Negative temperature. Not colder than zero.

Here's the strange part.

Negative absolute temperature sounds like a freezer setting that should not exist. In specialised systems, it describes an unusual distribution of energy, not something colder than absolute zero. Researchers created such a state in atoms confined with an upper energy limit. Ordinary hot-and-cold intuition is not enough here; the minus sign is a warning to read the definitions. (Phys.org / Science)

Go straight to the receipts.

Curious about the methods, dates or details? Open the original source and keep the caveat in view.