Angle-resolved photoemission study of the strongly correlated semiconductor FeSi

The temperature-dependent electronic states of FeSi have been studied by using high-resolution angle-resolved photoemission spectroscopy (ARPES) and using low-energy tunable photons. At low temperatures, a peak indicating the valence-band maximum (VBM) exists at a binding energy of $\sim 20\text{meV}$ along the $\Gamma R$ direction. The observed dispersional width of the energy bands is narrower than that given by the band-structure calculation, and the width of the ARPES peak near the VBM rapidly broadens as the binding energy increases. Analysis of a model self-energy reveals the importance of electron correlation, especially near the VBM. We observed an unusual temperature dependence of the ARPES spectral features near the Fermi level $\left(E_F\right)$: Below $\sim 100\text{K}$, the peak at the VBM and the energy gap structures are almost unchanged, while at $\sim 100–350\text{K}$, the peak gradually moves toward $E_F$ and the gap is filled. The present results indicate that FeSi is a strongly correlated semiconductor, with a renormalized band near $E_F$ being responsible for the rapid collapse of the peak and the coherent energy gap upon heating.

Figure 1

(a) Brillouin zone of FeSi. The $\Gamma \text{XRM}$ plane (gray) was measured by ARPES experiment. (b) Relation between $k_{\perp }$ and $k_{\parallel }$. The solid curve shows the cross-section of the Brillouin zone measured by the ARPES with $h\nu =23.5\text{eV}$. (c) ARPES spectra of FeSi taken at $h\nu =23.5\text{eV}$. Structures are indicated by triangles.

Figure 2

(a) Intensity plot of the ARPES spectra. The open circles indicate the peak positions. (b) Calculated LSDA band structures along the solid line in Fig. 1b. (c) Real and imaginary parts of $\Sigma \left(\omega \right)$ assumed in the present analyses. (d) Intensity map of $A\left(\mathbf{k},\omega \right)$ with the self-energy correction. A Gaussian function with a width of 5 meV FWHM was convoluted to represent energy resolution.

Figure 3

(a) Normal emission ARPES spectra at 5 K. The vertical bars show the energy positions given by the LDA calculation. The energy position of the calculated VBM has been adjusted to coincide with the observed VBM. (b) Measured $k$ points in the BZ by means of normal emission ARPES. (c) Band structure given by the LDA calculation along the $\Gamma R$ line.

Figure 4

(Color online) Temperature-dependent spectra at $h\nu =8.44\text{eV}$. The inset shows the spectra close to $E_F$ for $h\nu =8.44$ and 5.48 eV.

Figure 5

(Color online) Temperature-dependence of photoemission spectra at 8.44 eV in normal emission. (a) Experimental $A\left(\mathbf{k},\omega \right)$. The positions of the peak and the gap are indicated by triangles. (b) Temperature dependence of the peak and gap intensity. (c) Peak position as a function of temperature.