Temperature and Pressure Dependencies of Sm Valence in SmB6 Kondo Insulator Probed by X-Ray Absorption Spectroscopy

Pressure and temperature induced valence changes in the Kondo insulator SmB 6 have been investigated using x-ray absorption spectroscopy (XAS) along several isobaric cooling runs and one isothermal compression. The average valence of Sm at various pressure and temperature conditions is extracted from high quality XAS data by peak fitting procedures. The temperature dependence of the Sm valence at ambient pressure shows changes of slope near the known characteristic temperatures of gap formation (120 K) and the in -gap state (50 K). At higher pressures the average valence also shows similar temperature dependent trends upon cooling but the slope changes are less pronounced. This behavior is discussed in relation to the possible existence of an underlying first order valence transition and its critical end point. At constant temperature (T = 20 K), the Sm valence increases rapidly with pressure; however, it remains far below the expected integer value (3+) near the onset of the magnetic ordering and gap closure. In addition, the local structure of SmB 6 at ambient conditions has been analyzed using extended x-ray absorption fine structure.