Electronically smectic-like phase in a nearly half-doped manganite

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Electronically smectic-like phase in a nearly
half-doped manganite

• J. A. Fernandez-Baca
• Oak Ridge National Laboratory,
• Oak Ridge, TN 37831, USA

INTERNATIONAL WORKSHOP ON SELF-ORGANIZED STRONGLY
CORRELATED ELECTRON SYSTEMS Seillac, France May
29-31, 2006
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Collaborators

• F. Ye (ORNL)
• P. Dai (Univ. of Tennessee, ORNL)
• J. W. Lynn (NIST)
• H. Kawano-Furukawa (Ochanomizu University, Japan)
• H. Yoshizawa (ISSP-University of Tokyo, Japan)
• Y. Tomioka (CERC, Japan)
• Y. Tokura (U. of Tokyo, CERC, Japan

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Outline

• Motivation Charge and orbital ordering in the
  CMR manganites are associated with the insulating
  state in the manganite perovskites.
• CE-type CO/OO and AF ordering is prevalent and
  not so far energetically from the metal state
• Need to better understand CE-type CO/OO and AF
  order
• Neutron studies in Pr1-x(Ca1-ySry)xMnO3 near
  x0.5
• Evidence of weakly coupled 1D spin chains
  (smetic-like) above TN
• Discussion

Ye et al Phys. Rev. B 72, 212404 (2005)
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CMR perovskite manganitesSubstitution of Ln3
for Ca2 results in mixed valence Mn

• Mn B site

• Ln1-xAxMnO3 perovskite structure
• Ln3Lanthanide (La,Pr,Nd)
• A2Ca,Sr,Ba

A site
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Rich phase diagram Metallic state is associated
with ferromagnetism due to Double exchange
P. Schiffer et al, PRL (1995)

• Insulator-Metal transition coupled with the
  Paramagnetic-Ferromagnetic (PM-FM) transition

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Pr0.5Sr0.5MnO3
Rich phase diagram Insulating state is
associated with Charge/orbital Ordering (CO-OO)
50 Mn3 with 50 Mn4
CE-type CO-OO
Tomioka et al, PRL 74
CO-OO promotes AF and insulating behavior
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Rich phase diagram Insulating state is
associated with Charge/orbital Ordering (CO-OO)
50 Mn3 with 50 Mn4
CE-type CO-OO
The prevalent CO-OO and AF order is of the
so-called CE-type originally proposed by
Goodenough (Phys. Rev. 1955)
Tomioka et al, PRL 74
CO-OO promotes AF and insulating behavior
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Neutron scattering of CE-type CO-OO
CO-OO peak at 1/4,1/4
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Pr 1-x(Ca1-ySry)xMnO3 y0.20, x0.45
Charge/orbital ordering CO-OO can be melted by an
external magnetic field
Tomioka and Tokura PRB 66, 104416 (2002)
Ye et al (unpublished)
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Short range CO/OO fluctuations in LCMO30 above TC
CO-OO peak at 1/4,1/4
CMR seems to result from the competition of the
tendency to CE CO/OO and DE
Dai et al., PRL (2000)
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Trying to understand the CE-type CO-OO

• Goodenough (1955)
• charge ordering at high temperature ( covalent
  bond energy)
• magnetic ordering at much lower temperature
  (exchange interaction).

• van den Brink et al. PRL (1999)
• Anisotropy of short range DE
• Competition between
• Kinetic Energy
• Magnetic Exchange Energy
• Orbital degeneracy

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Trying to understand the CE-type CO-OO
Zener polarons (Daoud-Aladine) Trapping of
electrons in Mn pairs (Zener polarons)

• Mn3/Mn4 picture challenged
• Resonant x-ray experiments suggets magnetic
  moments at Mn3 and Mn4 not so different
• Magnetic powder diffraction, small difference in
  the magnetic moments

• Popovic et al., PRL (2002)
• Jahn-Teller Distortion
• Onsite Coulomb repulsion
• Mn3/Mn4 picture ???

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Trying to understand the CE-type CO-OO

• Important questions
• Origin of CE-type structure
• Role of magnetism
• What about Mn3 and Mn4, are they really
  different? (in magnitude, in character)
• Are there really zigzag chains of Mn moments?

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Case study CO-OO Pr0.55(Ca1-ySry)0.45MnO3
(PCSMO45)
(Pr,Ca)MnO3
(Pr,Sr)MnO3
Tomioka et al. (2002)
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CE type structure in real and reciprocal space
Mn4 AFM (½,0,0)
Mn3 AFM (¼,¼,0)
Mn3 CO-OO (¼,¼,0)(2,0,0)
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Charge/Orbital Ordering of PCSMO45 (Synchrotron
X-ray)
CO-OO peak at large equivalent 1/4,1/4
T240K
T160K
T130K
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Resistivity, CO-OO and AF Ordering in PCSMO45
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Anisotropic AFM fluctuation in PCSMO45

• Highly anisotropic
• Suggest low-dimensional behavior (1D, smectic)

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Isotropic AFM fluctuations in RbMnF3

• PM to AFM transition in perovskite RbMnF3
• spin system is isolated from the lattice
• Paramagnetic ordering (diffuse scattering) above
  TN

• Isotropic, Lorentzian shape
• Spin-correlation only function of distance

Tucciarone, et al. Phys. Rev. B (1972)
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Comparison of Mn3 and Mn4 in PCSMO45
Scattering from Mn3 and Mn4 have distinct
orbital character F. Ye, et al., PRB (2005)
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Comparison of AFM (Mn3 and Mn4) in PCSMO45

• AFM from Mn3 shows strong hysteresis
• AFM from Mn4 nearly no hysteresis

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Discussion

• Conventional wisdom (Goodenough)
• charge ordering at high temperature ( covalent
  bond energy)
• magnetic ordering at much lower temperature
  (exchange interaction).
• Observations do not support this picture. These
  types of ordering occur at temperatures TCO and
  TN close to each other.
• Region between TCO and TN is quite unique
  (remarkable)
• Charge/orbital and magnetic orderings likely have
  a common origin.
• Hints of low-Dimensional AF (1D, smectic)
  behavior
• Mn3 and Mn4 sites have distinctive orbital
  character
• Observations support van den Brink, Solovyev
  approach CE ordering explained from kinetic
  energy, exchange interactions and orbital
  degeneracy.

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Evolution of CE-type CO-OO and magnetic ordering
TN lt T lt TCO
T lt TN
First observation of quasi long-range 1D spin
chains
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Discussion

• Quasi long range 1D zigzag spin changes are the
  building blocks of the CE-structure
• Consistent with the new soft phase proposed by
  Milward et al (Nature 2005), Dagotto et al
  (Science, 2005)

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General feature in strongly correlated electron
system
S. A. Kivelson, Nature (1998)
charge/spin order in cuprates (Stripes) Tranquada,
Nature (2004)