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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experimentalprobesleadtoperturbationsofthesystemunderconsideration.Quantummechanicslinks<br />
staticsanddynamicsalreadyattheequilibriumlevelofaquantumphasetransitionandmakesitnecessary<br />
totreatbothonequalfooting.Thisinvalidatesapproachesthathavebeensuccessfullyusedforclassical<br />
dynamicalphasetransitions.<br />
Intermetallicrareearthcompoundsareextendedlatticesystemswherethecompetitionbetweenlocalized<br />
anditinerantdegreesofphysicsgivesrisetoacomplexbehaviorandaplethoraofcompetinggroundstates<br />
wheretherelevantenergyscalesemergeoutofthebarecouplingsthatdeterminethehigh-temperature<br />
behavior. Asaconsequence, theoreticalmodelsliketheso-calledKondolatticemodelareeffective<br />
modelsdesignedtodescribetheuniversalaspectsoftheproblem.Thermodynamicand(bulk)transport<br />
measurements,i.e. thetraditionalbulkmeasuresthatareavailableatlowtemperaturesaremoreand<br />
moreaugmentedbylocalprobesthathavebecomeavailable,e.g.neutronscatteringtechniquesormost<br />
recentlyscanningtunnelingmicroscopyforheavyfermions. Thesenovelexperimentaltoolsgiveaccess<br />
tothemicroscopiccouplingconstantsthatentertheoreticalmodelsbutinturnrequireeffectivetheories<br />
thatgobeyondtheuniversalaspectsoftheproblem. Withthesedevelopmentsinthebroadareaof<br />
instabilitiesinstronglycorrelatedelectronsystemsinmind,thejointjuniorresearchgroup’Collective<br />
PhenomenainSolidStateandMaterialsScience’focusesonthefollowingtopics:<br />
• Developmentofeffectivetheoriesforlatticeproblemstounderstandtheemergentphasesnear<br />
electronicinstabilitiesandobtainingabettermicroscopicmodelingoflocaldegreesoffreedomand<br />
theeffectofbondandsitedisorder.Ofparticularexperimentalrelevancewillbetoaddresswhen<br />
quantumcriticalfluctuationscanleadtoCooperpairingandhencetosuperconductivity.<br />
• AddressingthenumericalobservationofsymmetryenhancementnearquantumcriticalityanddevelopingthecriticalfieldtheoryforcriticalKondodestruction.<br />
Thiscriticalfieldtheorywillhave<br />
tocapturetherecentlyobservedfinitetemperaturescalingpropertiesofthemagnetotransport<br />
propertiesaswellasthezero-temperaturejumpintheFermivolume.<br />
• Anyexperimentalmeasurementcreatesaperturbationtothesystem.Forasmallperturbationthe<br />
systemswillreacttoitinawaythatiscompatiblewiththeequilibriumfluctuationspectrum.This<br />
isthecontentofthefluctuation-dissipationtheorem. Inaquantumcriticalstatethesituationis<br />
moresubtleduetoitsscale-invariantspectrumandtherangeofapplicabilityofthefluctuationdissipationtheoremisaprioriunclear.<br />
Understandingtheeffectofnon-equilibriumdrivesase.g.<br />
biasvoltageortemperaturegradientsonthecriticalphysicsofappropriatequantumsystemsis<br />
thereforeanissueofimmensepraticalrelevance.<br />
ExternalCollaborations<br />
Inadditiontothejointprojectsbetweenthe mpipksandtheMPI-CPfS,thereexistseveralprojectswith<br />
externalcollaborators.Researchprojectscurrentlyexistwiththegroupof<br />
• Prof. Dr. LaurensMolenkampfromUniversitätWürzburgontherealizationoftheBose-Fermi<br />
Andersonmodelinself-assembledII-VIsemiconductorquantumdots.<br />
• Prof. Dr. QimiaoSifromRiceUniversity,HoustonandProf. Dr. KevinIngersent,Universityof<br />
FloridaatGainesville.Thelong-termgoalofthecollaborationwithQimiaoSiandKevinIngersent<br />
andthemembersoftheirgroupsistheconstructionofthecriticalfieldtheoryoftheparticular<br />
unconventionalquantumcriticalpointthatrecentlywereobservedinanumberofintermetallicrare<br />
earthcompounds. Wetrytotacklethisproblembyatwo-prongedapproach. Ontheonehand,<br />
weobtainamicroscopicunderstandingofthephysicsnearzero-temperatureinstabilitiesincertain<br />
models.Thishassofarbeenaccomplishedforthespin-isotropicBose-FermiKondomodelandthe<br />
so-calledpseudogapBose-FermiAndersonmodel. Ontheotherhand,symmetryconsiderations<br />
canbeusedtoposeimportantconstraintsontheunderlyingcriticalfieldtheory.Wee.g.recently<br />
arguedthatcriticalKondodestructionisaccompaniedbyanenhancedconformalsymmetry,absent<br />
inthebaremodel. Thisisreminiscentofthesymmetryrestorationthatoccurse.g. intheU(1)<br />
Thirringmodel,whichrestorestheSU(2)symmetrydynamically.<br />
1.9. JuniorResearchGroups 29
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