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Home , Solid, State of matter, Superconductivity, Superfluidity, Supersolid

Feature: physicsworld.com The return of supersolids Following a false alarm in 2004, two groups report what could be the first observation of supersolids, a theoretically predicted state of that is both a superfluid and a at the same time. Stephen Ornes reports

Stephen Ornes is a We learn it from a young age: hold their shapes; may now join the ranks of exotic, fundamental phases, science writer in flow. Physical states of matter are mutually like and , but also Nashville, exclusive. A solid occupies a particular position in because the has travelled a long and at times Tennessee, US, space, its fixed. A assumes the shape rocky path from prediction to experimental evidence. e-mail stephen@ of its container, its molecules in constant . “There are no scoops in science, only a slow con- stephenornes.com But a so-called , a predicted of mat- struction of truth,” says physicist Sébastien Balibar ter that forms only under extreme circumstances, at the École Normale Supérieure in Paris, who has doesn’t follow this idea of order. To describe super- conducted research on solids and was not solids is an exercise in contradictions. On the one involved in the new studies. “Discoveries are very hand, they form rigid crystalline structures. On the rarely made in one shot.” other, theory predicts that part of their mass also acts like a superfluid – a quantum phase of matter A false start that flows like a , but without . That The latest reports weren’t the first from physicists combination lets supersolids do things that seem who suspected they’d formed supersolids. In a study unfathomable to the humdrum, room-, published in 2004, Pennsylvania State University Newtonian world, like flow through themselves – physicist Moses Chan, together with his graduate without . student Eun-Seong Kim, reported extraordinary Although the Russian physicists Alexander Andreev results from experiments using -4, the most and Ilya Liftshitz first predicted in 1969 that supersol- abundant of helium on Earth ( 427 ids could form in helium close to , defi- 225). At , helium-4 can be encour- nite proof has been hard to come by, and this elusive aged to form either a solid (at high ) or a phase of matter has largely remained entrenched in superfluid (at standard pressure). Experiments in the the world of theory. That may have changed, though: 1930s showed that helium undergoes a phase transi- two independent groups of researchers – one at the tion to become a superfluid at 2.2 K, below which it Massachusetts Institute of (MIT) in the exhibits spectacularly bizarre behaviour, like flowing US, and the other at ETH Zurich in Switzerland – up the walls of its container and out down the sides. recently reported forming supersolids. Chan and Kim started with solid helium-4. They Both of the new papers were posted on the arXiv put the material in a torsional oscillator – a device preprint server in October (arXiv:1610.08194; that rotates in alternating directions – and lowered arXiv:1609.09053), though they have not yet been the temperature. At a sliver of a degree above abso- published in peer-reviewed journals. Experts in the lute zero, the rotation of the device increased in fre- field say that so far, the evidence for supersolids looks quency, which suggested that the amount of mass convincing, with the usual caveats: namely, that more that was rotating had decreased. That change was and replication are needed. Both teams report consistent with the 1969 predictions by Andreev and coaxing supersolids into existence by manipulating a Liftshitz, who hypothesized that some of the heli- Bose–Einstein condensate (BEC), a bizarre state of um’s mass would form a superfluid that could flow matter that forms when are chilled to within through the rest of the solid without friction. a fraction of a degree above absolute zero. Other groups reproduced the experiment and found The near-simultaneous reporting of two cases the same results, exciting the condensed-matter phys- of supersolids, found using different experimental ics community. Still, doubt lingered, and for years, the approaches, is exciting not only because supersolids results from Chan and Kim remained controversial. One team that set out to reproduce the experi- ment comprised John Reppy, a physicist at Cornell Supersolids may now join the ranks University in the US, and graduate student Sophie Rittner. In a paper published in 2006, they reported of exotic, fundamental phases, like that the frequency uptick was tied to defects in the solid helium. When they warmed the helium and superconductivity and superfluidity let it cool slowly – a process called annealing that smooths out defects – the signature of supersolid- ity vanished. Then, in a paper published in Nature in 2007, physicist John Beamish at the University of Alberta, Canada, and his collaborators challenged

24 World February 2017 physicsworld.com Feature: Supersolids MIT

Chilly chamber The new set-up at ’s lab at MIT.

Physics World February 2017 25 Feature: Supersolids physicsworld.com 150403 by permission of the APS 107

Phys. Rev. Lett. Reusedfrom National Institute of Standards and Technology/Science Photo Library

State of the art A Bose–Einstein condensate (simulation, left) is a starting point for supersolids (simulation, right).

Chan and Kim’s findings by suggesting that solid on supersolids. helium wasn’t perfectly stiff but instead had some In the two decades since a BEC was first observed, give, a “giant ”. This effect could allow some physicists have become adept at finding ways to con- to slide past each other, mimicking the prop- trol every term in the Hamiltonian – the mathemati- erties of supersolidity. In later experiments, Beam- cal description of the state of the material. It ish’s group worked with Balibar and his colleagues in is through tweaking the values of these terms that Paris to better understand this effect, and bolstered they’ve been able to probe new fundamental phases the case for the new explanation. of matter, like supersolids. Chan, ultimately, brought this chapter to its close. Physicists often characterize transitions between Reppy had been Chan’s adviser in graduate school, phases of matter by what kind of symmetry is bro- and Chan set out to redesign his own experiment to ken. Liquid , for example, at the molecular test alternative ideas about the supersolid state. In level, looks the same under any transformation. The a paper published in 2012 and based on a new set- arrangement of molecules at one place in the liquid up, he reported finding no increase in rotational fre- looks like the arrangement of molecules at another. quency – and thus no evidence for supersolids (Phys. But is a , which means its structure looks Rev. Lett. 109 155301). the same only when observed at periodic intervals. “This is a remarkable piece of science history,” says So the translational symmetry of the liquid is broken physicist Tilman Pfau, who studies interac- as it becomes a crystal. tions in BECs at the University of Stuttgart, in Ger- Both forming a crystal and forming a superfluid many. “The same author that claims something, gets are associated with breaking symmetry; thus, to form criticized, goes back to the lab, sees he was wrong a supersolid requires two kinds of symmetry to break and writes a paper about it.” simultaneously. First, a superfluid must be formed. An advantage of working with BECs is that it is well New experiments known how to make BECs behave like superfluids, While some researchers continue to pursue the for- making them a natural place to start; another is mation of supersolids in helium, many other labs that physicists know how to vary interactions have turned to BECs. first predicted in the material. Second, while this superfluidity is the existence of this in 1924, based on maintained, the superfluid must become regularly theoretical work by Indian physicist Satyendra Bose, ordered into regions of high and low , like but it took decades to develop the machinery needed atoms in a crystal. Physicists have posited a variety to test the prediction. The first BEC was created in of ways to stimulate atom interactions that to a lab in Colorado, US, in 1995, when physicists used a solid state while maintaining superfluidity, i.e. the and magnetic fields to trap a clutch of rubid- long-sought supersolid state. ium atoms as the temperature was reduced as much “Supersolidity is a paradoxical competition as possible. Just above absolute zero, the individual between two different and contradictory types atoms all began behaving like one giant superatom of order,” says Balibar. One of those is the order Supersolidity is – a single quantum entity at its lowest energy state. demanded by solidity, where individual atoms line a paradoxical Research into the discovery and properties of up on a lattice; the other is superfluidity, where the competition BECs netted Nobel prizes for physicists Eric Cor- atoms effectively combine, accumulating to the same between two nell, at the US National Institute of Standards and quantum state. “Atoms in a supersolid should be Technology, and , then at the Univer- localized and delocalized at the same time, distin- different and sity of Colorado Boulder and now at Stanford Uni- guishable and indistinguishable.” contradictory versity, as well as Wolfgang Ketterle at MIT, whose There may be more than one way to coax a solid types of order lab is one of the two that has produced new findings from a BEC superfluid. One group that reported its

26 Physics World February 2017 physicsworld.com Feature: Supersolids findings in October, led by at ETH Zurich, trapped the BEC at the intersection of cross- ing lasers, with each forming an optical cavity. The interaction of the and atoms in the BEC gave rise to self-organization – the hallmark of solid- ity – even though the material continued to look like a superfluid. Pfau says the new work “goes clearly beyond” what groups have done before; Balibar, in Paris, says that the results look “convincing” and “the fundamental effect is clearly there”. At the same time, Balibar ETH Zurich Institute for Quantum Electronics cautions that although Esslinger’s group claims evi- dence for spontaneous symmetry breaking, he’d like to see better confirmation. “That’s not totally obvi- ous to me since the period of the supersolid is fixed by the laser .” The other group, from Ketterle’s lab at MIT, also used lasers, but with a kind of BEC that takes advan- tage of the connections between the of an atom – an intrinsic quantum property that’s analogous to rotation – and its motion. (Spin–orbit coupling is for decades, they’ve figured out a lot about how to Intersection a physical interaction that underlies many unusual tame them and tune them to probe fundamental A Bose–Einstein physical phenomena, including topological insula- phases of matter. But they’re just getting started, condensate trapped tors and some behaviours in superconductors.) The says Busch. Now they’re looking for ways not only to by lasers is one physicists used a laser to transfer some to identify other exotic phases, but also to explore what method to create a the atoms in the BEC, which led to the formation of happens when these strange are combined, supersolid. interference patterns. From those patterns emerged or how they act under other experimental conditions. tiger-like stripes of alternating density – standing “How do these systems actually behave by them- waves – in the material. In its paper, Ketterle’s group selves? How do they react to external stimulation? reports that this density modulation breaks transla- What happens if we squeeze them?” Busch likens this tional symmetry, the requirement for a solid. era of discovery to what happened in the years after Physicist Thomas Busch, who studies quantum BECs were first discovered, when physicists couldn’t processes in ultracold atomic at the Okinawa wait to get to know the new condensates better. “The Institute of Science and Technology, in Japan, says first thing people did [to BECs] was to squeeze them theorists predicted a few years ago that the super- – the stuff you do when you get a new toy.” solid stripes should emerge. At the same time, he In addition, he says, physicists want to study the notes that experimental verification is exciting news effects of different long-range interactions and bet- to the community. ter understand how affect the properties Neither group explicitly showed that the material of the materials. Impurities could be critical in find- could flow through itself, though the papers do offer ing applications for supersolids. Busch notes that in arguments in favour of superfluidity. Despite past research, impurities added through controversies over what is or isn’t a supersolid, Busch doping can change the conductivity of a material and says that the vast majority of people will not have a make it fit a certain use. problem calling the entities in the two new studies Higher dimensions may also be in store. In the supersolids. “Figuring out the exact ‘super’ proper- preprint from Ketterle’s group, the researchers note ties of the states created is now an exciting task for a couple of possible future directions: more charac- the future,” he says. terization of the system, for example, or extending their method to a 2D spin–orbit coupling system. Beyond supersolids Achieving supersolidity in three dimensions would Finding new states of matter has been a driving be another major milestone, but breaking symme- force in cold-atom research for decades, and super- tries in three dimensions would be difficult to realize solids are the latest bizarre material to join a grow- in experiments. ing list that already includes things like superfluids Exotic states of matter, like supersolids, show and superconductors. For the last two years, Pfau’s that under extreme conditions our physical reality group, in Stuttgart, has been exploring quantum behaves in bizarre ways that aren’t easy to explain. ferro­ – magnetic droplets that can self-organ- “The physics of cold atoms is some kind of simula- ize out of BECs at low temperature. “Nobody would tion of fundamental problems that are well defined, have thought before [we observed the material in the but hard to calculate,” says Balibar. Theory may lab] that this was a stable state of matter,” he says. predict a of undiscovered properties that Last year, in a paper published in Nature, the group emerge in idealized matter, but controlling such reported that quantum can also break strange stuff under extreme conditions is difficult. translational symmetry, which means they might be “Real matter has defects and surface states,” he a good place to search for other supersolids. says, “so our understanding of real matter is far from Because scientists have been working with BECs being complete.”  n

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