Despite 30 years of intense study, the explanation behind the zero-resistance current displayed by high-temperature superconductors (HTSCs) is still shrouded in complexity. HTSCs tend to be heterogeneous materials with multiple phases, and disentangling their various electronic behaviors for analysis can be difficult. At the ALS, researchers used resonant soft x-ray diffraction (RSXD), a technique sensitive to both structure and electronic state at the nanoscale, to study layered thin-film heterostructures containing the cuprate high-temperature superconductor, YBa2Cu3O7-x (YBCO). They found a surprising three-dimensional, long-range charge order—the first of its kind ever reported in a cuprate—that competes with superconductivity. A better understanding of such phenomena could help in the design of more robust superconductors with higher transition temperatures.
Many HTSCs, especially members of the cuprate family that includes YBCO, have been found to exhibit multiple ordering tendencies, such as periodic ripples in the electronic charge, which compete with the same electronic states involved in superconductivity. Such orderings of the electronic charge typically manifest within the cuprates’ copper-oxide planes, which interact weakly (if at all) with each other. This two-dimensional charge order is typically a short-range effect, meaning that the order exists only weakly on a very local scale (a few atomic distances). So far, it is not yet known whether these various electronic orders (“supermodulations”) are truly distinct or rather originate from the same underlying mechanism, nor why they manifest in so many forms.
In this work, the researchers studied 30-nm thin films of YBCO grown epitaxially on a sublayer of the ferromagnetic material, La0.7Ca0.3MnO3 (LCMO). Interestingly, such heterostructures have previously been reported to exhibit “long-range proximity effects,” in which the two major competing orders in the system—the superconductivity in the YBCO and the ferromagnetism in the LCMO—tend to supress each other across the interface. RSXD, by combining diffraction (information about structure) with spectroscopy (information about electronic state), is ideally suited to explore spatial charge modulations in thin films and at buried interfaces in nanoscale heterostructures. The experiments were performed at ALS Beamline 4.0.2 at an x-ray scattering endstation specifically built for studies of electronically ordered phases.
The researchers expected their diffraction data to show a rod-shaped peak, indicating the presence of a short-range, two-dimensional supermodulation in the YBCO/LCMO film. Instead they discovered that the diffraction peaks were concentrated into a single pair of sharp points, indicating that the electronic supermodulation found in YBCO/LCMO bilayers exists across much larger distances than previously thought and extends in all directions, in sharp contrast to the traditional two-dimensional order. The correlation length (a measure of the extent of the supermodulation), estimated from the sharpness of the diffraction peaks, is about 42 unit cells in-plane (double the length reported for bulk YBCO) and about 19 unit cells out-of-plane (compared to 1 unit cell in bulk cuprates). Moreover, the periodicity of the supermodulation has both an in-plane component and an out-of-plane component, with the in-plane component repeating every four copper atoms and the out-of-plane component repeating every eight copper-oxide planes.
Discovery of this three-dimensional electronic supermodulation in YBCO/LCMO is a significant advance in our understanding of the mechanisms that underlie superconductivity and competing orders in cuprates. Furthermore, it may also help explain the seemingly paradoxical “long-range proximity effect” observed across several tens of nanometers near YBCO/LCMO interfaces, whereas mutual suppression of superconductivity in YBCO and ferromagnetism in LCMO is expected to extend only about one nanometer. The existence of competing orders in YBCO, stabilized by the LCMO, may help explain the long-range suppression of superconductivity even when ferromagnetism is limited to the interfacial region. Improving our understanding of the phases that compete with and degrade superconductivity helps theorists and experimentalists recognize and focus on the important physical interactions in condensed matter physics that are dominated by “electron correlation” effects.
Contact: Padraic Shafer
Research conducted by: J. He, T.R. Mion, J. Kong, M.J. Graf, and R.-H. He (Boston College); P. Shafer, Y.-D. Chuang, W.L. Yang, and E. Arenholz (ALS); V.T. Tra, J.-Y. Lin, and Y.-H. Chu (National Chiao Tung Univ., Taiwan); and Q. He (Durham Univ., UK).
Research funding: Boston College startup fund, National Science Foundation, Ministry of Science and Technology (R.O.C.), and Ministry of Education (R.O.C.). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this research: J. He, P. Shafer, T.R. Mion, V.T. Tra, Q. He, J. Kong, Y.-D. Chuang, W.L. Yang, M.J. Graf, J.-Y. Lin, Y.-H. Chu, E. Arenholz, and R.-H. He, “Observation of a three-dimensional quasi-long-range electronic supermodulation in YBa2Cu3O7-x/La0.7Ca0.3MnO3 heterostructures,” Nature Communications 7, 10852 (2016).
ALS SCIENCE HIGHIGHT #333