Unique determination of a single crack in a uniform simply supported beam in bending vibration

In this paper we consider one of the basic inverse problems in damage detection based on natural frequency data, namely the identification of a single open crack in a uniform simply supported beam from measurement of the first and the second natural frequency. It is commonly accepted in the literature that the knowledge of this set of spectral data allows for the unique determination of the severity and the position (up to symmetry) of the damage. However, in spite of the fact that many numerical evidences are in support of this property, the result is rigorously proved only when the severity of the crack is small. In this paper we definitely show, by means of an original constructive method, that the above result holds true for any level of crack severity. (C) 2016 Elsevier Ltd. All rights reserved

Ab-initio calculation of all-optical time-resolved calorimetry of nanosized systems: Evidence of nanosecond-decoupling of electron and phonon temperatures

The thermal dynamics induced by ultrashort laser pulses in nanoscale systems, i.e. all-optical time-resolved nanocalorimetry is theoretically investigated from 300 to 1.5 K. We report ab-initio calculations describing the temperature dependence of the electron-phonon interactions for Cu nanodisks supported on Si. The electrons and phonons temperatures are found to decouple on the ns time scale at 10 K, which is two orders of magnitude in excess with respect to that found for standard low-temperature transport experiments. By accounting for the physics behind our results we suggest an alternative route for overhauling the present knowledge of the electron-phonon decoupling mechanism in nanoscale systems by replacing the mK temperature requirements of conventional experiments with experiments in the time-domain.Comment: 5 pages, 3 figures. Accepted on Physical Review B

Structural and magnetic dynamics of a laser induced phase transition in FeRh

We use time-resolved x-ray diffraction and magnetic optical Kerr effect to study the laser induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (\tau_1 ~ 30ps). This is significantly faster than the magnetic response (\tau_2 ~ 60ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases co-exist on short time-scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.Comment: 5 pages, 3 figures - changed to reflect version accepted for PR

Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs

New insights into the laser-assisted photoelectric effect from solid-state surfaces

Photoemission from a solid surface provides a wealth of information about the electronic structure of the surface and its dynamic evolution. Ultrafast pump-probe experiments are particularly useful to study the dynamic interactions of photons with surfaces as well as the ensuing electron dynamics induced by these interactions. Time-resolved laser-assisted photoemission (tr-LAPE) from surfaces is a novel technique to gain deeper understanding of the fundamentals underlying the photoemission process. Here, we present the results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy experiment on two different metal surfaces conducted at the X-ray Free-Electron Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core levels using ultrashort soft X-ray pulses in combination with synchronized infrared (IR) laser pulses. When both pulses overlap in time and space, laser-assisted photoemission results in the formation of a series of sidebands that reflect the dynamics of the laser-surface interaction. We demonstrate a qualitatively new level of sideband generation up to the sixth order and a surprising material dependence of the number of sidebands that has so far not been predicted by theory. We provide a semi-quantitative explanation of this phenomenon based on the different dynamic dielectric responses of the two materials. Our results advance the understanding of the LAPE process and reveal new details of the IR field present in the surface region, which is determined by the dynamic interplay between the IR laser field and the dielectric response of the metal surfaces.Comment: 18 pages, 3 figure

Melting of magnetic order in NaOsO₃ by femtosecond laser pulses

NaOsO3 has recently attracted significant attention for the strong coupling between its electronic band structure and magnetic ordering. Here, we used time-resolved magnetic x-ray diffraction to determine the timescale of the photoinduced antiferromagnetic dynamics in NaOsO3. Our measurements are consistent with a sub-100 fs melting of the antiferromagnetic long-range order that occurs significantly faster than the lattice dynamics as monitored by the transient change in intensity of selected Bragg structural reflections, which instead show a decrease of intensity on a timescale of several ps

Multi-Mode Front Lens for Momentum Microscopy: Part II Experiments

We have experimentally demonstrated different operating modes for the front lenses of the momentum microscopes described in Part I. Measurements at energies from vacuum UV at a high-harmonic generation (HHG)-based source to the soft and hard X-ray range at a synchrotron facility validated the results of theoretical ray-tracing calculations. The key element is a ring electrode concentric with the extractor electrode, which can tailor the field in the gap. First, the gap-lens-assisted extractor mode reduces the field strength at the sample while mitigating image aberrations. This mode gave good results in all spectral ranges. Secondly, by compensating the field at the sample surface with a negative voltage at the ring electrode we can operate in zero-field mode, which is beneficial for operando experiments. Finally, higher negative voltages establish the repeller mode, which removes all slow electrons below a certain kinetic energy to eliminate the primary contribution to the space-charge interaction in pump-probe experiments. The switch from extractor to repeller mode is associated with a reduction in the k-field-of-view (10-20 % at hard-X-ray energies, increasing to ~50% at low energies). Real-space imaging also benefits from the new lens modes as confirmed by ToF-XPEEM imaging with 650 nm resolution.Comment: 22 pages, 9 figures, 56 reference

Substrate Induced Strain Field in FeRh Epilayers Grown on Single Crystal MgO (001) Substrates

Equi-atomic FeRh is highly unusual in that it undergoes a first order meta-magnetic phase transition from an antiferromagnet to a ferromagnet above room temperature (Tr ≈ 370 K). This behavior opens new possibilities for creating multifunctional magnetic and spintronic devices which can utilise both thermal and applied field energy to change state and functionalise composites. A key requirement in realising multifunctional devices is the need to understand and control the properties of FeRh in the extreme thin film limit (tFeRh < 10 nm) where interfaces are crucial. Here we determine the properties of FeRh films in the thickness range 2.5–10 nm grown directly on MgO substrates. Our magnetometry and structural measurements show that a perpendicular strain field exists in these thin films which results in an increase in the phase transition temperature as thickness is reduced. Modelling using a spin dynamics approach supports the experimental observations demonstrating the critical role of the atomic layers close to the MgO interface

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e–e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from −20 to −1100 V/mm for Ekin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field −21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments