Dancers hold hands and move in perfect sync. In an excitonic insulator, electrons and holes form bound pairs that condense into a single quantum state, like dance partners who never miss a step. But what happens when one of them stumbles? Researchers studied a Ta2Pd3Te5 crystal in which individual tellurium atoms are missing: these vacancies are like potholes on the dance floor. Using scanning tunneling microscopy, they saw that each vacancy creates a pair of spectral peaks inside the energy gap. These peaks are the trace of a broken pair: an electron and a hole that can no longer dance together.
The energy of these peaks depends on how the defect is positioned, and it can be smoothly tuned by the electric field from the microscope tip. This is like how the tension of a string changes its pitch: the stronger the field, the higher the energy of the states. Mapping showed that the pair's components are distributed differently in space and are electronically coupled. In regions where electrons and holes are strongly imbalanced, a second pair of states appears with even lower energy, as if the dance breaks twice, in two different ways.
These observations are the first atomic-scale 'fingerprint' of local destruction of excitonic pairing. Understanding how pairs break is critical for controlling quantum phases of matter and, potentially, for future devices based on excitonic condensates.
The excitonic insulator was predicted theoretically back in the 1960s, but only in recent years has it been observed experimentally thanks to advances in scanning tunneling microscopy.