Negative absolute temperatures: "hot" spins in spontaneous magnetic order.
Depending on the sign of the spin temperature in the picokelvin range, antiferromagnetic nuclear and ferromagnetic nuclear orders in silver are caused by the same interactions. In rhodium, the antiferromagnetic state is preferred both at temperatures greater than and temperatures less than zero. The lowest and "highest" temperatures ever produced on this scale and measured, 280 and -750 picokelvin, respectively, have been reached in the course of these experiments. The results on silver, in particular, show that negative temperatures are real, not fictitious, quantities.
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