Suchergebnisse

Low-temperature scanning tunneling microscopy and spectroscopy has been used to image the vortex core and the vortex lattice in FeSe single crystals. The local tunneling spectra acquired at the center of elliptical vortex cores display a strong particle-hole asymmetry with spatial oscillation, characteristic of the quantum-limit vortex core. Furthermore, a quasihexagonal vortex lattice at low magnetic field undergoes noticeable rhombic distortions above a certain field ∼1.5 T. This field H∗ also reveals itself as a kink in the magnetic field dependence of the specific heat. The observation of a nearly hexagonal vortex lattice at low field is very surprising for materials with an orthorhombic crystal structure and it is in apparent contradiction with the elliptical shape of the vortex cores. These observations can be directly connected to the multiband nature of superconductivity in this material, provided we attribute them to the suppression of superconducting order parameter in one of the energy bands. Above the field H∗ the superconducting coherence length for this band can well exceed the intervortex distance which strengthens the nonlocal effects. Therefore, in addition to multiple-band effects, other possible sources that can contribute to the observed evolution of the vortex-lattice structure include nonlocal effects which cause the field-dependent interplay between the symmetry of the crystal and vortex lattice or the magnetoelastic interactions due to the strain field generated by vortices. © 2019 American Physical Society. ; Citrus Research and Development Foundation, CRDF ; Government Council on Grants, Russian Federation ; Russian Science Foundation, RSF: 17-12-01383, 18-72-10027 ; Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR ; Foundation for the Advancement of Theoretical Physics and Mathematics: 17-11-109 ; Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR ; Kazan Federal University ; Office of Science, SC ; Division of Materials Sciences and Engineering, DMSE ; Russian Foundation for Basic Research, RFBR: 17-52-12044 ; Ministry of Education and Science of the Russian Federation, Minobrnauka ; Temple University, TU ; Argonne National Laboratory, ANL ; Nanjing University of Science and Technology, NUST: K2-2017-084 ; Drexel University ; The authors would like to acknowledge fruitful discussions with V. Kogan and T. Hanaguri. We also would like to acknowledge technical support during the early stage of these measurements from S. A. Moore. The work at Temple University, where low temperature scanning tunneling measurements were performed, was supported by US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division under Award No. DE-SC0004556. The work at Drexel University and at the M.V. Lomonosov Moscow State University was supported by the US Civilian Research and Development Foundation (CRDF Global). The work in Russia has been supported in part by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST MISiS Grant K2-2017-084, by Act 211 of the Government of Russian Federation, Contracts No. 02.A03.21.0004, No. 02.A03.21.0006, and No. 02.A03.21.0011 and by the Russian Government Program of Competitive Growth of Kazan Federal University. One of the authors (C.D.G.) would like to acknowledge partial support from MIUR (Ministry of Education, Universities and Research of the Italian Government). The work in IPM RAS (Nizhny Novgorod) was supported in part by the Russian Science Foundation (the calculation of the vortex-lattice characteristics Grant No. 18-72-10027; the calculation of the vortex-core deformation and the analysis of the experimental data Grant No. 17-12-01383), the Russian Foundation for Basic Research (Grant No. 17-52-12044), and Foundation for the Advancement of Theoretical Physics and Mathematics "BASIS" (Grant No. 17-11-109). The work at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.