Signature of hidden order and evidence for periodicity modification inURu2Si2

Abstract

The detail of electronic structures near the Fermi level in URu 2 Si 2 has been investigated employing stateof-art laser angle-resolved photoemission spectroscopy. The observation of a narrow dispersive band near the Fermi level in the ordered state as well as its absence in a Rh-substituted sample strongly suggest that the emergence of the narrow band is a clear signature of the hidden-order transition. The temperature dependence of the narrow band, which appears at the onset of the hidden-order transition, invokes the occurrence of periodicity modification in the ordered state, which is shown for the first time by any spectroscopic probe. We compare our data to other previous studies and discuss possible implications.

Unusual Magnetic Field-Dependence of the Electronic Dispersion Relations in a Hidden Ordered Phase.

“…The transition is driven by the spin-flip part of the Hund's rule exchange interaction which produces nesting between two sheets of the Fermi-surface that have different orbital characters. The transition results in a folding of the Brillouin zone through a commensurate vector Q which is consistent with Angle Resolved Photoemission [12], de Haasvan Alphen [13] and neutron scattering measurements [14]. Similar nesting conditions have been found to be satisfied in LDA calculations [15,16] of the electronic structure of URu 2 Si 2 .…”

Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2

“…This smallness of the lattice change implies that the hidden-order transition is driven by an electronic ordering, and small but finite electron-lattice coupling gives rise to the lattice distortion. It should be noted that in ironpnictides the large orthorhombicity is believed to be associated with the 'ferroic' (Q ¼ 0) orbital ordering, which is in sharp contrast to the URu 2 Si 2 case where the hidden order is most likely an antiferroic order with the wave vector Q ¼ (001) 6,7,[9][10][11][12] . Such an antiferroic ordering is expected to couple only weakly to the ferroic Q ¼ 0 orthorhombic distortion especially for high-rank multipole orders, in which the degree of local electronic distributions near the atoms are much smaller than that of dipoles in the antiferromagnetic case.…”

“…139 in the international tables for crystallography), which has 15 maximal non-isomorphic subgroups. Recent quantum oscillations 9 and angle-resolved photoemission spectroscopy [10][11][12] studies have revealed that the electronic structure in the hidden-order phase is similar to that of antiferromagneitc phase under pressure. This implies that in the hidden-order phase the Brillouin Zone is folded with the antiferroic wave vector Q ¼ (001) and the nested parts of Fermi surface are gapped as in the antiferromagnetic phase 13 .…”

Spin Nematic State as a Candidate of the Hidden Order Phase ofURu2Si2

“…The enigmatic features of this so-called "hidden order(HO)" phase are described as follows: (i) Despite large anomaly in thermodynamic quantities and drastic reconstruction of the Fermi surfaces at T = T HO , there is neither conventional magnetic order nor the change of the crystal structure [1][2][3][15][16][17][18][19]. (ii) However, under applied pressure, an antiferromagnetic (AF) state with large moment appears, and more surprisingly, the Fermi surfaces in the AF ordered state are almost the same as those found in the HO phase [20][21][22][23][24].Recently, an experimental breakthrough for this issue was achieved by Okazaki et al [25], who found spontaneous symmetry breaking in the spin space at T < T HO . They reported that the anisotropy of the spin susceptibility in the xy-plane, which is measured by the quantity χ xy = S x S y , becomes nonzero below T HO .…”