arXiv subjects

Johanna Richter

Publications and source records attributed to Johanna Richter.

6 recordsLinked to original sources

arXiv2026

Orientation-resolved ultrafast spin reorientation dynamics in ferrimagnetic DyCo$_5$

Under quasi-static conditions, ferrimagnetic DyCo$_5$ thin films exhibit a thermally induced spin-reorientation transition, in which the equilibrium magnetization changes from an out-of-plane to an in-plane orientation, mostly as a result of the competing magnetic anisotropy contributions of the Dy $4f$ rare-earth and Co $3d$ transition-metal sublattices. While this equilibrium transition has been studied, it remains an open question whether such a reorientation can be triggered on ultrafast timescales using femtosecond laser excitation. In this work, we investigate the ultrafast spin-reorientation dynamics in DyCo$_5$. The time-dependent orientation of the magnetization vector following femtosecond laser excitation is quantified by combining polar with transverse magneto-optical Kerr effect (MOKE) measurements in the extreme ultraviolet spectral range, which are sensitive to the out-of-plane and in-plane magnetization component, respectively. Both techniques are implemented at the Co M$_{3,2}$ resonance and are complemented by visible-light MOKE measurements. This combined approach allows us to resolve the canting of the magnetization from an out-of-plane toward an in-plane orientation and to determine the characteristic timescales of the transient spin-reorientation process.

cond-mat.mtrl-sci↗

arXiv2025

Spectroscopic probe of ultrafast magnetization dynamics in the extreme ultraviolet spectral range

The development of spectroscopic techniques in the extreme ultraviolet (XUV) spectral range has significantly advanced the understanding of ultrafast interactions in magnetic systems triggered by optical excitation. In this work, we introduce a previously missing geometry that facilitates the observation of the ultrafast magnetization dynamics of magnetic systems with an out-of-plane magnetization grown on XUV opaque substrates. This approach to probing ultrafast magnetization dynamics combines the magneto-optical Kerr effect with the strong dependence of a sample's reflectance near its Brewster angle. It therefore works with linearly polarized light and does not require any additional polarizing optics. We provide a comprehensive analysis of the technique by presenting both simulations and experimental data as a function of the energy and the polarization of the XUV probe radiation as well as of the delay time after optical excitation.

cond-mat.mtrl-sci↗

arXiv2025

Fluence dependent delay of Ni in an FeNi alloy supports an exchange based origin

The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct spin transfer from Ni to Fe. In this study, we employ element-resolved extreme ultraviolet spectroscopy to investigate the effect of excitation strength on this delay, aiming to resolve the controversy. The data indicate a significant reduction in the delay with increasing fluence, which is inconsistent with the theoretical predictions of OISTR. These findings, in conjunction with the observation of a saturation of Fe demagnetization at the onset of Ni demagnetization, indicate that a spin-wave instability within the IMG framework may provide a potential explanation for the experimental results.

cond-mat.mtrl-sci↗

arXiv2023

Probing the formation of dark interlayer excitons via ultrafast photocurrent

Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond timescale after the excitation. As proof of principle, we extract the interlayer exciton formation time 0.4~ps at 160 $μ$J/cm$^2$ fluence in a MoS$_2$/MoSe$_2$ heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches.

cond-mat.mes-hall↗

arXiv2023

Variations of vibronic states in densely-packed structures of molecules with intramolecular dipoles

Electrostatic potentials strongly affect molecular energy levels and charge states, providing the fascinating opportunity of molecular gating. Their influence on molecular vibrations remains less explored. Here, we investigate Ethyl-Diaminodicyanoquinone molecules on a monolayer of MoS$_2$ on Au(111) using scanning tunneling and atomic force microscopy and spectroscopy. These molecules exhibit a large dipole moment in gas phase, which we find to (partially) persist on the MoS$_2$ monolayer. The self-assembled structures consist of chains, where the dipoles of neighboring molecules are aligned anti-parallel. Thanks to the decoupling efficiency of the molecular states from the metal by the MoS$_2$ interlayer, we resolve vibronic states of the molecules, which vary in intensity depending on the molecular surrounding. We suggest that the vibrations are strongly damped by electrostatic interactions with the environment.

cond-mat.mes-hall↗

arXiv2017

Quantitative bounds versus existence of weakly coupled bound states for Schrödinger type operators

It is well-known that for usual Schroedinger operators weakly coupled bound states exist in dimensions one and two, whereas in higher dimensions the famous Cwikel-Lieb-Rozenblum bound holds. We show for a large class of Schrödinger-type operators with general kinetic energies that these two phenomena are complementary. In particular, we explicitly get a natural semi-classical type bound on the number of bound states precisely in the situation when weakly coupled bound states exist not.

math-ph↗