Electrons and holes can spontaneously form excitons and condense in a semimetal or semiconductor, as predicted decades ago. This type of Bose condensation can happen at much higher temperatures in comparison with dilute atomic gases. Two-dimensional (2D) materials with reduced Coulomb screening around the Fermi level are promising for realizing such a system. Here we report a change in the band structure accompanied by a phase transition at about 180 K in single-layer ZrTe₂ based on angle-resolved photoemission spectroscopy (ARPES) measurements. Below the transition temperature, gap opening and development of an ultra-flat band top around the zone center are observed. This gap and the phase transition are rapidly suppressed with extra carrier densities introduced by adding more layers or dopants on the surface. The results suggest the formation of an excitonic insulating ground state in single-layer ZrTe₂, and the findings are rationalized by first-principles calculations and a self-consistent mean-field theory. Our study provides evidence for exciton condensation in a 2D semimetal and demonstrates strong dimensionality effects on the formation of intrinsic bound electron–hole pairs in solids.

正如几十年前预测的那样，电子和空穴可以自发地形成激子并凝聚在半金属或半导体中。与稀原子气体相比，这种玻色凝聚可以在更高的温度下发生。在费米能级附近减少库仑屏蔽的二维 (2D) 材料有望实现这样的系统。在这里，我们报告了单层 ZrTe _{2中约 180 K 处的能带结构变化伴随相变}基于角分辨光电子能谱 (ARPES) 测量。在转变温度以下，观察到间隙打开和在区域中心周围形成超平带顶。通过在表面添加更多层或掺杂剂引入额外的载流子密度，可以迅速抑制这种间隙和相变。结果表明在单层 ZrTe ₂中形成了激子绝缘基态，并且通过第一性原理计算和自洽平均场理论合理化了这些发现。我们的研究为二维半金属中的激子凝聚提供了证据，并证明了对固体中本征束缚电子-空穴对形成的强维数效应。