arXiv subjects

Heiko B. Weber

Publications and source records attributed to Heiko B. Weber.

At least 19 recordsLinked to original sources

arXiv2025

pynxtools: A Python framework for generating and validating NeXus files in experimental data workflows

Scientific data across physics, materials science, and materials engineering often lacks adherence to FAIR principles (Barker et al., 2022; Jacobsen et al., 2020; M. D. Wilkinson et al., 2016; S. R. Wilkinson et al., 2025) due to incompatible instrument-specific formats and diverse standardization practices. pynxtools is a Python software development framework with a command line interface (CLI) that standardizes data conversion for scientific experiments in materials science to the NeXus format (Klosowski et al., 1997; Könnecke, 2006; Könnecke et al., 2015) across diverse scientific domains. NeXus defines data storage specifications for different experimental techniques through application definitions. pynxtools provides a fixed, versioned set of NeXus application definitions that ensures convergence and alignment in data specifications across, among others, atom probe tomography, electron microscopy, optical spectroscopy, photoemission spectroscopy, scanning probe microscopy, and X-ray diffraction. Through its modular plugin architecture pynxtools provides conversion of data and metadata from instruments and electronic lab notebooks to these unified definitions, while performing validation to ensure data correctness and NeXus compliance. pynxtools can be integrated directly into Research Data Management Systems (RDMS) to facilitate parsing and normalization. We detail one example for the RDM system NOMAD. By simplifying the adoption of NeXus, the framework enables true data interoperability and FAIR data management across multiple experimental techniques.

cond-mat.mtrl-sci↗

arXiv2024

On the experimental properties of the TS defect in 4H-SiC

When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence analyses on the single defect level. They reveal that the three occurring spectral signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose three different crystallographic orientations in the basal plane, which relate to the projections of the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies, when applying mechanical strain. This dependency is quantitatively calibrated. A complementary electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS defect at 0.6 eV above the valence band. For a future identification, this accurate characterization of its optical and electronic properties along with their response to mechanical strain is a milestone.

physics.app-ph↗

arXiv2024

NOMAD CAMELS: Configurable Application for Measurements, Experiments and Laboratory Systems

NOMAD CAMELS (short: CAMELS) is a configurable, open-source measurement software that records fully self-describing experimental data. It has its origins in the field of experimental physics where a wide variety of measurement instruments are used in frequently changing experimental setups and measurement protocols. CAMELS provides a graphical user interface (GUI) which allows the user to configure experiments without the need of programming skills or deep understanding of instrument communication. CAMELS translates user-defined measurement protocols into stand-alone executable Python code for full transparency of the actual measurement sequences. Existing large-scale, distributed control systems using e.g. EPICS can be natively implemented. CAMELS is designed with focus on full recording of data and metadata. When shared with others, data produced with CAMELS allow full understanding of the measurement and the resulting data in accordance with the FAIR (Findable, Accessible, Interoperable and Re-usable) principles.

physics.ins-det↗

arXiv2023

Electrochemical etching strategy for shaping monolithic 3D structures from 4H-SiC wafers

Silicon Carbide (SiC) is an outstanding material, not only for electronic applications, but also for projected functionalities in the realm of photonic quantum technologies, nano-mechanical resonators and photonics on-a-chip. For shaping 3D structures out of SiC wafers, predominantly dry-etching techniques are used. SiC is nearly inert with respect to wet-etching, occasionally photoelectrochemical etching strategies have been applied. Here, we propose an electrochemical etching strategy that solely relies on defining etchable volumina by implantation of p-dopands. Together with the inertness of the n-doped regions, very sharp etching contrasts can be achieved. We present devices as different as monolithic cantilevers, disk-shaped optical resonators and membranes etched out of a single crystal wafer. The high quality of the resulting surfaces can even be enhanced by thermal treatment, with shape-stable devices up to and even beyond 1550°C. The versatility of our approach paves the way for new functionalities on SiC as high-performance multi-functional wafer platform.

cond-mat.mes-hall↗

arXiv2022

Datenkompetenz im Physikstudium -- ein Erfahrungsbericht

Do our physics curricula provide the appropriate data management competences in a world where data are considered a crucial resource and substantial funding is available for building a national research data infrastructure (German: Nationale Forschungsdateninfrastuktur = NFDI)? Although basic data evaluation and systematic documentation are practiced when students first come into contact with data, particularly in experimental physics lab courses, they do not meet the increasing demands of research and professional practice to deal with the analysis of huge datasets. In many cases, the problem starts with the fact that there is no consensus on a suitable entry-level programming language. At the Department of Physics at Friedrich-Alexander-Universität Erlangen-Nürnberg, we have made minor adjustments to the physics curriculum in recent years, which we present in this article. We placed data management competences early in the bachelor curriculum, which has resulted in considerable advantages throughout the further course of studies. The authors feel that students are quickly moving into the fast lane in data management; we can already see in our research groups that they are becoming a driving force towards modern research data management. -- Vermitteln unsere Physikcurricula die passenden Datenverarbeitungskompetenzen in einer Welt, in der Daten als entscheidende Ressource betrachtet werden und erhebliche Fördermittel für eine Nationale Forschungsdateninfrastruktur (NFDI) bereitstehen? Beim Erstkontakt mit Daten, also insbesondere in den Praktika der Experimentalphysik, werden zwar elementare Datenevaluation und systematische Dokumentation eingeübt, diese genügen aber nicht den steigenden Ansprüchen der Forschung und der Berufspraxis, sich zunehmend mit der Analyse großer Datenmengen zu befassen. Es scheitert oft schon an einem Konsens über eine geeignete Einstiegsprogrammiersprache. Am Department Physik der Friedrich-Alexander-Universität Erlangen-Nürnberg haben wir in den letzten Jahren kleinere Anpassungen im Physikcurriculum vorgenommen, die wir in diesem Artikel vorstellen. Datenkompetenz wurde früh im Bachelorstudium platziert, woraus sich erhebliche Vorteile für den weiteren Studienverlauf ergeben haben. Die Autoren können sich des Eindrucks nicht erwehren, dass die Studierenden in puncto Datenkompetenz schnell auf die Überholspur gehen; wir sehen bereits jetzt in unseren Arbeitsgruppen, dass sie sich als treibende Kräfte hin zu einem modernen Forschungsdatenmanagement erweisen.

physics.ed-ph↗

arXiv2022

Light-field control of real and virtual charge carriers

Light-driven electronic excitation is a cornerstone for energy and information transfer. In the interaction of intense and ultrafast light fields with solids, electrons may be excited irreversibly, or transiently during illumination only. As the transient electron population cannot be observed after the light pulse is gone it is referred to as virtual, while the population remaining excited is called real. Virtual charge carriers have recently been associated with high-harmonic generation and transient absorption, while photocurrent generation may stem from real as well as virtual charge carriers. Yet, a link between the carrier types in their generation and importance for observables up to technological relevance is missing. Here we show that real and virtual carriers can be excited and disentangled in the optical generation of currents in a gold-graphene-gold heterostructure using few-cycle laser pulses. Depending on the waveform used for photoexcitation, real carriers receive net momentum and propagate to the gold electrodes, while virtual carriers generate a polarization response read out at the gold-graphene interfaces. Based on these insights, we further demonstrate a proof of concept of a logic gate for future lightwave electronics. Our results offer a direct means to monitor and excite real and virtual charge carriers. Individual control over each type will dramatically increase the integrated circuit design space and bring closer to reality petahertz signal processing.

physics.optics↗

arXiv2021

Electronic coherence and coherent dephasing in the optical control of electrons in graphene

Electronic coherence is of utmost importance for the access and control of quantum-mechanical solid-state properties. Using a purely electronic observable, the photocurrent, we measure an electronic coherence time of 22 +/- 4 fs in graphene. The photocurrent is ideally suited to measure electronic coherence as it is a direct result of quantum path interference, controlled by the delay between two ultrashort two-color laser pulses. The maximum delay for which interference between the population amplitude injected by the first pulse interferes with that generated by the second pulse determines the electronic coherence time. In particular, numerical simulations reveal that the experimental data yield a lower boundary on the electronic coherence time and that coherent dephasing masks a lower coherence time. We expect that our results will significantly advance the understanding of coherent quantum-control in solid-state systems ranging from excitation with weak fields to strongly driven systems.

cond-mat.mes-hall↗

arXiv2021

Narrow inhomogeneous distribution of spin-active emitters in silicon carbide

Optically active solid-state spin registers have demonstrated their unique potential in quantum computing, communication and sensing. Realizing scalability and increasing application complexity requires entangling multiple individual systems, e.g. via photon interference in an optical network. However, most solid-state emitters show relatively broad spectral distributions, which hinders optical interference experiments. Here, we demonstrate that silicon vacancy centres in semiconductor silicon carbide (SiC) provide a remarkably small natural distribution of their optical absorption/emission lines despite an elevated defect concentration of $\approx 0.43\,\rm μm^{-3}$. In particular, without any external tuning mechanism, we show that only 13 defects have to be investigated until at least two optical lines overlap within the lifetime-limited linewidth. Moreover, we identify emitters with overlapping emission profiles within diffraction limited excitation spots, for which we introduce simplified schemes for generation of computationally-relevant Greenberger-Horne-Zeilinger (GHZ) and cluster states. Our results underline the potential of the CMOS-compatible SiC platform toward realizing networked quantum technology applications.

quant-ph↗

arXiv2020

Thermal origin of light emission in non-resonant and resonant tunnel junctions

Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide a novel experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In an open, non-resonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. These very clear results should resolve any ambiguity about the mechanism of light emission in nano contacts.

cond-mat.mes-hall↗

arXiv2020

Thermoelectricity of near-resonant tunnel junctions and their near-Carnot efficiency

The resonant tunneling model is the simplest model for describing electronic transport through nanoscale objects like individual molecules. A complete understanding includes not only charge transport but also thermal transport and their intricate interplay. Key linear response observables are the electrical conductance G and the Seebeck coefficient S. Here we present experiments on unspecified resonant tunnel junctions and molecular junctions that uncover correlations between $G$ and $S$, in particular rigid boundaries for $S(G)$. We find that these correlations can be consistently understood by the single-level resonant tunneling model, with excellent match to experiments. In this framework, measuring $I(V)$ and $S$ for a given junction provides access to the full thermoelectric characterization of the electronic system. A remarkable result is that without targeted chemical design, molecular junctions can expose thermoelectric conversion efficiencies which are close to the Carnot limit. This insight allows to provide design rules for optimized thermoelectric efficiency.

cond-mat.mes-hall↗

arXiv2020

Attosecond-fast internal photoemission

The photoelectric effect has a sister process relevant in optoelectronics called internal photoemission. Here an electron is photoemitted from a metal into a semiconductor. While the photoelectric effect takes place within less than 100 attoseconds, the attosecond time scale has so far not been measured for internal photoemission. Based on the new method CHArge transfer time MEasurement via Laser pulse duration-dependent saturation fluEnce determinatiON, CHAMELEON, we show that the atomically thin semi-metal graphene coupled to bulk silicon carbide, forming a Schottky junction, allows charge transfer times as fast as (300 $\pm$ 200) attoseconds. These results are supported by a simple quantum mechanical model simulation. With the obtained cut-off bandwidth of 3.3 PHz for the charge transfer rate, this semimetal-semiconductor interface represents the first functional solid-state interface offering the speed and design space required for future light-wave signal processing.

physics.app-ph↗

arXiv2019

Interaction of carrier envelope phase-stable laser pulses with graphene: the transition from the weak-field to the strong-field regime

Ultrafast control of electron dynamics in solid state systems has recently found particular attention. By increasing the electric field strength of laser pulses, the light-matter interaction in solids might turn from a perturbative into a novel non-perturbative regime, where interband transitions from the valence to the conduction band become strongly affected by intraband motion. We have demonstrated experimentally and numerically that this combined dynamics can be controlled in graphene with the electric field waveform of phase-stabilized few-cycle laser pulses. Here we show new experimental data and matching simulation results at comparably low optical fields, which allows us to focus on the highly interesting transition regime where the light-matter interaction turns from perturbative to non-perturbative. We find a 5th order power-law scaling of the laser induced waveform-dependent current at low optical fields, which breaks down for higher optical fields, indicating the transition.

physics.optics↗

arXiv2018

Coherent electron trajectory control in graphene

We investigate coherent electron dynamics in graphene, interacting with the electric field waveform of two orthogonally polarized, few-cycle laser pulses. Recently, we demonstrated that linearly polarized driving pulses lead to sub-optical-cycle Landau-Zener quantum path interference by virtue of the combination of intraband motion and interband transition [Higuchi $\textit{et al.}$, Nature $\textbf{550}$, 224 (2017)]. Here we introduce a pulsed control laser beam, orthogonally polarized to the driving pulses, and observe the ensuing electron dynamics. The relative delay between the two pulses is a tuning parameter to control the electron trajectory, now in a complex fashion exploring the full two-dimensional reciprocal space in graphene. Depending on the relative phase, the electron trajectory in the reciprocal space can, for example, be deformed to suppress the quantum path interference resulting from the driving laser pulse. Intriguingly, this strong-field-based complex matter wave manipulation in a two-dimensional conductor is driven by a high repetition rate \textit{laser oscillator}, rendering unnecessary complex and expensive amplified laser systems.

cond-mat.mes-hall↗

arXiv2018

Fractional Quantum Hall plateaus in mosaic-like conductors

We report a simple route to generate magnetotransport data that results in fractional quantum Hall plateaus in the conductance. Ingredients to the generating model are conducting tiles with integer quantum Hall effect and metallic linkers, further Kirchhoff rules. When connecting few identical tiles in a mosaic, fractional steps occur in the conductance values. Richer spectra representing several fractions occur when the tiles are parametrically varied. Parts of the simulation data are supported with purposefully designed graphene mosaics in high magnetic fields. The findings emphasize that the occurrence of fractional conductance values, in particular in two-terminal measurements, does not necessarily indicate interaction-driven physics. We underscore the importance of an independent determination of charge densities and critically discuss similarities with and differences to the fractional quantum Hall effect.

cond-mat.mes-hall↗

arXiv2017

Spin-split bands cause the indirect band gap of (CH$_3$NH$_3$)PbI$_3$: Experimental evidence from circular photogalvanic effect

Long carrier lifetimes and diffusion lengths form the basis for the successful application of the organic-inorganic perovskite (CH$_3$NH$_3$)PbI$_3$ in solar cells and lasers. The mechanism behind the long carrier lifetimes is still not completely understood. Spin-split bands and a resulting indirect band gap have been proposed by theory. Using near band-gap left-handed and right-handed circularly polarized light we induce photocurrents of opposite directions in a single-crystal (CH$_3$NH$_3$)PbI$_3$ device at low temperature ($4~\mathrm{K}$). The phenomenom is known as the circular photogalvanic effect and gives direct evidence for phototransport in spin-split bands. Simultaneous photoluminecence measurements show that the onset of the photocurrent is below the optical band gap. The results prove that an indirect band gap exists in (CH$_3$NH$_3$)PbI$_3$ with broken inversion symmetry as a result of spin-splittings in the band structure. This information is essential for understanding the photophysical properties of organic-inorganic perovskites and finding lead-free alternatives. Furthermore, the optically driven spin currents in (CH$_3$NH$_3$)PbI$_3$ make it a candidate material for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2017

Charge Transport in C$_{60}$-based Single-Molecule Junctions with Graphene Electrodes

We investigate charge transport in C$_{60}$-based single-molecule junctions with graphene electrodes employing a combination of density functional theory (DFT) electronic structure calculations and Landauer transport theory. In particular, the dependence of the transport properties on the conformation of the molecular bridge and the type of termination of the graphene electrodes is investigated. Furthermore, electron pathways through the junctions are analyzed using the theory of local currents. The results reveal, in agreement with previous experiments, a pronounced dependence of the transport properties on the bias polarity, which is rationalized in terms of the electronic structure of the molecule. It is also shown that the edge states of zigzag-terminated graphene induce additional transport channels, which dominate transport at small voltages. The importance of the edge states for transport depends profoundly on the interface geometry of the junctions.

cond-mat.mes-hall↗

arXiv2016

Anomalous Dirac point transport due to extended defects in bilayer graphene

Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the conductivity distribution $P(σ)$ is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2016

On the Origin of Non-Saturating Linear Magnetoresistivity

The observation of non-saturating classical linear magnetoresistivity has been an enigmatic phenomenon in solid state physics. We present a study of a two-dimensional ohmic conductor, including local Hall effect and a self-consistent consideration of the environment. An equivalent-circuit scheme delivers a simple and convincing argument why the magnetoresistivity is linear in strong magnetic field, provided that current and biasing electric field are misaligned by a nonlocal mechanism. A finite-element model of a two-dimensional conductor is suited to display the situations that create such deviating currents. Besides edge effects next to electrodes, charge carrier density fluctuations are efficiently generating this effect. However, mobility fluctuations that have frequently been related to linear magnetoresistivity are barely relevant. Despite its rare observation, linear magnetoresitivity is rather the rule than the exception in a regime of low charge carrier densities, misaligned current pathways and strong magnetic field.

cond-mat.dis-nn↗