Many properties of materials are the result of correlated behaviour of electrons. Magnetic order, metallic conductivity, and superconductivity are just a few examples of different states of matter, each characterized by different types of correlation between the charge and spin of the electrons in a solid.
Theoretical work faces the tremendous challenge posed by the difficulty to define a minimal model for such a large group of materials. Clearly, from a reductionist point of view, the full Schrödinger equation, including all electrons and details of the various different stoichiometry and crystal structures, is not a satisfactory starting point; moreover, exact solutions including the many-body aspects are not a realistic option with the available computational techniques.
Experimentally, one faces the challenge as to how to identify quantities that (i) can be determined experimentally and (ii) provide insights independent from theoretical bias. Apart from direct phenomenological quantities such as resistivity, magnetization, entropy, our understanding is rooted in fundamental quantities related to symmetry, conservation laws and topology. Particularly powerful examples are Mott-insulators (Sr2VO4), Fermi-liquid-like phases (Sr2RuO4), correlation-induced metal-insulator transitions (SmNiO3), hidden order (URu2Si2), topological states of matter, unconventional pairing, and so forth.
In our group we obtain reliable insights in all of these subjects, using various kind of optical spectroscopies, as well as other spectroscopies using synchrotron radiation and neutron sources.
Top image: real part of the dielectric function (top) and optical conductivity (bottom) of NdNiO3 on a NdGaO3 (110) substrate (a), NdNiO3 on a NdGaO3 (101) substrate (b), and for SmNiO3 on a LaAlO3 (001) substrate. Bottom image: real part of the optical conductivity for some temperatures, and energy/temperature color maps of samples (a) NNO/NGO-110, (b) NNO/NGO-101, and (c) SNO/LAO-001. Arrows on the color map mark metal-insulator phase transitions. A and B designate two peaks in the insulating phase. Data at 0 eV come from DC measurements. [J Ruppen et al (2015)]