arXiv subjects

Jacob Holder

Publications and source records attributed to Jacob Holder.

3 recordsLinked to original sources

arXiv2026

Localized Photon Absorption in a Single-Crystalline Material

The absorption of light is one of the most fundamental processes in condensed-matter physics and optics. Here we investigate under which conditions laser light is absorbed by a crystalline material as an electromagnetic wave with delocalized properties or rather as photons that cause discrete, localized, nanometer-sized consequences. We excite the first-order phase transition of vanadium dioxide with laser pulses of sufficient frequency to overcome the band gap but with insufficient pulse energy to overcome the latent heat. According to Maxwell's equations and Bloch theory, no transition should occur, because nowhere in the material is enough energy. Nevertheless, we observe with ultrafast electron diffraction a disordered crystal geometry with nanometer-sized spots of switched material that grow and diminish with time. The amount of localized spots matches approximately to the number of photons in the absorbed laser wave. Two optical experiments substantiate this phenomenon, and simulations reproduce all measurements results. We discuss whether crystals defects, temperature, or a genuine wavefunction collapse can explain the discovered phenomenon. Practically, the reported absorption mechanism enables local consequences at substantially higher energy than average and provides insight into symmetry breaks and non-thermal fluctuations within complex materials.

physics.optics↗

arXiv2025

Two-electron quantum walks can probe entanglement and decoherence in an electron microscope

Classical physics is often a good approximation for quantum systems composed of many interacting particles, although wavepacket dispersion and scattering processes continuously induce delocalization and entanglement. According to decoherence theory, an entangled ensemble can appear classical when only a subset of all particles is observed. This emergence of macroscopic phenomena from quantum interactions is, for example, relevant for phase transitions, quantum thermalization, hydrodynamics, spin liquids, or time crystals. However, entanglement and decoherence in free electrons have not yet been explored, although the electron is a fundamental elementary particle with extraordinary technological relevance. Here, we investigate the degree of coherence and entanglement in a free-space electron gas in the beam of an ultrafast electron microscope. We introduce a two-electron quantum walk that transforms the quantum state into different bases for quantum state tomography of entangled or partially entangled electron-electron pairs. We apply this novel diagnostic to study quantum effects in short pulses of hundreds of electrons under strong Coulomb correlation. We observe a high contrast interference in the electron-electron correlations but no significant signs of electron-electron entanglement which we explain by limited purity of the initial states and decoherence effects from unmeasured reservoir electrons. The ability to characterize quantum states of multiple free electrons may allow verification of electron-electron entanglement for use in fundamental studies and quantum electron microscopy.

quant-ph↗

arXiv2023

Determination of the nearest-neighbor interaction in VO$_2$ via fractal dimension analysis

The Ising model is one of the simplest and most well-established concepts to simulate phase transformations in complex materials. However, its most central constant, the interaction strength J between two nearest neighbors, is hard to obtain. Here we show how this basic constant can be determined with a fractal dimension analysis of measured domain structures. We apply this approach to vanadium dioxide, a strongly correlated material with a first-order insulator-to-metal phase-transition with enigmatic properties. We obtain a nearest-neighbor interaction of 13.8 meV, a value close to the thermal energy at room temperature. Consequently, even far below the transition temperature, there are spontaneous local phase-flips from the insulating into the metallic phase. These fluctuations explain several measured anomalies in VO$_2$, in particular the low thermal carrier activation energy and the finite conductivity of the insulating phase. As a method, our fractal dimension analysis links the Ising model to macroscopic material constants for almost any first-order phase transition.

cond-mat.stat-mech↗