Anisotropic heavy Fermi-liquid formation in the valence fluctuating   $\alpha$-YbAlB$_4$
  $\alpha$-YbAlB$_4$ is the locally isostructural polymorph of $\beta$-YbAlB$_4$, the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above $T_{\rm c}$. Interestingly, both $\alpha$-YbAlB$_4$ and $\beta$-YbAlB$_4$ have strongly intermediate valence. Our single crystal study of the specific heat, magnetization and resistivity has confirmed the Fermi liquid ground state of $\alpha$-YbAlB$_4$ ~in contrast with the quantum criticality observed in $\beta$-YbAlB$_4$. Both systems exhibit Kondo lattice behavior with the characteristic temperature scale $T^* \sim$ 8 K in addition to a valence fluctuation scale $\sim 200$ K. Below $T^*$, $\alpha$-YbAlB$_4$ a heavy Fermi liquid state with an electronic specific heat coefficient $\gamma\sim$ 130 mJ/mol K$^2$ and a large Wilson ratio more than 7, which indicates ferromagnetic correlation between Yb moments. A large anisotropy in the resistivity suggests that the hybridization between 4$f$ and conduction electrons is much stronger in the $ab$-plane than along the c-axis. The strongly anisotropic hybridization as well as the large Wilson ratio is the key to understand the unusual Kondo lattice behavior and heavy fermion formation in mixed valent compounds. 
Yosuke Matsumoto, K. Kuga, T. Tomita, Y. Karaki and S. Nakatsuji
cond-mat.str-el