arXiv subjects

Felix Kurtz

Publications and source records attributed to Felix Kurtz.

5 recordsLinked to original sources

arXiv2026

Surface-Sensitive Mapping of Anisotropic Phonon Cascades in T$_{d}$-WTe$_{2}$

Understanding how energy flows from photoexcited carriers into the lattice is essential for describing nonequilibrium phenomena in low-symmetry quantum materials. Here, we use ultrafast low-energy electron diffraction and diffuse scattering to probe momentum-resolved phonon dynamics at the surface of T$_d$-WTe$_2$, a strongly anisotropic semimetal. Following optical excitation, the Debye--Waller suppression of Bragg peaks exhibits a biexponential increase of the mean-squared atomic displacement, indicating sequential lattice relaxation. Analysis of the diffuse background reveals a preferential intensity build-up parallel to the tungsten-chain axis in the material, attributed to anisotropic electron--phonon coupling during electronic cooling which precedes anharmonic phonon--phonon scattering and subsequent thermalization across the surface Brillouin zone. The results identify a hierarchical relaxation pathway in which energy is first deposited into selected finite-momentum phonons before spreading through the broader lattice bath. Our work highlights the importance of momentum-resolved diffuse scattering for disentangling electron--phonon and phonon--phonon relaxation in anisotropic topological semimetals.

cond-mat.mtrl-sci↗

arXiv2024

Evidence for reduced periodic lattice distortion within the Sb-terminated surface layer of the kagome metal CsV$_3$Sb$_5$

The discovery of the kagome metal CsV$_3$Sb$_5$ sparked broad interest, due to the coexistence of a charge density wave (CDW) phase and possible unconventional superconductivity in the material. In this study, we use low-energy electron diffraction (LEED) with a $\mu$m-sized electron beam to explore the periodic lattice distortion at the antimony-terminated surface in the CDW phase. We recorded high-quality backscattering diffraction patterns in ultrahigh vacuum from multiple cleaved samples. Unexpectedly, we did not find superstructure reflexes at intensity levels predicted from dynamical LEED calculations for the reported $2 \times 2 \times 2$ bulk structure. Our results suggest that in CsV$_3$Sb$_5$ the periodic lattice distortion accompanying the CDW is less pronounced at Sb-terminated surfaces than in the bulk.

cond-mat.str-el↗

arXiv2024

Optical switching of ferro-rotational charge-density wave states

Tailored optical excitations can steer a system along non-equilibrium pathways to metastable states with specific structural or electronic properties. The light-induced hidden state of 1T-TaS$_{2}$, with its strongly enhanced conductivity and exceptionally long lifetime, represents a unique model system for studying the ultrafast switching of correlated electronic states. We use surface-sensitive electron diffraction in combination with a femtosecond optical quench to reveal the coexistence of both charge-density-wave (CDW) 2D chiralities as a structural characteristic of the hidden state, corresponding to coexisting ferro-rotational CDW states. Density functional theory (DFT) simulations of interfaces between opposite CDW 2D chiralities predict a higher-level, fractal-type moir'{e} superstructure with a kagome band structure near the Fermi energy. More broadly, these findings suggest that heterochiral interfaces in CDW systems provide an additional structural degree of freedom, expanding the possibilities for electronic control via twist-angle engineering.

cond-mat.mes-hall↗

arXiv2024

Valley-controlled photoswitching of metal-insulator nanotextures

Spatial heterogeneity and phase competition are hallmarks of strongly-correlated materials, promising tunable functionality on the nanoscale. Light-induced switching of a correlated insulator to a metallic state is well established. However, optical excitation generally lacks the specificity to select sub-wavelength domains and control final textures. Here, we employ valley-selective photodoping to drive the domain-specific quench of a textured Peierls insulator. Polarized excitation leverages the anisotropy of quasi-one-dimensional states at the correlated gap to initiate an insulator-to-metal transition with minimal electronic heating. We find that averting dissipation facilitates domain-specific carrier confinement, control over nanotextured phases, and a prolonged lifetime of the metastable metallic state. Complementing existing manipulation schemes, valley-selective photoexcitation will enable the activation of electronic phase separation beyond thermodynamic limitations, facilitating optically-controlled hidden states, engineered heterostructures, and polarization-sensitive percolation networks.

cond-mat.str-el↗

arXiv2019

Nonlinear Spectroscopy and All-Optical Switching of Femtosecond Soliton Molecules

The emergence of confined structures and pattern formation are exceptional manifestations of concurring nonlinear interactions found in a variety of physical, chemical and biological systems[1]. Optical solitons are a hallmark of extreme spatial or temporal confinement enabled by a variety of nonlinearities. Such particle-like structures can assemble in complex stable arrangements, forming "soliton molecules"[2,3]. Recent works revealed oscillatory internal motions of these bound states, akin to molecular vibrations[4-8]. These observations beg the question as to how far the "molecular" analogy reaches, whether further concepts from molecular spectroscopy apply in this scenario, and if such intra-molecular dynamics can be externally driven or manipulated. Here, we probe and control such ultrashort bound-states in an optical oscillator, utilizing real-time spectroscopy and time-dependent external perturbations. We introduce two-dimensional spectroscopy of the linear and nonlinear bound-state response and resolve anharmonicities in the soliton interaction leading to overtone and sub-harmonic generation. Employing a non-perturbative interaction, we demonstrate all-optical switching between distinct states with different binding separation, opening up novel schemes of ultrafast spectroscopy, optical logic operations and all-optical memory.

physics.optics↗