Discovery of high-temperature charge order and time-reversal symmetry-breaking in the kagome superconductor YRu$_3$Si$_2$

Identifying high-temperature unconventional charge order and superconductivity in kagome systems is crucial for understanding frustrated, correlated electrons and enabling future quantum technologies. Here, we report that the kagome superconductor YRu$_3$Si$_2$ hosts an exceptional interplay of charge order, magnetism, and superconductivity, revealed through a comprehensive suite of muon spin rotation (μSR), magnetotransport, X-ray diffraction, and density functional theory (DFT). We identify a high-temperature charge-ordered state with propagation vector (1/2,0,0) and a record onset temperature of 800 K, unprecedented in kagome systems and quantum materials more broadly. μSR measurements further reveal time-reversal symmetry-breaking below 25 K and field-induced magnetism near 90 K, features mirrored in the magnetoresistance, which reaches 45% at low temperatures. Band-structure calculations show two van Hove singularities near the Fermi level, including one within a flat band. At low temperatures, YRu$_3$Si$_2$ becomes superconducting below Tc = 3.4 K with either two full isotropic gaps or an anisotropic nodeless gap. These results establish YRu$_3$Si$_2$ as a prime platform for studying correlated kagome physics.