Visualizing the MIT in V2O3
Visualizing the MIT in V<sub>2</sub>O<sub>3</sub>

Despite 60 years of research, the metal-to-insulator transition in complex oxide materials still holds some mysteries. In a study published in Nature Physics, we show that the electronic transition in V2O3 evolves in real space by forming intertwined `mazes' of insulating and metallic patches.

The first order metal-to-insulator in  has been widely studied as a typical example of a phase transition driven by strong correlation effects. In this transition the resistivity changes by several decades turning the material from bad metal at high temperature into a so-called Mott insulator at low temperatures. During a period as visiting scholar in the group of Dimitri Basov at the University of California: San Diego I was involved in the first low temperature measurements made with an upcoming technique in materials science: scattering-type scanning near-field optical spectroscopy. Near field optical spectroscopy makes use of a standard atomic force microscope, coupled to a michelson-morley type interferometer, to probe materials on length scales much smaller than the diffraction limit. In our experiment we used infrared radiation with a wavelength of 11 μm and used the near-field interaction between a metallic AFM needle and a material to probe an area of 25 nm radius. Together with PhD student Alex S. McLeod, we visualize the spatial dependence of the electronic response in thin films of V2O3 as the material is slowly cycled through the transition. Unexpectedly, the electronic response shows stark metallic and insulating patterns. By analyzing these patterns we find that the macroscopic response of the system results from a percolative type of transition: as temperature increases metallic patches start to grow until they form a connected network of metallic stripes (the percolation threshold). At this temperature the resistivity shows the strongest decrease and starts to behave metallic. Finally, we show that the structural transition, which was thought to occur concomitantly, is decoupled from the electronic transition. By careful thermometry between different experiments we show that structural transition temperature is higher than the electronic transition temperature.

Read more on complex oxides.
Fermi liquid like ground state in the iron-pnictides
Fermi liquid like ground state in the iron-pnictides

There are two key ingredients necessary to gain further understanding of high-temperature superconductivity. The first is the identification of the pairing interaction and the second a correct description of the normal state from which superconductivity emerges. Our optical experiments show that the normal state of a carefully annealed, electron-doped iron-pnictide superconductor is well described by Fermi liquid theory.

The analysis of the optical properties of materials is usually done using either the Drude-Lorentz model or the extended Drude model. The former is a semi-classical approach providing a description of the optical properties in terms of free and bound charges and provides useful information on for example the plasma frequency or the size of a bandgap. The extended Drude model on the other hand can provide information on the interactions between electrons or between electrons and the lattice (see my thesis for more info). One of the drawbacks of the extended Drude model is that it fails when the free charge optical response overlaps in energy with interband transitions, which is the case for multi-band systems like the iron-pnictide superconductors. Our work shows a new route forward by analysing the full frequency and temperature dependence of the optical conductivity. This is made possible by the high temperature resolution of 2 Kelvin in our experiments, which is comparable with the frequency resolution. Our analysis shows that the experimentally determined complex optical conductivity can be accurately predicted using basic Fermi liquid theory over a broad range of temperatures and frequencies.

Read more in the pdf.
Optical spectroscopy in Amsterdam
Optical spectroscopy in Amsterdam

We are proud to announce that construction of the new optical spectroscopy lab has finished. Our home-build cryostat combined with a Bruker vertex 80v allows us to measure optical spectra in the photon energy range between 2 meV and 6 eV at temperatures between 7.5 K and 400 K.

Erik van Heumen joined the Institute of Physics in August 2013 where he will focus on optical spectroscopy of quantum materials. As part of the Quantum Matter group the new lab also has access to crystal growth and characterisation facilities, angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Having gained experience with ARPES and STM as Veni Laureate, Erik will continue to combine multiple spectroscopies in the investigation of new functional materials. Alona Tytarenko joined the group in January 2014 and is currently finishing the first experiments in the new lab. Her first paper will involve superconductivity in iron-pnictide superconductors. Stay tuned for the first papers, but in the mean time you can get an impression of the new lab in the image gallery.