Conferences, Workshops, Lectures (archive)

[more]

Abstract: We investigate the lattice relaxation effect on moir´e band structures in twisted bilayer MoTe2 with two approaches: (a) large-scale plane-wave basis first principle calculation down to 2.88 deg, (b) transfer learning structure relaxation + local-basis first principles calculation down to 1.1 deg. Two types of van der Waals corrections have been examined: the D2 method of Grimme and the density-dependent energy correction. We note the density-dependent energy correction yields a continuous evolution of bandwidth with twist angles. Including second harmonic of intralayer potential/interlayer tunneling and the strain induced gauge field, we develop a more complete continuum model with a single set of parameters for a wide range of twist angles, and perform many-body simulations at v=-1,-2/3, -1/3. We further identify a series of C=1 Chern bands around 2 from Wilson loop and edges state calculation, which serves as the foundation point for even-denominator non-Abelian states. [more]

Abstract: Non-collinear chiral antiferromagnet (AFM) Mn₃Sn is an attractive material since this AFM exhibits a Néel temperature of 420 K and a non-negligible magneto-optical Kerr effect (MOKE) due to magnetic-octupole order and topological Weyl nature. Despite extensive research on ultrafast magnetization dynamics in ferromagnets and ferrimagnets, investigations into such chiral AFMs are still in their very early stages. In particular, magnetic damping significantly influences both the energy efficiency and operational speed of the device aiming to utilize the electronic spin degree of freedom, thus further understanding is demanded. Here we investigate, for the first time, time-resolved magneto-optical Kerr effect for Mn₃Sn films with a perpendicular magnetic anisotropy to gain insight into the physics of octupole dynamics. The films were prepared using magnetron sputtering technique. The film stacking structure is single crystalline MgO (110) sub./W(2) /Ta(3) /Mn₃Sn(30) /MgO_(1.3) /Ru(1) (thickness is in nm). The time-resolved MOKE (TR-MOKE) was measured using an all-optical pump-probe technique. The out-of-plane magnetic-octupole dynamics is induced by the pump laser pulse and is detected via MOKE for a probe laser pulse. We observed an ultrafast change of Kerr rotation angle at the zero delay, which is attributed to ultrafast collapse of magnetic octupole order. We also observed the damped-oscillation which would be attributed to GHz-frequency magnetic octupole order dynamics. [more]

[more]

[more]

[more]

[more]

[more]

[more]

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 [more]