Results

In figure we display 1/T₁(T/H) measured at the Boron sites of SrB₆ with different external fields from 0.58 T to 7.56 T. As already discussed SrB₆ enters a metallic state at low temperatures and one would expect a Korringa-type relaxation mechanism which would show up in a linear temperature-dependence of the relaxation rate 1/T₁. But instead one measures a very different behaviour, including two different temperature régimes with a field dependent crossover temperature T_</I>B between 0.5 K and 10 K. Above T_</I>B, 1/T₁ shows a slightly decreasing, but almost constant temperature dependence of 1/T₁. The relaxation rate decreases faster for low fields than for high fields. Below T_</I>B the relaxation rate decreases very rapidly. Note that in the crossover régime the nuclear spin of ¹¹B in SrB₆ relaxes even faster than it does in LaB₆ in spite of the much higher electron density in that metallic compound.

  

Figure: The temperature and field dependence of the ¹¹B spin lattice relaxation rate 1/T₁ in SrB₆. The temperature in the plot is scaled with the applied field, in order to illustrate that T_</I>B/H seems to be a constant.

The ratio T_</I>B/H, where H denotes the strength of the external field, is a constant and K/T.

In figure we display 1/T₁(H) measured at the ¹¹B sites in CaB₆ at a field of 5.2 T. The data coincides with the relaxation rate measured in SrB₆; the same effects show up and we measure again . The deviation towards low temperatures could be artificial because the data in SrB₆ was taken with a shorter comb such that we were in the crossover regime between slow and fast irradiation. Therefore we can only guarantee the self-consistency of the SrB₆ data at very low temperatures, but not the complete equivalence to the measurements in CaB₆.

  

Figure: The temperature dependence of the ¹¹B spin lattice relaxation rate 1/T₁ in CaB₆. The spin-lattice relaxation rate of CaB₆ seems to coincide with that of SrB₆.

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