Superconducting state in Ba_((1-x)) Sr_(x)Ni_(2)As_(2) near the quantum critical point

In the phase diagram of the nickel-based superconductor Ba_(1-x)Sr_(x)Ni_(2)As_(2),T_(C) has been found to be enhanced sixfold near the quantum critical point(QCP) x=0.71 compared with the parent compound.However,the mechanism is still under debate.Here,we report a detailed investigation of the superconducting properties near the QCP(x≈0.7) by utilizing scanning tunneling microscopy and spectroscopy.The temperature-dependent superconducting gap and magnetic vortex state were obtained and analyzed in the framework of the Bardeen-Cooper-Schrieffer model.The ideal isotropic s-wave superconducting gap excludes the long-speculated nematic fluctuations while preferring strong electron-phonon coupling as the mechanism for T_(C) enhancement near the QCP.The lower than expected gap ratio of Δ/(k_(B) T_(C)) is rooted in the fact that Ba_(1-x)Sr_(x)Ni_(2)As_(2) falls into the dirty limit with a serious pair breaking effect similar to the parent compound.

Revisit of the anisotropic vortex states of 2H-NbSe_(2) towards the zero-field limit

We revisited the vortex states of 2H-Nb Se2towards zero fields by a low-temperature scanning tunneling microscope.Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splitting zero bias peak, and the other is an in-gap conductance anomaly resembling evolved crossing features around the center of the three nearest vortices.Both of them distribute solely along the next nearest neighboring direction of the vortex lattice and become unresolved in much higher magnetic fields, implying an important role played by the vortex–vortex interactions. To clarify these issues,we have studied the intrinsic vortex states of the isolated trapped vortex in zero fields at 0.45 K. It is concluded that the anisotropic zero bias peak is attributed to the superconducting gap anisotropy, and the spatially evolved crossing features are related to the vortex–vortex interaction. The vortex core size under the zero-field limit is determined. These results provide a paradigm for studying the inherent vortex states of type-II superconductors especially based on an isolated vortex.

Superconductivity and pseudogap features of an isolated FeAs layer in KCa_(2)Fe_(4)As_(4)F_(2) unraveled by STM/STS

To reveal the intrinsic properties of the basic superconducting units,i.e.,the single layered FeSe/FeAs in iron-based superconductors or CuO_(2) planes in cuprate superconductors,is a necessary prerequisite for understanding the mechanism of high-Tc superconductivity.Up to now,an isolated FeAs layer has rarely been studied due to the difficulty in materials synthesis.Here,we report a scanning tunneling microscopy/spectroscopy(STM/STS)study on the iron-based superconductor KCa_(2)Fe_(4)As_(4)F_(2).In situ cleavage produced a single FeAs layer covered by a reconstructed K surface,which is isolated from the bulk by the underlying CaF layer and shows multi-band superconductivity with a much lower T_(c) than its bulk counterpart.In the exposed As-terminated regions with coverage of scattered K atoms,a pseudogap was observed,leading to an inhomogeneous superconductivity without long-range phase coherence in real space,which is remarkably similar to the high-T_(c) cuprate superconductors.These results provide a new perspective to understanding the origin of superconductivity in iron-based superconductors.

Emergent superconducting fluctuations in compressed kagome superconductor CsV_(3)Sb_(5)

The recent discovery of superconductivity(SC)and charge density wave(CDW)in kagome metals AV3Sb5(A=K,Rb,Cs)provides an ideal playground for the study of emergent electronic orders.Application of moderate pressure leads to a two-dome-shaped SC phase regime in CsV_(3)Sb_(5) accompanied by the destabilizing of CDW phase.Nonetheless,the nature of this pressure-tuned SC state and its interplay with the CDW are yet to be explored.Here,we perform soft point-contact spectroscopy(SPCS)measurements in CsV_(3)Sb_(5) to investigate the evolution of superconducting order parameter with pressure.Surprisingly,we find that the superconducting gap is significantly enhanced between the two SC domes,at which the zero-resistance temperature is suppressed and the transition is remarkably broadened.Moreover,the temperature-dependence of the SC gap in this pressure range severely deviates from the conventional Bardeen-Cooper-Schrieffer(BCS)behavior,evidencing for strong Cooper pair phase fluctuations.These findings reveal the complex intertwining of the CDW with SC in the compressed CsV_(3)Sb_(5),suggesting striking parallel to the cuprate superconductor La2
xBaxCuO4.Our results point to the essential role of charge degree of freedom in the development of intertwining electronic orders,and thus provide new constraints for theories.

通过微量锰掺杂直接合成体超导的Fe(Te,Se)单晶

无砷的铁硒基超导体Fe(Te,Se)在高场应用方面具有广阔的前景,但至今仍缺乏制备体超导块材的有效方法.本文通过微量的Mn掺杂,一步法直接合成了无间隙铁的厘米尺寸高性能体超导单晶.通过电阻、磁化率与比热表征,发现Mn掺杂浓度为1%时,样品具有最好的体超导电性,且上临界场和临界电流密度相比未掺杂时显著提高.最佳掺杂样品在2 K时自场下的临界电流密度高达4.5×10^(5)A cm^(-2),且在高场下衰减很小.利用扫描隧道显微镜观测,证实了微量锰掺杂有效消除了层间的间隙铁原子.本工作首次提供了一种大规模制备高性能Fe(Te,Se)超导块材的实用方法,为其未来高场应用奠定了基础.