Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSiSlide 2IntroductionSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSiNirmal GhimireApril 20, 2010In Class PresentationSolid State Physics IIInstructor: Elbio DagottoIntroduction Fermi Liquid Theory Non-Fermi Liquid SystemNon-Fermi Liquid State in MnSi Magnetic Ordering and Spin StructureConclusionOutlineOutlineIntroductionIntroductionThere are two basic mechanism for the observed magnetic moments in magnetic materialsLocal magnetic moments Itinerant magnetic momentsW. K. Heisenberg(1901-1976)E. C. Stoner(1899-1968)Cases of complete localization or complete ionization are hardly ever foundBoth phenomena exist side by side: A unified theory of solid state magnetism is needed1scqq1scqqIntroductionIntroductionL. D. Landau(1908-1968)1957: Fermi Liquid TheoryModel for metallic state:Pauli exclusion principle + screening effectSuccessfully described some near or weak ferromagnetic d-electron metalsMnSi, a weakly magnetic d-electron compound, also shows Non-Fermi Liquid (NFL) behaviorIntroductionFermi Liquid Theory Non-Fermi Liquid SystemNon-Fermi Liquid State in MnSiMagnetic Ordering and Spin StructureConclusionOutlineOutlineFermi-Liquid TheoryFermi-Liquid Theory•Quasiparticle excitation of interacting Fermi system•Fermi liquids have spin and obey Fermi statistics21One to one correspondence of quasiparticle and free electron:Interaction of the quasiparticle Energy of the system Energy of N quasiparticlesFermi-Liquid TheoryFermi-Liquid Theory Energy of a quasiparticle is:Energy of quasiparticle at T =0Mean field effect of interaction with other quasi particles•Scattering amplitude of two quasi particles•Accounts for the deviation of density of states from the equilibrium value nFermi (n- nFermi)Fermi Liquid TheoryFermi Liquid Theory Total energy: Prediction: Electrical resistivity of CeCl3 Specific heat of CeCl3Experimental confirmationNon-Fermi Liquid SystemNon-Fermi Liquid System Physical Properties:Experimental confirmationIntroductionFermi Liquid Theory Non-Fermi Liquid SystemNon-Fermi Liquid State in MnSiMagnetic Ordering and Spin StructureConclusionOutlineOutlineNon-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSiStructure of MnSi•B20 Cubic structure with a =4.588 Å•Lacks space inversion symmetryConsequence of the broken inversion symmetryHelical spin density waveNon-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi•Magnetic phase transition at Tc=29.1 from paramagnetic to helical magnetic structure•Wavelength of spiral = 180 Å in (111) direction Magnetic phase diagramMagnetic properties:•Curie-Weiss fit of susceptibility: Effective magnetic moment = 1.4 μB•Observation: spontaneous magnetic moment of 0.4 μB at 0K.Weak itinerant ferromagnetNon-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSiVariation of resistivity with temperature•Resistivity drops monotonically with decreasing temperature5.55 Kbar8.35 Kbar8.35 Kbar14.3 Kbar15.5 Kbar•Peak position indicates the transition temperature•Below pc=14.6 Kbar, there is quadratic behavior•At pc, quadratic behavior collapses•Above pc, temperature variation of resistivity is slower than quadraticNon-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSiComparison between experiment and FFL TheoryHigh T: FFL model in agreement with experimentLow T: T dependence deviates from experimental observationIntroductionFermi Liquid Theory Non-Fermi Liquid SystemNon-Fermi Liquid State in MnSiMagnetic Ordering and Spin StructureConclusionOutlineOutlineMagnetic Order in NFL State Magnetic Order in NFL State Results from Neutron Scattering experiment:Critical pressure = 14.6 KbarMagnetic Ordering above critical pressure? Helical with same periodicity and long range orderUnusual thing:Considerable degree of disorderness in the direction of magnetic propagation vectorA broad angular distribution around <110>: not expected to be favored by the crystal field in cubic symmetryThere exists magnetic moment even above pcPartial magnetic orderingNon-Trivial Spin Structure!Non-Trivial Spin Structure!Two Possible Scenarios for the partial magnetic orderingBreaking of helical structure into multi-domain stateUnlocking of helix direction from <111> and no strict directional orderResult of polarized neutron scattering : partial order on local scale is not related to helical structureNo experimental or theoretical supportAny other possibility?Non-Trivial Spin Structure!Non-Trivial Spin Structure!Quantum critical phenomena?NFL resistivity emerges under pressure without quantum criticalitySpin ordering other than plain pining of the helix or a multi-domain stateA non-trivial spin structure!!ConclusionConclusion•MnSi , a weak itinerant ferromagnet, shows a behavior of resistivity which is not consistent with current model of itinerant ferromagnetism •Temperature dependence of resistivity may lie in the novel form of magnetic ordering •Currently, there is no theoretical account for the NFL resistivity and how it is related to the partial magnetic ordering.•There is need of more experimental evidences.23TThank
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