Research topics

We investigate emergent phenomena of quantum matter such as low-dimensional magnetism, unconventional superconductivity, or electronic topological states. Transport of charge and entropy, in either bulk materials or nanoscale devices, provide important information about unconventional quasiparticles. Their spectral and scattering properties are probed in bulk samples, whereas the quasi-particle phase, their quantum interference, and decoherence are accessed in quantum transport experiments in nanoelectronic devices. At even smaller scales, we map out real-space variations of the electronic local density of states (LDOS) and of the magnetic structure with low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and magnetic force microscopy (MFM), respectively.

Quantum magnetism

We investigate quantum magnetism primarily by transport measurements. Low-dimensional quantum magnets, such as spin chains, ladders, planes, often possess a highly unusual mode of heat transport which is carried by the elementary excitations of those systems. We use this quantum-magnetic heat transport in order to probe the low-energy physics of these excitations, viz. the nature of their thermal generation and their scattering. Selected Publications

Unconventional superconductivity

We use transport measurements for investigating the normal state of unconventional superconductors. This includes the exploration of electronic phase diagrams of novel superconducting materials, as well as dedicated Hall-, Seebeck-, and Nernst effect measurements which aim at revealing peculiar electronic properties of selected compounds. As a very powerful method for addressing the superconducting state, we apply high-resolution STM/STS, because superconductivity is probed on a local scale. From registering spatial variations of the LDOS, e.g. caused by a magnetic field or impurities, we extract information on microscopic parameters, the momentum space electronic structure, and even the pairing symmetry. Selected Publications

Collective phenomena of charge, spin and orbitals

The partial filling of electronic d-shells in transition metal compounds often is of paramount importance for the compounds’ physical properties. Usually, electronic correlation is strong, which can lead to a number of exciting physical effects based on the interplay of charge, magnetism and orbitals and related collective ordering phenomena. Well known examples are collective charge, spin and orbital ordering states in manganites and nickelates, multiferroicity, and also charge/spin density waves in itinerant systems. We employ transport experiments as well as STM/STS for the exploration and rationalization of such phenomena. Selected Publications

Carbon-based materials and molecular nanostructures

Carbon-based materials and molecular nanostructures, such as graphene and molecular magnets possess a proven application potential. We use STM/STS for exploring the real-space electronic structure of pertinent materials and systems. Selected Publications

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Topological Insulators

Topology is a powerful paradigm in quantum condensed-matter physics, and the importance of geometric phases in band theory has recently gained new prominence with the discovery of Z2 topological insulators (TIs). We study the physics of their gapless states in nanostructures of 3D TIs, such as Bi2Se3, where surface-state transport results from topologically-protected electronic states (spin-chiral Dirac fermions). Such quasi-particles have unique properties (suppression of localization, weak scattering by disorder,...) which can be revealed by quantum transport studies of nanowires/ribbons (quantum interferences, proximity effect with superconducting or ferromagnetic contacts). Selected Publications

Oxide Heterostructures

Transition metal oxide heterostructures offer interesting opportunities to investigate strongly correlated electronic systems in reduced dimensionality. Among them, the intriguing formation of a quasi-2D metallic state at the interface between two insulating materials gives broad perspectives to study new collective phases induced by electronic interactions. We focus our work on a new type of oxide heterostructure γ-Al2O3/SrTiO3 with enhanced carrier mobility and phase-coherence length, as compared to optimized LaAlO3/SrTiO3 heterostructures. Selected Publications

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Nanomagnets based on carbon nanotubes

Our nanoscale chemistry group supported by the synthesis and crystal growth group successfully synthesized single crystalline nanomagnets embedded in carbon nanotubes (CNT). These hybrid nanosystems are characterized by a very high mechanical and chemical stability, high magnetic shape anisotropy and exceptionally narrow switching field distributions. We investigate magnetic properties including anisotropy and magnetization reversal of individual single domain nanomagnets. Furthermore we are developing nanomagnetometry concepts to bridge the gap between conventional cantilever force detection methods and ultrasensitive CNT oscillator approaches. Studies of the interplay of kinematics and magnetism of individual ferromagnetic nanowires include the investigation of rapidly repeated magnetization reversal that is induced by the flexural vibration of cantilevered nanowires. Our working plan might be indicated and motivated by the following consideration: Frequencies of magnetic resonances are roughly independent of the magnets’ size. In contrast, the ratio of mechanical resonance frequencies is proportional to the reciprocal of the ratio of their sizes. Thus, downscaling of a ferromagnetic mechanical oscillator may finally allow for matching of mechanical and magnetization frequencies resulting in exciting magnetomechanical couplings. Selected Publications

Development of MFM probes

Our development of monopole-type magnetic force microscopy (MFM) probes based on filled CNT allows for straightforward high-resolution quantitative MFM measurements. The recently developed bimodal sensor type is simultaneously sensitive to magnetic field derivatives parallel and perpendicular to the sample’s surface. We are developing and intending to implement design concepts of MFM sensors with modulated tip magnetization that would allow removing the non-magnetic force background. Selected Publications

Nanoscale graphitization in carbon films

Currently, we are revisiting a topic already covered ten years ago (Appl. Phys. Lett. 85, 5727 (2004)). Under ambient or vacuum conditions diamond and related materials like tetrahedral amorphous carbon (ta-C) are in a metastable state. By application of energy ta‑C is able to undergo a phase change to a graphitic or sp2-hybridised state. We induce local phase changes in ta-C thin films by eV and low keV electron beams in ultrahigh vacuum provided by a scanning tunneling microscope (STM) and a scanning electron microscope (SEM), respectively. The local graphitization is analyzed by STM-based current-distance (I(z)) spectroscopy, conductive atomic force microscopy (c-AFM), as well as ultraviolet and x-ray photoemission spectroscopy (UPS/XPS). Selected Publications

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NANO UKULELE – A short and funny movie showing preparation steps of nanotube-based oscillator devices by Julia Körner and Christopher F. Reiche