Time-reversal symmetry-breaking charge order in a kagome superconductor (2106.13443v2)

Abstract: The kagome lattice, the most prominent structural motif in quantum physics, benefits from inherent nontrivial geometry to host diverse quantum phases, ranging from spin-liquid phases, topological matter to intertwined orders, and most rarely unconventional superconductivity. Recently, charge sensitive probes have suggested that the kagome superconductors AV_3Sb_5 (A = K, Rb, Cs) (A = K, Rb, Cs) exhibit unconventional chiral charge order, which is analogous to the long-sought-after quantum order in the Haldane model or Varma model. However, direct evidence for the time-reversal symmetry-breaking of the charge order remains elusive. Here we utilize muon spin relaxation to probe the kagome charge order and superconductivity in KV_3Sb_5. We observe a striking enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Remarkably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV_3Sb_5 and that the T_c/lambda_{ab}^{-2} ratio is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

• The paper reveals that time-reversal symmetry-breaking charge order emerges at the charge ordering temperature, persisting into the superconducting state.
• The paper demonstrates a multigap superconducting state in KV3Sb5, with T₍c₎/λ₍ab₎⁻² ratios akin to those in high-temperature superconductors.
• The paper uncovers anisotropic magnetic responses that support chiral charge states linked to orbital currents, suggesting potential anomalous Hall effects.

Time-Reversal Symmetry-Breaking Charge Order in a Kagome Superconductor

The paper investigates the interplay between time-reversal symmetry-breaking charge order and unconventional superconductivity within the recently unveiled kagome superconductor KV$_3$Sb$_5$. This system, characterized by its kagome lattice structure, is notable for the diverse quantum phases it supports due to the geometric frustration inherent in its triangular motifs. Recent theoretical propositions have suggested that such materials could demonstrate unconventional phases, including chiral charge orders akin to phenomena described in the Haldane and Varma models. However, experimental confirmation of these orders and their consequences on time-reversal symmetry in its superconducting state has remained incomplete.

Key Experimental Outcomes

This work employs muon spin relaxation techniques (${\mu}$SR) to probe the intimate details of charge ordering and superconductivity in KV$_3$Sb$_5$. The critical observations from these experiments are as follows:

1. Time-Reversal Symmetry-Breaking Charge Order: Upon lowering the temperature, the muon ensemble experiences a distinct enhancement in internal field width coinciding with the charge ordering temperature. This anomaly, persisting into the superconducting ground state, denotes a compelling indicator for time-reversal symmetry breaking originating from the charge order.
2. Multigap Nature of Superconductivity: The findings reveal that KV$_3$Sb$_5$ is indicative of multigap superconductivity. The $T_{\rm c}$/$\lambda_{ab}^{-2}$ ratio parallels those of unconventional high-temperature superconductors, suggesting similar underlying Coulomb interaction mechanisms.
3. Magnetic Response and Anisotropic Behavior: The magnetic response across the charge-ordering transition is notably anisotropic. This anisotropy reinforces theoretical predictions regarding orbital current induced chiral charge states in kagome systems, which potentially generates large anomalous Hall effects.
4. High-Field Muon Spin Relaxation: The significant field-induced enhancement of the electronic response reveals the intricacy of the relationship between applied magnetic fields and the symmetry-breaking transitions within KV$_3$Sb$_5$.

Implications and Speculative Outlook

The paper advances the understanding of how geometric frustration in kagome lattices facilitates complex quantum states, notably offering evidence for time-reversal symmetry-breaking charge order intertwined with superconductivity. This constitutes credible support for theories predicting exotic chiral states and possible non-trivial topological phenomena.

Fundamentally, these results propel forward the investigation into high-temperature superconductivity where electronic correlations and symmetry-breaking orders could play pivotal roles. The interplay of time-reversal symmetry-breaking and multigap superconductivity hints at innovative avenues for exploring analogous phenomena in related systems. From a materials science and condensed matter physics perspective, these revelations could prove instrumental in guiding the design of materials tailored to harness the unique properties of kagome lattices for technological applications.

Concluding Remark

While this investigation significantly contributes to the landscape of quantum materials, it also opens new discussions regarding the detailed nature of the orbital currents and their implication on macro-scale phenomena like the anomalous Hall effect. Future studies directed towards neutron scattering and further muon-based analyses could help delineate the swirling currents' configuration and substantiate these theoretical models. Such inquiries will continue to illuminate the multifaceted behavior of quantum materials found in kagome frameworks, broadening the horizons for uncovering novel states of matter.