Understanding why and how certain materials are high-temperature superconductors will allow for novel, efficient electrical devices that conduct with zero resistance at practical temperatures. One such material is iron selenide (FeSe), an archetypical member of the iron-based superconductor family. Interestingly, single-atom-thick layers of the material synthesized on $\mathrm{SrTi}{\mathrm{O}}_3$ have been found to exhibit spectroscopic signatures of superconductivity at much higher temperatures than bulk FeSe. A better understanding of the nature and origin of this enhanced superconducting state holds potential for opening up a new frontier for high-temperature superconductivity through engineering atomic interfaces. To achieve this, we employ, for the first time, a combination of angle-resolved photoemission spectroscopy and in situ resistivity measurements to simultaneously probe both the electronic states and superconducting behavior of pristine monolayer $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3$.

Our experiments reveal a striking dichotomy between the spectroscopic and transport properties of monolayer $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3$. While spectroscopic measurements indicate the initial formation of a superconducting gap at temperatures as high as 70 K, a true zero-resistance state is not achieved until below 30 K. We show that this discrepancy is due to an unprecedentedly large “pseudogap regime”—a suppression in the density of states extending above the superconducting critical temperature—not previously observed in iron-based superconductors, but arising here from the intrinsic 2D nature of the system.

This work not only clarifies many of the mysteries and apparent inconsistencies surrounding monolayer $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3$ but also provides insights into the important role of reduced dimensionality in driving the unique behavior of interfacial high-temperature superconductors.