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Register a new userDecorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2
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We report the microscopic magnetic model for the spin-1/2 Heisenberg system CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization plateau. Recent neutron diffraction experiments on this compound [M. Hase et al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order, inconsistent with the previously suggested phenomenological magnetic model of isolated dimers and spin chains. Based on extensive density-functional theory band structure calculations, exact diagonalizations, quantum Monte Carlo simulations, third-order perturbation theory, as well as high-field magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2 are accounted for by a frustrated quasi-2D magnetic model featuring four inequivalent exchange couplings: the leading antiferromagnetic coupling J_d within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2, forming magnetic tetramers, and a ferromagnetic coupling J_it between the tetramers. Based on comparison to the experimental data, we evaluate the ratios of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30, with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the frustration and triggers long-range antiferromagnetic ordering. The proposed model accounts correctly for the different magnetic moments localized on structurally inequivalent Cu atoms in the ground-state magnetic configuration. We extensively analyze the magnetic properties of this model, including a detailed description of the magnetically ordered ground state and its evolution in magnetic field with particular emphasis on the 1/2-magnetization plateau. Our results establish remarkable analogies to the Shastry-Sutherland model of SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.
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Motivated by the intriguing properties of the Shastry-Sutherland compound SrCu2(BO3)2 under pressure, with a still debated intermediate plaquette phase appearing at around 20 kbar and a possible deconfined critical point at higher pressure upon entering the antiferromagnetic phase, we have investigated its high-field properties in this pressure range using tunnel diode oscillator (TDO) measurements. The two main new phases revealed by these measurements are fully consistent with those identified by infinite Projected Entangled Pair states (iPEPS) calculations of the Shastry-Sutherland model, a 1/5 plateau and a 10 x 2 supersolid. Remarkably, these phases are descendants of the full-plaquette phase, the prominent candidate for the intermediate phase of SrCu2(BO3)2. The emerging picture for SrCu2(BO3)2 is shown to be that of a system dominated by a tendency to an orthorhombic distortion at intermediate pressure, an important constraint on any realistic description of the transition into the antiferromagnetic phase.
Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTOs quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (vertical to b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.
The Shastry-Sutherland model and its generalizations have been shown to capture emergent complex magnetic properties from geometric frustration in several quasi-two-dimensional quantum magnets. Using an $sd$ exchange model, we show here that metallic Shastry-Sutherland magnets can exhibit topological Hall effect driven by magnetic skyrmions under realistic conditions. The magnetic properties are modelled with competing symmetric Heisenberg and asymmetric Dzyaloshinskii-Moriya exchange interactions, while a coupling between the spins of the itinerant electrons and the localized moments describes the magnetotransport behavior. Our results, employing complementary Monte Carlo simulations and a novel machine learning analysis to investigate the magnetic phases, provide evidence for field-driven skyrmion crystal formation for extended range of Hamiltonian parameters. By constructing an effective tight-binding model of conduction electrons coupled to the skyrmion lattice, we clearly demonstrate the appearance of topological Hall effect. We further elaborate on effects of finite temperatures on both magnetic and magnetotransport properties.
We present 11B NMR studies of the 2D frustrated dimer spin system SrCu2(BO3)2 in the field range 27-31 T covering the upper phase boundary of the 1/8 magnetization plateau, identified at 28.4 T. Our data provide a clear evidence that above 28.4 T the spin-superlattice of the 1/8 plateau is modified but does not melt even though the magnetization increases. Although this is precisely what is expected for a supersolid phase, the microscopic nature of this new phase is much more complex. We discuss the field-temperature phase diagram on the basis of our NMR data.
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