Gervasi Herranz

The origin of superconductivity in oxide interfaces and its relation to ferroelectricity remains an open question. At LaAlO₃/SrTiO₃ interfaces, quantum confinement and inversion symmetry breaking create a two-dimensional electron gas near a ferroelectric quantum critical point, yet direct evidence linking phonon dynamics to electron pairing has been lacking. Here we directly probe lattice vibrations and atomic structure at LaAlO₃/SrTiO₃ interfaces across the superconducting phase diagram using vibrational spectroscopy with momentum selectivity in a scanning transmission electron microscope. We find that superconductivity across the doping series correlates with inversion symmetry breaking and the appearance of high-frequency localized phonons. These tunable, polar vibrations-confined near the interface-exhibit strong electron-phonon coupling and evolve systematically with carrier density. Our findings establish a link between lattice instability, superconductivity and strong electron-phonon coupling mediated by tunable localized phonons, providing new insights into possible microscopic pairing pathways in quantum paraelectric systems.

Topological photonics offers a robust platform for controlling light, with applications such as backscattering‐immune edge‐transport and slow‐light propagation. A comprehensive and automated framework is presented for the design and characterization of symmetry‐protected interface modes in 2D photonic crystals. The main tool in this approach is an iterative band‐connection algorithm that ensures symmetry consistency across the Brillouin zone, enabling reliable reconstruction of bands even near degeneracies. Complementing this, a data‐driven symmetry classification method is introduced that constructs comparator functions directly from eigenmode data, removing the need for predefined symmetry operations or irreducible representations. These tools are particularly suited for generative or parametrized geometries where symmetries may vary. Using this framework, example structures exhibiting obstructed atomic limits, characterized by Wannier center displacements and mode inversions, are identified. The tradeoffs between interface mode dispersion and bulk bandgap size are analyzed, and how the number of photonic crystal periods at the interface governs the emergence and robustness of topological modes is shown. Finally, the scalability of this approach across material platforms and operating wavelengths, including the telecommunication range, is demonstrated. These contributions enable physically grounded and fully automated design of topological photonic interfaces, paving the way for large‐scale exploration and optimization of complex photonic structures.

Recent developments in photonic topological phases have revealed that protected edge modes can emerge not only from global topological invariants, but also from symmetry-enforced polarization mismatches between distinct bulk phases. In this work, we investigate the capabilities and limitations of a square-lattice (C ₄-symmetric) photonic crystal (PC) composed of a single dielectric material that supports interface-localized modes at the boundary between regions characterized by distinct obstructed atomic limits (OALs). These modes are confined to a common band gap and exhibit high transmission, even in the presence of structural perturbations. Our analysis reveals that the interface modes are stabilized by a mismatch in the position of Wannier centers between the two adjoining crystals. We demonstrate nearly lossless transmission around sharp turns and through localized defects, though the robustness depends asymmetrically on the side of perturbation, reflecting the partial nature of the protection. We also show that increasing the number of PC periods surrounding the interface enhances both modal confinement and spectral stability. These findings establish polarization mismatch between OALs as a practical and fabrication-compatible mechanism for engineering robust photonic transport in C ₄-symmetric systems.

The observation of unconventional superconductivity in oxide quantum paraelectrics–materials exhibiting incipient ferroelectricity at low temperatures– raises fundamental questions about the role of ferroelectricity in the pairing mechanism. In oxide interfaces, such as LaAlO3/SrTiO3 (LAO/STO), quantum confinement and inversion symmetry breaking give rise to intriguing electronic phases, with charge doping driving the system toward a ferroelectric quantum critical point. However, direct experimental evidence of coupling between confined electrons and active phonon modes in a polar lattice has remained elusive. Here, we use momentum-selective vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) to simultaneously probe the atomic structure and phonon spectra across a series of LAO/STO interfaces with varying carrier concentrations and superconducting behavior. We find that the emergence of superconductivity correlates with interfacial lattice polarization and the appearance of new high-frequency, localized phonon modes. These modes, primarily involving in-plane and out-of-plane vibrations of equatorial and apical oxygen atoms in the TiO2 and SrO layers, respectively, are highly sensitive to carrier concentration. Our results provide compelling evidence for the coexistence of inversion symmetry breaking, superconductivity in the dilute limit, and strong electron-phonon coupling mediated by localized phonons, offering new insights into the microscopic pairing mechanism in quantum paraelectric systems.

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO 2 . A combination of oxide molecular beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to a possibly sign-changing order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity.

Topological photonics provides a robust and flexible platform for controlling light, enabling functionalities such as backscattering-immune edge transport and slow-light propagation. In this work, we design and characterize photonic topological interfaces in two-dimensional photonic crystals. We introduce an iterative band connection algorithm that preserves mode symmetry and present a general framework for band symmetry recognition, essential for identifying Z2 topological phases. Design strategies for unit cell geometries are developed to achieve targeted band inversions, overlapping bandgaps, and tailored dispersions. Furthermore, the approach can be readily adapted to specific material platforms and operating wavelengths, including the telecommunication range, by appropriately scaling the lattice parameter as long as absorption remains low. We investigate the trade-off between bandgap size and band flatness, identifying exceptions governed by lattice geometry. Additionally, we demonstrate how photonic crystal periodicity influences the stability of topological modes and enhances unidirectional energy transport.

Spin-orbit entanglement in 4d and 5d transition metal systems can enhance electronic correlations, leading to nontrivial ground states and the emergence of exotic excitations. There is also an interest to investigate spin-orbit entanglement in 3d compounds, though this is challenging due to their smaller spin-orbit coupling. Here we demonstrate that the Jahn-Teller effect in Mn³⁺ reduces the energy gap between high- and low- spin-orbital states that lead to enhanced spin-orbit entanglement. Our results show a rare example of synergistic effects of Jahn-Teller and spin-orbit interactions and provide a way to entangle different degrees of freedom in d-metal oxides, which may allow paths to explore the interplay of orbital, lattice and spins in 3d correlated systems.

SrTiO₃-based conducting interfaces, which exhibit coexistence of gate-tunable 2D superconductivity and strong Rashba spin-orbit coupling (RSOC), are candidates to host topological superconductive phases. Yet, superconductivity is usually in the dirty limit, which tends to suppress nonconventional pairing and therefore challenges these expectations. Here we report on LaAlO₃/SrTiO₃ (LAO/STO) interfaces with large mobility and mean free paths comparable to the superconducting coherence length, approaching the clean limit for superconductivity. We further show that the carrier density, mobility, and formation of the superconducting condensate are controlled by the fine-tuning of La/Al chemical ratio in the LAO film. We find a region in the superconducting phase diagram where the critical temperature is not suppressed below the Lifshitz transition, at odds with previous experimental investigations. These findings point out the relevance of achieving a clean-limit regime to enhance the observation of unconventional pairing mechanisms in these systems.