Microscopic evidence for imaginary charge density wave in a kagome metal

TL;DR

This paper provides microscopic spectroscopic evidence for an imaginary charge density wave (iCDW) in a kagome metal, revealing a novel quantum order associated with chiral loop currents and spontaneous time-reversal symmetry breaking.

Contribution

It is the first direct microscopic detection of a pure iCDW in a kagome metal, confirming its existence and characteristics.

Findings

Anomalous broadening in NQR spectra below 120 K indicates iCDW formation.

Spectra asymmetry under magnetic fields confirms magnetic origin of broadening.

Local magnetic fields (~1 mT) are consistent with chiral loop currents.

Abstract

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.