Roton pair density wave in a strong-coupling kagome superconductor

Abstract

The recently discovered family of vanadium-based kagome metals with topological band structures offer new opportunities to study frustrated, correlated topological quantum states. These layered compounds are nonmagnetic and undergo charge density wave (CDW) transitions before developing superconductivity at low temperatures. Here we report the observation of unconventional superconductivity and pair density wave (PDW) in the vanadium-based kagome metal CsV3Sb5 using scanning tunneling microscope/spectroscopy (STM/STS) and Josephson STS. The differential conductance exhibits a V-shaped pairing gap Δ~0.5 meV below a transition temperature Tc~2.3 K. Superconducting phase coherence is observed by Josephson effect and Cooper-pair tunneling using a superconducting tip. We find that CsV3Sb5 is a strong-coupling superconductor (2∆/kBTc~5) and coexists with long-range 4a0 unidirectional and 2×2 charge order. Remarkably, we discover a bidirectional PDW accompanied by 4a0/3 spatial modulations of the coherence peak and gap-depth in the tunneling conductance. We term this novel quantum state a roton-PDW that can produce a commensurate vortex-antivortex lattice and account for the observed conductance modulations. Above Tc, we observe a large V-shaped pseudogap in the 4a0 unidirectional and 2a0 bidirectional CDW state. Electron-phonon calculations attribute the 2×2 CDW to phonon softening induced structural reconstruction, but the phonon mediated pairing cannot describe the observed strong-coupling superconductor. Our findings show that electron correlations drive the 4a0 unidirectional CDW, unconventional superconductivity, and roton-PDW with striking analogies and distinctions to the phenomenology of high-Tc cuprate superconductors, and provide groundwork for understanding their microscopic origins in vanadium-based kagome superconductors.

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.…”

“…Indeed, charge-density waves and superconductivity have been discovered [12,15,16], with ample evidence showing that both types of orders are exotic. For example, the CDW transition is accompanied possibly by a large anomalous Hall effect [17,18], and the superconductivity features a pair-density wave state [19]. The weakly bonded layers can be mechanically exfoliated [11], enabling the exploration of the effects of dimensionality on these phenomena and their carrier-density tuning [20,21].…”

“…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].…”

Tri-Hexagonal charge order in kagome metal CsV$_{3}$Sb$_{5}$ revealed by $^{121}$Sb NQR

“…The superconducting state is found to be spin singlet with s-wave pairing symmetry in the bulk state [6][7][8]. STM studies observed possible Majorana modes [9] and pair density wave [10] in the surface state. There are also experimentally observed residual density of states in the superconducting state [11], which may be due to the competition between the superconductivity and the CDW [11][12][13][14][15][16].…”