Footprint of time-reversal symmetry breaking in CsV3Sb5 marked in photoemission intensity

Spontaneous symmetry breaking in kagome metals has prompted intense debate, particularly regarding the charge density wave (CDW) phase in CsV ₃ Sb ₅ and whether time-reversal symmetry (TRS) is broken, and if so, when it occurs. Here, we try to resolve this issue by performing circular dichroism measurements in angle-resolved photoemission spectroscopy (ARPES) on CsV ₃ Sb ₅ across an extended temperature range. We observe clear signatures of chiral symmetry breaking emerging at T ^* ≈ 145 K well above the conventional CDW temperature, manifested by a pronounced dichroic response in the photoemission intensity. Importantly, this phenomenon is unlikely to arise solely from structural distortions or domain effects, and rather indicates the presence of such an intriguing phase as loop current order that breaks time-reversal symmetry. Upon entering the CDW phase at T _CDW ≈ 90K, the dichroic response evolves in a complex manner, reflecting the mixed nature of charge bonding and current order. These observations are supported by first-principles calculations and atomic orbital angular momentum (OAM) analysis, which together identify the dichroism as a characteristic of orbital polarization resulting from time-reversal symmetry breaking. Our findings indicate that CD-ARPES is an effective tool for investigating certain types of hidden orders, and provide insight into the symmetry-breaking characteristics of the CDW phase in kagome metals.