Showing papers by "Wolf Gero Schmidt published in 2018"

Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition

Abstract: Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.

Beyond the molecular movie: dynamics of bands and bonds during a photo-induced phase transition

Abstract: Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.

Signatures of transient Wannier-Stark localization in bulk gallium arsenide.

Abstract: Many properties of solids result from the fact that in a periodic crystal structure, electronic wave functions are delocalized over many lattice sites. Electrons should become increasingly localized when a strong electric field is applied. So far, this Wannier–Stark regime has been reached only in artificial superlattices. Here we show that extremely transient bias over the few-femtosecond period of phase-stable mid-infrared pulses may localize electrons even in a bulk semiconductor like GaAs. The complicated band structure of a three-dimensional crystal leads to a strong blurring of field-dependent steps in the Wannier–Stark ladder. Only the central step emerges strongly in interband electro-absorption because its energetic position is dictated by the electronic structure at an atomic level and therefore insensitive to the external bias. In this way, we demonstrate an extreme state of matter with potential applications due to e.g., its giant optical non-linearity or extremely high chemical reactivity. In strong enough electric fields the non-linear response of electrons in crystals is expected to lead to spatial localization but so far this has only been seen in artificial structures. Schmidt et al. present evidence of this Wannier-Stark localization effect in bulk GaAs driven by intense mid-infrared pulses.

Calculation of spin-spin zero-field splitting within periodic boundary conditions: Towards all-electron accuracy

Abstract: For high-spin centers, one of the key spectroscopic fingerprints is the zero-field splitting (ZFS) addressable by electron paramagnetic resonance. In this paper, an implementation of the spin-spin contribution to the ZFS tensor within the projector augmented-wave (PAW) formalism is reported. We use a single-determinant approach proposed by M. J. Rayson and P. R. Briddon [Phys. Rev. B 77, 035119 (2008)], and complete it by adding a PAW reconstruction term which has not been taken into account before. We benchmark the PAW approach against a well-established all-electron method for a series of diatomic radicals and defects in diamond and cubic silicon carbide. While for some of the defect centers the PAW reconstruction is found to be almost negligible, in agreement with the common assumption, we show that in general it significantly improves the calculated ZFS towards the all-electron results.

Imaging of 180∘ ferroelectric domain walls in uniaxial ferroelectrics by confocal Raman spectroscopy: Unraveling the contrast mechanism

Abstract: In recent years, Raman spectroscopy has been used to visualize and analyze ferroelectric domain structures. The technique makes use of the fact that the intensity or frequency of certain phonons is strongly influenced by the presence of domain walls. Although the method is used frequently, the underlying mechanism responsible for the changes in the spectra is not fully understood. This inhibits deeper analysis of domain structures based on this method. Two different models have been proposed. However, neither model completely explains all observations. In this work, we have systematically investigated domain walls in different scattering geometries with Raman spectroscopy in the common ferroelectric materials used in integrated optics, i.e., KTiOPO4, LiNbO3, and LiTaO3. Based on the two models, we can demonstrate that the observed contrast for domain walls is in fact based on two different effects. We can identify on the one hand microscopic changes at the domain wall, e.g., strain and electric fields, and on the other hand a macroscopic change of selection rules at the domain wall. While the macroscopic relaxation of selection rules can be explained by the directional dispersion of the phonons in agreement with previous propositions, the microscopic changes can be explained qualitatively in terms of a simplified atomistic model.

Probing quasi-one-dimensional band structures by plasmon spectroscopy

Abstract: T. Lichtenstein,1 Z. Mamiyev,1,2 C. Braun,3 S. Sanna,4 W. G. Schmidt,3 C. Tegenkamp,1,2 and H. Pfnür1,2,* 1Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany 2Laboratorium für Nanound Quantenengineering (LNQE), Leibniz Universität Hannover, Schneiderberg 39, 30167 Hannover, Germany 3Theoretische Materialwissenschaften, Universität Paderborn, D-33095 Paderborn, Germany 4Institut für Theoretische Physik, Justus-Liebig-Universität Gießen, Heinrich-Buff-Weg 16, D-35392 Gießen, Germany

Spin pairing versus spin chains at Si(553)-Au surfaces

Abstract: Density-functional theory is used to probe the spin structure of the Si(553)-Au surface. A diamagnetic $s{p}^{2}+p$ rehybridized structure, where the dangling bonds are either filled with two spin-paired electrons or are empty, is more favorable and in better agreement with experiment than the generally accepted spin-chain model. The shallow potential energy surface of Si(553)-Au, together with the ordered array of empty dangling bonds, suggests this surface as susceptible for spin polarization by doping, however.

Unraveling the Oxidation and Spin State of Mn-Corrole through X-ray Spectroscopy and Quantum Chemical Analysis.

Abstract: The interplay between Mn ions and corrole ligands gives rise to complex scenarios regarding the metal centers' electronic properties expressing a range of high oxidation states and spin configurations. The resulting potential of Mn-corroles for applications such as catalysts or fuel cells has recently been demonstrated. However, despite being crucial for their functionality, the electronic structure of Mn-corroles is often hardly accessible with traditional techniques and thus is still under debate, especially under interfacial conditions. Here, we unravel the electronic ground state of the prototypical Mn-5,10,15-tris(pentafluorophenyl)corrole complex through X-ray spectroscopic investigations of ultrapure thin films and quantum chemical analysis. The theory-based interpretation of Mn photoemission and absorption fine structure spectra (3s and 2p and L2,3-edge, respectively) evidence a Mn(III) oxidation state with an S = 2 high-spin configuration. By referencing density functional theory calculations with the experiments, we lay the basis for extending our approach to the characterization of complex interfaces.

Plasmon spectroscopy: Robust metallicity of Au wires on Si(557) upon oxidation

Abstract: Z. Mamiyev,1,2 T. Lichtenstein,1 C. Tegenkamp,1,2 C. Braun,3 W. G. Schmidt,3 S. Sanna,4,* and H. Pfnür1,2,† 1Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany 2Laboratorium für Nanound Quantenengineering (LNQE), Leibniz Universität Hannover, Schneiderberg 39, 30167 Hannover, Germany 3Lehrstuhl für Theoretische Materialphysik, Universität Paderborn, 33095 Paderborn, Germany 4Institut für Theoretische Physik, Justus-Liebig-Universität Gießen, Heinrich-Buff-Weg 16, D-35392 Gießen, Germany

Vibrational properties of the Au-(3×3)/Si(111) surface reconstruction

Abstract: The vibrational properties of the Au-induced ( √ 3 × √3)R30◦ reconstruction of the Si(111) surface are investigated by polarized surface Raman spectroscopy and density-functional theory. The Raman measurements are performed in situ at room temperature as well as 20 K, and they reveal the presence of vibrational eigenmodes in the spectral range from 20 to 450 cm−1. In particular, two peaks of E symmetry at 75 and 183 cm−1 dominate the spectra. No substantial difference between roomand low-temperature spectra is observed, suggesting that the system does not undergo a phase transition down to 20 K. First-principles calculations are performed based on the structural models discussed in the literature. The thermodynamically stable conjugate honeycomb-chained-trimer model (CHCT) [Surf. Sci. 275, L691 (1992)] leads to phonon eigenvalues compatible with the experimental observations in the investigated spectral range. On the basis of the phonon eigenfrequencies, symmetries, and Raman intensities, we assign the measured spectral features to the calculated phonon modes. The good agreement between measured and calculated modes provides a strong argument in favor of the CHCT model.

Structural dynamics upon photoexcitation-induced charge transfer in a dicopper(i)–disulfide complex

Abstract: The structural dynamics of charge-transfer states of nitrogen-ligated copper complexes has been extensively investigated in recent years following the development of pump-probe X-ray techniques In this study we extend this approach towards copper complexes with sulfur coordination and investigate the influence of charge transfer states on the structure of a dicopper(i) complex with coordination by bridging disulfide ligands and additionally tetramethylguanidine units [CuI2(NSSN)2]2+ In order to directly observe and refine the photoinduced structural changes in the solvated complex we applied picosecond pump-probe X-ray absorption spectroscopy (XAS) and wide-angle X-ray scattering (WAXS) Additionally, the ultrafast evolution of the electronic excited states was monitored by femtosecond transient absorption spectroscopy in the UV-Vis probe range DFT calculations were used to predict molecular geometries and electronic structures of the ground and metal-to-ligand charge transfer states with singlet and triplet spin multiplicities, ie S0, 1MLCT and 3MLCT, respectively Combining these techniques we elucidate the electronic and structural dynamics of the solvated complex upon photoexcitation to the MLCT states In particular, femtosecond optical transient spectroscopy reveals three distinct timescales of 650 fs, 10 ps and >100 ps, which were assigned as internal conversion to the ground state (Sn → S0), intersystem crossing 1MLCT → 3MLCT, and subsequent relaxation of the triplet to the ground state, respectively Experimental data collected using both X-ray techniques are in agreement with the DFT-predicted structure for the triplet state, where coordination bond lengths change and one of the S-S bridges is cleaved, causing the movement of two halves of the molecule relative to each other Extended X-ray absorption fine structure spectroscopy resolves changes in Cu-ligand bond lengths with precision on the order of 001 A, whereas WAXS is sensitive to changes in the global shape related to relative movement of parts of the molecule The results presented herein widen the knowledge on the electronic and structural dynamics of photoexcited copper-sulfur complexes and demonstrate the potential of combining the pump-probe X-ray absorption and scattering for studies on photoinduced structural dynamics in copper-based coordination complexes

Identifying On-Surface Site-Selective Chemical Conversions by Theory-Aided NEXAFS Spectroscopy: The Case of Free-Base Corroles on Ag(111)

Abstract: We demonstrate here that theory-assisted near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy enables the site-sensitive monitoring of on-surface chemical reactions, thus, providing information not accessible by other techniques. As a prototype example, we have used free-base 5,10,15-tris(pentafluorophenyl)corroles (3H-TpFPC) adsorbed on Ag(111) and present a detailed investigation of the angle-dependent NEXAFS of this molecular species as well as of their thermally induced derivatives. For this, we have recorded experimental C and N K-edge NEXAFS spectra and interpret them based on XAS cross-section calculations by using a continuous fraction approach and core-hole including multiprojector PAW pseudopotentials within DFT. We have characterized the as-deposited low temperature (200 K) phase and unraveled the subsequent changes induced by dehydrogenation (at 330 K) and ring-closure reactions (at 430 K). By exemplarily obtaining profound insight into the on-surface chemistry of free-base corrolic species adsorbed on a noble metal this work highlights how angle-dependent XAS combined with accurate theoretical modeling can serve for the investigation of on-surface reactions, whereby even highly similar molecular structures, such as tautomers and isomers, can be distinguished.

Erratum: Optical properties of titanium-doped lithium niobate from time-dependent density-functional theory [Phys. Rev. Materials 1, 034401 (2017)]

Abstract: In the presentation of our results in Sec. III D, the numerical values of the ordinary (n⊥) and the extraordinary (n‖) refractive index were inadvertently reversed, affecting the discussion in the main text as well as the figures. Consequently, whenever we refer to our results for the ordinary refractive index or n⊥ in the original article, the extraordinary refractive index n‖ is actually meant, and vice versa. In the revised versions of Figs. 7 and 8 included here, the labels n⊥ and n‖ are now exchanged. Furthermore, we have taken this opportunity to incorporate improved convergence offsets in Fig. 7 that follow the procedure described at the beginning of Sec. III D and slightly raise the numerical values of the refractive indices compared to the original version. While the error does not affect the conclusions drawn from our results, the dependence of the refractive indices on the titanium concentration in Fig. 8 must be reinterpreted. We find that both the ordinary and extraordinary refractive index are approximately linear functions of the Ti concentration. Minor deviations from the linearity are due to the presence of pairs of Ti atoms situated closely along the crystal c axis in our structural models. Our calculations thus confirm the experimental finding [1,2] that, within the range of the investigated concentrations, the extraordinary refractive index is more strongly affected by Ti indiffusion than the ordinary refractive index. The experimentally observed concave behavior of the ordinary refractive index cannot be reproduced in our simulations due to the finite size of the supercells that can be treated with the required level of accuracy. Indeed, doping concentrations lower than 1.85 mol%, achieved using the larger 3×3×3 supercell, are currently not accessible. Moreover, other defect configurations, such as the substitution of NbLi antisites or NbNb by titanium atoms [3], might in fact be responsible for the experimentally observed deviation from the linear behavior.

Photo-Excited Surface Dynamics from Massively Parallel Constrained-DFT Calculations

Abstract: Constrained density-functional theory (DFT) calculations show that the recently observed optically induced insulator-metal transition of the In/Si(111)(8×2)/(4×1) nanowire array (Frigge et al., Nature 544:207, 2017) corresponds to the non-thermal melting of a charge-density wave (CDW). Massively parallel numerical simulations allow for the simulation of the photo-excited nanowires and provide a detailed microscopic understanding of the CDW melting process in terms of electronic surface bands and selectively excited soft phonon modes. Excited-state molecular dynamics in adiabatic approximation shows that the insulator-metal transition can be as fast as 350 fs.