Professor, Boston University & Distinguished Professor, Institute of Physics, CAS

Abstract
Theoretical descriptions of quantum phase transitions have indicated the existence of critical points with higher symmetry than those of the underlying Hamiltonian. Points of emergent symmetry have not been expected at discontinuous (first-order) transitions, however, but our recent work shows that this is also a possibility [1]. I will discuss an apparent symmetry enhanced first-order phase transition in a two-dimensional quantum magnet, where phase coexistence at the transition takes the form of an enhanced rotational symmetry in a space of two order parameters. The system hosts the same kind of antiferromagnetic (AFM) and plaquette singlet (PS) states recently detected in SrCu2(BO3)2 under high pressure. Using quantum Monte Carlo simulations, we observe that the O(3) symmetric AFM order and the Z2 symmetric PS order form an O(4) vector at the transition. The control parameter (a coupling ratio) rotates the vector from the AFM sector to the PS sector, with the length of the combined order parameter vector always remaining non-zero. This phenomenon should be observable in neutron scattering experiments on SrCu2(BO3)2, and we also propose the possibility to detect the PS phase and observe manifestations of the enhanced symmetry in high-pressure studies of the specific heat.
 [1] B. Zhao, P. Weinberg, A. W. Sandvik (in preparation)

Brief CV of Prof. Anders W. Sandvik:
Prof. Sandvik completed his M.Sc. at Åbo Akademi University in Finland and his Ph.D. at the University of California, Santa Barbara, in 1993. He carried out postdoctoral work at Florida State University, the University of Illinois at Urbana-Champaign, and Los Alamos National Laboratory, before returning to Finland as a Senior Fellow of the Academy of Finland in 2000. He joined the Boston University as an Associate Professor of Physics in 2004 and was promoted to Full Professor in 2008. In 2018 he joined the Institute of Physics of the Chinese Academy of Sciences as a Distinguished Professor. He has published more than 100 research articles on computational algorithms and their applications to condensed-matter systems at the electronic level. In recognition of his work on quantum magnetism, he was elected a Fellow of the American Physical Society in 2007. His recent research has been focused on quantum phase transitions beyond the standard paradigms.

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