Deconfined Quantum Criticality in a Shastry-Sutherland Compound SrCu2(BO3)2

Landau theory predicts that there is no continuous quantum phase transition (QPT) between two states with different types of symmetry breaking. However, about 20 years ago, field theory and quantum many-body calculations based on some specifically designed models support a beyond Landau paradigm: a continuous deconfined quantum critical point (QDCP) may exist which hosts emergent symmetries and fractional excitations [1,2]. DQCP became a frontier research in strongly correlated electrons, which may also help to understand novel properties of high temperature superconductors. Unfortunately, experimental evidence of DQCP has never been found after original theoretical proposals. A frustrated antiferromagnet SrCu2(BO3)2 [3], which is called a Shastry-Sutherland material, provides the possibility of exploring DQCP, where pressure-induced QPTs from dimerized spin singlet (DS), to plaquette singlet state (PS), and then to antiferromagnetic state (AFM) were suggested by specific heat and neutron scattering studies [4,5,6].

In this talk, I will present our 11B nuclear magnetic resonance (NMR) studies on SrCu2(BO3)2 with combined high-pressure and high-field tuning, and cooled in a dilution refrigerator, to approach DQCP [7]. At pressures above 2 GPa, we discovered microscopic experimental evidence of a full-plaquette (FP) singlet state, where the FWHM of the spectra behaves as an order parameter below a transition temperature of 1.8 K. Furthermore, at pressures of 2.1 and 2.4 GPa, our NMR spectra reveal a field-induced weakly first-order QPT from the FP state to the AFM state at a field about 6 T, where the coexistence temperature of two phases is as low as 0.07 K and the order parameter of the AFM phase becomes very small at the QPT. Furthermore, both transition temperatures of the PS and the AFM phases scale with |H-HC| when approaching the transition field HC with the same power-law exponent; such duality supports the emergence of enhanced symmetries, consistent with an enhanced O(3) symmetry by numerical simulations. The spin-lattice relaxation rate data 1/T1 at 2.4 GPa also reveals a quantum critical scaling behavior in the low-energy spin dynamics, which does not follow a traditional QCP. Therefore, our study provides concrete experiment evidences for realization of a proximate DQCP and a new platform for exploring DQCP. Note that the system may go beyond the Shastry-Sutherland model; many properties, such as the nature of a plaquette-liquid phase and existence of fractional excitations, call for further experimental studies.

References:
[1] R. R. P. Singh et al., Science 303, 1490 (2004).
[2] A. W. Sandvik, Phys. Rev. Lett 98, 227202 (2007).
[3] H. Kageyama et al., Phys. Rev. Lett 82, 3168 (1999).
[4] M. E. Zayed et al., Nature Phys. 13, 962 (2017).
[5] J. Guo et al., Phys. Rev. Lett 124, 206602 (2020).
[6] J. Larrea Jimenez et al., Nature 592, 370 (2021).
[7] Y. Cui et al., Science 380, 1179 (2023).