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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 113; no. 49; pp. 13989 - 13994
• Main Authors: Sundermann, Martin, Haverkort, Maurits W., Agrestini, Stefano, Al-Zein, Ali, Moretti Sala, Marco, Huang, Yingkai, Golden, Mark, de Visser, Anne, Thalmeier, Peter, Tjeng, Liu Hao, Severing, Andrea
• Format: Journal Article
• Language: English
• Published: 06.12.2016
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1612791113

Significance The hidden order problem in URu 2 Si 2 is an unanswered question in the field of strongly correlated electron materials. Although it has been studied for several decades, there is still no consensus about how this new phase forms. Understanding the hidden order phase formation is not only an intellectual problem, it will also advance concepts for designing quantum materials with new exotic properties. Many hidden order scenarios are based on the assumption of certain ground-state symmetries and the present study addresses this aspect. A spectroscopic technique, nonresonant inelastic X-ray scattering, that has become available through the use of high-brilliance synchrotrons, allows us to measure directly in a bulk-sensitive experiment the symmetry of the 5 f ground-state wave function in URu 2 Si 2 . The second-order phase transition into a hidden order phase in URu 2 Si 2 goes along with an order parameter that is still a mystery, despite 30 years of research. However, it is understood that the symmetry of the order parameter must be related to the symmetry of the low-lying local electronic f -states. Here, we present results of a spectroscopic technique, namely core-level nonresonant inelastic X-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and is bulk-sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5 f ground-state wave function is composed mainly of the Γ 1 with majority J z = | 4 ⟩ + | − 4 ⟩ and/or Γ 2 singlet states. The incomplete dichroism indicates the possibility that quantum states of other irreducible representation are mixed into the ground state.