View source for Physics:Supersolid

{{Short description|State of matter}}
{{Condensed matter physics|expanded=States of matter}}

In [[Physics:Condensed matter physics|condensed matter physics]], a '''supersolid''' is a [[Physics:Solid|spatially ordered]] material with superfluid properties. In the case of [[Physics:Helium-4|helium-4]], it has been conjectured since the 1960s that it might be possible to create a supersolid.<ref>{{Cite journal|last=Chester|first=G. V.|date=1970|title=Speculations on Bose–Einstein Condensation and Quantum Crystals|journal=Physical Review A|volume=2|issue=1|pages=256–258|bibcode=1970PhRvA...2..256C|doi=10.1103/PhysRevA.2.256}}</ref> Starting from 2017, a definitive proof for the existence of this state was provided by several experiments using atomic [[Physics:Bose–Einstein condensate|Bose–Einstein condensate]]s.<ref>{{Cite journal|last=Donner|first=Tobias|date=2019-04-03|title=Viewpoint: Dipolar Quantum Gases go Supersolid|journal=Physics|volume=12|page=38 |doi=10.1103/Physics.12.38|doi-access=free}}</ref> The general conditions required for supersolidity to emerge in a certain substance are a topic of ongoing research.

== Background ==
A supersolid is a special [[Physics:Quantum|quantum]] state of matter where particles form a rigid, spatially ordered structure, but also flow with zero [[Physics:Viscosity|viscosity]]. This is in contradiction to the intuition that flow, and in particular [[Physics:Superfluidity|superfluid]] flow with zero viscosity, is a property exclusive to the [[Physics:Fluid|fluid]] state, e.g., [[Physics:Superconductivity|superconducting]] electron and neutron fluids, gases with [[Physics:Bose–Einstein condensate|Bose–Einstein condensate]]s, or unconventional liquids such as helium-4 or [[Physics:Helium-3|helium-3]] at sufficiently low temperature. For more than 50 years it was thus unclear whether the supersolid state can exist.<ref>{{Cite journal|last=Balibar|first=Sebastien|date=March 2010|title=The enigma of supersolidity|journal=Nature|volume=464|issue=7286|pages=176–182|doi=10.1038/nature08913|pmid=20220834|bibcode=2010Natur.464..176B|s2cid=4303097}}</ref>

== Experiments using helium ==
While several experiments yielded negative results, in the 1980s, John Goodkind discovered the first anomaly in a solid by using [[Physics:Ultrasound|ultrasound]].<ref name="physworld">{{cite news|url=http://physicsworld.com/cws/article/print/27733|title=The quantum solid that defies expectation|last=Chalmers|first=Matthew|date=2007-05-01|access-date=2009-02-25|publisher=Physics World}}</ref> Inspired by his observation, in 2004 Eun-Seong Kim and [[Biography:Moses H. W. Chan|Moses Chan]] at [[Organization:Pennsylvania State University|Pennsylvania State University]] saw phenomena which were interpreted as supersolid behavior.<ref>{{Cite journal |last1= Kim |first1=E. |last2=Chan |first2=M. H. W.  |date=2004 |title=Probable Observation of a Supersolid Helium Phase |journal=Nature |volume=427 |issue=6971 |pages=225–227 |bibcode=2004Natur.427..225K |doi=10.1038/nature02220 |pmid=14724632|s2cid=3112651 }}</ref> Specifically, they observed a non-classical rotational [[Physics:Moment of inertia|moment of inertia]]<ref>{{Cite journal|last=Leggett|first=A. J.|date=1970-11-30|title=Can a Solid Be "Superfluid"?|journal=Physical Review Letters|volume=25|issue=22|pages=1543–1546|doi=10.1103/PhysRevLett.25.1543|bibcode=1970PhRvL..25.1543L|s2cid=122591300}}</ref> of a torsional oscillator. This observation could not be explained by classical models but was consistent with superfluid-like behavior of a small percentage of the helium atoms contained within the oscillator.

This observation triggered a large number of follow-up studies to reveal the role played by crystal defects or helium-3 impurities. Further experimentation has cast some doubt on the existence of a true supersolid in helium. Most importantly, it was shown that the observed phenomena could be largely explained due to changes in the elastic properties of the helium.<ref>{{Cite journal|last1=Day|first1=James|last2=Beamish|first2=John|date=December 2007|title=Low-temperature shear modulus changes in solid 4 He and connection to supersolidity|journal=Nature|volume=450|issue=7171|pages=853–856|doi=10.1038/nature06383|pmid=18064007|arxiv=0709.4666|bibcode=2007Natur.450..853D|s2cid=4411989}}</ref> In 2012, Chan repeated his original experiments with a new apparatus that was designed to eliminate any such contributions. In this experiment, Chan and his coauthors found no evidence of supersolidity.<ref>{{Cite journal|last=Voss|first=David|date=2012-10-08|title=Focus: Supersolid Discoverer's New Experiments Show No Supersolid|journal=Physics|volume=5|pages=111|bibcode=2012PhyOJ...5..111V|doi=10.1103/physics.5.111}}</ref>

== Experiments using ultracold quantum gases ==
In 2017, two research groups from ETH Zurich and from MIT reported on the creation of an ultracold quantum gas with supersolid properties. The Zurich group placed a [[Physics:Bose–Einstein condensate|Bose–Einstein condensate]] inside two optical resonators, which enhanced the atomic interactions until they started to spontaneously crystallize and form a solid that maintains the inherent superfluidity of Bose–Einstein condensates.<ref>{{Cite news |url=https://www.ethz.ch/content/main/en/news-und-veranstaltungen/eth-news/news/2017/03/Kristallin-und-fluessig-zugleich.html |title=Crystalline and liquid at the same time |last=Würsten |first=Felix |date=1 March 2017 |access-date=2018-01-18 |publisher=[[Organization:ETH Zurich|ETH Zurich]] }}</ref><ref>{{Cite journal |last1=Léonard |first1=Julian |last2=Morales |first2=Andrea |last3=Zupancic |first3=Philip |last4=Esslinger |first4=Tilman |last5=Donner |first5=Tobias |date=1 March 2017 |title=Supersolid formation in a quantum gas breaking a continuous translational symmetry |journal=Nature |volume=543 |issue=7643 |pages=87–90 |arxiv=1609.09053 |bibcode=2017Natur.543...87L |doi=10.1038/nature21067 |pmid=28252072 |s2cid=4459567 }}</ref> This setting realises a special form of a supersolid, the so-called lattice supersolid, where atoms are pinned to the sites of an externally imposed lattice structure. The MIT group exposed a Bose–Einstein condensate in a double-well potential to light beams that created an effective [[Physics:Spin–orbit interaction|spin–orbit coupling]]. The interference between the atoms on the two spin–orbit coupled lattice sites gave rise to a characteristic density modulation.<ref>{{Cite news |url=https://news.mit.edu/2017/mit-researchers-create-new-form-matter-0302 |title=MIT researchers create new form of matter |last=Keller |first=Julia C. |date=March 2, 2017 |work=MIT News |access-date=2018-01-18}}</ref><ref>{{Cite journal |last1=Li |first1=Jun-Ru |last2=Lee |first2=Jeongwon |last3=Huang |first3=Wujie |last4=Burchesky |first4=Sean |last5=Shteynas |first5=Boris |last6=Top |first6=Furkan Çağrı |last7=Jamison |first7=Alan O. |last8=Ketterle |first8=Wolfgang |date=1 March 2017 |title=A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates |journal=Nature |volume=543 |issue=7643 |pages=91–94 |arxiv=1610.08194 |bibcode=2017Natur.543...91L |doi=10.1038/nature21431 |pmid=28252062 |s2cid=4463520 }}</ref>

In 2019, three groups from Stuttgart, Florence, and Innsbruck observed supersolid properties in dipolar [[Physics:Bose–Einstein condensate|Bose–Einstein condensate]]s<ref>{{Cite news |url=http://physics.aps.org/articles/v12/38 |title=Viewpoint: Dipolar Quantum Gases go Supersolid |last= Donner |first=Tobias |date=April 3, 2019 |work=APS Physics |access-date=2019-04-19}}</ref> formed from [[Chemistry:Lanthanide|lanthanide]] atoms. In these systems, supersolidity emerges directly from the atomic interactions, without the need for an external optical lattice. This facilitated also the direct observation of superfluid flow and hence the definitive proof for the existence of the supersolid state of matter.<ref>{{Cite journal|last1=Guo|first1=Mingyang|last2=Böttcher|first2=Fabian|last3=Hertkorn|first3=Jens|last4=Schmidt|first4=Jan-Niklas|last5=Wenzel|first5=Matthias|last6=Büchler|first6=Hans Peter|last7=Langen|first7=Tim|last8=Pfau|first8=Tilman|date=October 2019|title=The low-energy Goldstone mode in a trapped dipolar supersolid|journal=Nature|volume=574|issue=7778|pages=386–389|doi=10.1038/s41586-019-1569-5|pmid=31499511|arxiv=1906.04633|bibcode=2019Natur.574..386G|s2cid=184488202}}</ref><ref>{{Cite journal|last1=Tanzi|first1=L.|last2=Roccuzzo|first2=S. M.|last3=Lucioni|first3=E.|last4=Famà|first4=F.|last5=Fioretti|first5=A.|last6=Gabbanini|first6=C.|last7=Modugno|first7=G.|last8=Recati|first8=A.|last9=Stringari|first9=S.|date=October 2019|title=Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas|journal=Nature|volume=574|issue=7778|pages=382–385|doi=10.1038/s41586-019-1568-6|pmid=31499510|arxiv=1906.02791|bibcode=2019Natur.574..382T|s2cid=174801325}}</ref>

In 2021, [[Chemistry:Dysprosium|dysprosium]] was used to create a 2-dimensional supersolid quantum gas.<ref>{{Cite journal|last1=Norcia|first1=Matthew A.|last2=Politi|first2=Claudia|last3=Klaus|first3=Lauritz|last4=Poli|first4=Elena|last5=Sohmen|first5=Maximilian|last6=Mark|first6=Manfred J.|last7=Bisset|first7=Russell N.|last8=Santos|first8=Luis|last9=Ferlaino|first9=Francesca|date=August 2021|title=Two-dimensional supersolidity in a dipolar quantum gas|journal=Nature|volume=596|issue=7872|pages=357–361|doi=10.1038/s41586-021-03725-7|pmid=34408330 |arxiv=2102.05555|bibcode=2021Natur.596..357N |s2cid=231861397 }}</ref> In 2022, the same team created a supersolid disk in a round trap.<ref>{{cite journal |last1=Bland |first1=Thomas |last2=Poli |first2=Elena |last3=Politi |first3=Claudia |last4=Lauritz |first4=Klaus |last5=Norcia |first5=Matthew A. |last6=Ferlaino |first6=Francesca |last7=Santos |first7=Luis |last8=Bisset |first8=Russell N. |title=Two-dimensional supersolidity in a circular trap |journal=Phys. Rev. Lett. |date=13 May 2022 |volume=128 |issue=19 |page=195302 |doi=10.1103/PhysRevLett.128.195302 |pmid=35622047 |arxiv=2107.06680 |s2cid=248781709 }}</ref> 

In 2021, confocal [[Cavity quantum electrodynamics|cavity quantum electrodynamics]] with a Bose–Einstein condensate was used to create a supersolid that possesses a key property of solids, vibration.  That is, a supersolid was created that possesses lattice [[Physics:Phonon|phonon]]s with a Goldstone mode dispersion exhibiting a 16 cm/s speed of sound.<ref>{{Cite journal|last1=Guo|first1=Yudan|last2=Kroeze|first2=Ronen M.|last3=Marsh|first3=Brendan P.|last4=Gopalakrishnan|first4=Sarang|last5=Keeling|first5=Jonathan|last6=Lev|first6=Benjamin L.|date=2021|title=An optical lattice with sound|journal=Nature|volume=599|issue=7884|pages=211–215|arxiv=2104.13922| doi=10.1038/s41586-021-03945-x|pmid=34759361 |bibcode=2021Natur.599..211G |hdl=10023/25329 |s2cid=233423569 }}</ref>

== Theory ==
In most theories of this state, it is supposed that vacancies – empty sites normally occupied by particles in an ideal crystal – lead to supersolidity. These vacancies are caused by [[Physics:Zero-point energy|zero-point energy]], which also causes them to move from site to site as [[Wave|wave]]s. Because vacancies are [[Physics:Boson|boson]]s, if such clouds of vacancies can exist at very low temperatures, then a Bose–Einstein condensation of vacancies could occur at temperatures less than a few tenths of a Kelvin. A coherent flow of vacancies is equivalent to a "superflow" (frictionless flow) of particles in the opposite direction. Despite the presence of the gas of vacancies, the ordered structure of a crystal is maintained, although with less than one particle on each lattice site on average. Alternatively, a supersolid can also emerge from a superfluid. In this situation, which is realised in the experiments with atomic Bose–Einstein condensates, the spatially ordered structure is a modulation on top of the superfluid density distribution.

== See also ==
* [[Physics:Superfluid film|Superfluid film]]
* [[Physics:Superglass|Superglass]]

== References ==
{{reflist|colwidth=30em}}

== External links ==
* [http://www.nature.com/nsu/040112/040112-7.html ''Nature'' story on a supersolid experiment]
* [https://physics.aps.org/articles/v14/159 APS ''Physics Magazine'' on a vibrating supersolid experiment] 
* [https://web.archive.org/web/20051111133522/http://www.rps.psu.edu/probing/supersolid.html Penn State: What is a Supersolid?]
* {{cite web |title=Supersolid behaviour spotted in dipolar quantum gases |url=https://physicsworld.com/a/supersolid-behaviour-spotted-in-dipolar-quantum-gases/ |website=Physics World |date=20 April 2019}}

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{{Sourceattribution|Supersolid}}