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Register a new userTwo distinct superconducting states controlled by orientation of local wrinkles in LiFeAs
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We observe two types of superconducting states controlled by orientations of local wrinkles on the surface of LiFeAs. Using scanning tunneling microscopy/spectroscopy, we find type-I wrinkles enlarge the superconducting gaps and enhance the transition temperature, whereas type-II wrinkles significantly suppress the superconducting gaps. The vortices on wrinkles show a C2 symmetry, indicating the strain effects on the wrinkles. A discontinuous switch of superconductivity occurs at the border between two different wrinkles. Our results demonstrate that the local strain effect could affect superconducting order parameter of LiFeAs with a possible Lifshitz transition, by alternating crystal structure in different directions.
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As the simplest iron-based superconductor, FeSe forms a tetragonal structure with transition temperature Tc ~ 8 K. With assistance of pressure, or other techniques, Tc can be greatly enhanced, even to above liquid nitrogen temperature. The newly discovered superconducting tetragonal FeS (Tc ~ 4.5 K), a sulfide counterpart of FeSe, promotes us on its high pressure investigation. The transport and structure evolution of FeS with pressure have been studied. A rapid suppression of Tc and vanishing of superconductivity at 4.0 GPa are observed, followed by a second superconducting dome with a 30% enhancement in maximum Tc. An onsite tetragonal to hexagonal phase transition occurs around 7.0 GPa, followed by a broad pressure range of phase coexistence. The residual deformed tetragonal phase is considered as the source of second superconducting dome. The observation of two superconducting domes in iron-based superconductors poses great challenges for understanding their pairing mechanism.
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High resolution laser-based angle-resolved photoemission measurements were carried out on an overdoped $Bi_2Sr_2CaCu_2O_{8+delta}$ superconductor with a Tc of 75 K. Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels: the bonding sheet corresponds to a doping level of 0.14 which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped, giving an overall doping level of 0.20 for the sample. Different superconducting gap sizes on the two Fermi surface sheets are revealed for the first time. The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet. The maximum gap difference between the two Fermi surface sheets near the antinodal region is $sim$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
While it is known that the nature and the arrangement of defects in complex oxides have an impact on the material functionalities little is known on control of superconductivity by oxygen interstitial organization in cuprates. Here we report direct compelling evidence for the control of Tc, by manipulation of the superconducting granular networks of nanoscale puddles, made of ordered oxygen stripes, in a single crystal of YBa2Cu3O6.5+y with average formal hole doping p close to 1/8. Upon thermal treatments we were able to switch from a first network of oxygen defects striped puddles with OVIII modulation (qOVIII(a*)=(h+3/8,k,0) and qOVIII(a*)=(h+5/8,k,0)), to second network characterized by OXVI modulation (qOXVI(a*)=(h+7/16,k,0) and qOXVI(a*)=(h+9/16,k,0)), and finally to a third network with puddles of OV periodicity (qOV(a*)=(4/10,1,0) and qOV(a*)=(6/10,1,0)). We map the microscopic spatial evolution of the out of plane OVIII, OXVI and OV puddles nano-size distribution via scanning micro-diffraction measurements. In particular, we calculated the number of oxygen chains (n) and the charge density (holes concentration p) inside each puddle, analyzing areas of 160x80 {mu}m2, and recording 12800 diffraction patterns to reconstruct each spatial map. The high spatial inhomogeneity shown by all the reconstructed spatial maps reflects the intrinsic granular structure that characterizes cuprates and iron-chalcogenides, disclosing the presence of several complex networks of coexisting superconducting domains with different lattice modulations, charge density and different gaps like in the proposed multi-gaps scenario called superstripes.
Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s$^{pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.
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