Observation of Phase Separation in a Strongly-Interacting Im

We have observed phase separation between the superfluid and the normal component in a strongly interacting Fermi gas with imbalanced spin populations. The in situ distribution of the density difference between two trapped spin components is obtained using

ObservationofPhaseSeparationinaStrongly-InteractingImbalancedFermiGas

Y.Shin, M.W.Zwierlein,C.H.Schunck,A.Schirotzek,andW.Ketterle

DepartmentofPhysics,MIT-HarvardCenterforUltracoldAtoms,andResearchLaboratoryofElectronics,

MassachusettsInstituteofTechnology,Cambridge,Massachusetts,02139

(Dated:February3,2008)

Wehaveobservedphaseseparationbetweenthesuper uidandthenormalcomponentinastronglyinteractingFermigaswithimbalancedspinpopulations.Theinsitudistributionofthedensitydi erencebetweentwotrappedspincomponentsisobtainedusingphase-contrastimagingand3Dimagereconstruction.Ashellstructureisclearlyidenti edwherethesuper uidregionofequaldensitiesissurroundedbyanormalgasofunequaldensities.Thephasetransitioninducesadramaticchangeinthedensitypro lesasexcessfermionsstartbeingexpelledfromthesuper uid.

Jun 2006

81PACSnumbers:03.75.Ss,03.75.Hh,05.70.Fh

]chCooperpairingistheunderlyingmechanismfortheeBardeen-Cooper-Schrie er(BCS)super uidstateofanmequalmixtureoftwofermionicgases.Aninterestingsit--uationariseswhenthetwocomponentshaveunequaltapopulations.Doestheimbalancequenchsuper uidity,tsdoesitleadtophaseseparationbetweenabalancedand.timbalancedregion,ordoesitgiverisetonewformsofasuper uidity?Asearchforexoticsuper uidstatesismpromisinginimbalancedmixtures,sincetheimbalance-ddestabilizesBCS-types-wavepairingwhichisusuallythenstrongestpairingmechanismRecently,thisoproblemhasbeenexperimentallyaddressedinultracoldcatomicFermicloudswithcontrolledpopulationimbal-[ ancesSuper uiditywasobservedinastrongly 2interactingregimewithabroadrangeofimbalancesandvthePauliorClogstonlimitofsuper uiditywaschar-2acterized7].

34Thephaseseparationscenariosuggeststhatunpaired6fermionsarespatiallyseparatedfromaBCS-super uidof0equaldensitiesduetothepairinggapinthesuper uid6region[9,10,11].Inourpreviousexperiments[5,7],0/weobservedastrongcentraldepletioninthedi erencetapro lesofexpandingcloudsindicatingthatexcessatomsmareexpelledfromthesuper uidregion.Ref.reports-depletionofexcessatomsatthetrapcenter.Ithasbeendclari edthatnoneoftheseexperimentsanswerednthequestion:whetherthedensitiesofthetwospincom-coponentsareequalinthesuper uidregionandwhether:phaseseparationorratherdistortionsofthecloudduetoviinteractionshaveoccurred.

XHerewereportthedirectobservationofphasesepa-rarationbetweenthesuper uidandthenormalregioninastronglyinteractingFermigaswithimbalancedspinpopulations.Thedensitydi erencebetweenthetwospincomponentsisdirectlymeasuredinsituusingaspecialphase-contrastimagingtechniqueand3Dimagerecon-struction.Weclearlyidentifyashellstructureinanim-balancedFermigaswherethesuper uidregionofequaldensitiesissurroundedbyanormalgasofunequalden-sities.ThisphaseseparationisobservedthroughoutthestronglyinteractingregimenearaFeshbachresonance.

FIG.1:Phase-contrastimagingofthedensitydi erenceoftwospinstates.(a)Theprobebeamistunedtotheredforthe|1 →|e transitionandtotheblueforthe|2 →|e transition.Theresultingopticalsignalinthephase-contrastimageisproportionaltothedensitydi erencend≡n1 n2,wheren1andn2arethedensitiesofthestates|1 and|2 ,respectively.(b)Phase-contrastimagesoftrappedatomiccloudsinstate|1 (left)andstate|2 (right),andofanequalmixtureofthetwostates(middle).

Furthermore,wecharacterizethenormal-to-super uidphasetransitionofanimbalancedFermimixtureusinginsituphase-contrastimaging.Theonsetofsuper uid-ityinducesadramaticchangeinthedensitypro lesasexcessfermionsareexpelledfromthesuper uid.

AdegenerateFermigasofspin-polarized6Liatomswaspreparedinanopticaltrapafterlasercoolingandsympatheticcoolingwithsodiumatoms[14,15].Thepopulationimbalanceδofthetwolowesthyper nestates|1 and|2 wasadjustedwitharadio-frequencysweep[5].Here,δ≡(N1 N2)/(N1+N2),whereN1andN2aretheatomnumbersin|1 and|2 ,respectively.InteractionsbetweenthesetwostateswerestronglyenhancednearabroadFeshbachresonanceatB0=834G.The nalevap-orativecoolingwasperformedatB=780Gbyloweringthetrapdepth.Subsequently,theinteractionstrengthwasadiabaticallychangedtoatargetvaluebyadjustingthevalueofthemagnetic-bias eldBwitharampspeedof≤0.4G/ms.Fortypicalconditions,thetotalatomnumberwasNt=N1+N2≈1×107andtheradial(axial)trapfrequencywasfr=130Hz(fz=23Hz).ThecondensatefractionintheimbalancedFermi

We have observed phase separation between the superfluid and the normal component in a strongly interacting Fermi gas with imbalanced spin populations. The in situ distribution of the density difference between two trapped spin components is obtained using

FIG.2:InsitudirectimagingoftrappedFermigaseswithpopulationimbalance.Theintegrated2Ddistributionsofthedensitydi erencen d(x,z)≡ nd( r)dyweremeasuredusingphase-contrastimaging.ThedatawereobtainedatB=834GfortotalatomnumberNt≈1×107andvariouspopulationimbalanceδ.Forδ≤75%,adistinctivecorewasobservedshowingtheshellstructureofthecloud.The eldofviewforeachimageis160µm×800µm.Thethreeleft-mostimagesaredisplayedwithdi erentcontrastlevelsforclarity.Theimagewithδ=5(3)%wastakenforNt≈1.7×107.

mixturewasdeterminedviatherapidtransfertech-nique[16,17].Immediatelyafterturningo thetrap,themagnetic eldwasquicklyrampedtoB=690G(1/kFa≈2.6,wherekFisde nedastheFermimomen-tumofanon-interactingequalmixturewiththesametotalatomnumberandaisthescatteringlength)inap-proximately130µs.Thedensitypro leoftheexpandingminoritycloudwas tbyaGaussianfornormalcom-ponents(thermalmoleculesandunpairedatoms)andaThomas-Fermi(TF)pro leforthecondensate[18].

Thedensitydi erencebetweenthetwocomponentswasdirectlymeasuredusingaphase-contrastimagingtechniquedescribedinFig.1.Inthisimagingscheme,thesignsofthephaseshiftsduetothepresenceofatomsineachstateareoppositesothattheresultingp …… 此处隐藏：6102字，全部文档内容请下载后查看。喜欢就下载吧 ……