2level system: level 1 能量E1有n1个粒子；level 2 能量E2有n2个粒子 E2 > E1
波兹曼分布:n2/n1 = EXP{-(E2-E1)/RT}
正常情况你怎么加热最多能5/5开 n2/n1=1
此时T高的话体系整体能量n1*E1+n2*E2必然大
如果有第三个能级给level 2灌粒子使n2/n1 >1 就推出了负温度 应用n2/n1 = EXP{-(E2-E1)/RT}就得出了负温度 而体系整体能量实际上是增加了
By injecting energy into these systems in the right fashion, it is possible to create a system in which there are more particles in the higher energy states than in the lower ones. The system can then be characterised as having a negative temperature. A substance with a negative temperature is not colder than absolute zero, but rather it is hotter than infinite temperature. As Kittel and Kroemer (p. 462) put it, "The temperature scale from cold to hot runs:
+0 K, ... , +300 K, ... , +∞ K, −∞ K, ... , −300 K, ... , −0 K."