Emergent charge order in pressurized kagome superconductor CsV3Sb5.

The discovery of several electronic orders in kagome superconductors AV3Sb5 (A means K, Rb, Cs) provides a promising platform for exploring unprecedented emergent physics1-9. Under moderate pressure (<2.2 GPa), the triple-Q charge density wave (CDW) order is monotonically suppressed by pressure, while the superconductivity shows a two-dome-like behaviour, suggesting an unusual interplay between superconductivity and CDW order10,11. Given that time-reversal symmetry breaking and electronic nematicity have been revealed inside the triple-Q CDW phase8,9,12,13, understanding this CDW order and its interplay with superconductivity becomes one of the core questions in AV3Sb5 (refs. 3,5,6). Here, we report the evolution of CDW and superconductivity with pressure in CsV3Sb5 by 51V nuclear magnetic resonance measurements. An emergent CDW phase, ascribed to a possible stripe-like CDW order with a unidirectional 4a0 modulation, is observed between Pc1 ≅ 0.58 GPa and Pc2 ≅ 2.0 GPa, which explains the two-dome-like superconducting behaviour under pressure. Furthermore, the nuclear spin-lattice relaxation measurement reveals evidence for pressure-independent charge fluctuations above the CDW transition temperature and unconventional superconducting pairing above Pc2. Our results not only shed new light on the interplay of superconductivity and CDW, but also reveal new electronic correlation effects in kagome superconductors AV3Sb5.

Giant Negative Magnetoresistance beyond Chiral Anomaly in Topological Material YCuAs2.

The large negative magnetoresistance (MR) effect, which usually emerges in various magnetic systems, is a technologically important property for spintronics. Recently, the so-called "chiral anomaly" in topological semimetals offers an alternative to generate a considerable negative MR effect without utilizing magnetism. However, it requires that the applied magnetic field must be strictly along the electric current direction, which sets a strong limit for practical applications. Here, a giant negative MR effect is discovered with a value of up to -40% in 9 T at 2 K in the nonmagnetic Dirac material YCuAs2 , which is not restricted to the specific configuration for applied magnetic fields. Based on magnetic susceptibility and NMR measurements, the giant negative MR effect is tightly connected with the unexpected spin-dependent scattering from vacancy-induced local moments, which is also beyond the classical Kondo effect. The present work not only offers an alternative route for spintronics based on nonmagnetic topological materials, but also helps to further understand the negative MR effect in topological materials.

Charge-density-wave-driven electronic nematicity in a kagome superconductor.

Electronic nematicity, in which rotational symmetry is spontaneously broken by electronic degrees of freedom, has been demonstrated as a ubiquitous phenomenon in correlated quantum fluids including high-temperature superconductors and quantum Hall systems1,2. Notably, the electronic nematicity in high-temperature superconductors exhibits an intriguing entanglement with superconductivity, generating complicated superconducting pairing and intertwined electronic orders. Recently, an unusual competition between superconductivity and a charge-density-wave (CDW) order has been found in the AV3Sb5 (A = K, Rb, Cs) family with two-dimensional vanadium kagome nets3-8. Whether these phenomena involve electronic nematicity is still unknown. Here we report evidence for the existence of electronic nematicity in CsV3Sb5, using a combination of elastoresistance measurements, nuclear magnetic resonance (NMR) and scanning tunnelling microscopy/spectroscopy (STM/S). The temperature-dependent elastoresistance coefficient (m11 minus m12) and NMR spectra demonstrate that, besides a C2 structural distortion of the 2a0 × 2a0 supercell owing to out-of-plane modulation, considerable nematic fluctuations emerge immediately below the CDW transition (approximately 94 kelvin) and finally a nematic transition occurs below about 35 kelvin. The STM experiment directly visualizes the C2-structure-pinned long-range nematic order below the nematic transition temperature, suggesting a novel nematicity described by a three-state Potts model. Our findings indicate an intrinsic electronic nematicity in the normal state of CsV3Sb5, which sets a new paradigm for revealing the role of electronic nematicity on pairing mechanism in unconventional superconductors.