Topological superconductivity is the quantum condensate of paired electrons with an odd parity of the pairing function. By using a Corbino-shape like electrode configuration, we measure the c-axis resistivity of the recently discovered superconductor SrxBi2Se3 with the magnetic field rotating within the basal planes, and find clear evidence of two-fold superconductivity. The Lane diffraction measurements on these samples show that the maximum gap direction is either parallel or perpendicular to the main crystallographic axis. This observation is consistent with the theoretical prediction and strongly suggests that SrxBi2Se3 is a topological superconductor.
Guan DuYuFeng LiJ. SchneelochR. D. ZhongGenDa GuHuan YangHai LinHai-Hu Wen
Since the discovery of high-temperature superconductivity(SC) in quasi-two-dimensional copper oxides,a few layered compounds,which bear similarities to the cuprates,have also been found to host the unconventional SC. Our recent observation of SC at 6.1 K in correlated electron material K2Cr3As3 represents an obviously different paradigm,primarily because of its quasi-one-dimensional(Q1D) nature. The new material is structurally featured by the [(Cr3As3)2-]∞ double-walled subnano-tubes composed of face-sharing Cr6/2(As6/2) octahedron linear chains,which are well separated by the columns of K+ counterions. Later,an isostructural superconducting Rb2Cr3As3 was synthesized,thus forming a new superconducting family. Here we report the third member,Cs2Cr3As3,which possesses the largest interchain distance. SC appears below 2.2 K. Similar to the former two sister compounds,Cs2Cr3As3 exhibits a non-Fermi liquid behavior with a linear temperature dependence of resistivity in the normal state,and a high upper critical field beyond the Pauli limit as well,suggesting a common unconventional SC in the Q1 D Cr-based material.
The second class of high-temperature superconductors （HTSCs）, iron-based pnictides and chalcogenides, necessarily contain Fe2X2 （＂X＂ refers to a pnictogen or a chalcogen element） layers, just like the first class of HTSCs which possess the essential CuO2 sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing ＂hard and soft acids and bases （HSAB）＂ concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.
In this article, we review the recent theoretical works on the spin fluctuations and superconductivity in iron-based superconductors. Using the fluctuation exchange approximation and multi-orbital tight-binding models, we study the char- acteristics of the spin fluctuations and the symmetries of the superconducting gaps for different iron-based superconductors. We explore the systems with both electron-like and hole-like Fermi surfaces （FS） and the systems with only the electron-like FS. We argue that the spin-fluctuation theories are successful in explaining at least the essential part of the problems, indicating that the spin fluctuation is the common origin of superconductivity in iron-based superconductors.
Impurity scattering in a superconductor may serve as an important probe for the nature of superconducting pairing state. Here we re- port the impurity effect on superconducting transition temperature Te in the newly discovered Cr-based superconductor K2Cr3As3. The resistivity measurements show that the crystals prepared using high-purity Cr metal （≥99.99%） have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various non- magnetic impurities, however, the Tc decreases significantly, in accordance with the generalized Abrikosov-Gor＇kov pair-breaking theory. This finding supports a non-s-wave superconductivity in K2Cr3As3.
Yi LiuJin-Ke BaoHao-Kun ZuoAbduweli AblimitZhang-Tu TangChun-Mu FengZeng-Wei ZhuGuang-Han Cao
We have synthesized two iron fluo-arsenides ACa2Fe4As4Fz with A = Rb and Cs, analogous to the newly discovered superconductor KCazFe4As4F2. The quinary inor- ganic compounds crystallize in a body-centered tetragonal lattice with space group I4/mmm, which contain double Fe2As2 layers that are separated by insulating Ca2F2 layers. The electrical and magnetic measurements on the polycrys- talline samples demonstrate that the new materials undergo superconducting transitions at Tc = 30.5 and 28.2 K, respec- tively, without extrinsic doping. The correlations between Tc and structural parameters are discussed.
Superconductivity （SC） and ferromagnetism （FM） are in general antagonistic, which makes their coexistence very rare. Following our recent discovery of robust coexistence of SC and FM in RbEuFe4As4 （Liu et al. in Phys Rev B 93：214503, 2016）, here we report another example of such a coexistence in its sister com- pound CsEuFe4As4, synthesized for the first time. The new material exhibits bulk SC at 35.2 K and Eu2＋-spin ferro- magnetic ordering at 15.5 K, demonstrating that it is a new robust ferromagnetic superconductor.
Recently, new Cr-based superconductors, A2Cr3As3(A = K, Rb, Cs), have gained intense interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr3As3(A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A2Cr3As3. The new compounds remain one-dimensional(Cr3As3)∞ linear chains, and the interchain distance can be tuned by the incorporation of the alkali-metal cations with different sizes. The physical property measurements indicate a local-moment behavior at high temperatures, and the moments freeze into a cluster spin-glass state below 5–6 K. No superconductivity was observed in both materials. We also found that, with increasing the interchain distance, the Cr effective moments increase monotonically, accompanied with the enhancement of semi-conductivity. Our results shed light on the understanding of occurrence of superconductivity in A2Cr3As3.
