arXiv subjects

Henrik Lemke

Publications and source records attributed to Henrik Lemke.

9 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

Emergent Antiphase Stacking in a Transient Charge Density Wave

Photoexcitation can induce novel states in materials that are inaccessible in equilibrium, a recent example being the light-induced charge density wave (CDW) observed in $\text{LaTe}_3$. Here, we investigate this transient CDW using infrared-pump x-ray-probe scattering at a free-electron laser, with high momentum and time resolution. We find that the transient CDW Bragg peak is broad in reciprocal space, indicating a highly disordered state. The ordering wavevector of the transient state is different from the equilibrium orders that develop in this class of materials - the transient peak appears near (2/7, 0, 0) reciprocal lattice units, whereas the equilibrium $a$ order and $c$ order occur at $\approx (5/7, 0, 0)$ and $(0, 0, 2/7)$, respectively. The transient CDW is therefore distinct from the equilibrium $a$ order, differing in the relative phase of the CDW displacement between the two equivalent nearly-square Te-Te nets in the conventional unit cell. Our work highlights how photoexcitation can access states with no equilibrium analog, and how x-ray scattering can provide microscopic insight into such elusive phases.

cond-mat.str-el↗

arXiv2026

A textured polar phase in strained SrTiO3

Quantum materials can harbour hidden phases whose microscopic structures differ from conventional ordered states while reproducing their macroscopic signatures, making them easy to miss. Strontium titanate is a longstanding puzzle of this kind: on cooling it shows every hallmark of an incipient ferroelectric, yet never orders, and is usually described as a quantum paraelectric in which fluctuations suppress ferroelectricity. Here we combine uniaxial strain, single-cycle terahertz excitation and femtosecond x-ray scattering to measure the polar collective modes of strontium titanate as a function of momentum and strain. Under modest tensile strain, we observe a new vibrational mode that emerges not at the Brillouin zone centre, as a ferroelectric transition would require, but at finite wavevector, identifying the ordered state as a polar texture on nanometre length scales rather than a uniform ferroelectric. Unstrained quantum paraelectric strontium titanate is then naturally understood as the disordered precursor of this textured phase, offering a resolution to a decades-old puzzle and illustrating how finite-momentum collective excitations can unmask hidden phases in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Dimensionality reduction of optically generated vortex strings in a charge density wave

In phase transitions, mesoscale structures such as topological defects, vortex strings, and domain walls control the path towards equilibrium, and thus the functional properties of many active devices. In photoinduced phase transitions driven by femtosecond laser excitation, the temporal (pulse duration) and spatial (penetration depth) structure of the optical excitation present opportunities for control and creating structures with unique topologies. By performing time-resolved optical pump, x-ray probe experiments on the CDW system Pd-intercalated ErTe$_{3}$, we gain access to the nanoscale dynamics of the mesoscale topological features (vortex strings) produced after a quench, which have a different apparent dimensionality than the topological defects predicted from the bulk system. We show that these vortex strings persist for much longer than the electronic recovery time. The critical exponent obtained from power-law scaling of the intensity as a function of wavevector shows a reduction in the effective dimensionality of the topological defects in the system, corroborated by time-dependent Ginzburg-Landau simulations. Our results demonstrate a novel pathway to use light to control the dimensionality and orientation of topological defects in quantum materials, which could be used to stabilize competing quantum states.

cond-mat.mes-hall↗

arXiv2024

Dynamical Scaling Reveals Topological Defects and Anomalous Evolution of a Photoinduced Phase Transition

Nonequilibrium states of quantum materials can exhibit exotic properties and enable unprecedented functionality and applications. These transient states are inherently inhomogeneous, characterized by the formation of topologically protected structures, requiring nanometer spatial resolution on femtosecond timescales to resolve their evolution. Using ultrafast total x-ray scattering at a free electron laser and a sophisticated scaling analysis, we gain unique access to the dynamics on the relevant mesoscopic lengthscales. Our results provide direct evidence that ultrafast excitation of LaTe$_3$ leads to formation of topological vortex strings of the charge density wave. These dislocations of the charge density wave exhibit anomalous, subdiffusive dynamics, slowing the equilibration process, providing rare insight into the nonequilibrium mesoscopic response in a quantum material. Our findings establish a general framework to investigate properties of topological defects, which are expected to be ubiquitous in nonequilibrium phase transitions and may arrest equilibration and enhance competing orders.

cond-mat.mtrl-sci↗

arXiv2022

Melting of magnetic order in ${\mathrm{NaOsO}}_{3}$ by fs laser pulses

NaOsO$_3$ has recently attracted significant attention for the strong coupling between its electronic band structure and magnetic ordering. Here, we used time-resolved magnetic X-ray diffraction to determine the timescale of the photoinduced \afm dynamics in NaOsO$_3$. Our measurements are consistent with a sub-100~fs melting of the \afm long-range order, that occurs significantly faster than the lattice dynamics as monitored by the transient change in intensity of selected Bragg structural reflections, which instead show a decrease of intensity on a timescale of several ps.

cond-mat.str-el↗

arXiv2020

Ultrafast electron localization in a correlated metal

Ultrafast electron delocalization induced by a fs laser pulse is a well-known process and is the initial step for important applications such as fragmentation of molecules or laser ablation in solids. It is well understood that an intense fs laser pulse can remove several electrons from an atom within its pulse duration. [1] However, the speed of electron localization out of an electron gas, the capture of an electron by ion, is unknown. Here, we demonstrate that electronic localization out of the conduction band can occur within only a few hundred femtoseconds. This ultrafast electron localization into 4f states has been directly quantified by transient x-ray absorption spectroscopy following photo-excitation of a Eu based correlated metal with a fs laser pulse. Our x-ray experiments show that the driving force for this process is either an ultrafast reduction of the energy of the 4f states, a change of their bandwidth or an increase of the hybridization between the 4f and the 3d states. The observed ultrafast electron localization process raises further basic questions for our understanding of electron correlations and their coupling to the lattice.

cond-mat.str-el↗

arXiv2018

Dynamics of the photoinduced insulator-to-metal transition in a nickelate film

The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO$_3$ 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 to 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 phase transitions.

cond-mat.str-el↗

arXiv2018

The Ultrafast Einstein-De Haas Effect

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum. Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation. The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these sub-picosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a large fraction of the angular momentum lost from the spin system on the laserinduced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we roughly estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system.

cond-mat.str-el↗