negative absolute temperature!

https://www.mpg.de/research/negative-absolute-temperature

I browsed through it.

It's very interesting study. I understand it is so abnormal condition, thinking under finite temperature. But I cannot understand why they definite the experimental system as negative absolute temperature. By attracting each other and pressure? 

This is gas phase study,  

And I want to pursue the idea of absolute zero and temperature by order or disorder in spin frustration system......

ざっと読んだだけですが、

絶対零度より下の温度が、かなり特殊な状況を作った実験系では定義できる！って話。

なかなか興味深いです。

ただ、「『絶対マイナス温度』は、絶対有限温度（絶対零度より高い温度）よりも”hotter“ 」の意図がよくわからないかなぁ・・・

Today, it's so fine and beautful day.

I spent this afternoon reading paper and writing essays in our lab,