Specific Heat of theS=1/2 Two-Dimensional Shastry–Sutherland Antiferromagnet SrCu₂(BO₃)₂in High Magnetic Fields

Abstract

We characterize the field-induced magnetic phases of SrCu2(BO3)2, a frustrated spin-1/2 Heisenberg antiferromagnet in the two-dimensional Shastry-Sutherland lattice, using specific heat in magnetic fields up to 33 T. We find that the spin gap persists above the expected critical field Hc = ∆/gµB of 21 T despite the appearance of magnetic moment in the ground state. At the magnetization plateau at 1/8 of the saturation, the Sz = +1 triplets that carry the magnetization of the ground state are observed to form a two-dimensional spin gas of massive bosons. A spin gas consisting of the same number of massive particles continues to completely dominate the specific heat in the field region above the plateau, although the magnetization increases with increasing field. Ordering is observed at a temperature immediately below the spin-gas regime.PACS numbers: 75.45.+j, 75.40.Cx, 05.30.Jp, 67.40.Db, 75.50.Ee Geometrical frustration is one of the important subjects in strongly correlated systems [1]. In particular, the fascinating interplay of geometrical frustration and quantum fluctuation in two dimensions provides a fertile ground for new physics, as has been theoretically demonstrated for Heisenberg antiferromagnets in the triangular lattice and the kagomé lattice. For the S = 1/2 triangular antiferromagnet, there now appears to be a consensus that Néel order must exist despite the strong frustration [2]. The S = 1/2 kagomé antiferromagnet has been predicted to possess a gap separating the ground state from upper triplet levels and a band of low-lying singlet excitations within the triplet gap, although the exact nature of the ground state is still under debate [3]. These predictions are yet to be borne out by experiment, because no isotropic S = 1/2 model system has been identified in the laboratory for these lattices.In the last few years, there has been a rapid progress in the understanding of the S = 1/2 Heisenberg antiferromagnet in the Shastry-Sutherland lattice [4], another two-dimensional frustrated geometry, for which the exact ground state is known at zero and low magnetic fields despite geometrical frustration. The progress owes largely to the discovery of the spin gap and other novel magnetic properties in SrCu 2 (BO 3 ) 2 [5], the only known laboratory model for the geometrically frustrated S = 1/2 two-dimensional Heisenberg antiferromagnet.The low-field ground state of the Shastry-Sutherland antiferromagnet is simply a product of spin-dimer singlet wave functions, and the dimerization by the diagonal bond J causes a gap to form [4]. The gapped triplet excitations, which are also spin dimers at least to a good approximation, have a large mass as a result of the unique lattice topology involving orthogonal arrangement of dimers [6,7,8,9]. In SrCu 2 (BO 3 ) 2 , the coupling strengths extracted from experiments are J = 85 K for the intra-dimer exchange and J ′ = 54 K for the interdimer exchange [10] or J = 70 K and J ′ = 42 K [11]. The large J ′ makes the energy gap ∆ substantially smaller than J [5,12]. In...

Effect of Dzyaloshinskii-Moriya interactions on the phase diagram and magnetic excitations of SrCu2(BO3)Romhányi1, Totsuka2, Penc3 2011Phys. Rev. B

“…Later another splitting was found 22 at the q = (π, 0) point indicating the relevance of in-plane components of the DM interaction. The ESR study of Nojiri et al 23 shows an anti-level crossing at the critical magnetic field where the lowest-lying triplet excitation would cross the singlet level, which is consistent with the persistent spin gap found in the specific-heat 24 -and the NMR measurements 25 . These splittings and the antilevel crossing mean that states corresponding to different magnetization (singlets and triplets) are mixing and S z is no longer a good quantum number.…”

Novel Ordered Phases in the Orthogonal Dimer Spin System SrCu2(BO3)2

“…The transition temperatures plotted in Fig. 12(b) extrapolates smoothly to the plateau region and also to the specific heat anomaly observed at higher fields up to 33 T. 45) The full 11 B NMR spectra are displayed in Fig. 13, including one spectrum in the 1/8 plateau (27.5 T) and two spectra above the plateau (28.7 and 29.9 T).…”

Degeneracy of the 1/8 Plateau and Antiferromagnetic Phases in the Shastry-Sutherland Magnet TmB4

“…Among the FPPs that such theories need to replicate, the 1/8-FPP has presented a puzzle. While both the Heisenberg SrCu 2 (BO 3 ) 2 as well as Ising TmB 4 exhibit this phase in both thermodynamic 8,9,[22][23][24][25][26][27][28][29][30] 10,27 , and charge transport 30 , and the major features are shown in the inset of and hysteresis in the value of the magnetization in the plateau region, as opposed to the location of its boundaries, is seen. In this region, Siemensmeyer et al 27 and Wierschem et al 29 observe M/M sat fractions of 1/7, 1/9, and 1/11, in addition to 1/8.…”