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(This message was added in version 6.7.0.) in /home4/scienrds/scienceandnerds/wp-includes/functions.php on line 6114Source:https:\/\/www.quantamagazine.org\/meet-strange-metals-where-electricity-may-flow-without-electrons-20231127\/#comments<\/a><\/br> After a year of trial and error, Liyang Chen had managed to whittle down a metallic wire into a microscopic strand half the width of an E.coli<\/em> bacterium \u2014 just thin enough to allow a trickle of electric current to pass through. The drips of that current might, Chen hoped, help settle a persistent mystery about how charge moves through a bewildering class of materials known as strange metals.<\/p>\n Chen, then a graduate student, and his collaborators at Rice University measured the current flowing through their atoms-thin strand of metal. And they found that it flowed smoothly and evenly. So evenly, in fact, that it defied physicists\u2019 standard conception of electricity in metals.<\/p>\n Canonically, electric current results from the collective movement of electrons, each carrying one indivisible chunk of electric charge. But the dead steadiness of Chen\u2019s current implied that it wasn\u2019t made of units at all. It was like finding a liquid that somehow lacked individually recognizable molecules.<\/p>\n While that might sound outlandish, it\u2019s exactly what some physicists expected from the metal the group tested, which along with its unusual kin has beguiled and bewildered physicists since the 1980s. \u201cIt\u2019s a very beautiful piece of work,\u201d said Subir Sachdev<\/a>, a theoretical physicist at Harvard University who specializes in strange metals.<\/p>\n The observation, reported last week<\/a> in the journal Science<\/em>, is one of the most straightforward indications yet that whatever carries current through these unusual metals doesn\u2019t look anything like electrons. The new experiment strengthens suspicions that a new quantum phenomenon is arising within strange metals. It also provides new grist for theoretical physicists attempting to understand what it might be.\u00a0<\/em><\/strong><\/p>\n \u201cStrange metals, no one has any earthly idea where they\u2019re coming from,\u201d said Peter Abbamonte<\/a>, a physicist at the University of Illinois, Urbana-Champaign. \u201cIt used to be considered an inconvenience, but now we realize it\u2019s really a different phase of matter living in these things.\u201d<\/p>\n The first challenge to the conventional understanding of metals came in 1986, when Georg Bednorz and Karl Alex M\u00fcller rocked the physics world with their discovery of high-temperature superconductors \u2014 materials that perfectly carry an electric current even at relatively warm temperatures. Familiar metals like tin and mercury become superconductors only when chilled to within a few degrees of absolute zero. Bednorz and M\u00fcller measured the electrical resistance in a copper-based (\u201ccuprate\u201d) material and saw that it vanished at a relatively balmy 35 kelvins. (For their breakthrough discovery, Bednorz and M\u00fcller pocketed a Nobel Prize just a year later.)<\/p>\n Physicists soon realized that high-temperature superconductivity was only the beginning of the mysterious behavior of the cuprates.<\/p>\n The cuprates got really weird when they stopped superconducting and started resisting. As all metals warm, resistance increases. Warmer temperatures mean atoms and electrons jiggle more, creating more resistance-inducing collisions as electrons shuttle current through a material. In normal metals, such as nickel, resistance rises quadratically at low temperatures \u2014 slowly at first and then faster and faster. But in the cuprates, it rose linearly: Each degree of warming brought the same increase in resistance \u2014 a bizarre pattern that continued over hundreds of degrees and, in terms of strangeness, overshadowed the material\u2019s superconducting ability. The cuprates were the strangest metals researchers had ever seen.<\/p>\n \u201cSuperconductivity is a mouse,\u201d said Andrey Chubukov<\/a>, a theoretical physicist at the University of Minnesota. \u201cThe elephant \u2026 is this strange metal behavior.\u201d<\/p>\n The linear rise in resistance threatened a celebrated explanation of how electric charge moves through metals. Proposed in 1956, Lev Landau\u2019s \u201cFermi liquid\u201d theory placed electrons at the center of it all. It built upon earlier theories that, for simplicity, assumed that electrons carry electric current, and that the electrons move through a metal like a gas; they flit freely between atoms without interacting with each other.<\/p>\n Landau added a way of handling the crucial but complicated fact that electrons interact. They are negatively charged, which means they constantly repel each other. Considering this interaction between the particles transformed the electron gas into something of an ocean \u2014 now, as one electron moved through the fluid of electrons, it disturbed the nearby electrons. Through a complicated series of interactions involving mutual repulsion, these now gently interacting electrons ended up traveling in crowds \u2014 in clumps known as quasiparticles.<\/p>\n The miracle of Fermi liquid theory was that each quasiparticle behaved almost exactly as if it were a single, fundamental electron. One major difference, though, was that these blobs moved more sluggishly or more nimbly (depending on the material) than a bare electron, effectively acting heavier or lighter. Now, just by adjusting the mass terms in their equations, physicists could continue to treat current as the movement of electrons, only with an asterisk specifying that each electron was really a quasiparticle clump.<\/p>\n A major triumph of Landau\u2019s framework was that in normal metals, it nailed the complicated way in which resistance rises quadratically with temperature. Electron-like quasiparticles became the standard way of understanding metals. \u201cIt\u2019s in every textbook,\u201d Sachdev said.<\/p>\n But in the cuprates, Landau\u2019s theory failed dramatically. Resistance rose in an immaculate line rather than the standard quadratic curve. Physicists have long interpreted this line as a sign that cuprates are home to a new physical phenomenon.<\/p>\n \u201cYou pretty much have to believe that nature is either giving you a clue or nature is incredibly cruel,\u201d said Gregory Boebinger<\/a>, a physicist at Florida State University who has spent much of his career studying the cuprates\u2019 linear response. \u201cTo put up such a terribly simple and beguiling signature and to have it not be physically important would just be too much to bear.\u201d<\/p>\n And the cuprates were just the beginning. Researchers have since discovered a host of disparate materials<\/a> with the same alluring linear resistance, including organic \u201cBechgaard salts\u201d and misaligned sheets of graphene. As these \u201cstrange metals\u201d proliferated, scientists wondered why Landau\u2019s Fermi fluid theory seemed to break down in all these different materials. Some came to suspect that it was because there were no quasiparticles at all; the electrons were somehow organizing themselves in a strange new way that obscured any individuality, much as the discrete nature of grapes gets lost in a bottle of wine.<\/p>\n \u201cIt\u2019s a phase of matter where an electron really has no identity,\u201d Abbamonte said. \u201cNevertheless, [a strange metal] is a metal; it somehow carries current.\u201d<\/p>\n But one does not simply abolish electrons. To some scientists, a potentially continuous electric current \u2014 one that isn\u2019t divvied up into electrons \u2014 is too radical. And some strange metal experiments<\/a> continue to match certain predictions of Landau\u2019s theory. The persisting controversy prompted Chen\u2019s thesis adviser, Douglas Natelson<\/a> of Rice University, along with his colleague Qimiao Si<\/a>, to consider how they might more directly scrutinize the anatomy of the charge moving through a strange metal.<\/p>\n \u201cWhat could I measure that would actually tell me what\u2019s going on?\u201d Natelson wondered.<\/p>\n The team\u2019s goal was to dissect the current in a strange metal. Did it come in electron-size chunks of charge? Did it come in chunks at all? To find out, they took inspiration from a classic way of measuring fluctuations in a flow \u2014 the \u201cshot noise\u201d \u2014 a phenomenon that can be understood if we think of the ways that rain might fall during a rainstorm.<\/p>\n Imagine you\u2019re sitting in your car, and you know from a trustworthy weather forecast that 5 millimeters of rain will fall over the next hour. Those 5 millimeters are like the total electrical current. If that rain is parceled into a handful of giant drops, the variation in when those drops hit your roof will be high; sometimes drops will splatter back to back, and at other times they will be spaced out. In this case, the shot noise is high. But if the same 5 millimeters of rain is spread into a constant mist of tiny droplets, the variation in arrival time \u2014 and therefore the shot noise \u2014 will be low. The mist will smoothly deliver almost the same amount of water from moment to moment. In this way, shot noise reveals the size of the drops.<\/p>\n \u201cJust measuring the rate at which water shows up doesn\u2019t tell you the whole picture,\u201d Natelson said. \u201cMeasuring the fluctuations [in that rate] tells you a lot more.\u201d<\/p>\n
\nMeet Strange Metals: Where Electricity May Flow Without Electrons<\/br>
\n2023-11-28 21:58:26<\/br><\/p>\nA Cuprate Wrench<\/strong><\/h2>\n
The Anatomy of Electricity<\/strong><\/h2>\n