Microscopic evidence for imaginary charge density wave in a kagome metal
Dissipationless charge transport without any energy loss is one of the most fascinating phenomena in condensed matter physics. This extraordinary state manifests in two well-established systems: superconductors and quantum Hall systems. A proposed third category is associated with chiral loop current order, characterized by the spontaneous formation of microscopic electric current loops. The microscopic origin of these currents stems from imaginary hopping terms, conceptualized as an imaginary charge density wave (iCDW). Despite extensive investigations, its existence remains highly controversial. Here we report site-selective spectroscopic evidence for a pure iCDW in the kagome nonmagnetic metal CsV$_3$Sb$_5$. Nuclear quadrupole resonance spectra at out-of-plane $^{121}$Sb site sensitive to in-plane currents reveal anomalous broadening below $T^*\approx$120 K, coinciding with the nematic transition well above the real charge density wave (CDW). Under magnetic fields, the spectra exhibit asymmetric lineshapes, demonstrating that this broadening purely originates from magnetic effects rather than from electric quadrupolar effects associated with CDW fluctuations. The observed lineshapes are quantitatively consistent with ~1 mT local fields induced by chiral loop currents, indicating spontaneous time-reversal symmetry breaking. This microscopic identification of the long-sought pure iCDW establishes a novel form of quantum order, potentially revolutionizing our understanding of exotic electronic states in quantum materials.