Evidence of quantum dimer excitations inSr3Ir2O7

Abstract

The magnetic excitation spectrum in the bilayer iridate Sr 3 Ir 2 O 7 has been investigated using high-resolution resonant inelastic x-ray scattering (RIXS) performed at the iridium L 3 edge and theoretical techniques. A study of the systematic dependence of the RIXS spectrum on the orientation of the wave-vector transfer Q, with respect to the iridium-oxide bilayer, has revealed that the magnon dispersion is comprised of two branches well separated in energy and gapped across the entire Brillouin zone. Our results contrast with those of an earlier study which reported the existence of a single dominant branch. While these earlier results were interpreted as two overlapping modes within a spin-wave model of weakly coupled iridium-oxide planes, our results are more reminiscent of those expected for a system of weakly coupled dimers. In this latter approach, the lower-and higher-energy modes find a natural explanation as those corresponding to transverse and longitudinal fluctuations, respectively. We have therefore developed a bond-operator theory which describes the magnetic dispersion in Sr 3 Ir 2 O 7 in terms of quantum dimer excitations. In our model, dimerization is produced by the leading Heisenberg exchange J c , which couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also includes in-plane exchange J , as well as further neighbor couplings and relevant anisotropies. The bond-operator theory provides an excellent account of the dispersion of both modes, while the measured Q dependence of the RIXS intensities is in reasonable qualitative accord with the spin-spin correlation function calculated from the theory. We discuss our results in the context of the quantum criticality of bilayer dimer systems in the presence of anisotropic interactions derived from strong spin-orbit coupling.

Disordered dimer state in electron-dopedSr3Ir2O7

“…Excitations become overdamped as carriers are introduced, yet the large spin gap inherent to the AF parent state survives into the disordered regime. The spectral weight of magnons in the metallic state becomes nearly momentum independent and exhibits a dispersion best described using the BO representation appropriate for a dimer state [21]. Supporting this, static spin susceptibility measurements resolve the emergence of local moments which grow with increasing La-content and are consistent with a picture where each electron doped breaks a dimer and creates an uncompensated moment.…”

“…1 (c) [20,21]. Specifically, the data show that the gap energies of M -peaks at the (π, π) and (0, 0) positions become increasingly inequivalent upon doping.…”

“…While its ground state is antiferromagnetic (AF) [17][18][19], measurements of magnetic excitations in Sr 3 Ir 2 O 7 observe anomalous spectra with large spin gaps (∆E ≈ 90 meV) whose values exceed that of the single magnon bandwidth [20,21]. This has led to models shown in Figs.…”

“…This has led to models shown in Figs. 1 (a) and (b) that cast the underlying exchange into two extremes: A linear spin wave approach (LSW) with a large anisotropy gap and predominantly intraplane exchange [20] versus a bond operator (BO) mean field approach [22] with a dominant interplane, dimerlike, exchange [21,23]. Recently, an additional excitation attributed to a longitudinal mode associated with triplon excitations was observed supporting the latter approach [24].…”

“…The BO approach however allows for nondegenerate spectral weight at these positions through inequivalent transverse mode E π,π and E 0,0 gap values whose ratio is governed by the anisotropy term cot(θ) = E 0,0 /E π,π . Therefore, to parameterize the dispersion in electron-doped Sr-327 samples, the BO model was utilized [21,23].…”