We investigate emergent phenomena of quantum matter such as low-dimensional magnetism, unconventional superconductivity, or topological states. Transport of charge and entropy, in either bulk materials or nanoscale devices, provides 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.
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.
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.
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.
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).
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 development of monopole-type magnetic force microscopy (MFM) probes based on filled carbon nanotubes allows for straightforward high-resolution quantitative MFM measurements. In particular the bidirectional sensor type is simultaneously sensitive to magnetic field derivatives parallel and perpendicular to the sample’s surface.
Furthermore, we develop MFM and magnetometry sensors based on coupled mechanical oscillators. We combine the high sensitivity of a nanowire resonator with the easy detection capability of a state-of-the-art micron-sized cantilever by co-resonant coupling of the two mechanical resonators. The term “co-resonant” refers to the important case when the eigenfrequencies of the two mechanical beams are close to each other. Even though the nanowire’s mass and stiffness might be orders of magnitude smaller than those of the cantilever, very strong coupling phenomena can be expected in that case.
Percolative Mott insulator-metal transition in doped Sr2IrO4,
Z. Sun, J. M. Guevara, S. Sykora, E. M. Pärschke, J. van den Brink, K. Manna, A. Maljuk, S. Wurmehl, B. Büchner, C. Hess,
Evolution of the nematic susceptibility in LaFe1-xCoxAsO,
X. Hong, F. Caglieris, R. Kappenberger, S. Wurmehl, S. Aswartham, B. Büchner, C. Hess,
Topological electronic structure and intrinsic magnetization in MnBi4Te7: a Bi2Te3-derivative with a periodic Mn sublattice,
R. C. Vidal, A. Zeugner, J. I. Facio, R. Ray, M. H. Haghighi, A. U. B. Wolter, L. T. Corredor Bohorquez, F. Caglieris, S. Moser, T. Figgemeier, T. R. F. Peixoto, H. B. Vasili, M. Valvidares, S. Jung, C. Cacho, A. Alfonsov, K. Mehlawat, V. Kataev, C. Hess, M. Richter, B. Büchner, J. van den Brink, M. Ruck, F. Reinert, H. Bentmann, A. Isaeva,
Orbital-driven elasto-Seebeck and elasto-Nernst effects in 1111 iron based superconductors,
F. Caglieris, C. Wuttke, X. Hong, S. Sykora, R. Kappenberger, S. Aswartham, S. Wurmehl, B. Büchner, C. Hess,
Berry curvature unravelled by the anomalous Nernst effect in MnGe3,
C. Wuttke, F. Caglieris, S. Sykora, F. Scaravaggi, A. U. B. Wolter, K. Manna, V. Süss, C. Shekhar, C. Felser, B. Büchner, C. Hess,
Phys. Rev. B 100, 085111 (2019)
Spectroscopic evidence of nematic fluctuations in LiFeAs,
Z. Sun, P. K. Nag, S. Sykora, J. M. Guevara, S. Hoffmann, C. Salazar, T. Hänke, R. Kappenberger, S. Wurmehl, B. Büchner, C. Hess,
Phys. Rev. B 100, 024506 (2019)
Heat transport of cuprate-based low-dimensional quantum magnets with strong exchange coupling,
Physics Reports 811, 1 (2019)
Chemical Aspects of the Candidate Antiferromagnetic Topological Insulator MnBi2Te4,
A. Zeugner, F. Nietschke, A. U. B. Wolter, S. Gaß, R. C. Vidal, T. R. F. Peixoto, D. Pohl, C. Damm, A. Lubk, R. Hentrich, S. K. Moser, C. Fornari, C. H. Min, S. Schatz, K. Kißner, M. Ünzelmann, M. Kaiser, F. Scaravaggi, B. Rellinghaus, K. Nielsch, C. Hess, B. Büchner, F. Reinert, H. Bentmann, O. Oeckler, T. Doert, M. Ruck, A. Isaeva,
Chem. Mater. 31, 2795 (2019)
Spin-polaron ladder spectrum of the spin-orbit-induced Mott insulator Sr2IrO4 probed by scanning tunneling spectroscopy,
J. M. Guevara, Z. Sun, E. M. Pärschke, S. Sykora, K. Manna, J. Schoop, A. Maljuk, S. Wurmehl, J. van den Brink, B. Büchner, C. Hess,
Phys. Rev. B 99, 121114(R) (2019)
Large Thermal Hall Effect in α-RuCl3: Evidence for Heat Transport by Kitaev-Heisenberg Paramagnons,
R. Hentrich, M. Roslova, A. Isaeva, T. Doert, W. Brenig, B. Büchner, C. Hess,
Phys. Rev. B 99, 085136 (2019)
Magnetization reversal and local switching ﬁelds of ferromagnetic Co/Pd microtubes with radial magnetization
N. Puwenberg, C.F. Reiche, R. Streubel, M. Khan, D. Mukherjee, I.V. Soldatov, M. Melzer, O.G. Schmidt, B. Büchner, T. Mühl