Plutonium and quantum criticality (continued)

From its onset the Manhattan Project was bedeviled with a peculiar problem: the lattice structure and density of elemental plutonium seemed to have two different values, depending on how it was prepared. Perhaps even more remarkably, despite the importance of this problem and the passing of more than sixty years, we still don’t understand why plutonium occurs in these two allotropic forms.

Another mystery is why plutonium is not magnetic. By the usual atomic rules, the valence f-electrons of plutonium should have a non-vanishing angular momentum and associated magnetic moment. However, there is considerable evidence that there are no local magnetic moments in elemental plutonium. More than thirty years ago Bill Nellis showed that at very low concentrations, plutonium impurities in platinum exhibited the expected magnetic moment. However, Nellis discovered that as the concentration increases the magnetic moment rapidly decreases, and quite surprisingly becomes vanishingly small for plutonium concentrations above a few percent.

Fortunately, a solution to these puzzles may be on the horizon. A hint that elemental plutonium lies close to a quantum critical point (QCP) of some kind is that, as a function of relatively modest changes in temperature and pressure, there are a variety of phase transitions that involve changes in both the lattice structure and the electronic state of the 5f electrons. (See the introduction by Jim Smith and me to “Challenges in Plutonium Science,” Los Alamos Science, 2000.)

The negative thermal expansion of delta (δ) plutonium is also indicative of critical behavior. Recent studies of radioactivity-induced damage in plutonium carried out at Lawrence Livermore National Laboratory by Mike Fluss and Scott McCall have yielded evidence that these strange phenomena may all be the result of a QCP associated with the existence of a quantum-order parameter in plutonium coupled to lattice distortions. Our guess is that elemental plutonium is a gossamer superconductor, i.e., there is a condensate ground state with off-diagonal long-range order but no bulk superconductivity. More specifically, we believe that in the actinides there is a non-vanishing density of paired itinerant charge carriers and that plutonium lies close to a critical point for this order parameter.

In general, if one plots the pressure of a quantum fluid versus condensate density one will obtain a van der Waals-like curve. For some particular values of the parameters in the Hamiltonian this curve can have a critical point, and I believe that the δ form of plutonium can be identified with such a point.

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