Chirality density wave of the “hidden order” phase in URu ₂ Si ₂

Abstract

A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu 2 Si 2 , the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state. 1 arXiv:1410.6398v1 [cond-mat.str-el]

Direct bulk-sensitive probe of 5 f symmetry in URu 2 Si 2

“…Here, +1 k denotes an electron in the host conduction band. We recall that a hybridization gap of 13 meV opens up below 27 K (13), which is also seen in the DMFT calculation that includes these Kondo processes (19,29). The stabilization energy of this Kondoesque wave function should be of the same order as the hybridization gap and the contributing CEF configurations should also be within this energy range.…”

“…29 and supplementary materials in Kung et al (19)], but the experiment yields the additional information that the Jz = +4 and −4 in the Γ1 is dominating. We further would like to stress that linear polarized XAS data at the U O4,5-edge (42) also agree with our findings in the sense that both, the NIXS and XAS dichroism, rule out the Γ 5 doublet as ground state might be due to the higher surface sensitivity of the XAS experiment.…”

“…Also, the interpretation of the resonance intensity in the main A2g and other Raman channels in terms of a staggered chirality density wave requires a mixing of Γ1 and Γ2 singlet states that support a hexadecapole-type hidden order parameter. The model of Kung et al (19) contains both dominant A2g and subdominant B1g symmetry parts. The latter involves higher energy CEF states and coupling to the lattice leads to a secondary orthorhombic distortion that leaves only twofold symmetry.…”

“…This second-order transition into an electronically ordered state involves a reconstruction of the Fermi surface (9, 10) and a change of quasiparticle scattering rate (11). The Fermi surfaces of the HO and high-pressure LMAF phase are very similar (12).In URu2Si2 three-energy scales have been identified: a hybridization gap of ∆ hyb ≈ 13 meV [150 K] (13) that opens below 27 K, another gap that opens in the HO phase with ∆HO ≈ 4.1 meV [50 K] in the charge (9, 10, 14, 15) as well as spin channel (16,17), and a resonance mode that appears in the HO gap at ∼ =1.6 meV [18 K], also in both channels (18)(19)(20). Furthermore, with entering the HO phase, the breaking of the fourfold rotational symmetry has been reported from torque experiments (21), high resolution X-ray diffraction on high quality crystals (22), and elastoresistance measurements (23).…”

Single Crystal Growth of URu2Si2 by the Modified Bridgman Technique

“…The fact that it also hosts unconventional superconductivity makes this material even more attractive. Recent success in producing high quality specimens [5][6][7] has resulted in a surge of experimental work, including electronic Raman spectroscopy [8], elastoresistance [9], resonant ultrasound [10], Kerr rotation [11], X-ray scattering [12], and quantum oscillation measurements [13][14][15], which have provided unprecedented insight into the electronic, ordered state, and superconducting behavior. However, progress is limited by restricted access to high quality specimens, particularly due to challenges that are inherent to the mainstream growth techniques.…”