Hybridization effects in $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_5$ observed by angle-resolved photoemission

We have investigated the low-energy electronic structure of the heavy fermion superconductor $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_5$ by angle-resolved photoemission. We focus on the dispersion and the peak width of the prominent quasi-two-dimensional Fermi surface sheet at the corner of the Brillouin zone as a function of temperature along certain $k$ directions with a photon energy of $h\nu =100\mathrm{eV}$. We find slight changes of the Fermi vector and an anomalous broadening of the peak width when the Fermi energy is approached. Additionally, we performed resonant angle-resolved photoemission spectroscopy experiments with $h\nu =121\mathrm{eV}$. A flat $f$-derived band is observed with a distinct temperature dependence and a $k$-dependent spectral weight. These results, including both off- and on-resonant measurements, fit qualitatively to a two-level mixing model derived from the periodic Anderson model.

Figure 1

(Color online) Fermi surface map at $T=25\mathrm{K}$ with $h\nu =100\mathrm{eV}$ photon energy. Bright yellow denotes regions of high intensity. The first Brillouin zone (BZ) is highlighted by dashed lines. The Fermi surface consists of closed squarish structures centered at the corner of the BZ and bright complex structures around the BZ center. The light blue bars mark the positions of the measurements shown in Figs. 2, 3.

Figure 2

(Color online) Energy distribution maps along the zone diagonal (see the diagonal bar in Fig. 1) for (a) $T=130\mathrm{K}$ and (b) $T=25\mathrm{K}$ and $h\nu =100\mathrm{eV}$. The white arrows highlight the slight temperature dependence of the dispersion. (c) Momentum distribution curves (MDCs) extracted from (a) and (b). The energy positions of the blue/red MDC pairs are given by the roman numbers. The upper blue/red MDC pair corresponds to the Fermi energy. The peak heights have been normalized to one. The MDCs are integrated over energy windows of 12 and of $24\mathrm{meV}$ directly at the Fermi energy. The black/green MDC pair is a comparison of different energies. The $E=147\mathrm{meV}$ MDC has been shifted along $k$ to enable comparison of the peak width. The black dot highlights the approximate position of a second weak feature.

Figure 3

(Color online) Energy distribution maps parallel to the zone boundary (see the horizontal bar in Fig. 1) for (a) $T=130\mathrm{K}$ and (b) $T=25\mathrm{K}$ and $h\nu =100\mathrm{eV}$. (c) MDCs extracted from (a) and (b). The energy positions of the blue/red MDC pairs are given by the roman numbers. The upper blue/red MDC pair corresponds to the Fermi energy. The peak heights have been normalized to one. The MDCs are integrated over energy windows of 12 and of $24\mathrm{meV}$ directly at the Fermi energy. The black/green MDC pair is a comparison of different energies. The $E=180\mathrm{meV}$ MDC has been shifted along $k$ to enable comparison of the peak width.

Figure 4

(Color online) Comparison of typical valence band spectra measured off $\left(117\mathrm{eV}\right)$ and on $\left(121\mathrm{eV}\right)$ resonance and with high photon energy $\left(1486.6\mathrm{eV}\right)$. The off-resonant spectra is dominated by a $\mathrm{Co}3d$ derived peak at $E=-0.7\mathrm{eV}$. On resonance there is a strong enhancement around $E=-1.4\mathrm{eV}$ and a smaller enhancement in the vicinity of $E_F$. The high energy valence band features are broadened but match the low-energy features. The spectra have been normalized to the peak maximum.

Figure 5

(Color online) (Upper row) Energy distribution maps (EDMs) along the horizontal blue line in Fig. 1 measured off and on resonance as a function of temperature. Note the parabola shaped band (red circles) and the appearance of the additional flat band (black circles) near $E_F$ in panel (c). [(b), (d), (f), and (h)] Energy distribution curves integrated from the regions enclosed by the pink or black dashed lines. (Upper row) to the left from $k_F$ (inside the parabola); (lower row) to the right from $k_F$ (outside the parabola). The dashed red line in the lower panels is a guide for the eyes indicating approximately the background contribution derived from panel (b).

Figure 6

(Color online) (a) Schematic view on the Anderson lattice model.[2] The dashed lines are a dispersing valence band and the dispersionless renormalized $f$ states above the Fermi energy. For a finite hybridization, a gap opens and two branches with $k$ dependent orbital weight are formed $(E_1,E_2)$. The $E_F$ crossing is adjusted to accommodate the additional $f$ electrons. The Fermi velocity is reduced and the effective mass is enhanced. (b) Off-resonant spectral weight as seen by photoemission. (c) On-resonant spectral weight.