Interplay between electronic and atomic short-range order in La2−xCaxCuO4−δ : Temperature and doping dependence

Complex oxides are well-known to develop oxygen ordering patterns with well-defined periodicities, but their interplay with the electronic structure remains largely unexplored. Here, we report resonant and nonresonant x-ray diffraction data indicating a four-unit-cell periodic superstructure related to oxygen vacancy ordering in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>La</a:mi><a:mrow><a:mn>2</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi>Ca</a:mi><a:mi>x</a:mi></a:msub><a:msub><a:mi>CuO</a:mi><a:mrow><a:mn>4</a:mn><a:mo>−</a:mo><a:mi>δ</a:mi></a:mrow></a:msub></a:mrow></a:math> films with Ca dopant concentrations from the underdoped (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>x</b:mi><b:mo>=</b:mo><b:mn>0.15</b:mn></b:mrow></b:math>) to the extremely overdoped (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>x</c:mi><c:mo>=</c:mo><c:mn>0.50</c:mn></c:mrow></c:math>) regime. Whereas the oxygen superstructure is temperature independent up to 300 K, a strongly temperature-dependent electronic charge density wave (CDW) is observed in the underdoped and slightly overdoped regimes (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>x</d:mi><d:mo>≤</d:mo><d:mn>0.20</d:mn></d:mrow></d:math>). The periodicity, in-plane, and out-of-plane correlation lengths of the CDW are locked in by the oxygen superstructure. Our results highlight the interdependence of electronic and atomic ordering phenomena and underscore the necessity of determining the temperature dependence to distinguish electronic and lattice atomic contributions to superstructures.