Delong Fang

Giant superconducting fluctuation and anomalous semiconducting normal state in NdO1−xFxBi1−yS2 single crystals

We report the successful growth of the NdO1-xFxBi1-yS2 single crystals and prove the intrinsic superconductivity. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at T-c = 4.83 K. Measurements of excess conductivity and the in-plane angle-dependent resistance reveal a giant superconducting fluctuation far above T-c (extending to 2T(c)-4T(c)). This is supported by the Nernst and the scanning tunneling measurements. Analysis based on the anisotropic Ginzburg-Landau theory gives a very large anisotropy gamma = root m(c)/m(ab) approximate to 30-50. Two gap features with magnitudes of about 3.5 +/- 0.3 meV and 7.5 +/- 1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio 2 Delta(1)(s)/k(B)T(c) = 16.8, and the gapped feature remains up to 20-30 K. Another fascinating phenomenon is that the normal state recovered by applying a high magnetic field along the c-axis shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor may have an exotic reason which is beyond the BCS picture. Copyright (C) EPLA, 2014

Influence of microstructure on superconductivity in KxFe₂-ySe₂ and evidence for a new parent phase K₂Fe₇Se₈.

The search for new superconducting materials has been spurred on by the discovery of iron-based superconductors whose structure and composition is qualitatively different from the cuprates. The study of one such material, KxFe2−ySe2 with a critical temperature of 32 K, is made more difficult by the fact that it separates into two phases—a dominant antiferromagnetic insulating phase K2Fe4Se5, and a minority superconducting phase whose precise structure is as yet unclear. Here we perform electrical and magnetization measurements, scanning electron microscopy and microanalysis, X-ray diffraction and scanning tunnelling microscopy on KxFe2−ySe2 crystals prepared under different quenching processes to better understand the relationship between its microstructure and its superconducting phase. We identify a three-dimensional network of superconducting filaments within this material and present evidence to suggest that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √8 x √10 parallelogram structure. The structure of the superconducting phase of iron-based superconductor KxFe2−ySe2 is difficult to determine because it coexists with an predominant insulating phase. Ding et al. identify the superconducting filaments that provide clues to the structure of the parent phase of superconductivity.Since the discovery of high temperature superconductivity in F-doped LaFeAsO, many new iron based superconductors with different structures have been fabricated2. The observation of superconductivity at about 32 K in KxFe2-ySe2 with the iso-structure of the FeAs-based 122 superconductors was a surprise and immediately stimulated the interests because the band structure calculation8 predicted the absence of the hole pocket which was supposed to be necessary for the theoretical picture of S+- pairing. Soon later, it was found that the material may separate into the insulating antiferromagnetic K2Fe4Se5 phase and the superconducting phase. It remains unresolved that how these two phases coexist and what is the parent phase for superconductivity. In this study we use different quenching processes to produce the target samples with distinct microstructures, and apply multiple measuring techniques to reveal a close relationship between the microstructures and the global appearance of superconductivity. In addition, we clearly illustrate three dimensional spider-web-like superconducting filamentary paths, and for the first time propose that the superconducting phase may originate from a state with one vacancy in every eight Fe-sites with the root8*root10 parallelogram structure.

Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

Electric transport and scanning tunneling spectrum (STS) have been investigated on polycrystalline samples of the new superconductor Bi4O4S3. A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field. Interestingly, a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed. This kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation. From the tunneling spectra, a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kBTC ∼ 16.6. This value is much larger than the one predicted by the BCS theory in the weak coupling regime (2Δ/kBTC ∼ 3.53), which suggests the strong coupling superconductivity in the present system. Furthermore, the gapped feature persists on the spectra until 14 K in the STS measurement, which suggests a prominent fluctuation region of superconductivity. Such a superconducting fluctuation can survive at very high magnetic fields, which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.Resistivity, Hall effect and magnetization have been investigated on the new superconductor Bi4O4S3. A weak insulating behavior has been induced in the normal state when the superconductivity is suppressed. Hall effect measurements illustrate clearly a multiband feature dominated by electron charge carriers, which is further supported by the magnetoresistance data. Interestingly, a kink appears on the temperature dependence of resistivity at about 4 K at all high magnetic fields when the bulk superconductivity is completely suppressed. This kink can be well traced back to the upper critical field Hc2(T) in the low field region, and is explained as the possible evidence of residual Cooper pairs on the one dimensional chains.

Scrutinizing the double superconducting gaps and strong coupling pairing in (Li 1− x Fe x )OHFeSe

In the field of iron-based superconductors, one of the frontier studies is about the pairing mechanism. The recently discovered (Li1−xFex)OHFeSe superconductor with the transition temperature of about 40 K provides a good platform to check the origin of double superconducting gaps and high transition temperature in the monolayer FeSe thin film. Here we report a scanning tunnelling spectroscopy study on the (Li1−xFex)OHFeSe single crystals. The tunnelling spectrum mimics that of the monolayer FeSe thin film and shows double gaps at about 14.3 and 8.6 meV. Further analysis based on the quasiparticle interference allows us to rule out the d-wave gap, and for the first time assign the larger (smaller) gap to the outer (inner) Fermi pockets (after folding) associating with the dxy (dxz/dyz) orbitals, respectively. The gap ratio amounts to 8.7, which demonstrates the strong coupling mechanism in the present superconducting system.

Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As

Different experimental probes have found different bosonic modes in the iron-based superconductors. A scanning tunnelling spectroscopy study of two separate superconductors now links the tunnelling mode with the ‘neutron resonance’, both of which vanish when superconductivity disappears.

Drive the Dirac electrons into Cooper pairs in SrxBi2Se3

Topological superconductors are a very interesting and frontier topic in condensed matter physics. Despite the tremendous efforts in exploring topological superconductivity, its presence is however still under heavy debate. The Dirac electrons have been proven to exist on the surface of a topological insulator. It remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. Here we show the systematic study of scanning tunnelling microscope/spectroscopy on the possible topological superconductor SrxBi2Se3. We first demonstrate that only the intercalated Sr atoms can induce superconductivity. Then we show the full superconducting gaps without any in-gap density of states as expected theoretically for a bulk topological superconductor. Finally, we find that the surface Dirac electrons will simultaneously condense into the superconducting state within the superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs.

In-gap quasiparticle excitations induced by non-magnetic Cu impurities in Na(Fe 0.96 Co 0.03 Cu 0.01 )As revealed by scanning tunnelling spectroscopy

The origin of superconductivity in the iron pnictides remains unclear. One suggestion is that superconductivity in these materials has a magnetic origin, which would imply a sign-reversal s± pairing symmetry. Another suggests it is the result of orbital fluctuations, which would imply a sign-equal s++ pairing symmetry. There is no consensus yet which of these two distinct and contrasting pairing symmetries is the right one in iron pnictide superconductors. Here we explore the nature of the pairing symmetry in the superconducting state of Na(Fe0.97−xCo0.03Cux)As by probing the effect of scattering of Cooper pairs by non-magnetic Cu impurities. Using scanning tunnelling spectroscopy, we identify the in-gap quasiparticle states induced by the Cu impurities, showing signatures of Cooper pair breaking by these non-magnetic impurities–a process that is only consistent with s± pairing. This experiment provides strong evidence for the s± pairing.

Anisotropic superconducting gap and elongated vortices with Caroli-De Gennes-Matricon states in the new superconductor Ta4Pd3Te16.

The superconducting state is formed by the condensation of a large number of Cooper pairs. The normal state electronic properties can give significant influence on the superconducting state. For usual type-II superconductors, the vortices are cylinder like with a round cross-section. For many two dimensional superconductors, such as Cuprates, albeit the in-plane anisotropy, the vortices generally have a round shape. In this paper we report results based on the scanning tunnelling microscopy/spectroscopy measurements on a newly discovered superconductor Ta4Pd3Te16. The chain-like conducting channels of PdTe2 in Ta4Pd3Te16 make a significant anisotropy of the in-plane Fermi velocity. We suggest at least one anisotropic superconducting gap with gap minima or possible node exists in this multiband system. In addition, elongated vortices are observed with an anisotropy of ξ||b/ξ&bottom⊥b ≈ 2.5. Clear Caroli-de Gennes-Matricon states are also observed within the vortex cores. Our results will initiate the study on the elongated vortices and superconducting mechanism in the new superconductor Ta4Pd3Te16.

Unexpected weak spatial variation in the local density of states induced by individual Co impurity atoms in superconducting Na(Fe1-xCox)As crystals revealed by scanning tunneling spectroscopy

We use spatially resolved scanning tunneling spectroscopy in Na(Fe1-xCox)As to investigate the impurity effect induced by Co dopants. The Co impurities are successfully identified, and the spatial distributions of local density of state at different energies around these impurities are investigated. It is found that the spectrum shows negligible spatial variation at different positions near the Co impurity, although there is a continuum of the in-gap states which lifts the zero-bias conductance to a finite value. Our results put constraints on the S +/- and S++ models and sharpen the debate on the role of scattering potentials induced by the Co dopants. DOI: 10.1103/PhysRevB.86.214512

Strong-coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

We investigate the electronic properties of the tetragonal FeS superconductor by using scanning tunneling microscope/spectroscopy. It is found that the typical tunneling spectrum on the top layer of sulfur can be nicely fitted with an anisotropic s-wave or a combination of two superconducting components in which one may have a highly anisotropic or nodal like superconducting gap. The fittings lead to the maximum superconducting gap