Charge Ordering in Transition Metal Oxides

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Charge Ordering in Transition Metal Oxides
In the name of God
Mohammad E. Ghazi Shahrood University Of
Technology, Iran
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Oxides display a range of different structural,
electronic and magnetic phases due to electron
correlations
superconductivity in copper oxides
La2-xSrxCuO4 stripe phases in nickel oxides
La2-xSrxNiO4 La2NiO4? colossal
magnetoresistance in manganese oxides
La1-xCaxMnO3, Nd1-xSrxMnO3
A common theme of charge, spin and orbital
ordering
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La2-xSrxCuO4 and La2-xSrxNiO4

• The parent compounds of La2CuO4 and La2NiO4 are
  an AFM insulator

• Doping with Sr and creating hole
• In cuprites gt5 causes the formation of a
  superconducting phase.

• La2-xSrxNiO4 does not exhibit superconductivity
  for any value of x. But displays charge ordering
  (periodic ordering of doped holes at low
  temperature).
• Attraction To understand the possible
  relationship between charge ordering and
  superconductivity

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The properties of La2-xSrxNiO4

• Resistivity and susceptibility measurements show
  that x 1/3 is special.
• However for x 1/3 there are no structural phase
  transitions at low temperatures.
• But strong effect of charge ordering on physical
  properties

Temperature derivative of logarithmic resistivity
(upper panel) and susceptibility multiplied by
temperature (lower panel) versus temperature for
Sr concentrations of 0.2 ?x ?0.4 Phys. Rev. B
49,7088,1994.
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Properties of La5/3Sr1/3NiO4

• Charge ordering at TCO ? 240 K and ordering of
  the Nickel spins at TSO ? 190 K.
• The stripes run along diagonal directions in
  NiO2 planes
• The transition into the charge striped phase is
  entirely second order.

• Anomalies associated with charge and spin
  ordering in these materials have also been
  observed in sound velocity, specific heat,
  resistivity, and magnetic susceptibility.
• Stripe correlations are maximised at x 1/3 and
  x 1/2.

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Mechanisms of Stripe Ordering
Polaron ordering due to the large magnetic
coupling between Ni and hole spin with large
electron phonon coupling induces
self-localisation of holes (a small polaron)
Competition between long-range Coulombic
repulsion of doped holes and short-range
attractive interactions such as electron-lattice
coupling and magnetic confinement effect, etc.
Frustrated phase separation The holes minimise
their kinetic energy by attempting to separate
into hole-rich regions and regions with local
antiferromagnetic correlations
Leads to
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Experimental Probes of Charge Stripes
Electron Diffraction Sensitive to small
charge/lattice distortions, but low resolution
and intensities difficult to interpret

• Neutron Diffraction
• Used for study of phonons , spin ordering etc.
  but low intensity, low resolution and needs large
  samples.

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X-rays as a probe of charge stripes
Higher wavevector resolution, Higher
intensities, and can use smaller crystal (better
quality)
Mainly using synchrotron radiation at BM28,
ESRF, and SRS, in the UK at Daresbury .
Experiments involve cryostat, triple crystal
geometry with high-resolution analyser crystals.
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Results and Discussion 1 Intensity
New, weak satellites found at low temperatures
surrounding Bragg reflections at non-integer
positions such as (4.66, 0,5), (5.33, 0, 7).
These disappear above 240 K. All indexed as q
(h2?, 0, l) with h even and l odd. ? 0..33
(i.e. Sr stoichiometry).
Intensity of charge stripe reflections close to
the melting point fit the power law
TCO 239.2 0.2 2? 0.23 0.02
Exponent indicates 2D nature of charge stripes
and critical scattering above TCO.
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Results and Discussion 2 Width

• The width of the charge stripe satellites gives
  us a measure of the inverse correlation length.
  Above TCO the width increases dramatically
  because of critical scattering. Fit to the power
  law

TCO 239.2 ?H,K 1.080.2 ?L 1.005 0.03
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Results and Discussion 3 Order, Disorder Liquid

• Above the melting point (240 K) scattering
  observed due to fluctuations into the charge
  stripe phase.

• Below the melting temperature displays an
  anisotropic broadening suggestive of an
  order-disorder transition.

• At low temperatures the stripes are correlated,
  but this does not improve as the temperature is
  decreased, suggesting quenched and partially
  ordered stripes.

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What happens away from x 1/3?

• The width (wavevector) of the stripes is both
  stoichiometry and temperature dependant.
• The deviation away from nh ? starts below x
  1/3 and increases as x decreases.
• The density of the holes within an average stripe
  is nh/ ?.. So the variation in ? is inversely
  related to the hole density (nh is a constant).
• The variation of ? occurs principally at higher
  temperatures and ceases at low temperatures.

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Manganese Oxides, Nd0.5 Sr0.5 MnO3
Attractions CMR, charge-, spin-and orbital
ordering due to the presence of the multi valence
Mn (Mn3, Mn4) ion. Mn 3 (d4)has an
electronic configuration with 3 electrons in the
lower t2g band and the outer electron in the
degenerate eg orbital. Different phases include
ferromagnetic metal, antiferromagnetic
insulators, canted antiferromagnetic etc.
Spin and orbital ordering is common over a wide
range of stoichiometry but charge ordering exists
only over a narrow range around Nd½Sr½MnO3.
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Manganese Oxides, Nd0.5 Sr0.5 MnO3
Nd0.5Sr05MnO3 is a ferromagnetic metal with a TC
of 250 K and transforms to an insulating CO
state around 160 K. The CO transition is first
order.
The FMM CO transition is accompanied by spin
and orbital ordering, and the CO insulator is
antiferromagnetic (CE type).
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Manganese Oxides, Nd0.5 Sr0.5 MnO3
Below TCO160 K,, (Periodic ordering of Mn3 and
Mn4 ions at low temperature) a structural
modulation arises from the Jahn-Teller (J-T)
distortion associated with charge and orbital
ordering. This causes the neighbouring
undistorted Mn4O6 octahedra to displace in
opposite directions and a doubling of the unit
cell along the a-axis .
We report X-ray scattering measurements of J-T
distortion ordering satellite in the low
temperature phase . These peaks had a wavevector
(1/2, 0, 0) and an intensity of approximately
10-3 that of the Bragg reflections.
The projection of Nd0.5Sr0.5MnO3 superstructure
in the ac-plane at low temperature. Arrows show
the displacements of the Mn4O6 octahedra P. G.
Radaelli et al..
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First-order structural phase transition in
Nd½Sr½MnO3
Measurement of Bragg reflections show transition
from ferromagnetic metallic phase into low
temperature charge ordered phase. The structural
transition is first order and displays
considerable hysteresis.
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Melting of charge order satellites with
temperature

• Inverse width (correlation length) of charge
  order satellites also displays a dramatic
  collapse at the structural transition.

Intensity of charge order satellites as a
function of temperature displays large
hysteresis, typical of first-order structural
phase transition
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Charge, Spin and Orbital Ordering in Transition
Metal Oxides

• Different types of charge, spin or orbital
  ordering have been found in a large number of
  different transition metal oxides.
• Iron oxides, Fe3O4, La1-xSrxFeO3, LuFe2O4
• Cobalt Oxides, YBa2Co2O5,
• La2-xSrxCoO4,
• Vanadium Oxides, V2O3, NaV2O5,

In nearly all cases, such ordering causes
dramatic changes in physical properties,
metal-insulator transitions, magnetic and
electrical properties etc. Charge and spin
ordering is found to exist in some high-TC oxide
superconductors. The understanding of charge,
spin and orbital ordering is a new, and very
lively, area of research.
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Acknowledgements and collaborators

• Prof. Peter Hatton (Durham, UK)
• Dr. S. Wilkins, and Dr. P. Spencer
• Prof. S-W. Cheong (Rutgers, USA)
• Dr. A. Boothroyd (Oxford)
• Dr. S. Brown ( XMaS, ESRF, Grenoble)
• Dr. S. Collins (16.3 SRS, UK)

And Thank you for your attention