Fine Structure in the Electronic Density of States near the Fermi Energy of Al-Ni-Co Decagonal Quasicrystal from Ultrafast Time-Resolved Optical Reflectivity

We measured the temperature and fluence dependence of the time-resolved photoinduced optical reflectivity in a decagonal ${\mathrm{Al}}_{71.9}{\mathrm{Ni}}_{11.1}{\mathrm{Co}}_{17.0}$ quasicrystal. We find no evidence for the relaxation of a hot thermalized electron gas as observed in metals. Instead, a quick diffusion of the hot nonthermal carriers $\sim 40\mathrm{nm}$ into the bulk is detected, enhanced by the presence of a broad pseudogap. From the relaxation dynamics we find evidence for the suppression of the electronic density of states (DOS) at the Fermi energy with respect to the electronic DOS at $\sim 13\mathrm{meV}$ away from the Fermi energy which is consistent with recent theoretical calculations.

Figure 1

The probe polarization and photon energy dependence of photoinduced-reflectivity transients at 5 K (a). The scans are normalized to enable easier comparison. Temperature dependence of photoinduced-reflectivity transients (b) and fluence dependence of photoinduced-reflectivity transients at two different temperatures (c) and (d) at 1.55-eV probe-photon energy and polarization along the periodic direction.

Figure 2

Fits of Eq. (1) to $\Delta R/R$ at $\mathcal{F}=320\mu \mathrm{J}/{\mathrm{cm}}^2$ (a) and at $\mathcal{F}=32\mu \mathrm{J}/{\mathrm{cm}}^2$ (b) shown by the thick solid lines. For comparison a function proportional to $1/\sqrt{t}$ in (a) and a single exponential fit in (b) are shown by the thin solid lines. The inset shows the temperature dependence of the fit parameter. The temperature dependence of the initial energy-distribution depth (c) and the initial surface-temperature rise at $32\mathrm{\text{−}}\mu \mathrm{J}/{\mathrm{cm}}^2$ excitation fluence (d).

Figure 3

Temperature dependencies of the intermediate–time scale peak magnitude of $\Delta R/R$ (a) and the intermediate–time scale rise time (b) at the two different fluences. In (a) the small open symbols connected with dotted lines represent $\Delta f_T(E_p)$ discussed in text. The solid line is a fit of Eq. (3) to the data.

Figure 4

Fluence dependence of the intermediate–time scale peak magnitude of $\Delta R/R$ at 5 K. For comparison $\Delta f_T(E_p)$ is shown with $E_p/k_B=150\mathrm{K}$ as the solid line assuming 45-nm initial energy deposition depth and published values of the heat capacity data [15, 16, 17] in similar decagonal quasicrystals.