arXiv subjects

Urs Staub

Publications and source records attributed to Urs Staub.

At least 19 recordsLinked to original sources

arXiv2026

Quantifying angular momentum of coherently driven circular phonons

The use of intense terahertz (THz) pulses to manipulate low-energy excitations offers a powerful approach for ultrafast control of electronic and magnetic properties in materials. Theory suggests that circular ionic motions driven by THz fields carry angular momentum, potentially generating internal magnetic fields. Recent experiments in nonmagnetic SrTiO3 (STO) have hinted at such THz-induced fields, but their origin remains debated. Here, we employ ultrafast x-ray diffraction to resolve the time-dependent ionic trajectories in STO following excitation by circularly polarized THz pulses. Our analysis reveals that oxygen ions, despite their lower mass, contribute around 90% of the phonon angular momentum. The resulting imbalance between the negatively and positively charged ions provides a clear explanation for the mechanism behind induced magnetism in STO. This work further provides the first quantitative measurement of circular ionic motions and their angular momentum and establishes a general methodology for the investigation of angular momentum transfer in solids, paving the way for new strategies to control topological phonon transport and phonon-driven magnetism in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

THz-induced phonon mode mixing and collective dynamics in a polar nanolattice

Manipulating phonons through symmetry is a fundamental approach to alter the dynamic responses of materials. Most often new phases are sought through control of the unit-cell (e.g. via strain, doping, light, etc.). By comparison, there is vast potential to look beyond the unit-cell into higher-order architectures to control wave scattering and interference effects that remains less explored. We describe the THz-induced dynamics of an SrTiO$_{3}$ thin film with a nanoscale ordered interfacial dislocation network probed with 2- and 3-dimensional time-resolved x-ray diffraction and classical molecular dynamics simulations. We find that symmetry breaking at all scales is an effective approach to create a dynamical electric polarization and to control phonon mixing that generates previously unreported collective modes in the THz regime with circular vortex-like displacements. This work opens a new pathway to explore dynamical functional properties that can be extended to magnetic, electric, and ferroelectric systems by controlling the real-space topology via epitaxy.

cond-mat.str-el↗

arXiv2026

Electric field switching of chiral phonons

Lattice vibrations carrying angular momentum, known as chiral phonons, have emerged as a promising route to control and understand complex material properties, yet their deterministic manipulation remains largely unexplored. Here we demonstrate electric-field switching of phonon angular momentum in the technologically relevant ferroelectric BaTiO3. Using circularly dichroic resonant inelastic X-ray scattering (CD-RIXS) at the oxygen K edge, we directly probe the phonon angular momentum and compare the measured dichroism with first-principles predictions of phonon-mode chirality. We find excellent agreement, revealing a momentum-dependent circular-dichroism contrast that exhibits a reversible gyroelectric effect, stable for at least 15 hours. Our results establish a robust mechanism for non-volatile control of chiral phonons and point towards new opportunities for phonon-based information and energy technologies.

cond-mat.mtrl-sci↗

arXiv2026

Hierarchical quasiparticle dynamics in antiferromagnets revealed by time- and momentum-resolved X-ray scattering

Energy flows among coupled subsystems are essential for ultrafast dynamics and high-speed technologies. In magnetic materials, spin fluctuations -- magnons -- mediate these flows in ultrafast magnetism. Yet momentum-resolved access to low-energy magnons governing the microscopic dynamics has been lacking. Using time-resolved resonant diffuse scattering alongside complementary time-resolved X-ray techniques and quantum-kinetic simulations, we unveil the hierarchical energy pathways among correlated systems in the photoexcited antiferromagnet CuO. Above-bandgap excitation triggers near-instantaneous spin disorder, generating non-thermal magnons throughout reciprocal space within femtoseconds. Real-time momentum-resolved tracking reveals picosecond magnon quasi-thermalization, followed by nanosecond recovery via momentum-selective magnon-phonon scattering. The quasiparticle dispersion mismatch creates recovery bottlenecks that control non-equilibrium lifetimes. This microscopic framework transcends phenomenological models and generalizes across materials, establishing design principles for ultrafast control of material properties.

cond-mat.str-el↗

arXiv2026

Ultrafast Decoherence of Charge Density Waves in K$_{0.3}$MoO$_{3}$

Recent works have suggested that transient suppression of a charge density wave (CDW) by an ultra-short excitation can lead to an inversion of the CDW phase. We experimentally investigate the dynamics of the CDW in K$_{0.3}$MoO$_{3}$ by time resolved x-ray diffraction after excitation with optical pulses. Our results indicate a transient inversion of the CDW phase close to the surface that evolves into a highly disordered state in less than one picosecond. Numerical simulations solving the Ginzburg-Landau equation including disorder from strong pinning defects reproduce our main observations. Our findings highlight the critical role of disorder in schemes for coherent control in condensed matter systems.

cond-mat.str-el↗

arXiv2026

Spin waves excited by hard x-ray transient gratings

Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and non-magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.

physics.optics↗

arXiv2025

High frame rate RIXS spectroscopy using a JUNGFRAU detector with an iLGAD sensor

Resonant inelastic X-ray scattering (RIXS) is a powerful photon-in, photon-out spectroscopy technique for probing electronic, magnetic, and lattice excitations in matter. Time-resolved RIXS extends this capability through a stroboscopic optical pump-probe scheme to characterize the time evolution of the photoexcitation and subsequent relaxation dynamics of a sample. This technique is, however, extremely photon-hungry, requiring high-repetition-rate and intense X-ray facilities. The Heisenberg RIXS (hRIXS) spectrometer at the Spectroscopy and Coherent Scattering (SCS) instrument of the European X-ray Free-Electron Laser (EuXFEL) is designed to exploit high-repetition-rates, while maintaining optimal time and energy resolution. In this work, we demonstrate the successful deployment of a JUNGFRAU detector equipped with an inverse Low Gain Avalanche Diode (iLGAD) sensor for time-resolved RIXS studies in the soft X-ray range, using the hRIXS spectrometer. A spatial resolution of $19.71 \pm 0.7~\mu\mathrm{m}$ and a resolving power exceeding 10,000 were achieved at an unprecedented frame rate of 47 kHz. Intra-train resolved data measured with a high FEL peak fluence of $1.8~\mathrm{mJ/cm^{2}}$ for a 928.5 eV ph photon energy and 1.1 MHz repetition rate from cupric oxide (CuO) revealed a decrease in the emitted signal by ~10% over a time interval of $340~\mu\mathrm{s}$, indicating FEL-induced effects that require monitoring when conducting high-repetition-rate experiments. These results establish the JUNGFRAU-iLGAD as a promising detector to harvest the full potential of the hRIXS spectrometer, and validate its suitability for soft X-ray applications.

physics.ins-det↗

arXiv2025

Low energy excitations in A-site ordered SmBaMn2O6

The electron in a solid can be considered a bound state of the three independent, fundamental degrees of freedom creating quasi-particles: spinons, carrying the electron spin; plasmons carrying the collective charge mode and orbitons carrying its orbital degree of freedom. These fundamental degrees of freedom could form ordering states in which dynamics or collective motions could occurr and manifest as low-energy excitations. The exotic properties that appear in the materials exhibiting these electronic orderings are associated with these low-energy excitations. Although the orbital order (OO) and its coupling to the spin system creates very interesting phenomena, the microscopic origin of OO has been much less explored than other electronic properties as it is very difficult to directly access experimental information from OO. Due to the recent improvement in energy resolution and flux, soft x-ray resonant inelastic scattering (RIXS) allows for a re-examination of orbital excitations in manganites. Here, we present a study of low energy excitations in half doped A-site ordered SmBaMn_{2}O_{6} through a combination of RIXS and soft x-ray resonant elastic scattering (REXS) measurements. The obtained experimental data confirm the OO at \mathbf{q} = (0.25, 0.25, 0) and find various low energy excitations below 200 meV. while several excitations can be assigned to be of magnetic and phononic origin, a group of excitations between 80 and 200 meV show a temperature dependence distinctively following that of the OO making them possible candidates for orbital excitations.

cond-mat.str-el↗

arXiv2025

Tuning chirality amplitude at ultrafast timescales

Chirality is a fundamental symmetry concept describing discrete states, i.e., left-handed, right-handed, or achiral, and existing at disparate scales and in many categories of scientific fields. Even though symmetry breaking is indispensable for describing qualitatively distinct phenomena, symmetry cannot quantitatively predict measurable quantities. One can continuously distort an object, introducing the concept of chirality amplitude, similar to representing magnetization as the amplitude of time-reversal symmetry breaking. Considering the role of magnetization in emergent phenomena with time-reversal symmetry breaking, chirality amplitude is intuitively a key quantity for controlling chirality-related emergent phenomena. Here, we propose two types of chiral lattice distortions and demonstrate the tunability of their amplitude in ultrafast timescales. Resonant X-ray diffraction with circular polarization is an established technique to measure crystal chirality directly. We quantify the ultrafast change in chirality amplitude in real time after an optical excitation. Using instead a THz excitation, we observe oscillations in the resonant diffraction intensities corresponding to specific phonon frequencies. This indicates the creation of additional asymmetry, which could also be described as an enhancement in chirality amplitude. Our proposed concept of chirality amplitude and its ultrafast control may lead to a unique approach to control chirality-induced emergent phenomena in ultrafast timescales.

cond-mat.str-el↗

arXiv2025

Chiral phonons in polar LiNbO3

Quasiparticles describe collective excitations in many-body systems, and their symmetry classification is of fundamental importance because they govern physical processes on various timescales, e.g., excited states, transport phenomena, and phase transitions. Recent studies have revealed that quasiparticles can possess chirality and that this degree of freedom leads to various important phenomena. Among them, chiral phonons have recently attracted significant interest because of their intrinsic magnetism, which non-trivially bridges the spin system and the lattice. Here, we directly prove the presence of chiral phonons in a prototypical polar crystal LiNbO3. Our demonstration adds a polar crystal in the showcase of materials hosting chiral phonons and, furthermore, creates a substantial potential in chiral phononics because of its expected in-situ switchable phonon chirality and associated control of phonon angular momentum.

cond-mat.str-el↗

arXiv2025

Circular dichroism in resonant inelastic x-ray scattering from birefringence in CuO

Resonant inelastic x-ray scattering (RIXS) has become a prominent technique to study quasiparticle excitations. With advances in polarization analysis capabilities at different facilities, RIXS offers exceptional potential for investigating symmetry-broken quasiparticles like chiral phonons and magnons. At optical wavelengths birefringence can severely affect polarization states in low-symmetry systems. Here we show its importance for soft x-ray resonances. Given the growing interest in Circular Dichroism (CD) in RIXS, it is important to evaluate how birefringence may affect the RIXS spectra of anisotropic systems. We investigate CuO, a well-known anisotropic material, using Cu $L_3$-edge RIXS and detect significant CD in both magnetic and orbital excitations in the collinear antiferromagnetic phase. We demonstrate that the CD can be modeled by a proper treatment of RIXS scattering amplitudes derived from single-ion calculations with birefringence. Recognizing these effects is crucial for unambiguous identification of subtle dichroic effects induced by symmetry-broken quasiparticles. Furthermore, the combined sensitivity of RIXS and birefringence to local symmetry presents an opportunity to study microscopic changes driven by external perturbations.

cond-mat.mtrl-sci↗

arXiv2024

Non-chiral 1T-TiSe2 creates circular dichroism in resonant X-ray diffraction via multipole scattering interference

Resonant X-ray diffraction (RXD) provides a unique capability to investigate electronically ordered states in matter. Importantly, circular dichroism in RXD can determine the absolute chirality formed by anisotropic multipole arrangement. Here, we demonstrate that the scattering interference between distinct atomic electric multipoles can create circular contrast in resonant RXD even in a non-chiral crystal. These general considerations are applied to 1T-TiSe2, where recently reported circular dichroism in RXD was used to claim a chiral charge density wave order. We show that the experimental observations are well reproduced by our ab initio calculations based on the centrosymmetric crystal structure. Our results not only illuminate a way for a better understanding of possible chiral electronic order but also propose a general approach to extract buried high-order atomic multipoles via circular contrast in RXD. Besides, we discuss the difference in the circular dichroism of RXD originated by crystal chirality in chiral crystals and the interference effect in non-chiral crystals.

cond-mat.str-el↗

arXiv2024

Element-specific, non-destructive profiling of layered heterostructures

Fabrication of semiconductor heterostructures is now so precise that metrology has become a key challenge for progress in science and applications. It is now relatively straightforward to characterize classic III-V and group IV heterostructures consisting of slabs of different semiconductor alloys with thicknesses of $\sim$5 nm and greater using sophisticated tools such as X-ray diffraction, high energy X-ray photoemission spectroscopy, and secondary ion mass spectrometry. However, profiling thin layers with nm or sub-nm thickness, e.g. atomically thin dopant layers ($\delta$-layers), of impurities required for modulation doping and spin-based quantum and classical information technologies is more challenging. Here, we present theory and experiment showing how resonant-contrast X-ray reflectometry meets this challenge. The technique takes advantage of the change in the scattering factor of atoms as their core level resonances are scanned by varying the X-ray energy. We demonstrate the capability of the resulting element-selective, non-destructive profilometry for single arsenic $\delta$-layers within silicon, and show that the sub-nm electronic thickness of the $\delta$-layers corresponds to sub-nm chemical thickness. In combination with X-ray fluorescence imaging, this enables non-destructive three-dimensional characterization of nano-structured quantum devices. Due to the strong resonances at soft X-ray wavelengths, the technique is also ideally suited to characterize layered quantum materials, such as cuprates or the topical infinite-layer nickelates.

quant-ph↗

arXiv2024

Phonon dispersion of quantum paraelectric SrTiO3 in electric fields

Here we report on an elastic and inelastic neutron scattering study addressing the effect of electric fields on quantum paraelectric SrTiO3. Our elastic scattering results find small changes as a function of field in a superlattice reflection that sample the octahedral rotations, which is indicative of only weak coupling of octahedral rotation and electric polarization. By collecting not only the change in gap, but also of the dispersion, we can better quantify the changes in the lattice dynamics. The findings are put in context to recent field DFT calculations predicting the E-field effect on the atomic motions of the lowest lying transverse optical (soft) mode. We find hints of non-linear coupling to the acoustic mode and to the phonon with polarization perpendicular to the E-field, which shows the non-linearity in the chemical potential that is also relevant when strongly driving SrTiO3 with E-field (THz) pulses.

cond-mat.mtrl-sci↗

arXiv2024

Controlling 4f antiferromagnetic dynamics via itinerant electronic susceptibility

Optical manipulation of magnetism holds promise for future ultrafast spintronics, especially with lanthanides and their huge, localized 4f magnetic moments. These moments interact indirectly via the conduction electrons (RKKY exchange), influenced by interatomic orbital overlap, and the conduction electron susceptibility. Here, we study this influence in a series of 4f antiferromagnets, GdT2Si2 (T=Co, Rh, Ir), using ultrafast resonant X-ray diffraction. We observe a twofold increase in ultrafast angular momentum transfer between the materials, originating from modifications in the conduction electron susceptibility, as confirmed by first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2024

Octahedral and polar phase transitions in freestanding films of SrTiO3

From extreme strain to bending, the possibilities in the manipulation of freestanding films of oxide perovskites bring a novel landscape to their properties and brings them one step closer to their application. It is therefore of great importance to fully understand the inherent properties of such films, in which dimensionality and surface effects can play a major role in defining the properties of the materials ground state. This paper reports the properties of freestanding (FS) films of the canonical oxide, SrTiO3 (STO) with thicknesses 20, 30, 40 and 80 nm. We show that the relaxed ultrathin STO FS films become polar at temperatures as high as 85 K, in contrast to the quantum paraelectric behavior of bulk. Our findings are based on the softening of the ferroelectric mode towards the ferroelectric transition temperature Tc and its consecutive hardening below Tc with further decreasing temperature, probed with THz time domain spectroscopy in transmission mode. We find almost no thickness dependence in Tc. Moreover, we characterize the antiferrodistortive (AFD) phase transition in STO FS by X-ray diffraction (XRD) probing superlattice reflections characteristic for the rotation of the TiO6 octahedra. Our results point to a higher phase transition temperature in comparison to bulk STO, as well as an unbalanced domain population favoring the rotation axis to be in plane. X-ray linear dichroism results further show a preferential Ti xz/yz orbital occupancy at the surface, but with a complete degeneracy in the t2g states in the inner part of the film indicating that the AFD distortion does not strongly affect the t2g splitting. These findings demonstrate that STO FS films have clearly different properties than bulk.

cond-mat.mtrl-sci↗

arXiv2024

Transient grating spectroscopy on a DyCo$_5$ thin film with femtosecond extreme ultraviolet pulses

Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo$_5$ alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser (FEL) with the wavelength of 20.8\,nm matching the Co $M$-edge, resulting in a SAW wavelength of $Λ=44$\,nm. Using the pump-probe transient grating scheme in a reflection geometry the excited SAWs could be followed in the time range of -10 to 100\,ps in the thin film. Coherent generation of TGs by ultrafast EUV pulses allows to excite SAW in any material and to investigate their couplings to other dynamics such as spin waves and orbital dynamics. In contrast, we encountered challenges in detecting electronic and magnetic signals, potentially due to the dominance of the larger SAW signal and the weakened reflection signal from underlying layers. A potential solution for the latter challenge involves employing soft X-ray probes, albeit introducing additional complexities associated with the required grazing incidence geometry.

cond-mat.mtrl-sci↗

arXiv2023

Provoking topology by octahedral tilting in strained SrNbO$_3$

Transition metal oxides with a wide variety of electronic and magnetic properties offer an extraordinary possibility to be a platform for developing future electronics based on unconventional quantum phenomena, for instance, the topology. The formation of topologically non-trivial states is related to crystalline symmetry, spin-orbit coupling, and magnetic ordering. Here, we demonstrate how lattice distortions and octahedral rotation in SrNbO$_3$ films induce the band topology. By employing angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we verify the presence of in-phase $a^0a^0c^+$ octahedral rotation in ultra-thin SrNbO$_3$ films, which causes the formation of topologically-protected Dirac band crossings. Our study illustrates that octahedral engineering can be effectively exploited for implanting and controlling quantum topological phases in transition metal oxides.

cond-mat.mtrl-sci↗