Cascade of correlated electron states in the kagome superconductor CsV3Sb5

Abstract

The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology, which continues to bear surprises. In this work, using spectroscopic imaging scanning tunneling microscopy, we discover a cascade of different symmetry-broken electronic states as a function of temperature in a new kagome superconductor, CsV3Sb5. At a temperature far above the superconducting transition Tc ~ 2.5 K, we reveal a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe a prominent V-shape spectral gap opening at the Fermi level and an additional breaking of the six-fold rotation symmetry, which persists through the superconducting transition. This rotation symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the V kagome bands. Our experiments reveal a complex landscape of electronic states that can co-exist on a kagome lattice, and provide intriguing parallels to high-Tc superconductors and twisted bilayer graphene.Quantum solids composed of atoms arranged on a lattice of corner-sharing triangles (kagome lattice) are a fascinating playground for the exploration of novel correlated and topological electronic phenomena [1][2][3][4] . Due to their intrinsic geometric frustration, kagome systems are predicted to host to a slew of exotic electronic states [5][6][7][8][9][10][11][12][13][14][15][16][17][18] , such as bond and charge ordering 7,8,10,[16][17][18] , spin liquid phases 5,15 and chiral superconductivity 9,10,17 . The majority of the experimental efforts thus far have focused on transition-metal kagome magnets, for example Co3Sn2S2 [19][20][21][22][23] FeSn 24,25 and Fe3Sn2 26,27 , in which different forms of magnetism dominate the low-temperature electronic ground state. Electronic correlations in the absence of magnetic ordering could in principle favor the emergence of new symmetry-broken electronic states, but this has been difficult to explore in many of the existing kagome materials due to a tendency towards magnetic ordering.

Observation of anomalous amplitude modes in the kagome metal CsV$_3$Sb$_5$

“…The A 2 mode was also observed in another Raman study [53] and by time-resolved pump-probe spectroscopy [29,54]. However, in these works, it was suggested to emerge below ∼60 K [29,53,54], hence ascribed to another phase transition associated with the unidirectional 1 × 4 order [19,31,32]. According to our data, shown in Fig.…”

“…The mechanism for the collapse of the crystal lattice in atomically thin CsV 3 Sb 5 could be related with the high chemical activity of the alkaline metal Cs. STM studies found nonuniform cleaved surfaces with clustered Cs atoms [31]. Refinement of X-ray diffraction data revealed high atomic displacement parameters for the Cs atoms, indicating their rattling-like motion owing to their weakly bonded nature [57].…”

“…Currently, there is still no consensus on the structural configuration of the CDW ground state. X-ray diffraction [12] and scanning tunneling microscopy (STM) found a 2×2 modulation of the crystal lattice [19,[31][32][33]. The CDW was shown to be three-dimensional, featuring a c-axis modulation that either doubles [33] or quadruples the original periodicity [34].…”

“…The CDW was shown to be three-dimensional, featuring a c-axis modulation that either doubles [33] or quadruples the original periodicity [34]. STM also found a 1 × 4 unidirectional modulation emerging only below 50 K [19,31,32], which was suggested to be of surface origin [32,35]. It was further proposed that the CDW is chiral and exhibits time-reversal symmetry breaking [32,[36][37][38].…”

Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by its anisotropic compression

“…Here, the presence of linear bands, Dirac points, and band saddle points (van Hove singularities) in the electronic structure of a nearest-neighbor kagome metal causes multiple electronic instabilities that have been studied theoretically [1][2][3][4] but did not find their way into the lab until the recent discovery of the AV 3 Sb 5 compound family with A = K, Rb, Cs [5]. Both charge-density wave (CDW) order [6][7][8][9][10][11] and superconductivity [12][13][14][15][16][17] observed experimentally in these compounds closely resemble theoretical findings for a simple kagome metal with the Fermi energy lying near the van Hove singularity at the filling level of 5 12 [18,19]. In AV 3 Sb 5 , the corresponding saddle points are clearly visible in the vanadium bands around the M point slightly below the Fermi level (Fig.…”