials bidimensionals (2D), centrant-se particularment en el grafè. Contribueix a connectar la recerca de física fonamental en materials 2D, típicament realitzada per físics de la matèria condensada, i el punt de vista enginyeril propi de la comunitat d'investigadors que estudien els qubits superconductors. En aquesta tesi s'introdueixen els dos camps i es destaquen els possibles avantatges de fusionar-los. A continuació, es demostra la fabricació d'unions de Josephson basades en grafè utilitzant les instal·lacions disponibles al MIT, establint les bases per a la fabricació de circuits superconductors híbrids que poden utilitzar-se per fer qubits i construir els ordinadors quàntics del futur. En aquesta mateixa línia, aquesta tesi contribueix a l'avanç del camp en fabricar i caracteritzar qubits superconductors de tipus gatemon (controlats amb una porta o "gate" electroestàtica) basats en grafè amb una coherència més alta que els últims qubits gatemon de grafè més avançats. A més, en aquest treball s'aconsegueix reproduir una tècnica per donar la forma desitjada a flocs (o escates) de grafè fent servir oxidació anòdica local amb un microscopi de força atòmica i es presenta la primera demostració de la incorporació d'aquest tipus d'escates de grafè en dispositius de grafè monocapa. En general, amb aquesta investigació es contribueix a la fiabilitat i robustesa dels mètodes de fabricació necessaris per fer qubits superconductors basats en grafè.Este trabajo de final de grado (TFG) explora la sinergia entre los campos de los qubits superconductores y los materiales bidimensionales (2D), centrándose particularmente en el grafeno. Contribuye a conectar la investigación de física fundamental en materiales 2D, típicamente realizada por físicos de la materia condensada, y el punto de vista ingenieril propio de la comunidad de investigadores que estudian los qubits superconductores. En esta tesis se introducen los dos campos y se destacan las posibles ventajas de fusionarlos. A continuación, se demuestra la fabricación de uniones de Josephson basadas en grafeno utilizando las instalaciones disponibles en el MIT, sentando las bases para la fabricación de circuitos superconductores híbridos que pueden utilizarse para hacer qubits y construir los ordenadores cuánticos del futuro. En esta misma línea, esta tesis contribuye al avance del campo al fabricar y caracterizar qubits superconductores de tipo gatemon (controlados con una puerta o "gate" electroestática) basados en grafeno con una coherencia más alta que los últimos qubits gatemon de grafeno más avanzados. Además, en este trabajo se consigue reproducir una técnica para dar la forma deseada a copos (o escamas) de grafeno utilizando oxidación anódica local con un microscopio de fuerza atómica y se presenta la primera demostración de la incorporación de este tipo de copos de grafeno en dispositivos de grafeno monocapa. En general, con esta investigación se contribuye a la fiabilidad y robustez de los métodos de fabricación necesarios para hacer qubits superconductores basados en grafeno.This thesis explores the synergy between the fields of superconducting qubits and two-dimensional (2D) materials, particularly focusing on graphene. It contributes to bridging the gap between fundamental physics research in 2D materials, typically performed by condensed matter physicists and the engineering-focused approach of the superconducting qubits community. The thesis introduces the two fields and highlights the potential advantages of merging them. Then, it demonstrates the fabrication of graphene-based Josephson junctions utilizing the facilities at MIT, laying the groundwork for fabricating hybrid superconducting circuits that can be used to make qubits and build quantum computers in the future. Following this path, this thesis contributes to advancing the field by fabricating and characterizing superconducting graphene-based gatemon qubits (controlled with an electrostatic back gate) with higher coherence than the original demonstration of graphene gatemon qubits. Furthermore, it applies a technique for patterning graphene flakes using local anodic oxidation with an atomic force microscope and presents the first demonstration of incorporating these flakes into monolayer graphene devices. Overall, this work contributes to the reliability and robustness of the fabrication methods necessary to make graphene-based superconducting qubits.Outgoin
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