arXiv subjects

Adam Burke

Publications and source records attributed to Adam Burke.

13 recordsLinked to original sources

arXiv2026

Heat Conduction and Energy Relaxation in an InAs Nanowire Approaching the Clean One-Dimensional Limit

We investigate heat conduction and energy relaxation in an InAs semiconductor nanowire using a hybrid semiconductor-superconductor architecture. Local electronic temperatures are measured with an in-situ grown quantum dot thermometer, while controlled Joule heating is applied at different locations along the wire to probe temperature gradients at sub-kelvin temperatures. With a onedimensional heat transport model, we calculate an electron-phonon heat flow that scales as Q_{e-ph} \propto T^2.6, which is in close agreement with the T^3 dependence predicted for a clean one-dimensional electron gas coupled to a phonon bath. We further estimate a characteristic length l_{eq} = 370 nm, beyond this length scale, phonon-mediated heat transport dominates over heat conduction in our nanowire. Our results provide a quantitative measure of energy relaxation mechanisms in a onedimensional semiconductor and provide a framework for studying heat flow in low-dimensional nanostructures.

cond-mat.mes-hall↗

arXiv2025

Radial etching of strongly confined crystal-phase defined quantum dots

We realize strongly confined quantum dots (QDs) in InAs nanowires (NWs) by combining epitaxial crystal-phase control with chemical wet etching. A strong axial confinement is first introduced by growing closely spaced wurtzite (WZ) tunnel barriers in NWs to enclose a zinc blende (ZB) QD. The NW cross-section is then reduced by isotropic etching to obtain very small QDs, with a maximum observed charging energy > 30 meV. Using low-temperature electrical characterization and finite-element method simulations, we study how charging energies and the onset of electron filling scale with QD diameter. For extremely small diameters, we identify a regime where stray capacitances become non-negligible, limiting further increase in charging energy by diameter reduction alone. This approach to increasing confinement is particularly relevant for understanding the strong spin-orbit interaction observed in crystal-phase QDs, possibly linked to polarization charges at the WZ/ZB interfaces. Small diameter QDs allow considerably weaker interfering electric fields when studied, but the QDs cannot be realized with epitaxial growth alone due to a loss of crystal phase control.

cond-mat.mes-hall↗

arXiv2025

Irish-BLiMP: A Linguistic Benchmark for Evaluating Human and Language Model Performance in a Low-Resource Setting

We present Irish-BLiMP (Irish Benchmark of Linguistic Minimal Pairs), the first dataset and framework designed for fine-grained evaluation of linguistic competence in the Irish language, an endangered language. Drawing on a variety of linguistic literature and grammar reference works, we manually constructed and reviewed 1020 minimal pairs across a taxonomy of 11 linguistic features, through a team of fluent Irish speakers. We evaluate both existing Large Language Models (LLMs) and fluent human participants on their syntactic knowledge of Irish. Our findings show that humans outperform all models across all linguistic features, achieving 16.6% higher accuracy on average. Moreover, a substantial performance gap of 18.1% persists between open- and closed-source LLMs, with even the strongest model (gpt-5) reaching only 73.5% accuracy compared to 90.1% by human. Interestingly, human participants and models struggle on different aspects of Irish grammar, thus highlighting a difference in representation learned by the models. Overall, Irish-BLiMP provides the first systematic framework for evaluating the grammatical competence of LLMs in Irish and offers a valuable benchmark for advancing research on linguistic understanding in low-resource languages.

cs.CL↗

arXiv2025

Combined Light Excitation and Scanning Gate Microscopy on Heterostructure Nanowire Photovoltaic Devices

Nanoscale optoelectronic components achieve functionality via spatial variation in electronic structure induced by composition, defects, and dopants. To dynamically change the local band alignment and influence defect states, a scanning gate electrode is highly useful. However, this technique is rarely combined with photoexcitation by a controlled external light source. We explore a setup that combines several types of light excitation with high resolution scanning gate and atomic force microscopy (SGM/AFM). We apply the technique to InAs nanowires with an atomic scale defined InP segment, that have attracted considerable attention for studies of hot carrier devices. Using AFM we image the topography of the nanowire device. SGM measurements without light excitation show how current profiles can be influenced by local gating near the InP segment. Modelling of the tip and nanowire can well predict the results based on the axial band structure variation and an asymmetric tip. SGM studies including light excitation are then performed using both a white light LED and laser diodes at 515 and 780nm. Both negative and positive photoconductance can be observed and the combined effect of light excitation and local gating is observed. SGM can then be used to discriminate between effects related to the wire axial compositional structure and surface states. The setup explored in the current work has significant advantages to study optoelectronics at realistic conditions and with rapid turnover.

physics.app-ph↗

arXiv2024

Exploring Complexity: An Extended Study of Formal Properties for Process Model Complexity Measures

A good process model is expected not only to reflect the behavior of the process, but also to be as easy to read and understand as possible. Because preferences vary across different applications, numerous measures provide ways to reflect the complexity of a model with a numeric score. However, this abundance of different complexity measures makes it difficult to select one for analysis. Furthermore, most complexity measures are defined for BPMN or EPC, but not for workflow nets. This paper is an extended analysis of complexity measures and their formal properties. It adapts existing complexity measures to the world of workflow nets. It then compares these measures with a set of properties originally defined for software complexity, as well as new extensions to it. We discuss the importance of the properties in theory by evaluating whether matured complexity measures should fulfill them or whether they are optional. We find that not all inspected properties are mandatory, but also demonstrate that the behavior of evolutionary process discovery algorithms is influenced by some of these properties. Our findings help analysts to choose the right complexity measure for their use-case.

cs.FL↗

arXiv2023

Geometric symmetry breaking and nonlinearity can increase thermoelectric power

Direct thermal-to-electric energy converters typically operate in the linear regime, where the ratio of actual maximum power relative to the ideal maximum power, the so-called fill factor (FF), is 0.25. By increasing the FF one can potentially increase maximum power by up to four times, but this is only possible in the nonlinear regime of transport and has previously rarely been considered. Here we show, based on fundamental symmetry considerations, that the leading order non-linear terms that can increase the FF require devices with broken spatial symmetry. To experimentally demonstrate such a system, we study nonlinear, thermoelectric transport across an asymmetric energy barrier epitaxially defined in a single semiconductor nanowire. We find in both experiment and theory that we can increase the FF above the linear-response limit of 0.25, accompanied by a drastic increase in short circuit current, open-circuit voltage and maximum power. Our results show that geometric symmetry breaking combined with the design of nonlinear behaviour represent a design strategy for increasing the performance of thermal-to-electric energy converters such as in hot-carrier photovoltaics, thermophotovoltaics or in anisotropic thermoelectric materials.

cond-mat.mes-hall↗

arXiv2021

Gate control, g-factors and spin orbit energy of p-type GaSb nanowire quantum dot devices

Proposals for quantum information applications are frequently based on the coherent manipulation of spins confined to quantum dots. For these applications, p-type III-V material systems promise a reduction of the hyperfine interaction while maintaining large $g$-factors and strong spin-orbit interaction. In this work, we study bottom-gated device architectures to realize single and serial multi-quantum dot systems in Schottky contacted p-type GaSb nanowires. We find that the effect of potentials applied to gate electrodes on the nanowire is highly localized to the immediate vicinity of the gate electrode only, which prevents the formation of double quantum dots with commonly used device architectures. We further study the transport properties of a single quantum dot induced by bottom-gating, find large gate-voltage dependent variations of the $g^*$-factors up to $8.1\pm 0.2$ as well as spin-orbit energies between $110$-$230\,\mu$eV.

cond-mat.mes-hall↗

arXiv2020

Hot-Carrier Separation in Heterostructure Nanowires observed by Electron-Beam Induced Current

The separation of hot carriers in semiconductors is of interest for applications such as thermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowires offer several potential advantages for effective hot-carrier separation such as: a high degree of control and flexibility in heterostructure-based band engineering, increased hot-carrier temperatures compared to bulk, and a geometry well suited for local control of light absorption. Indeed, InAs nanowires with a short InP energy barrier have been observed to produce electric power under global illumination, with an open-circuit voltage exceeding the Shockley-Queisser limit. To understand this behaviour in more detail, it is necessary to maintain control over the precise location of electron-hole pair-generation in the nanowire. In this work we perform electron-beam induced current measurements with high spatial resolution, and demonstrate the role of the InP barrier in extracting energetic electrons. We interprete the results in terms of hot-carrier separation, and extract estimates of the hot carrier mean free path.

cond-mat.mes-hall↗

arXiv2020

Heat driven transport in serial double quantum dot devices

Studies of thermally induced transport in nanostructures provide access to an exciting regime where fluctuations are relevant, enabling the investigation of fundamental thermodynamic concepts and the realization of thermal energy harvesters. We study a serial double quantum dot formed in an InAs/InP nanowire coupled to two electron reservoirs. By means of a specially designed local metallic joule-heater, the temperature of the phonon bath in the vicinity of the double quantum dot can be enhanced. This results in phonon-assisted transport, enabling the conversion of local heat into electrical power in a nano-sized heat engine. Simultaneously, the electron temperatures of the reservoirs are affected, resulting in conventional thermoelectric transport. By detailed modelling and experimentally tuning the interdot coupling we disentangle both effects. Furthermore, we show that phonon-assisted transport gives access to the energy of excited states. Our findings demonstrate the versatility of our design to study fluctuations and fundamental nanothermodynamics.

cond-mat.mes-hall↗

arXiv2019

Selective tuning of spin-orbital Kondo contributions in parallel-coupled quantum dots

We use co-tunneling spectroscopy to investigate spin-, orbital-, and spin-orbital Kondo transport in a strongly confined system of InAs double quantum dots (QDs) parallel-coupled to source and drain. In the one-electron transport regime, the higher symmetry spin-orbital Kondo effect manifests at orbital degeneracy and no external magnetic field. We then proceed to show that the individual Kondo contributions can be isolated and studied separately; either by orbital detuning in the case of spin-Kondo transport, or by spin splitting in the case of orbital Kondo transport. By varying the inter-dot tunnel coupling, we show that lifting of the spin degeneracy is key to confirming the presence of an orbital degeneracy, and to detecting a small orbital hybridization gap. Finally, in the two-electron regime, we show that the presence of a spin-triplet ground state results in spin-Kondo transport at zero magnetic field.

cond-mat.mes-hall↗

arXiv2018

Spectroscopy and level detuning of few-electron spin states in parallel InAs quantum dots

We use tunneling spectroscopy to study the evolution of few-electron spin states in parallel InAs nanowire double quantum dots (QDs) as a function of level detuning and applied magnetic field. Compared to the much more studied serial configuration, parallel coupling of the QDs to source and drain greatly expands the probing range of excited state transport. Owing to a strong confinement, we can here isolate transport involving only the very first interacting single QD orbital pair. For the (2,0)-(1,1) charge transition, with relevance for spin-based qubits, we investigate the excited (1,1) triplet, and hybridization of the (2,0) and (1,1) singlets. An applied magnetic field splits the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet, we clearly resolve transport through all triplet states near the avoided singlet-triplet crossings. Transport calculations, based on a simple model with one orbital on each QD, fully replicate the experimental data. Finally, we observe an expected mirrored symmetry between the 1-2 and 2-3 electron transitions resulting from the two-fold spin degeneracy of the orbitals.

cond-mat.mes-hall↗

arXiv2017

Thermoelectric power factor limit of a 1D nanowire

In the past decade, there has been significant interest in the potentially advantageous thermoelectric properties of one-dimensional (1D) nanowires, but it has been challenging to find high thermoelectric power factors based on 1D effect in practice. Here we point out that there is an upper limit to the thermoelectric power factor of non-ballistic 1D nanowires, as a consequence of the recently established quantum bound of thermoelectric power output. We experimentally test this limit in quasi-ballistic InAs nanowires by extracting the maximum power factor of the first 1D subband through I-V characterization, finding that the measured maximum power factors conform to the theoretical limit. The established limit predicts that a competitive power factor, on the order of mW/m-K^2, can be achieved by a single 1D electronic channel in state-of-the-art semiconductor nanowires with small cross-section and high crystal quality.

cond-mat.mes-hall↗

arXiv2017

Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage

Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated "hot carriers" before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where the potential efficiency gain is highest. Recently, a high open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire with a low bandgap of 0.39 eV, and was interpreted in terms of a photothermoelectric effect. Here, we point out that this device is a hot carrier solar cell and discuss its performance in those terms. In the demonstrated devices, InP heterostructures are used as energy filters in order to thermoelectrically harvest the energy of hot electrons photogenerated in InAs absorber segments. The obtained photovoltage depends on the heterostructure design of the energy filter and is therefore tunable. By using a high-resistance, thermionic barrier an open-circuit voltage is obtained that is in excess of the Shockley-Queisser limit. These results provide generalizable insight into how to realize high voltage hot carrier solar cells in low-bandgap materials, and therefore are a step towards the demonstration of higher efficiency hot carrier solar cells.

physics.app-ph↗