Thermal phase transitions surround us. Ice melts. Water boils. Triggered by thermal fluctuations, these transitions are typically characterized by a reorganization of matter at a critical temperature from one distinct phase to another. Less obvious in our everyday lives are phase transitions that occur in quantum systems at zero temperature. These quantum phase transitions exist in many condensed matter systems, in which tuning such variables as pressure, atom concentration, or magnetic field brings one quantum phase to a “critical point” with another. Instead of thermal fluctuations, these transitions are driven by the quantum fluctuations dictated by the uncertainty principle, and they often involve competing interactions (for example, repulsive versus attractive) between the phases.
Now, theorists simulating the interactions between protons and neutrons in a nucleus have found that certain light nuclei exist near a quantum phase transition. The transition is from a state with a uniform distribution of protons and neutrons (a Fermi liquid) to a state consisting of several alpha particles (two protons and two neutrons) [1]. The finding, from Dean Lee at North Carolina State University, Raleigh, and colleagues, might provide clues as to why clustering occurs in the ground states of some nuclei but only in the excited states of others—such as the excited Hoyle state of carbon that is thought to be essential to life.
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