Anisotropic Thermal Expansion of α-YbAlB4

Abstract

Thermal expansion measurements were performed for α-YbAlB4 using powder X-ray diffraction and our high resolution capacitance dilatometry. Highly anisotropic thermal expansion, which is much larger along the c-axis compared to the a- and b-axes, was observed. In addition, high resolution capacitance dilatometry revealed negative thermal expansion appears below 90 K only along the a-axis, not along the c-axis. The observation is consistent with anisotropic hybridization stronger in the ab-plane, suggesting the formation of anisotripic heavy fermion well below the valence fluctuation temperature scale of 200 ∼ 300 K. Furthermore, linear thermal expansion coefficient along the a-axis starts to decrease below 115 K. The characteristic temperature scale of 90 - 115 K may correspond to the crystal electric field excited level. The in-plane negative thermal expansion can be regarded as a development of anisotropic hybridization due to |Jz = ±5/2 > crystal electric field ground states which becomes dominant on cooling below the excited states.