Unconventional charge order and superconductivity in kagome-lattice systems as seen by muon-spin rotation

Abstract

Kagome lattices are intriguing and rich platforms for studying the intertwining of topology, electron correlation, and magnetism. These materials have been subject to tremendous experimental and theoretical studies not only due to their exciting physical properties but also as systems that may solve critical technological problems. We will review recent experimental progress on superconductivity and magnetic fingerprints of charge order in several kagome-lattice systems from the local-magnetic probe point of view by utilizing muon-spin rotation under extreme conditions, i.e., hydrostatic pressure, ultra low temperature and high magnetic field. The systems include: (1) The series of compounds AV3Sb5 (A = K, Rb, Cs) with V kagome lattice which form the first kagome-based family that exhibits a cascade of symmetry-broken electronic orders, including charge order and superconductivity. In these systems, we find a time-reversal symmetry-breaking charge ordered state and tunable unconventional time-reversal symmetry-breaking superconductivity. (2) The system LaRu3Si2 with distorted kagome layers of Ru, in which our experiments and calculations taken together point to nodeless moderate coupling superconductivity. It was also found that the electron-phonon coupling alone can only explain a small fraction of Tc from calculations, which suggests other factors enhancing Tc such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and the high density of states from the narrow kagome bands. (3) CeRu2 with a pristine Ru kagome lattice, which we classify as an exceedingly rare nodeless (with anisotropic s-wave gap symmetry) magnetic kagome superconductor.

Discovery of charge order above room-temperature in the prototypical kagome superconductor La(Ru1−xFex)3Si2

“…Our results identify LaRu 3 Si 2 as the kagome superconductor with the highest known charge ordering temperature, offering a promising avenue for researching room temperature quantum phases and developing related technologies.Charge order is prevalent in materials exhibiting strong electronic correlations and pronounced electron-phonon interactions. This phenomenon is observed in various systems such as cuprate high-temperature superconductors 1-3 , colossal magnetoresistive manganites 4 , transitionmetal dichalcogenides (TMDs) [5][6][7] , and more recently, kagome lattice metals 8,9 . Charge order is usually associated with unconventional quantum phases, which highlights the importance of charge order in understanding and manipulating novel phases of matter.…”

“…However, the lack of room-temperature charge-order materials has limited the development of charge-orderbased electronic devices. Thus, it is of importance both from fundamental and technological aspects to find more materials with room-temperature charge orders.Along these lines, several kagome metals [8][9][10][11][12][13][14][15][16][17] have appeared as promising platforms to study unconventional correlated quantum phases, including charge order. There are three material classes of kagome lattice systems that were recently shown to exhibit charge order: the AV 3 Sb 5 (A = K, Rb, Cs) 9,18,19 family of materials, ScV 6 Sn 6 [20][21][22][23][24][25] and FeGe [26][27][28] .…”

“…The system ScV 6 Sn 6 has a similar vanadium structural motif as AV 3 Sb 5 and exhibits charge order below T co ≃ 90 K, however it is not superconducting down to the lowest measured temperature. FeGe is a correlated magnetic kagome system and exhibits an A-type antiferromagnetic (AFM) order below 400 K and a charge order below 100 K. The unique features of AV 3 Sb 5 and ScV 6 Sn 6 are the emergence of possibly time-reversal symmetry (TRS) breaking chiral charge orders with both magnetic 8,11,21,[32][33][34][35][36][37][38][39][40][41][42][43][44] and electronic anomalies 45,46 . Theoretically, these features could be explained by a complex…”

“…Along these lines, several kagome metals [8][9][10][11][12][13][14][15][16][17] have appeared as promising platforms to study unconventional correlated quantum phases, including charge order. There are three material classes of kagome lattice systems that were recently shown to exhibit charge order: the AV 3 Sb 5 (A = K, Rb, Cs) 9,18,19 family of materials, ScV 6 Sn 6 [20][21][22][23][24][25] and FeGe [26][27][28] .…”