Photoemission Imaging of 3D Fermi Surface Pairing at the Hidden Order Transition

We report angle-resolved photoemission spectroscopy (ARPES) experiments probing deep into the hidden order (HO) state of URu2Si2, utilizing tunable photon energies with sufficient energy and momentum resolution to detect the near Fermi surface (FS) behavior. Our results reveal: (i) the full itinerancy of the 5f electrons; (ii) the crucial three-dimensional (3D) k-space nature of the FS and its critical nesting vectors, in good comparison with density-functional theory calculations, and (iii) the existence of hot-spot lines and pairing of states at the FS, leading to FS gapping in the HO phase. PACS numbers: 74.25.Jb,71.18.+y,74.70.Tx,79.60.-i The physics underlying the Hidden Order (HO) tran-sition in URu2Si2 remains elusive after over 25 years of intensive research [1–4]. This second-order phase transi-tion at T0 = 17.5 K is marked by a jump in specific heat and removal of significant fraction of total entropy [1, 3], however, with no sign of static dipolar magnetic order except under pressure [5, 6]. The appearance of a new 5f electronic phase with apparent lack of ordered mag-netism has given birth to the mystery of the unknown, or ‘hidden ’ order parameter, addressed so far by a few dozens of theories (e.g., [4, 7–16]), and has recently been proposed to occur in a wide range of materials [17]. One of the important dividing lines between different HO theories is related to time-reversal symmetry break-ing [9, 18, 19] as opposed to time-reversal invarianc