Correlated Plasmons in La1.85Sr0.15CuO4 Enable New Pathways to High-Temperature Superconductivity

High-temperature superconductivity in complex materials continues to challenge physicists, and understanding the underlying mechanisms remains a central goal. Xiongfang Liu, from Shanghai, alongside H. Breese from the Singapore Synchrotron Light Source, National University of Singapore, and Shengwei Zeng, now demonstrate a crucial link between interfacial strain and the emergence of correlated plasmons in the superconducting material La1.85Sr0.15CuO4. Their research reveals these unique plasmons, absent in non-superconducting versions of the material, arise from collective electronic behaviour within the copper-oxide planes, and are strongly influenced by strain at the material’s interface. This discovery, supported by theoretical calculations, suggests that long-range electronic correlations, finely tuned by interfacial strain, may be fundamental to achieving and controlling high-temperature superconductivity, offering new avenues for materials design and optimisation.

Tunable Plasmons and High-Tc Superconductivity

Investigating Plasmons and Superconducting Cuprate Materials

This research investigates the connection between collective electronic excitations, known as plasmons, and superconductivity in high-temperature cuprate superconductors, specifically lanthanum strontium copper oxide. The study proposes that these plasmons, significantly modified by strong interactions between electrons within the material, play a vital role in enabling superconductivity and can be tuned by external factors like strain and doping, potentially offering a pathway to enhance superconducting properties. Researchers combined optical spectroscopy with sophisticated theoretical calculations to understand the underlying electronic structure and electron correlations. The research reveals that tuned plasmons can enhance the coupling between electrons and vibrations, a key mechanism believed to drive superconductivity, facilitating attractive interactions between electrons and encouraging them to pair up and conduct electricity without resistance. Observations show mid-infrared plasmons connected to the copper-oxygen planes, the superconducting component of the material, and a transfer of spectral weight indicating increased electron correlations and collective excitations, highlighting the importance of understanding the interplay between these factors.

Strain Tuning Reveals Correlated Plasmons in Superconductors

Scientists investigated high-temperature superconductivity in lanthanum strontium copper oxide by meticulously controlling interfacial strain, a powerful method for tuning electronic interactions. The study pioneered a technique using spectroscopic ellipsometry to observe correlated plasmons exclusively within superconducting films, demonstrating their absence in non-superconducting counterparts. Detailed analysis revealed these plasmons originate from collective excitations within bands influenced by strong electron correlations, driven by long-range electronic interactions in the copper-oxygen planes. To model these interactions, researchers employed advanced computational methods combining dynamical cluster approximation and Monte Carlo calculations, incorporating both nearest and next-nearest neighbor Coulomb interactions. By extracting the local density of states, the team directly compared theoretical predictions with experimental data, finding that incorporating longer-range interactions regularized the calculations and aligned with observed spectral features, confirming that an optimal balance of interactions governs long-range electronic correlations and impacts the superconducting transition temperature.

Discovery of Novel Correlated Plasmons in Superconducting Films

Correlated Plasmons Emerge in Superconducting Films

Scientists have discovered a novel form of correlated plasmons exclusively within superconducting lanthanum strontium cuprate films. These plasmons, absent in non-superconducting counterparts, originate from collective excitations within bands influenced by strong electron correlations and are driven by long-range electronic interactions in the copper-oxygen planes. Detailed optical measurements and analyses reveal that these unique plasmonic states and anomalous optical signals are attributable to shifts and modifications to the electronic structure mediated by high-energy electronic excitations, influencing long-range electronic correlations. Experiments demonstrate a strong link between interfacial strain and the emergence of these correlated plasmons, suggesting that strain plays a pivotal role in regulating the superconducting transition temperature. Researchers found that varying the substrate upon which the films are grown modifies the interfacial strain and, consequently, the superconducting properties, confirming that interfacial strain regulates the transition temperature by tuning the long-range electronic correlations within the copper-oxygen planes.

Understanding Superconductivity Through Electronic Excitation Pathways

Correlated Plasmons Reveal Superconductivity’s Electronic Origins

This research demonstrates the discovery of correlated plasmons within the superconducting material lanthanum strontium cuprate, a finding exclusive to its superconducting state. These plasmons, originating from collective electronic excitations within the material, are driven by long-range electronic interactions present in the copper-oxygen planes and are demonstrably influenced by interfacial strain, playing a crucial role in the emergence of superconductivity and potentially in tuning its critical temperature. Supporting these experimental findings, theoretical calculations employing advanced computational methods confirm the importance of long-range Coulomb interactions within the material. These calculations show that incorporating longer-range interactions into models of the material leads to a more accurate representation of its electronic behaviour, specifically rectifying asymmetries in the calculated density of states, suggesting an optimal balance of interactions exists, governing the strength of these long-range correlations and their impact on the material’s properties.

The observed collective electronic excitations, or plasmons, suggest a breakdown of the simple picture of electron-pairing mediated solely by lattice vibrations (phonons). Instead, the strong coupling between the plasmon mode and the oxygen bond-stretching phonon modes implies a complex, coupled excitation continuum. Understanding the precise mixing ratio between these two types of excitations is critical, as it allows physicists to delineate whether the attractive pairing interaction is primarily electronic (plasmons-mediated) or structural (phonon-mediated), thereby refining theories beyond conventional BCS models.

From a materials science perspective, the ability to manipulate these plasmons via interfacial strain opens a pathway for epitaxially grown thin films. By precisely engineering the lattice mismatch between the superconducting film and the substrate, researchers can apply controlled biaxial strain. This method offers a means to fine-tune the orbital overlap within the copper-oxide planes, potentially stabilizing desirable electronic states that are metastable or difficult to achieve through bulk chemical doping alone.

Further theoretical advancement requires integrating Density Functional Theory (DFT) calculations with advanced correlation methods, such as Dynamical Mean Field Theory (DMFT). While current models effectively describe the low-energy plasmon features, a full quantitative description of the pairing mechanism necessitates incorporating strong on-site Coulomb repulsion ($U$) and the proximity to Mott insulating states. Bridging the gap between these highly correlated electronic models and measurable optical spectra remains a formidable computational challenge.