We have investigated the specific heat of optimally-doped iron chalcogenide superconductor Fe(Te 0.57 Se 0.43 ) with a high-quality single crystal sample. The electronic specific heat C e of this sample has been successfully separated from the phonon contribution using the specific heat of a non-superconducting sample (Fe 0.90 Cu 0.10 )(Te 0.57 Se 0.43 ) as a reference. The normal state Sommerfeld coefficient n  of the superconducting sample is found to be ~ 26.6 mJ/mol K 2 , indicating intermediate electronic correlation. The temperature dependence of e C in the superconducting state can be best fitted using a double-gap model with c 2 (0)/ 3.92 s B k T   and c 2 (0)/ 5.84 l B k T   . The large gap magnitudes derived from fitting, as well as the large specific heat jump of e c n c ( ) / ~2.11 C T T   , indicate strong-coupling superconductivity.Furthermore, the magnetic field dependence of specific heat shows strong evidence for multiband superconductivity.

The nature of metallicity and the level of electronic correlations in the antiferromagnetically ordered parent compounds are two important open issues for the iron-based superconductivity. We perform a temperature-dependent angle-resolved photoemission spectroscopy study of Fe1.02Te, the parent compound for iron chalcogenide superconductors. Deep in the antiferromagnetic state, the spectra exhibit a "peak-dip-hump" line shape associated with two clearly separate branches of dispersion, characteristics of polarons seen in manganites and lightly-doped cuprates. As temperature increases towards the Neel temperature (TN ), we observe a decreasing renormalization of the peak dispersion and a counterintuitive sharpening of the hump linewidth, suggestive of an intimate connection between the weakening electron-phonon (e-ph) coupling and antiferromagnetism. Our finding points to the highly-correlated nature of Fe1.02Te ground state featured by strong interactions among the charge, spin and lattice and a good metallicity plausibly contributed by the coherent polaron motion. The role of many-body interactions is one of the central questions for unconventional superconductivity. For the recently discovered iron-based superconductors, the strength of electronic correlations is still an unsettled issue [1,2]. For one of them, iron chalcogenides, a strong correlation scenario has been proposed by theory [3,4] and supported by experiments [5][6][7][8][9][10][11][12]. For their parent compound Fe 1+y Te, while the high-temperature paramagnetic (PM) state shows similar signs for localized physics as in the undoped high-Tc cuprates in transport [11] and optical [12] experiments, the metallic behavior in the low-temperature antiferromagnetic (AFM) state (at T

Superconductors with a chiral p-wave pairing are of great interest because they could support Majorana modes that could enable the development of topological quantum computing technologies that are robust against decoherence. Sr 2 RuO 4 is widely believed to be a chiral p-wave superconductor. Yet, the mechanism by which superconductivity emerges in this, and indeed most other unconventional superconductors, remains unclear. Here we show that the local superconducting transition temperature in the vicinity of lattice dislocations in Sr 2 RuO 4 can be up to twice that of its bulk. This is all the more surprising for the fact that disorder is known to easily quench superconductivity in this material. With the help of a phenomenological theory that takes into account the crystalline symmetry near a dislocation and the pairing symmetry of Sr 2 RuO 4 , we predict that a similar enhancement should emerge as a consequence of symmetry reduction in any superconductor with a two-component order parameter.

Odd-parity, spin-triplet superconductor Sr2RuO4 has been found to feature exotic vortex physics including half-flux quanta trapped in a doubly connected sample and the formation of vortex lattices at low fields. The consequences of these vortex states on the low-temperature magnetoresistive behavior of mesoscopic samples of Sr2RuO4 were investigated in this work using ring device fabricated on mechanically exfoliated single crystals of Sr2RuO4 by photolithography and focused ion beam. With the magnetic field applied perpendicular to the in-plane direction, thin-wall rings of Sr2RuO4 were found to exhibit pronounced quantum oscillations with a conventional period of the full-flux quantum even though the unexpectedly large amplitude and the number of oscillations suggest the observation of vortex-flow-dominated magnetoresistance oscillations rather than a conventional Little-Parks effect. For rings with a thick wall, two distinct periods of quantum oscillations were found in high and low field regimes, respectively, which we argue to be associated with the "lock-in" of a vortex lattice in these thick-wall rings. No evidence for half-flux-quantum resistance oscillations were identified in any sample measured so far without the presence of an in-plane field.