Itisnowwellacceptedthatthesuppressionoforderedstates,suchasmagnetism,cangiverisetoanovelstatewithhighlyanomalousmetalliccharacteristics.ItremainsachallengetounderstandtheroleofthequantumcriticalfluctuationsassociatedwiththeT=0phasetransitionininducingthisnewstate,andifthereisfeedbackbetweenthefluctuationsandtheessentialpropertiesofthequasiparticlesinthenon-Fermiliquidelectronicstate.AlackofdetailedexperimentalresultsonsuitableQCsystemshasslowedprogresstowardsthisunderstanding.Thequasi-twodimensionalmetalYFe2Al10isaverypromisingsystem,comprisedoflayersofnearlysquarenetsofFeatoms.Despitethestrongdivergenceofthesusceptibility,[)<[)<(T)~T-1.4thereisnoevidenceformagneticorderabove0.02K.Inelasticneutronscatteringmeasurementsfindthatthescatteringhasnoindicationofincipientmagneticorder,ratherthereisnomeasureablewavevectordependence,beyondthatoftheformfactor.However,thescatteringdisplaysastrongenergydivergence,andtheKramers-Kroniganalysisindicatesthatitisthesequantumcriticalexcitationsthatareresponsibleforthedivergenceinthemagneticsusceptibility.Thescatteringisalsotemperatureindependent,evidencethattheimaginarypartofthedynamicalsusceptibility[)<”(E,T)displaysE/Tscaling,wheretheabsenceofanycharacteristicenergyscalebeyondtemperatureisthehallmarkofquantumcriticalsystems.ThequantumcriticalfluctuationsareverystronginYFe2Al10,signalingthatitisveryclosetoaT=0phasetransition.Theneutronscatteringmeasurementsrevealthatthecriticalfluctuationsarecompletelylocal,witheachmomentfluctuatingincoherently,eachwiththesamespectrumofexcitations.ThesefindingsruleoutthepossibilitythatYFe2Al10isnearmagneticordering,andinsteaditseemslikelythatthephasetransitioncorresponds to a purely electronic phase transition, possibly an orbital selective Mott transition, where localized magnetic moments first emerge.

Study of the phase diagramof cuprates inanultrafast fashion

Unconventional superconductivity probed usinguniaxial stress: an NMR applicationto Sr2RuO4

StuartBrown,UniversityofCalifornia,LosAngeles

ThesuperconductingstateofSr2RuO4haslongbeenheldupasasolidstateanalogtosuperuid3He-A,withstrongnormalstatecorrelations,andanodd-parityorderparameterthatalsobreakstimereversalsymmetry.Knownasthechiralp-wavestate,aconsequenceoftheproposedtwo-componentorderparameterisasplitsuperconductingtransitionwhensubjectedtoin-planemagneticfieldsoruniaxialstress.Recentstudiesinsteadrevealedasteepriseandincreaseintransitiontemperaturebyafactorof2.5instressedsamples,motivatingustostudytheevolutionofthenormalandsuperconductingstatesusing17ONMR.Understressedconditions,thenormalstateKnightshiftsareconsistentwithtuningthroughavanHovesingularity,alongwithanassociatedenhancementofmagneticfluctuations.Deepinthesuperconductingstate,areductionofKnightshiftsKisobserved,
indicating a drop in the spin polarization. Implications for the order
parameter, in the context of a much broader set of experimental
findings, are discussed.

Weyl-Kondosemimetalbehavior in Ce3Bi4Pd3

Silke Bühler-Paschen, Vienna University of Technology

Recentprogressesonheavyfermionquantumcriticality:unconventionalsuperconductivityand entwined degrees of freedom

AngCai,RiceUniversity

Heavyfermionssystemsprovideavaluablesettingtostudyquantumcriticality.Here,quantumcriticalpoint(QCP)isbelievedtogobeyondtheLandauparadigm,featuringthephysicsofKondodestruction[1].In this context,wereporton recentdevelopmentsregardingtwo prominentandoverarchingissues.First,weanalyzetheimplicationsoftheKondodestructionquantumcriticalityforsuperconductivity,anissuethathasbeenhighlightedinparticularbytheobservedhigh-TcsuperconductivityinCeRhIn5[2].

WehavestudiedanAndersonlatticemodelusingtherecentlydevelopedClusterExtended-DMFTapproach.WehavedemonstratedthattheKondodestructionQCPisrobustgoingfromsingle-siteEDMFTtotheClusterEDMFT[3],andshownthatthistypeofQCPpromoteshighTcunconventionalsuperconductivity,withamaximumTcontheorderofseveralpercentofthebareKondotemperature[3].OurresultsprovideanaturalunderstandingforthephasediagramoftheCe-115systems,andhighlight howunconventional superconductivity develops fromastrange-metal normal state.

Second,weaddresshowKondomaterialsprovideasettingtostudytheinterplaybetweenspinandotherlocaldegreesoffreedom.ExperimentsperformedinViennaoncubicheavyfermionsystemCe3Pd20Si6uncoveredevidencefortwoconsecutiveFermisurfacecollapsingQCPsunderasingleparametertuning[4].WeconsideranSU(4)Bose-FermiKondomodelwithbothspinandorbitaldegreesoffreedomcoupledtobosonicbaths,whichrepresentsaneffectivemodelintheEDMFTapproachofamultipolarKondolatticesystemassociatedwithGamma8multipletsandKugel–Khomskiitypeinteractions.WefindthatagenericparametertuningtrajectorycontainstwoQCPs,eachbeingassociatedwiththeKondodestructionofspinororbitaldegreeoffreedom[4].Ourresultsrevealhowquantumcriticalitysimplifiestheinterplaybetweentheentwineddegreesoffreedomandsupportelectron localizationas a unified framework for strongly correlated materials.

MottinsulatorisacentralconceptinstronglycorrelatedphysicsandmanifestswhentherepulsiveCoulombinteractionbetweenelectronsdominatesovertheirkineticenergy.AtunableMottinsulator,wherethecompetitionbetweentheCoulombinteractionandthekineticenergycanbevariedinsitu,canprovideaninvaluablemodelsystemforthestudyofMottphysics.Inthistalk,Iwilldiscussageneralroutetoengineerstronglycorrelatedphysicsintwo-dimensionalmoirésuperlattices,andshowtheexperimentalrealizationofatunableMottinsulatorintheABCstackedtrilayergraphene(TLG)/hBN moirésuperlattice.ThemoirésuperlatticeinTLG/hBNheterostructuresleadstonarrowelectronicminibandsandallowsfortheobservationofgate-tunableMottinsulatorstatesat1/4and1/2fillings.Inaddition,signaturesofsuperconductivityareobservedatlowtemperaturenearthe1/4fillingMottinsulating state in the TLG/hBN heterostructures.

Inthefirststudy,wedemonstrateanewapproachforexploringintrinsicvalleydynamicsby measuringnoisecorrelations[1].Underconditionsofstrictthermalequilibrium,weuseopticalFaradayrotationtopassively“listen”tothethermodynamicfluctuationsofthevalleypolarizationinaFermiseaofresidentelectronsorholes,duetotheirspontaneousscatteringbetweentheKandK’valleysoftheBrillouinzone.Thenoisecorrelationfunctionrevealsverylongexponentially-decayingintrinsicvalleyrelaxation,providingaviableroutetowardquantitativemeasurementsofthetrulyintrinsicvalleydynamics,freefromanyexternalperturbation,pumping,orexcitation.Inthesecondstudy,wemeasuretheopticalresponseofanelectronFermiseainmonolayerWSe2.Lowenergyfeaturesintheopticalspectrumsuggesttheemergenceofnewquasiparticlesduetocouplingtoshort-wavelengthinter-valleyplasmons[2], which become discretized in large applied magnetic fields to 65 T.

Topological Spin Excitations in Honeycomb Ferromagnet CrI3

PengchengDai,RiceUniversity

Intwo-dimensionalhoneycombferromagnets,bosonicmagnonquasiparticles(spinwaves)mayeitherbehaveasmasslessDiracfermionsorformtopologicallyprotectededgestates.Thekeyingredientdefiningtheirnatureisthenext-nearest-neighborDzyaloshinskii-Moriyainteractionthatbreakstheinversionsymmetryofthelatticeanddiscriminateschiralityoftheassociatedspin-waveexcitations.Usinginelasticneutronscattering,wefindthatspinwavesoftheinsulatinghoneycombferromagnetCrI3(TC=61K)havetwodistinctivebandsofferromagneticexcitationsseparatedbya~4meVgapattheDiracpoints.TheseresultscanonlybeunderstoodbyconsideringaHeisenbergHamiltonianwithDzyaloshinskii-Moriyainteraction,thusprovidingexperimentalevidencethatspinwavesinCrI3canhave robust topological properties potentially useful for dissipationless spintronic applications.

What Non-EquilibriumPhoton Spectroscopies Can Tell Us About Materials

TomDevereaux, Stanford University

Photon-basedspectroscopieshavehadasignificantimpactonbothfundamentalscienceandapplicationsbyprovidinganefficientapproachtoinvestigatethemicroscopicphysicsofmaterials.TogetherwiththedevelopmentofsynchrotronX-raytechniques,theoreticalunderstandingofthespectroscopiesthemselvesandtheunderlyingphysicsthattheyrevealhasprogressedthroughadvancesinnumericalmethodsandscientificcomputing.Inthistalk,Iwillprovideanoverviewofultrafasttechniquesforout-of-equilibriumspectroscopies,fromcharacterizingequilibriumpropertiesto generating transient or metastable states, mainly froma theoretical point of view.

Developments in Correlated Materials in Low Dimensions

GregoryFiete,NortheasternUniversity

Inthistalk,Iwillhighlightrecenttheoreticalandexperimentaldevelopmentsincorrelatedmaterialsinlowdimensions,focusingontwo-dimensionalsystemsthatexhibitinterestingpropertiesdrivenbyelectroniccorrelations.Theseincludevariousformsofsuperconductivity,magnetism,chargedensitywaves,topologicalproperties,andtheinterplayofthese.Theaimofthisbrieftalkistosetthestagefor the following talks and related scientific discussion.

Supermetal

LiangFu,MassachusettsInstituteofTechnology

Iwilldescribepropertiesoftwo-dimensionalmetalswithdivergentdensityofstatesduetoFermisurfacesingularity.Asidefromthewell-knownvanHovesingularity,Iwillintroduceawidearrayof Fermisurfacesingularitiesassociatedwithhigh-ordersaddlepointsandpower-law(insteadoflogrithmic)divergentdensityofstates.Thesesupermetalscanbequantumcritical,orformorderedstatessuchassuperconductivityanddensitywaveatlowtemperature.Candidateexamplesofsupermetal include magic-angle twisted bilayer graphene and bilayer ruthenates.

Kondobreakdown and non-Fermi liquid infrustrated Kondo lattice models

TarunGrover,UniversityofCalifornia,SanDiego

OneofthecentralthemesinheavyfermionsystemsisthebreakdownofKondoscreeningasafunctionofexternalparameters[1,2].OnepossibleroutetoKondobreakdownisviageometricfrustrationoflocalmoments.Forexample,geometricfrustrationmaycauselocalmomentstoenter a quantum spinliquidstatetherebydecouplingthemfromtheconductionelectronsatlowenergies[3].Numericalstudyofmodelsthatmayexhibitsuchphysicsischallenging,bothduetothefermionsignproblemandthesignproblemthatarisesfromgeometricfrustration.InthistalkIwilldiscussrecentprogressintacklingsuchproblemsandshowthatalargeclassoffrustratedKondolatticemodelscanbeformulatedwithoutanysignproblem[4].Asanapplication,IwilldiscussamodelwhereDiracelectronsonthehoneycomblatticeareKondocoupledtoquantumspinsonthekagomelattice[5].Iwillshowthatinacertainparameterregime,oneobtainsanon-FermiliquidphasewhereKondoscreeningisnotoperativeandspinsformatopologicallyorderedquantumspinliquidstate.Iwilldiscussthepropertiesofthisnon-Fermiliquidphaseviaconventionalobservablessuchasthespectralfunction,andalsoviamutualinformation between the electrons and the spins.

Thermalization and Possible Signatures of QuantumChaos in Complex Crystalline Materials

Aharon Kapitulnik, Stanford University

Analysesofthermaldiffusivitydata on complexinsulatorsand on stronglycorrelated electron systemshostedinsimilarcomplexcrystalstructuressuggestthatquantumchaosisagooddescriptionforthermalizationprocessesinthesesystems,particularlyinthehightemperatureregimewherethemanyphononbandsandtheirinteractionsdominatethethermaltransport.Hereweobservethatforsuchsystemsdiffusivethermaltransportiscontrolledbya universalPlanckiantimescale$\tau\sim\hbar/k_BT$,andaunique(butterfly)velocity$v_B$.Specifically,$v_B\approxv_{ph}$forcomplexinsulators,and$v_{ph}\lesssimv_B\llv_{F}$inthepresenceofstronglycorrelateditinerantelectrons($v_{ph}$and$v_F$arethephononsandelectronsvelocitiesrespectively).Forthecomplexcorrelatedelectronsystemswefurthershowthatchargediffusivity,whilealsoreachingthePlanckianrelaxationbound,islargelydominatedbytheFermi velocityoftheelectrons, hencesuggestingthatitisonlythethermal (energy) diffusivity that describes chaos diffusivity.

Topology and Correlation in Kitaev Materials

Yong-BaekKim,UniversityofToronto

WediscussrecentprogressintheoryandexperimentonemergentcorrelatedtopologicalphasesinKitaevmaterials.Herethecompetitionbetweendifferentanisotropicspin-exchangeinteractionsmayleadtoanumberofexoticphasesofmatter.Weinvestigatepossibleemergenceofquantumspinliquid,topologicalmagnons,andtopologicalsuperconductivityintwoandthreedimensionalsystems.Wemake connections to existing and future experiments.

Weyl-KondoSemimetal-Magnetism and Topology in Heavy-Fermion Systems

Hsin-HuaLai,RiceUniversity

StronglycorrelatedandtopologicallynontrivialinsulatorsgobackbydecadestoquantumHallsystems.Bycontrast,topologicalconductorswithstrongcorrelationsarestillbeingidentified.Heavyfermionmetalsexemplifysystemsofstrongcorrelationsandaretypicallylocatedattheedgeofmagnetism.Giventheirinherentlystrongspin-orbitcouplings,heavyfermionmetalspresentanaturalsettingto studyhowstrongcorrelationsingeneralandmagneticdegreesoffreedominparticularcanproducecorrelatedtopologicalstates.Recently,wehaveadvancedaWeyl-Kondosemimetalphase[1]withinawell-definedthree-dimensionallatticemodelthatbreakstheinversionsymmetry.ThequasiparticlesneartheWeylnodesdevelopoutofthemagneticcorrelationsintheformofKondoeffect,asdothesurfacestatesthatfeatureFermiarcs.Importantly,theWeylnodesarepinnedneartheFermienergybytheKondoeffectalongwithspacegroupsymmetry.ThishasallowedustoproposeakeythermodynamicsignatureoftheWeyl-Kondosemimetalphase,viz.thespecificheatCgoingasTcubedwithaprefactorenhancedbyasmuchas9ordersofmagnitudecomparedtotheexpectedvalueforweaklycorrelatedsystems.ThisthermodynamicsignaturehasbeenidentifiedintheheavyfermionsemimetalCe3Bi4Pd3[2].Furthermore,recentexperimentalfindingofaspontaneousHalleffectinCe3Bi4Pd3providesdirectevidenceforthetopologicalnatureofthisphase[3].Ourresultsopenupanewavenueforthecreationofcorrelatedtopologicalconductorsbythefluctuationsofmagneticdegrees of freedom.

Recentexperimentshaverevealedtheexistenceofmultiple-Qspintexture,suchasmagnetichelix,bubbleandskyrmioncrystals,inmagneticinsulators.Westudythenovelchargetransportinthesesystemsinthepresenceofathermalgradient.Thethermalgradientdrivesthemultiple-Qspintextureintomotion.Themotionofthespintexturecanpumpchargebothinthelongitudinalandtransversedirections.Wefindthatthechargepumpisassociatedwiththenon-trivialtopologyoftheelectronicstatetwistedbythemagneticspintexture.ByintroducingancillarydimensionsassociatedwiththeGoldstonemodeofthespintexture,thesystemcanmapintohigherdimensionalquantumHallsystems.The direction of the charge current depends on the topological index of the quantumHall systems.

Emergence of Flat-BandMagnetism and Half-Metallicity in Twisted Bilayer Graphene

TwistedbilayergrapheneisaplanarstructurecomposedoftwovanderWaalsbondedgraphenesheetswhereonesheetistwistedwithrespecttotheother.First-principlescalculationsarepresentedtodemonstratethatdynamicband-structureengineeringintwistedbilayergrapheneispossiblebycontrollingthechemicalcompositionwithextrinsicdoping,theinterlayercouplingstrengthwithpressure,andthemagneticorderingwithexternalelectricfield.Theinterplaybetweenelectronicandgeometricdegreesoffreedomincompressedbilayersleadstoanunbalanceddistributionofchargedensityresultinginthespontaneousapparitionoflocalizedmagneticmomentswithoutdisruptingthestructuralintegrityofthebilayer.Weakexchangecorrelationbetweenmagneticmomentsisestimatedin large unit cells and evidence of half-metallicity provided.

Non-Fermi liquid at the FFLO quantum critical point

Ipsita Mandal, Cornell University

When a 2D superconductoris subjected to a strong in-plane magnetic field, Zeeman polarization of theFermisurfacecangiverisetoinhomogeneousFFLOorderwithaspatiallymodulatedgap.Furtherincreaseofthemagneticfieldeventuallydrivesthesysteminto a normalmetalstate.Weperformed arenormalizationgroupanalysisofthisquantumphasetransition,startingfromanappropriatelow-energytheory.Wecomputedone-loopflowequationswithinthecontrolleddimensionalregularizationschemewithfixeddimensionofFermisurface,expandingin\epsilon=5/2−d.Wefoundanewstablenon-Fermiliquidfixedpointandwilldiscussitscriticalproperties.OneofthemostinterestingaspectsoftheFFLOnon-Fermiliquidscenarioisthatthequantumcriticalpointispotentiallynaked,withthescalingregimeobservabledowntoarbitrarylowtemperatures.Inordertostudythispossibility,wealsoperformedageneralanalysisofcompetinginstabilities,whichsuggestedthatonlychargedensitywaveorder is enhanced in the vicinity of the quantumcritical point.

Possible Kitaev spin liquid physics and topologicaltransitions in RuCl3

StephenNagler,OakRidgeNationalLaboratory

TheS=1/2honeycomblatticemagneticinsulatoralpha-RuCl3hasattractedattentionasapossiblemanifestationofphysicsrelatedtotheKitaevmodel.Atlowtemperaturesthematerialordersantiferromagneticallyinthezigzagphase.Despitethisordering,neutronscatteringandspectroscopicmeasurementshaveprovidedevidenceforafractionalizedmagneticexcitationspectrum.Themagneticorder can be suppressed by a magnetic fieldof 7.5 Tesla applied appropriately in the honeycomb plane,andthisresultsinadisorderedquantumphasethatislikelysomesortofquantumspinliquid.Thistalkemphasizestheexcitationspectrumasafunctionoffieldasmeasuredwithinelasticneutronscattering,alongwithdiffractionandthermodynamicmeasurements.Thedatashowsfeaturesthatmaybeevidence for a high field topological phase transition.

Interacting Majorana fermions in strained nodal superconductors

Usingultrashortopticalpulsestounraveltheinterplaybetweenelectronicandmagnetic phenomena in strongly correlated systems

RohitPrasankumar,LosAlamosNationalLaboratory

Ahallmarkofcorrelatedelectronmaterialsisthesensitivityoftheirpropertiestotheinterplayandcouplingbetweendifferentorderparameters.Forexample,heavyfermionmaterialsdisplayawidespectrumoflowtemperaturestatesdependingontheinteractionsbetweencoherentheavyelectronsandlocalizedf-electronspins.Similarly,multiferroicoxideshaveattractedmuchattentioninrecentyearsduetotheircoexisting,sometimescoupledmagneticandelectricorders,whichcouldleadtocontrollingmagnetismwithanelectricfieldandferroelectricity(FE)withamagneticfield.Inthepastseveralyears,wehavedemonstratedthatultrafastopticalspectroscopy(UOS),atfrequenciesrangingfromsoftX-raystoterahertz,isauniquetoolforexploringMEcouplinginbothcanonicalmultiferroics(e.g.,HoMnO3)andmultiferroicoxideheterostructures.WehavealsoshownthatUOScanprovideinsightintotheelectronicstructureofheavyfermionmaterialsthatcannotbeobtainedthroughotherexperimentaltechniques.Here,Iwilldescribeourworkintheseareas,alongwithnewconceptsforultrafast studies of these material systems.

Inrareearthintermetallicswithweaklyboundf-electronsandaKondoenergyscalemuchlargerthanmagneticexchangeorcrystalfieldsplittings,thescreeningoflocalmomentsmayresultinanon-magneticFermiliquidgroundstate.Atlowtemperatures,thequantumfluctuationsbetweenmagneticandnon-magneticvalenceconfigurationscanacquireacooperative(lattice)character,whichhasaprofoundimpactonanumberofbulkphysicalproperties.Onaphenomenologicalbasis,asoundunderstandingofthisphenomenonhasbeenachieved.However,amicroscopictheoryofthisprototypecoherentlyentangledquantummanybodystateremainsanoutstandingchallenge.ThecooperativecharacterofsuchAndersonLatticesisaccessiblebymomentum-resolvedspectroscopies,suchasangle-resolvedphotoemissionandinelasticneutronscattering.Thesemethodsprobesingle-particleexcitationsinthechargeandmagneticchannels,respectively.Bycontrast,novelsoftx-rayresonantinelasticx-rayscattering(RIXS)experimentsprovideamoresubtlepointofview,couplingtobothchargeandspindegreesoffreedominanon-trivialway.IfcalculationsoftheunderlyingKramers-Heisenbergtermonabasisofstronglycorrelatedelectronicbandscouldbeachieved,thiscouldunlockunprecedented microscopic insight into one of the longest-standing issues in quantummatter.

Nematicity and quantumcriticality in CeRhIn5 probed by dilatometry

PriscilaRosa,LosAlamosNationalLaboratory

CeRhIn5,a heavy-fermion antiferromagnet,exhibits remarkable behaviorwhensubjectedtoeitherhighmagneticfieldsorhighappliedpressures.Fromthesuperconductingdomearoundaputativeunconventionalquantumcriticalpointat2:3GPatotherecentlydiscoveredXYnematicphaseatH*>30 T, this material serves as a platformfor the study of strongly correlated phenomena.Inthistalk,high-resolutionthermalexpansionandmagnetostrictionmeasurementswillbeusedtoprobeCeRhIn5atextremes.OpticalfiberscontainingfiberBragggrating(FBG)sensorsareusedtomeasurethelengthchangeinmillimeter-sizedsinglecrystalssubjectedtoDCfieldsto45T.AnFBGopticalapparatusformeasuringthermalexpansionunderappliedhydrostaticpressuresto2.5GPawillalsobepresented.Ourresultssupportascenarioinwhichanisotropichybridizationsetsthestageforelectronic nematicity and quantumcriticality in CeRhIn5.

Thequantumdimermagnet(QDM)isthecanonicalexampleof'quantummagnetism'.Thisstateconsistsofentanglednearest-neighborspindimersandoftenexhibitsafield-induced'triplon'Bose-Einsteincondensate(BEC)phase.Iwilldiscuss a newQDMinthestronglyspin-orbitcoupled,distortedhoneycomb-latticematerialYb2Si2O7[1].Singlecrystalneutronscattering,specificheat,andultrasoundvelocitymeasurementsrevealagappedsingletzerofieldgroundstatewithsharp,dispersiveexcitations.Wefindafield-inducedmagneticallyorderedphasereminiscentofaBECphase,withexceptionallylowcriticalfieldsofHc1~0.4TandHc2~1.4T.UsinginelasticneutronscatteringweobserveaGoldstonemodethatpersiststhroughouttheentirefield-inducedmagneticallyorderedphase,suggestiveofthespontaneousbreakingofU(1)symmetryexpectedforatriplonBEC.However,incontrasttootherwell-knowncasesofthisphase,thehigh-field(H>1.2T)partofthephasediagraminYb2Si2O7isinterruptedbyanunusualregimesignaledby a changeinthefielddependenceoftheultrasoundvelocityandnetmagnetization,aswellasthedisappearanceofasharpanomalyinthespecificheat.Thesemeasurementsraisethequestionofhowanisotropyinstronglyspin-orbitcoupledmaterialsmodifiesthe field induced phases of QDMs.

Strange Metal Transport Properties of Electron-DopedLa2-xCexCuO4

Tarapada Sarkar, University of Maryland-College Park

Thenormalstatemagnetoresistanceexhibitsananomalouslinear-in-Hbehavior[1]atthesamedopingandtemperaturewherea linear-in-TresistivitywaspreviouslyobservedforH>Hc2[2],i.e.abovetheFermisurfacereconstructionat x =0.14uptotheendofthesuperconductingdome(x~ 0.175). For doping above thedome conventional Fermi liquid behavior is found.

ThenormalstateSeebeckcoefficient,S/T,exhibitsanunconventionallowtemperature–lnTdependenceatthesamedopingwherelinear-in-Tandlinear-in-Hresistivityisfound[3].Conventional S/T = constant behavior is found above the superconducting dome.

ThenormalstateresistivityaboveTc,from80 K to400K,followsananomalous~A(x)T2behaviorat zero field for all doping(x), with no indication of a MIR limit[4].

WeconcludethatconventionalFermiliquidtheorycannotexplainanyoftheseresults.Moreover,themagnitudeoftheanomalousmagnetoresistanceandthermopowerscaleswithTc,suggestingthattheoriginofthesuperconductivityiscorrelatedwiththeanomalousnormalstateproperties.Iftimeallows,we will discuss the surprising origin of the QCP at the end of the SC dome [5].

Is Superfluid 3He-A a Precursor to Magnetically OrderedSolid 3He?

JamesSauls, Northwestern University

Liquid 3 He is a strongly correlated Fermi liquid with heavy quasiparticles that become superconducting at low temperatures. There are two broken symmetry phases, both of which are spin-triplet, p-wave BCS condensates. The bulk of the pressure-temperature phase diagram is occupied by the time-reversal invariant B phase, a condensate of entangled spin-triplet, p-wave Cooper pairs with a pair amplitude jBi = Y1;-1(p)j ""i+Y1;+1(p)j ##i+Y1;0(p)j "# + #"i . This is the ground state predicted by Balian and Werthamer in 1963 based on weak-coupling BCS theory for p-wave pairing valid for any pressure. By contrast, the high pressure A phase is a condensate of antiferromagnetically ordered, chiral p-wave Cooper pairs, jAi = Y1;+1(p) (j ""i+j ##i) . Thus, the A phase breaks time-reversal and mirror reflection symmetries, as well as gauge, spin and orbital rotational symmetries.

Despite our detailed understanding of the physical properties of the phases of superfluid 3 He, a quantitative theory of the pairing mechanism, phase diagram and thermodynamics of the high-pressure superfluid phases has been elusive. Above the tri-critical pressure of pPCP =21 bar, the A phase is stabilized in a window of temperatures, TAB < T < Tc , separated from the B phase by a pressure and temperature dependent first-order transition at TAB(p) . The stability of the A phase requires a microscopic pairing theory based on strong-correlation physics that goes beyond weak-coupling BCS theory. The “feedback” model proposed by Anderson and Brinkman in which spin-triplet pairing correlations modify the spin-fluctuation-mediated pairing interaction based on paramagnon exchange was a key insight pointing towards a mechanism to stabilize the equal-spin-pairing A phase over the B phase. However, paramagnon exchange theory fails to provide quantitative predictions for the stability of the A phase, specifically the pressure-temperature phase diagram.

I present a strong-coupling theory of superfluid 3 He based on a generalized fluctuation-mediated theory of paring, combined with next-to-leading order corrections to weak-coupling pairing theory based on quasiparticle-quasparticle interactions that accurately describes the thermodynamic potentials for the A and B phases at all pressures and temperatures below Tc(p).

The interaction potentials that describe the quasiparticle scattering amplitudes exhibit a broad ferromagnetic spin-fluctuation peak near q = 0, reminiscent of paramagnon theory, but also resonances corresponding to antiferromagnetic spin-fluctuations and density fluctuations at wavevector Q = 2kf = 0: 82. This wavevector corresponds to a reciprocal lattice vector of bcc solid 3 He at melting pressure. The results provide a quantitative strong-coupling theory for the stability of the A phase, and imply that liquid 3 He at high pressures is an almost localized Fermi liquid near a Mott transition, and suggests that the equal-spin pairing A phase is the precursor of the UUDD phase of solid 3He.

Magnetic Topological Semimetal in Square-Net materials

LeslieSchoop, Princeton University

Materials containing the structural motif of a square-net, have been heavily investigated in respect to their topological semimetal nature. While many of these are nonmagnetic, magnetism can be introduced to these compounds due to their range of chemical tunability. In this talk I will introduce recently developed guidelines that characterize square net materials based on chemical concepts and are suitable to separate trivial ones form topological ones, just by considering atomic distances and electron counts. I will also discuss our recent progress in synthesizing and characterizing magnetically ordered members of this family.

Mapping the Fermi Surface in HgBa2CuO4+d with Angular Magnetoresistance

Katherine Schreiber, Los Alamos National Laboratory

The Fermi surfaces of underdoped cuprates are reconstructed by charge density waves. As a result, the Fermi surfaces may be quite complex, featuring small pockets and interlayer warping. The shape of the reconstructed Fermi surface, including warping of the surface along the c-axis, provides important information towards discerning the symmetry of the charge density waves. With this knowledge, the relationship between the charge density wave and superconductivity may become better understood. In this work, we present a study of the Fermi surface in underdoped HgBa2CuO4+d (Hg1201) samples, obtained through angular magnetoresistance measurements. Transport along the c-axis was measured in pulsed magnetic fields of up to 65 T, applied along many polar and azimuthal angles in order to map out the Fermi surface, over a broad range of temperatures. We discuss the implication of our measurements for the shape of the Fermi surface, including the interlayer warping.

Novel electronic nematicity in (Ba,Rb)Fe2As2 iron-based superconductors

TakasadaShibauchi, University of Tokyo

Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics), resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba_{1−x}Rb_{x}Fe_{2}As_{2} iron-based superconductors. While for x=0 the nematic director points along the in-plane diagonals of the tetragonal lattice, for x=1 it points along the horizontal and vertical axes. Remarkably, for intermediate doping, the susceptibilities of these two symmetry-irreducible nematic channels display comparable Curie-Weiss behavior, thus revealing a nearly XY-nematic state [1]. This opens a new route to assess this elusive electronic quantum liquid-crystalline state, which is a candidate to host unique phenomena not present in the Ising-nematic case.

Entropy plateaus in Spin-S Kitaev Models

RajivSingh, University of California, Davis

Spin-S Kitaev models on the Honeycomb lattice share many features of the soluble spin-half Kitaev model. They have conserved Z_2 fluxes on each elementary hexagonal plaquette and spin-spin correlations are zero beyond nearest-neighbor. We present a study of the thermodynamic properties of these quantum spin-liquids using high-temperature series expansions and thermal pure-quantum methods. We find evidence for plateaus in entropy, where the value of the entropy equals half the total entropy, which implies a highly degenerate low-energy manifold. Anisotropy in the Kitaev couplings destroys these plateaus and instead produces robust plateaus at much smaller entropy values. We discuss possible reasons for the origin of these plateaus.

Unconventional superconductivity? The strange case of CeCu2Si2

FrankSteglich, Max Planck Institute for Chemical Physics of Solids

According to the so - called ‘quantum critical paradigm’ any antiferromagnetic quantum critical point (QCP) in a clean, stoichiometric heavy - fermion (HF) metal should give rise to unconventional superconductivity (SC). In this talk, the first HF superconductor CeCu2Si2 [1] will be addressed as an exemplary material which exhibits SC close to a three - dimensional spin - density - wave - type QCP [2, 3]. For a long time, CeCu2Si2 was considered a (single - band) d - wave superconductor [4, 5]. However, a few years ago its specific heat was found to exhibit two - band behavior with exponential temperature dependence at very low temperatures, typical for a conventional BCS superconductor [6]. Based upon atomic - substitution [7, 8], neutron - scattering [9, 10], specific - heat and London penetration – depth [11] measurements it will be argued that CeCu2Si2, rather than being an (isotropic or anisotropic) s - wave superconductor, is a fully - gapped, two - band d - wave superconductor. This is most likely due to an orbital - selective pairing mechanism [12], similar to the one proposed for Fe - based chalcogenides [13].

Exotic spin-orbital entangled phases in 4d and 5d transition metal oxides

HidenoriTakagi, Max Planck Institute for Solid State Research

The exploration of novel phases of interacting electrons (correlated electrons) has long been a major stream of condensed-matter research. Many-body interactions among electrons give rise to a huge variety of phases, grouped into electron-solid, -liquid-crystal, -liquid and -gas states. The wealth of possibilities arises from a complicated interplay of lattice geometry, quantum effects and the multiple degrees of freedom of the electron (charge, spin and orbital). In the past, the two dominant areas of exploration have been the 3d transition-metal (TM) oxides and the 4f intermetallic compounds but recently 5d TM oxides and related compounds have emerged as the next arena of correlated-electron physics. Significant new physics is expected due to the presence of a large spin-orbit coupling in heavy 5d elements, tying together the otherwise independent spin and orbital degrees of freedom. This can be of order 0.5eV and is often larger than the crystal-field splitting of the orbital states, resulting in a spin-orbital-entangled state of correlated electrons. The nature of the spin-orbital entanglement depends significantly on the d-electron number and the chemical bonding, and it is anticipated that, in combination with electron correlations, a rich variety of novel electronic phases are waiting to be discovered. To name just a few, the proposed phases include Kitaev quantum spin liquids, correlated topological semimetals, excitonic magnets and multipolar-ordered states.

In this talk, I will present our recent exploration of such exotic faces of spin-orbital entangled matter in 5d (and 4d) transition metal oxides. Topics will include the following.

Spin-orbital quantum liquid on honeycomb lattice in 5d^{5}H_{3}LiIr_{2}O_{6} [1].

J_{eff} = 0 Mott insulator [2] in 4d^{4 }Ru^{4+} oxides and proximity to excitonic magnetism.

Multipolar ordering in 5d^{1} Ta^{4+} chlorides [3].

Revisiting the dimensional crossover observed in the BEC material BaCuSi2O6 with new experimental results on Sr-doped samples

Franziska Weickert, Florida State University

In the last 20 years, more than a dozen quantum paramagnets have been identified to show properties consistent with the formalism of Bose-Einstein condensation (BEC) [1,2]. Among them, BaCuSi2O6 attracted considerable attention due to its small spin gap _= 3-5 meV and moderate energy dispersion, leading to ordering between critical fields Hc1 = 23.5T and Hc2 = 49T that are accessible in high magnetic field laboratories and allow thorough investigations of the field-induced ground states [3]. On the down side, BaCuSi2O6 undergoes a phase transition at 90K that changes the I41/acd tetragonal crystal structure at high temperatures to a slightly distorted monoclinic lattice at lower temperature causing inequivalent intradimer exchange couplings and different spin gaps (ΔB/ΔA = 1.16) in every second plane along the c-axis [3].

Thermodynamic investigations of pure BaCuSi2O6 close to Hc1 uncovered BEC behavior in the critical exponent of the phase boundary. Furthermore, they revealed a dimensional crossover from 3D for T > 1K towards 2D for lower temperatures [4] that is unique for BaCuSi2O6 compared to other BEC materials. This unexpected discovery is most surprising, because low-energy coupling terms are usually amplified close to quantum critical points, which increases rather than decreases the dimension when lowering the temperature [1]. First interpretations argued that frustrated interlayer exchange causes the dimensional reduction by effectively decoupling the dimer layers along the c-axis [5]. In contrast, a theoretical study based on DFT calculations excluded magnetic frustration to be present in this material [6] and opened up again the discussion on the effect of structural lattice distortion on the dimensionality of the BEC.

In the presentation, we discuss experimental results on newly synthesized single crystals BaCuSi2O6 doped with 10% Sr [7,8], which do not exhibit a structural phase transition at 90K and allow to investigate BEC in the absence of lattice distortions. We establish the H-T phase diagram based on magnetization, specific heat and magnetocaloric effect (MCE) measurements in pulsed magnetic fields revealing a dome-shaped BEC phase with slightly lower critical fields compared to pure BaCuSi2O6 as displayed in Fig. 1. We furthermore discuss magnetic torque measurements carried out in high DC fields up to 31T and down to 0.3K that address the dimensional crossover in the critical exponent of the phase boundary close to Hc1.

Role of Orbital Physics in Iron Chalcogenides

MingYi, Rice University

Electron correlation effects give rise to a variety of emergent phenomena in quantum materials—high temperature superconductivity, electronic nematicity, Mott insulating phase, magnetism. The family of Fe(Se,Te) superconductors plays a remarkable host to all of these phenomena in different parameter regimes. In this talk, I will present angle-resolved photoemission results on two aspects of electron correlation effects in this material family—i) orbital-selective Mott insulating behaviors towards the FeTe end of the phase diagram, and ii) electronic nematicity in completely detwinned FeSe. Both examples showcase the phenomenal way that correlation effects rewrite the low energy electronic states of a material system, and reveal the exceptional role the orbital degree of freedom plays in composing the fundamental physics in iron chalcogenide superconductors.

Recent developments on the electronic nematicity in iron-based superconductors -- the role of electron correlations

RongYu, Renmin University of China

Electronic order in general, and electronic nematic order in particular, has been the topic of considerable interest in the area of iron-based superconductors. New type of questions continues to emerge. In this talk, I will focus on two recent developments along this direction, with a particular attention paid to the role of electron correlations. First, I will show that the multiorbital aspect of electron correlations, in the form of orbital selectivity, interplays with the nematicity in a striking way [1]. Illustrated in the context of the bulk FeSe, I will show that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, when the various types of bond and site nematic orders are combined, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small. These results explain the seemingly unusual observation of strong orbital selectivity in the nematic phase of FeSe [2]. In the second example, I will show, based on a Ginzburg-Landau theory with symmetry analysis, that the spin correlations in the system allow for a variety of nematic orders, in particular an unusual $B_{2g}$ nematicity [3]. Using qualitative considerations as well as microscopic calculations, I will discuss the types of magnetic fluctuations that stabilize this $B_{2g}$ nematicity and how our proposed mechanism provides a natural understanding of the recent experimental observations in the heavily hole doped iron pnictides (Rb,Cs)Fe$_2$As$_2$ [4,5].