Strain engineering of the charge and spin-orbital interactions in Sr ₂ IrO ₄

Abstract

In the high spin–orbit-coupled Sr2IrO4, the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir–O bond geometry in Sr2IrO4 and perform momentum-dependent resonant inelastic X-ray scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low-energy elementary excitations. We observe that the pseudospin-wave dispersion for tensile-strained Sr2IrO4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven cross-over from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron–hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr2IrO4, originating from the modified hopping elements between the t2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr2IrO4 and provides valuable information toward the control of the ground state of complex oxides in the presence of high spin–orbit coupling.

Modeling multiorbital effects in Sr2IrO4 under strain and a Zeeman field

“…[16] the strain is shown to cause a decrease in the AFM order manifested in a lowered Néel temperature. As found in our model, the increased importance, due to strain, of the J = 3/2 states also agrees with the observed intensity increase in optical transitions between J = 3/2 and 1/2 states found in other studies [45][46][47] . In addition, transport measurements observe a steady decrease in resistivity as the compressive epitaxial strain is increased 48 .…”

“…Experiments show both decreasing resistivity for tensile strain values 48 and a lower Néel temperature for samples with a tensile strain of = 0.4% than for those with a compressive strain of = −0.7% 20 . However, ab initio calculations at tensile strain 46 point towards an increased charge gap which agrees with that observed in RIXS spectra 47 . Accurately modelling the tensile regime seems to require the inclusion of effects not included in this work.…”

Charge ordering in Ir dimers in the ground state of Ba5AlIr2O11

“…1 The lowest nine quartet (s = 3 2 ) and 24 doublet (s = 1 2 ) scalar relativistic states (first column in Table I) are first computed and then are allowed to admix via the SOC, resulting in 84 states (see the second column of Table I). It can be seen that the excitation energies obtained from CASSCF + NEVPT2 + SOC calculations are in excellent agreement with the peaks observed in RIXS experiments, except for peak D. This peak is related to the electron-hole exciton which is also observed in other iridate materials such as Sr 2 IrO 4 [29,38,39] and Na 2 IrO 3 [40]. Such excitations are not considered in the current QC calculations [41].…”

Unveiling the Mechanism of Bulk Spin‐Orbit Torques within Chemically Disordered FexPt1‐x Single Layers

“…After we have excluded non‐uniformities in magnetic moment density, electron scattering strength, and exchange stiffness as the dominant mechanisms of inversion asymmetry, the lattice strain gradient is left as the most likely symmetry‐breaking mechanism for the bulk dampinglike SOT. Microscopically, a strain gradient can non‐uniformly modify the strengths of the SHE, [ 31–35 ] spin‐orbit interaction, [ 36 ] orbital polarization, [ 37 ] spin states at the Fermi level, [ 37 ] and strain‐spin coupling, [ 38 ] ultimately leading to inversion symmetry breaking in the generation and relaxation of spin current within the sample. As shown in Figure 4c, the lattice constant of the chemically disordered Fe x Pt 1‐ x indeed increases by 2.3% as x from 0.25 to 0.75, [ 39 ] suggesting a very strong strain gradient in the composition‐gradient samples.…”