2020-12-4, Utech, Toni, Pötschke, Petra, Simon, Frank, Janke, Andreas, Kettner, Hannes, Paiva, Maria, Zimmerer, Cordelia

Electrochemically exfoliated graphene (eeG) layers possess a variety of potential applications, e.g. as susceptor material for contactless induction heating in dynamic electro-magnetic fields, and as flexible and transparent electrode or resistivity heating elements. Spray coating of eeG dispersions was investigated in detail as a simple and fast method to deposit both, thin conducting layers and ring structures on polycarbonate substrates. The spray coating process was examined by systematic variation of dispersion concentration and volume applied to heated substrates. Properties of the obtained layers were characterized by UV-VIS spectroscopy, SEM and Confocal Scanning Microscopy. Electrical conductivity of eeG ring structures was measured using micro-four-point measurements. Modification of eeG with poly(dopamine) and post-thermal treatment yields in the reduction of the oxidized graphene proportion, an increase in electrical conductivity, and mechanical stabilization of the deposited thin layers. The chemical composition of modified eeG layer was analyzed via x-ray photoelectron spectroscopy pointing to the reductive behavior of poly(dopamine). Application oriented experiments demonstrate the direct electric current heating (Joule-Heating) effect of spray-coated eeG layers.

2020, Zhang, Yang, Xu, Qiunan, Koepernik, Klaus, Fu, Chenguang, Gooth, Johannes, van den Brink, Jeroen, Felser, Claudia, Sun, Yan

Since spin currents can be generated, detected, and manipulated via the spin Hall effect (SHE), the design of strong SHE materials has become a focus in the field of spintronics. Because of the recent experimental progress also the spin Nernst effect (SNE), the thermoelectrical counterpart of the SHE, has attracted much interest. Empirically strong SHEs and SNEs are associated with d-band compounds, such as transition metals and their alloys—the largest spin Hall conductivity (SHC) in a p-band material is $\sim 450\left(\hslash /e\right){\left({\Omega}\enspace \mathrm{c}\mathrm{m}\right)}^{-1}$ for a Bi–Sb alloy, which is only about a fifth of platinum. This raises the question whether either the SHE and SNE are naturally suppressed in p-bands compounds, or favourable p-band systems were just not identified yet. Here we consider the p-band semimetal InBi, and predict it has a record SHC ${\sigma }_{xy}^{z}\approx 1100\enspace \left(\hslash /e\right){\left({\Omega}\enspace \mathrm{c}\mathrm{m}\right)}^{-1}$ which is due to the presence of nodal lines in its band structure. Also the spin-Nernst conductivity ${\alpha }_{zx}^{y}\approx 1.2\enspace \left(\hslash /e\right)\left(A/m\cdot K\right)$ is very large, but our analysis shows its origin is different as the maximum appears in a different tensor element compared to that in SHC. This insight gained on InBi provides guiding principles to obtain a strong SHE and SNE in p-band materials and establishes a more comprehensive understanding of the relationship between the SHE and SNE.

2020, Senfftleben, B., Kretschmar, M., Hoffmann, A., Sauppe, M., Tümmler, J., Will, I., Nagy, T., Vrakking, M.J.J., Rupp, D., Schütte, B.

Intense extreme-ultraviolet (XUV) pulses enable the investigation of XUV-induced non-linear processes and are a prerequisite for the development of attosecond pump - attosecond probe experiments. While highly non-linear processes in the XUV range have been studied at free-electron lasers (FELs), high-harmonic generation (HHG) has allowed the investigation of low-order non-linear processes. Here we suggest a concept to optimize the HHG intensity, which surprisingly requires a scaling of the experimental parameters that differs substantially from optimizing the HHG pulse energy. As a result, we are able to study highly non-linear processes in the XUV range using a driving laser with a modest (˜ 10 mJ) pulse energy. We demonstrate our approach by ionizing Ar atoms up to Ar5 + , requiring the absorption of at least 10 XUV photons. © 2020 The Author(s). Published by IOP Publishing Ltd

2020, Major, B., Kretschmar, M., Ghafur, O., Hoffmann, A., Kovács, K., Varjú, K., Senfftleben, B., Tümmler, J., Will, I., Nagy, T., Rupp, D., Vrakking, M.J.J., Tosa, V., Schütte, B.

The ongoing development of intense high-harmonic generation (HHG) sources has recently enabled highly non-linear ionization of atoms by the absorption of at least 10 extreme-ultraviolet (XUV) photons within a single atom (Senfftleben et al, arXiv:1911.01375). Here we investigate how the generation of these very intense HHG pulses in our 18-m-long beamline is aided by the reshaping of the fundamental, few-cycle, near-infrared (NIR) driving laser within a 30-cm-long HHG Xe medium. Using an incident NIR intensity that is higher than what is required for phase-matched HHG, signatures of reshaping are found by measuring the NIR blueshift and the fluorescence from the HHG medium along the propagation axis. These results are well reproduced by numerical calculations that show temporal compression of the NIR pulses in the HHG medium. The simulations predict that after refocusing an XUV beam waist radius of 320 nm and a clean attosecond pulse train can be obtained in the focal plane, with an estimated XUV peak intensity of 9 × 1015 W cm-2. Our results show that XUV intensities that were previously only available at large-scale facilities can now be obtained using moderately powerful table-top light sources. © 2020 The Author(s). Published by IOP Publishing Ltd