Charge-density Wave Transitions and Phonon Instabilities in α-U

Uranium is one of the only elemental metals in which charge density waves have been observed (the phases are labeled α₁, α₂, and α₃). The first transition takes place at 43 K (α₁), the second at 38 K (α₂), and the last one stabilizes below 25 K (α₃). The structure below the transition is complex, consisting of small displacements of the atoms along all three of the orthorhombic axes. Despite the large experimental and theoretical effort, the nature of these transitions is still elusive, but it is believed to be associated with the unique coupling of 5f states, residing in the vicinity of the Fermi level, and the lattice vibrations. Here we present detailed experimental and theoretical studies of low-temperature thermal, thermodynamic, and electronic transport properties of the high-quality single crystals of α-U, across the CDW transitions.