SrCu2(BO3)2
Chemical formula:
SrCu2(BO3)2
Lattice type:
Tetragonal, space group I4̄2m
How to grow:
Floating zone image furnace
Magnetic model:
Fully frustrated two-dimensional network of Heisenberg spin dimers (Shastry-Sutherland model)
Why is it cool:
A pressure-induced transition between two qualitatively different quantum paramagnetic phases
The fully frustrated Shastry-Sutherland model is famous for being a quantum paramagnet of two entirely different kinds: depending on the ratio of its exchange constants, the ground state is either exactly dimerized or built of four-spin plaquette singlets. SrCu2(BO3)2 is the only known good realization of the model, and it sits in the dimer phase. Hydrostatic pressure distorts the lattice, continuously tunes the superexchange, and eventually drives the material into the plaquette phase. But how is the change to be detected? Both states are nonmagnetic, neither has any magnetic order, and their thermodynamics are practically identical. What actually distinguishes them is buried deep in the wave function: whether the quantum-fluctuating spins on each dimer bond are correlated antiferromagnetically or ferromagnetically.
We found a way to measure exactly that, with the sample compressed inside a diamond anvil cell [1]. Raman optical spectroscopy tracks one particular optic phonon, a "pantograph" mode that does little other than stretch the dimer bond. As the spin correlations develop at low temperatures, the frequency shift of this phonon becomes a precise gauge of the sign and magnitude of the dimer spin-spin correlator. The pressure-induced sign switching is thereby confirmed and quantified experimentally.
Temperature dependence of the "pantograph" phonon frequency in SrCu2(BO3)2 at different pressures [1]. The low-temperature shift measures the dimer spin-spin correlator directly: it points down in the dimer phase, is progressively suppressed under pressure, and reverses sign above about 22 kbar, where the dimer correlations turn ferromagnetic. The dashed line marks the temperature corresponding to the ambient-pressure spin gap.
- [1] S. Bettler, L. Stoppel, Z. Yan, S. Gvasaliya, A. Zheludev, Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure , Phys. Rev. Research 2, 012010(R) (2020); arXiv:1907.09316.