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- ItemChiral Spin Liquid Ground State in YBaCo3FeO7(College Park, Md. : APS, 2022) Schweika, W.; Valldor, M.; Reim, J.D.; Rößler, U.K.A chiral spin liquid state is discovered in the highly frustrated, noncentrosymmetric swedenborgite compound YBaCo3FeO7, a layered kagome system of hexagonal symmetry, by advanced polarized neutron scattering from a single domain crystalline sample. The observed diffuse magnetic neutron scattering has an antisymmetric property that relates to its specific chirality, which consists of three cycloidal waves perpendicular to the c axis, forming an entity of cylindrical symmetry. Chirality and symmetry agree with relevant antisymmetric exchanges arising from broken spatial parity. Applying a Fourier analysis to the chiral interference pattern, with distinction between kagome sites and the connecting trigonal interlayer sites of threefold symmetry, the chiral spin correlation function is determined. Characteristic chiral waves originate from the trigonal sites and extend over several periods in the kagome planes. The chiral spin liquid is remarkably stable at low temperatures despite strong antiferromagnetic spin exchange. The observation raises a challenge, since the commonly accepted ground states in condensed matter either have crystalline long-range order or form a quantum liquid. We show that, within the classical theory of magnetic order, a disordered ground state may arise from chirality. The present scenario, with antisymmetric exchange acting as a frustrating gauge background that stabilizes local spin lumps, is similar to the avoided phase transition in coupled gauge and matter fields for subnuclear particles.
- ItemRobust metastable skyrmions with tunable size in the chiral magnet FePtMo3 N(Woodbury, NY : Inst., 2020) Sukhanov, A.S.; Heinemann, A.; Kautzsch, L.; Bocarsly, J.D.; Wilson, S.D.; Felser, C.; Inosov, D.S.The synthesis of new materials that can host magnetic skyrmions and their thorough experimental and theoretical characterization are essential for future technological applications. The β-Mn-type compound FePtMo3N is one such novel material that belongs to the chiral space group P4132, where the antisymmetric Dzyaloshinskii-Moriya interaction is allowed due to the absence of inversion symmetry. We report the results of small-angle neutron scattering (SANS) measurements of FePtMo3N and demonstrate that its magnetic ground state is a long-period spin helix with a Curie temperature of 222 K. The magnetic field-induced redistribution of the SANS intensity showed that the helical structure transforms to a lattice of skyrmions at ∼13 mT at temperatures just below TC. Our key observation is that the skyrmion state in FePtMo3N is robust against field cooling down to the lowest temperatures. Moreover, once the metastable state is prepared by field cooling, the skyrmion lattice exists even in zero field. Furthermore, we show that the skyrmion size in FePtMo3N exhibits high sensitivity to the sample temperature and can be continuously tuned between 120 and 210 nm. This offers different prospects in the control of topological properties of chiral magnets. © 2020 authors. Published by the American Physical Society.
- ItemAll-optical Stückelberg spectroscopy of strongly driven Rydberg states(College Park, MD : APS, 2022) Bengs, Ulrich; Patchkovskii, Serguei; Ivanov, Misha; Zhavoronkov, NickolaiThe AC Stark shift of electronic levels is ubiquitous in the interaction of intense light fields with atoms and molecules. As the light intensity changes on the rising and falling edges of a femtosecond laser pulse, it shifts the Rydberg states in and out of multiphoton resonances with the ground state. The two resonant pathways for transient excitation arising at the leading and the trailing edges of the pulse generate Young's type interference, generally referred to as the Stückelberg oscillations. Here we report the observation of the Stückelberg oscillations in the intensity of the coherent free-induction decay following resonant multiphoton excitation. Moreover, combining the experimental results with accurate numerical simulations and a simple model, we use the Stückelberg oscillations to recover the population dynamics of strongly driven Rydberg states inside the laser pulse by all-optical measurements after the end of the pulse. We demonstrate the potential of this spectroscopy to characterize lifetimes of Rydberg states dressed by laser fields with strengths far exceeding the Coulomb field between the Rydberg electron and the core.
- ItemIntracluster Coulombic decay following intense NIR ionization of clusters(Bristol : IOP Publ., 2015) Schütte, Bernd; Arbeiter, Mathias; Fennel, Thomas; Jabbari, Ghazal; Gokhberg, Kirill; Kuleff, Alexander I.; Vrakking, Marc J. J.; Rouzée, ArnaudWe report on the observation of a novel intracluster Coulombic decay process following Rydberg atom formation in clusters ionized by intense near-infrared fields. A new decay channel emerges, in which a Rydberg atom relaxes to the ground state by transferring its excess energy to a weakly bound electron in the environment that is emitted from the cluster. We find evidence for this process in the electron spectra, where a peak close to the corresponding atomic ionization potential is observed. For Ar clusters, a decay time of 87 ps is measured, which is significantly longer than in previous time-resolved studies of interatomic Coulombic decay.
- ItemCorrelated electronic decay following intense near-infrared ionization of clusters(Bristol : IOP Publ., 2015) Schütte, Bernd; Arbeiter, Mathias; Fennel, Thomas; Jabbari, Ghazal; Kuleff, Alexander I.; Vrakking, Marc J. J.; Rouzée, ArnaudWe report on a novel correlated electronic decay process following extensive Rydberg atom formation in clusters ionized by intense near-infrared fields. A peak close to the atomic ionization potential is found in the electron kinetic energy spectrum. This new contribution is attributed to an energy transfer between two electrons, where one electron decays from a Rydberg state to the ground state and transfers its excess energy to a weakly bound cluster electron in the environment that can escape from the cluster. The process is a result of nanoplasma formation and is therefore expected to be important, whenever intense laser pulses interact with nanometer-sized particles.
- ItemAtomic processes in bicircular fields(Bristol : IOP Publ., 2016) Odžak, S.; Hasović, E.; Becker, W.; Milošević, D.B.We investigate laser-assisted electron-ion recombination (LAR), high-order harmonic generation (HHG) and above-threshold ionization (ATI) of argon atoms by a bicircular laser field, which consists of two coplanar counter-rotating circularly polarized fields of frequencies rω and sω. The energy of soft x rays generated in the LAR process is analyzed as a function of the incident electron angle and numerical results of direct recombination of electrons with Ar+ ions are presented. We also present the results of HHG by a bicircular field and confirm the selection rules derived earlier for inert-gas atoms in a p ground state. We show that the photoelectron spectra in the ATI process, presented in the momentum plane, as well as the LAR spectra exhibit the same discrete rotational symmetry as the applied field.
- ItemTerahertz emission from lithium doped silicon under continuous wave interband optical excitation(Bristol : IOP Publ., 2015) Andrianov, A.V.; Zakhar'in, A.O.; Zhukavin, R.K.; Shastin, V.N.; Abrosimov, N.V.We report on experimental observation and study of terahertz emission from lithium doped silicon crystals under continuous wave band-to-band optical excitation. It is shown that radiative transitions of electrons from 2P excited states of lithium donor to the 1S(A1) donor ground state prevail in the emission spectrum. The terahertz emission occurs due to capture of nonequilibrium electrons to charged donors, which in turn are generated in the crystal as a result of impurity assisted electron-hole recombination. Besides the intracentre radiative transitions the terahertz emission spectrum exhibits also features at about 12.7 and 15.27 meV, which could be related to intraexciton transitions and transitions from the continuum to the free exciton ground state.
- ItemEvidence of the dominant production mechanism of ammonia in a hydrogen plasma with parts per million of nitrogen([Melville, NY] : American Institute of Physics, 2021) Ellis, J.; Köpp, D.; Lang, N.; van Helden, J. H.Absolute ground state atomic hydrogen densities were measured, by the utilization of two-photon absorption laser induced fluorescence, in a low-pressure electron cyclotron resonance plasma as a function of nitrogen admixtures - 0 to 5000 ppm. At nitrogen admixtures of 1500 ppm and higher, the spectral distribution of the fluorescence changes from a single Gaussian to a double Gaussian distribution; this is due to a separate, nascent contribution arising from the photolysis of an ammonia molecule. At nitrogen admixtures of 5000 ppm, the nascent contribution becomes the dominant contribution at all investigated pressures. Thermal loading experiments were conducted by heating the chamber walls to different temperatures; this showed a decrease in the nascent contributions with increasing temperature. This is explained by considering how the temperature influences recombination coefficients, and from which, it can be stated that the Langmuir-Hinshelwood recombination mechanism is dominant over the Eley-Rideal mechanism.
- ItemPressure-driven magnetic moment collapse in the ground state of MnO(Milton Park : Taylor & Francis, 2007) Kasinathan, Deepa; Koepernik, K.; Pickett, W.E.The zero temperature Mott transition region in antiferromagnetic, spin S = 5/2 MnO is probed using the correlated band theory LSDA + U method. The first transition encountered is an insulator-insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating 'spin-flip' moment collapse. This spin-flip to S = 1/2 takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.
- ItemChirp-control of resonant high-order harmonic generation in indium ablation plumes driven by intense few-cycle laser pulses(Washington, DC : Optical Society of America, OSA, 2018) Abdelrahman, Z.; Khokhlova, M.A.; Walke, D.J.; Witting, T.; Zair, A.; Strelkov, V.V.; Marangos, J.P.; Tisch, J.W.G.We have studied high-order harmonic generation (HHG) in an indium ablation plume driven by intense few-cycle laser pulses centered at 775 nm as a function of the frequency chirp of the laser pulse. We found experimentally that resonant emission lines between 19.7 eV and 22.3 eV (close to the 13th and 15th harmonic of the laser) exhibit a strong, asymmetric chirp dependence, with pronounced intensity modulations. The chirp dependence is reproduced by our numerical time-dependent Schrödinger equation simulations of a resonant HHG by the model indium ion. As demonstrated with our separate simulations of HHG within the strong field approximation, the resonance can be understood in terms of the chirp-dependent HHG photon energy coinciding with the energy of an autoionizing state to ground state transition with high oscillator strength. This supports the validity of the general theory of resonant four-step HHG in the few-cycle limit.