Dynamic pathway of the photoinduced phase transition of TbMnO3_3

We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic TbMnO$_3$ by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. Using orthogonal linear x-ray polarizations, we suceeded in disentangling the response of the multiferroic cycloidal spin order from the sinusoidal antiferromagnetic order in the time domain. This enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond timescale. The transient phase is shown to be a spin density wave, as in the adiabatic case, which nevertheless retains the wave vector of the cycloidal long range order. Two different pump photon energies, 1.55 eV and 3.1 eV, lead to population of the conduction band predominantly via intersite $d$-$d$ transitions or intrasite $p$-$d$ transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a timescale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic and orbital orders. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such complex theoretical studies

Dynamics of the photoinduced insulator-to-metal transition in a nickelate film

The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO$_3$ film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions.Comment: 17 pages, 6 figure

Dynamics of the photoinduced insulator-to-metal transition in a nickelate film

Material properties can be controlled via strain, pressure, chemical composition, or dimensionality. Nickelates are particularly susceptible due to their strong variations of the electronic and magnetic properties on such external stimuli. Here, we analyze the photoinduced dynamics in a single crystalline NdNiO3 film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally, we compare our results with similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced electronic ordering phase transitions. © 2018 Author(s)

Ultrafast studies on the coupling of magnetic and structural dynamics in solid-state ferromagnets

The thesis describes primarily two experiments which investigate the coupling of structural and magnetic properties in solid-state ferromagnets on an ultrafast ($\approx$100~fs) timescale. To achieve this goal, both studies use ultrafast optical and x-ray pump-probe methods in combination, integrating information about magnetic and structural dynamics to form a more complete picture of the underlying physics. In the first experiment, the well-known shape-memory ferromagnetic Heusler alloy $\text{Ni}_2\text{MnGa}$ is studied. This compound is a multiferroic material stabilised by the correlations between electronic, magnetic, and structural order. The shape-memory effect is based on a structural phase transition between a low-temperature martensite phase and a high-temperature austenite phase. In both phases, the material is ferromagnetic. It has been observed for different stoichiometries that the martensite phase exhibits slightly incommensurate structural modulations whenever the shape-memory effect is present, but the exact connection between the two is unclear. We performed ultrafast optical measurements of the magnetisation of $\text{Ni}_2\text{MnGa}$, using 100~fs, 800~nm pulses for both the pump and the magneto-optical Kerr effect probe. We found a rapid demagnetisation with a timescale of $320\pm50$~fs, followed by a very slow recovery. At the FEMTO slicing source, we performed ultrafast x-ray diffraction on the weak (20201) satellite reflection of the (202) Bragg peak, which is a measure of the structural modulation. At high fluences of the 800~nm near-IR pump, we were able to observe a rapid and almost complete suppression of the structural modulation on a timescale of $300\pm40$~fs, very similar to the behaviour of the magnetisation. However, the longer timescale dynamics are markedly different from the one observed in the magnetisation: we observed an intermediate recovery of the structural modulation, before a second drop was caused by the thermal phase transition to the high-temperature austenite structure, which does not support the structural modulation. Our conclusion is that magnetisation and structural modulation are not strongly coupled in $\text{Ni}_2\text{MnGa}$, as the structural modulation was able to recover even while the material was demagnetised. The second experiment studied the movement of the lattice during ultrafast demagnetisation of a thin single-crystal iron film. While ultrafast demagnetisation is a very well-established effect, its microscopic mechanism remains unclear, especially with respect to the flow of angular momentum. Our goal was therefore to find a signature of the ultrafast Einstein--De Haas effect, i.e. the flow of spin angular momentum from the magnetisation to the mechanical angular momentum of the lattice. We identified that transverse shear waves would be launched from the surfaces of a rapidly demagnetising crystal, with an initial deflection whose direction depends on the magnetisation. A double-grazing x-ray diffraction geometry to measure the crystal truncation rod (CTR) of an in-plane and in-surface Bragg peak of the iron was chosen for the experiment. This choice maximises the sensitivity to the transverse strain wave and simultaneously minimises sensitivity to longitudinal strain waves caused by laser heating. The ultrafast pump-probe experiment was performed at the XPP endstation of the LCLS free-electron-laser, using a 6.9~keV x-ray probe beam and a 800~nm near-IR pump. The intensity along the (2~2~L) CTR was recorded simultaneously for each shot by using an imaging detector, with L in the range 0.02--0.09. Measurements were made for two opposite directions of magnetisation by selecting one of the two polarities of the sample electromagnet for each shot. By taking the difference of the time-resolved diffraction intensities for the two magnetisation directions along the CTR, we were able to positively identify a transverse mechanical strain wave in the data. Comparison with a detailed lattice dynamics and diffraction model allowed us to extract the timescale of the appearance of angular momentum in the lattice as around 200~fs, and the magnitude of angular momentum as corresponding to 8\% of the saturation magnetisation of iron. Using magneto-optical data for calibration, where we observed a 10\% demagnetisation under the same conditions, we conclude that a majority of about 80\% of the angular momentum lost from the spin system appear as mechanical angular momentum in the lattice; a complete transfer is also plausibe with the given experimental uncertainties. This shows that the interaction with the lattice plays an essential role in the ultrafast demagnetisation of iron

The dose-effect relation of interferon beta therapy to the endogene interferon beta produktion of patients medicated with interferon beta 1a 22 µg and interferon beta 1a 22 µg/44 µg, was analysed

Die Multiple Sklerose ist die häufigste chronisch- entzündliche ZNS- Erkrankung in der westlichen Welt und Australien. Die Pathophysiologie beruht auf autoreaktiven T- Zellen, welche sich vornehmlich gegen das myelobasische Protein von Oligodendrozyten richten. Nachfolgend kommt zu einer Zerstörung der Myelinscheiden, Axonalschaden und sekundärer Sklerosierung. In der Behandlung der schubförmigen multiplen Sklerose (relapsing remitting (RR)) werden, gemäß der Therapieleitlinien der Multiple Sklerose Therapie Konsensus Gruppe (MSTKG), Interferon beta- Präparate (Rebif®, Avonex®, Betaferon®), eingesetzt. Zahlreiche klinische Studien zeigten, dass Interferon b- Präparate die Grundaktivität der RR- MS- Erkrankung stabil halten oder senken können, so dass es letztendlich zu einer geringeren Schubrate, als auch zu einer Verlangsamung der Progredienz der Erkrankung kommt (Befundkonstanz der MRT- Bildgebung, Stabilisierung des EDSS- Scores). In dieser Arbeit sollte zum einen die interindividuellen Unterschiede in Bezug auf das Immunprofil und die Klinik zwischen unbehandelten und mit Interferon beta 1a behandelten Patienten dargestellt werden. Zum anderen wurde die Dosis- Wirkungsbeziehung einer Interferon beta Therapie auf das Immunprofil, insbesondere auf die endogene Interferon beta Produktion, von Interferon beta 1a 22 µg und Interferon beta 1a 22 µg/44 µg behandelten Patienten untersucht. Es wurden insgesamt 50 Patienten (16 Pat. untherapiert; 17 Pat. Interferon beta 1a 22 µg, 17 Pat., welche unter einer einjährigen Therapie mit Interferon beta 1a 22 µg einen Krankheitsprogress zeigten und daraufhin eine Dosisverdopplung auf Interferon beta 1a 44 µg erhielten) über einen Zeitraum von einem Jahr bzw. zwei Jahren (Interferon beta 1a 22 µg/44 µg) beobachtet. Die Patienten wurden alle drei Monate in unsere hiesige MS- Sprechstunde zur Blutprobengewinnung und ärztlicher Untersuchung einbestellt. Das Immunmonitoring erfolgte mit sequenz- spezifischen Hyb- probes mittels Real- Time- PCR. Hierbei wurden zum einem die proinflammatorischen Zytokine TNF R1, R2 und Interferon gamma sowie die antiinflammatorischen Zytokine IL4, IL4 R, und Interferon beta gemessen. Es ergaben sich keine signifikanten Unterschiede während des gesamten Beobachtungszeitraumes von einem bzw. zwei Jahren zwischen Kontrollgruppe, 22µg Gruppe, sowie der 22µg/44µg Gruppe bei folgenden gemessenen Parameter: TNF R1 und R2, sowie bei IL 4 und dessen Rezeptor. Die m-RNA Interferon beta- Produktion war nach sechs und neun Monaten nach Wechsel von Interferon beta 1a 22 µg auf Interferon beta 1a 44 µg signifikant höher als zu Therapiebeginn (T 6 und T 9 (44µg) p<0,001 vgl. mit T -9); ebenso im Vergleich zur Kontrollgruppe, dem Interferon beta 1a 22 µg- Kollektiv. Die Interferon beta 1a 22 µg- Monogruppe zeigte ebenfalls 3 Monate nach Therapiebeginn einen, zwar nicht signifikanten, aber dennoch messbaren Anstieg von Interferon beta. Die in der Literatur beschrieben Effekte von Interferon beta eine Verlagerung des Th1/Th2- Gleichgewichtes zu Gunsten einer Th2 Antwort mit nachfolgender Ausschüttung antiinflammatorischer Zytokine wie IL- 4 zu induzieren, konnten nicht verifiziert werden. Eine Interferon beta Therapie stimuliert die endogene Interferon beta Produktion. Die Dosis, welche zur Stimulation benötigt wird, ist abhängig von der initialen Ausgangsmenge von Interferon gamma. Patienten, welche initial nur eine geringe Menge von Interferon gamma m- RNA aufwiesen, sprachen immunologisch als auch klinisch gut auf die Dosierung von Interferon beta 1 a 22 µg an. Patienten mit anfänglich hohen Interferon gamma zeigten immunologisch als auch klinisch keine Ansprache auf eine Interferon beta 1a 22 µg Therapie. Erst nach einer Dosiseskalation auf 44 µg kam es zu einer signfikant erhöhten endogenen Interferon beta Produktion und einem nachfolgenden klinischen Benefit. Diese Beobachtung unterstützt die These von Petereit et al 2002: „Low Interferon gamma producers are better treatment responders“. Eine Therapie mit Interferon beta Präparaten hat somit keinen supprimierenden Effekt auf die endogene Interferon beta Produktion wie es bei anderen Hormonachsen bekannt ist (z. B. Cortison), sondern hat vielmehr eine agonistische Wirkung.Multiple sclerosis is the most common chronic inflammatoric disease of the CNS in the western world and Australia. Autoreactive T-cells, mostly attack myelobasic protein of oligodentrocytes, are held to be responsible for the disease. Consequently causes destruction of the myelin sheat, as well as axonal damage and sclerosis occurs. According to the guidelines of the Multiple Sklerose Therapie Konsensus Gruppe, interferon beta biologics are used in the therapy of the relapsing remitting MS (Interferon beta 1a®, Avonex®, Betaferon®). Numerous clinical studies showed a stable or less activity of disease by using interferon beta, resulting in a lower rate of relapses and a decelation of progress. (constant MRI status and EDSS-Score) In this work, the interindividual differences according due to the immune profile and clinical appearance of untreated and interferon beta 1a medicated patients, should be shown. Additionally the dose-effect relation of interferon beta therapy to the immune profile, especially to the endogene production of interferon beta, of patients medicated with interferon beta 1a 22 µg and interferon beta 1a 22 µg/44 µg, was analysed. All in all 50 patients (16 un-treated, 17 interferon beta 1a 22 µg, 17 patients, showing a progress in disease by a one year of therapy with interferon beta 1a 22 µg and therefore were treatet with interferon beta 1a 44 µg) were observed for one or two yeares (interferon beta 1a 22 µg/44 µg). The patients were seen in a three month term in our MS-consultation for acquirement of blood samples and medical examination. The immune monitoring was performed by real time PCR with sequence specific Hyb-probes. By real- time PCR the pro- inflammatoric cytocine TNF R1, R2 and interferon gamma, as well as the anti- inflammatoric cytocines IL4, IL4-R, and interferon beta were measured. As a result there were no signific differences in TNF R1 and R2, as well as in IL4 and IL4-R during the obervation period of one or two years measured in the controll group, the 22 µg group and the 22 µg/44 µg group. The production of mRNA interferon beta was significant higher after a period of six and nine months after a change from interferon beta 1a 22 µg to interferon beta 1 a 44 µg; also in comparison to the control group and to the interferon beta 1a 22 µg group. Also the interferon beta 1a 22 µg group showed a non significant rise of interferon beta after three month of treatment. Previously other authors described effects of interferon beta on a shift to the Th1/Th2 balance in favour of a Th2 answer with successive release of anti- inflammatory cytokines like IL-4. This could not be verified in our work. An interferon beta treatment stimulates the endogene production of interferon beta in human beings. The dose, necessary for stimulation, depends on the initial amount of interferon gamma. There was a good respond of treatment with interferon beta 1 a 22 µg by patients with initial low doses of interferon gamma m-RNA. No response according to immunological and clinical changes were seen by patients with initial high interferon gamma levels and a treatment with interferon beta 1 a 22 µg. Just after a increase of the dosage to 44 µg a significant higher endogene beta production and a clinical benefit were observed. Our study confirms the thesis of Petereit et al 2002: “Low interferon gamma producers are better treatment responders.” An interferon beta treatment has no suppressive effect on the endogene interferon beta production in vivo, as we have seen in the hypothalamic- pituitary adrenal axis. The existing data show an agonistic effect

Two and three-photon double ionization of lithium

Differential studies on two and three-photon double ionization (DI) of atomic lithium have been performed for photon energies of 50 eV and 59 eV at I 5 × 1013 W/cm2. At 50 eV DI proceeds via the simultaneous absorption of two photons resulting in the co

Ultrafast demagnetization in iron: Separating effects by their nonlinearity

The laser-driven ultrafast demagnetization effect is one of the long-standing problems in solid-state physics. The time scale is given not only by the transfer of energy, but also by the transport of angular momentum away from the spin system. Through a double-pulse experiment resembling two-dimensional spectroscopy, we separate the different pathways by their nonlinear properties. We find (a) that the loss of magnetization within 400 fs is not affected by the previous excitations (linear process), and (b) we observe a picosecond demagnetization contribution that is strongly affected by the previous excitations. Our experimental approach is useful not only for studying femtosecond spin dynamics, but can also be adapted to other problems in solid-state dynamics.ISSN:2329-777