Strongly Correlated Electronic Oxides
Research focus lies on electronic correlations in oxide thin films. Due to Coulomb electron-electron interaction and strong electron-phonon and spin-phonon coupling the 3d-transition-metal oxides demonstrate a number of coupled phase transitions, i.e. Metal-Insulator, Ferro(Antiferro)-Paramagnetic, Charge/Orbital Ordering and Structural Phase Transition. The strongly correlated electronic oxides (SCEO) include materials with perovskite structure, ABO3, containing 3d-(V, Mn, Co, Fe) and 4d-(Mo, Ru) transition metal ions at the B-site and rare earth (La, Pr) and alkali (Ca, Sr, Ba) ions occupying A-sites. Electronic correlations and phase transitions in bulk SCEO can be controlled by chemical doping and/or isovalent substitutions. In thin films an additional nanoscale control by tuning the film architecture, dimensionality and interfaces is possible.
Growth challenges
We are developing novel approaches and technologies to grow thin films and hetero-structures of SCEO using in-situ atomic layer growth control by optical ellipsometry. Our technique (see Fig), Metalorganic Aerosol Deposition (MAD), is a chemical deposition route, which uses aerosols of metalorganic precursors to control the stoichiometry of the grown film. MAD is a vacuum-free technique and provides growth conditions close to the equilibrium (temperature, deposition rate and oxygen partial pressure), which could be advantageous for growth of SCEO. Our current interests are:
a) fine controlling of the growth atmosphere (Ar/O2 ratio);
b) an adaptive MAD growth by getting feedback between the ellipsometry signal and precursor dosing units and
c) in-situ growth of microstructured films with extremely high deposition rates ~1 ?/s.
Structure
The crystal structure and architecture of SCEO heterostructures is studied by global (X-ray diffraction and reflectometry) and local (STM/AFM, SEM) techniques. In addition, Raman & Tip-Enhanced Raman Spectroscopy (TERS) allow us to correlate the phonon spectra and crystal symmetry, which could be especially advantageous by monitoring phase transitions, driven by temperature, electric and magnetic field as well as by light.
Electronic properties and magnetism
Transport and magnetism is studied by dc/ac electric (PPMS) and magnetic (SQUID, MOKE) techniques in a wide range of temperatures and applied magnetic fields. The main aim is to study emergent exchange coupling (AFM/FM) phenomena mediated by correlated Jahn-Teller polarons at the 1 order transition in electronically and/or structurally phase separated manganites. A possibility to tune the exchange coupling and crystal structure in manganite/titanite superlattices by means of interface engineering and strain was demonstrated. Electronic reconstructions at atomically sharp interfaces in A-AFM/G-AFM superlattices were shown to result in a new interfacial high-Tc ferromagnetic phase.
Figure: A scheme of MAD installation with in-situ growth control by optical ellipsometry.