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Abstract A hallmark of unconventional superconductors is a complex electronic phase diagram where intertwined orders of charge-spin-lattice degrees of freedom compete and coexist. While the kagome metals such as CsV3Sb5 also exhibit complex behavior, involving coexisting charge density wave order and superconductivity, much is unclear about the microscopic origin of the superconducting pairing. We study the vortex lattice in the superconducting state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and enhances superconductivity. Using small-angle neutron scattering, a strictly bulk probe, we show that the vortex lattice exhibits a strikingly conventional behavior. This includes a triangular symmetry with a period consistent with 2e-pairing, a field dependent scattering intensity that follows a London model, and a temperature dependence consistent with a uniform superconducting gap. Our results suggest that optimal bulk superconductivity in Cs(V1−x Ta x )3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling, different from spin fluctuation mediated unconventional copper- and iron-based superconductors.

Abstract The thermal Hall effect has emerged as a potential probe of exotic excitations in spin liquids. In the Kitaev magnet $${{\alpha }}$$ α -RuCl3, the thermal Hall conductivity $${{{\kappa }}}_{{{xy}}}$$ κ x y has been attributed to Majorana fermions, chiral magnons, or phonons. Theoretically, the former two types of heat carriers can generate a “planar” $${{{\kappa }}}_{{{xy}}}$$ κ x y , whereby the magnetic field is parallel to the heat current, but it is unknown whether phonons also could. Here we show that a planar $${{{\kappa }}}_{{{xy}}}$$ κ x y is present in another Kitaev candidate material, Na2Co2TeO6. Based on the striking similarity between $${{{\kappa }}}_{{{xy}}}$$ κ x y and the phonon-dominated thermal conductivity $${{{\kappa }}}_{{{xx}}}$$ κ x x , we attribute the effect to phonons. We observe a large difference in $${{{\kappa }}}_{{{xy}}}$$ κ x y between different configurations of heat current and magnetic field, which reveals that the direction of heat current matters in determining the planar $${{{\kappa }}}_{{{xy}}}$$ κ x y . Our observation calls for a re-evaluation of the planar $${{{\kappa }}}_{{{xy}}}$$ κ x y observed in $$\,{{\alpha }}$$ α -RuCl3.

Severe capacity degradation at high operating voltages and poor interphase stability at elevated temperature have thus far precluded the practical application of LiNi0.5Mn1.5O4 (LNMO) as a cathode material for lithium-ion batteries. Addressing these challenges through a combination of experimental and theoretical methods in this work, we demonstrate how a fluorinated carbonate electrolyte enables both high-voltage and high temperature operation by mitigating the traditional interfacial reactions observed in electrolytes with conventional carbonate solvents. Computational studies confirm the exceptional oxidation stability of fluorinated carbonate electrolyte which reduces deprotonation at high voltage. The mitigated deprotonation will then minimize the formation of HF acid which corrodes the LNMO surface and leads to phase transformation and poor interphases. With fluorinated carbonate electrolyte at elevated temperature, it was found on LNMO’s subsurface a reduced amount of Mn3O4 phase which can block Li+ transfer and result in drastic cell failure. Leveraging this approach, LNMO/graphite full cells with a high loading of 3.0 mAh/cm2 achieve excellent cycling stability, retaining ∼84 % of their initial capacity at room temperature (25 °C) after 200 cycles and ∼68 % after 100 cycles at 55 °C. This advanced electrolyte also shows promise for improving calendar life, retaining >30 % more capacity than the carbonate baseline after high temperature storage. These results indicate that electrolytes based on fluorinated carbonates are a promising strategy for overcoming the remaining challenges toward practical commercial application of LNMO.

Abstract The report of a half-quantized thermal Hall effect and oscillatory structures in the magnetothermal conductivity in the Kitaev material α-RuCl3 have sparked a strong debate on whether it is generated by Majorana fermion edge currents, spinon Fermi surface, or whether other more conventional mechanisms are at its origin. Here, we report low temperature thermal conductivity (κ) of another candidate Kitaev material, Na2Co2TeO6. The application of a magnetic field (B) along different principal axes of the crystal reveals a strong directional-dependent B impact on κ, while no evidence for mobile quasiparticles except phonons can be concluded at any field. Instead, severely scattered phonon transport prevails across the B−T phase diagram, revealing cascades of phase transitions for all B directions. Our results thus cast doubt on recent proposals for significant itinerant magnetic excitations in Na2Co2TeO6, and emphasize the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.

As a candidate Kitaev material, Na_{2}Co_{2}TeO_{6} exhibits intriguing magnetism on a honeycomb lattice that is believed to be C_{3} symmetric. Here we report a neutron diffraction study of high-quality single crystals under a-axis magnetic fields. Our data support the less common notion of a magnetic ground state that corresponds to a triple-q magnetic structure with C_{3} symmetry, rather than the multidomain zigzag structure typically assumed in prototype Kitaev spin liquid candidates. In particular, we find that the field is unable to repopulate the supposed zigzag domains, where the only alternative explanation is that the domains are strongly pinned by hitherto unidentified structural reasons. If the triple-q structure is correct, then this requires reevaluation of many candidate Kitaev materials. We also find that fields beyond about 10 Tesla suppress the long-range antiferromagnetic order, allowing magnetic behavior to emerge different from that expected for a spin liquid.

The quest for a half-quantized thermal Hall effect of a Kitaev system represents an important tool to probe topological edge currents of emergent Majorana fermions. Pertinent experimental findings for α-RuCl_{3} are, however, strongly debated, and it has been argued that the thermal Hall signal stems from phonons or magnons rather than from Majorana fermions. Here, we investigate the thermal Hall effect of the Kitaev candidate material Na_{2}Co_{2}TeO_{6}, and we show that the measured signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the longitudinal and transversal heat conductivities share clear phononic signatures, while the transversal signal changes sign upon entering the low-temperature, magnetically ordered phase. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in Kitaev topological quantum spin liquids such as α-RuCl_{3} and Na_{2}Co_{2}TeO_{6} requires disentangling phonon vs magnetic contributions, including potentially fractionalized excitations such as the expected Majorana fermions.