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To access superconductivity via the electric field effect in a clean, two-dimensional device is a central goal of nanoelectronics. Recently, superconductivity has been realized in graphene moiré heterostructures.sup.1-4; however, many of these structures are not mechanically stable, and experiments show signatures of strong disorder. Here we report the observation of superconductivity--manifesting as low or vanishing resistivity at sub-kelvin temperatures--in crystalline rhombohedral trilayer graphene.sup.5,6, a structurally metastable carbon allotrope. Superconductivity occurs in two distinct gate-tuned regions (SC1 and SC2), and is deep in the clean limit defined by the ratio of mean free path and superconducting coherence length. Mapping of the normal state Fermi surfaces by quantum oscillations reveals that both superconductors emerge from an annular Fermi sea, and are proximal to an isospin-symmetry-breaking transition where the Fermi surface degeneracy changes.sup.7. SC1 emerges from a paramagnetic normal state, whereas SC2 emerges from a spin-polarized, valley-unpolarized half-metal.sup.17 and violates the Pauli limit for in-plane magnetic fields by at least one order of magnitude.sup.8,9. We discuss our results in view of several mechanisms, including conventional phonon-mediated pairing.sup.10,11, pairing due to fluctuations of the proximal isospin order.sup.12, and intrinsic instabilities of the annular Fermi liquid.sup.13,14. Our observation of superconductivity in a clean and structurally simple two-dimensional metal provides a model system to test competing theoretical models of superconductivity without the complication of modelling disorder, while enabling new classes of field-effect controlled electronic devices based on correlated electron phenomena and ballistic electron transport. Superconductivity is observed in rhombohedral trilayer graphene in the absence of a moiré superlattice, with two distinct superconducting states both occurring at a symmetry-breaking transition where the Fermi surface degeneracy changes., Author(s): Haoxin Zhou [sup.1] , Tian Xie [sup.1] , Takashi Taniguchi [sup.2] , Kenji Watanabe [sup.3] , Andrea F. Young [sup.1] Author Affiliations: (1) Department of Physics, University of California, [...]

Strange metals possess highly unconventional electrical properties, such as a linear-in-temperature resistivity.sup.1-6, an inverse Hall angle that varies as temperature squared.sup.7-9 and a linear-in-field magnetoresistance.sup.10-13. Identifying the origin of these collective anomalies has proved fundamentally challenging, even in materials such as the hole-doped cuprates that possess a simple bandstructure. The prevailing consensus is that strange metallicity in the cuprates is tied to a quantum critical point at a doping p* inside the superconducting dome.sup.14,15. Here we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond p*. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high values of the ratio of the field and the temperature, indicating that the strange-metal regime extends well beyond p*. Moreover, the magnitude of the magnetoresistance is found to be much larger than predicted by conventional theory and is insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange-metal region hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant 'Planckian' dissipators. Measurements of high-field magnetotransport in overdoped cuprates indicate that the strange-metal regime exists beyond the critical doping, and that it has both coherent and incoherent contributions., Author(s): J. Ayres [sup.1] [sup.2] [sup.3] , M. Berben [sup.2] [sup.3] , M. Culo [sup.2] [sup.3] [sup.10] , Y.-T. Hsu [sup.2] [sup.3] , E. van Heumen [sup.4] [sup.5] , Y. [...]

Moiré quantum matter has emerged as a materials platform in which correlated and topological phases can be explored with unprecedented control. Among them, magic-angle systems constructed from two or three layers of graphene have shown robust superconducting phases with unconventional characteristics.sup.1-5. However, direct evidence of unconventional pairing remains to be experimentally demonstrated. Here we show that magic-angle twisted trilayer graphene exhibits superconductivity up to in-plane magnetic fields in excess of 10 T, which represents a large (2-3 times) violation of the Pauli limit for conventional spin-singlet superconductors.sup.6,7. This is an unexpected observation for a system that is not predicted to have strong spin-orbit coupling. The Pauli-limit violation is observed over the entire superconducting phase, which indicates that it is not related to a possible pseudogap phase with large superconducting amplitude pairing. Notably, we observe re-entrant superconductivity at large magnetic fields, which is present over a narrower range of carrier densities and displacement fields. These findings suggest that the superconductivity in magic-angle twisted trilayer graphene is likely to be driven by a mechanism that results in non-spin-singlet Cooper pairs, and that the external magnetic field can cause transitions between phases with potentially different order parameters. Our results demonstrate the richness of moiré superconductivity and could lead to the design of next-generation exotic quantum matter. A large violation of the Pauli limit and re-entrant superconductivity in a magnetic field is reported for magic-angle twisted trilayer graphene, suggesting that the spin configuration of the superconducting state of this material is unlikely to consist of spin singlets., Author(s): Yuan Cao [sup.1] , Jeong Min Park [sup.1] , Kenji Watanabe [sup.2] , Takashi Taniguchi [sup.3] , Pablo Jarillo-Herrero [sup.1] Author Affiliations: (1) Department of Physics, Massachusetts Institute of [...]

Superconductivity and magnetism have been observed in layered graphene in which the sheets are twisted with respect to each other. But a simpler, more stable graphene system also exhibits these phases. Trilayer graphene system without a twist shows different phases., Author(s): Thiti Taychatanapat Author Affiliations: Untwisted trilayer graphene hosts superconductivity and magnetism Electrons typically propagate without interacting in graphene, a single layer of carbon atoms arranged in a honeycomb lattice. [...]

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivitywhich cannot be explained by weak electronphonon interactionsin a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1the first magic anglethe electronic band structure of this twisted bilayer graphene exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperaturecarrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 10[sup.11] per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids., Author(s): Yuan Cao (corresponding author) [1]; Valla Fatemi [1]; Shiang Fang [2]; Kenji Watanabe [3]; Takashi Taniguchi [3]; Efthimios Kaxiras [2, 4]; Pablo Jarillo-Herrero (corresponding author) [1] Strong interactions among [...]

A material system known as magic-angle twisted trilayer graphene exhibits superconductivity. The observation that this superconductivity persists under a strong magnetic field could lead to advances in quantum computation. Evidence of spin-triplet superconductivity in a quasi-2D system., Author(s): Yi-Ting Hsu Author Affiliations: Superconductivity in a graphene system survives a strong magnetic field Quantum phases of matter known as superconductors transmit electrical current with zero resistance. Microscopically, this [...]

The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry. In the almost three decades since, much has been learned about the novel forms of quantum matter that are exhibited in these strongly correlated electron systems. A qualitative understanding of the nature of the superconducting state itself has been achieved. However, unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the 'normal' state at elevated temperatures., The discovery of high-temperature superconductivity in the copper oxide perovskite [La.sub.2-x][Ba.sub.x]Cu[O.sub.4] (ref. 1) ranks among the major scientific events of the twentieth century. The superconducting transition temperatures in the copper [...]

Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures (1-3). In complex oxides, this method has been used to melt electronic order (4-6), drive insulator-to-metal transitions (7) and induce superconductivity (8). Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in Y[Ba.sub.2][Cu.sub.3][O.sub.6+x] (refs 9,10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state Y[Ba.sub.2][Cu.sub.3][O.sub.6+x] at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-[O.sub.2] intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] character of the in-plane electronic structure is likely to favour superconductivity., The response of a crystal lattice to strong, resonant excitation of an infrared-active phonon mode can be described by separating the crystal Hamiltonian into its linear and nonlinear terms: H [...]

Physicists at the University of British Columbia (UBC), Canada, report creating the first-ever superconducting graphene sample by coating it with lithium atoms. Although superconductivity has already been observed in intercalated [...]