Field-induced bound-state condensation and spin-nematic phase in SrCu2(BO3)2 revealed by neutron scattering up to 25.9 T
Ellen Fogh, Mithilesh Nayak, O. Prokhnenko, Maciej Bartkowiak, Koji Munakata, Jian-Rui Soh, A. A. Turrini, Mohamed Zayed, E. Pomjakushina, Hiroshi Kageyama, Hiroyuki Nojiri, Kazuhisa Kakurai, B. Normand, Frédéric Mila, H. M. Rønnow
Abstract
Abstract In quantum magnetic materials, ordered phases induced by an applied magnetic field can be described as the Bose-Einstein condensation (BEC) of magnon excitations. In the strongly frustrated system SrCu 2 (BO 3 ) 2 , no clear magnon BEC could be observed, pointing to an alternative mechanism, but the high fields required to probe this physics have remained a barrier to detailed investigation. Here we exploit the first purpose-built high-field neutron scattering facility to measure the spin excitations of SrCu 2 (BO 3 ) 2 up to 25.9 T and use cylinder matrix-product-states (MPS) calculations to reproduce the experimental spectra with high accuracy. Multiple unconventional features point to a condensation of S = 2 bound states into a spin-nematic phase, including the gradients of the one-magnon branches and the persistence of a one-magnon spin gap. This gap reflects a direct analogy with superconductivity, suggesting that the spin-nematic phase in SrCu 2 (BO 3 ) 2 is best understood as a condensate of bosonic Cooper pairs.