Chern Fermi-pockets and chiral topological pair density waves in kagome superconductors

The field of transition-metal kagomé lattice materials has leapt forward with the recent discovery of superconductivity in a new family of vanadium-based kagomé metals AV3Sb5 (A = K, Rb, Cs) [4, 5]. In contrast to the insulating kagomé compounds extensively studied for quantum spin liquids and doped Mott insulators [6–8], AV3Sb5 are nonmagnetic correlated metals with itinerant electrons traversing the unique kagomé lattice structure that geometrically frustrates kinetic motion due to quantum interference. They are complementary to the (Fe, Co, Mn)-based itinerant kagomé magnets that exhibit correlated topological phenomena such as massive spinpolarized Dirac fermions [9, 10], magnetic Weyl semimetals [11–13], Berry curvature induced orbital magnetism [14], but have remained nonsuperconducting at low temperatures. All AV3Sb5 undergo charge density wave (CDW) transitions below Tcdw ∼ 78 – 103 K and superconducting (SC) transitions below Tc ∼ 0.9 – 2.5 K to become rare kagomé lattice superconductors. Both the diagonal and the off-diagonal long-range ordered states turn out to be highly intriguing and unconventional. While charge order has been confirmed to be a 2a0 × 2a0 3Q-CDW in the kagomé plane stacked along the c-axis [1–3, 15–18], it surprisingly produces giant anomalous Hall effect (AHE) despite the absence of magnetism [19, 20]. Scanning tunneling microscopy (STM) imaging of KV3Sb5 observed that the CDW responds to an applied magnetic field differently when the field direction is reversed along the caxis [1], suggesting spontaneously time-reversal symmetry (TRS) breaking. Although the robustness of the STM observation is controversial [21, 22], evidence for TRS breaking has been detected in μSR experiments [23, 24]. At low temperatures, STM experiments in CsV3Sb5 reveal the coexistence of strong-coupling superconductivity with 2a0 × 2a0 CDW and 4a0 unidirectional charge order [2, 3]. Remarkably, a 3Q pair density wave (PDW) with 4 3 a0 × 4 3 a0 period was discovered that spatially modulates the SC gap and coherence peaks [3]. Moreover, the phenomenology of the PDW is striking compared to that in high-Tc cuprates: it is detectable in the vortex core and above Hc2, emerges as a “mother state” above Tc, and is responsible for the observed pseudogap behavior. Here, we demonstrate theoretically that the essential part of these highly unusual properties is captured by a doped orbital Chern insulator on the kagomé lattice, and the emergent Chern Fermi surface pockets provide a new mechanism for orbital-driven intrinsic chiral topological superconductivity.