| Whether unconventional high-temperature superconductivity can be understood within the electron-phonon coupling BCS theory remains one of the most important and challenging problems in condensed matter physics. To solve this problem, one can grow heterostructure to simulate the layered structure of unconventional high-temperature superconductors, and check how the interface can significantly enhance transition temperature and investigate whether the BCS theory for conventional superconductors is valid. Based on this idea, in this thesis, by combining molecular beam epitaxy (MBE), scanning tunneling microscope/spectroscopy (STM/S), angle resolved photoelectron spectroscopy (ARPES) and transport and magnetic measurements, we study the growth and superconducting properties of single unit-cell (1-UC) FeSe films (as a superconducting layer) on SrTiO3substrates (as a charge reservoir layer). The main results and conclusions are summarized as follows:(1) Atomically flat FeSe films have been successfully obtained on insulating SrTiO3(001) substrates in a well-controlled layer-by-layer growth mode by MBE. Crystalline FeTe protection layers have been also well prepared on FeSe films. By in situ STM, we have studied the surface morphologies of SrTiO3substrates, FeSe films and FeTe protection layers. Scanning transmission electron microscopy (STEM) reveals that the interface of FeSe/SrTiO3or FeTe/FeSe is atomically sharp without interlayer chemical mixing. The success of growing non-superconducting FeTe protection layers allows us to carry out ex situ transport measurements of the FeSe/SrTiO3heterostructure.(2) By transport, high-temperature superconductivity in1-UC FeSe films on SrTiO3is established. The superconducting transition temperature is above40K, which is5times higher than the bulk value of FeSe. The voltage-current curves indicate that the superconductivity transition behaves in a Berezinskii-Kosterlitz-Thouless (BKT) manner of two-dimensional system. The1-UC films exhibit extremely a very high critical current density of1MA/cm2(2K,0T), which is two orders of magnitude larger than that of bulk FeSe. The ARPES shows a very simple Fermi surface and band structure. There are only electron-like pockets near the Brillouin zone corner without any indication of Fermi surface around the zone center, which is distinct from other Fe-based superconductors. The superconducting gap is nearly isotropic and nodeless.(3) We have investigated the evolution of the superconducting properties of1-UC FeSe/SrTiO3films at different annealing temperatures by in situ STM/S, ARPES and ex situ transport measurements. We find that the superconductivity develops with the formation of stoichiometric FeSe films and is indeed enhanced by charge transfer from SrTiO3substrates to FeSe films. Moreover, the superconductivity is independent of the bulk property of the SrTiO3substrate, regardless of whether it is insulating or conductive. Our results reveal that the high-temperature superconductivity of1-UC FeSe/SrTiO3films occurs at the FeSe/SrTiO3interface, where the electron doping into FeSe films plays an important role in this interfacial superconductivity. The scenario is further supported by field effect experiment.(4) By electrical transport measurements, we have calculated the upper critical magnetic field, which is approximately140T and exhibits very strong anisotropy. In the thermal-active flux-flow region of superconducting mixed states, we have found the vortex pinning in the form of single-vortex hopping in low magnetic field and the collective flux creep in high magnetic field. We have also observed a quantum linear magnetoresistance in thicker FeSe films on SrTiO3, which indicates Dirac-cone states in their electronic structure.In summary, we have unambiguously demonstrated the interface enhanced superconductivity in the1-UC FeSe/SrTiO3system. Our work not only opens up a window of studying new quantum phenomena and fundamental physical properties at interface, but also provides a promising avenue for exploring new high-temperature superconducting materials. |
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