We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of ${\mathrm{Co}}_{0.975}{\mathrm{Ge}}_{0.025}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ (Ge-CCO) and ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$ (CCO) under low and high magnetic fields from 0.2 to 6.5 T. A characterization of Ge-CCO at a low magnetic field was performed, and the results were compared with those of pure CCO. The ferrimagnetic phase transition temperature ${T}_{C}\ensuremath{\approx}95\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the multiferroic transition temperature ${T}_{S}\ensuremath{\approx}27\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in Ge-CCO are comparable with those observed in CCO. In Ge-CCO, the ordering wave vector ($qq0$) observed below ${T}_{S}$ is slightly larger compared with that of CCO, and unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity, and therefore the electric polarization, was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.
We photoexcite SrTiO 3 and EuTiO 3 in their purely soft-mode-driven structurally distorted phase and trace the structural order parameter via ultra-short x-rays. We observe a rapid decay for SrTiO 3 and an intriguing transient enhancement for EuTiO3.
The objective of this thesis is to elucidate the femtosecond dynamics of coupled low-energy excitations in both strongly correlated materials and artificially engineered quantum structures. By means of near-infrared pump/multi-THz probe spectroscopy and a series of technological innovations (a novel collinear four-pass Ti:Sapphire amplifier and shot-noise reduced electro-optic sampling), fundamentally new insights into the many-body physics of two representative strongly correlated materials are obtained:
- It is clarified which microscopic mechanisms underlie the formation of the charge density wave in the transition-metal dichalcogenide titanium diselenide (1T-TiSe2).
- A study of the high-temperature superconductor YBCO reveals that there exists no temporal hierarchy between electron-electron and electron-phonon scattering processes in this system.
Furthermore, non-adiabatic activation of ultrastrong light-matter interaction between a tailor-cut photonic resonance and an electronic excitation is realized and studied on a sub-cycle timescale. This further paves the way towards the observation of novel quantum-electrodynamical phenomena.
Time- and angle-resolved photoemission spectroscopy data of bulk terbium tritelluride (TbTe3, unidirectional charge-density-wave phase, T=100K) using a laser-based femtosecond XUV source and a hemispherical analyzer for photoelectron detection at the Fritz-Haber-Institute, Berlin, Germany. The 3D (angle, energy, pump-probe-delay) datasets include the photoemission intensities for various pump-laser fluences. The time-resolved X-ray diffraction data were obtained at the Femto hard X-ray slicing source at the Swiss Light Source, and include the charge-density-wave superlattice (2 10 1+q_CDW) peak intensities as functions of pump-probe-delay for various pump-laser fluences. The data and associated metadata are stored in the NeXus data format (https://www.nexusformat.org/).
Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, due to their direct energy gaps in the optical range. Their electronic and optical properties, however, are complicated by exotic room-temperature excitons, whose fundamental structure and dynamics has been under intense investigation. While interband spectroscopy probes energies of excitons with vanishing centre-of-mass momenta, the majority of excitons has remained elusive, raising questions about their unusual internal structure, symmetry, many-body effects, and dynamics. Here we report the first direct experimental access to all relevant excitons in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates a record fast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.
We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic ${\mathrm{TbMnO}}_{3}$ by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. The use of orthogonal linear x-ray polarizations provides information on the contribution from the different magnetic moment directions, which can be interpreted as signatures from multiferroic cycloidal spin order and sinusoidal spin order. Tracking these signatures in the time domain enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond time scale. The transient phase is shown to exhibit mostly spin density wave character, as in the adiabatic case, while nevertheless retaining the wave vector of the cycloidal long-range order. Two different pump photon energies, 1.55 and 3.1 eV, lead to population of the conduction band predominantly via intersite $d\ensuremath{-}d$ or intrasite $p\ensuremath{-}d$ transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a time scale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic order and orbital reconstruction. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such theoretical studies.
In ultrabroadband terahertz electro-optic sampling (EOS), spectral filtering of the gate pulse can strongly reduce the quantum noise while the signal level is only weakly affected. The concept is tested for phase-matched electro-optic detection of field transients centered at 45 THz with 12 fs near-infrared gate pulses in AgGaS2. Our new approach increases the experimental signal-to-noise ratio by a factor of 3 compared to standard EOS. Under certain conditions an improvement factor larger than 5 is predicted by our theoretical analysis.
Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter coupled excitations, such as exciton polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized upon macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates.
Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter-coupled excitations, such as exciton-polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates.
