Zengyi Du

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

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.

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.

Signature of topological quantum phase transitions via Wigner-Yanase skew information

We apply the Wigner-Yanase skew information approach to analyze two typical models that exhibit a topological quantum phase transition. Based on the exact solutions of the ground states, the Wigner-Yanase skew information between two nearest sites for each of the two models is obtained. For the one-dimensional Kitaev chain model, the first-order derivative of the Wigner-Yanase skew information is non-analytical around the critical point. The scaling behavior and the universality are verified numerically. In particular, the skew information can also detect the factorization transition in such a model. For the two-dimensional Kitaev honeycomb model, the first-order derivative of the Wigner-Yanase skew information shows some singularities at the critical points where the system transits from the gapless phase to the gapped one. Our results suggest that the Wigner-Yanase skew information can serve as a good indicator of the topological phase transitions in these models and shed considerable light on the relationships between topological quantum phase transition and information theory.

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.

Sign reversal of the order parameter in (Li 1− x Fe x )OHFe 1− y Zn y Se

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is spin-fluctuation driven and intimately related to their fermiology, specifically, hole and electron pockets separated by the same wave vector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs. This picture was questioned after the discovery of a new family, based on the FeSe layers, either intercalated or in the monolayer form. The critical temperatures there reach ~40 K, the same as in optimally doped bulk FeSe - despite the fact that intercalation removes the hole pockets from the Fermi level and, seemingly, undermines the basis for the spin-fluctuation theory and the idea of a sign-changing OP. In this paper, using the recently proposed phase-sensitive quasiparticle interference technique, we show that in LiOH intercalated FeSe compound the OP does change sign, albeit within the electronic pockets, and not between the hole and electron ones. This result unifies the pairing mechanism of iron based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.

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

Magnetic-charge ordering and phase transitions in monopole-conserved square spin ice

\Delta_{max}\approx

Fully gapped superconductivity in In-doped topological crystalline insulator Pb0.5Sn0.5Te

0.90

Giant electronic polarization induced by a helical spin order in SrMn7O12: First-principles calculations

\;