Hai-Hu Wen

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

Strong electron pairing at the iron 3d(xz) (yz) orbitals in hole-doped BaFe2As2 superconductors revealed by angle-resolved photoemission spectroscopy

D.V.Evtushinsky,1 V.B. Zabolotnyy,1 T.K.Kim,1, 2 A.A.Kordyuk,1, 3 A.N. Yaresko,4 J.Maletz,1 S.Aswartham,1 S.Wurmehl,1 A.V. Boris,4 D.L. Sun,4 C.T. Lin,4 B. Shen,5 H.H.Wen,6 A.Varykhalov,7 R.Follath,7 B.Büchner,1 and S.V.Borisenko1 Institute for Solid State Research, IFW Dresden, P.O.Box 270116, D-01171 Dresden, Germany Diamond Light Source Ltd., Didcot, Oxfordshire, OX11 0DE, United Kingdom Institute of Metal Physics of National Academy of Sciences of Ukraine, 03142 Kyiv, Ukraine Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China BESSY GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, T-c. We investigated the temperature dependence of the lower critical field H-c1(T) of a Ca0.32Na0.68Fe2As2 (T-c approximate to 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same T-c. We estimated the range of the large gap value Delta(L) = 6-8 meV (depending on small variation of T-c) and its a k space anisotropy of about 30%, and the small gap Delta(S) approximate to 1.7 +/- 0.3 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.

Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2

In a Josephson junction, a current flows from one superconductor to another through a barrier without any voltage being applied. SQUIDs, for example, are based on this phenomenon. Now, an iron-based multi-band superconductor shows signs of intrinsic Josephson junctions, opening up prospects for applications.

Evidence for nodeless superconductivity in NdO1−xFxBiS2 (x = 0.3 and 0.5) single crystals

We study the superconducting pairing states of NdO(1-x)F(x)BiS2 (x = 0.3 and 0.5) by measuring the magnetic penetration depth Δλ(T) using the tunnel-diode-oscillator (TDO) technique. An upturn is observed in Δλ(T) as well as the magnetic susceptibility χ(T) in the low-temperature limit, which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth Δλ(T) follows an exponential-type temperature dependence at T ≪ T(c), providing evidence of nodeless superconductivity for NdO(1-x)F(x)BiS2. This is further supported by the analyses of superfluid density ρ(s)(T), which can be well described by a BCS model with an energy gap of Δ(0) ∼ 2.15 k(B)T(c).

Evolution from incoherent to coherent electronic states and its implications for superconductivity in FeTe1-xSex

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe1-x Se-x (0 <= x <= 0.45) to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a well-defined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergence of superconductivity.

Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting KxFe2 ySe2 single crystals

In-plane resistivity, magnetoresistance and Hall effect measurements have been conducted on quenched K

Critical behavior of the spin density wave transition in underdoped Ba(Fe1-xCox)(2)As-2 (x <= 0.05): As-75 NMR investigation

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Interplane resistivity of underdoped single crystals (Ba1-xKx)Fe2As2(0<= x < 0.34)

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Competition between superconductivity and magnetic/nematic order as a source of anisotropic superconducting gap in underdoped Ba1-xKxFe2As2

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