• Guiding a superconducting future with gr

    From ScienceDaily@1:317/3 to All on Tue Apr 19 22:30:44 2022
    Guiding a superconducting future with graphene quantum magic

    Date:
    April 19, 2022
    Source:
    Nagoya University
    Summary:
    Superconductors are materials that conduct electrical current
    with practically no electrical resistance at all. This ability
    makes them extremely interesting and attractive for a plethora
    of applications such as loss-less power cables, electric motors
    and generators, as well as powerful electromagnets that can be
    used for MRI imaging and for magnetic levitating trains. Now,
    researchers have detailed the superconducting nature of a new class
    of superconducting material, magic-angle twisted bilayer graphene.



    FULL STORY ========================================================================== Superconductors are materials that conduct electrical current with
    practically no electrical resistance at all. This ability makes them
    extremely interesting and attractive for a plethora of applications such
    as loss-less power cables, electric motors and generators, as well as
    powerful electromagnets that can be used for MRI imaging and for magnetic levitating trains. Now, researchers from Nagoya University have detailed
    the superconducting nature of a new class of superconducting material, magic-angle twisted bilayer graphene.


    ==========================================================================
    For a material to behave as a superconductor, low temperatures are
    required.

    Most materials only enter the superconducting phase at extremely low temperatures, such as -270DEGC, lower than those measured in outer
    space! This severely limits their practical applications because such
    extensive cooling requires very expensive and specialized liquid helium
    cooling equipment. This is the main reason superconducting technologies
    are still in their infancy.

    High temperature superconductors (HTS), such as some iron and copper-based ones, enter the superconducting phase above -200DEGC, a temperature that
    is more readily achievable using liquid nitrogen which cools down a system
    to ?195.8DEGC. However, the industrial and commercial applications of HTS
    have been thus far limited. Currently known and available HTS materials
    are brittle ceramic materials that are not malleable into useful shapes
    like wires. In addition, they are notoriously difficult and expensive
    to manufacture. This makes the search for new superconducting materials critical, and a strong focus of research for physicists like Prof. Hiroshi Kontani and Dr. Seiichiro Onari from the Department of Physics, Nagoya University.

    Recently, a new material has been proposed as a potential superconductor
    called magic-angle twisted bilayer graphene (MATBG). In MATBG, two layers
    of graphene, essentially single two-dimensional layers of carbon arranged
    in a honeycomb lattice, are offset by a magic angle (about 1.1 degrees)
    that leads to the breakage of rotational symmetry and the formation of a high-order symmetry known as SU(4). As temperature changes, the system experiences quantum fluctuations, like water ripples in the atomic
    structure, that lead to a novel spontaneous change in the electronic
    structure and a reduction in symmetry.

    This rotational symmetry breaking is known as the nematic state and
    has been closely associated with superconducting properties in other
    materials.

    In their work published recently in Physical Review Letters, Prof. Kontani
    and Dr. Onari use theoretical methods to better understand and shine light
    on the source of this nematic state in MATBG. "Since we know that high temperature superconductivity can be induced by nematic fluctuations in strongly correlated electron systems such as iron-based superconductors, clarifying the mechanism and origin of this nematic order can lead
    to the design and emergence of higher temperature superconductors,"
    explains Dr. Onari.

    The researchers found that nematic order in MATBG originates from the interference between the fluctuations of a novel degree-of-freedom that combines the valley degrees of freedom and the spin degrees of freedom, something that has not been reported from conventional strongly correlated electron systems. The superconducting transition temperature of twisted
    bilayer graphene is very low, at 1K (-272DEGC), but the nematic state
    manages to increase it by several degrees. Their results also show that although MATBG behaves in some ways like an iron-based high temperature superconductor, it also has some distinct properties that are quite
    exciting, such as a net charge loop current giving rise to a magnetic
    field in a valley polarized state, while the loop current is canceled
    out by each valley in the nematic state. Besides, the malleability of
    graphene can also play an important role in increasing the practical applications of these superconductors. With a better understanding of
    the underlying mechanisms of superconductivity, science and technology
    inch closer to a conducting future that is indeed super.


    ========================================================================== Story Source: Materials provided by Nagoya_University. Note: Content
    may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Seiichiro Onari, Hiroshi Kontani. SU(4) Valley+Spin Fluctuation
    Interference Mechanism for Nematic Order in Magic-Angle Twisted
    Bilayer Graphene: The Impact of Vertex Corrections. Physical Review
    Letters, 2022; 128 (6) DOI: 10.1103/PhysRevLett.128.066401 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/04/220419092330.htm

    --- up 7 weeks, 1 day, 10 hours, 51 minutes
    * Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)