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Rotational anisotropic nth harmonic generation (RA-nHG) of Sr2IrO4 perovskite structure

Filed in: Blog.Rotational-anisotropic-nth-harmonic-generation-RA-nHG-of-Sr2IrO4-perovskite-structure · Modified on : Wed, 25 Oct 17

Title : Structural distortion-induced magnetoelastic locking in Sr2IrO4 revealed through nonlinear optical harmonic generation Author : D. H. Torchinsky et al. DOI : 10.1103/PhysRevLett.114.096404

The Sr2IrO4 was predicted to be an unconventional superconductor upon chemical doping with a pseudo gap behavior because of its crystallographic, electronic and magnetic structure to the parent compound La2CuO4 of the high-Tc cuprates. But it's unclear why no signature of superconductivity are detected. Recent experiments suggest that the structural and magnetic properties of Sr2IrO4 are not completely understood, in particular neutron diffraction studies report new Bragg peaks that challenge its long accepted crystal structure. On the other hand, resonant x-ray diffraction studies report a near perfect locking of the magnetic moment canting and oxygen octahedra rotation angles that cannot be fully explained by existing theoretical models.

In this letter, they report a global bulk structural distortion in Sr2IrO4 observed using a combination of spatially resolved optical second harmonic generation (SHG) and third harmonic generation (THG) experiments. Nonlinear optical harmonic generation is a process by which light of frequency "w" is converted into higher harmonics "nw" (n=2,3,4...) through its nonlinear interaction with a material. By Neumann's principle, the nonlinear optical susceptibility tensors that relate the incident electric field to induced electric dipole, electric quadrupole, magnetic dipole, or even higher order multiple densities, which act as sources of higher harmonic radiation and must be invariant under every symmetry operation of the crystal. The structure of the susceptibility therefore encodes the symmetries of a crystal, with higher rank allowing for more accurate levels of refinement. The components of the susceptibility can be measured through rotational anisotropy (RA) experiments, where the intensity of high harmonic light reflected from a crystal is recorded as a function of the angle subtended between the light scattering plane and a crystalline axis. Different tensor components are probed by selecting the incident and reflected light to be either P of S polarized.

Previous work have assigned Sr2IrO4 to a centrosymmetric tetragonal 4/mmm crystallographic point group (I41/acd space group). However, recent single crystal neutron diffraction studies observe additional nuclear Bragg peaks that violate the I41/acd space group. These forbidden peaks may originate from structural defects such as oxygen vacancies that distort the local symmetry or from a subtle global symmetry reduction. Although a number of alternative centrosymetric and noncentrosymetric space groups have been proposed. The RA-SHG and RA-THG patterns together show that Sr2IrO4 exhibits a globally reduced bulk structural symmetry that is best describe by the I41/a space group.

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