Strongly hybridized electronic structure of YbAl${}_2$: An angle-resolved photoemission study

We report the electronic structure of a prototypical valence fluctuation system, YbAl${}_2$, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4$f$ bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization suppresses band renormalization and is responsible for the valence fluctuations in YbAl${}_2$.

Figure 1

(a) BZ of YbAl${}_2$, in which $k_z$ corresponds to the (111) direction of the fcc BZ. In the $k_x$-$k_y$ plane, the midpoint of the zone edge ($K$) is referred to as $M^{\prime }$ point, which is also equivalent to the midpoint between $L$ and $U$ points. (b) and (c) FS mapping in the $k_x$-$k_z$ and $k_x$-$k_y$ planes, respectively. The FS is obtained by a plot of the integrated intensity within $E_{\mathrm{F}}\pm 50$ meV in the ARPES spectra. The color scale is shown at the right of each map. The FSs obtained by the LDA calculation are overlapped in (b) and (c).

Figure 2

(a) Intensity plot of the ARPES spectra of YbAl${}_2$ taken along the $\Gamma$-$K$ direction. The color scale is shown at the bottom left. (b) Angle-integrated spectrum obtained from (a). The 4$f_{7/2}$ peak is fitted by a Lorentzian multiplied by an FDD and an integral background. “S” and “SS” denote the surface and subsurface states, respectively.

Figure 3

(a) and (c) Series of ARPES spectra near $E_{\mathrm{F}}$ along the $\Gamma$-$K$ and $\Gamma$-$M^{\prime }$ directions, respectively. (b) and (d) ARPES intensity plots of (a) and (c), respectively. (e)–(h) The same data as in (a)–(d), normalized to the angle-integrated spectrum. The band dispersions obtained by the LDA calculation are overlapped in (b), (d), (f), and (h).

Figure 4

(a) and (b) Analysis of band dispersions along the $\Gamma$-$K$ and $\Gamma$-$M^{\prime }$ directions, respectively, by $c$-$f$ hybridized bands based on PAM. The dotted lines indicate the $c$ bands and $f$ states before hybridization. The experimental data points were obtained by a Lorentzian fitting of MDC or EDC. (c) and (d) Expansion of dispersion for the 4$f_{7/2}$ band. The angle-integrated data were obtained by the fitting of the angle-integrated spectra.