arXiv subjects

T. Schumann

Publications and source records attributed to T. Schumann.

6 recordsLinked to original sources

arXiv2022

Circular photogalvanic effects in topological insulator/ferromagnet hybrid structures

We study laser driven spin-current effects at ferromagnet/topological-insulator interfaces by two complementary experimental approaches. The DC photocurrent is studied in ferromagnet/topological-insulator bilayers with high spatial resolution. Dynamic interface currents are explored via the emission of terahertz radiation emitted by these currents with high temporal resolution. From our experiments, we reveal a lateral and dynamic interaction of the ferromagnet and the topological insulator interface.

cond-mat.mes-hall↗

arXiv2019

Determination of the crystal field splitting energy in Cd3As2 using magnetooptics

Symmetry considerations are of extreme importance to the topological properties of crystals. A crystal field splitting δ yields Dirac nodes near the Brillouin zone center in Cd3As2, but its value has yet to be determined with precision. We study the band structure of Cd3As2 using magnetooptical infrared spectroscopy measurements on epilayers with low carrier density grown by molecular beam epitaxy. By combining angular dependent cyclotron resonance with Landau level spectroscopy measurements in the Faraday geometry, we determine that δ is positive and equal to 15+/-5 meV in Cd3As2. Our results lead to a more accurate knowledge of the details of the band structure of this Dirac semimetal such as the position its Dirac nodes in momentum space and their splitting into Weyl nodes under a magnetic field.

cond-mat.mes-hall↗

arXiv2019

A Large Effective Phonon Magnetic Moment in a Dirac Semimetal

We investigated the magnetoterahertz response of the Dirac semimetal Cd$_3$As$_2$ and observed a particularly low frequency optical phonon, as well as a very prominent and field sensitive cyclotron resonance. As the cyclotron frequency is tuned with field to pass through the phonon, the phonon become circularly polarized as shown by a notable splitting in their response to right- and left-hand polarized light. This splitting can be expressed as an effective phonon magnetic moment that is approximately 2.7 times the Bohr magneton, which is almost four orders of magnitude larger than ab initio calculations predict for phonon magnetic moments in nonmagnetic insulators. This exceedingly large value is due to the coupling of the phonons to the cyclotron motion and is controlled directly by the electron-phonon coupling constant. This field tunable circular-polarization selective coupling provides new functionality for nonlinear optics to create light-induced topological phases in Dirac semimetals.

cond-mat.mes-hall↗

arXiv2018

Observation of spin Nernst photocurrents in topological insulators

The theoretical prediction of topological insulators in 2007 triggered tremendous interest. They are of fundamental interest because of their topological twist in k-space, which comes along with unidirectional, spin-polarized surface-state currents, required for spin-optoelectronics. This property makes topological insulators on one hand perfect materials for optically generated, ultrafast spin-bunches spin-current sources for the generation of THz radiation. On the other hand, those spin-polarized surface-state currents when generated by a voltage lead to large spin Hall effects, or when generated by a temperature gradient to the thermal analogue, the spin Nernst effect. Both mutually convert charge/ heat currents into transverse spin currents leading to spin accumulations. By connecting both research fields, we show the evidence of heat-transport related spin Hall effects that can be extracted from opto-transport experiments. This heat-driven spin Nernst effect drives a transverse spin-current and affects the optical spin-orientation in the three-dimensional topological insulator. This manifests as a modification of the circular polarization-dependent photocurrent. We illuminate the detailed thermocurrent distribution, including the influence of edges and contacts, in spatially resolved current maps.

cond-mat.mes-hall↗

arXiv2015

Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

cond-mat.mtrl-sci↗

arXiv2009

Graphite based Schottky diodes formed on Si, GaAs and 4H-SiC substrates

We demonstrate the formation of semimetal graphite/semiconductor Schottky barriers where the semiconductor is either silicon (Si), gallium arsenide (GaAs) or 4H-silicon carbide (4H-SiC). Near room temperature, the forward-bias diode characteristics are well described by thermionic emission, and the extracted barrier heights, which are confirmed by capacitance voltage measurements, roughly follow the Schottky-Mott relation. Since the outermost layer of the graphite electrode is a single graphene sheet, we expect that graphene/semiconductor barriers will manifest similar behavior.

cond-mat.mtrl-sci↗