Importance ofXYanisotropy inSr2IrO4revealed by magnetic critical scattering experiments

Abstract

The magnetic critical scattering in Sr 2 IrO 4 has been characterized using x-ray resonant magnetic scattering (XRMS) both below and above the three-dimensional antiferromagnetic ordering temperature T N . The order parameter critical exponent below T N is found to be β = 0.195(4), in the range of the two-dimensional (2D) XY h 4 universality class. Over an extended temperature range above T N , the amplitude and correlation length of the intrinsic critical fluctuations are well described by the 2D Heisenberg model with XY anisotropy. This contrasts with an earlier study of the critical scattering over a more limited range of temperature, which found agreement with the theory of the isotropic 2D Heisenberg quantum antiferromagnet, developed to describe the critical fluctuations of the conventional Mott insulator La 2 CuO 4 and related systems. Our study therefore establishes the importance of XY anisotropy in the low-energy effective Hamiltonian of Sr 2 IrO 4 , the prototypical spin-orbit Mott insulator. The Ruddlesden-Popper series Sr n+1 Ir n O 3n+1 of perovskite iridates has emerged as a fruitful arena in which to explore the effects of electron correlations in the strong spin-orbit coupling limit. The first two members of this series, single-layer Sr 2 IrO 4 (n = 1) and bilayer Sr 3 Ir 2 O 7 (n = 2), are believed to exemplify a new class of spin-orbit Mott insulators. Of central importance to our understanding of these materials is the emergence of a j eff = 1/2 ground state by the combined action of a strong cubic crystal field and spin-orbit interactions on the 5d 5 electrons of the Ir 4+ ions [1]. The weakened electron correlations typical of the 5d elements then split the j eff = 1/2 band, opening a gap, leading to a Mott-like state.Sr 2 IrO 4 in particular has attracted considerable attention because of its striking similarities to La 2 CuO 4 in terms of both its structural and magnetic properties. The magnetic structures and excitations of Sr 2 IrO 4 have been investigated in a number of x-ray resonant magnetic scattering (XRMS) studies [1][2][3][4][5][6][7] which have allowed an effective low-energy Hamiltonian to be proposed and refined. Sr 2 IrO 4 forms an antiferromagnetic structure below T N ∼ 225 K in which the moments are confined to the a − b planes and canted to follow rigidly the correlated rotation of the oxygen octahedra of the I 4 1 /acd crystal structure [8]. A resonant inelastic x-ray scattering (RIXS) experiment [2] has revealed a dispersion relation somewhat reminiscent of that displayed by La 2 CuO 4 , albeit with a lower-energy scale and much stronger further neighbor couplings, which can be derived from a smaller ratio of on-site repulsion over hopping amplitude [9]. This result * j.vale@ucl.ac.uk suggests that the low-energy isospin dynamics of the j eff = 1/2 states in Sr 2 IrO 4 may, to leading order, be mapped onto an effective isotropic two-dimensional Heisenberg Hamiltonian, in agreement with predictions by Jackeli and Khaliullin [10].Critical scattering studies provide informa...

Dispersive magnetic and electronic excitations in iridate perovskites probed by oxygen K -edge resonant inelastic x-ray scattering

“…The large magnon gap (∼90 meV) in Sr 3 Ir 2 O 7 was explained to be driven by the strong magnetic anisotropy associated with SOC [17,18]. The smaller magnon gap in Sr 2 IrO 4 (∼40 meV) is consistent with an Ir L 3 RIXS report [16] and was described by including an XY anisotropic term [45,46] into the Heisenberg model described in Ref. [15].…”

“…The black curve in (a) is a theoretical fit of the magnon dispersion using a Heisenberg model reported in Refs. [45,46]. The black and red curves in (b) are transverse and longitudinal magnon dispersions for Sr 3 Ir 2 O 7 using the quantum-dimer model reported in Ref.…”

Strong anisotropy within a Heisenberg model in the Jeff=12 insulating state of Sr2Ir0.8Calder1, Kim2, Taylor3 et al. 2016Phys. Rev. B7040View full textAdd to dashboardCite

“…A further surprising aspect of the excitation spectra for Sr 2 IrO 4 was the absence of a spin-gap in several measurements 2,11-13 , however a separate reanalysis of the data in Ref. 11 indicated a spin-gap 14 . Overall any spin-gap would be expected to be large in a SOC dominated system with strong exchange interactions and the absence was explained as being due to an accidental balance of symmetric and antisymmetric anisotropic terms for the basal plane moment case 15 .…”

“…Therefore following this reasoning in Sr 2 Ir 0.8 Ru 0.2 O 4 that has c-axis aligned pseudospins this quasi-degeneracy is broken and a large gap can emerge. This observation has further implications since in the undoped case if a large spin-gap does indeed emerge then it is a result of dominant XY -anisotropy 14 …”

Possibility of an unconventional spin state ofIr4+inBa21Ir9O43Yang1, Jeong2, Arakcheeva3 et al. 2016Phys. Rev. BSelf Cite0000View full textAdd to dashboardCite

“…Additional effects of SOC include XY anisotropy [11,12], locking of the magnetic moment to the correlated rotation of oxygen octahedra in Sr 2 IrO 4 [13], anisotropic dimer excitations in Sr 3 Ir 2 O 7 [14], and Kitaev-type magnetic interactions in Na 2 IrO 3 [15]. Structural distortions from the ideal octahedral environment have been observed in the pyrochlore family of the type R 2 Ir 2 O 7 (R =rare earth) where trigonal compression induces a reconstruction of energy levels [16,17].…”