Resonant x-ray diffraction measurements in charge ordered kagome superconductors KV3Sb5and RbV3Sb5.

We report on (resonant) x-ray diffraction experiments on the normal state properties of kagome-lattice superconductors KV3Sb5and RbV3Sb5. We have confirmed previous reports indicating that the charge density wave (CDW) phase is characterized by a doubling of the unit cell in all three crystallographic directions. By monitoring the temperature dependence of Bragg peaks associated with the CDW phase, we ascertained that it develops gradually over several degrees, as opposed to CsV3Sb5, where the CDW peak intensity saturates promptly just below the CDW transition temperature. Analysis of symmetry modes indicates that this behavior arises due to lattice distortions linked to the formation of CDWs. These distortions occur abruptly in CsV3Sb5, while they progress more gradually in RbV3Sb5and KV3Sb5. In contrast, the amplitude of the mode leading to the crystallographic symmetry breaking fromP6/mmmtoFmmmappears to develop more gradually in CsV3Sb5as well. Diffraction measurements close to the V K edge and the Sb L1edge show no sensitivity to inversion- or time-symmetry breaking, which are claimed to be associated with the onset of the CDW phase. The azimuthal angle dependence of the resonant diffraction intensity observed at the Sb L1edge is associated with the difference in the population of unoccupied states and the anisotropy of the electron density of certain Sb ions.

Determination of sub-ps lattice dynamics in FeRh thin films.

Understanding the ultrashort time scale structural dynamics of the FeRh metamagnetic phase transition is a key element in developing a complete explanation of the mechanism driving the evolution from an antiferromagnetic to ferromagnetic state. Using an X-ray free electron laser we determine, with sub-ps time resolution, the time evolution of the (-101) lattice diffraction peak following excitation using a 35 fs laser pulse. The dynamics at higher laser fluence indicates the existence of a transient lattice state distinct from the high temperature ferromagnetic phase. By extracting the lattice temperature and comparing it with values obtained in a quasi-static diffraction measurement, we estimate the electron-phonon coupling in FeRh thin films as a function of laser excitation fluence. A model is presented which demonstrates that the transient state is paramagnetic and can be reached by a subset of the phonon bands. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

Route to tunable room temperature electric polarization in SrTiO3-CoFe2O4 heterostructures.

Utilizing the magnetostrictive properties of CoFe2O4, we demonstrate reversible room temperature control of the Ti electronic structure in SrTiO3-CoFe2O4 heterostructures, by inducing local and reversible strain in the SrTiO3. By means of X-ray absorption spectroscopy, we have ascertained the changes that take place in the energy levels of the Ti 3d orbitals under the influence of an external magnetic field. The observed Ti electronic state when the sample is subjected to moderately large external magnetic fields and the disappearance of the induced phase upon their removal indicates lattice distortions that are suggestive of the development of a net electric polarization.

Spin-Wave Dynamics and Symmetry Breaking in an Artificial Spin Ice.

Artificial spin ices are periodic arrangements of interacting nanomagnets which allow investigating emergent phenomena in the presence of geometric frustration. Recently, it has been shown that artificial spin ices can be used as building blocks for creating functional materials, such as magnonic crystals. We investigate the magnetization dynamics in a system exhibiting anisotropic magnetostatic interactions owing to locally broken structural inversion symmetry. We find a rich spin-wave spectrum and investigate its evolution in an external magnetic field. We determine the evolution of individual modes, from building blocks up to larger arrays, highlighting the role of symmetry breaking in defining the mode profiles. Moreover, we demonstrate that the mode spectra exhibit signatures of long-range interactions in the system. These results contribute to the understanding of magnetization dynamics in spin ices beyond the kagome and square ice geometries and are relevant for the realization of reconfigurable magnonic crystals based on spin ices.

Time-resolved imaging of three-dimensional nanoscale magnetization dynamics.

Understanding and control of the dynamic response of magnetic materials with a three-dimensional magnetization distribution is important both fundamentally and for technological applications. From a fundamental point of view, the internal magnetic structure and dynamics in bulk materials still need to be mapped1, including the dynamic properties of topological structures such as vortices2, magnetic singularities3 or skyrmion lattices4. From a technological point of view, the response of inductive materials to magnetic fields and spin-polarized currents is essential for magnetic sensors and data storage devices5. Here, we demonstrate time-resolved magnetic laminography, a pump-probe technique, which offers access to the temporal evolution of a three-dimensional magnetic microdisc with nanoscale resolution, and with a synchrotron-limited temporal resolution of 70 ps. We image the dynamic response to a 500 MHz magnetic field of the complex three-dimensional magnetization in a two-phase bulk magnet with a lateral spatial resolution of 50 nm. This is achieved with a stroboscopic measurement consisting of eight time steps evenly spaced over 2 ns. These measurements map the spatial transition between domain wall motion and the dynamics of a uniform magnetic domain that is attributed to variations in the magnetization state across the phase boundary. Our technique, which probes three-dimensional magnetic structures with temporal resolution, enables the experimental investigation of functionalities arising from dynamic phenomena in bulk and three-dimensional patterned nanomagnets6.

Collective magnetism in an artificial 2D XY spin system.

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.

Generation of coherent extreme ultraviolet radiation from α-quartz using 50 fs laser pulses at a 1030 nm wavelength and high repetition rates.

Coherent extreme ultraviolet (EUV) radiation using high-harmonic generation (HHG) in α-quartz is demonstrated from 10 to 200 kHz, using 50 fs laser pulses at the center wavelength of 1030 nm. The EUV radiation extends beyond 25 eV in the nondamaging regime. The number of photons generated in a single harmonic order at 15.6 eV is measured to be ≈(3.5±2.5)×1010 per second which, to the best of our knowledge, is a first and record value reported to date using EUV HHG from solids. This Letter demonstrates one of the first all-solid-state EUV sources based on industrial-grade fiber lasers, enabling the possibility of bringing reliable EUV sources to the mass market.

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.

Three-dimensional magnetization structures revealed with X-ray vector nanotomography.

In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities-Bloch points-occur. These were predicted more than fifty years ago but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure and a twisted state that appears to correspond to an 'anti-Bloch point'. Our imaging method enables the nanoscale study of topological magnetic structures in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications.

Element-specific X-ray phase tomography of 3D structures at the nanoscale.

Recent advances in fabrication techniques to create mesoscopic 3D structures have led to significant developments in a variety of fields including biology, photonics, and magnetism. Further progress in these areas benefits from their full quantitative and structural characterization. We present resonant ptychographic tomography, combining quantitative hard x-ray phase imaging and resonant elastic scattering to achieve ab initio element-specific 3D characterization of a cobalt-coated artificial buckyball polymer scaffold at the nanoscale. By performing ptychographic x-ray tomography at and far from the Co K edge, we are able to locate and quantify the Co layer in our sample to a 3D spatial resolution of 25 nm. With a quantitative determination of the electron density we can determine that the Co layer is oxidized, which is confirmed with microfluorescence experiments.

Linear polarization scans for resonant X-ray diffraction with a double-phase-plate configuration.

An in-vacuum double-phase-plate diffractometer for performing polarization scans combined with resonant X-ray diffraction experiments is presented. The use of two phase plates enables the correction of some of the aberration effects owing to the divergence of the beam and its energy spread. A higher rate of rotated polarization is thus obtained in comparison with a system with only a single retarder. Consequently, thinner phase plates can be used to obtain the required rotated polarization rate. These results are particularly interesting for applications at low energy (e.g. 4 keV) where the absorption owing to the phase plate(s) plays a key role in the feasibility of these experiments. Measurements by means of polarization scans at the uranium M(4) edge on UO(2) enable the contributions of the magnetic and quadrupole ordering in the material to be disentangled.

Chirality, magnetic charge and other strange entities in resonant x-ray Bragg diffraction.

Subtleties in the electronic structure of complex materials can be directly observed, in great detail, by means of the Bragg diffraction of x-rays whose energy matches an atomic resonance. Strange atomic multipoles can be encountered in the interpretation of measured Bragg intensities, e.g., chirality and magnetic charge. Additionally, the x-ray technique allows the direct observation of the enantiomorphic screw-axis in chiral crystals, such as tellurium, low quartz and berlinite.