High pressure transport measurements on the rare earth systems samarium hexaboride, cerium bismuth platinide and europium hexaboride.

Abstract

High pressure transport and magnetotransport measurements on the rare earth systems $\rm SmB\sb6,\ Ce\sb3Bi\sb4Pt\sb3$ and EuB$\sb6$ as well as high pressure magnetic measurements on EuB$\sb6$ are reported. In the case of SmB$\sb6$ the resistivity, magnetoresistance and Hall effect were measured at pressures to 150 kbar, and fields as high as 18 T, over temperatures from 1 K to 300 K. At 1 bar SmB$\sb6$ is a metal with a resistivity well in excess of the unitarity limit at low temperature. The mean field hybridization model used for Kondo Insulators cannot explain the sudden collapse of the gap in this material at a sample dependent critical pressure or the gap's weak quadratic field dependence. At high pressure the resistivity of SmB$\sb6$ resembles that of a non ordering heavy fermion material with a $T\sp2$ temperature dependence. The field dependence of the high pressure resistivity suggests a Fermi surface orbit shaped similarly to one in LaB$\sb6$. The resistivity of the Kondo Insulator $\rm Ce\sb3Bi\sb4Pt\sb3$ was measured at pressures up to 150 kbar at temperatures between 1 K and 300 K. The gap $\Delta$ increases by an order of magnitude in this pressure range. From $\Delta$, the pressure dependences of the Kondo temperature and Ce valence were determined and found consistent with the predictions of the mean field hybridization model in the weak coupling limit. Three dimensional variable range hopping was found to dominate the low temperature transport at pressures above 30 kbar. In EuB$\sb6$ the resistivity and magnetoresistance were measured at pressures from 1 bar to 169 kbar in fields to 7 T over temperatures from 1 K to 300 K. The magnetoresistance near the ordering temperature $T\sb{C}$ is negative and scales with the magnetization while at low temperature it is large, $\Delta\rho/\rho\sb0 > 10,$ and positive. Shubnikov-de Haas oscillations about a small branch of the Fermi surface were observed. In addition $T\sb{C}$ was measured using a magnetic susceptibility technique at pressures from 1 bar to 57 kbar. The apparatus is described. $T\sb{c}$ increases with pressures up to $\sim$100 kbar, then becomes nearly pressure independent. Finally evidence of an unusual two step magnetic ordering process was found in the temperature dependence of the spontaneous magnetization and resistivity.