arXiv subjects

Lukas Powalla

Publications and source records attributed to Lukas Powalla.

3 recordsLinked to original sources

arXiv2026

Stabilizing Magnetic Bubble Domains in Epitaxial 2D Magnet/Topological Insulator Heterostructures through Interfacial Interactions

Epitaxial heterostructures of two-dimensional van der Waals magnets and topological insulators offer a powerful platform for probing interfacial spin interactions that govern magnetic textures in low-dimensional quantum systems, while simultaneously enabling highly efficient, atomically thin spin-orbit-torque memory and computing architectures. Despite this promise, the fundamental role of these interfacial interactions in determining magnetic domain-phase stability remain largely uncharted. Here, we perform scanning transmission X-ray microscopy to image nanoscale magnetic textures in epitaxial Fe3GeTe2 Bi2Te3 heterostructures, enabled by a thermal-release-tape dry transfer process onto X-ray transparent silicon-rich nitride membranes. Under zero-field-cooled conditions, we observe robust bubble domain phases from 75 to 165 K, and across different number of folds of the multilayer Fe3GeTe2 Bi2Te3 heterostructures. This is in stark contrast with exfoliated single-crystal Fe3GeTe2 flakes, where ZFC stripe domains are observed for flakes thicker than 20 nm and no domains have been reported for thin flakes less than 15 nm. First-principles calculations and micromagnetic simulations reveal that interfacial coupling to Bi2Te3 modifies the magnetic anisotropy and introduces interfacial Dzyaloshinskii-Moriya interaction, shifting the magnetic phase space towards bubble-domain stabilization without field-cooling. Together, our results offer a new strategy for phase-selective control of magnetic domains through interfacial engineering.

cond-mat.mtrl-sci↗

arXiv2021

Fluxons in high-impedance long Josephson junctions

The dynamics of fluxons in long Josephson junctions is a well-known example of soliton physics and allows for studying highly nonlinear relativistic electrodynamics on a microscopic scale. Such fluxons are supercurrent vortices that can be accelerated by a bias current up to the Swihart velocity, which is the characteristic velocity of electromagnetic waves in the junction. We experimentally demonstrate slowing down relativistic fluxons in Josephson junctions whose bulk superconducting electrodes are replaced by thin films of a high kinetic inductance superconductor. Here, the amount of magnetic flux carried by each supercurrent vortex is significantly smaller than the magnetic flux quantum $ Φ_0 $. Our data show that the Swihart velocity is reduced by about one order of magnitude compared to conventional long Josephson junctions. At the same time, the characteristic impedance is increased by an order of magnitude, which makes these junctions suitable for a variety of applications in superconducting electronics.

cond-mat.supr-con↗

arXiv2021

Berry curvature-induced local spin polarisation in gated graphene/WTe$_2$ heterostructures

Full experimental control of local spin-charge interconversion is of primary interest for spintronics. Heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have provided so far global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we utilize magneto-optical Kerr microscopy to resolve the gate-tunable, local current-induced spin polarisation in graphene/WTe$_2$ van der Waals (vdW) heterostructures. It turns out that even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We develop a theoretical model which explains the gate- and bias-dependent onset and spatial distribution of the massive Kerr signal on the basis of interlayer tunnelling, along with the reduced point group symmetry and inherent Berry curvature of the heterostructure. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important further step toward electrical spin control on the nanoscale.

cond-mat.mes-hall↗