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- ItemExchange-Striction Driven Ultrafast Nonthermal Lattice Dynamics in NiO(College Park, Md. : APS, 2021) Windsor, Y.W.; Zahn, D.; Kamrla, R.; Feldl, J.; Seiler, H.; Chiang, C.-T.; Ramsteiner, M.; Widdra, W.; Ernstorfer, R.; Rettig, L.We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AFM) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce Q-resolved effective temperatures describing the transient lattice. We identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ∼0.3 ps and persists for 25 ps. We associate this with transient changes to the AFM exchange striction-induced lattice distortion, supported by the observation of a transient Q asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order.
- ItemUnraveling the Orbital Physics in a Canonical Orbital System KCuF3(College Park, Md. : APS, 2021) Li, Jiemin; Xu, Lei; Garcia-Fernandez, Mirian; Nag, Abhishek; Robarts, H.C.; Walters, A.C.; Liu, X.; Zhou, Jianshi; Wohlfeld, Krzysztof; van den Brink, Jeroen; Ding, Hong; Zhou, Ke-JinWe explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF3 using the Cu L3-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu2+ dd orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF3, which predominantly drives orbital ordering.
- ItemDeveloping a Laser Induced Liquid Beam Ion Desorption Spectral Database as Reference for Spaceborne Mass Spectrometers(Malden, Mass. : American Geophysical Union, 2022) Klenner, Fabian; Umair, Muhammad; Walter, Sebastian H. G.; Khawaja, Nozair; Hillier, Jon; Nölle, Lenz; Zou, Zenghui; Napoleoni, Maryse; Sanderink, Arnaud; Zuschneid, Wilhelm; Abel, Bernd; Postberg, FrankSpaceborne impact ionization mass spectrometers, such as the Cosmic Dust Analyzer on board the past Cassini spacecraft or the SUrface Dust Analyzer being built for NASA's upcoming Europa Clipper mission, are of crucial importance for the exploration of icy moons in the Solar System, such as Saturn's moon Enceladus or Jupiter's moon Europa. For the interpretation of data produced by these instruments, analogue experiments on Earth are essential. To date, thousands of laboratory mass spectra have been recorded with an analogue experiment for impact ionization mass spectrometers. Simulation of mass spectra of ice grains in space is achieved by a Laser Induced Liquid Beam Ion Desorption (LILBID) approach. The desorbed cations or anions are analyzed in a time-of-flight mass spectrometer. The amount of unstructured raw data is increasingly challenging to sort, process, interpret and compare with data from space. Thus far this has been achieved manually for individual mass spectra because no database containing the recorded reference spectra was available. Here we describe the development of a comprehensive, extendable database containing cation and anion mass spectra from the laboratory LILBID facility. The database is based on a Relational Database Management System with a web server interface and enables filtering of the laboratory data using a wide range of parameters. The mass spectra can be compared not only with data from past and future space missions but also mass spectral data generated by other, terrestrial, techniques. The validated and approved subset of the database is available for general public (https://lilbid-db.planet.fu-berlin.de).
- ItemMeasurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures(College Park, Md. : APS, 2021) Ricci, Alessandro; Poccia, Nicola; Campi, Gaetano; Mishra, Shrawan; Müller, Leonard; Joseph, Boby; Shi, Bo; Zozulya, Alexey; Buchholz, Marcel; Trabant, Christoph; Lee, James C. T.; Viefhaus, Jens; Goedkoop, Jeroen B.; Nugroho, Agustinus Agung; Braden, Markus; Roy, Sujoy; Sprung, Michael; Schüßler-Langeheine, ChristianWe study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations.
- ItemEvolution of a molecular shape resonance along a stretching chemical bond(College Park, Md. : APS, 2020) Brausse, Felix; Bach, Florian; Krečinić, Faruk; Vrakking, Marc J.J.; Rouzée, ArnaudWe report experiments on laser-assisted electron recollisions that result from strong-field ionization of photoexcited I2 molecules in the regime of low-energy electron scattering (<25 eV impact energy). By comparing differential scattering cross sections extracted from the angle-resolved photoelectron spectra to differential scattering cross sections from quantum-scattering calculations, we demonstrate that the electron-scattering dynamics is dominated by a shape resonance. When the molecular bond stretches during the evolution of a vibrational wave packet this shape resonance shifts to lower energies, both in experiment and theory. We explain this behavior by the nature of the resonance wave function, which closely resembles an antibonding molecular orbital of I2.
- ItemUltrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges(College Park, Md. : APS, 2020) Schneider, Michael; Pfau, Bastian; Günther, Christian M.; von Korff Schmising, Clemens; Weder, David; Geilhufe, Jan; Perron, Jonathan; Capotondi, Flavio; Pedersoli, Emanuele; Manfredda, Michele; Hennecke, Martin; Vodungbo, Boris; Lüning, Jan; Eisebitt, StefanWe systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M3,2 absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16 mJ/cm2/pulse to 10 000 mJ/cm2/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse—implying an increased XUV peak electric field—results in a reduced quenching of the resonant diffraction at the Co M3,2 edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.
- ItemHorizontal Wavenumber Spectra of Vertical Vorticity and Horizontal Divergence of Mesoscale Dynamics in the Mesosphere and Lower Thermosphere Using Multistatic Specular Meteor Radar Observations(Malden, Mass. : American Geophysical Union, 2022) Poblet, Facundo L.; Chau, Jorge L.; Conte, J. Federico; Avsarkisov, Victor; Vierinen, Juha; Charuvil Asokan, HarikrishnanSpecular meteor radars (SMRs) have significantly contributed to the understanding of wind dynamics in the mesosphere and lower thermosphere (MLT). We present a method to estimate horizontal correlations of vertical vorticity (Qzz) and horizontal divergence (P) in the MLT, using line-of-sight multistatic SMRs velocities, that consists of three steps. First, we estimate 2D, zonal, and meridional correlation functions of wind fluctuations (with periods less than 4 hr and vertical wavelengths smaller than 4 km) using the wind field correlation function inversion (WCFI) technique. Then, the WCFI's statistical estimates are converted into longitudinal and transverse components. The conversion relation is obtained by considering the rotation about the vertical direction of two velocity vectors, from an east-north-up system to a meteor-pair-dependent cylindrical system. Finally, following a procedure previously applied in the upper troposphere and lower stratosphere to airborne wind measurements, the longitudinal and transverse spatial correlations are fitted, from which Qzz, P, and their spectra are directly estimated. The method is applied to a special Spread spectrum Interferometric Multistatic meteor radar Observing Network data set, obtained over northern Germany for seven days in November 2018. The results show that in a quasi-axisymmetric scenario, P was more than five times larger than Qzz for the horizontal wavelengths range given by ∼50–400 km, indicating a predominance of internal gravity waves over vortical modes of motion as a possible explanation for the MLT mesoscale dynamics during this campaign.
- ItemUltrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet(College Park, Md. : APS, 2021) Golias, E.; Kumberg, I.; Gelen, I.; Thakur, S.; Gördes, J.; Hosseinifar, R.; Guillet, Q.; Dewhurst, J.K.; Sharma, S.; Schüßler-Langeheine, C.; Pontius, N.; Kuch, W.We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.
- ItemHidden Charge Order in an Iron Oxide Square-Lattice Compound(College Park, Md. : APS, 2021) Kim, Jung-Hwa; Peets, Darren C.; Reehuis, Manfred; Adler, Peter; Maljuk, Andrey; Ritschel, Tobias; Allison, Morgan C.; Geck, Jochen; Mardegan, Jose R. L.; Bereciartua Perez, Pablo J.; Francoual, Sonia; Walters, Andrew C.; Keller, Thomas; Abdala, Paula M.; Pattison, Philip; Dosanjh, Pinder; Keimer, BernhardSince the discovery of charge disproportionation in the FeO2 square-lattice compound Sr3Fe2O7 by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained “hidden” to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO2 planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.
- ItemCoexistence of Superconductivity and Charge Density Waves in Tantalum Disulfide : Experiment and Theory(College Park, Md. : APS, 2020) Kvashnin, Y.; VanGennep, D.; Mito, M.; Medvedev, S.A.; Thiyagarajan, R.; Karis, O.; Vasiliev, A.N.; Eriksson, O.; Abdel-Hafiez, M.The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter. © 2020 authors. Published by the American Physical Society.