Electronically soft phases in manganites
G. Millward, M.J. Calderon, and P.B. Littlewood,
Nature 433, 607 (2005). cond-mat/0407727.
The phenomenon of colossal magnetoresistance in manganites
is generally agreed to be a result of competition between crystal
phases with different electronic, magnetic and structural order; a
competition which can be strong enough to cause phase separation
between metallic ferromagnetic and insulating chargemodulated
states. Nevertheless, closer inspection of phase
diagrams in many manganites reveals complex phases where
the two order parameters of magnetism and charge modulation
unexpectedly coexist. Here we show that such experiments can
be naturally explained within a phenomenological Ginzburg–
Landau theory. In contrast to models where phase separation
originates from disorder or as a strain-induced kinetic phenomenon,
we argue that magnetic and charge modulation coexist in
new thermodynamic phases. This leads to a rich diagram of
equilibrium phases, qualitatively similar to those seen experimentally.
The success of this model argues for a fundamental
reinterpretation of the nature of charge modulation in these
materials, from a localized to a more extended ‘charge-density
wave’ picture. The same symmetry considerations that favour
textured coexistence of charge and magnetic order may apply to
many electronic systems with competing phases. The resulting ‘electronically soft’ phases of matter with incommensurate,
inhomogeneous and mixed order may be general phenomena
in correlated systems.
Limited local electron-lattice coupling in manganites.
D. Sánchez, M.J. Calderón, J. Sánchez-Benítez, A.J. Williams, J.P. Attfield, P.A. Midgley and
N.D. Mathur.
Phys. Rev. B 77, 092411 (2008). arXiv:0801.4626.
(Pr,Ca)MnO3 is the archetypal charge-ordered manganite, but in Pr0.48Ca0.52MnO3 we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice, and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.
Strain control of superlattice implies weak charge-lattice coupling in La0.5Ca0.5MnO3.
S. Cox, E. Rosten, J.C. Loudon, J.C. Chapman, S. Kos, M.J. Calderón, D.J. Kang, P.B. Littlewood, P.A. Midgley, and N.D. Mathur,
Phys. Rev. B 73, 132401 (2006).
cond-mat/0504476.
We have recently argued that manganites do not possess stripes of charge order, implying that the electron-lattice
coupling is weak [Loudon et al., Phys. Rev. Lett. 94, 097202 (2005)]. Here we independently argue the
same conclusion based on transmission electron microscopy measurements of a nanopatterned epitaxial film of
La0.5Ca0.5MnO3. In strain relaxed regions, the superlattice period is modified by 2% to 3% with respect to the
parent lattice, suggesting that the two are not strongly tied.
Strain selection of charge and orbital ordering patterns in half-doped manganites.
M.J. Calderon, A.J. Millis and K.H. Ahn,
Phys. Rev. B 96, R100401 (2003). cond-mat/0305440.
Theoretical and computational results are presented clarifying the role of long-ranged strain interactions in determining the charge and orbital ordering in colossal magnetoresistance manganites. The strain energy contribution is found to be of order 20–30 meV/Mn and in particular stabilizes the anomalous “zigzag chain” order observed in many half-doped manganites.
