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.

The Ginzburg-Landau functional for a two-gap superconductor is derived within the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then, applied to investigate various magnetic properties of MgB2 including an upturn temperature dependence of the transverse upper critical field and a core structure of an isolated vortex. Orientation of vortex lattice relative to crystallographic axes is studied for magnetic fields parallel to the c-axis. A peculiar 30 • -rotation of the vortex lattice with increasing strength of an applied field observed by neutron scattering is attributed to the multi-gap nature of superconductivity in MgB2.

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...

The effect of biaxial strain on the oxygen octahedra rotations in a LaVO 3 thin film is investigated using synchrotron radiation. First, we find that the film adopts a distorted orthorhombic structure under the compressive stress induced by the SrTiO 3 substrate. Second, we separate the contribution to the superstructure peaks arising from cation displacement and VO 6 rotations in order to quantify the rotation angles. Finally, we find an original a − a + c − tilt system, which is induced by the biaxial strain imposed by the substrate. These quantitative results may open up new directions for understanding the modification of electronic properties of engineered oxide films.