Emergence of one-dimensional physics from the distorted Shastry-Sutherland lattice

Abstract

Motivated by the on-going investigation of SrCu2(BO3)2 under pressure, we study a variant of the twodimensional Shastry-Sutherland (SS) spin-1/2 model with two types of dimers. Combined with the frustration of the SS model, this modification induces, in a large parameter range, a dimensional reduction at low energies, with nearly decoupled effective S = 1 Haldane chains forming along one of the diagonals of the lattice. We also present evidence that the intermediate plaquette solid phase of the undistorted SS model remains stable in a finite region of the phase diagram. Introduction.-An exciting route for exploring new phases in frustrated magnets consists in applying hydrostatic pressure so as to considerably modify the dominant magnetic interactions and thus alter the relevant low-energy degrees of freedom. The quasi-2D compound SrCu 2 (BO 3 ) 2 [1] offers such a possibility since it is in the close vicinity of a quantum phase transition from an exact tensor product of singlets toward a plaquette solid [2,3]. Indeed, Nuclear Magnetic Resonance (NMR) [4,5], Inelastic Neutron Scattering (INS) [6], as well as earlier susceptibility measurements [7], show strong evidence that SrCu 2 (BO 3 ) 2 undergoes at least one phase transition under high hydrostatic pressure, the nature of which is still not fully understood. What is clear however is that the four-fold (C 4 ) symmetry around the void plaquettes is quickly lost under pressure, so that nearest-neighbor dimers pointing in different directions become inequivalent (see Fig.1). This naturally leads to an extension of the Shastry-Sutherland (SS) model with two different diagonal bonds J 1 and J 2 described by the spin S = 1/2 Heisenberg Hamiltonian

Decorated Shastry-Sutherland lattice in the spin-12magnet CdCu2(BO3)Janson1, Rousochatzakis2, Tsirlin3 et al. 2012Phys. Rev. BSelf Cite

“…For example, external pressure leads to at least one phase transition at low temperatures, 11 as evidenced by nuclear magnetic resonance (NMR) studies, 12 while a recent theoretical model 13 with two inequivalent dimer couplings J predicts an unanticipated, effectively one-dimensional Haldane phase. 13,14 An alternative way to tune the magnetic couplings is the directed substitution of nonmagnetic atoms or anionic groups. The impact of this substitution on the magnetic coupling regime can be very different.…”

Phase diagram of the Shastry-Sutherland Compound SrCu2(BO3)2 under extreme combined conditions of field and pressure

“…Additionally, Dzyaloshinskii-Moriya (DM) interactions [13][14][15][16][17] of order of a few percent of J may affect the location of the phase boundaries. We note finally that the empty plaquette state is different from the full plaquette state implied by experiments, which can be obtained using a deformed Shastry-Sutherland model [24,46] that includes two types of intra-and interdimer interactions. These modifications of the model may also affect the magnetization process, particularly at low fields.…”

“…In the full plaquette phase, the lattice symmetry is broken, and half the dimers, say the horizontal ones, are singlets, while the other ones are triplets. These triplets build effective Haldane spin-1 chains [46], leading to the full plaquette phase (called for that reason the Haldane phase in earlier papers). Similarly, in the 1/5 plateau, all horizontal dimers are singlets, but a subset of vertical dimers along vertical paths also form dimer singlets, effectively cutting the system into vertical stripes, see Fig.…”

Fluctuation-induced ferrimagnetism in sublattice-imbalanced antiferromagnets with application to SrCu$_2$(BO$_3$)$_2$ under pressure

“…The ferrimagnetism discussed in this paper appears upon breaking the glide symmetry, such that two different types of (mutually parallel) dimers emerge. Such symmetry breaking corresponds to an orthorhombic distortion where half of the intra-dimer bonds elongate and the other half contract [29,30]. In the AF state, each of the intra-dimer couplings acts on one AF sublattice only, such that the symmetry between the two sublattices is broken.…”