Michael Zuerch1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2
Michael Zuerch1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2
Extreme ultraviolet second harmonic generation spectroscopy (XUV-SHG) is an emerging technique used to study inversion symmetry breaking with core-state specificity. This novel technique was only recently demonstrated for the first time measuring the surface spectrum of carbon films [1]. Pushing nonlinear spectroscopic techniques to the x-ray regime has several advantages. For example, higher energy x-rays can penetrate materials providing access to buried interfaces and symmetry-broken states in bulk material with specificity to a single atomic species. Recent experiments demonstrated quantification of the interfacial bond geometry of an organic-inorganic interface [2] and measurement of a surface spectrum of titanium [3]. Measuring the angular distribution of SXR-SHG has enabled additional sensitivities such as to the nature of the symmetry state itself [4]. Here, we utilize XUV-SHG spectroscopy to investigate the polar metal phase of LiOsO<sub>3</sub>. In polar metals the coexistence of polarity and metallicity is unexpected as the itinerant conducting electrons in metals are expected to screen long-range electrostatic forces that are typically required to stabilize a macroscopic polarization. The large difference of atomic number renders it challenging to study this material with electron and X-ray scattering techniques. We apply XUV-SHG to study the symmetry properties in this material with specificity to the lithium atoms in the lattice. In the experiment we focus an intense femtosecond X-ray laser beam obtained by a free-electron laser onto the material with photon energies in the range of 28 to 33 eV, which enables reaching a resonance condition for the Li 1s electrons around the K-edge at ~54 eV. A sensitivity to broken inversion symmetry appears above the Li K-edge. We compare the experimental spectra with numerical calculations based on time-dependent density functional theory that show how the spectrally-resolved SHG varies with Li-displacement. As the first demonstration of XUV-SHG spectroscopy around a phase transition, these results pave the way for using nonlinear XUV methods to investigate broken symmetry from an element-specific perspective. In addition, inherent femtosecond temporal resolution will enable studying phase transitions on the electronic timescale.<br/><br/>[1] R. K. Lam, et al., Phys. Rev. Lett. 120, 023901 (2018)<br/>[2] C. P. Schwartz, et al., arXiv:2005.01905 (2020).<br/>[3] T. Helk, et al., Sci. Adv. 7, eabe2265 (2021).<br/>[4] C. Uzundal, et al., arXiv:2010.03134 (2021), in press PRL.<br/>[5] E. Berger at al., Nano Letters 21, 6095–6101 (2021).