Electrical transport properties of ${\mathrm{CaB}}_6$

We report results from a systematic electron-transport study in a broad temperature range on $12 {\mathrm{CaB}}_6$ single crystals. None of the crystals were intentionally doped. The different carrier densities observed presumably arise from slight variations in the Ca:B stoichiometry. In these crystals, the variation of the electrical resistivity and of the Hall effect with temperature can be consistently accounted for by the model we propose, in which B-antisite defects (B atom replacing Ca atom) are “amphoteric.” The magnetotransport measurements reveal that most of the samples we have studied are close to a metal-insulator transition at low temperatures. The magnetoresistance changes smoothly from negative—for weakly metallic samples—to positive values—for samples in a localized regime.

Figure 1

Variation of electrical resistivity with temperature for various stoichiometric ${\mathrm{CaB}}_6$ single crystals.

Figure 2

Variation of Hall coefficient with temperature for various stoichiometric ${\mathrm{CaB}}_6$ single crystals. The inset schematically shows the density of states vs energy in our model.

Figure 3

Electron concentration vs temperature for various ${\mathrm{CaB}}_6$ single-crystal samples. Solid lines are fits to experimental data points using the model we propose here. The inset shows the same for the sample with high impurity concentration.

Figure 4

(a) The fraction of occupied $E_1$ defects vs temperature for various doping levels is shown. The inset shows the Fermi energy vs temperature. (b) Electron concentration vs temperature in the conduction band for various doping levels.

Figure 5

Hall mobility vs temperature for various ${\mathrm{CaB}}_6$ single crystals. The inset shows Hall mobility vs electron concentration at $T=2$ K.

Figure 6

Low–temperature magnetoresistance for two ${\mathrm{CaB}}_6$ single crystals in different localization regimes.