The strange behaviour of a magnet near absolute zero temperature provides the first direct evidence that some quantum phase transitions proceed very differently than the conventional phase transitions that occur at higher temperatures. Researchers in Germany applied a magnetic field to a metallic compound and watched it transform from a magnet to a non-magnet -- just as expected. The surprise came at higher field strengths, where a puzzling change in the character of the metal was observed. As the temperature was lowered both the magnetic phase transition and the mysterious change converged on the same magnetic field value -- the "quantum critical point" -- defying the conventional method of characterizing phase transitions in terms of a single "universality class" (Science 315 969).
In a conventional continuous phase transition, a change in temperature causes matter to transform from one state to another (magnetic to non-magnetic, for example). As the material approaches the critical temperature, thermal fluctuations cause “bubbles” of the new state to appear and grow within the old state, eventually taking over the material. During the transition, the difference in energy between the two states approaches zero in a manner that defines the “universality class” of the transition. All know continuous phase transitions can be described using one of a small number of universality classes.
At extremely low temperatures near absolute zero, there is little energy available for thermal fluctuations and quantum “zero-point” fluctuations are expected to play a role in phase transitions. These fluctuations keep matter in constant motion — even at zero temperature. However, it isn’t clear whether “quantum phase transitions” driven by these fluctuations will belong to the same universality classes as phase transitions driven by thermal fluctuations.
Now, Philip Gegenwart and co-workers at the Max Planck Institute for the Chemistry of Solids in Germany along with colleagues in the US have reported that quantum fluctuations appear to be driving two very distinct phenomena in an antiferromagnetic metal. This unexpected behaviour was observed at temperatures below 0.8 K in YRS, which is a compound of ytterbium, rhodium and silicon.
The researchers believe that the unexpected change they observed at higher fields could be related to the “entanglement” of magnetic spins and conduction electrons. Magnetic spins are electrons that are fixed to individual atoms and normally have little to do with the conduction electrons. But at higher magnetic fields and very low temperatures they appear to become entangled with conduction electrons to create quasi-particles that behave like very heavy electrons. Gegenwart and collegues may have observed a transition to this “heavy electron liquid” state of matter.
According to Andrew Schofield of the UK’s Birmingham University, if quantum fluctuations were simply taking over the role of thermal fluctuations, only the magnetic transition should have been observed. He told Physics Web that the appearance of a second distinct feature in the phase diagram associated with the quantum critical point defies our current understanding of universality classes and a new physical theory is required to describe what is a purely quantum phenomenon.
