arXiv subjects

Jakob Kjærulff Svaneborg

Publications and source records attributed to Jakob Kjærulff Svaneborg.

4 recordsLinked to original sources

arXiv2026

Spectroscopic signatures of persistent exciton condensation in a bulk magnetic topological insulator

Exciton condensates are long-sought correlated quantum states arising from macroscopic coherence of bound electron-hole pairs. Although equilibrium and transient excitonic states have been reported in several material platforms, direct evidence for a light-induced exciton condensate state has been challenging to achieve as an intrinsic property of a bulk quantum material. Here, we use time- and angle-resolved photoemission spectroscopy to investigate long-lived photoexcited carriers in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ with the chemical potential tuned to the topological surface state by Sb substitution. Following optical excitation, we observe the delayed emergence of a transient state whose formation coincides with depopulation of the bulk conduction band and whose lifetime extends to the microsecond timescale. The quasiparticle dispersion exhibits a pronounced flattening and develops a Mexican-hat-like profile. These spectral signatures are consistent with the formation of a metastable excitonic condensate state. Our results establish magnetic topological insulators as a promising platform for investigating long-lived photoinduced many-body states and their interplay with topology and magnetism.

cond-mat.mes-hall↗

arXiv2026

Rigorous foundations of the effective mass approximation: analyticity and symmetry of band extrema

The effective mass approximation is widely used across models of carrier transport, optical response, and excitons in semiconductors and insulators, but its validity hinges on the assumption that the band dispersion $E_n(\mathbf{k})$ at the relevant extremum is analytic. We prove that analyticity holds at any non-degenerate extremum for Kohn-Sham density functional theory (DFT) with local or hybrid exchange-correlation functionals, for Hartree-Fock, and for band-edge $G_0 W_0$ quasiparticle energies in gapped systems. Band non-analyticity (or warping) in these settings is therefore intrinsically tied to degeneracy. We then use group theory to determine the symmetry-allowed form of the effective mass tensor for each of the 32 crystallographic point groups, providing a stringent consistency check on first-principles calculations. As a representative application, we show that the electron and hole effective masses at the $K$ point of monolayer MoS$_2$ must be strictly isotropic at the DFT and $G_0W_0$ levels.

cond-mat.mtrl-sci↗

arXiv2026

Quantitative prediction of excitons in lattice-mismatched van der Waals heterostructures

Accurate modeling of dielectric screening in van der Waals (vdW) heterostructures is essential for predicting photonic and optoelectronic properties - yet conventional first-principles methods are often hindered by incommensurate lattices and prohibitive computational costs. In this work, we introduce the microscopic Quantum Electrostatic Heterostructure (mQEH) method. mQEH employs a hierarchical and systematically improvable basis set to describe potentials and induced densities, eliminating the need for arbitrary geometric cutoffs and ensuring accurate screening descriptions at all length scales. The mQEH method is combined with a layer projected Bethe-Salpeter Equation (BSE) to enable calculations of optical spectra of experimentally relevant lattice-mismatched vdW heterostructures. Applying the mQEH-BSE framework to a series of transition-metal dichalcogenide (TMD) heterobilayers, we obtain absorption spectra and momentum-indirect exciton energies in excellent agreement with experiment. The framework provides a computationally efficient route to predictive modeling and design of vdW heterostructures with tailored optical properties.

cond-mat.mtrl-sci↗

arXiv2023

Manipulation of magnetization and spin transport in hydrogenated graphene with THz pulses

Terahertz (THz) field pulses can now be applied in Scanning Tunnelling Microscopy (THz-STM) junction experiments to study time resolved dynamics. The relatively slow pulse compared to the typical electronic time-scale calls for approximations based on a time-scale separation. Here, we contrast three methods based on non-equilibrium Green's functions (NEGF): (i) the steady-state, adiabatic results, (ii) the lowest order dynamic expansion in the time-variation (DE), and (iii) the auxiliary mode (AM) propagation method without approximations in the time-variation. We consider a concrete THz-STM junction setup involving a hydrogen adsorbate on graphene where the localized spin polarization can be manipulated on/off by a local field from the tip electrode and/or a back-gate affecting the in-plane transport. We use steady-state NEGF combined with Density Functional Theory (DFT-NEGF) to obtain a Hubbard model for the study of the junction dynamics. Solving the Hubbard model in a mean-field approximation, we find that the near-adiabatic first order dynamical expansion provides a good description for STM voltage pulses up to the 1 V range.

cond-mat.mes-hall↗