### Recent Submissions

• #### Damage-free substrate removal technique: Wet undercut etching of semipolar (20-21) laser structures by incorporation of un/relaxed sacrificial layer single quantum well

(Japanese Journal of Applied Physics, IOP Publishing, 2021-03-30) [Article]
We applied a damage-free substrate removal technique using photoelectrochemical etching (PECE) by incorporating sacrificial layer In0.12Ga0.88N single quantum well (SL-SQW) types in semipolar (202 ̅1) flip-chip laser diode (FC-LD) structures. Although 40-nm type I promoted the development of high-quality green active region (AR) devices in terms of managing strain relaxation, processing was required under low-temperature KOH. However, 10-nm type II exhibited a smooth n-type GaN surface with room-temperature KOH, thereby promoting the applicability of the proposed technique for either a short light emitter or a combination with type I. The temperature-dependent PECE of SL-SQW types is important in realizing advanced FC-LDs.
• #### Physics-informed Learning for Identification and State Reconstruction of Traffic Density

(arXiv, 2021-03-25) [Preprint]
This paper deals with traffic density reconstruction using measurements from Probe Vehicles (PVs). The main difficulty arises when considering a low penetration rate, meaning that the number of PVs is small compared to the total number of vehicles on the road. Moreover, the formulation assumes noisy measurements and a partially unknown first-order model. All these considerations make the use of machine learning to reconstruct the state the only applicable solution. We first investigate how the identification and reconstruction processes can be merged and how a sparse dataset can still enable a good identification. Secondly, we propose a pre-training procedure that helps the hyperparameter tuning, preventing the gradient descent algorithm from getting stuck at saddle points. Examples using numerical simulations and the SUMO traffic simulator show that the reconstructions are close to the real density in all cases.
• #### Effective strain manipulation of the antiferromagnetic state of polycrystalline NiO

(arXiv, 2021-03-24) [Preprint]
As a candidate material for applications such as magnetic memory, polycrystalline antiferromagnets offer the same robustness to external magnetic fields, THz spin dynamics, and lack of stray field as their single crystalline counterparts, but without the limitation of epitaxial growth and lattice matched substrates. Here, we first report the detection of the average Neel vector orientiation in polycrystalline NiO via spin Hall magnetoresistance (SMR). Secondly, by applying strain through a piezo-electric substrate, we reduce the critical magnetic field required to reach a saturation of the SMR signal, indicating a change of the anisotropy. Our results are consistent with polycrystalline NiO exhibiting a positive sign of the in-plane magnetostriction. This method of anisotropy-tuning offers an energy efficient, on-chip alternative to manipulate a polycrystalline antiferromagnets magnetic state.
• #### Automated Applications of Acoustics for Stored Product Insect Detection, Monitoring, and Management

(Insects, MDPI AG, 2021-03-19) [Article]
Acoustic technology provides information difficult to obtain about stored insect behavior, physiology, abundance, and distribution. For example, acoustic detection of immature insects feeding hidden within grain is helpful for accurate monitoring because they can be more abundant than adults and be present in samples without adults. Modern engineering and acoustics have been incorporated into decision support systems for stored product insect management, but with somewhat limited use due to device costs and the skills needed to interpret the data collected. However, inexpensive modern tools may facilitate further incorporation of acoustic technology into the mainstream of pest management and precision agriculture. One such system was tested herein to describe Sitophilus oryzae (Coleoptera: Curculionidae) adult and larval movement and feeding in stored grain. Development of improved methods to identify sounds of targeted pest insects, distinguishing them from each other and from background noise, is an active area of current research. The most powerful of the new methods may be machine learning. The methods have different strengths and weaknesses depending on the types of background noise and the signal characteristic of target insect sounds. It is likely that they will facilitate automation of detection and decrease costs of managing stored product insects in the future.
• #### Catalytic Mechanism of Interfacial Water in the Cycloaddition of Quadricyclane and Diethyl Azodicarboxylate

(The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2021-03-18) [Article]
“On-water” catalysis, the unusual activity of water molecules at the organic solvent–water interface, has been demonstrated in many organic reactions. However, the catalytic mechanism has remained unclear, largely because of the irreproducibility of the organic–water interface under the common stirring condition. Here, the interfacial area was controlled by employing adsorbed water on mesoporous silica nanoparticles as the catalyst. Reliable kinetics of the cycloaddition reaction of quadricyclane and diethyl azodicarboxylate (DEAD) at the toluene–water interface within the nanoparticle pores were measured. Data reveal an Eley–Rideal mechanism, wherein DEAD adsorbs at the toluene–water interface via hydrogen bonds formed with interfacial water, which lower the activation energy of the cycloaddition reaction. The mechanistic insights gained and preparation of surface water in silica pores described herein may facilitate the future design of improved “on-water” catalysts.
• #### Photosymbiosis in Late Triassic scleractinian corals from the Italian Dolomites

(PeerJ, PeerJ, 2021-03-16) [Article]
During the Carnian, oligotrophic shallow-water regions of the western Tethys were occupied by small, coral-rich patch reefs. Scleractinian corals, which already contributed to the formation of the reef structure, owed their position most probably to the symbiosis with dinoflagellate algae (zooxanthellae). Using microstructural (regularity of growth increments) and geochemical (oxygen and carbon stable isotopes) criteria of zooxanthellae symbiosis, we investigated whether this partnership was widespread among Carnian scleractinians from the Italian Dolomites (locality Alpe di Specie). Although corals from this locality are renowned from excellent mineralogical preservation (aragonite), their skeletons were rigorously tested against traces of diagenesis Irrespective of their growth forms, well preserved skeletons of corals from the Dolomites, most frequently revealed regular growth bands (low values of coefficient of variation) typical of modern zooxanthellate corals. Paradoxically, some Carnian taxa (Thamnasteriomorpha frechi and Thamnasteriomorphasp.)with highly integrated thamnasterioid colonies which today are formed exclusively by zooxanthellate corals, showed irregular fine-scale growth bands (coefficient of variation of 40% and 41% respectively) that could suggest their asymbiotic status. However, similar irregular skeletal banding is known also in some modern agariciids (Leptoseris fragilis) which are symbiotic with zooxanthellae. This may point to a similar ecological adaptation of Triassic taxa with thamnasterioid colonies. Contrary to occasionally ambiguous interpretation of growth banding, all examined Carnian corals exhibited lack of distinct correlation between carbon (δ$^{13}$C range between 0.81‰ and 5.81‰) and oxygen (δ$^{18}$O values range between −4.21‰ and −1.06‰) isotope composition of the skeleton which is consistent with similar pattern in modern zooxanthellates. It is therefore highly likely, that Carnian scleractinian corals exhibited analogous ecological adaptations as modern symbiotic corals and that coral-algal symbiosis that spread across various clades of Scleractinia preceded the reef bloom at the end of the Triassic.
• #### A unified first-order hyperbolic model for nonlinear dynamic rupture processes in diffuse fracture zones

(Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, The Royal Society, 2021-03-15) [Article]
Earthquake fault zones are more complex, both geometrically and rheologically, than an idealized infinitely thin plane embedded in linear elastic material. To incorporate nonlinear material behaviour, natural complexities and multi-physics coupling within and outside of fault zones, here we present a first-order hyperbolic and thermodynamically compatible mathematical model for a continuum in a gravitational field which provides a unified description of nonlinear elasto-plasticity, material damage and of viscous Newtonian flows with phase transition between solid and liquid phases. The fault geometry and secondary cracks are described via a scalar function ξ ∈ [0, 1] that indicates the local level of material damage. The model also permits the representation of arbitrarily complex geometries via a diffuse interface approach based on the solid volume fraction function α ∈ [0, 1]. Neither of the two scalar fields ξ and α needs to be mesh-aligned, allowing thus faults and cracks with complex topology and the use of adaptive Cartesian meshes (AMR). The model shares common features with phase-field approaches, but substantially extends them. We show a wide range of numerical applications that are relevant for dynamic earthquake rupture in fault zones, including the co-seismic generation of secondary off-fault shear cracks, tensile rock fracture in the Brazilian disc test, as well as a natural convection problem in molten rock-like material. This article is part of the theme issue ‘Fracture dynamics of solid materials: from particles to the globe’.
• #### Substrate-Independent Laser-Induced Graphene Electrodes for Microfluidic Electroanalytical Systems

(ACS Applied Nano Materials, American Chemical Society (ACS), 2021-03-11) [Article]
Laser-induced graphene’s (LIG) inherent graphene-like and highly porous characteristics and its simple, scalable, and inexpensive fabrication render it a desirable electrode material for bio- and chemosensors. The best LIG electrodes are made in polyimide foils using a CO2 laser scriber, which unfortunately limits their integration into more sophisticated analytical devices due to polyimide’s inertness. The transfer of LIG electrodes onto standard polymer substrates used in microfluidic systems and their use in microfluidic assays were therefore studied and the resulting electrodes characterized morphologically, chemically, and electroanalytically. It was found that a direct pressure-driven transfer produces highly functional transfer-LIG (tLIG) electrodes. tLIG differed from LIG electrodes with respect to a much smoother surface and hence a lower active surface area, a loss of the graphene characteristic Raman 2D peak, and a slight decrease in electron transfer rates. However, their performance in amperometric detection strategies were comparable also when used in adhesive-tape-enabled microfluidic channels for the detection of p-aminophenol. tLIG outperformed LIG electrodes in their ability to be integrated into more advanced microfluidic channel systems made of an all-polymethyl methacrylate (PMMA) substrate for the biosensing detection of alkaline phosphatase, commonly used as a biomarker and as a biosensor amplification system. LIG and tLIG have hence the potential to change electroanalytical sensing in diagnostic systems as their fabrication requires minimal resources, is highly scalable, and allows their integration into simple and, as tLIG, also sophisticated analytical systems.
• #### High Thermal Effusivity Nanocarbon Materials for Resonant Thermal Energy Harvesting

(Small, Wiley, 2021-03-06) [Article]
Carbon nanomaterials have extraordinary thermal properties, such as high conductivity and stability. Nanocarbon combined with phase change materials (PCMs) can yield exceptionally high thermal effusivity composites optimal for thermal energy harvesting. The progress in synthesis and processing of high effusivity materials, and their application in resonant energy harvesting from temperature variations is reviewed.
• #### Assessing Margin-Wide Rupture Behaviors along the Cascadia Megathrust with 3-D Dynamic Rupture Simulations

(California Digital Library (CDL), 2021-03-05) [Preprint]
• #### A unified first order hyperbolic model for nonlinear dynamic rupture processes in diffuse fracture zones

(Copernicus GmbH, 2021-03-04) [Presentation]
Earthquake fault zones are more complex, both geometrically and rheologically, than an idealised infinitely thin plane embedded in linear elastic material. To incorporate nonlinear material behaviour, natural complexities and multi-physics coupling within and outside of fault zones, here we present a first order hyperbolic and thermodynamically compatible mathematical model for a continuum in a gravitational field which provides a unified description of nonlinear elasto-plasticity, material damage and of viscous Newtonian flows with phase transition between solid and liquid phases. The fault geometry and secondary cracks are described via a scalar function $\xi \in [0,1]$ that indicates the local level of material damage. The model also permits the representation of arbitrarily complex geometries via a diffuse interface approach based on the solid volume fraction function $\alpha \in [0,1]$. Neither of the two scalar fields $\xi$ and $\alpha$ needs to be mesh-aligned, allowing thus faults and cracks with complex topology and the use of adaptive Cartesian meshes (AMR). The model shares common features with phase-field approaches, but substantially extends them. We show a wide range of numerical applications that are relevant for dynamic earthquake rupture in fault zones, including the co-seismic generation of secondary off-fault shear cracks, tensile rock fracture in the Brazilian disc test, as well as a natural convection problem in molten rock-like material.
• #### Designing optimal experiments: an application to proton Compton scattering

(The European Physical Journal A, Springer Science and Business Media LLC, 2021-02-27) [Article]
Interpreting measurements requires a physical theory, but the theory’s accuracy may vary across the experimental domain. To optimize experimental design, and so to ensure that the substantial resources necessary for modern experiments are focused on acquiring the most valuable data, both the theory uncertainty and the expected pattern of experimental errors must be considered. We develop a Bayesian approach to this problem, and apply it to the example of proton Compton scattering. Chiral Effective Field Theory (χEFT) predicts the functional form of the scattering amplitude for this reaction, so that the electromagnetic polarizabilities of the nucleon can be inferred from data. With increasing photon energy, both experimental rates and sensitivities to polarizabilities increase, but the accuracy of χEFT decreases. Our physics-based model of χEFT truncation errors is combined with present knowledge of the polarizabilities and reasonable assumptions about experimental capabilities at HIγS and MAMI to assess the information gain from measuring specific observables at specific kinematics, i.e. to determine the relative amount by which new data are apt to shrink uncertainties. The strongest gains would likely come from new data on the spin observables Σ 2x and Σ2x′ at ω≃ 140 to 200 MeV and 40 ∘ to 120 ∘. These would tightly constrain γE1E1- γE1M2. New data on the differential cross section between 100 and 200 MeV and over a wide angle range will substantially improve constraints on αE1- βM1, γπ and γM1M1- γM1E2. Good signals also exist around 160 MeV for Σ 3 and Σ2z′. Such data will be pivotal in the continuing quest to pin down the scalar polarizabilities and refine understanding of the spin polarizabilities.
• #### The one-sided Lipschitz condition in the follow-the-leader approximation of scalar conservation laws

(arXiv, 2021-02-26) [Preprint]
We consider the follow-the-leader particle approximation scheme for a $1d$ scalar conservation law with nonnegative $L^\infty_c$ initial datum and with a $C^1$ concave flux, which is known to provide convergence towards the entropy solution $\rho$ to the corresponding Cauchy problem. We provide two novel contributions to this theory. First, we prove that the one-sided Lipschitz condition satisfied by the approximating density $\rho^n$ is a discrete version of an entropy condition; more precisely, under fairly general assumptions on $f$ (which imply concavity of $f$) we prove that the continuum version $\left(f(\rho)/\rho\right)_x\leq 1/t$ of said condition allows to select a unique weak solution, despite $\left(f(\rho)/\rho\right)_x\leq 1/t$ is apparently weaker than the classical Oleinik-Hoff one-sided Lipschitz condition $f'(\rho)_x\leq 1/t$. Said result relies on an improved version of Hoff's uniqueness proof. A byproduct of it is that the entropy condition is encoded in the particle scheme prior to the many-particle limit, which was never proven before. Second, we prove that in case $f(\rho)=\rho(A-\rho^\gamma)$ the one-sided Lipschitz condition can be improved to a discrete version of the classical (and sharp) Oleinik-Hoff condition. In order to make the paper self-contained, we provide proofs (in some cases alternative ones) of all steps of the convergence of the particle scheme.
• #### Investigation of the Deactivation and Reactivation Mechanism of a Heterogeneous Palladium(II) Catalyst in the Cycloisomerization of Acetylenic Acids by In Situ XAS

(ACS Catalysis, American Chemical Society (ACS), 2021-02-22) [Article]
A well-studied heterogeneous palladium(II) catalyst used for the cycloisomerization of acetylenic acids is known to be susceptible to deactivation through reduction. To gain a deeper understanding of this deactivation process and to enable the design of a reactivation strategy, in situ X-ray absorption spectroscopy (XAS) was used. With this technique, changes in the palladium oxidation state and coordination environment could be studied in close detail, which provided experimental evidence that the deactivation was primarily caused by triethylamine-promoted reduction of palladium(II) to metallic palladium nanoparticles. Furthermore, it was observed that the choice of the acetylenic acid substrate influenced the distribution between palladium(II) and palladium(0) species in the heterogeneous catalyst after the reaction. From the mechanistic insight gained through XAS, an improved catalytic protocol was developed that did not suffer from deactivation and allowed for more efficient recycling of the catalyst.
• #### Numerical study of COVID-19 spatial-temporal spreading in London

(arXiv, 2021-02-19) [Preprint]
Recent study reported that an aerosolised virus (COVID-19) can survive in the air for a few hours. It is highly possible that people get infected with the disease by breathing and contact with items contaminated by the aerosolised virus. However, the aerosolised virus transmission and trajectories in various meteorological environments remain unclear. This paper has investigated the movement of aerosolised viruses from a high concentration source across a dense urban area. The case study looks at the highly air polluted areas of London: University College Hospital (UCH) and King Cross and St Pancras International Station (KCSPI). We explored the spread and decay of COVID-19 released from the hospital and railway stations with the prescribed meteorological conditions. The study has three key findings: the primary result is that it is possible for the virus to travel from meters up to hundred meters from the source location. The secondary finding shows viruses released into the atmosphere from entry and exit points at KCSPI remain trapped within a small radial distance of < 50m. This strengthens the case for the use of face coverings to reduce the infection rate. The final finding shows that there are different levels of risk at various door locations for UCH, depending on which door is used there can be a higher concentration of COVID-19. Although our results are based on London, since the fundamental knowledge processes are the same, our study can be further extended to other locations (especially the highly air polluted areas) in the world.
• #### FluxPMU - A Maker's Guide of a DIY Synchronized Phasor Measurement Unit

(IEEE, 2021-02-16) [Conference Paper]
Synchrophasor technology is transforming the way the power grid is being monitored thanks to the benefits provided by synchronized Phasor Measurement Units (PMUs). To exploit these benefits, future power engineers need better understanding of how PMUs work at all levels, from hardware components to phasor estimation methods implemented in software. However, hands-on experiences with these technologies allowing students to “tinker” with a PMU are limited. The FluxPMU project aims to enable students to explore and build an open source, low cost PMU. Building from the legacy of OpenPMU V1, FluxPMU provides a Do-it-Yourself (DIY) guide with all documentation and software sources required to build the FluxPMU, which can be found in the following Github repository: https://github.com/alsetlab/fluxpmu. This paper summarizes the authors’ efforts in creating this Maker’s Guide.
• #### A non-unitary metasurface enables continuous control of quantum photon–photon interactions from bosonic to fermionic

(Nature Photonics, Springer Science and Business Media LLC, 2021-02-11) [Article]
Photonic quantum information processing, one of the leading platforms for quantum technologies1–5, critically relies on optical quantum interference to produce an indispensable effective photon–photon interaction. However, such an effective interaction is fundamentally limited to bunching6 due to the bosonic nature of photons7 and the restricted phase response from conventional unitary optical elements8,9. Here we propose and experimentally demonstrate a new degree of freedom in the optical quantum interference enabled by a non-unitary metasurface. Due to the unique anisotropic phase response that creates two extreme eigen-operations, we show dynamical and continuous control over the effective interaction of two single photons such that they show bosonic bunching, fermionic antibunching or arbitrarily intermediate behaviour, beyond their intrinsic bosonic nature. This quantum operation opens the door to both fundamental quantum light–matter interaction and innovative photonic quantum devices for quantum communication, quantum simulation and quantum computing.
• #### Synthesis of nickel and cobalt oxide nanoparticles by pulsed underwater spark discharges

(Journal of Applied Physics, AIP Publishing, 2021-02-10) [Article]
Electrical discharges in liquids are considered an efficient and ecological technique of nanoparticle synthesis via controlled erosion of electrodes. Herein, we use spark discharges between Co–Co, Ni–Ni, Co–Ni, or Ni–Co electrodes immersed in distilled water to synthesize Co and/or Ni nanoparticles, as well as their oxides. When mixed electrodes are used (Co–Ni or Ni–Co), both Co and Ni nanoparticles are produced, and the major species is dictated by the nature of the anode pin. The characteristics of nanoparticles synthesized under varying conditions of pulse width (100 and 500 ns) and voltage amplitude (5 and 20 kV) are analyzed by transmission electron microscopy. Within the investigated discharge conditions, it is not possible to produce Co–Ni nanoalloys; however, core–shell nanoparticles are observed among the Ni and Co nanoparticles. Finally, the direct optical bandgaps of the nanomaterials are determined using UV-visible absorption spectroscopy.
• #### Seismic source tracking with six degree-of-freedom ground motion observations

(Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), 2021-02-04) [Article]
Back azimuth information can be determined from combined measurements of rotations and translations at a single site. Such six degree-of-freedom (6-DoF) measurements are reasonably stable in delivering similar information compared to a small-scale array of three-component seismometers. Here we investigate whether a 6-DoF approach is applicable to tracking seismic sources. While common approaches determining the timing and location of energy sources generating seismic waves rely on the information of P-waves, here we use S-waves. We track back azimuths of directly arriving SH-waves in the 2-D case, P-converted SV-waves, direct SV- and direct SH-waves in the 3-D case. For data analysis, we compare a cross-correlation approach using a grid-search optimization algorithm with a polarization analysis method. We successfully recover the rupture path and rupture velocity with only one station, under the assumption of an approximately known fault location. Using more than one station, rupture imaging in space and time is possible without a priori assumptions. We discuss the effects of rupture directivity, supershear rupture velocity, source-receiver geometry, wavefield interference, and noise. We verify our approach with the analysis of moving traffic noise sources using 6-DoF observations. The collocated classic seismometer and newly-built ring laser gyroscope ROMY near Munich, Germany, allow us to record high-fidelity, broadband 6-DoF (particle velocity and rotational rate) ground motions. We successfully track vehicles and estimate their speed while traveling along a nearby highway using the estimated BAz as a function of time of a single station observation.
• #### DFT insights into hydrogen activation on the doping Ni2P surfaces under the hydrodesulfurization condition

(Applied Surface Science, Elsevier BV, 2021-02) [Article]
Hydrogen activation on the different Ni2P surfaces were explored under the traditional hydrodesulfurization conditions using density function theory (DFT) calculations. Firstly, the H2 dissociative adsorption phase diagrams were calculated and compared on the Ni(I)- and Ni(II)-exposed surfaces. The H2 dissociative adsorption on the Ni(I)-exposed surface had high activity below 4 H2 coverage, and the H2 dissociative adsorption on the Ni(II)-exposed surface was preferable below 2 H2 coverage. By contrast, the Ni(I)-exposed surface exhibited higher H2 activation activity than that on the Ni(II)-exposed surface. Moreover, the dissociative H atoms on Ni(II)-exposed surface would lead the rearrangement of surface Ni atoms to form the quasi tetrahedral coordination structure, which was similar to that of Ni(I)-exposed surface. Secondly, the surface phase diagrams were discussed on the different transition metals (Cr, Fe, Co, Cu or Mo) doping Ni(I)-exposed surfaces. The results revealed that H2 activation ability followed the order: Fe > Co > Mo > Cr > Ni > Cu. Furthermore, the difference charge densities and bader charge analysis were calculated to provide the additional information for understanding the phenomenon clearly, and the results indicated that the difficulty sequence of charge transfer between transition metals atom and H atoms was completely consistent with the above research.