The river splits in two. One branch is wide and fast, with water flowing smoothly, but as temperature rises, the flow slows: eddies and waves appear. The second branch is narrow, with rapids, where water leaps from rock to rock. This flow, on the contrary, becomes more lively with warming. Roughly like this, according to a new study, electric current behaves in the mysterious family of metals — the actinides.
Actinides include uranium and plutonium, as well as their less famous neighbors in the periodic table — thorium, neptunium, curium. Their electrical resistance has long defied a unified explanation: the graphs for different elements looked as if they were drawn for completely different substances. The author of the study proposed a simple model with two parallel conduction channels. The first channel is classical: electrons scatter and slow down when they hit thermal vibrations of atoms (this mechanism is described by the Bloch–Grüneisen equation). As temperature rises, atoms vibrate more strongly, and this channel works worse.
The second channel is hopping: electrons jump from atom to atom like a relay baton. This process requires activation energy and follows the Arrhenius equation: the hotter the metal, the more readily electrons set off. For neptunium, the energy of such a jump turned out to be a record among all actinides — about 16 millielectronvolts (a tiny amount, roughly a hundred times less than the energy of visible light).
The Debye temperatures obtained from this model agree well with those previously measured from heat capacity. This confirms that the model is not a trick but a reflection of real physics. The two channels of current are not just a metaphor but a convenient way to describe what actually happens in these unusual metals.
The most famous actinide, uranium, changes its resistance at room temperature by almost a factor of two between different crystalline modifications, even though chemically it is the same element.
In Isaac Asimov's novel The Gods Themselves, physicists struggle with the mystery of 'plutonium-186', which violates all laws. The real mysteries of plutonium also forced scientists to reconsider simple models, and find an elegant solution.