@misc{LomadzeKopyshevBargheeretal.2017, author = {Lomadze, Nino and Kopyshev, Alexey and Bargheer, Matias and Wollgarten, Markus and Santer, Svetlana}, title = {Mass production of polymer nanowires filled with metal nanoparticles}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-402712}, pages = {10}, year = {2017}, abstract = {Despite the ongoing progress in nanotechnology and its applications, the development of strategies for connecting nano-scale systems to micro- or macroscale elements is hampered by the lack of structural components that have both, nano- and macroscale dimensions. The production of nano-scale wires with macroscale length is one of the most interesting challenges here. There are a lot of strategies to fabricate long nanoscopic stripes made of metals, polymers or ceramics but none is suitable for mass production of ordered and dense arrangements of wires at large numbers. In this paper, we report on a technique for producing arrays of ordered, flexible and free-standing polymer nano-wires filled with different types of nano-particles. The process utilizes the strong response of photosensitive polymer brushes to irradiation with UV-interference patterns, resulting in a substantial mass redistribution of the polymer material along with local rupturing of polymer chains. The chains can wind up in wires of nano-scale thickness and a length of up to several centimeters. When dispersing nano-particles within the film, the final arrangement is similar to a core-shell geometry with mainly nano-particles found in the core region and the polymer forming a dielectric jacket.}, language = {en} } @article{ShaydukHallmannRodriguezFernandezetal.2022, author = {Shayduk, Roman and Hallmann, J{\"o}rg and Rodriguez-Fernandez, Angel and Scholz, Markus and Lu, Wei and B{\"o}senberg, Ulrike and M{\"o}ller, Johannes and Zozulya, Alexey and Jiang, Man and Wegner, Ulrike and Secareanu, Radu-Costin and Palmer, Guido and Emons, Moritz and Lederer, Max and Volkov, Sergey and Lindfors-Vrejoiu, Ionela and Schick, Daniel and Herzog, Marc and Bargheer, Matias and Madsen, Anders}, title = {Femtosecond x-ray diffraction study of multi-THz coherent phonons in SrTiO3}, series = {Applied physics letters}, volume = {120}, journal = {Applied physics letters}, number = {20}, publisher = {AIP Publishing}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/5.0083256}, pages = {5}, year = {2022}, abstract = {We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in SrTiO3 (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial SrRuO3 film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude.}, language = {en} } @article{SchickHerzogWenetal.2014, author = {Schick, Daniel and Herzog, Marc and Wen, Haidan and Chen, Pice and Adamo, Carolina and Gaal, Peter and Schlom, Darrell G. and Evans, Paul G. and Li, Yuelin and Bargheer, Matias}, title = {Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO3}, series = {Physical review letters}, volume = {112}, journal = {Physical review letters}, number = {9}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.112.097602}, pages = {6}, year = {2014}, abstract = {We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data.}, language = {en} } @article{HerzogSchickLeitenbergeretal.2012, author = {Herzog, Marc and Schick, Daniel and Leitenberger, Wolfram and Shayduk, Roman and van der Veen, Renske M. and Milne, Christopher J. and Johnson, Steven Lee and Vrejoiu, Ionela and Bargheer, Matias}, title = {Tailoring interference and nonlinear manipulation of femtosecond x-rays}, series = {New journal of physics : the open-access journal for physics}, volume = {14}, journal = {New journal of physics : the open-access journal for physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {1367-2630}, doi = {10.1088/1367-2630/14/1/013004}, pages = {9}, year = {2012}, abstract = {We present ultrafast x-ray diffraction (UXRD) experiments on different photoexcited oxide superlattices. All data are successfully simulated by dynamical x-ray diffraction calculations based on a microscopic model, that accounts for the linear response of phonons to the excitation laser pulse. Some Bragg reflections display a highly nonlinear strain dependence. The origin of linear and two distinct nonlinear response phenomena is discussed in a conceptually simpler model using the interference of envelope functions that describe the diffraction efficiency of the average constituent nanolayers. The combination of both models facilitates rapid and accurate simulations of UXRD experiments.}, language = {en} } @article{SchickBojahrHerzogetal.2012, author = {Schick, Daniel and Bojahr, Andre and Herzog, Marc and von Korff Schmising, Clemens and Shayduk, Roman and Leitenberger, Wolfram and Gaa, P. and Bargheer, Matias}, title = {Normalization schemes for ultrafast x-ray diffraction using a table-top laser-driven plasma source}, series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, volume = {83}, journal = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0034-6748}, doi = {10.1063/1.3681254}, pages = {7}, year = {2012}, abstract = {We present an experimental setup of a laser-driven x-ray plasma source for femtosecond x-ray diffraction. Different normalization schemes accounting for x-ray source intensity fluctuations are discussed in detail. We apply these schemes to measure the temporal evolution of Bragg peak intensities of perovskite superlattices after ultrafast laser excitation.}, language = {en} } @phdthesis{Pudell2020, author = {Pudell, Jan-Etienne}, title = {Lattice dynamics}, doi = {10.25932/publishup-48445}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-484453}, school = {Universit{\"a}t Potsdam}, pages = {XII, 259}, year = {2020}, abstract = {In this thesis I summarize my contribution to the research field of ultrafast structural dynamics in condensend matter. It consists of 17 publications that cover the complex interplay between electron, magnon, and phonon subsystems in solid materials and the resulting lattice dynamics after ultrafast photoexcitation. The investigation of such dynamics is necessary for the physical understanding of the processes in materials that might become important in the future as functional materials for technological applications, for example in data storage applications, information processing, sensors, or energy harvesting. In this work I present ultrafast x-ray diffraction (UXRD) experiments based on the optical pump - x-ray probe technique revealing the time-resolved lattice strain. To study these dynamics the samples (mainly thin film heterostructures) are excited by femtosecond near-infrared or visible light pulses. The induced strain dynamics caused by stresses of the excited subsystems are measured in a pump-probe scheme with x-ray diffraction (XRD) as a probe. The UXRD setups used during my thesis are a laser-driven table-top x-ray source and large-scale synchrotron facilities with dedicated time-resolved diffraction setups. The UXRD experiments provide quantitative access to heat reservoirs in nanometric layers and monitor the transient responses of these layers with coupled electron, magnon, and phonon subsystems. In contrast to optical probes, UXRD allows accessing the material-specific information, which is unavailable for optical light due to the detection of multiple indistinguishable layers in the range of the penetration depth. In addition, UXRD facilitates a layer-specific probe for layers buried opaque heterostructures to study the energy flow. I extended this UXRD technique to obtain the driving stress profile by measuring the strain dynamics in the unexcited buried layer after excitation of the adjacent absorbing layers with femtosecond laser pulses. This enables the study of negative thermal expansion (NTE) in magnetic materials, which occurs due to the loss of the magnetic order. Part of this work is the investigation of stress profiles which are the source of coherent acoustic phonon wave packets (hypersound waves). The spatiotemporal shape of these stress profiles depends on the energy distribution profile and the ability of the involved subsystems to produce stress. The evaluation of the UXRD data of rare-earth metals yields a stress profile that closely matches the optical penetration profile: In the paramagnetic (PM) phase the photoexcitation results in a quasi-instantaneous expansive stress of the metallic layer whereas in the antiferromagnetic (AFM) phase a quasi-instantaneous contractive stress and a second contractive stress contribution rising on a 10 ps time scale adds to the PM contribution. These two time scales are characteristic for the magnetic contribution and are in agreement with related studies of the magnetization dynamics of rare-earth materials. Several publications in this thesis demonstrate the scientific progress in the field of active strain control to drive a second excitation or engineer an ultrafast switch. These applications of ultrafast dynamics are necessary to enable control of functional material properties via strain on ultrafast time scales. For this thesis I implemented upgrades of the existing laser-driven table-top UXRD setup in order to achieve an enhancement of x-ray flux to resolve single digit nanometer thick layers. Furthermore, I developed and built a new in-situ time-resolved magneto-optic Kerr effect (MOKE) and optical reflectivity setup at the laser-driven table-top UXRD setup to measure the dynamics of lattice, electrons and magnons under the same excitation conditions.}, language = {en} } @article{DebPopovaJaffresetal.2022, author = {Deb, Marwan and Popova, Elena and Jaffr{\`e}s, Henri-Yves and Keller, Niels and Bargheer, Matias}, title = {Polarization-dependent subpicosecond demagnetization in iron garnets}, series = {Physical review : B, covering condensed matter and materials physics}, volume = {106}, journal = {Physical review : B, covering condensed matter and materials physics}, number = {18}, publisher = {American Institute of Physics, American Physical Society}, address = {Woodbury, NY}, issn = {2469-9950}, doi = {10.1103/PhysRevB.106.184416}, pages = {7}, year = {2022}, abstract = {Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators BixY3-xFe5O12 with 1 xBi 3. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of 0.6 ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.}, language = {en} } @article{DebPopovaJaffresetal.2022, author = {Deb, Marwan and Popova, Elena and Jaffr{\`e}s, Henri-Yves and Keller, Niels and Bargheer, Matias}, title = {Controlling high-frequency spin-wave dynamics using double-pulse laser excitation}, series = {Physical review applied}, volume = {18}, journal = {Physical review applied}, number = {4}, publisher = {American Physical Society}, address = {College Park}, issn = {2331-7019}, doi = {10.1103/PhysRevApplied.18.044001}, pages = {7}, year = {2022}, abstract = {Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetiza-tion and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.}, language = {en} } @misc{SteteKoopmanBargheer2018, author = {Stete, Felix and Koopman, Wouter-Willem Adriaan and Bargheer, Matias}, title = {Signatures of strong coupling on nanoparticles}, series = {Quantum Nano-Photonics}, journal = {Quantum Nano-Photonics}, publisher = {Springer}, address = {Dordrecht}, isbn = {978-94-024-1546-9}, issn = {1871-465X}, doi = {10.1007/978-94-024-1544-5_53}, pages = {445 -- 447}, year = {2018}, abstract = {The electromagnetic coupling of molecular excitations to plasmonic nanoparticles offers a promising method to manipulate the light-matter interaction at the nanoscale. Plasmonic nanoparticles foster exceptionally high coupling strengths, due to their capacity to strongly concentrate the light-field to sub-wavelength mode volumes. A particularly interesting coupling regime occurs, if the coupling increases to a level such that the coupling strength surpasses all damping rates in the system. In this so-called strong-coupling regime hybrid light-matter states emerge, which can no more be divided into separate light and matter components. These hybrids unite the features of the original components and possess new resonances whose positions are separated by the Rabi splitting energy h Omega. Detuning the resonance of one of the components leads to an anticrossing of the two arising branches of the new resonances omega(+) and omega(-) with a minimal separation of Omega = omega(+) - omega(-).}, language = {en} } @misc{SteteSchossauKoopmanetal.2018, author = {Stete, Felix and Schossau, Phillip Gerald and Koopman, Wouter-Willem Adriaan and Bargheer, Matias}, title = {Size Dependence of the Coupling Strength in Plasmon-Exciton Nanoparticles}, series = {Quantum Nano-Photonics}, journal = {Quantum Nano-Photonics}, publisher = {Springer}, address = {Dordrecht}, isbn = {978-94-024-1546-9}, issn = {1871-465X}, doi = {10.1007/978-94-024-1544-5_26}, pages = {381 -- 383}, year = {2018}, abstract = {The coupling between molecular excitations and nanoparticles leads to promising applications. It is for example used to enhance the optical cross-section of molecules in surface enhanced Raman scattering, Purcell enhancement or plasmon enhanced dye lasers. In a coupled system new resonances emerge resulting from the original plasmon (ωpl) and exciton (ωex) resonances as ω±=12(ωpl+ωex)±14(ωpl-ωex)2+g2---------------√, (1) where g is the coupling parameter. Hence, the new resonances show a separation of Δ = ω+ - ω- from which the coupling strength can be deduced from the minimum distance between the two resonances, Ω = Δ(ω+ = ω-).}, language = {en} } @phdthesis{Haseeb2023, author = {Haseeb, Haider}, title = {Charge and heat transport across interfaces in nanostructured porous silicon}, doi = {10.25932/publishup-61122}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-611224}, school = {Universit{\"a}t Potsdam}, pages = {84}, year = {2023}, abstract = {This thesis discusses heat and charge transport phenomena in single-crystalline Silicon penetrated by nanometer-sized pores, known as mesoporous Silicon (pSi). Despite the extensive attention given to it as a thermoelectric material of interest, studies on microscopic thermal and electronic transport beyond its macroscopic characterizations are rarely reported. In contrast, this work reports the interplay of both. PSi samples synthesized by electrochemical anodization display a temperature dependence of specific heat 𝐶𝑝 that deviates from the characteristic 𝑇^3 behaviour (at 𝑇<50𝐾). A thorough analysis reveals that both 3D and 2D Einstein and Debye modes contribute to this specific heat. Additional 2D Einstein modes (~3 𝑚𝑒𝑉) agree reasonably well with the boson peak of SiO2 in pSi pore walls. 2D Debye modes are proposed to account for surface acoustic modes causing a significant deviation from the well-known 𝑇^3 dependence of 𝐶𝑝 at 𝑇<50𝐾. A novel theoretical model gives insights into the thermal conductivity of pSi in terms of porosity and phonon scattering on the nanoscale. The thermal conductivity analysis utilizes the peculiarities of the pSi phonon dispersion probed by the inelastic neutron scattering experiments. A phonon mean-free path of around 10 𝑛𝑚 extracted from the presented model is proposed to cause the reduced thermal conductivity of pSi by two orders of magnitude compared to p-doped bulk Silicon. Detailed analysis indicates that compound averaging may cause a further 10-50\% reduction. The percolation threshold of 65\% for thermal conductivity of pSi samples is subsequently determined by employing theoretical effective medium models. Temperature-dependent electrical conductivity measurements reveal a thermally activated transport process. A detailed analysis of the activation energy 𝐸𝐴𝜎 in the thermally activated transport exhibits a Meyer Neldel compensation rule between different samples that originates in multi-phonon absorption upon carrier transport. Activation energies 𝐸𝐴𝑆 obtained from temperature-dependent thermopower measurements provide further evidence for multi-phonon assisted hopping between localized states as a dominant charge transport mechanism in pSi, as they systematically differ from the determined 𝐸𝐴𝜎 values.}, language = {en} } @article{DebPopovaHehnetal.2019, author = {Deb, Marwan and Popova, Elena and Hehn, Michel and Keller, Niels and Petit-Watelot, Sebastien and Bargheer, Matias and Mangin, Stephane and Malinowski, Gregory}, title = {Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets}, series = {Physical review letters}, volume = {123}, journal = {Physical review letters}, number = {2}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.123.027202}, pages = {6}, year = {2019}, abstract = {We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (similar to 33 mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics.}, language = {en} } @article{SarhanKoopmanSchuetzetal.2019, author = {Sarhan, Radwan Mohamed and Koopman, Wouter-Willem Adriaan and Schuetz, Roman and Schmid, Thomas and Liebig, Ferenc and Koetz, Joachim and Bargheer, Matias}, title = {The importance of plasmonic heating for the plasmondriven photodimerization of 4-nitrothiophenol}, series = {Scientific Reports}, volume = {9}, journal = {Scientific Reports}, publisher = {Macmillan Publishers Limited}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-019-38627-2}, pages = {8}, year = {2019}, abstract = {Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.}, language = {en} } @article{IurchukSchickBranetal.2016, author = {Iurchuk, V. and Schick, D. and Bran, J. and Colson, D. and Forget, A. and Halley, D. and Koc, Azize and Reinhardt, Mathias and Kwamen, C. and Morley, N. A. and Bargheer, Matias and Viret, M. and Gumeniuk, R. and Schmerber, G. and Doudin, B. and Kundys, B.}, title = {Optical Writing of Magnetic Properties by Remanent Photostriction}, series = {Physical review letters}, volume = {117}, journal = {Physical review letters}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.117.107403}, pages = {5}, year = {2016}, abstract = {We present an optically induced remanent photostriction in BiFeO3, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO3/Ni structure. The 75\% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO3. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.}, language = {en} } @article{ShaydukHerzogBojahretal.2013, author = {Shayduk, Roman and Herzog, Marc and Bojahr, Andre and Schick, Daniel and Gaal, Peter and Leitenberger, Wolfram and Navirian, Hengameh and Sander, Mathias and Goldshteyn, Jevgenij and Vrejoiu, Ionela and Bargheer, Matias}, title = {Direct time-domain sampling of subterahertz coherent acoustic phonon spectra in SrTiO3 using ultrafast x-ray diffraction}, series = {Physical review : B, Condensed matter and materials physics}, volume = {87}, journal = {Physical review : B, Condensed matter and materials physics}, number = {18}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.87.184301}, pages = {7}, year = {2013}, abstract = {We synthesize sub-THz longitudinal quasimonochromatic acoustic phonons in a SrTiO3 single crystal using a SrRuO3/SrTiO3 superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast x-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/omega(2) power law known from Akhiezer's model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher q is discussed in the framework of nonlinear effects appearing due to the high amplitude of the synthesized phonons.}, language = {en} } @article{ReinhardtKocLeitenbergeretal.2016, author = {Reinhardt, Matthias and Koc, Azize and Leitenberger, Wolfram and Gaal, Peter and Bargheer, Matias}, title = {Optimized spatial overlap in optical pump-X-ray probe experiments with high repetition rate using laser-induced surface distortions}, series = {Journal of synchrotron radiation}, volume = {23}, journal = {Journal of synchrotron radiation}, publisher = {International Union of Crystallography}, address = {Chester}, issn = {1600-5775}, doi = {10.1107/S1600577515024443}, pages = {474 -- 479}, year = {2016}, abstract = {Ultrafast X-ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X-ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented.}, language = {en} } @phdthesis{TchoumbaKwamen2018, author = {Tchoumba Kwamen, Christelle Larodia}, title = {Investigating the dynamics of polarization reversal in ferroelectric thin films by time-resolved X-ray diffraction}, doi = {10.25932/publishup-42781}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427815}, school = {Universit{\"a}t Potsdam}, pages = {xvii, 126, xxiii}, year = {2018}, abstract = {Ferroic materials have attracted a lot of attention over the years due to their wide range of applications in sensors, actuators, and memory devices. Their technological applications originate from their unique properties such as ferroelectricity and piezoelectricity. In order to optimize these materials, it is necessary to understand the coupling between their nanoscale structure and transient response, which are related to the atomic structure of the unit cell. In this thesis, synchrotron X-ray diffraction is used to investigate the structure of ferroelectric thin film capacitors during application of a periodic electric field. Combining electrical measurements with time-resolved X-ray diffraction on a working device allows for visualization of the interplay between charge flow and structural motion. This constitutes the core of this work. The first part of this thesis discusses the electrical and structural dynamics of a ferroelectric Pt/Pb(Zr0.2,Ti0.8)O3/SrRuO3 heterostructure during charging, discharging, and polarization reversal. After polarization reversal a non-linear piezoelectric response develops on a much longer time scale than the RC time constant of the device. The reversal process is inhomogeneous and induces a transient disordered domain state. The structural dynamics under sub-coercive field conditions show that this disordered domain state can be remanent and can be erased with an appropriate voltage pulse sequence. The frequency-dependent dynamic characterization of a Pb(Zr0.52,Ti0.48)O3 layer, at the morphotropic phase boundary, shows that at high frequency, the limited domain wall velocity causes a phase lag between the applied field and both the structural and electrical responses. An external modification of the RC time constant of the measurement delays the switching current and widens the electromechanical hysteresis loop while achieving a higher compressive piezoelectric strain within the crystal. In the second part of this thesis, time-resolved reciprocal space maps of multiferroic BiFeO3 thin films were measured to identify the domain structure and investigate the development of an inhomogeneous piezoelectric response during the polarization reversal. The presence of 109° domains is evidenced by the splitting of the Bragg peak. The last part of this work investigates the effect of an optically excited ultrafast strain or heat pulse propagating through a ferroelectric BaTiO3 layer, where we observed an additional current response due to the laser pulse excitation of the metallic bottom electrode of the heterostructure.}, language = {en} } @article{DebPopovaHehnetal.2019, author = {Deb, Marwan and Popova, Elena and Hehn, Michel and Keller, Niels and Petit-Watelot, Sebastien and Bargheer, Matias and Mangin, Stephane and Malinowski, Gregory}, title = {Damping of Standing Spin Waves in Bismuth-Substituted Yttrium Iron Garnet as Seen via the Time-Resolved Magneto-Optical Kerr Effect}, series = {Physical review applied}, volume = {12}, journal = {Physical review applied}, number = {4}, publisher = {American Physical Society}, address = {College Park}, issn = {2331-7019}, doi = {10.1103/PhysRevApplied.12.044006}, pages = {7}, year = {2019}, abstract = {We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.}, language = {en} } @phdthesis{Jay2020, author = {Jay, Raphael Martin}, title = {Principles of charge distribution and separation}, school = {Universit{\"a}t Potsdam}, pages = {xi, 162}, year = {2020}, abstract = {The electronic charge distributions of transition metal complexes fundamentally determine their chemical reactivity. Experimental access to the local valence electronic structure is therefore crucial in order to determine how frontier orbitals are delocalized between different atomic sites and electronic charge is spread throughout the transition metal complex. To that end, X-ray spectroscopies are employed in this thesis to study a series of solution-phase iron complexes with respect to the response of their local electronic charge distributions to different external influences. Using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) at the iron L-edge, changes in local charge densities are investigated at the iron center depending on different ligand cages as well as solvent environments. A varying degree of charge delocalization from the metal center onto the ligands is observed, which is governed by the capabilities of the ligands to accept charge density into their unoccupied orbitals. Specific solvents are furthermore shown to amplify this process. Solvent molecules of strong Lewis-acids withdraw charge from the ligand allowing in turn for more metal charge to be delocalized onto the ligand. The resulting local charge deficiencies at the metal center are, however, counteracted by competing electron-donation channels from the ligand towards the iron, which are additionally revealed. This is interpreted as a compensating effect which strives to maintain local charge densities at the iron center. This mechanism of charge density preservation is found to be of general nature. Using time-resolved RIXS and XAS at the iron L-edge, an analogous interplay of electron donation and back-donation channels is also revealed for the case of charge-transfer excited states. In such transient configurations, the electronic occupation of iron-centered frontier orbitals has been altered by an optical excitation. Changes in local charge densities that are expected to follow an increased or decreased population of iron-centered orbitals are, however, again counteracted. By scaling the degree of electron donation from the ligand onto the metal, local charge densities at the iron center can be efficiently maintained. Since charge-transfer excitations, however, often constitute the initial step in many electron transfer processes, these findings challenge common notions of charge-separation in transition metal dyes.}, language = {en} } @article{BojahrHerzogSchicketal.2012, author = {Bojahr, Andre and Herzog, Marc and Schick, Daniel and Vrejoiu, Ionela and Bargheer, Matias}, title = {Calibrated real-time detection of nonlinearly propagating strain waves}, series = {Physical review : B, Condensed matter and materials physics}, volume = {86}, journal = {Physical review : B, Condensed matter and materials physics}, number = {14}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.86.144306}, pages = {5}, year = {2012}, abstract = {Epitaxially grown metallic oxide transducers support the generation of ultrashort strain pulses in SrTiO3 (STO) with high amplitudes up to 0.5\%. The strain amplitudes are calibrated by real-time measurements of the lattice deformation using ultrafast x-ray diffraction. We determine the speed at which the strain fronts propagate by broadband picosecond ultrasonics and conclude that, above a strain level of approx. 0.2\%, the compressive and tensile strain components travel at considerably different sound velocities, indicating nonlinear wave behavior. Simulations based on an anharmonic linear-chain model are in excellent accord with the experimental findings and show how the spectrum of coherent phonon modes changes with time.}, language = {en} }