Study of the Possibility of CN- Background in CO all Sky Survey: A New Region Proposed

Non-Fermi Liquid Behavior inWeak Itinerant Ferromagnet MnSi

Nirmal Ghimire

April 20, 2010

In Class Presentation

Solid State Physics II

Instructor: Elbio Dagotto

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Introduction

 Fermi Liquid Theory

Non-Fermi Liquid System

Non-Fermi Liquid State in MnSi

 Magnetic Ordering and Spin Structure

Conclusion

OutlineOutline

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IntroductionIntroduction

There are two basic mechanism for the observed magnetic moments inmagnetic materials

Local magnetic moments

Itinerant magnetic moments

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W. K. Heisenberg

(1901-1976)

E. C. Stoner

(1899-1968)

Cases of completelocalization orcompleteionization arehardly ever found

Both phenomenaexist side by side: Aunified theory ofsolid statemagnetism is needed

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IntroductionIntroduction

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L. D. Landau

(1908-1968)

1957: Fermi Liquid Theory

Model for metallic state:

Pauli exclusion principle + screening effect

Successfully described some near or weakferromagnetic d-electron metals

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MnSi, a weakly magnetic d-electron compound, also shows Non-Fermi Liquid (NFL) behavior

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Introduction

Fermi Liquid Theory

Non-Fermi Liquid System

Non-Fermi Liquid State in MnSi

Magnetic Ordering and Spin Structure

Conclusion

OutlineOutline

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Fermi-Liquid TheoryFermi-Liquid Theory

•Quasiparticle excitation of interacting Fermi system

•Fermi liquids have spin and obey Fermi statistics

One to one correspondence of quasiparticle and free electron:

Interaction of the quasiparticle

Energy of the system

Energy of N quasiparticles

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Fermi-Liquid TheoryFermi-Liquid Theory

Energy of a quasiparticle is:

Energy of quasiparticle at T =0

Mean field effect of interactionwith other quasi particles

•Scattering amplitude of two quasi particles

•Accounts for the deviation of density of states from theequilibrium value nFermi (n- nFermi)

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Fermi Liquid TheoryFermi Liquid Theory

Total energy:

Prediction:

Electrical resistivity of CeCl3

Specific heat of CeCl3

Experimental confirmation

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Non-Fermi Liquid SystemNon-Fermi Liquid System

Physical Properties:

Experimental confirmation

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Introduction

Fermi Liquid Theory

Non-Fermi Liquid System

Non-Fermi Liquid State in MnSi

Magnetic Ordering and Spin Structure

Conclusion

OutlineOutline

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Non-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi

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Structure of MnSi

•B20 Cubic structure with a =4.588 Å

•Lacks space inversion symmetry

Consequence of the broken inversion symmetry

Helical spin density wave

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Non-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi

•Magnetic phase transition at Tc=29.1 fromparamagnetic to helical magnetic structure

•Wavelength of spiral = 180 Å in (111)direction

Magnetic phase diagram

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Magnetic properties:

•Curie-Weiss fit of susceptibility: Effective magnetic moment = 1.4 μB

•Observation: spontaneous magnetic moment of 0.4 μB at 0K.

Weak itinerantferromagnet

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Non-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi

Variation of resistivity with temperature

•Resistivity drops monotonicallywith decreasing temperature

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5.55Kbar

8.35Kbar

14.3Kbar

15.5Kbar

•Peak position indicates thetransition temperature

•Below pc=14.6 Kbar, there isquadratic behavior

•At pc, quadratic behaviorcollapses

•Above pc, temperature variationof resistivity is slower thanquadratic

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Non-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi

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Comparison between experiment and FFL Theory

High T: FFL model in agreementwith experiment

Low T: T dependence deviatesfrom experimental observation

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Introduction

Fermi Liquid Theory

Non-Fermi Liquid System

Non-Fermi Liquid State in MnSi

Magnetic Ordering and Spin Structure

Conclusion

OutlineOutline

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Magnetic Order in NFL State Magnetic Order in NFL State

Results from Neutron Scattering experiment:

Critical pressure = 14.6 Kbar

Magnetic Ordering above critical pressure?

Helical with same periodicity and long range order

Unusual thing:

Considerable degree of disorderness in thedirection of magnetic propagation vector

A broad angular distribution around <110>:not expected to be favored by the crystal fieldin cubic symmetry

There exists magnetic moment even above pc

Partial magnetic ordering

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Non-Trivial Spin Structure!Non-Trivial Spin Structure!

Two Possible Scenarios for the partial magnetic ordering

Breaking of helical structureinto multi-domain state

Unlocking of helix direction from<111> and no strict directionalorder

Result of polarizedneutron scattering : partialorder on local scale is notrelated to helical structure

No experimental ortheoretical support

Any otherpossibility?

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Non-Trivial Spin Structure!Non-Trivial Spin Structure!

Quantum critical phenomena?

NFL resistivity emerges under pressure without quantum criticality

Spin ordering other than plain pining of thehelix or a multi-domain state

A non-trivial spin structure!!

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ConclusionConclusion

•MnSi , a weak itinerant ferromagnet, shows a behavior ofresistivity which is not consistent with current model of itinerantferromagnetism

•Temperature dependence of resistivity may lie in the novel form ofmagnetic ordering

•Currently, there is no theoretical account for the NFL resistivity andhow it is related to the partial magnetic ordering.

•There is need of more experimental evidences.

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Thank You