Lianzhi Yang | Searcharxiv

arXiv2026

Spectral Visualization of Excitonic Pair Breaking at Individual Impurities in Ta2Pd3Te5

Excitonic insulators host the condensates of bound electron-hole pairs, offering a platform for studying correlated bosonic quantum states. Yet, how macroscopic coherence emerges from locally collapsed pairing remains elusive. Here, using scanning tunnelling spectroscopy, we report the impurity-induced pair breaking in an excitonic insulator Ta2Pd3Te5. Individual Te vacancies are found to generate a pair of spectral peaks within the excitonic gap. Their energies depend sensitively on the defect configurations and are continuously tunable by tip electric field, indicating controllable impurity scatterings. Spectral mapping shows spatially anisotropic and electronically coupled electron-hole components of the subgap states. These observations, together with mean-field modelling, suggest an excitonic pair-breaking origin. In the strongly electron-hole imbalanced region, a secondary pair-breaking effect, manifesting as an additional pair of subgap states with distinctly lower energies, can emerge, presenting the interplay of pairing breakings with different excitonic order parameters. Our findings demonstrate the spectroscopic 'fingerprint' of local excitonic depairing at the atomic level, offering a crucial clue to the critical behavior across excitonic condensation.

cond-mat.str-el↗

arXiv2024

Visualizing uniform lattice-scale pair density wave in single-layer FeSe/SrTiO3 films

Typical BCS superconductors are microscopically homogeneous in real space governed by the coherent Cooper pairs with high phase stiffness of superfluid density, which is characterized by a coherence length. However, a periodic oscillation of superconducting order parameter may develop driven by breaking the time-reversal or translational invariance. To date, such modulated orders were specific to each material systems, with a periodicity much larger than the lattice constant. Here we report the direct observation of a uniform lattice-scale pair density wave (PDW) in single-layer FeSe/SrTiO3 films, enforced by peculiar interfacial structure of crystal symmetries breaking. Our spectroscopic imaging scanning tunneling microscopy unravels a spatial modulation of Cooper-pairing gap within a single unit-cell, depending on inequivalent atomic sites. Prominent periodic variation of superfluid density is visualized via Josephson current by a superconducting tip, indicating a real-space oscillation of phase stiffness. Such a lattice-scale superconducting modulation, which coexists with a larger length scale of PDW order, indicates the lattice-scale variation of both pairing strength and phase stiffness. Our findings provide new insights into the intertwined density-wave orders of quasiparticle character in correlated electronic systems, and provoke future studies on the unconventional pairing interaction and phase stiffness in the two-dimensional limit.

cond-mat.supr-con↗