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  lectricity flows without resistance on the rim of molybdenum ditelluride


  The material molybdenum ditelluride can host a superconducting current that is constrained to travel around its edge,physicists report.


  Superconductors are getting edgy.


  For the first time,scientists have spotted a superconducting current traveling along the edge of a material,like a trail of ants crawling along the rim of a dinner plate without venturing into its middle.


  Normally,such superconducting currents,in which electricity flows without any loss of energy,permeate an entire material.But in a thin sheet of molybdenum ditelluride chilled to near absolute zero,the interior and edge make up two distinct superconductors,physicist Nai Phuan Ong and colleagues report in the May 1 Science.The two superconductors are“basically ignoring each other,”says Ong,of Princeton University.

  通常,电流流过而没有任何能量损失的这种超导电流会渗透到整个材料中。但是物理学家奈Phuan Ong和他的同事在5月1日的《Science》杂志上报告说,在冷却至绝对零附近的二碲化钼薄板中,内部和边缘构成了两个截然不同的超导体。普林斯顿大学的Ong说,这两种超导体“基本上互相忽略”。

  This distinction between exterior and interior makes molybdenum ditelluride an example of what are called topological materials.Their behavior is closely tied to the mathematical field of topology,in which shapes are considered distinct only if one can’t be molded into another without cutting or melding(SN:10/4/16).In topological insulators,electric currents can flow on the surface of a material but not the interior,like a potato covered in tinfoil(SN:5/7/10).



  Likewise,topological superconductors are superconducting in their interiors and behave differently on their surfaces.Although some researchers suspected topological superconductors might also host superconducting current on their edges,none had yet been found.But the new observation is“extremely convincing,”says physical chemist Claudia Felser of the Max Planck Institute for Chemical Physics of Solids in Dresden,Germany,who was not involved with the research.“It’s really,really super exciting.”

  同样,拓扑超导体在其内部也具有超导体,并且在其表面上具有不同的行为。尽管一些研究人员怀疑拓扑超导体也可能在其边缘承载超导电流,但尚未发现。但是德国德累斯顿的马克斯·普朗克固体化学物理研究所的物理化学家Claudia Felser说,但是这一新发现“极具说服力”。“真的,真的非常令人兴奋。”

  Molybdenum ditelluride is a metal-like compound called a Weyl semimetal(SN:7/16/15).Its unusual properties might mean it could harbor Majorana fermions,disturbances within a material that scientists hope to use to create better quantum computers.Such topological quantum computers are expected to resist the jitter that impairs quantum calculations(SN:7/20/17).


  In their experiment,Ong and colleagues gradually ramped up the magnetic field on the material.They simultaneously measured how much they could increase the electric current before the superconducting state was lost,a value known as the critical current.As the magnetic field increased,the critical current oscillated,getting larger,smaller,and larger again in a repeating pattern—a hallmark of an edge superconductor.


  The oscillation results from the weird physics of superconductors,in which electrons form partnerships called Cooper pairs.The pairs act as a unified whole,all taking on the same quantum state,or wave function,which determines the probability of a particle being found at a particular location.


  A superconducting current(white arrows)runs around the edge of a thin sheet of molybdenum ditelluride(illustrated on right)in a magnetic field(black arrows).A similar effect in a ring of superconductor(left)was seen in a classic study known as the Little-Parks experiment.



  A property of the wave function called the phase is analogous to twists in a party streamer hung around the edges of a room,Ong says.If connected at the ends,the party streamer can twist once or twice,but never 1.2 times,for example,because the ends wouldn’t align.Similarly,the phase must make a full number of twists around the material.The interplay between the increasing magnetic field and the twisting constraint causes the critical current to oscillate.


  A classic 1960s study known as the Little-Parks experiment is closely tied to the new work.In that study,a superconductor shaped like a cylinder exhibited related oscillations in a changing magnetic field.But in Ong and colleagues’version,the superconducting current runs around the edge of a solid chunk of material rather than a physical cylinder.


  “It’s a very clever and beautiful way of assessing whether or not there’s an edge current”that is superconducting,says physicist Smitha Vishveshwara of the University of Illinois at Urbana-Champaign,who was not involved with the research.

  伊利诺伊大学厄巴纳-香槟分校的物理学家Smitha Vishveshwara说:“这是一种评估是否存在边缘电流的非常聪明而优美的方法。”,他没有参与这项研究。


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