Continuous and discontinuous quantum phase transitions in a model two-dimensional magnet.
The Shasty-Sutherland model, which consists of a set of spin 1/2 dimers on a 2D square lattice, is simple and soluble but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu(2)(BO(3))(2), in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution X-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure-dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.
Advanced Photon Source, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA.
S
S
Haravifard
A
A
Banerjee
J C
JC
Lang
G
G
Srajer
D M
DM
Silevitch
B D
BD
Gaulin
H A
HA
Dabkowska
T F
TF
Rosenbaum