Ruben Hühne’s group uses deposition techniques such as pulsed laser deposition (PLD) and magnetron sputtering to prepare thin films and heterostructures. The main emphasis is on the study of epitaxially grown superconducting, ferroelectric and magnetic thin films in order to understand the basic phenomena in these materials and to tune their functional properties for applications. The research on superconducting thin films focuses on unconventional superconductors as high-Tc cuprates or Fe-based materials, whereas layers showing an electrocaloric effect are studied in the area of ferroelectricity. Additionally, the group has a large expertise in the application of superconductors in superconducting magnetic bearings (SMB). Here, we develop and optimize together with our cooperation rotational bearings for different applications partners as for example ring spinning textile machines. This research includes detailed measurements of the dynamic properties at high-speed rotational speed as well as numerical simulations in order to improve the properties of the complete setup.
T. Espenhahn, M. Sparing, A. Berger, K. Nielsch, R. Hühne, Supercond. Sci. Technol. 34 (2021) 125004.
Superconducting magnetic bearings enable friction-free rotation, which is beneficial for different applications. Whereas mechanical losses are drastically reduced, additional hysteretic losses might be generated as for example by a process-related tilt of the field-cooled permanent magnet of the superconducting bearing. We designed a caloric measurement setup with a resolution down to 5 mW in order to study such losses in more detail. Therefore, a field-cooled magnet ring is deliberately tilted up to 3°before it rotates with a speed of up to 5000 rpm above the superconducting ring. The generated losses inside the superconductor lead to an increased evaporation rate in the attached liquid nitrogen bath, which in turn results to a measurable volume flow. The determined losses increase almost linear with speed, whereas an exponential increase was observed for the tilt angle. The results were confirmed by 2D simulations using a two-component model leading to similar dependencies for the hystertic losses on speed and tilt.
B. Magalhaes, S. Engelhardt, C. Molin, S.E. Gebhardt, K. Nielsch, R. Hühne, J. Alloys Com. 891 (2021) 162041.
The basic aim of our study is to investigate the correlation between structural parameters and the electrocaloric effect in lead-free epitaxial Ba1−xSrxTiO3 (BSTO) based thin film architectures. Therefore, BSTO thin films with Sr contents of x = 0 to x = 0.3 were grown on SrRuO3 buffered SrTiO3 single crystalline substrates by pulsed laser deposition. Structural characterization verified an epitaxial growth for all Sr contents with an additional tetragonal distortion compared to bulk material. Temperature and frequency- dependent measurements of dielectric properties revealed increased permittivity values for thicker films with broad maxima indicating a diffuse phase transition. The temperature of maximum permittivity decreases with increasing Sr content, whereas polarization measurements indicate a relaxor-like behaviour in particular above room temperature. Adiabatic temperature changes were determined with the indirect method resulting in |ΔT| values of up to 2.9 K for a 680 nm thick BSTO layer with x = 0.3 at an applied electric field of 750 kV cm−1.
S. Holleis, A. Anna Thomas, I.A. Shipulin, R. Hühne, A. Steiger-Thirsfeld, J. Bernardi, M. Eisterer, Supercond. Sci. Technol. 35 (2022) 074001.
Iron-based superconductors are a popular candidate in the search for affordable and simple superconductors for high-field applications. In particular, the relaxed texture requirements fuel hope that films deposited on RABiTS with simple buffer layer architectures could enable cheap coated conductors. We find that a single Yttrium oxide buffer layer can act as a suitable diffusion barrier and epitaxial Fe(Se,Te) thin films were successfully grown by pulsed laser deposition. An analysis of the local current distribution by means of scanning Hall probe microscopy reveals current densities exceeding 1 MA cm−2, however, granularity still seems to be an issue. Transmission electron microscopy images and analysis by transmission Kikuchi diffraction show that the out-of-plane orientation of underlying Ni-W grains in the substrate has a severe impact on the growth of Fe(Se,Te) films.
S. Holleis, I.A. Shipulin, R. Hühne, J. Bernardi, M. Eisterer, Supercond. Sci Technol. 35 (2022) 104001.
REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO3 (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron
microscopy of undoped and BHO-doped YBa2Cu3O7-δ(YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.
S. Guo, B. Wang, D. Wolf, A. Lubk, W. Xia, M. Wang, Y. Xiao, J. Cui, D. Pravarthana, Z. Dou, K. Leistner, R.W. Li, R. Hühne, K. Nielsch ACS Nano 17 (2023) 2517-2528.
Colossal magnetoresistance is of great fundamental and technological significance in condensed-matter physics, magnetic memory, and sensing technologies. However, its relatively narrow working temperature window is still a severe obstacle for potential applications due to the nature of the material-inherent phase transition. Here, we realized hierarchical La0.7Sr0.3MnO3 thin films with well-defined (001) and (221) crystallographic orientations by combining substrate modification with conventional thin-film deposition. Microscopic investigations into its magnetic transition through electron holography reveal that the hierarchical microstructure significantly broadens the temperature range of the ferromagnetic−paramagnetic transition, which further widens the response temperature range of the macroscopic colossal magnetoresistance under the scheme of the double-exchange mechanism. Therefore, this work puts forward a method to alter the magnetic transition and thus to extend the magnetoresistance working window by nanoengineering, which might be a promising approach also for other phase-transition-related effects in functional oxides.
Correlation between microstructure and superconductivity in advanced coated conductors architectures based on technical templates (DFG research grant)
Ruben Hühne (IFW Dresden), Michael Eisterer (TU Wien, Austria), since 2019
Direct measurement of the electrocaloric efficiency in lead-free microstructured film architectures (DFG research grant)
Ruben Hühne (IFW Dresden), Peter Gaal (IKZ Berlin), since 2020
Development and study of a three-axis-stable superconducting bearing prepared from coated conductors (DFG research grant)
Tilo Espenhahn (IFW Dresden), since 2020
Three-dimensional yarn path and bearing dynamics of the high-speed ring spinning process with a superconducting magnetic bearing twisting system including transient operation modes (DFG research grant)
Ruben Hühne (IFW Dresden), Michael Beitelschmidt (TU Dresden), Chokri Cherif (TU Dresden), since 2021
Development of superconducting tape stacks for implementation in contactless magnetic bearings for high rotational speed (DFG research grant)
Ruben Hühne (IFW Dresden), since 2023