"Quantum matter"

"Quantum matter" refers to materials whose properties emerge from quantum mechanical interactions in many-body systems. The profound understanding of these systems is one of the central challenges for modern solid-state physics. Quantum matter shows a multitude of fascinating phenomena and new surprising effects are discovered again and again. Examples within the large family of transition-metal oxides with strong electronic correlations are high-temperature superconductivity, quantum magnetism, orbital fluctuations, multiferroicity and many others. Recently, systems with strong spin-orbit coupling became the focus of international attention. One example are topological insulators which harbor Dirac electrons in surface states. The optical properties of topological insulators are predicted to show a series of exotic phenomena such as the topological magnetoelectric effect, a universal conductance of the metallic surfaces at high energies, the coupling of spin and charge degrees of freedom to a spin plasmon, and the universal Faraday effect. Promising challenges for optical spectroscopy!

Optical investigations of these systems over a broad frequency range are of paramount importance for a microscopic understanding of the fundamental phenomena. A short example: using Fourier spectroscopy, we recently were able to establish the existence of a new bulk phenomenon in topological insulators: the formation of puddles at low temperatures and their rapid destruction with increasing temperature. The insulating bulk corresponds to a perfectly compensated semiconductor. Coulomb interactions between disordered donors and acceptors give rise to strong band bending. Puddles are formed as soon as valence band or conduction band touch the Fermi level. The observation that puddles are destroyed by heating to 50 K came as a complete surprise. However, this behavior can be described quantitatively by thermally excited carriers which screen the Coulomb fluctuations and reduce band bending. 

Experimental investigations of such novel materials and the observation of entirely new effects constitute a fascinating experience. Questions concerning Bachelor or Master projects within the group can be clarified in a personal discussion.

Optical spectroscopy (THz spectroscopy, Fourier spectroscopy, ellipsometry)

The combination of ellipsometry, Fourier spectroscopy, and THz spectroscopy and the close collaboration with the group of Prof. J. Hemberger – employing e.g. dielectric spectroscopy – allows us to cover a very broad frequency range over 18 orders of magnitude, from mHz to PHz (i.e., UV).

Measurements using Fourier spectroscopy are preformed from the far infrared up to the UV (2 meV - 6 eV). This allows us to investigate many different types of excitations such as free-carrier absorption, phonons, orbital and magnetic excitations, excitons, or interband excitations. One focus of the group lies on optically “forbidden” excitations such as spinons or orbitons which become optically active e.g. via the additional excitation of a phonon or spin-orbit coupling.

Ellipsometry is a self-normalizing method which determines the polarization state – in general elliptical – of the reflected light for oblique angle of incidence. The self-normalizing property in particular allows for a very precise determination of the temperature dependence of the optical properties, e.g. the change of the spectral weight of the optical conductivity across a magnetic phase transition. In the vanadates, we for instance employed this precise temperature dependence to distinguish between an orbital liquid with strong quantum fluctuations and a more classical type of long-range orbital order. Another example is our recent detection of a change of the electronic excitations at the heavily debated Verwey transition in magnetite Fe3O4. 

THz spectroscopy has experienced enormous progress recently.  In close collaboration with the group of Prof. Hemberger, we employ photomixers which emit circularly polarized THz radiation at the difference frequency of two narrow-band tunable near-infrared lasers. Coherent detection allows us to determine both amplitude and phase of the THz radiation. This exceptional setup covers the range from 60 GHz to 2 THz with a resolution of the order of a few MHz. The possibility to employ the photomixers inside a magneto cryostat at liquid He temperatures is particularly rewarding since it allows us to perform THz measurements with circularly polarized light in high magnetic fields (up to 8T) at low temperatures without the disturbing influence of optical windows. This method is particularly well suited for investigations of e.g. magnetic excitations or of the universal Faraday effect in topological insulators. Recently, we developed a self-normalizing method for measuring the phase with significantly enhanced accuracy. Based on photomixing of three lasers, this method eliminates the (e.g. thermal) drift of the optical path-length difference, achieving an accuracy of 10-8 of the optical path length. 

RIXS (resonant inelastic x-ray scattering)