Evidence for an odd-parity nematic phase above the charge density wave transition in kagome metal CsV${}_3$Sb${}_5$

The quest for fascinating quantum states arising from the interplay between correlation, frustration, and topology is at the forefront of condensed-matter physics. Recently discovered nonmagnetic kagome metals $A$V${}_3$Sb${}_5$ ($A=$ K, Cs, Rb) with charge density wave (CDW) and superconducting instabilities may host such exotic states. Here we report that an odd electronic nematic state emerges above the CDW transition temperature ($T_{\mathrm{C}\mathrm{D}\mathrm{W}}=94$ K) in CsV${}_3$Sb${}_5$. High-resolution torque measurements reveal a distinct twofold in-plane magnetic anisotropy that breaks the crystal rotational symmetry below $T^{\ast }\approx 130$ K. However, no relevant anomalies are detected in the elastoresistance data near $T^{\ast }$, which excludes the even-parity ferro-orbital nematicity often found in other superconductors. Moreover, in the temperature range between $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ and $T^{\ast }$, conical rotations of magnetic field yield a distinct first-order phase transition, indicative of time-reversal symmetry breaking. These results provide thermodynamic evidence for the emergence of an odd-parity nematic order, implying that an exotic loop-current state precedes the CDW in CsV${}_3$Sb${}_5$.