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-NbSe_(2) towards 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.

Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

The superconducting ground state of kagome metals AV_(3)Sb_(5)(where A stands for K,Rb,or Cs)emerges from an exotic charge density wave(CDW)state that potentially breaks both rotational and time reversal symmetries.However,the specifics of the Cooper pairing mechanism,and the nature of the interplay between these two states remain elusive,largely due to the lack of momentum-space(k-space)superconducting energy gap structure.By implementing Bogoliubov quasiparticle interference(B QPI)imaging,we obtain k-space information on the multiband superconducting gap structureΔ_(SC)^(i)(k)in pristine CsV_(3)Sb_(5).We show that the estimated energy gap on the vanadium d_(xy/x^(2)-y^(2))orbital is anisotropic but nodeless,with a minimal value located near the M point.Interestingly,a comparison ofΔ_(SC)^(i)(k)with the CDW gapΔ_(CDW)^(i)(k)obtained by angle-re solved photoemission spectro scopy(ARPES)reveals direct k-space competition between the se two order parameters,i.e.,the opening of a large(small)CDW gap at a given momentum corresponds to a small(large)superconducting gap.When the long-range CDW order is suppressed by replacing vanadium with titanium,we find a nearly isotropic energy gap on both the V and Sb bands.This information will be critical for identifying the microscopic pairing mechanism and its interplay with intertwined electro nic orders in this kagome superconductor family.

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.