Abstract
The ground-state electronic order in doped manganites is frequently associated with a lattice modulation, contributing to their many interesting properties. However, measuring the thermal evolution of the lattice superstructure with reciprocal-space probes alone can lead to ambiguous results with competing interpretations. Here we provide direct observations of the evolution of the superstructure in La1/3Ca2/3MnO3 in real space, as well as reciprocal space, using transmission electron microscopic (TEM) techniques. We show that the transitions are the consequence of a proliferation of dislocations plus electronic phase separation. The resulting states are well described by the symmetries associated with electronic-liquid-crystal (ELC) phases. Moreover, our results resolve the long-standing controversy over the origin of the incommensurate superstructure and suggest a new structural model that is consistent with recent theoretical calculations.
Highlights
The modulation is a consequence of charge-density-wave or orbital ordering[13,14,15,16,17]
La1/3Ca2/3MnO3 is known to have a unidirectional superstructure aligned along the a-axis at low temperatures[3,4,5]
With distinct structures along the a and c axes compared to the fundamental lattice, the superstructure breaks the point group rotational symmetry, as well
Summary
La1/3Ca2/3MnO3 is known to have a unidirectional superstructure aligned along the a-axis at low temperatures[3,4,5]. When the disordered patches are small, the shift in the average charge density should be small, but when they proliferate, the increased charge density in the regions with the superstructure order should become noticeable, resulting in an increase in xeff, qualitatively consistent with the observed change in q (Fig. 1b) This “charge rich” and “charge poor” electronic phase separation scenario was theoretically proposed by previous work[37], but is lacking of direct experimental observations to confirm. It is very interesting that the results from the END and EELS analyses, which measure distinct local properties, are quantitatively consistent concerning the size, temperature dependence and local charge deviation from nominal doping level, revealing a scenario of electronic phase separation at the nanoscale (see supplemental material for more details) during the ELC phase transitions in La1/3Ca2/3MnO3. A similar situation, involving the growth of nematic order in a crystal with pre-existing broken rotation symmetry is known to occur in YBa2Cu3O6+x21,22,41–44
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