Lectures, Workshops, Conferences (Archiv)

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Abstract: Ni-Mn based Heusler alloys undergo a first-order magneto-structural transition (FOMST) from high-temperature high magnetic austenite to low-temperature low magnetic martensite. The FOMST can be induced by external stimuli such as magnetic fields, uniaxial stress, or temperature. The sensitivity of FOMST to external stimuli results in large caloric effects and can be used for multicaloric solid-state cooling. A disadvantage of Ni-Mn-based Heusler alloys is their brittleness, limiting the shaping of those alloys required for further application. We used microstructure design by suction casting or additive manufacturing (AM) to process alloys with beneficiary microstructure to increase mechanical stability and cyclic performance. In addition, AM also enables the processing of complex geometries. We also investigate the correlation of microstructure and the thermal hysteresis of the FOMST and the transition width. By in-situ microscopy, we can identify the nucleation sides of the FOMST and the defect acting as pinning sides for the propagation of the phase transition. The FOMST and thermal hysteresis can be specifically designed for multicaloric cooling cycles by tailoring the processing method, parameters, and particle size. In addition, microstructural design by DED, PBF-LB, SPS, or hot compaction can significantly improve the mechanical and cyclic stability of brittle Heusler alloys. This work is supported by ERC (Adv. Grant "Cool Innov") and DFG 527201505 and CRC/TRR 270 "HoMMage". [1] F. Scheibel et al., Energy Techn. 6, 1397 (2018), DOI :10.1002/ente.201800264 [2] L. Pfeuffer et al., Acta Mater. 221, 117390 (2021), DOI:10.1016/j.actamat.2021.117390 [3] F. Scheibel et al., Materialia 29, 101783 (2023), DOI:10.1016/j.mtla.2023.101783 [4] F. Scheibel et al., Adv. Eng. Mater. 24, 2200069 (2022), DOI:10.1002/adem.202200069 [mehr]

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Frontiers of topological quantum matter: linked Weyl rings and ideal Weyl ferromagnets Quantum science is driven by the synthesis of state-of-the-art quantum materials and the characterization of their exotic topological states [1-7]. In the first part of this talk, I introduce our discovery of linked Weyl rings in the room temperature Heusler ferromagnet Co₂MnGa using high-resolution soft X-ray ARPES [1,2]. By combining ideas in condensed matter physics and knot theory, I explicitly draw the Weyl link diagram for the quantum state and show a linking number of (2,2,2), providing a direct experimental measurement of a new kind of topological invariant in physics. In the second part of this talk, I introduce our observation of a semimetallic Weyl ferromagnet in thin films of (Cr,Bi)₂Te₃ [3]. In transport, we find a record bulk anomalous Hall angle > 0.5 along with non-metallic conductivity, a regime sharply distinct from established Weyl materials and conventional ferromagnets. Together with density functional theory (DFT), our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation > 75% of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using non-equilibrium molecular beam epitaxy (MBE), we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a Murakami-type topological phase diagram with broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our discovery of a topological quantum link and semimetallic Weyl ferromagnet suggests new approaches to non-Abelian quantum states, as well as materials synthesis methods relevant to quantum technology. 1. I.B. et al. Nature 604, 647 (2022) 2. I.B. et al. Science 365, 6459 (2019) 3. I.B. et al. Nature, under review 4. Max T. Birch, I.B. et al. Nature, in press (2024) 5. M. Z. Hasan, G. Chang, I.B. et al. Nat. Rev. Mat. 6, 784 (2021) 6. D. Sanchez*, I.B.* et al. Nature 567, 500 (2019) 7. S. Xu*, I.B.* et al. Science 349, 613 (2015) [mehr]

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Abstract: The heavy fermion metal YbRh₂Si₂ is superconducting below about 10 mK, and is a candidate odd parity, and hence topological, superconductor. Previous 4-terminal electrical transport study on a number of samples demonstrates the important role played by sample inhomogeneity in the measured response. However, the magnetic susceptibility reaches full diamagnetic screening at T = 0 and thus we use these measurements to determine the complex conductivity of the superconducting state. A slab-like sample 380 µm thick is placed in magnetically shielded environment to < 20 nT, and studied as a function of magnetic field applied perpendicular to the c-axis. The setup operates at frequencies < 1 kHz.As a first step, the results are analysed in terms of an effective London penetration depth λ, which shows an unusual temperature dependence with the effective λ(T=0) significantly larger than anticipated. On cooling, a distinct downward step in λ clearly shows a transition into a new superconducting regime at TA ~ 2 mK, coinciding with a sharp heat capacity signature and by a drop in sample inductance seen in electrical transport. Since the onset of more robust superconductivity below TA in zero field is associated with a new SDW antiferromagnetic order, it is a candidate odd-parity PDW.We will also report the strong field dependence of the complex conductivity in the context of non-linear Meissner effect, and discuss evidence for line nodes in the gap structure, and topological surface states. [mehr]