Discovery of hidden order in the Shastry-Sutherland magnet Nd2Be2GeO7
Hidden order typically manifests as a thermodynamic phase transition without a conventional order parameter, leaving its true nature concealed even at the lowest temperatures. In the frustrated Shastry-Sutherland magnet Nd$_2$Be$_2$GeO$_7$, we observe a related yet fundamentally distinct phenomenon. A sharp specific-heat anomaly appears at 250 mK, but zero-field neutron diffraction and muon spin relaxation detect no static magnetism down to 100 and 30 mK, respectively, pointing to a hidden-order state. Remarkably, this hidden order does not emerge under an applied magnetic field, but instead reveals itself only after the field is applied and subsequently removed where magnetic Bragg peaks appear, albeit with strongly suppressed moments. A glassy state is ruled out by ac susceptibility and specific heat measurements. Complementary $μ$SR measurements reveal coherent spin fluctuations at a rate on the order of gigahertz. Taken together, these results suggest that the system lies in close proximity to the quantum spin liquid and long-range magnetic order state such that a small perturbation can effectively drive the system towards distinct ground states. These findings also distinguish Nd$_2$Be$_2$GeO$_7$ from known frustrated systems, establishing it as a unique platform where the synergistic interplay among the spin-orbit coupling, crystal field, and magnetic frustration leads to unexpected quantum states.