UCAS Researchers Reveal the Role of Localized Phonons in Oxide Interface Superconductivity
Recently, A research team led by Professor Wu Zhou from the School of Physical Sciences at the University of Chinese Academy of Sciences (UCAS), in collaboration with scientists from the Institute of Materials Science of Barcelona (ICMAB-CSIC) and Nanomaterials and Nanotechnology Research Center (CINN-CSIC) in Spain, has directly observed localized polar phonons at superconducting LaAlO3/SrTiO3 (LAO/STO) interfaces with atomic resolution. Their work establishes a direct link between inversion symmetry breaking, localized electron–phonon coupling, and interfacial superconductivity, providing new experimental insight into the microscopic pairing mechanism of oxide interface superconductors.
This study, entitled "Electron-phonon coupling and symmetry breaking in superconducting oxide interfaces near ferroelectric quantum criticality," was published online in Nature Materials on June 30, 2026.
Superconductivity, the complete disappearance of electrical resistance, is one of the most remarkable quantum phenomena in condensed matter physics. In conventional superconductors, electrons form Cooper pairs through interactions mediated by lattice vibrations, or phonons. Whether a similar mechanism governs superconductivity in dilute oxide superconductors has remained an open question for decades, largely because the relevant phonons have never been directly observed at the atomic scale.
The LaAlO3/SrTiO3 (LAO/STO) interface has become one of the most important model systems for exploring this question. Although both LAO and STO are insulating materials, their interface hosts a two-dimensional electron gas only a few nanometers thick that becomes superconducting at low temperatures. Combining quantum confinement, inversion symmetry breaking, and proximity to a ferroelectric quantum critical point, this interface provides an ideal platform for investigating unconventional superconductivity and electron–phonon interactions. However, directly probing lattice vibrations within such an ultrathin superconducting layer has long been beyond the reach of conventional experimental techniques.
To overcome this challenge, the research team developed and applied an atomic-resolution momentum-selective vibrational electron energy-loss spectroscopy (momentum-selective vibrational EELS) technique. By combining sub-angstrom spatial resolution with millielectronvolt energy resolution and momentum selectivity, the method enables direct measurement of lattice vibrations with different momentum-transfer directions while simultaneously mapping the local atomic structure, electronic states, and lattice dynamics at buried interfaces.
The researchers systematically tuned the carrier density of LAO/STO interfaces by controlling the La/Al stoichiometry during film growth, spanning the phase diagram from the non-superconducting regime to optimal superconductivity. They found that increasing carrier density progressively broadens the interfacial two-dimensional electron gas, enhances lattice polarity, and ultimately induces inversion symmetry breaking near the interface. These structural changes closely track the evolution of superconductivity.
Most importantly, the team directly observed high-frequency localized polar phonons confined within approximately 1.5 nanometers of the interface. Unlike bulk phonons, these localized vibrational modes emerge only at the interface as a consequence of electronic reconstruction and primarily involve localized vibrations of oxygen atoms. The experiments further revealed that phonons vibrating parallel and perpendicular to the interface evolve in opposite directions as carrier density increases, providing direct evidence for highly anisotropic electron–phonon coupling. First-principles calculations confirmed that these localized polar phonons strongly couple to electrons in the two-dimensional electron gas and that the coupling strength closely follows the superconducting phase diagram.
Figure 1 | Atomic-resolution momentum-selective vibrational EELS reveals the evolution of phonon modes and strong electron–phonon coupling mediated by localized polar phonons at the LAO/STO interface.
Together, these findings establish, for the first time, an atomic-scale connection between interfacial polar lattice distortions, inversion symmetry breaking, localized polar phonons, strong electron–phonon coupling, and interfacial superconductivity. Beyond providing new insight into superconducting pairing in quantum paraelectric systems, the work also demonstrates the unique capability of momentum-selective vibrational EELS for probing local lattice dynamics at buried interfaces. The technique is expected to find broad applications in oxide heterostructures, interfacial superconductors, polar metals, and other low-dimensional quantum materials.
Dr. Roger Guzman, formerly a research scientist at UCAS and now a tenured scientist at the Institute of Materials Science of Barcelona, is the first author of the paper. Dr. Miguel Pruneda (CINN-CSIC) led the theoretical calculations. Dr. Mingquan Xu (now an Associate Professor at Hunan University) and Dr. Aowen Li (now a postdoctoral researcher at the University of Tokyo) co-developed the momentum-selective vibrational EELS methodology and data analysis framework, with additional contributions from UCAS Ph.D. student Ang Li. The corresponding authors are Prof. Wu Zhou (UCAS, School of Physical Sciences), Dr. Roger Guzman (UCAS), Dr. Gervasi Herranz (ICMAB-CSIC), and Dr. Miguel Pruneda (ICN2).
This research was supported by the Beijing Outstanding Young Scientist Program, the National Natural Science Foundation of China, the CAS Youth Team Program for Basic Research, and the Electron Microscopy Center at the University of Chinese Academy of Sciences.
Link to the research article: https://doi.org/10.1038/s41563-026-02647-x
Link to Prof. Wu Zhou’s group: https://zhouwu.ucas.ac.cn/
