Pressure and magnetic field reveal a new gapless low-energy state in a frustrated quantum magnet
High-pressure calorimetry uncovers T-linear specific heat near the pressure-induced magnetic transition in SrCu2(BO3)2
image: (a) The zero-field pressure-temperature (P-T) phase diagram. (b-g) The T-linear specific heat region (red area) is concentrated near the zero-field first-order PS–AF transition pressure, highlighting the role of pressure and magnetic field in stabilizing the new low-energy response state. (h)The pressure–magnetic field phase diagram of SrCu2(BO3)2 at 0.4 K summarizes the evolution of distinct magnetic ground states under external tuning.
Credit: ©Science Bulletin
Quantum magnets with strong correlations and geometric frustration provide a unique platform for exploring unconventional quantum states and emergent low-energy excitations. Among them, SrCu2(BO3)2, a Shastry–Sutherland lattice magnet, has attracted considerable attention because of its competing magnetic phases and rich quantum phase diagram.
SrCu2(BO3)2 hosts a plaquette-singlet (PS) state and a pressure-induced antiferromagnetic (AF) state under external tuning. The transition between these competing phases provides an important opportunity to investigate unusual quantum phenomena. However, previous studies have shown that the zero-field PS–AF transition is first order, raising new questions about the nature of low-energy excitations near this phase boundary.
In a recent study, the team investigated the low-temperature thermodynamic properties of SrCu2(BO3)2 under combined pressure and magnetic field conditions. Using high-pressure low-temperature AC calorimetry, the team performed measurements under pressures up to approximately 3.2 GPa, temperatures down to 0.4 K, and magnetic fields up to 14 T.
They discovered a robust T-linear specific heat response near the zero-field PS–AF transition pressure. This behavior appears within a narrow pressure range of approximately 2.68–2.9 GPa and is most pronounced around 2.75 GPa, where it extends over a broad magnetic field range.
Further measurements confirmed that SrCu2(BO3)2 remains a Mott insulating state throughout the investigated pressure and magnetic field range, indicating that the observed T-linear specific heat does not originate from conventional metallic electrons. Instead, it likely reflects unconventional low-energy spin excitations associated with the frustrated magnetic background.
By constructing detailed pressure–temperature, magnetic field–temperature, and pressure–magnetic field phase diagrams, the team demonstrated that the T-linear specific heat region is concentrated around the zero-field first-order PS–AF transition boundary. The results suggest that pressure and magnetic field can work together to enhance low-energy responses near competing magnetic phases.
This study reveals a previously unexplored gapless low-energy response regime in a frustrated quantum magnet. The findings provide a new experimental platform for investigating quantum fluctuations and emergent excitations in strongly correlated insulating systems.