Ultrafast Dynamics Evidence of High Temperature Superconductivity in Single Unit Cell FeSe on ${\mathrm{SrTiO}}_3$

We report the time-resolved excited state ultrafast dynamics of single unit cell (1 UC) thick FeSe films on ${\mathrm{SrTiO}}_3$ (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 $(-5/+2)\mathrm{K}$ and a SC gap of $\mathrm{\Delta }(0)=20.2\pm 1.5\mathrm{meV}$. The obtained electron-phonon coupling strength $\lambda$ 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.

Figure 1

Normalized photoinduced differential reflectivity. Scanning traces at different temperatures are illustrated, with zoom-in view shown in insets. The purple arrows mark the hump varying trends on temperature increase, respectively. Also shown is the schematic sample configuration.

Figure 2

Qualitative evidence of the SC transition. The two dimensional mapping of the differential reflectivity as a function of both temperature and time illustrates qualitatively that a SC phase transition exists at $\sim 68\mathrm{K}$. The white line marks the SC $T_c$. The temporal step sizes are 0.23 ps for the $-2.8-43.5\mathrm{ps}$ range and 3.33 ps for the 43.5–173 ps range; the temperature step size varies, as explicitly shown in Fig. 3. Appropriate interpolation is used to make smoother coloring.

Figure 3

Quantitative evidence for SC transition at $68(-5/+2)\mathrm{K}$. Temperature dependences of the amplitude (a) and lifetime (b) of the fast component, where the red dots in (b) are the deconvoluted data. Temperature dependences of the amplitude (c) and lifetime (d) of the slow component, where the red lines are theoretical simulation curves. The orange stripes mark the $T_c$ value of 63–70 K. The light-blue stripe marks the non-SC phase transition in the FeTe capping layer. Inset: Density of thermally excited quasiparticles (c) and zoom-in view of the data around 8 K (d).

Figure 4

Coherent acoustic phonon in the capping layer and the decay of the gluing bosons. (a) Ultrafast dynamics of the 2 UC $\mathrm{FeTe}/\mathrm{STO}$ sample. Solid curves: single exponential fittings. The superimposed coherent acoustic phonons are further displayed in the lower panel. Inset: Fourier transformation. (b) Schematic phonon decay tunneling through the FeSe layer. Red wave: optical phonons in STO and at the interface. Yellow waves: acoustic phonons in the FeTe capping layer. (c) Corresponding $K$-space schematic diagram. Thick red arrows: new channels of relaxation. LO, TO, and LA: longitudinal optical, transverse optical, and longitudinal acoustic phonons.