arXiv subjects

Gerd Schönhense

Publications and source records attributed to Gerd Schönhense.

27 records · Page 2Linked to original sources

arXiv2021

Subpicosecond metamagnetic phase transition driven by non-equilibrium electron dynamics

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.

cond-mat.str-el↗

arXiv2019

Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of non-equilibrium electronic processes, transient states in chemical reactions or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical and structural analysis requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. The PG2 beamline at FLASH (DESY, Hamburg) provides a high pulse rate of 5000 pulses/s, 60 fs pulse duration and 40 meV bandwidth in an energy range of 25-830 eV with a photon beam size down to 50 microns in diameter. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines FEL capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in ($k_x$, $k_y$, $E$) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å $^{-1}$ diameter in a binding-energy range of several eV, resolving about $2.5\times10^5$ data voxels. As an example, we present results for the ultrafast excited state dynamics in the model van der Waals semiconductor WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2018

Matrix-element induced spin polarization of photoelectrons from bulk bands

Angular- or $k$-resolved photoelectron spectroscopy in the soft X-ray range gives access to the bulk electronic structure of materials. Here this technique is extended to the spin degree of freedom. A non-magnetic material (tungsten) was chosen, in order to exclude any initial-state spin polarization from exchange-split bands. By measurement of two spin-polarizations for different light helicity, one can distinguish between contributions of optical spin-orientation by circularly-polarized X-rays (Fano componen) and a second component originating from interference of final-state partial waves. Both phenomena have so far only been observed at low photon energies. Their detection in the X-ray range is a milestone on the way to the "complete experiment" in bulk photoemission.

cond-mat.mtrl-sci↗

arXiv2016

Vectorial spin-polarization detection in multichannel spin-resolved photoemission spectroscopy using an Ir(001) imaging spin filter

We report on spin- and angular resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over $10^3$ compared to standard single channel detectors. Furthermore, we review the spin filter preparation, and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

physics.ins-det↗

arXiv2016

Crystal Growth of new charge-transfer salts based on $π$-conjugated molecules

New charge transfer crystals of $π$-conjugated, aromatic molecules (phenanthrene and picene) as donors were obtained by physical vapor transport. The melting behavior, optimization of crystal growth and the crystal structure is reported for charge transfer salts with (fluorinated) tetracyanoquinodimethane (TCNQ-F$_x$, x=0, 2, 4), which was used as acceptor material. The crystal structures were determined by single-crystal X-ray diffraction. Growth conditions for different vapor pressures in closed ampules were applied and the effect of these starting conditions for crystal size and quality is reported. The process of charge transfer was investigated by geometrical analysis of the crystal structure and by infrared spectroscopy on single crystals. With these three different acceptor strengths and the two sets of donor materials, it is possible to investigate the distribution of the charge transfer systematically. This helps to understand the charge transfer process in this class of materials with $π$-conjugated donor molecules.

physics.chem-ph↗

arXiv2014

Temperature dependence of electronic and magnetic properties of (DOEO)$_4$[HgBr$_4$]TCE single crystals

The temperature dependence of electronic and magnetic properties of the organic charge-transfer salt (DOEO)$_4$[HgBr$_4$]TCE was investigated using magnetometry. Electronic transport properties revealed three distinct phases which are related to different magnetic coupling phenomena. In the low-temperature insulating phase (T<70 K) the antiferromagnetic coupling between two distinct sites of magnetic moments causes antiferromagnetic order below the Néel temperature T$_N$=40 K. In the temperature region 70-120 K the (DOEO)$_4$[HgBr$_4$]TCE shows metallic-like behavior and with further increasing of temperature it becomes a bad metal due to loss of itinerant character and increase of hopping conductivity of charge carriers.

cond-mat.str-el↗

arXiv2006

Electronic structure and spectroscopy of the quaternary Heusler alloy Co$_2$Cr$_{1-x}$Fe$_{x}$Al

Quaternary Heusler alloys Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy ($\geq 3.5$keV) hard X-ray photo emission was used to probe the density of the occupied states in Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al.

cond-mat.mtrl-sci↗

arXiv2006

Correlation in the transition metal based Heusler compounds Co$_2$MnSi and Co$_2$FeSi

Half-metallic ferromagnets like the full Heusler compounds with formula X$_2$YZ are supposed to show an integer value of the spin magnetic moment. Calculations reveal in certain cases of X = Co based compounds non-integer values, in contrast to experiments. In order to explain deviations of the magnetic moment calculated for such compounds, the dependency of the electronic structure on the lattice parameter was studied theoretically. In local density approximation (LDA), the minimum total energy of Co$_2$FeSi is found for the experimental lattice parameter, but the calculated magnetic moment is about 12% too low. Half-metallic ferromagnetism and a magnetic moment equal to the experimental value of $6μ_B$ are found, however, only after increasing the lattice parameter by more than 6%. To overcome this discrepancy, the LDA$+U$ scheme was used to respect on-site electron correlation in the calculations. Those calculations revealed for Co$_2$FeSi that an effective Coulomb-exchange interaction $U_{eff}=U-J$ in the range of about 2eV to 5eV leads to half-metallic ferromagnetism and the measured, integer magnetic moment at the measured lattice parameter. Finally, it is shown in the case of Co$_2$MnSi that correlation may also serve to destroy the half-metallic behavior if it becomes too strong (for Co$_2$MnSi above 2eV and for Co$_2$FeSi above 5eV). These findings indicate that on-site correlation may play an important role in the description of Heusler compounds with localized moments.

cond-mat.mtrl-sci↗

arXiv2005

Design of magnetic materials: Co$_2$Cr$_{1-x}$Fe$_{x}$Al

Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al ($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure calculations. The ordered compounds turned out to be ferromagnetic with small Al magnetic moment being aligned anti-parallel to the 3d transition metal moments. All compounds show a gap around the Fermi-energy in the minority bands. The pure compounds exhibit an indirect minority gap, whereas the ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism (MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. The experimentally observed reduction of the magnetic moment of Cr can be explained by Co-Cr site-disorder. The presence of the gap in the minority bands of Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl (001)-planes in the $L2_1$ structure. It is retained in structures with different order of the CrAl planes but vanishes in the $X$-structure with alternating CoCr and CoAl planes.

cond-mat.mtrl-sci↗