Roton

{{Short description|Collective excitation in superfluid helium-4 (a quasiparticle)}} {{Other uses}} thumb|Roton dispersion relation, showing the quasiparticle energy E(p) as a function of momentum p. A quasiparticle with momentum generated in the local energy minimum is called a roton. In theoretical physics, a '''roton''' is an elementary excitation, or quasiparticle, seen in superfluid helium-4 and Bose–Einstein condensates with long-range dipolar interactions or spin-orbit coupling. The dispersion relation of elementary excitations in this superfluid shows a linear increase from the origin, but exhibits first a maximum and then a minimum in energy as the momentum increases. Excitations with momenta in the linear region are called phonons; those with momenta close to the minimum are called rotons. Excitations with momenta near the maximum are called maxons.

The term "roton-like" is also used for the predicted eigenmodes in 3D metamaterials using beyond-nearest-neighbor coupling.<ref>{{cite journal |last1=Wang |first1=Ke |last2=Chen |first2=Yi |last3=Kadic |first3=Muamer |last4=Wang |first4=Changguo |last5=Wegener |first5=Martin |date=24 May 2022 |title=Nonlocal interaction engineering of 2D roton-like dispersion relations in acoustic and mechanical metamaterials |journal=Communications Materials |volume=3 |issue=1 |page=35 |bibcode=2022CoMat...3...35W |doi=10.1038/s43246-022-00257-z |doi-access=free |s2cid=248991736}}</ref><ref>{{cite journal |last1=Chen |first1=Yi |last2=Kadic |first2=Muamer |last3=Wegener |first3=Martin |date=2 June 2021 |title=Roton-like acoustical dispersion relations in 3D metamaterials |journal=Nature Communications |volume=12 |issue=1 |page=3278 |bibcode=2021NatCo..12.3278C |doi=10.1038/s41467-021-23574-2 |pmc=8172548 |pmid=34078904}}</ref> A "roton-like" dispersion relation was demonstrated under ambient conditions for both acoustic pressure waves in a channel-based metamaterial at audible frequencies and transverse elastic waves in a microscale metamaterial at ultrasound frequencies.<ref>{{Cite journal |last1=Iglesias Martínez |first1=Julio Andrés |last2=Groß |first2=Michael Fidelis |last3=Chen |first3=Yi |last4=Frenzel |first4=Tobias |last5=Laude |first5=Vincent |last6=Kadic |first6=Muamer |last7=Wegener |first7=Martin |date=2021-12-03 |title=Experimental observation of roton-like dispersion relations in metamaterials |journal=Science Advances |language=en |volume=7 |issue=49 |article-number=eabm2189 |doi=10.1126/sciadv.abm2189 |issn=2375-2548 |pmc=8635434 |pmid=34851658|bibcode=2021SciA....7.2189I }}</ref>

==Models== Originally, the roton spectrum was phenomenologically introduced by Lev Landau in 1947.<ref>{{cite journal |last1=Landau |first1=L. |title=Theory of the Superfluidity of Helium II |journal=Physical Review |date=15 August 1941 |volume=60 |issue=4 |pages=356–358 |doi=10.1103/PhysRev.60.356 |bibcode=1941PhRv...60..356L }}</ref> Currently there exist helium-4 based models which try to explain the roton spectrum with varying degrees of success and fundamentality.

The requirement for any model of this kind is that it must explain not only the shape of the spectrum itself but also other related observables, such as the speed of sound and structure factor of superfluid helium-4. Microwave and Bragg spectroscopy has been conducted on helium to study the roton spectrum.<ref>{{Cite journal|title = Microwave Spectroscopy of Condensed Helium at the Roton Frequency|date = 4 Nov 2009|journal = Journal of Low Temperature Physics|doi = 10.1007/s10909-009-0025-6|bibcode = 2010JLTP..158..244R |last1 = Rybalko|first1 = A.|last2 = Rubets|first2 = S.|last3 = Rudavskii|first3 = E.|last4 = Tikhiy|first4 = V.|last5 = Poluectov|first5 = Y.|last6 = Golovashchenko|first6 = R.|last7 = Derkach|first7 = V.|last8 = Tarapov|first8 = S.|last9 = Usatenko|first9 = O.|volume = 158|issue = 1–2|pages = 244–249|s2cid = 120191282}}</ref>

==Bose–Einstein condensation== Bose–Einstein condensation of rotons has been also proposed and studied.<ref>{{Cite journal|title = The role of the condensate in the existence of phonons and rotons|date = December 1993|journal = Journal of Low Temperature Physics|doi = 10.1007/BF00692035|bibcode = 1993JLTP...93..861G |last1 = Glyde|first1 = Henry R.|volume = 93|issue = 5–6|pages = 861–878|s2cid = 122151606}}</ref> In Bose-Einstein condensates, of magnetic atoms rotons are expected to occur<ref>{{cite journal |title=Roton-Maxon Spectrum and Stability of Trapped Dipolar Bose-Einstein Condensates |last1 = Santos|first1 =L.|journal=Physical Review Letters |volume=90 |pages=250403 |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.90.250403 |access-date=15 December 2025}}</ref> <ref>{{Cite journal|title = Fingerprinting Rotons in a Dipolar Condensate: Super-Poissonian Peak in the Atom-Number Fluctuations|date = 26 June 2013|journal = Phys. Rev. Lett. |volume=110 |article-number=265302|doi = 10.1103/PhysRevLett.110.265302|arxiv = 1304.3605 |bibcode = 2013PhRvL.110z5302B |last1 = Bisset|first1 = R. N.|last2 = Blakie|first2 = P. B.|issue = 26|pmid = 23848891|s2cid = 24788775}}</ref><ref>{{Cite journal|title = Roton spectroscopy in a harmonically trapped dipolar Bose–Einstein condensate|date = Aug 15, 2012|journal = Phys. Rev. A |volume=86 |article-number=021604 |doi = 10.1103/PhysRevA.86.021604|arxiv = 1206.2770 |bibcode = 2012PhRvA..86b1604B |last1 = Blakie|first1 = P. B.|last2 = Baillie|first2 = D.|last3 = Bisset|first3 = R. N.|issue = 2|s2cid = 119285430}}</ref> caused by the magnetic dipole-dipole interactions. Rotons were first detected experimentally in 2018 with a Bose-Einstein condensate of Erbium atoms.<ref>{{cite journal|last1=Chomaz|first1=L.|title=Observation of roton mode population in a dipolar quantum gas|journal=Nature Physics|date=2018|volume=14|issue=5|pages=442–446|doi=10.1038/s41567-018-0054-7|pmid=29861780|pmc=5972007|arxiv=1705.06914|bibcode=2018NatPh..14..442C}}</ref>

Under specific conditions the roton minimum gives rise to a crystal solid-like structure called the supersolid, detected experimentally in 2019.<ref>{{cite journal |last1=Donner |first1=Tobias |title=Dipolar Quantum Gases go Supersolid |journal=Physics |date=3 April 2019 |volume=12 |article-number=38 |doi=10.1103/Physics.12.38 |bibcode=2019PhyOJ..12...38D |doi-access=free }}</ref><ref>{{cite web | url=https://phys.org/news/2019-04-teams-independently-dipolar-quantum-gasses.html | title=Three teams independently show dipolar quantum gasses support state of supersolid properties }}</ref><ref>{{cite journal |last1=Henkel |first1=N. |last2=Nath |first2=R. |last3=Pohl |first3=T. |title=Three-Dimensional Roton Excitations and Supersolid Formation in Rydberg-Excited Bose-Einstein Condensates |journal=Physical Review Letters |date=11 May 2010 |volume=104 |issue=19 |article-number=195302 |doi=10.1103/PhysRevLett.104.195302 |pmid=20866972 |arxiv=1001.3250 |bibcode=2010PhRvL.104s5302H |s2cid=14445701 }}</ref>

==References== {{Reflist}}

Category:Quasiparticles Category:Bose–Einstein condensates Category:Superfluidity Category:Lev Landau