{'fraction_non_alphanumeric': 0.025934065934065935, 'fraction_numerical': 0.017582417582417582, 'mean_word_length': 5.201634877384196, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Observing a previously hidden structural-phase transition onset through heteroepitaxial cap response

Significance We provide an approach to study structural transitions by using appropriate heteroepitaxial caps. Previously unobservable phase transitions that involve small changes in lattice constants, but may have important impact on the physical properties of the material, can now be detected in real space. Using the coherent nanotwinning waves of epitaxial La0.7Sr0.3MnO3/SrTiO3 thin films as a sensitive detector, we are able to uncover the onset stage of the cubic-to-tetragonal structural-phase transition of SrTiO3 in real space. Researchers involved in the study of material properties as well as technologists in a variety of industries, such as electronics, will greatly benefit from this approach. Characterization of the onset of a phase transition is often challenging due to the fluctuations of the correlation length scales of the order parameters. This is especially true for second-order structural-phase transition due to minute changes involved in the relevant lattice constants. A classic example is the cubic-to-tetragonal second-order phase transition in SrTiO3 (STO), which is so subtle that it is still unresolved. Here, we demonstrate an approach to resolve this issue by epitaxially grown rhombohedral La0.7Sr0.3MnO3 (LSMO) thin films on the cubic STO (100) substrate. The shear strain induced nanotwinning waves in the LSMO film are extremely sensitive to the cubic-to-tetragonal structural-phase transitions of the STO substrate. Upon cooling from room temperature, the development of the nanotwinning waves is spatially inhomogeneous. Untwinned, atomically flat domains, ranging in size from 100 to 300 nm, start to appear randomly in the twinned phase between 265 and 175 K. At ∼139 K, the untwinned, atomically flat domains start to grow rapidly into micrometer scale and finally become dominant at ∼108 K. These results indicate that the low-temperature tetragonal precursor phase of STO has already nucleated at 265 K, significantly higher than the critical temperature of STO (∼105 K). Our work paves a pathway to visualize the onset stages of structural-phase transitions that are too subtle to be observed using direct-imaging methods.