arXiv subjects

Christoph Emeis

Publications and source records attributed to Christoph Emeis.

7 recordsLinked to original sources

arXiv2026

X-ray driven displacive excitation of coherent phonons

Modulating electron-phonon coupling offers a route to control structural displacements and tune material functionality. Valence-to-conduction band transitions, however, provide limited leverage over the driving force. Here, we demonstrate coherent lattice dynamics in trigonal tellurium using free-electron laser pulses tuned to the Te N4,5-edge. Over a broad fluence range, the oscillation amplitude obeys the displacive excitation of coherent phonons framework, extended to core resonance with twice the driving efficiency of a visible pump. Ab initio calculations decompose the force into competing multiband contributions, inaccessible to optical excitation, whose balance shifts as carriers relax. Tunable extreme-ultraviolet and X-ray pulses thus open a regime in which the displacive response is set by band-dependent coupling to the lattice, not by the number and temperature of the photocarriers alone.

cond-mat.mtrl-sci↗

arXiv2026

Nonlinear electron-phonon coupling drives light-induced symmetry switching in charge-density waves

Ultrafast optical excitation in charge-density wave (CDW) crystals can transiently suppress long-range order, driving the lattice toward higher symmetry on femtosecond timescales. Here, we formulate and implement a first-principles theory of light-induced melting of CDW order. The approach is based on the structural dynamics in the Heisenberg picture, and it explicitly accounts for quartic lattice anharmonicities, nonlinear electron-phonon interactions, and photoexcitation-induced modifications of the potential energy surface. We illustrate these concepts through first-principles calculations of the ultrafast melting of CDW order in monolayer TiSe$_2$ - a prototypical CDW crystal with a 2$\times$2 structural reconstruction. The simulations are in good agreement with existing experiments, and they capture the defining features of CDW melting, such as the damped coherent structural motion, the transient renormalization of the soft mode, and the restoration of CDW order over timescales of a few picoseconds. Besides identifying nonlinear electron-phonon interactions as the primary mechanism driving symmetry switching in CDW systems, our work establishes a generally applicable theoretical framework to treat quartic anharmonicities and light-induced phase transitions in first-principles ultrafast dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2026

Coherent Phonon-Driven Band Renormalizations in 1T$'$-MoTe$_2$

Here, we investigate phonon mode- and electron band-selective electron-phonon couplings in centrosymmetric 1T$'$-MoTe$_2$ using time- and angle-resolved photoemission spectroscopy combined with frequency-domain analysis. Femtosecond near-infrared pulses excite coherent $A_g$-symmetric phonon modes at 2.34 THz, 3.34 THz, and 3.86 THz, which manifest as oscillatory modulations in photoemission intensity and binding energy across the valence bands. Pixel-wise Fourier analysis using recently developed methodologies reveals pronounced band selectivity with distinct coupling strengths for different electronic states and phonon modes, enabling the evaluation of band-renormalization amplitudes in the range of few meV. Ab initio calculations qualitatively reproduce the experimentally observed coupling patterns and relative trends, demonstrating the capability of combined experimental and theoretical approaches to resolve ultrafast electron-phonon interactions in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

Watching Polarons Form in Real Time

Polaron formation in pump-probe experiments is an inherently non-equilibrium phenomenon, driven by the ultrafast coupled dynamics of electrons and phonons, and culminating in the emergence of a localized quasiparticle state. In this work, we present a first-principles quantum-kinetic theory of polaron formation that captures the real-time evolution of electronic and lattice degrees of freedom in presence of electron-phonon coupling. We implement this framework to investigate the ultrafast polaron formation in the prototypical polar insulator MgO. This approach allows us to determine the characteristic timescales of polaron localization and to identify its distinctive dynamical fingerprint. Our results establish clear and experimentally accessible criteria for identifying polaron formation in pump-probe experiments.

cond-mat.mtrl-sci↗

arXiv2025

Coherent phonon control beyond amplitude saturation in a sliding ferroelectric

The breakdown of Hooke's law marks the onset of nonlinear behaviour: when displacements become large, restoring forces weaken and conventional proportionality fails. In quantum materials, intense optical excitation can drive the crystal lattice into a similar regime, where established linear relations between light, electrons, and phonons no longer hold. Sliding ferroelectrics are particularly susceptible, as controlling their polarization requires large interlayer shifts. Displacive excitation of coherent phonons, the principal mechanism for launching structural motion, typically assumes that lattice-driving forces scale linearly with the photo-excited carrier density. Whether this linearity survives at high excitation, however, remains largely unexplored, and its breakdown can fundamentally limit accessible lattice displacements. Here we show that such nonlinear limitations can be surpassed in a sliding ferroelectric by timing, rather than strengthening the optical drive. Time-resolved second-harmonic generation reveals that the interlayer sliding phonon governing ferroelectricity saturates and even diminishes under single-pulse excitation. First-principles calculations attribute this nonlinearity to band-specific electron-phonon coupling that induces competing forces on the lattice. By splitting the optical energy into two well-timed pulses that avoid populating counteracting states, we achieve markedly larger phonon amplitudes at fixed total fluence. The resulting enhanced sliding motion exposes a regime of anharmonic phonon coupling that emerges only far from equilibrium. Our findings show that nonlinear limits in driven solids can be overcome, opening new pathways for steering lattice motion in quantum materials.

physics.optics↗

arXiv2025

Origin of phonon decoherence

Phonon decoherence determines the characteristic timescales over which coherent lattice vibrations decay, making it a crucial process for understanding the non-equilibrium dynamics of crystal lattices after excitation by a pump pulse. Here, we report a theoretical and computational investigation of the origin of phonon decoherence within a first-principles many-body framework. We derive quantum kinetic equations for the dynamics of coherent phonons by explicitly accounting for dissipation processes induced by electron-phonon and phonon-phonon interactions. The decoherence rate and frequency renormalization are formulated in terms of the non-equilibrium phonon self energy, providing a framework amenable for ab initio calculations. To validate this approach, we conduct a first-principles study of phonon decoherence for the elemental semimetals antimony and bismuth. The robust agreement with available temperature- and fluence-dependent experimental data confirms the accuracy of our theoretical and computational framework. More generally, our findings reveal that either electron-phonon and phonon-phonon coupling can prevail in determining the decoherence time, depending on the temperature and driving conditions. Overall, this work fills a critical gap in the theoretical understanding of phonon decoherence, providing a predictive framework for determining the timescales of light-induced structural dynamics in driven solids.

cond-mat.str-el↗

arXiv2023

Plasmonic polarons induced by alkali-atom deposition in hafnium disulfide (1$T$-HfS$_2$)

We combine ab-initio calculations based on many-body perturbation theory and the cumulant expansion with angle-resolved photoemission spectroscopy (ARPES) to quantify the electron-plasmon interaction in the highly-doped semiconducting transition metal dichalcogenide 1$T$-HfS$_2$. ARPES reveals the emergence of satellite spectral features in the vicinity of quasiparticle excitations at the bottom of the conduction band, suggesting coupling to bosonic excitations with a characteristic energy of 200 meV. Our first-principles calculations of the photoemission spectral function reveal that these features can be ascribed to electronic coupling to carrier plasmons (doping-induced collective charge-density fluctuations). We further show that reduced screening at the surface enhances the electron-plasmon interaction and is primarily responsible for the emergence of plasmonic polarons.

cond-mat.str-el↗