arXiv subjects

Martin Otto

Publications and source records attributed to Martin Otto.

At least 19 recordsLinked to original sources

arXiv2026

A Class of Generalised Quantifiers for k-Variable Logics

We introduce k-quantifier logics -- logics with access to k-tuples of elements and very general quantification patterns for transitions between k-tuples. The framework is very expressive and encompasses e.g. the k-variable fragments of first-order logic, modal logic, and monotone neighbourhood semantics. We introduce a corresponding notion of bisimulation and prove variants of the classical Ehrenfeucht-Fraisse and Hennessy-Milner theorem. Finally, we show a Lindstrom-style characterisation for k-quantifier logics that satisfy Los' theorem by proving that they are the unique maximally expressive logics that satisfy Los' theorem and are invariant under the associated bisimulation relations.

math.LO↗

arXiv2024

Gate-defined single-electron transistors in twisted bilayer graphene

Twisted bilayer graphene (tBLG) near the magic angle is a unique platform where the combination of topology and strong correlations gives rise to exotic electronic phases. These phases are gate-tunable and related to the presence of flat electronic bands, isolated by single-particle band gaps. This enables gate-controlled charge confinement, essential for the operation of single-electron transistors (SETs), and allows to explore the interplay of confinement, electron interactions, band renormalisation and the moir\'e superlattice, potentially revealing key paradigms of strong correlations. Here, we present gate-defined SETs in near-magic-angle tBLG with well-tunable Coulomb blockade resonances. These SETs allow to study magnetic field-induced quantum oscillations in the density of states of the source-drain reservoirs, providing insight into gate-tunable Fermi surfaces of tBLG. Comparison with tight-binding calculations highlights the importance of displacement-field-induced band renormalisation crucial for future advanced gate-tunable quantum devices and circuits in tBLG including e.g. quantum dots and Josephson junction arrays.

cond-mat.mes-hall↗

arXiv2024

Tunable Doping and Mobility Enhancement in 2D Channel Field-Effect Transistors via Damage-Free Atomic Layer Deposition of AlOX Dielectrics

Two-dimensional materials (2DMs) have been widely investigated because of their potential for heterogeneous integration with modern electronics. However, several major challenges remain, such as the deposition of high-quality dielectrics on 2DMs and the tuning of the 2DM doping levels. Here, we report a scalable plasma-enhanced atomic layer deposition (PEALD) process for direct deposition of a nonstoichiometric aluminum oxide (AlOX) dielectric, overcoming the damage issues associated with conventional methods. Furthermore, we control the thickness of the dielectric layer to systematically tune the doping level of 2DMs. The experimental results demonstrate successful deposition without detectable damage, as confirmed by Raman spectroscopy and electrical measurements. Our method enables tuning of the Dirac and threshold voltages of back-gated graphene and MoS${_2}$ field-effect transistors (FETs), respectively, while also increasing the charge carrier mobility in both device types. We further demonstrate the method in top-gated MoS${_2}$ FETs with double-stack dielectric layers (AlOX+Al${_2}$O${_3}$), achieving critical breakdown field strengths of 7 MV/cm and improved mobility compared with the back gate configuration. In summary, we present a PEALD process that offers a scalable and low-damage solution for dielectric deposition on 2DMs, opening new possibilities for precise tuning of device characteristics in heterogeneous electronic circuits.

physics.app-ph↗

arXiv2023

On the expressive power of inquisitive epistemic logic

Inquisitive modal logic, InqML, in its epistemic incarnation, extends standard epistemic logic to capture not just the information that agents have, but also the questions that they are interested in. We use the natural notion of bisimulation equivalence in the setting of InqML, as introduced in [Ciardelli/Otto: JSL 2021], to characterise the expressiveness of InqML as the bisimulation invariant fragment of first-order logic over natural classes of two-sorted first-order structures that arise as relational encodings of inquisitive epistemic (S5-like) models. The non-elementary nature of these classes crucially requires non-classical model-theoretic methods for the analysis of first-order expressiveness, irrespective of whether we aim for characterisations in the sense of classical or of finite model theory.

math.LO↗

arXiv2022

Plasma-enhanced atomic layer deposition of Al$_2$O$_3$ on graphene using monolayer hBN as interfacial layer

The deposition of dielectric materials on graphene is one of the bottlenecks for unlocking the potential of graphene in electronic applications. In this paper we demonstrate the plasma enhanced atomic layer deposition of 10 nm thin high quality Al$_2$O$_3$ on graphene using a monolayer of hBN as protection layer. Raman spectroscopy was performed to analyze possible structural changes of the graphene lattice caused by the plasma deposition. The results show that a monolayer of hBN in combination with an optimized deposition process can effectively protect graphene from damage, while significant damage was observed without an hBN layer. Electrical characterization of double gated graphene field effect devices confirms that the graphene did not degrade during the plasma deposition of Al$_2$O$_3$. The leakage current densities were consistently below 1 nA/mm for electric fields across the insulators of up to 8 MV/cm, with irreversible breakdown happening above. Such breakdown electric fields are typical for Al$_2$O$_3$ and can be seen as an indicator for high quality dielectric films.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast Electron Diffraction: Visualizing Dynamic States of Matter

Since the discovery of electron-wave duality, electron scattering instrumentation has developed into a powerful array of techniques for revealing the atomic structure of matter. Beyond detecting local lattice variations in equilibrium structures, recent research efforts have been directed towards the long sought-after dream of visualizing the dynamic evolution of matter in real-time. The atomic behavior at ultrafast timescales carries critical information on phase transition and chemical reaction dynamics, the coupling of electronic and nuclear degrees of freedom in materials and molecules, the correlation between structure, function and previously hidden metastable or nonequilibrium states of matter. Ultrafast electron pulses play an essential role in this scientific endeavor, and their generation has been facilitated by rapid technical advances in both ultrafast laser and particle accelerator technologies. This review presents a summary of the remarkable developments in this field over the last few decades. The physics and technology of ultrafast electron beams is presented with an emphasis on the figures of merit most relevant for ultrafast electron diffraction (UED) experiments. We discuss recent developments in the generation, manipulation and characterization of ultrashort electron beams aimed at improving the combined spatio-temporal resolution of these measurements. The fundamentals of electron scattering from atomic matter and the theoretical frameworks for retrieving dynamic structural information from solid-state and gas-phase samples are described, together with essential experimental techniques and several landmark works. Ultrafast electron probes with ever improving capabilities, combined with other complementary photon-based or spectroscopic approaches, hold tremendous potential for revolutionizing our ability to observe and understand energy and matter at atomic scales.

physics.ins-det↗

arXiv2022

Direct view of phonon dynamics in atomically thin MoS$_{2}$

Transition metal dichalcogenide monolayers and heterostructures are highly tunable material systems that provide excellent models for physical phenomena at the two-dimensional (2D) limit. While most studies to date have focused on electrons and electron-hole pairs, phonons also play essential roles. Here, we apply ultrafast electron diffraction and diffuse scattering to directly quantify, with time and momentum resolution, electron-phonon coupling (EPC) in monolayer molybdenum disulfide (MoS$_{2}$) and phonon transport from the monolayer to a silicon nitride (Si$_3$N$_4$) substrate. Optically generated hot carriers result in a profoundly anisotropic distribution of phonons in the monolayer on the $\sim$ 5 ps time scale. A quantitative comparison with ab-initio ultrafast dynamics simulations reveals the essential role of dielectric screening in weakening EPC. Thermal transport from the monolayer to the substrate occurs with the phonon system far from equilibrium. While screening in 2D is known to strongly affect equilibrium properties, our findings extend this understanding to the dynamic regime.

cond-mat.mtrl-sci↗

arXiv2022

Role of substrate surface morphology on the performance of graphene inks for flexible electronics

Two-dimensional (2D) materials, such as graphene, are seen as potential candidates for fabricating electronic devices and circuits on flexible substrates. Inks or dispersions of 2D materials can be deposited on flexible substrates by large-scale coating techniques, such as inkjet printing and spray coating. One of the main issues in coating processes is nonuniform deposition of inks, which may lead to large variations of properties across the substrates. Here, we investigate the role of surface morphology on the performance of graphene ink deposited on different paper substrates with specific top coatings. Substrates with good wetting properties result in reproducible thin films and electrical properties with low sheet resistance. The correct choice of surface morphology enables high-performance films without postdeposition annealing or treatment. Scanning terahertz time-domain spectroscopy (THz-TDS) is introduced to evaluate both the uniformity and the local conductivity of graphene inks on paper. A paper-based strain gauge is demonstrated and a variable resistor acts as an on-off switch for operating an LED. Customized surfaces can thus help in unleashing the full potential of ink-based 2D materials.

physics.app-ph↗

arXiv2021

Terahertz rectennas on flexible substrates based on one-dimensional metal-insulator-graphene diodes

Flexible energy harvesting devices fabricated in scalable thin-film processes are important components in the field of wearable electronics and the Internet of Things. We present a flexible rectenna based on a one-dimensional junction metal-insulator-graphene diode, which offers low-noise power detection at terahertz (THz) frequencies. The rectennas are fabricated on a flexible polyimide film in a scalable process by photolithography using graphene grown by chemical vapor deposition. A one-dimensional junction area reduces the junction capacitance and enables operation in the D-band (110 - 170 GHz). The rectenna on polyimide shows a maximum voltage responsivity of 80 V/W at 167 GHz in free space measurements and minimum noise equivalent power of 80 pW/$\sqrt{\text{Hz}}$.

physics.app-ph↗

arXiv2021

Optimizing the Stability of FETs Based on Two-Dimensional Materials by Fermi Level Tuning

Despite the enormous progress achieved during the past decade, nanoelectronic devices based on two-dimensional (2D) semiconductors still suffer from a limited electrical stability. This limited stability has been shown to result from the interaction of charge carriers originating from the 2D semiconductors with defects in the surrounding insulating materials. The resulting dynamically trapped charges are particularly relevant in field effect transistors (FETs) and can lead to a large hysteresis, which endangers stable circuit operation. Based on the notion that charge trapping is highly sensitive to the energetic alignment of the channel Fermi-level with the defect band in the insulator, we propose to optimize device stability by deliberately tuning the channel Fermi-level. Our approach aims to minimize the amount of electrically active border traps without modifying the total number of traps in the insulator. We demonstrate the applicability of this idea by using two differently doped graphene layers in otherwise identical FETs with Al$_2$O$_3$ as a gate oxide mounted on a flexible substrate. Our results clearly show that by increasing the distance of the Fermi-level to the defect band, the hysteresis is significantly reduced. Furthermore, since long-term reliability is also very sensitive to trapped charges, a corresponding improvement in reliability is both expected theoretically and demonstrated experimentally. Our study paves the way for the construction of more stable and reliable 2D FETs in which the channel material is carefully chosen and tuned to maximize the energetic distance between charge carriers in the channel and the defect bands in the insulator employed.

physics.app-ph↗

arXiv2021

A First-Order Framework for Inquisitive Modal Logic

We present a natural standard translation of inquisitive modal logic InqML into first-order logic over the natural two-sorted relational representations of the intended models, which captures the built-in higher-order features of InqML. This translation is based on a graded notion of flatness that ties the inherent second-order, team-semantic features of InqML over information states to subsets or tuples of bounded size. A natural notion of pseudo-models, which relaxes the non-elementary constraints on the intended models, gives rise to an elementary, purely model-theoretic proof of the compactness property for InqML. Moreover, we prove a Hennessy-Milner theorem for InqML, which crucially uses $ω$-saturated pseudo-models and the new standard translation. As corollaries we also obtain van Benthem style characterisation theorems.

math.LO↗

arXiv2021

Dispersive sensing of charge states in a bilayer graphene quantum dot

We demonstrate dispersive readout of individual charge states in a gate-defined few-electron quantum dot in bilayer graphene. We employ a radio frequency reflectometry circuit, where an LC resonator with a resonance frequency close to 280 MHz is directly coupled to an ohmic contact of the quantum dot device. The detection scheme based on changes in the quantum capacitance operates over a wide gate-voltage range and allows to probe excited states down to the single-electron regime. Crucially, the presented sensing technique avoids the use of an additional, capacitively coupled quantum device such as a quantum point contact or single electron transistor, making dispersive sensing particularly interesting for gate-defined graphene quantum dots.

cond-mat.mes-hall↗

arXiv2020

Oxidising and carburising catalyst conditioning for the controlled growth and transfer of large crystal monolayer hexagonal boron nitride

Hexagonal boron nitride (h-BN) is well-established as a requisite support, encapsulant and barrier for 2D material technologies, but also recently as an active material for applications ranging from hyperbolic metasurfaces to room temperature single-photon sources. Cost-effective, scalable and high quality growth techniques for h-BN layers are critically required. We utilise widely-available iron foils for the catalytic chemical vapour deposition (CVD) of h BN and report on the significant role of bulk dissolved species in h-BN CVD, and specifically, the balance between dissolved oxygen and carbon. A simple pre-growth conditioning step of the iron foils enables us to tailor an error-tolerant scalable CVD process to give exceptionally large h-BN monolayer domains. We also develop a facile method for the improved transfer of as-grown h-BN away from the iron surface by means of the controlled humidity oxidation and subsequent rapid etching of a thin interfacial iron oxide; thus, avoiding the impurities from the bulk of the foil. We demonstrate wafer-scale (2 inch) production and utilise this h-BN as a protective layer for graphene towards integrated (opto) electronic device fabrication.

cond-mat.mtrl-sci↗

arXiv2020

Electrostatic detection of Shubnikov-de-Haas oscillations in bilayer graphene by Coulomb resonances in gate-defined quantum dots

A gate-defined quantum dot in bilayer graphene is utilized as a sensitive electrometer for probing the charge density of its environment. Under the influence of a perpendicular magnetic field, the charge carrier density of the channel region next to the quantum dot oscillates due to the formation of Landau levels. This is experimentally observed as oscillations in the gate-voltage positions of the Coulomb resonances of the nearby quantum dot. From the frequency of the oscillations, we extract the charge carrier density in the channel and from the amplitude the shift of the quantum dot potential. We compare these experimental results with an electrostatic simulation of the device and find good agreement.

cond-mat.mes-hall↗

arXiv2020

Driving-induced resonance narrowing in a strongly coupled cavity-qubit system

We study a system consisting of a superconducting flux qubit strongly coupled to a microwave cavity. Externally applied qubit driving is employed in order to manipulate the spectrum of dressed states. We observe resonance narrowing in the region where the splitting between the two dressed fundamental resonances is tuned to zero. The narrowing in this region of overlapping resonances can be exploited for long-time storage of quantum states. In addition, we measure the response to strong cavity mode driving, and find a qualitative deviation between the experimental results and the predictions of a semiclassical model. On the other hand, good agreement is obtained using theoretical predictions obtained by numerically integrating the master equation governing the system's dynamics. The observed response demonstrates a process of a coherent cancellation of two meta-stable dressed states.

quant-ph↗

arXiv2020

Flexible One-Dimensional Metal-Insulator-Graphene Diode

In this work, a novel one-dimensional geometry for metal-insulator-graphene (1D-MIG) diode with low capacitance is demonstrated. The junction of the 1D-MIG diode is formed at the 1D edge of Al2O3-encapsulated graphene with TiO2 that acts as barrier material. The diodes demonstrate ultra-high current density since the transport in the graphene and through the barrier is in plane. The geometry delivers very low capacitive coupling between the cathode and anode of the diode, which shows frequency response up to 100 GHz and ensures potential high frequency performance up to 2.4 THz. The 1D-MIG diodes are demonstrated to function uniformly and stable under bending conditions down to 6.4 mm bending radius on flexible substrate.

physics.app-ph↗

arXiv2019

Does carrier velocity saturation help to enhance fmax in graphene field-effect transistors?

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get the highest possible maximum oscillation frequency (fmax). This paper numerically investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced fmax. For such a purpose, we used a drift-diffusion simulator that includes several factors that influence output conductance, especially at short channel lengths and-or large drain bias: short-channel electrostatics, saturation velocity, graphene-dielectric interface traps, and self-heating effects. As a testbed for our investigation, we analyzed fabricated GFETs with high extrinsin cutoff frequency fT,x (34 GHz) and fmax (37 GHz). Our simulations allow for a microscopic (local) analysis of the channel parameteres such as carrier concentration, drift and saturation velocities. For biases far away from the Dirac voltage, where the channel behaves as unipolar, we confirmed that the higher is the drift velocity, as close as possible to the saturation velocity, the greater fmax is. However, the largest fmax is recorded at biases near the crossover between unipolar and bipolar behavior, where it does not hold that the highest drift velocity maximizes fmax. In fact, the position and magnitude of the largest fmax depend on the complex interplay between the carrier concentration and total velocity which, in turn, are impacted by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the maximum fmax from around 60 to 40 GHz.

physics.app-ph↗

arXiv2019

Graded modal logic and counting bisimulation

This note sketches the extension of the basic characterisation theorems as the bisimulation-invariant fragment of first-order logic to modal logic with graded modalities and matching adaptation of bisimulation. We focus on showing expressive completeness of graded multi-modal logic for those first-order properties of pointed Kripke structures that are preserved under counting bisimulation equivalence among all or among just all finite pointed Kripke structures.

math.LO↗