The Experts below are selected from a list of 24561 Experts worldwide ranked by ideXlab platform
Kenji Watanabe - One of the best experts on this subject based on the ideXlab platform.
-
multiple flat bands and topological hofstadter butterfly in twisted bilayer graphene close to the second Magic Angle
Proceedings of the National Academy of Sciences of the United States of America, 2021Co-Authors: Biao Lian, Kenji Watanabe, Takashi Taniguchi, Andrei B Bernevig, Gaurav Chaudhary, B A Piot, Giulio Romagnoli, M Poggio, Allan H Macdonald, Dmitri K EfetovAbstract:Moire superlattices in two-dimensional van der Waals heterostructures provide an efficient way to engineer electron band properties. The recent discovery of exotic quantum phases and their interplay in twisted bilayer graphene (tBLG) has made this moire system one of the most renowned condensed matter platforms. So far studies of tBLG have been mostly focused on the lowest two flat moire bands at the first Magic Angle θm1 ∼ 1.1°, leaving high-order moire bands and Magic Angles largely unexplored. Here we report an observation of multiple well-isolated flat moire bands in tBLG close to the second Magic Angle θm2 ∼ 0.5°, which cannot be explained without considering electron–election interactions. With high magnetic field magnetotransport measurements we further reveal an energetically unbound Hofstadter butterfly spectrum in which continuously extended quantized Landau level gaps cross all trivial band gaps. The connected Hofstadter butterfly strongly evidences the topologically nontrivial textures of the multiple moire bands. Overall, our work provides a perspective for understanding the quantum phases in tBLG and the fractal Hofstadter spectra of multiple topological bands.
-
Gate-defined Josephson junctions in Magic-Angle twisted bilayer graphene
Nature Nanotechnology, 2021Co-Authors: Folkert K De Vries, Kenji Watanabe, Takashi Taniguchi, Elias Portoles, Giulia Zheng, Thomas Ihn, K Ensslin, Peter RickhausAbstract:In situ electrostatic control of two-dimensional superconductivity^ 1 is commonly limited due to large charge carrier densities, and gate-defined Josephson junctions are therefore rare^ 2 , 3 . Magic-Angle twisted bilayer graphene (MATBG)^ 4 – 8 has recently emerged as a versatile platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal^ 9 – 14 . Although MATBG appears to be an ideal two-dimensional platform for gate-tunable superconductivity^ 9 , 11 , 13 , progress towards practical implementations has been hindered by the need for well-defined gated regions. Here we use multilayer gate technology to create a device based on two distinct phases in adjustable regions of MATBG. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable d.c. and a.c. Josephson effects^ 15 , 16 . The ability to tune the superconducting state within a single material circumvents interface and fabrication challenges, which are common in multimaterial nanostructures. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics^ 17 , 18 and quantum information technology^ 19 , 20 . In situ electrostatic control of two-dimensional superconductivity is commonly limited due to large charge carrier densities. Now, by means of local gates, electrostatic gating can define a Josephson junction in a Magic-Angle twisted bilayer graphene device, a single-crystal material.
-
nematicity and competing orders in superconducting Magic Angle graphene
Science, 2021Co-Authors: Yuan Cao, Daniel Rodanlegrain, Kenji Watanabe, Takashi Taniguchi, Jeong Min Park, Noah F Q Yuan, Rafael M Fernandes, Pablo JarilloherreroAbstract:Strongly interacting electrons in solid-state systems often display multiple broken symmetries in the ground state. The interplay between different order parameters can give rise to a rich phase diagram. We report on the identification of intertwined phases with broken rotational symmetry in Magic-Angle twisted bilayer graphene (TBG). Using transverse resistance measurements, we find a strongly anisotropic phase located in a "wedge" above the underdoped region of the superconducting dome. Upon its crossing with the superconducting dome, a reduction of the critical temperature is observed. Furthermore, the superconducting state exhibits an anisotropic response to a direction-dependent in-plane magnetic field, revealing nematic ordering across the entire superconducting dome. These results indicate that nematic fluctuations might play an important role in the low-temperature phases of Magic-Angle TBG.
-
entropic evidence for a pomeranchuk effect in Magic Angle graphene
Nature, 2021Co-Authors: Asaf Rozen, Yuan Cao, Daniel Rodanlegrain, Kenji Watanabe, Takashi Taniguchi, Jeong Min Park, Uri Zondiner, Yuval Oreg, Ady Stern, Erez BergAbstract:In the 1950s, Pomeranchuk1 predicted that, counterintuitively, liquid 3He may solidify on heating. This effect arises owing to high excess nuclear spin entropy in the solid phase, where the atoms are spatially localized. Here we find that an analogous effect occurs in Magic-Angle twisted bilayer graphene2–6. Using both local and global electronic entropy measurements, we show that near a filling of one electron per moire unit cell, there is a marked increase in the electronic entropy to about 1kB per unit cell (kB is the Boltzmann constant). This large excess entropy is quenched by an in-plane magnetic field, pointing to its magnetic origin. A sharp drop in the compressibility as a function of the electron density, associated with a reset of the Fermi level back to the vicinity of the Dirac point, marks a clear boundary between two phases. We map this jump as a function of electron density, temperature and magnetic field. This reveals a phase diagram that is consistent with a Pomeranchuk-like temperature- and field-driven transition from a low-entropy electronic liquid to a high-entropy correlated state with nearly free magnetic moments. The correlated state features an unusual combination of seemingly contradictory properties, some associated with itinerant electrons—such as the absence of a thermodynamic gap, metallicity and a Dirac-like compressibility—and others associated with localized moments, such as a large entropy and its disappearance under a magnetic field. Moreover, the energy scales characterizing these two sets of properties are very different: whereas the compressibility jump has an onset at a temperature of about 30 kelvin, the bandwidth of magnetic excitations is about 3 kelvin or smaller. The hybrid nature of the present correlated state and the large separation of energy scales have implications for the thermodynamic and transport properties of the correlated states in twisted bilayer graphene. Magic-Angle graphene is found to have an exotic phase transition where, on heating, entropy is transferred from motional to magnetic degrees of freedom, analogously to the Pomeranchuk effect in 3He.
-
tuning electron correlation in Magic Angle twisted bilayer graphene using coulomb screening
Bulletin of the American Physical Society, 2021Co-Authors: Xiaoxue Liu, Kenji Watanabe, Takashi Taniguchi, Zhi Wang, Oskar VafekAbstract:Controlling the strength of interactions is essential for studying quantum phenomena emerging in systems of correlated fermions. We introduce a device geometry whereby Magic-Angle twisted bilayer graphene is placed in close proximity to a Bernal bilayer graphene, separated by a 3-nanometer-thick barrier. By using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effects on the insulating and superconducting states: As Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity at the optimal doping is enhanced. The results provide important constraints on theoretical models for understanding the mechanism of superconductivity in Magic-Angle twisted bilayer graphene.
Takashi Taniguchi - One of the best experts on this subject based on the ideXlab platform.
-
multiple flat bands and topological hofstadter butterfly in twisted bilayer graphene close to the second Magic Angle
Proceedings of the National Academy of Sciences of the United States of America, 2021Co-Authors: Biao Lian, Kenji Watanabe, Takashi Taniguchi, Andrei B Bernevig, Gaurav Chaudhary, B A Piot, Giulio Romagnoli, M Poggio, Allan H Macdonald, Dmitri K EfetovAbstract:Moire superlattices in two-dimensional van der Waals heterostructures provide an efficient way to engineer electron band properties. The recent discovery of exotic quantum phases and their interplay in twisted bilayer graphene (tBLG) has made this moire system one of the most renowned condensed matter platforms. So far studies of tBLG have been mostly focused on the lowest two flat moire bands at the first Magic Angle θm1 ∼ 1.1°, leaving high-order moire bands and Magic Angles largely unexplored. Here we report an observation of multiple well-isolated flat moire bands in tBLG close to the second Magic Angle θm2 ∼ 0.5°, which cannot be explained without considering electron–election interactions. With high magnetic field magnetotransport measurements we further reveal an energetically unbound Hofstadter butterfly spectrum in which continuously extended quantized Landau level gaps cross all trivial band gaps. The connected Hofstadter butterfly strongly evidences the topologically nontrivial textures of the multiple moire bands. Overall, our work provides a perspective for understanding the quantum phases in tBLG and the fractal Hofstadter spectra of multiple topological bands.
-
Gate-defined Josephson junctions in Magic-Angle twisted bilayer graphene
Nature Nanotechnology, 2021Co-Authors: Folkert K De Vries, Kenji Watanabe, Takashi Taniguchi, Elias Portoles, Giulia Zheng, Thomas Ihn, K Ensslin, Peter RickhausAbstract:In situ electrostatic control of two-dimensional superconductivity^ 1 is commonly limited due to large charge carrier densities, and gate-defined Josephson junctions are therefore rare^ 2 , 3 . Magic-Angle twisted bilayer graphene (MATBG)^ 4 – 8 has recently emerged as a versatile platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal^ 9 – 14 . Although MATBG appears to be an ideal two-dimensional platform for gate-tunable superconductivity^ 9 , 11 , 13 , progress towards practical implementations has been hindered by the need for well-defined gated regions. Here we use multilayer gate technology to create a device based on two distinct phases in adjustable regions of MATBG. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable d.c. and a.c. Josephson effects^ 15 , 16 . The ability to tune the superconducting state within a single material circumvents interface and fabrication challenges, which are common in multimaterial nanostructures. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics^ 17 , 18 and quantum information technology^ 19 , 20 . In situ electrostatic control of two-dimensional superconductivity is commonly limited due to large charge carrier densities. Now, by means of local gates, electrostatic gating can define a Josephson junction in a Magic-Angle twisted bilayer graphene device, a single-crystal material.
-
nematicity and competing orders in superconducting Magic Angle graphene
Science, 2021Co-Authors: Yuan Cao, Daniel Rodanlegrain, Kenji Watanabe, Takashi Taniguchi, Jeong Min Park, Noah F Q Yuan, Rafael M Fernandes, Pablo JarilloherreroAbstract:Strongly interacting electrons in solid-state systems often display multiple broken symmetries in the ground state. The interplay between different order parameters can give rise to a rich phase diagram. We report on the identification of intertwined phases with broken rotational symmetry in Magic-Angle twisted bilayer graphene (TBG). Using transverse resistance measurements, we find a strongly anisotropic phase located in a "wedge" above the underdoped region of the superconducting dome. Upon its crossing with the superconducting dome, a reduction of the critical temperature is observed. Furthermore, the superconducting state exhibits an anisotropic response to a direction-dependent in-plane magnetic field, revealing nematic ordering across the entire superconducting dome. These results indicate that nematic fluctuations might play an important role in the low-temperature phases of Magic-Angle TBG.
-
entropic evidence for a pomeranchuk effect in Magic Angle graphene
Nature, 2021Co-Authors: Asaf Rozen, Yuan Cao, Daniel Rodanlegrain, Kenji Watanabe, Takashi Taniguchi, Jeong Min Park, Uri Zondiner, Yuval Oreg, Ady Stern, Erez BergAbstract:In the 1950s, Pomeranchuk1 predicted that, counterintuitively, liquid 3He may solidify on heating. This effect arises owing to high excess nuclear spin entropy in the solid phase, where the atoms are spatially localized. Here we find that an analogous effect occurs in Magic-Angle twisted bilayer graphene2–6. Using both local and global electronic entropy measurements, we show that near a filling of one electron per moire unit cell, there is a marked increase in the electronic entropy to about 1kB per unit cell (kB is the Boltzmann constant). This large excess entropy is quenched by an in-plane magnetic field, pointing to its magnetic origin. A sharp drop in the compressibility as a function of the electron density, associated with a reset of the Fermi level back to the vicinity of the Dirac point, marks a clear boundary between two phases. We map this jump as a function of electron density, temperature and magnetic field. This reveals a phase diagram that is consistent with a Pomeranchuk-like temperature- and field-driven transition from a low-entropy electronic liquid to a high-entropy correlated state with nearly free magnetic moments. The correlated state features an unusual combination of seemingly contradictory properties, some associated with itinerant electrons—such as the absence of a thermodynamic gap, metallicity and a Dirac-like compressibility—and others associated with localized moments, such as a large entropy and its disappearance under a magnetic field. Moreover, the energy scales characterizing these two sets of properties are very different: whereas the compressibility jump has an onset at a temperature of about 30 kelvin, the bandwidth of magnetic excitations is about 3 kelvin or smaller. The hybrid nature of the present correlated state and the large separation of energy scales have implications for the thermodynamic and transport properties of the correlated states in twisted bilayer graphene. Magic-Angle graphene is found to have an exotic phase transition where, on heating, entropy is transferred from motional to magnetic degrees of freedom, analogously to the Pomeranchuk effect in 3He.
-
tuning electron correlation in Magic Angle twisted bilayer graphene using coulomb screening
Bulletin of the American Physical Society, 2021Co-Authors: Xiaoxue Liu, Kenji Watanabe, Takashi Taniguchi, Zhi Wang, Oskar VafekAbstract:Controlling the strength of interactions is essential for studying quantum phenomena emerging in systems of correlated fermions. We introduce a device geometry whereby Magic-Angle twisted bilayer graphene is placed in close proximity to a Bernal bilayer graphene, separated by a 3-nanometer-thick barrier. By using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effects on the insulating and superconducting states: As Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity at the optimal doping is enhanced. The results provide important constraints on theoretical models for understanding the mechanism of superconductivity in Magic-Angle twisted bilayer graphene.
Amir Goldbourt - One of the best experts on this subject based on the ideXlab platform.
-
Magic Angle spinning nmr spectroscopy a versatile technique for structural and dynamic analysis of solid phase systems
Analytical Chemistry, 2015Co-Authors: Tatyana Polenova, Rupal Gupta, Amir GoldbourtAbstract:Magic Angle Spinning (MAS) NMR spectroscopy is a powerful method for analysis of a broad range of systems, including inorganic materials, pharmaceuticals, and biomacromolecules. The recent developments in MAS NMR instrumentation and methodologies opened new vistas to atomic-level characterization of a plethora of chemical environments previously inaccessible to analysis, with unprecedented sensitivity and resolution.
-
biomolecular Magic Angle spinning solid state nmr recent methods and applications
Current Opinion in Biotechnology, 2013Co-Authors: Amir GoldbourtAbstract:The link of structure and dynamics of biomolecules and their complexes to their function and to many cellular processes has driven the quest for their detailed characterization by a variety of biophysical techniques. Magic-Angle spinning solid-state nuclear magnetic resonance spectroscopy provides detailed information on the structural properties of such systems and in particular contributes invaluable information on non-soluble, large molecular-weight and non-crystalline biomolecules. This review summarizes the recent progress that has been made in the characterization of macromolecular assemblies, viruses, membrane proteins, amyloid fibrils, protein aggregates and more by Magic-Angle spinning solid-state NMR.
Andreas Brinkmann - One of the best experts on this subject based on the ideXlab platform.
-
Magic-Angle-Spinning Nuclear Magnetic Resonance
2015Co-Authors: Andreas Brinkmann, Stockholms Universitet IsbnAbstract:This thesis concerns the development of radio-frequency pulse sequences in Magic-Angle-spinning solid-state nuclear magnetic resonance. First, two classes of pulse sequences are presented which are synchronized with the sample rotation. Symmetry theorems are described which link the symmetry of the pulse sequences to selection rules for the recoupling and/or decoupling of certain spin interactions. Pulse sequences are demonstrated which recouple direct homonuclear dipolar interactions at high sample spinning frequencies. Several applications are shown, including the ecient excitation of double-quantum coherences, two-dimensional double-quantum spectroscopy, transfer of longitudinal magnetization and two-dimensional correlation spec-troscopy. In addition, generalized Hartmann-Hahn sequences are demonstrated in which radio-frequency irradiation is applied simultaneously to two isotopic spin species. These sequences selectively recouple direct heteronuclear dipolar interactions and suppress all homonuclear interactions for both spin species
-
14n overtone nmr spectra under Magic Angle spinning experiments and numerically exact simulations
Journal of Chemical Physics, 2013Co-Authors: Luke A Odell, Andreas BrinkmannAbstract:It was recently shown that high resolution 14N overtone NMR spectra can be obtained directly under Magic Angle spinning (MAS) conditions [L. A. O’Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)]10.1016/j.cplett.2011.08.030. Preliminary experimental results showed narrowed powder pattern widths, a frequency shift that is dependent on the MAS rate, and an apparent absence of spinning sidebands, observations which appeared to be inconsistent with previous theoretical treatments. Herein, we reproduce these effects using numerically exact simulations that take into account the full nuclear spin Hamiltonian. Under sample spinning, the 14N overtone signal is split into five (0, ±1, ±2) overtone sidebands separated by the spinning frequency. For a powder sample spinning at the Magic Angle, the +2ωr sideband is dominant while the others show significantly lower signal intensities. The resultant MAS powder patterns show characteristic quadrupolar lineshapes from which the 14N quadrupolar parameters and ...
-
dipolar recoupling in Magic Angle spinning nuclear magnetic resonance
2001Co-Authors: Andreas BrinkmannAbstract:This thesis concerns the development of radio-frequency pulse sequences in Magic-Angle-spinning solid-state nuclear magnetic resonance.First, two classes of pulse sequences are presented which are ...
Tatyana Polenova - One of the best experts on this subject based on the ideXlab platform.
-
Magic Angle spinning nmr spectroscopy a versatile technique for structural and dynamic analysis of solid phase systems
Analytical Chemistry, 2015Co-Authors: Tatyana Polenova, Rupal Gupta, Amir GoldbourtAbstract:Magic Angle Spinning (MAS) NMR spectroscopy is a powerful method for analysis of a broad range of systems, including inorganic materials, pharmaceuticals, and biomacromolecules. The recent developments in MAS NMR instrumentation and methodologies opened new vistas to atomic-level characterization of a plethora of chemical environments previously inaccessible to analysis, with unprecedented sensitivity and resolution.
-
recoupling of chemical shift anisotropy by r symmetry sequences in Magic Angle spinning nmr spectroscopy
Journal of Chemical Physics, 2012Co-Authors: In Ja L. Byeon, Angela M. Gronenborn, Tatyana PolenovaAbstract:13C and 15N chemical shift (CS) interaction is a sensitive probe of structure and dynamics in a wide variety of biological and inorganic systems, and in the recent years several Magic Angle spinning NMR approaches have emerged for residue-specific measurements of chemical shift anisotropy (CSA) tensors in uniformly and sparsely enriched proteins. All of the currently existing methods are applicable to slow and moderate Magic Angle spinning (MAS) regime, i.e., MAS frequencies below 20 kHz. With the advent of fast and ultrafast MAS probes capable of spinning frequencies of 40–100 kHz, and with the superior resolution and sensitivity attained at such high frequencies, development of CSA recoupling techniques working under such conditions is necessary. In this work, we present a family of R-symmetry based pulse sequences for recoupling of 13C/15N CSA interactions that work well in both natural abundance and isotopically enriched systems. We demonstrate that efficient recoupling of either first-rank (σ1) or se...
