arXiv subjects

Karl Bergson Hallberg

Publications and source records attributed to Karl Bergson Hallberg.

4 recordsLinked to original sources

arXiv2026

Real-space microscopic description of laser-pulse induced melting of superconductivity

Quenching quantum order via laser pulses has proven a useful tool to access exotic physical effects in systems that are strongly perturbed out of equilibrium. However, theoretical modelling of experimental measurements is typically done phenomenologically or by assuming translational invariance due to the complexity of the problem. Here, we solve a microscopic real-space model of the time dynamics of a superconductor following an intense laser-pulse. We are able to reproduce recent experimental findings displaying a critical slowing-down of the melting of the order parameter for laser fluences close to the condensation energy. Moreover, we leverage the real-space resolution of our model to predict how phase fluctuations and currents in the system behave both spatially and temporally. We discover an unusual current flow in the superconductor after the pulse has subsided, resembling backward waves that normally require special engineering in metamaterials or wave guides. Our results predict a rich behavior of the superconducting order parameter at a microscopic level which is manifested in current textures that can be probed using radiation detection.

cond-mat.supr-con↗

arXiv2026

Electric and spin current vortices in altermagnets

Altermagnets constitute a class of collinear magnets with momentum-dependent spin splitting and vanishing net magnetization. Direct observation of the characteristic altermagnetic spin splitting, however, remains challenging. Indirect signatures can be obtained via transport studies, which so far have only considered homogeneous driving fields. We propose to leverage nonuniform electric fields and spin density gradients to probe the shape and the spin polarization of altermagnetic Fermi surfaces via transport measurements. By using both a semiclassical Boltzmann approach and a lattice Keldysh formalism, we show that altermagnets excite swirling electric and spin currents whose profiles depend on the relative orientation of altermagnetic lobes with respect to the sample boundaries. These currents can be measured via magnetometry techniques. Unlike previous proposals considering the hydrodynamic regime of transport, swirling currents are observed even in the Ohmic regime and rely exclusively on the altermagnetic spin splitting, with no swirls observed in ferromagnets. The electric and spin current vortices predicted here provide a different altermagnetic signature in an experimentally accessible setup.

cond-mat.mes-hall↗

arXiv2025

Orbital splitter effect and spatial resolution of current-induced orbital accumulation

The emergence of an orbital angular momentum (OAM) response to a charge current holds promise for technological applications, allowing electrical control of magnetization dynamics. Often, the OAM current is invoked in explaining experimental results for very large orbital transport effects, but this is conceptually challenging as the OAM current is not a conserved quantity. Instead of utilizing the orbital conductivity associated with the non-conserved OAM current, we here use non-equilibrium Green's functions to directly image the OAM density in real space under an applied electric current bias. We find strong spatial variations in OAM density, with the lattice acting as a source and sink of OAM. Moreover, we show that the OAM response depends sensitively on the angle between the charge current and the crystal axis. This enables the generation of a transverse OAM response in one current direction and solely a longitudinal response in another. We refer to this as an orbital splitter effect, analogous to the spin splitter effect in altermagnets.

cond-mat.mes-hall↗

arXiv2025

Visualization of spin-splitter effect in altermagnets via non-equilibrium Green functions on a lattice

When a charge current is injected into an altermagnet along a suitable crystallographic direction, a transverse spin current can be generated. This so-called spin-splitter effect does not rely on spin-orbit coupling, and is thus distinct from the spin Hall effect. The spin-splitter effect was predicted by \textit{ab initio} calculations and has been experimentally confirmed. To utilize the spin-splitter effect for practical purposes in spintronic devices, it is important to understand (i) how the system parameters affect the transverse spin current, such as filling fraction, altermagnetic strength, interface parameters, spin-orbit interactions, and impurities and (ii) determine the properties of any associated spin accumulation, which is the measurable quantity. Here, we determine the answer to these questions and provide a real-space visualization of the spin flow and spin accumulation due to the spin-splitter effect. We utilize the non-equilibrium Keldysh Green function method on a 2D square lattice to this end. We find that the presence of edges induces oscillations in the spin accumulation and strongly modify the signal for small samples. At half-filling, the spin accumulation acquires an anomalous pattern and the spin-splitter effect vanishes. We prove analytically that this follows from a combined particle-hole and spin-reversal symmetry of the model used for the altermagnetic state. Increasing the altermagnetic strength leads to a larger spin accumulation, as expected. However, when adding Rashba spin-orbit interaction, providing an additional spin Hall signal, we find that the spin accumulation is not simply the sum of the spin Hall and spin-splitter contribution. Finally, we show that the spin-splitter effect is robust towards moderate impurity scattering with a potential of the same order as the hopping parameter, which facilitates its observation in real materials.

cond-mat.mes-hall↗