The electronic structure of UCoGe by ab initio calculations and XPS experiment.

The crystal and electronic structures of the orthorhombic compound UCoGe are presented and discussed. It has been either refined by the x-ray diffraction on a single crystal or computed within the local spin density functional theory, employing the fully relativistic version of the full-potential local-orbital band structure code, respectively. We particularly give our attention to investigating the Fermi surface and de Haas-van Alphen quantities of UCoGe. The calculated electronic density is then examined by x-ray photoelectron spectroscopy (XPS). Fairly good agreement is achieved between theoretical and experimental XPS results in the paramagnetic state. A small difference in the position (in energy scale) of the U 5f bands is caused by the electron localization effect observed in the experimental XPS. There is also some discrepancy for the Co 3d electron contributions below E(F). The Fermi surface in the non-magnetic state is of a semimetallic type while that in the ferromagnetic state, with the ordered moment of -0.47 µ(B)/f.u. along the c axis, is more metallic, with nesting properties that may favour superconductivity.

Hidden order in URu2Si2 originates from Fermi surface gapping induced by dynamic symmetry breaking.

Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu(2)Si(2), where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.

Magnetic-field-induced band-structure change in CeBiPt.

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.