We report the time-resolved excited state ultrafast dynamics of single unit cell (1 UC) thick FeSe films on SrTiO_{3} (STO), with FeTe capping layers. By measuring the photoexcited quasiparticles' density and lifetime, we unambiguously identify a superconducting (SC) phase transition, with a transition temperature T_{c} of 68 (-5/+2) K and a SC gap of Δ(0)=20.2±1.5 meV. The obtained electron-phonon coupling strength λ is as large as 0.48, demonstrating the likely crucial role of electron-phonon coupling in the high temperature superconductivity of the 1 UC FeSe on STO systems. We further find a 0.05 THz coherent acoustic phonon branch in the capping layer, which provides an additional decay channel to the gluing bosons.
The
structure of self-assembled methylthiolate monolayers on Au(111) as
a function of surface coverage has been investigated using scanning
tunneling microscopic imaging, and complete structural phases are
obtained. A full layer (1/3 ML) of methylthiolate organizes into a
long-range ordered (3 × 4) phase with chiral domains. The domain
boundary has a local (3 × 4√3)-rect structure. Gradual
removal of methylthiolate by partial thermal desorption leads first
to the formation of a local (7 × √3)-rect phase with coverage
equal to 0.286 ML. This (7 × √3)-rect phase is found to
coexist with the (3 × 4) phase and a disordered phase. As the
coverage is reduced to below 0.28 ML, stripes parallel to the ⟨112̅⟩
directions appear. All the phases are made from the basic, CH3S–Au–SCH3, structural motif. Cis–trans transformation of the
CH3S–Au–SCH3 unit is observed.
For an isolated CH3S–Au–SCH3 unit,
both methyl groups have equal probability of contributing to the cis–trans transformation at 77 K.
Within the stripe phase, only one of the methyl groups actively participates
in cis–trans transformation
while the other group remains as a spectator.
Recent reports [1,2,3] of interface--induced superconductivity in the unit--cell films of FeSe on SrTiO 3 with a T c up to ~ 65±5 K, highest among the Fe--based superconductors and second only to that of the cuprates, have generated great excitement. Here we show results of the first magnetic and resistive investigation on the 1--4 unit--cell FeSe--films on SrTiO 3 . The samples display the Meissner state below ~ 20 K with a penetration field as low as 0.1 Oe due to large edge effect at 2 K, a mesoscopic superconducting state comprising patches of dimension < 1 µm up to 45~55 K and an unusual relaxation of the diamagnetic signal up to 80 K, suggesting possible superconductivity at this high temperature. The observations demonstrate the heterogeneous nature of the superconducting state in these films and offer new insights into the role of interfaces in high temperature superconductivity proposed and future superconducting electronics fabricated.
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