No abstract

We report the detailed phase diagram and anomalous transport properties of Fe-based high-T_{c} superconductors SmFeAsO1-xFx. It is found that superconductivity emerges at x approximately 0.07, and optimal doping takes place in the x approximately 0.20 sample with the highest T_{c} approximately 54 K. T_{c} increases monotonically with doping; the anomaly in resistivity from structural phase or spin-density-wave order is rapidly suppressed, suggesting a quantum critical point around x approximately 0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the x<0.14 regime but at low temperatures just above T_{c} in the x>0.14 regime; a drop in carrier density evidenced by a pronounced rise in the Hall coefficient is observed below the temperature of the anomaly peak in resistivity. A scaling behavior is observed between the Hall angle and temperature: cottheta_{H} proportional, variantT;{1.5} for all samples with different x in SmFeAsO1-xFx system.

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.

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.

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.

No abstract

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The BiS 2 -based superconductors were discovered recently 1-2 . The superconductivity has been proved by many other groups 3-10 . Since the previous experiments were all done on polycrystalline samples, therefore there remains a concern whether the superconductivity is really derived from the materials intrinsically or from some secondary phases 11 . Experiments on single crystals are highly desired. In this paper, we report the successful growth of the In the Bardeen-Cooper-Schrieffer (BCS) theory, the electron pairing and condensation occur simultaneously at T c . The superconducting fluctuation may extend to above T c in a very narrow region (less than 10% T c ) 12 . Therefore it is a surprise that the superconducting fluctuation appears at temperatures far above T c in the cuprates [13][14][15][16] . In some thin but dirty metallic films, people were pursuing a man-made wide superconducting fluctuation region in terms of the Cooper pair gas state [17][18] . In addition, in the BCS scheme, the normal state after the superconductivity is suppressed should show a metallic behavior. Any semiconducting or insulating behavior appearing in the normal state should have its special reasons. Within the weak electron-phonon coupling BCS picture, one has=3.5 with ∆ s the superconducting gap. This ratio can be slightly higher in the conventional superconductors with strong coupling, but can reach about 8 in the cuprates 19 . In this paper we will show that 3 all three features mentioned above for the BCS scheme will be violated in the newly discovered superconductor NdO 1-x F x Bi 1-y S 2 single crystal samples.The single crystals NdO 1-x F x Bi 1-y S 2 were grown using flux method with KCl/LiCl as the flux. The details about the sample growth are given in Methods. The crystals are very shiny with the dark-grey color. In Fig.1b, we show the Laue diffraction pattern on one crystal. The clear and symmetric spots can be well fitted to the model calculations of crystallography, indicating high quality of the crystals. From the data of Laue diffraction pattern, we can determine the in-plane crystalline axes, which is very helpful for the precise in-plane resistive measurements. In Fig.1c, we show the X-ray diffraction (XRD) patterns for the samples grown with different nominal concentrations of fluorine. It is clear that only (00l) reflections can be observed yielding a c-axis lattice constant c = 13.49±0.04 Å. In Fig.1d, we show a picture of one crystal on which the composition is analyzed using the energy-dispersion-spectrum (EDS). From the EDS data, one can see that our crystal has a formula like NdO 1-x F x Bi 0.84 S 1.94 . Since the oxygen and fluorine are both light elements, the value given here about them are not reliable, although the nominal compositions of them are well documented. We found that the samples with the nominal fluorine concentration less than 30% are not or bad superconductive. Due to the error bars of the EDS measurements, we can conclude that the composition of sulfur here is close to 2, but Bi is ...