Inhomogeneous Level Splitting in PrxBi2xRu2O7

1
Inhomogeneous Level Splitting in PrxBi2-xRu2O7

• Collin Broholm and Joost van Duijn
• Department of Physics and Astronomy
• Johns Hopkins University

2
Collaborators

• Ruthenium pyrochlores
• K. H. Kim Rutgers
• N. Hur Rutgers
• D. Adroja ISIS
• Q. Huang NIST
• S.-W. Cheong Rutgers
• T. G. Perring ISIS
• Iridium pyrochlores
• Satoru Nakatsuji Kyoto
• Yo Machida Kyoto
• Yoshiteru Maeno Kyoto
• Toshiro Sakakibara ISSP
• Takashi Tayama ISSP

3
Outline

• Introduction
• Bulk properties of 4dn and 5dn pyrochlores
• Spin correlations on TM sites
• Crystal field excitations on RE sites
• Level splitting in PrxBi2-xRu2O7
• Ground state doublet spin dynamics
• Model of inhomogeneous level splitting
• Discussion and Conclusions
• Possible relevance for other non-Kramers doublet
  systems

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The pyrochlore structure A2B2O7

• A and B sites on vertices of corner-sharing
  tetrahedra
• A3 site is trivalent RE with 8-fold O2-
  coordination
• B4 site is tetravalent TM with 6-fold O2-
  coordination
• Can have both A3 and B4 magnetism

Nearest neighbor exchange selects a manifold of
states characterized by Zero-spin tetrahedra
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BRu site magnetism of Y2Ru2O7
Taira et al. (2000)
Taira et al. (1999)

• Neutron Diffraction reveals long range magnetic
  order
• ZFC/FC hysteresis suggests some form of disorder
  that produces
• glassy canted AFM

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Strong Coupling Transition in Y2Ru2O7
T90 K
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PM and AFM Spin Fluctuations

• Phase Transition pushes significant spectral
  weight into resonance
• The Q-dependence of scattering reflects the form
  factor for AFM
• cluster degrees of freedom

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2 Magnetic Ions 2 Phase Transitions
R2Ru2O7
Er2Ru2O7
Pr3
M. Ito et al. (2001)
Yb3
N. Taira et al. (2003)
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Metal Insulator Transition

• Bi-doping increase bandwidth causing Mott Hubbard
  MIT
• Magnet order is found only in the insulating
  state
• Electronic DOS at EF is enhanced close to the MIT

Yoshii and Sato (1999)
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A Highly Entropic Metallic State
K. H. Kim et al.
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Praseodymium Crystal Field levels
116 meV
105 meV
5 K
Pr1.2Bi0.8Ru2O7
85 meV
50 meV
200 K
10 meV
0
Five transitions from GS implies it is doublet
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Fluctuations in metallic Pr1.2Bi0.8Ru2O7
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Inhomogeneous Level Splitting

• Broad Spectrum unchanged
• Upon heating to

• Wave vector dependence
• Follows Pr form factor2

Neutron Scattering measures the level splitting
spectrum
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Inhomogeneous two level system

• The level distribution function
• Singlet-singlet susceptibility
• Sample averaged susceptibility
• Fluctuation-dissipation theorem yields

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Data Collapse Confirms quenched broadening
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Specific heat of r(D)-split doublet
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Susceptibility of r(D)-split doublet
For gapless spectrum low T limiting form is lnT
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Clues to origins of level splitting

• T-independent distribution function r(D)
• Static not dynamic phenomenon
• Continuous not discrete spectrum
• Large rare defect or density wave producing
  distribution of environments
• Same distribution describes all xlt1.2
• Distribution may not come from Bi doping
• Magnetic field enters in quadrature
• Electrostatic not magnetostatic inhomogeneity

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Where else might this occur?

• Elements that can have non-Kramers doublet ground
  states
• Pr3 , Pm3, Sm2, Eu3, Tb3, Ho3, Tm3,
  Yb2, and U4
• Nominally cubic and stoichiometric systems may
  have clandestine level splitting disorder
• Doping effects may be controlled by the induced
  level splitting

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Other Materials to Reconsider

• Tb2Ti2O7 A non ordering magnet with mysterious
  low energy mode
• Ho2Ti2O7 Long range ordered with unusual
  thermodynamics
• YbBiPt Ultra heavy fermion system with
  mysterious low E mode (Robinson et al (1995).
• LiHoxY1-xF4 quantum spin glass. Strain induced
  level splitting adds an effective transverse
  field
• TbxY2-xTi2O7 Dilution has added effect of
  neutralizing Tb through level splitting

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Metal insulator transition in R2Ir2O7
Yanagishima and Maeno (2001)
Nakatsuji et al.
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Conclusions

• Ir and Ru pyrochlores offer MIT transitions with
  frustrated magnetism
• Y2Ru2O7 strong coupling transition
• Spectral weight pushed to finite E resonance
• Q-dependence unaffected by ordering
• PrxBi2-xRu2O7 static inhomogeneous distribution
  of level splittings
• Neutrons T-independent broad spectrum
• C(T)/T broadened Schottky anomaly
• c(T) possible logarithmic divergence at low T
• Watch out for level splitting in non-Kramers
  ions, which are not really magnets