Zouyouwei Lu

Symmetry elegantly governs the fundamental properties and derived functionalities of condensed matter. For instance, realizing the superconducting diode effect (SDE) demands breaking space-inversion and time-reversal symmetries simultaneously. Although the SDE is widely observed in various platforms, its underlying mechanism remains debated, particularly regarding the role of vortices. Here, we systematically investigate the nonreciprocal transport in the chiral type-I superconductor NbGe₂. Moreover, we induce type-II superconductivity with elevated superconducting critical temperature on the artificial surface by focused ion beam irradiation, enabling control over vortex dynamics in NbGe₂ devices. Strikingly, we observe negligible diode efficiency (Q < 2%) at low magnetic fields, which rises significantly to Q ~ 50% at high magnetic fields, coinciding with an abrupt increase in vortex creep rate when the superconductivity of NbGe₂ bulk is suppressed. These results unambiguously highlight the critical role of vortex dynamics in the SDE, in addition to the established symmetry rules.

Vortex pinning is a crucial factor that determines the critical current of practical superconductors and enables their diverse applications. However, the underlying mechanism of vortex pinning has long been elusive without a clear microscopic description^1,2. Here using high-resolution scanning tunneling microscopy, we studied single vortex pinning induced by point defect in layered FeSe-based superconductors. We found the defect-vortex interaction drives low-energy vortex bound states away from E _F, resulting a “mini” gap which effectively lowered the system energy and caused the pinning. By measuring the local density-of-states, we directly obtained the elementary pinning energy and estimated the pinning force through the spatial gradient of pinning energy. The results align with the bulk critical current measurement. We further show that a general microscopic quantum model with considering defect-vortex interaction can naturally capture our observation. It indicates the local pairing near pinned vortex core is actually enhanced, which is beyond the traditional understanding that non- superconducting regions pin vortices^1-3. Our study thus reveals a general microscopic mechanism of vortex pinning in superconductors, and provides guidance for enhancing the critical current of real superconductors.

The Kagome lattice, naturally encompassing Dirac fermions, flat bands, and van Hove singularities, tends to intertwine exotic electronic states. Revealing the characteristics of its Fermi surface will help on clarifying the nature of the complex quantum phenomena in kagome material. Here we report the Fermi surface properties of the new kagome superconductor CsTi₃Bi₅ by the de Haas–van Alphen oscillations. The observed oscillations are clear and consist of six principal frequencies ranging from 214 T to 1013 T. The angular dependence of the frequency implies a quasi-two-dimensional electronic structure. In addition, the Berry phase corresponding to 281 T, determined by the Landau level fan diagram method and full Lifshitz-Kosevich formula fitting, yields a value close to π, which indicates a band structure with nontrivial topological property. These results underscore the potential of CsTi₃Bi₅ as a promising new platform to explore the interplay between topological order, electronic nematicity, and superconductivity.

Intertwined spin and charge orders have been widely studied in high-temperature superconductors, since their fluctuations may facilitate electron pairing; however, they are rarely identified in heavily electron-doped iron selenides. Here, using scanning tunneling microscopy, we show that when the superconductivity of (Li_0.84Fe_0.16OH)Fe_1-xSe is suppressed by introducing Fe-site defects, a short-ranged checkerboard charge order emerges, propagating along the Fe-Fe directions with an approximately 2a_Fe period. It persists throughout the whole phase space tuned by Fe-site defect density, from a defect-pinned local pattern in optimally doped samples to an extended order in samples with lower T_c or non-superconducting. Intriguingly, our simulations indicate that the charge order is likely driven by multiple-Q spin density waves originating from the spin fluctuations observed by inelastic neutron scattering. Our study proves the presence of a competing order in heavily electron-doped iron selenides, and demonstrates the potential of charge order as a tool to detect spin fluctuations.

The electronic and superconducting properties of Fe_1–δ Se single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered FeSe by chemical-vapor-transport (CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal crossover with increasing temperature, is detected in the normal state of the present hydrothermal samples. Interestingly, a higher superconducting critical temperature T _c of 13.2 K is observed compared to a suppressed T _c of 9 K in the presence of the nematicity in the CVT FeSe. Moreover, the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of ∼42 T, which breaks the Pauli limit by a factor of 1.8.