Optical spectroscopy

Our research interests are devoted to optical and magneto-optical properties of novel strongly correlated electron systems exhibiting unconventional electronic transport and magnetic phenomena. We use wide-band, from sub-mm to deep ultraviolet, spectroscopic tools such as ellipsometry, reflectance and transmittance measurements to explore the interplay between charge, orbital and spin degrees of freedom in transition metal oxides and the mechanism of high-temperature superconductivity in ferropnictides and cuprates. Recently our research has been focused on the interface induced optical anomalies and unusual physics occurs in oxide-based artificially fabricated nanostructures.

Dielectric response and transverse field muon spin rotation measurements show that superlattices with LaNiO₃ as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions upon cooling, whereas superlattices with thicker LaNiO₃ layers remain metallic and paramagnetic at all temperatures.

Spectral ellipsometry allows measurements of the real and imaginary parts of the dielectric function of solids with significantly higher accuracy and reproducibility than regular reflection spectroscopy, because reference measurements and Kramers-Kronig transformations are not required. Our group has used synchrotron radiation to extend the frequency range of this method into the far-infrared regime. A Fourier-transform infrared spectrometer and ellipsometer was implemented and commissioned at the ANKA synchrotron at the Forschungszentrum Karlsruhe, building on experience previously acquired using a prototype instrument at the Brookhaven National Laboratory (USA). The apparatus now operates routinely and has been instrumental for many of our projects. The photon energy range from ≈ 1meV to ≈ 5.6 eV is now continuously covered with ellipsometers available in our group.
Highlights of recent research with spectral ellipsometry include the determination of collective modes and spectral weight shifts in cuprate superconductors; the discovery of magneto-polaron excitations in cobaltates; a quantitative analysis of the interplay between the magnetic structure high-energy charge excitations in a Mott-Hubbard insulator; the discovery of the enhancement of the “pseudogap” in a high-temperature superconductor by magnetic impurities; phonon anomalies associated with charge ordering in iron oxides; and the determination of the optical properties of oxide superlattices.
Current instrument development work is aimed at the use of coherent far-infrared radiation at ANKA, and at the development of infrared reflectance and ellipsometry methods in the presence of high magnetic fields. A high-field infrared reflectivity setup has just become operational in our laboratory.