Shubnikov-de Haas oscillations reveal at high fields an abrupt reconstruction of the Fermi surface within the hidden-order (HO) phase of URu2Si2. Taken together with reported Hall effect results, this implies an increase in the effective carrier density and suggests that the field suppression of the HO state is ultimately related to destabilizing a gap in the spectrum of itinerant quasiparticles. While hydrostatic pressure favors antiferromagnetism in detriment to the HO state, it has a modest effect on the complex H-T phase diagram. Instead of phase separation between HO and antiferromagnetism our observations indicate adiabatic continuity between both orderings with field and pressure changing their relative weight.
"Firstly, it has been advocated phenomenologically that even though the hidden order phase and the large moment phase have distinct order parameters, the behavior of many observables across the transition is remarkably similar. The term adiabatic continuity has been used to describe this situation  , but it is not justified on the theoretical grounds since the two phases are separated by a first order phase transition. The proposed order parameter ψ i , which unifies the no-moment and large moment phase, explains why even though the two phases are separated by a first order phase transition, they are in many respects very similar, for example in the critical temperature, and entropy change across the transition. "
[Show abstract][Hide abstract] ABSTRACT: Complex electronic matter shows subtle forms of self-organization, which are almost invisible to the available experimental tools. One prominent example is provided by the heavy-fermion material URu2Si2. At high temperature, the 5f electrons of uranium carry a very large entropy. This entropy is released at 17.5K by means of a second-order phase transition to a state that remains shrouded in mystery, termed a `hidden order' state. Here, we develop a first-principles theoretical method to analyse the electronic spectrum of correlated materials as a function of the position inside the unit cell of the crystal and use it to identify the low-energy excitations of URu2Si2. We identify the order parameter of the hidden-order state and show that it is intimately connected to magnetism. Below 70K, the 5f electrons undergo a multichannel Kondo effect, which is `arrested' at low temperature by the crystal-field splitting. At lower temperatures, two broken-symmetry states emerge, characterized by a complex order parameter psi. A real psi describes the hidden-order phase and an imaginary psi corresponds to the large-moment antiferromagnetic phase. Together, they provide a unified picture of the two broken-symmetry phases in this material.
"Our calculations are fully consistent with the known experimental properties of URu 2 Si 2 and provide a natural explanation for the reported " adiabatic continuity " , i.e., the similarity as well as continuous transition in the bulk physical properties of HO/SMAF and LMAF URu 2 Si 2 , which can be understood as an increase in the FS gapping. When we compare the symmetry operations of the body-centered tetragonal PM phase to those of the SMAF and LMAF phases, we evidently obtain that in the latter phases time-reversal symmetry is broken, as there exist small spin and (twotimes larger) orbital moments on U. The crystal symmetry remains tetragonal, but, as the two U atoms in the cell are not equivalent, the body-centering is also broken. "
[Show abstract][Hide abstract] ABSTRACT: Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter. The notion "hidden order" (HO) has recently been used for a variety of materials where a clear phase transition occurs to a phase without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2 where a mysterious HO transition occurs at 17.5K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Using state-of-the-art density-functional theory calculations, we provide here a microscopic explanation for the HO. 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 mechanism for the HO we propose spontaneous symmetry breaking through collective antiferromagnetic moment excitations. Comment: 14 pages, 4 figures
[Show abstract][Hide abstract] ABSTRACT: The H–T phase diagram of URu2Si2 is established under the hydrostatic pressure. The pressure effect on the complex phase transitions around quantum critical point (QCP) is a modest change of transition points to higher fields and lower temperatures. We propose an adiabatic continuity between hidden and antiferromagnetic (AFM) orderings as a function of the magnetic field and the pressure.
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