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- ItemTowards representing human behavior and decision making in Earth system models - An overview of techniques and approaches(München : European Geopyhsical Union, 2017) Müller-Hansen, Finn; Schlüter, Maja; Mäs, Michael; Donges, Jonathan F.; Kolb, Jakob J.; Thonicke, Kirsten; Heitzig, JobstToday, humans have a critical impact on the Earth system and vice versa, which can generate complex feedback processes between social and ecological dynamics. Integrating human behavior into formal Earth system models (ESMs), however, requires crucial modeling assumptions about actors and their goals, behavioral options, and decision rules, as well as modeling decisions regarding human social interactions and the aggregation of individuals' behavior. Here, we review existing modeling approaches and techniques from various disciplines and schools of thought dealing with human behavior at different levels of decision making. We demonstrate modelers' often vast degrees of freedom but also seek to make modelers aware of the often crucial consequences of seemingly innocent modeling assumptions. After discussing which socioeconomic units are potentially important for ESMs, we compare models of individual decision making that correspond to alternative behavioral theories and that make diverse modeling assumptions about individuals' preferences, beliefs, decision rules, and foresight. We review approaches to model social interaction, covering game theoretic frameworks, models of social influence, and network models. Finally, we discuss approaches to studying how the behavior of individuals, groups, and organizations can aggregate to complex collective phenomena, discussing agent-based, statistical, and representative-agent modeling and economic macro-dynamics. We illustrate the main ingredients of modeling techniques with examples from land-use dynamics as one of the main drivers of environmental change bridging local to global scales.
- ItemFull-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane(College Park, Md : APS, 2017) Patchkovskii, Serguei; Schuurman, Michael S.We present derivation and implementation of the multiconfigurational strong-field approximation with Gaussian nuclear wave packets (MC-SFA-GWP) - a version of the molecular strong-field approximation which treats all electronic and nuclear degrees of freedom, including their correlations, quantum mechanically. The technique allows realistic simulation of high-order-harmonic emission in polyatomic molecules without invoking reduced-dimensionality models for the nuclear motion or the electronic structure. We use MC-SFA-GWP to model isotope effects in high-order-harmonic-generation (HHG) spectroscopy of methane. The HHG emission in this molecule transiently involves the strongly vibronically coupled F22 electronic state of the CH4+ cation. We show that the isotopic HHG ratio in methane contains signatures of (a) field-free vibronic dynamics at the conical intersection (CI); (b) resonant features in the recombination cross sections; (c) laser-driven bound-state dynamics; as well as (d) the well-known short-time Gaussian decay of the emission. We assign the intrinsic vibronic feature (a) to a relatively long-lived (≥4 fs) vibronic wave packet of the singly excited ν4 (t2) and ν2 (e) vibrational modes, strongly coupled to the components of the F22 electronic state. We demonstrate that these physical effects differ in their dependence on the wavelength, intensity, and duration of the driving pulse, allowing them to be disentangled. We thus show that HHG spectroscopy provides a versatile tool for exploring both conical intersections and resonant features in photorecombination matrix elements in the regime not easily accessible with other techniques.
- ItemWedged Nd:YVO4 crystal for wavelength tuning of monolithic passively Q-switched picosecond microchip lasers(Washington, DC : Soc., 2021) Marianovich, André; Spiekermann, Stefan; Brendel, Moritz; Wessels, Peter; Neumann, Jörg; Weyers, Markus; Kracht, DietmarWe present a monolithic integrated passively Q-switched sub-150 ps microchip laser at 1064 nm with a wedged Nd:YVO4 crystal operating up to a repetition rate of 1 MHz. The wedge enables to change the cavity length by a small amount to fine tune the spectral cavity mode position over the full gain bandwidth of Nd:YVO4 and hence to optimize the output power. This additional degree of freedom may be a suitable approach to increase the wafer scale mass production yield or also to simplify frequency tuning of CW single-frequency microchip lasers.
- ItemObservation of orbital order in the van der Waals material 1T−TiSe2(College Park, MD : APS, 2022) Peng, Yingying; Guo, Xuefei; Xiao, Qian; Li, Qizhi; Strempfer, Jörg; Choi, Yongseong; Yan, Dong; Luo, Huixia; Huang, Yuqing; Jia, Shuang; Janson, Oleg; Abbamonte, Peter; van den Brink, Jeroen; van Wezel, JasperBesides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant x-ray-scattering experiments on the layered van der Waals compound 1T-TiSe2, we establish that the known charge density wave in this weakly correlated, quasi-two-dimensional material corresponds to an orbital ordered phase. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe2, and their importance throughout the family of correlated van der Waals materials.
- ItemOptical study of orbital excitations in transition-metal oxides(Milton Park : Taylor & Francis, 2005) Rückamp, R.; Benckiser, E.; Haverkort, M.W.; Roth, H.; Lorenz, T.; Freimuth, A.; Jongen, L.; Möller, A.; Meyer, G.; Reutler, P.; Büchner, B.; Revcolevschi, A.; Cheong, S.-W.; Sekar, C.; Krabbes, G.; Grüninger, M.The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R = La, Sm and Y), LaMnO3, Y2BaNiO5, CaCu2O3 and K4Cu4OCl10, ranging from early to late transition-metal ions, from t2g to eg systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yields good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g. the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular, in case of the orbital excitations at ≈0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.
- ItemMicromotor-mediated sperm constrictions for improved swimming performance(Berlin ; Heidelberg : Springer, 2021) Striggow, Friedrich; Nadporozhskaia, Lidiia; Friedrich, Benjamin M.; Schmidt, Oliver G.; Medina-Sánchez, MarianaSperm-driven micromotors, consisting of a single sperm cell captured in a microcap, utilize the strong propulsion generated by the flagellar beat of motile spermatozoa for locomotion. It enables the movement of such micromotors in biological media, while being steered remotely by means of an external magnetic field. The substantial decrease in swimming speed, caused by the additional hydrodynamic load of the microcap, limits the applicability of sperm-based micromotors. Therefore, to improve the performance of such micromotors, we first investigate the effects of additional cargo on the flagellar beat of spermatozoa. We designed two different kinds of microcaps, which each result in different load responses of the flagellar beat. As an additional design feature, we constrain rotational degrees of freedom of the cell’s motion by modifying the inner cavity of the cap. Particularly, cell rolling is substantially reduced by tightly locking the sperm head inside the microcap. Likewise, cell yawing is decreased by aligning the micromotors under an external static magnetic field. The observed differences in swimming speed of different micromotors are not so much a direct consequence of hydrodynamic effects, but rather stem from changes in flagellar bending waves, hence are an indirect effect. Our work serves as proof-of-principle that the optimal design of microcaps is key for the development of efficient sperm-driven micromotors.