Session D45: Kagome Superconductivity

A new class of topological kagome metals AV3Sb5 (A=K, Cs, Rb) with intertwined charge density wave order and superconductivity

Kagome metals are fascinating materials capable of hosting an interplay between topologically nontrivial electronic states and correlated electron phenomena. This interplay can arise from the Dirac points, flatbands, and saddle-points endemic to their band structures at select filling fractions. The discovery of a new class of kagome metals of the form AV₃Sb₅ with A=K, Cs, or Rb has recently provided a unique setting for exploring the interplay between Z2 electronic topology and intertwined charge density wave and superconducting orders. These quasi-two-dimensional compounds host a kagome net of nonmagnetic vanadium ions with an electron-filling that places the Fermi level near the saddle-points and their corresponding van Hove singularities. Nesting effects between these points are predicted to stabilize a variety of unusual states, ranging from orbital antiferromagnetism to unconventional superconductivity. Here I will present some of our recent work exploring the phase transitions and broken symmetries in these materials.   

Chiral charge order in kagome superconductor AV3Sb5

Kagome lattice has been introduced to quantum physics for seventy years. Over recent years, kagome lattice has been demonstrated as a novel quantum platform displaying unprecedented intertwining between lattice geometry, competing orders, and electronic topology [Nature555, 638-642 (2018); Nature562, 91-95(2018); Nature Physics, 443 (2019); Nature583, 533-536 (2020); Nature Review Physics, 249-263 (2021)]. While most of these materials exhibit quantum magnetism, the observation of chiral charge order in kagome superconductor AV3Sb5 (A=K, Cs, Sb) further pushes the research of kagome lattice to the quantum frontier. In this talk, I will present our scanning tunneling microscopy evidence of chiral charge order in all three AV3Sb5 compounds. This discovery is further supported by our muon spin rotation experiment which is bulk and extremely magnetic sensitive. I will discuss relevance of the chiral charge order to the Haldane model for the quantum anomalous Hall effect and Varma model for the hidden phase in cuprate high temperature superconductors.  

    

Time-reversal symmetry breaking and flux phases in Kagome Superconductor AV3Sb5

The quasi-2 dimensional Kagome materials AV₃Sb₅ (A=K,Rb,Cs) was found to be the first example of Kagome superconductor [1-3], which provides us a new platform to investigate the interplay between correlation, topology and geometric frustration. In this talk, I will give a brief review of the recent progress in Kagome materials AV₃Sb₅, especially the time-reversal symmetry breaking charge density wave order found by magnetic field dependent STM and zero-field muon spin spectroscopy. Since there is no long-range magnetic order in AV₃Sb₅, we argue that the topological charge density wave phase is a chiral flux phase [6]. By symmetry analysis and energy comparison, the explicit form of the chiral flux phase was constructed. To further confirm this, we search and classify all possible flux phases in Kagome lattice within 2*2 unit cell [7]. The consequence of this chiral flux phase and its influence on superconductivity will be covered [8].

[1] B. R. Ortiz et al., Phys. Rev. Mater. 3, 094407 (2019).

[2] B. R. Ortiz et al., Phys. Rev. Lett. 125, 247002 (2020).

[3] B. R. Ortiz et al., Phys. Rev. Materials 5, 034801 (2021).

[4] Y.-X. Jiang et al., arXiv:2012.15709.

[5] Li Yu et al., arXiv:2107.10714.

[6] Xilin Feng, Kun Jiang, Ziqiang Wang, Jiangping Hu, Sci. Bull. 66, 1384 (2021).

[7] Xilin Feng, Yi Zhang, Kun Jiang, Jiangping Hu, arXiv:2106.04395.

[8] Yuhao Gu, Yi Zhang, Xilin Feng, Kun Jiang, Jiangping Hu, arXiv:2108.04703.   

Topology and correlation driven exotic electronic phases in a kagome superconductor CsV3Sb5

Kagome compounds provide a fertile playground to explore the rich interplay between geometry, topology and electronic correlations. In this talk, we focus on a topological kagome metal CsV₃Sb₅ that exhibits cascade of symmetry-breaking electronic phases, including charge density wave, electronic nematicity and superconductivity. A giant anomalous Hall conductivity is observed exactly following the CDW transition, and the superconductivity shows an unusual competition with this CDW state under pressure. These findings point to an unconventional nature of the CDW. Deeply in the CDW state, we found the emergence of electronic nematicity that intertwines with CDW and superconductivity, drawing strike parallel to the high temperature superconductors.  At low temperatures, the topological surface states cooperate with superconductivity, and Majorana zero modes can be realized inside the vortex cores. All these results establish CsV₃Sb₅ as an ideal playground to study the topology and correlation driven exotic electronic orders.   

Time-reversal symmetry breaking charge order in a kagome superconductor

The kagome lattice, the most prominent structural motif in quantum physics, benefits from inherent nontrivial geometry to host diverse quantum phases [1,2,3], 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 AV₃Sb₅ (A = K, Rb, Cs) [4] exhibit unconventional chiral charge order [5]. However, direct evidence for the time-reversal symmetry-breaking of the charge order remained elusive. We utilized muon spin relaxation to probe the kagome charge order and superconductivity in KV₃Sb₅ [6]. 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 KV₃Sb₅ and that the T_c/λ⁻²_ab 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. While low-temperature time-reversal symmetry-breaking superconductivity has been discussed for many systems, high-temperature time-reversal symmetry-breaking charge order is extremely rare, and finds a direct comparison with the fundamental Haldane and Varma models.

[1] J.-X. Yin et. al., Nature 583, 533-536 (2020).

[2] Z. Guguchia et. al., Nature Communications 11, 559 (2020).

[3] C. Mielke III et al., and Z. Guguchia, Phys. Rev. Mat. 5, 034803 (2021).

[4] Ortiz, B. et al. Phys. Rev. Lett. 125, 247002 (2020).

[5] Y.-X. Jiang et. al., Nature Materials 20, 1353 (2021).

[6] C. Mielke III et. al., and Z. Guguchia, Nature (2022).