arXiv subjects

K. Dörr

Publications and source records attributed to K. Dörr.

7 recordsLinked to original sources

arXiv2026

Magneto-optical evidence for single-crystal-like magnetic switching of epitaxial antiferromagnetic LaFeO3 films

Strained epitaxial films of the antiferromagnetic orthoferrite LaFeO3 offer a promising platform for antiferromagnetic spintronics, yet their magnetic switching behavior and domain structure have remained largely unexplored due to the small magnitude of the weak ferromagnetic moment. Here, we demonstrate that longitudinal magneto-optical Kerr effect (MOKE) measurements provide a sensitive and direct probe of magnetic switching and domain processes in coherently strained LaFeO3 thin films grown on orthorhombic substrates. By employing DyScO3(110), GdScO3(110), and NdGaO3(110) substrates, we achieve straincontrolled, largely twin-free growth and identify the orientation of the orthorhombic c-axis through the presence or absence of a longitudinal MOKE signal. Compressively strained films exhibit large Kerr signals, rectangular hysteresis loops, and magnetic single-domain remanence over macroscopic areas. Tensile strain on orthorhombic substrates is associated with two competing structural effects on thin film orientation; in-plane magnetization has been identified in some films on GdScO3(110) by MOKE. Angle-dependent MOKE hysteresis follows the Kondorsky model, indicating domain-wall-controlled switching analogous to bulk single crystals. Kerr microscopy reveals abrupt domain nucleation and rapid domain-wall motion, with defects acting as pinning centers and governing the coercive field. Our results establish MOKE as an efficient optical tool for identifying orthorhombic orientation, probing magnetic switching of coupled weak magnetization and Neel vectors, and accessing domain dynamics in LaFeO3 films. This provides a foundation for strain-engineered orthoferrite thin films in antiferromagnetic spintronics and magnonics.

cond-mat.mtrl-sci↗

arXiv2025

Efficient broadband terahertz generation by above band-gap excitation of the pyroelectric ZnSnN2

Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 to above 30 THz. Our results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN2. Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. Our study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

cond-mat.mtrl-sci↗

arXiv2020

Growth-sequence-dependent interface magnetism of SrIrO$_3$ - La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers

Bilayers of the oxide 3d ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and the 5d paramagnet SrIrO$_{3}$ (SIO) with large spin-orbit coupling (SOC) have been investigated regarding the impact of interfacial SOC on magnetic order. For the growth sequence of LSMO on SIO, ferromagnetism is strongly altered and large out-of-plane-canted anisotropy associated with lacking magnetic saturation up to 4 T has been observed. Thin bilayer films have been grown coherently in both growth sequences on SrTiO$_3$ (001) by pulsed laser deposition and structurally characterized by scanning transmission electron microscopy (STEM) and x-ray diffraction (XRD). Measurements of magnetization and field-dependent Mn L$_{2,3}$ edge x-ray magnetic circular dichroism (XMCD) reveal changes of LSMO magnetic order which are strong in LSMO on SIO and weak in LSMO underneath of SIO. We attribute the impact of the growth sequence to the interfacial lattice structure/symmetry which is known to influence the interfacial magnetic coupling.

cond-mat.mtrl-sci↗

arXiv2011

Investigation of the In-Gap Electronic Structure of LaAlO3 - SrTiO3 Heterointerfaces by Soft X-ray Spectroscopy

We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at the Fermi energy and related to the filling of the \STO\ conduction band. It appears for low oxygen pressure only. The other one is centered at E$_{B}$ $\approx$ 1 eV and independent of the oxygen pressure during growth. It is probably due to defects. The magnitude of both excitations is comparable. It is shown that low oxygen pressure is detrimental for the Ti - O bonding. Our results shed light on the nature of the charge carriers in the vicinity of the \LAO\ - \STO\ interface.

cond-mat.str-el↗

arXiv2010

Effects of variable anisotropic-strain on the emission of neutral excitons confined in epitaxial quantum dots

We study the effect of elastic anisotropic biaxial strain on the light emitted by neutral excitons confined in different kinds of semiconductor quantum dots (QDs). We find that the light polarization rotates by up to 80 degree and the excitonic fine structure splitting varies by several tens of $μ$eVs as the strain is varied. By means of a continuum model we mainly ascribe the observed effects to substantial changes of the hole wave function. These results show that strain-fields of a few permill magnitude are suffcient to dramatically modify the electronic structure of QDs.

cond-mat.mes-hall↗

arXiv2010

Two Component Heat Diffusion Observed in CMR Manganites

We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.

cond-mat.str-el↗

arXiv2007

Metal to insulator transition in manganites - optical conductivity changes up to 24 eV

The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in $\rm La_{1-x}Ca_{x}MnO_3$ for $\rm (x=0.3,0.2)$ was investigated by dc conductivity, ellipsometry, and VUV reflectance for energies between 0 and 24 eV. A stablized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 24 eV at the metal to insulator transition. In the observed energy range, the spectral weight is conserved within $\rm 0.3 %$. The redistribution of spectral weight between low and high energies has important ramifications for the down-folding of low-energy Hamiltonians. We discuss the importance of the charge-transfer, Coulomb onsite, Jahn-Teller, and screening effects to the electronic structure.

cond-mat.str-el↗