YbB6 becoming a topological Kondo insulator under pressure
Topological Kondo insulator possesses characteristics of both the strong electron correlations and the topological configurations, as shown in a mixed-valence material SmB6. YbB6 is a structural analog of SmB6, but remains controversial whether compressed YbB6 material is a topological insulator or a Kondo topological insulator. A research team used HPCAT facility and performed x-ray measurements on YbB6samples. Both the high-pressure powder x-ray diffraction and optical Raman measurements show no trace of structural phase transitions in YbB6 up to 50 GPa. The x-ray absorption measurements reveal a gradual change of Yb valence from nonmagnetic Yb2+ to magnetic Yb3+above 18 GPa concomitantly with the increase in resistivity. A complete transition to insulating state occurs at pressures above 30 GPa, accompanied by an increase in the shear stress and anomalies in the pressure dependence of the Raman T2g mode and in the B atomic position. The resistivity at high pressures can be described by a model taking into account coexisting insulating and metallic channels with the activation energy for the insulating channel about 30 meV. These results indicate that YbB6 may become a topological Kondo insulator at high pressures above 35 GPa. More in J. Ying et al., Phys Rev. B, 97, 121101(R), 2018.