Low temperature properties of boron carbides

We have investigated boron carbides, B1-xCx with x=0.1, 0.13, and 0.2, especially below 1 K. The samples were characterized by ESR. The integrated intensities are roughly sample independent and correspond to about 1020 spins per gram. The temperature (T) dependences are Curie like. The line shapes were lorentzian and for x=0.1 and 0.13 are strongly temperature dependent, which is suggestive for a coupling of linewidths and charge carriers. The room temperature linewidths are 0.38 mT, 32.6 mT, and 23.5 mT for resp. x=0.2, 0.13 and 0.1. Also the AC susceptibility (χ) measured down to 100 mK points to a similar small number spins. There is no sign of a Pauli susceptibility within experimental error. No phase transition (e.g., to a superconducting state) is observed. For x=0.13 and 0.1 a very small specific heat, C is seen. The T dependence of C can be described by a Tα plus a small cubic term, α about 0.26. The relation between C and χ can be explained with a random exchange model. The C-value for x=0.2 is below the limit of our experimental accuracy