Contents - Max-Planck-Institut für Physik komplexer Systeme
Contents - Max-Planck-Institut für Physik komplexer Systeme
Contents - Max-Planck-Institut für Physik komplexer Systeme
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1.10 AdvancedStudyGroups<br />
AdvancedStudyGroup2010:UnconventionalMagnetisminHighFields<br />
(Convenor:Dr.MikeZhitomirsky)<br />
TheAdvancedStudyGroup(ASG)“UnconventionalMagnetisminHighFields”wasinstalledforafivemonthperiodfromApriltoSeptember,2010withtheaimtoinvestigatenovelfield-inducedphenomena<br />
ingeometricallyfrustratedandquantummagnets. TheASGactivitywascoordinatedwiththeDivision<br />
ofCondensedMatterandtheresearchgroup“NewStatesofQuantumMatter.”Asecondgoalwasto<br />
promotescientificcollaborationwithexperimentalgroupsfromtheDresdenHighMagneticFieldLaboratory(HMD),the<strong>Max</strong>Plank<strong>Institut</strong>eforChemicalPhysicsofSolids(MPI-CPfS),andthe<strong>Institut</strong>e<br />
forSolidStateandMaterialResearch(IFW).ThegroupmemberswereAndreyChubukov(University<br />
Wisconsin-Madison),PeterHoldsworth(ENS,Lyon),FredericMila(EPFL,Lausanne),andMikeZhitomirsky(C.E.A.,Grenoble).<br />
InadditiontheASGhosted30long-andshort-termvisitors. Aspecial<br />
seminarserieswasorganizedatthe mpipkswithmorethan25presentationsgivenbyourmembersand<br />
visitors,aswellasbyinvitedspeakersfromtheneighboringinstitutes.A.Chubukovwasalsooneofthe<br />
scientificcoordinatorsoftheworkshopon“EmergentQuantumStatesinComplexCorrelatedMatter”<br />
thattookplaceatthe mpipksonAugust23–27,2010.Principaldirectionsofresearchwereasfollows:<br />
Quantumspin-nematicstate. Enhancedquantumfluctuationsmaydestroyconventionallong-range<br />
magneticorderandstabilizeaspinanalogofliquidcrystals—astatewithpartiallybrokenrotational<br />
symmetrycharacterizedbytensororderparameter.Suchanewillusivestateofmatterhasattracteda<br />
lotoftheoreticalinterestinthepast,buthasneverbeendiscoveredsofarinrealmaterials.Motivated<br />
byanumberofrecentlystudiedfrustratedmagneticmaterialsweexploreanewroottothespin-nematic<br />
phasebasedontheformationofboundmagnonpairsinquantumantiferromagnetswithcompeting<br />
antiferro-andferromagneticinteractions[1],seealsotheresearchpart2.Theboundmagnonstatesare<br />
formedinthefullypolarizedphaseinhighmagneticfieldsandundergoaBosecondensationtransition<br />
withdecreasingfieldpriortotheconventionalsingle-magnoncondensation.Theresultingcondensateof<br />
magnonpairshasdirectsimilaritieswiththeCooperpaircondensateinsuperconductors.Inaddition,the<br />
paircondensateremainsinvariantunder π-rotationaboutthefielddirectionreflectingthequadrupolar<br />
natureofitsorderparameter.Thedevelopedtheorypredictspresenceofthespin-nematicphaseinthe<br />
frustratedchainmaterial LiCuVO4. Veryrecenthigh-fieldexperimentshaveindeedfoundanewphase<br />
inthiscompoundabove40T.Wehavefurtherexaminedthestabilityofboundmagnonpairsinavariety<br />
offrustratedmodelsincludingplanarmagneticsystems,whicharenowextensivelystudiedatMPI-CPfS.<br />
Monopoledynamicsinspinice. Thefractionalizationofmagneticdipolesintofreeanddeconfined<br />
magneticcharge(monopoles)isadirectconsequenceoftheextensivegroundstatedegeneracy,which<br />
constitutesavacuumforthequasi-particleexcitations.Wehavemodeledtheacsusceptibilitymeasurementsonthespinicematerial<br />
Dy2Ti2O7bythediffusivedynamicsofthisCoulombgasofquasiparticle<br />
interactions[2]. Inparallelwiththesedevelopmentsnovelexperimentshavebeendevisedtomeasure<br />
themagneticchargeofthequasi-particlesusingtheWieneffect.Weprovidedtheoreticalsupportfora<br />
Wieneffectexperiment[3]inwhichthemagneticresponseofacrystalof Dy2Ti2O7toasmallmagnetic<br />
field(1to10Gaussonly)wasmeasuredinaSQUIDmagnetometerspeciallydesignedtodealwithvery<br />
lowfields.Theresultswereinterpretedinanalogywiththedielectricresponseofaleakycapacitor,with<br />
magneticrelaxationduetobothboundandfreemagneticcharges.Throughthisquantitativeanalysiswe<br />
wereabletoconfirmthepredictedvalueofthemagneticchargeassociatedwithaMonopoleexcitation<br />
andillustratethevalidityoftheWieneffectforthismagneticCoulombgassystem.Finally,wecontinued<br />
projectsdedicatedtothetopologicalpropertiesofthevacuumofspinicestate. Wehavepreviously<br />
shownthat,applyingabonddistortion,reducingthesymmetryofthemanifoldofvacuumstatesto Z2<br />
leadstoamulticriticalmeanfieldphasetransition.Webeganinvestigatingthefinitesizescalingbehavior<br />
ofthistransition,withparticularinterestintheidentificationofGaussianfluctuationterms[4]. This<br />
systemmapsontothesix-vertexmodel,whichintwodimensionsisintegrable. Thetransitionhasnot<br />
previouslybeeninterpretedintermsofmeanfieldandGaussianfluctuations.<br />
Unconventionalphasesofcoldatomicgasesinopticallattices. Latticespinsystemsinamagnetic<br />
fieldcanbedirectlymappedontothehard-corebosonmodelsduetotheequivalencebetween SO(2)<br />
and U(1)symmetrygroups.Generalizationoftheeffectivespininteractionfromtheconventional SU(2)<br />
tothe SU(3)symmetricpotentialwasundertakenin[5]. Itfurtherallowstheuseofspinlanguageto<br />
1.10. AdvancedStudyGroups 35
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