Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.

We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (lambda{1}/xi{1}<1/sqrt[2]) and type-2 (lambda{2}/xi{2}>1/sqrt[2]) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These vortex clusters and their field dependence are directly visualized by scanning superconducting quantum interference device microscopy at very low vortex densities in MgB 2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime. DOI: 10.1103/PhysRevB.81.020506 PACS number͑s͒: 74.25.Uv, 74.70.Ad In magnesium diboride, 1 MgB 2 , the superconducting gaps open for both the two-dimensional ͑2D͒ band ͓gap size ⌬ = 7.1 meV ͑Refs. 2 and 3͔͒ and the three-dimensional band ͓⌬ = 2.2 meV ͑Refs. 3 and 4͔͒. The possibility of type-1.5 superconductivity 5 has been suggested for clean single crystals of MgB 2 , which lie in the type-1.5 regime: = 3.7Ͼ 1 / ͱ 2 ͑type-2͒ and = 0.66Ͻ 1 / ͱ 2 ͑type-1͒, where ␣ ͑␣ = , ͒ is the Ginzburg-Landau ͑GL͒ parameter for each band estimated from the band-structure calculations. 6In contrast to conventional type-2 superconductors, 7 in type-1.5 superconductors, vortex stripes and gossamerlike vortex patterns, i.e., vortex clusters emerge at relatively low applied fields, 5 which is due to a competition between attractive ͑type-1͒ and repulsive ͑type-2͒ vortex interactions governed by a two-gap GL theory. [8][9][10][11][12] Interestingly, a substantial difference in vortex structure between type-1.5 and type-2 superconductors, as a fingerprint of type-1.5 superconductivity, is expected at very low vortex densities because the intervortex distances in clusters are likely to be almost independent of the applied field in type-1.5 superconductors, whereas the intervortex distances in type-2 superconductors follow the conventional dependence ͑ 0 / B͒ 1/2 , where 0 ͑=2.07ϫ 10 −15 T m͒ is the flux quantum and B the magnetic field. This motivates strongly direct vortex visualization experiments aimed at investigating vortex structure at very low vortex densities in MgB 2 single crystals. Additionally, it is important to verify the existence of type-1.5 superconductivity in high quality MgB 2 crystals different from the ones used in Ref. 5.In order to study vortex structure in MgB 2 , we made scans with a scanning superconducting quantum interference device ͑SQUID͒ microscope on single crystals of MgB 2 which were grown by a pressure synthesis technique 13 ͑a crystal grown by this technique was used elsewhere 14 ͒. The superconducting transition temperatures T c of the crystals measured by a SQUID magnetometer are 38.5 K ͓the transition width ⌬T c = 0.8 K ͑10-90 % criterion͔͒. The surface of crystals was confirmed not to have any cracks or holes by using a field-emission scanning electron microscope in the secondary electron regime. High-resolution transmission electron microscope images and electron-diffraction patterns for the crystals show that the crystals have no grain boundaries. No impurity contamination was detected within 0.1% accuracy by an electron probe microanalyzer.Our scanning SQUID microscope is base...

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