When photoexcited by an ultrafast laser pulse, antiferromagnets allow direct angular momentum transfer between opposing spins, promising faster functionality than ferromagnets, which are intrinsically limited because their net angular momentum must dissipate to the lattice. The process of angular momentum transfer is closely linked to the nature of magnetic coupling in the system. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons (the Ruderman–Kittel–Kasuya–Yosida interaction, RKKY), and the effect of such conditions on the antiferromagnetic direct spin transfer is largely unexplored.
In our study we used resonant ultrafast X-ray diffraction to study ultrafast magnetization dynamics in a series of 4f antiferromagnets, and systematically varied the 4f [more...]

Interlayer charge- and energy transfer processes in atomically thin, layered van der Waals heterostructures are of fundamental importance for determining their properties in novel device concepts based on single active crystalline layers. In our recent preprint (arXiv link) we study the ultrafast excitation, relaxation and transfer processes in an epitaxial grown monolayer WSe₂/graphene heterostructure using time- and angle-resolved photoemission spectroscopy. By measuring the non-equilibrium electronic structure, we identify a novel interfacial energy transfer mechanism: Meitner-Auger energy transfer, which describes the conversion of an exciton in the semiconductor to an intraband electron-hole pair in graphene, characterized by the excitation of deep-lying valence holes [more...]

Phase transitions under quasi-equilibrium conditions, e.g., induced by a slow variation of temperature, are well described by Landau theory. In contrast, the situation far from equilibrium, e.g., after ultrafast laser excitation, differs fundamentally from a thermodynamic scenario, and it remains an open question how our understanding of static phase transitions in complex matter has to be adapted to capture a dynamical, photoinduced melting and recovery of order. In particular, even the thermal critical temperature might not provide a valid description in a system exhibiting strong non-equilibrium between different degrees of freedom, such as electrons and lattice.
In our study which was published in Nature Communications, we investigate an ultrafast charge-density-wave-to-metal transition after optical excitation by combining state-of-the-art time-resolved electronic and structural [more...]

The transport of electrons is governed by the shape of the Fermi surface. We found that the topology of the Fermi surface of a semimetal can be manipulated on ultrafast timescales through optical excitation. A change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. Combining time-resolved multidimensional photoemission spectroscopy and TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal T_d-MoTe₂. We show that this non-equilibrium topological transition finds its microscopic origin in the dynamical modification of the electronic correlations.
Beaulieu et al., Science Advances 7, eabd9275 (2021). Link [more...]

Our recent study of ultrafast lattice dynamics in NiO has demonstrated the existance of a nonthermal phonon population, and that this is directly linked to change in the antiferromagnetically-induced lattice distortion, known as “exchange striction”. The study was enabled by combining unique capabilities of two institutions: growth of a 20nm-thick free-standing NiO single crystal at the university of Halle, and state-of-the-art femtosecond electron diffraction at the Fritz Haber Instititue. The study open a new avenue towards ultrafast control of antiferromagnets via the crystal lattice.
Windsor et al, Phys. Rev. Lett. 126, 147202 (2021)