Brad Ramshaw<\/a>, associate professor of physics in the College of Arts and Sciences, who led the Cornell team. \u201cBut it\u2019s also not this story of basically fancy dirt, where the samples have impurities that are bouncing the heat one way instead of another. It\u2019s a new intrinsic effect that nobody had ever seen before.\u201d<\/p>\nMajoranas are unusual in that they are their own antiparticle. While they may not ever be produced in a particle accelerator, they can emerge from complex interactions between electrons in certain quantum materials. One such candidate is ruthenium trichloride, which is notable because it is an insulator and therefore should not have a thermal Hall effect. In that phenomenon, a magnetic field is applied to a material carrying a flow of heat and the heat flow bends \u2013 behavior that was thought to be impossible for an insulator.<\/p>\n
\u201cElectrons are charged, and so they feel a force from the field when they move, and they know whether that force is pushing them left or pushing them right. Heat flowing through an insulator is carried by vibrations of the lattice, and the lattice doesn\u2019t know about the field and therefore doesn\u2019t know left from right,\u201d Ramshaw said. \u201cFinding a thermal Hall Effect in ruthenium trichloride was surprising. It was even more surprising that it was quantized, suggesting Majorana fermions were carrying the heat.\u201d<\/p>\n
That was a big claim, according to Ramshaw, but the data seemed to back it up and the quantum materials community was terribly excited.\u00a0<\/p>\n
\u201cThen there were troubles with reproducibility and questions about who had the better samples \u2013 all the usual arguments,\u201d he said. \u201cBut ultimately, other people didn\u2019t get the same answer.\u201d<\/p>\n
The alternate explanation, of magnetic impurities deflecting the heat, was equally difficult to prove.<\/p>\n
\u201cThe problem is that, at the end of the day, all you\u2019re doing is flowing heat through something and measuring a change in temperature,\u201d Ramshaw said. \u201cYou don\u2019t know what\u2019s going on at the microscopic level. Heat is going one way, not another way, but you don\u2019t know why or how. So we wanted to design an experiment that could tell you how.\u201d<\/p>\n
Ramshaw and the paper\u2019s lead author, doctoral student Avi Shragai, designed a way to understand why that flow of heat bends: applying a magnetic field and following the movement of phonons, a type of lattice vibration that carries heat as it travels through the material as a soundwave \u2013 essentially the sonic equivalent of photons.<\/p>\n
Using ultrasonic measurements that tracked how the phonons moved in a magnetic field, the researchers found the phonons had twisted paths, like a corkscrew. This so-called acoustic Faraday effect demonstrated that the sample had Hall viscosity \u2013 also called gravitational Hall viscosity \u2013 which rotates phonon polarizations and also deflects their heat currents.\u00a0<\/p>\n
\u201cThe gravity analogy is not that far off,\u201d Ramshaw said. \u201cYou have probably seen those images of space and time being curved by gravity from a massive star. Hall viscosity adds a `twist\u2019 to that curvature. This doesn\u2019t seem to happen out in the universe, but it can emerge inside a quantum material like ruthenium chloride.\u201d\u00a0<\/p>\n
This Hall viscosity was what produced the thermal Hall effect in ruthenium trichloride.<\/p>\n
\u201cWhen we send sound pointing in one direction into the lattice, it moves like a helix and the soundwaves actually rotate their polarization,\u201d Ramshaw said. \u201cSoundwaves don\u2019t naively couple to magnetic fields, but it turns out there\u2019s a very special property of this material, called spin orbit coupling, that lets the soundwaves know left from right. That\u2019s basically what we showed.\u201d<\/p>\n
Researchers previously theorized Hall viscosity could be used to measure new and elusive states of matter, according to Ramshaw, but this is the first time it\u2019s been demonstrated.<\/p>\n
\u201cThis technique can now be used to make new discoveries,\u201d he said. \u201cI mean, essentially what we have here is a very elaborate null result on someone else\u2019s bold claim. Going forward, we can use this technique to make bold claims of our own.\u201d<\/p>\n
Co-authors include Ezekiel Horsley, Subin Kim and Young-June Kim of University of Toronto, who conducted sample growth and characterization.\u00a0<\/p>\n
The research was supported by the U.S. Air Force Office of Scientific Research, the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation and the Ontario Research Fund.<\/p>\n","protected":false},"excerpt":{"rendered":"It was a head-spinning discovery.\u00a0 In 2018, researchers in Japan claimed to find concrete evidence of an elusive…\n","protected":false},"author":2,"featured_media":602397,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[49],"tags":[199,79],"class_list":{"0":"post-602396","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-physics","9":"tag-science"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/602396","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/comments?post=602396"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/posts\/602396\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media\/602397"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/media?parent=602396"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/categories?post=602396"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us\/wp-json\/wp\/v2\/tags?post=602396"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}