Probing wave functions in complex oxides with X-rays

In general, gaining an understanding of the properties of any material depends on determining the wavefunctions of its many-body ground and excited states. As the rules of quantum mechanics preclude the direct observation of any wavefunction, experimentalists are constantly striving to develop new methods which probe physical quantities that are as directly related to the wavefunction as possible.

In this context, resonant X-ray scattering has been highlighted in a number of studies, as the incident X-ray energy can be tuned to excite transitions between specific electronic wave functions.

Reporting in Physical Review Letters, a team of researchers from the ESRF, Grenoble, France, and the London Centre for Nanotechnology (LCN) describe the results of their theoretical study of the extent to which it is possible to extract information on the wave function of complex oxides using resonant X-ray techniques.  The specific oxides considered by the team are those subject to strong spin-orbit coupling.

The spin-orbit coupling plays a pivotal role in determining the properties of a range of materials of current interest. Although the phenomenology can be very different, ranging from topological insulators to the anomalous Hall effect, in each case the spin-orbit interaction serves to entangle spin and orbital moments producing exotic magnetic states. One particular group of materials for which the spin-orbit coupling has been predicted to lead to novel electronic states is the transition metal oxides formed from a 5d metal. Of specific interest are compounds of iridium for which - in the ideal case – a so-called jeff = 1/2 ground state has been predicted to give rise to a Mott-like insulating state (see Fig. 1).

In their calculations, Moretti Sala et al. investigated how the ground and excited states wave functions of iridium oxides may be interrogated using resonant X-ray scattering techniques. Most importantly, they elucidated the dependence of the resonant X-ray scattering cross-sections on structural distortions of the local cubic environment and on the direction of the local magnetic moment, something which up to this point had not been fully considered. As well as providing a rigorous theoretical framework for interpreting existing data, their work is of use in planning further experiments to explore the unusual properties of the jeff =1/2 state.

Resonant X-Ray Scattering and the jeff=1/2 Electronic Ground State in Iridate Perovskites. M. Moretti Sala, S. Boseggia D. F. McMorrow and G. Monaco

Phys. Rev. Lett. 112, 026403 (2014)

Journal link: DOI: 10.1103/PhysRevLett.112.026403

Figure: The unusual extended, three dimensional  jeff=1/2 groundstate wave function of  an Ir4+ ion in an undistorted  IrO6 octahedron.  The IrO6 octahedron   forms the idealised structural motif for understanding the properties of a range of novel iridium based oxides. Moretti Sala et al. provide a theoretical framework for understanding  the extent to which the  jeff=1/2 wave function can be probed by resonant X-ray techniques depending on distortions from cubic symmetry and the direction of the iridium magnetic moment.

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