Strange metal in magic-angle graphene with near Planckian dissipation

Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. In this paper we show that magic-angle bilayer graphene exhibits another hallmark of strongly correlated systems --- a broad regime of $T-$linear resistivity above a small, density dependent, crossover temperature--- for a range of fillings near the correlated insulator. We also extract a transport "scattering rate", which satisfies a near Planckian form that is universally related to the ratio of $(k_BT/\hbar)$. Our results establish magic-angle bilayer graphene as a highly tunable platform to investigate strange metal behavior, which could shed light on this mysterious ubiquitous phase of correlated matter.Comment: 7 pages, 3 figures. (Supplementary material: 3 pages, 2 figures

Phonon Polaritons in Monolayers of Hexagonal Boron Nitride.

Phonon polaritons in van der Waals materials reveal significant confinement accompanied with long propagation length: important virtues for tasks pertaining to the control of light and energy flow at the nanoscale. While previous studies of phonon polaritons have relied on relatively thick samples, here reported is the first observation of surface phonon polaritons in single atomic layers and bilayers of hexagonal boron nitride (hBN). Using antenna-based near-field microscopy, propagating surface phonon polaritons in mono- and bilayer hBN microcrystals are imaged. Phonon polaritons in monolayer hBN are confined in a volume about one million times smaller than the free-space photons. Both the polariton dispersion and their wavelength-thickness scaling law are altered compared to those of hBN bulk counterparts. These changes are attributed to phonon hardening in monolayer-thick crystals. The data reported here have bearing on applications of polaritons in metasurfaces and ultrathin optical elements

Deep-Learning-Enabled Fast Optical Identification and Characterization of Two-Dimensional Materials

Advanced microscopy and/or spectroscopy tools play indispensable role in nanoscience and nanotechnology research, as it provides rich information about the growth mechanism, chemical compositions, crystallography, and other important physical and chemical properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, we use the optical characterization of two-dimensional (2D) materials as a case study, and demonstrate a neural-network-based algorithm for the material and thickness identification of exfoliated 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, segment sizes and their distributions, based on which we develop an ensemble approach topredict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other applications such as identifying layer numbers of a new 2D material, or materials produced by a different synthetic approach. Our artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials and potentially accelerate new material discoveries

Quantum coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures

Quantum coherence and control is foundational to the science and engineering of quantum systems. In van der Waals (vdW) materials, the collective coherent behavior of carriers has been probed successfully by transport measurements. However, temporal coherence and control, as exemplified by manipulating a single quantum degree of freedom, remains to be verified. Here we demonstrate such coherence and control of a superconducting circuit incorporating graphene-based Josephson junctions. Furthermore, we show that this device can be operated as a voltage-tunable transmon qubit, whose spectrum reflects the electronic properties of massless Dirac fermions traveling ballistically. In addition to the potential for advancing extensible quantum computing technology, our results represent a new approach to studying vdW materials using microwave photons in coherent quantum circuits

Líquids iònics com a dielèctrics de porta en dispositius encapsulats de alta qualitat hBN/MoS2/hBN.

The 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$_2$ has attracted particular interest because of its distinctive electronic and optical properties. For instance, in the bulk form, MoS$_2$ 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$_2$ 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$_2$ 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 $n_{2D} \simeq 10^{14} cm^{-2}$. 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$_2$ 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$_2$ 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$_2$ monolayers. However, the fabrication of high quality samples is far from being trivial. In order to perform multi-terminal transport measurements of MoS$_2$, we employed a van der Waals heterostructure device platform. Potential sources of disorder and scattering include defects such as sulfur vacancies in the MoS$_2$ itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, the MoS$_2$ 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$_{gate}$ 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 $n_{2D } = 10^{15}cm^{-2}$. Sheet resistance was minimized to below 50 Ohm/sq and record mobility values greater than 2000 $cm^2 V^{-1} s^{-1}$ were obtained in our samples. We equally report an on/off ratio on the order of 10$^5$ in our MoS$_2$ 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}$a 4K. La resistencia superficial medida es menor que 50 Ohm/sq y obtenemos movilidades récord en este material, del orden de 2000$cm^2 V^{-1} s^{-1}$. El ratio entre el estado apagado y encendido es de 10$^5$y mediante la aplicación de voltajes de puerta positivos y negativos se obtiene un comportamiento ambipolar del canal, es decir conducción tanto de electrones (voltaje positivo) como huecos (voltaje negativo).La habilitat de produir materials 2D de alta qualitat, i poder apilar-los els uns sobre els altres, és un assoliment major en el camp de la física de la materia condensada i la nanoelectrònica. Tals materials presenten nous fenòmens mai observats fins el moment només presents en sistemes de baixa dimensionalitat. En aquest projecte, es proposa la encapsulació de una monocapa de MoS2 entre dues escates de hBN ultra planes, inerts i aïllants. Els nanodispositius així fabricats tenen estructura de transistor, on s’usen líquids iònics com a dielèctrics de porta per a tal d’induir una altra densitat de portadors de càrrega al sistema i poder observar superconductivitat dos dimensional en capes atòmicament gruixudes. El projecte consisteix en la nanofabricació, caracterització i mesurament de mostres a baixes temperatures, amb presència de camps magnètics. Es reporta un comportament metàl•lic de la monocapa de MoS2 intrínsecament semiconductura amb la aplicació de voltatge al líquid iònic. Malgrat no observar superconductivitat a baixes temperaturas en tals sistemes, s’obté una densidad de portadors de càrrega del ordre de$10^{15}cm^{-2}$a 4K. La resistència superficial mesurada és menor que 50 Ohm/sq i obtenim mobilitats rècord en aquest material, del ordre de 2000$cm^2 V^{-1} s^{-1}$. El rati entat el estat apagat y encès és de 10$^5$. 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)