Particle Excitations Research Articles

Physico-chemical mechanism of surface dielectric barrier discharge product change based on spectral diagnosis

To gain an insight into the interaction mechanism among the gaseous products of atmospheric pressure air plasma, a surface dielectric barrier discharge is used as a study object. The dynamic processes of characteristic products (nitric oxide NO and ozone O<sub>3</sub>) are measured by in-situ Fourier infrared spectroscopy and UV absorption spectroscopy. The real energy density of the plasma is calculated by Lissajous figure and ICCD optical image. The gas temperature is obtained by fitting the emission spectrum of the second positive band of the nitrogen molecule. The results show that the real energy density and gas temperature are highly positively correlated with the applied voltage and frequency. Higher applied voltages and frequencies can lead to lower peak absorbance of O<sub>3</sub> and higher absorbance of NO, and accelerate the conversion of the products from O<sub>3</sub>-containing state into O<sub>3</sub>-free state. The microscopic mechanism of the product change is revealed by analyzing the effects of the real energy density and gas temperature on the major generation and quenching chemical reactions of the characteristic products. The analysis points out that there are two major reasons for the disappearance of O<sub>3</sub>, i.e. the quenching effect of O and O/O<sub>2</sub> excited state particles on O<sub>3</sub> and the quenching effect of NO on O<sub>3</sub>. And the mechanism that the disappearance of O<sub>3</sub> accelerates with the increase of energy density and gas temperature, is as follows. The increase of real energy density means that the energy injected into the discharge region is enhanced, which intensifies the collision reaction, thereby producing more energetic electrons and reactive oxygen and nitrogen particles. Since the discharge cavity is gas-tight, the rapid generation of O leads to a rapid increase in the ratio of O to O<sub>2</sub>, which accelerates the decomposition of O<sub>3</sub>; besides, the gas temperature is raised due to the intensification of the collision reaction. Whereas the gas temperature can change the rate coefficients of the chemical reactions involving the excited state particles of nitrogen and oxygen to regulate the production and quenching of the products. The increase of gas temperature has a negative effect on O<sub>3</sub>. The higher the gas temperature, the lower the rate of O<sub>3</sub> generation reaction is but the higher the rate of dissociation, which is thought to be the endogenous cause of the rapid disappearance of O<sub>3</sub>. In contrast, the gas temperature rising can significantly elevate the reaction rate of NO production and reduces its dissociation rate. This contributes to the faster production of massive NO, resulting in an accelerated quenching process of NO to O<sub>3</sub>, which can be considered as the exogenous cause of the rapid disappearance of O<sub>3</sub>. In a word, the present study contributes to a better understanding of the physico-chemical process in atmospheric pressure low-temperature plasma.

Silver nanoparticles with exceptional near-infrared absorbance for photoenhanced antimicrobial applications.

In this work, we outline a simple method for synthesizing decahedral and triangular silver nanoparticles using light to tune particle shape and spectral characteristics. Notably, we were able to generate triangular silver nanoparticles with exceptional absorbance in the near-infrared (NIR) region, with high spectral overlap with the biological window, making them particularly promising for biological applications. We further demonstrate that under complementary LED illumination, these excitable plasmonic particles display exceptional antibacterial properties, several orders of magnitude more potent than similar particles under dark conditions or under illumination that does not match particle absorbance. This work demonstrates the powerful effects that LED lights can have on the antibacterial activity of AgNPs, providing an inexpensive and easily implemented route to unlocking the full potential of AgNPs in photobiological applications.

Generation of a low-temperature plasma in a nozzle to initiate ion-cluster reactions in jets of mixtures of methane with a buffer gas

We have considered the option of generating cluster ions directly at the nozzle by initiating an electric discharge. Here, a low-temperature plasma in the flow is formed as a result of the outflow of a partially ionized gas. Conditions for the formation of clusters in the presence of excited particles are provided downstream. To implement this generation option, a scheme for creating an effective discharge in the diffuser part of a supersonic nozzle was proposed and implemented. The diagnostic system of the LEMPUS-2 gas-dynamic facility was adapted to register processes in a supersonic flow of ionized gas with clusters. Molecular nitrogen, which is weakly condensing under experimental conditions, was used as the primary reference gas. The methods of molecular beam mass spectrometry were used to study gas mixtures consisting of 20% methane in an inert carrier gas. Highly condensable argon and practically non-condensable helium were used as carrier gases. It was found that when using the ionized particle selection system for detection, only methane monomers and dimers are registered in mixtures with argon, whereas in mixtures with helium larger clusters are registered. A version of the explanation of the detected dependencies was presented.

Photothermal effects of gold nanorods in aqueous solution and gel media: Influence of particle size and excitation wavelength.

Gold nanorods (GNRs) have emerged as the most efficient photothermal agent in cancer therapy and photocatalysis. Understanding the influence of the surrounding medium, particle size, and excitation wavelength is critical to optimising the photothermal conversion rate. Here, three pairs of large and small gold nanorods of different aspect ratios and their heat generation under laser radiation at on and off surface plasmon resonance wavelengths in aqueous solution and gel-like media are investigated. In the aqueous solution, the temperature rise of the large gold nanorods is more than with small gold nanorods at resonance excitation. In contrast to the large gold nanorods (LGNRs), the small gold nanorods (SGNRs) were less sensitive to excitation wavelength. At off-resonance excitation, the temperature rise of the SGNRs is larger than that of the LGNRs. In the agarose gel, the photothermal effect of the SGNRs is greater than LGNRs excited at the wavelength near their solution phase longitudinal surface plasmon resonance wavelength. The temperature increase of LGNRs in gel is significantly less than in aqueous solution. These findings suggest that SGNRs could be more beneficial than the LGNRs for photothermal applications in biological systems and provides further insight when selecting GNRs.

Low energy excitation spectrum of magic-angle semimetals

We theoretically study the excitation spectrum of a two-dimensional Dirac semimetal in the presence of an incommensurate potential. Such models have been shown to possess magic-angle critical points in the single particle wavefunctions, signalled by a momentum space delocalization of plane wave eigenstates and flat bands due to a vanishing Dirac cone velocity. Using the kernel polynomial method, we compute the single particle Green's function to extract the nature of the single particle excitation energy, damping rate, and quasiparticle residue. As a result, we are able to clearly demonstrate the redistribution of spectral weight due to quasiperiodicity-induced downfolding of the Brillouin zone creating minibands with effective mini Brillouin zones that correspond to emergent superlattices. By computing the damping rate we show that the vanishing of the velocity and generation of finite density of states at the magic-angle transition coincides with the development of an imaginary part in the self energy and a suppression of the quasiparticle residue that vanishes in a power law like fashion. Observing these effects with ultracold atoms using momentum resolved radiofrequency spectroscopy is discussed.

One-Step Formation of Plasmonic Cu Nanodomains in p-Type Cu2O Matrix Films for Enhanced Photoconversion of n-ZnO/p-Cu2O Heterojunctions

Plasmonic Cu nanoparticles were in-situ grown into a Cu$_2$O semiconductor matrix by using reactive magnetron sputtering and adjusting the amount of oxygen available during the synthesis in order to prevent the oxidation of part of copper atoms landed on the film surface. Varying only the oxygen flowrate (OFR) and using a single Cu target it was possible to observe the evolution in the simultaneous formation of metallic Cu and Cu$_2$O phases for oxygen-poor conditions. Suchformation is accompanied by the development of the surface plasmon band (SPB) corresponding to Cu, as evidenced by UV-Vis spectrophotometry and spectroscopic ellipsometry. The bandgap values of the elaborated composites containing embedded Cu plasmonic nanodomains were lower than the bandgap of single-phased Cu$_2$O films, likely due to the higher defect density associated to the nanocrystalline nature of films, promoted by the presence of metallic Cu. The resistivity of the thin films increased with more oxidative deposition conditions and was associated to an increase in Cu$_2$O/Cu ratio and smaller and more isolated Cu particles, as evidenced by high resolution transmission electron microscopy and X-ray diffraction. Photoconversion devices based on the studied nanocomposites were characterized by I-V and spectral photocurrent measurements, showing an increase in the photocurrent density under light illumination as consequence of the plasmonic particles excitation leading to hot carrier's injection in the nearby ZnO and Cu$_2$O semiconductors.

New scintillator 6[formula omitted]:[formula omitted] for neutron sensitive screens

In the current study scintillation properties of a recently developed 6Li2CaSiO4:Eu2+ polycrystalline scintillation material have been evaluated. Samples of thin layers of scintillator with thicknesses of 54, 120, and 180 μm were prepared and the scintillation light yield of the samples under alpha particle excitation was estimated to be 12000 ph/MeV. The material with a layer thickness of 180μm shows similar sensitivity to the commercial ZnS:Ag-6LiF based 470μm Scintacor ND screen to moderated neutrons and has less sensitivity to the gamma-quanta. Scintillation kinetics has been estimated to be five times faster than that provided by ZnS:Ag, which makes the reported material prospective for high count rate measurements in pulse mode.

Particle manipulation with acoustic waves based on Hertz-Mindlin mechanics

Ultrasound noncontact particle manipulation (vibrotransportation) is of interest for a number of industrial applications: delivery of solid agents for defect healing, noninvasive excretion of undesirable particles, micromachine technology, and sorting of particles. It is known that a particle on a substrate can be moved by a combined action of acoustic waves and friction. A traditional theory describing the effect considers a particle as a material point capable of moving only against the wave, i.e., in a direction opposite to the wave propagation direction. Here, we use another approach based on Hertz-Mindlin mechanics in which a particle represents a deformable body whereas a zone of its contact with the substrate has a certain structure including evolving regions of stick and slip. The friction dynamics of the Hertz-Mindlin system with a mass comprises different regimes, including a consistent motion in a desired direction (not only the one opposite to the direction of wave propagation but also the one oriented like the wave), which allows a more elaborate manipulation or positioning of the particles. The presented numerical tool treats the problem of a particle on a substrate being moved by Rayleigh waves. We demonstrate various regimes in which steady movement is observed or not, depending on excitation parameters and particle inertia.

Cryogenic Scintillation Properties of Lead-Free Cs3Cu2I5 Single Crystals.

Perovskite scintillators have become increasingly popular in recent years because of their simple production and high sensitivity to X-ray. Due to large Stokes shifts, high light yield, eco-friendly fabrication, and good stability, the lead-free Cu-based perovskites have gained much attention. In this paper, we prepared the Cs3Cu2I5 single crystals (SCs) by the solution-processed method. At room temperature, we measured the emission band at 440 nm with an average decay time of 595 ns under X-ray excitation. Under 137Cs γ-ray excitation, we determined that the light yield of Cs3Cu2I5 SCs was 23 000 photons/MeV. Notably, under alpha particle excitation by 241Am, the light yield of Cs3Cu2I5 SCs is approximately 3.2 times higher than that of the commercial scintillator LYSO(Ce). In addition, we systematically investigated the cryogenic scintillation properties of Cs3Cu2I5 SCs at the temperature range of 60-300 K. With decreasing temperature, the intensity of the emission band at 440 nm significantly increases, and an additional emission band at 336 nm emerges below 100 K. Meanwhile, the temperature-dependent decay times were determined. The fast and slow decay time of Cs3Cu2I5 SCs are estimated to be 221 and 1193 ns, respectively, at 60 K. Our findings highlight the great potential for Cs3Cu2I5 SCs to be a cryogenic scintillator.

Suppression of Interband Heating for Random Driving.

Heating to high-lying states strongly limits the experimental observation of driving induced nonequilibrium phenomena, particularly when the drive has a broad spectrum. Here we show that, for entire families of structured random drives known as random multipolar drives, particle excitation to higher bands can be well controlled even away from a high-frequency driving regime. This opens a window for observing drive-induced phenomena in a long-lived prethermal regime in the lowest band.

Scattering strings off quantum extremal surfaces

We consider a Hayden & Preskill like setup for both maximally chaotic and sub-maximally chaotic quantum field theories. We act on the vacuum with an operator in a Rindler like wedge R and transfer a small subregion I of R to the other wedge. The chaotic scrambling dynamics of the QFT Rindler time evolution reveals the information in the other wedge. The holographic dual of this process involves a particle excitation falling into the bulk and crossing into the entanglement wedge of the complement to r = R\\I. With the goal of studying the locality of the emergent holographic theory we compute various quantum information measures on the boundary that tell us when the particle has entered this entanglement wedge. In a maximally chaotic theory, these measures indicate a sharp transition where the particle enters the wedge exactly when the insertion is null separated from the quantum extremal surface for r. For sub-maximally chaotic theories, we find a smoothed crossover at a delayed time given in terms of the smaller Lyapunov exponent and dependent on the time-smearing scale of the probe excitation. The information quantities that we consider include the full vacuum modular energy R\\I as well as the fidelity between the state with the particle and the state without. Along the way, we find a new explicit formula for the modular Hamiltonian of two intervals in an arbitrary 1+1 dimensional CFT to leading order in the small cross ratio limit. We also give an explicit calculation of the Regge limit of the modular flowed chaos correlator and find examples which do not saturate the modular chaos bound. Finally, we discuss the extent to which our results reveal properties of the target of the probe excitation as a “stringy quantum extremal surface” or simply quantify the probe itself thus giving a new approach to studying the notion of longitudinal string spreading.

All-in-One Optofluidic Chip for Molecular Biosensing Assays.

Integrated biosensor platforms have become subjects of high interest for consolidated assay preparation and analysis to reduce sample-to-answer response times. By compactly combining as many biosensor processes and functions as possible into a single lab-on-chip device, all-in-one point-of-care devices can aid in the accessibility and speed of deployment due to their compact size and portability. Biomarker assay preparation and sensing are functionalities that are often carried out on separate devices, thus increasing opportunity of contamination, loss of sample volume, and other forms of error. Here, we demonstrate a complete lab-on-chip system combining sample preparation, on-chip optofluidic dye laser, and optical detection. We first show the integration of an on-chip distributed feedback dye laser for alignment-free optical excitation of particles moving through a fluidic channel. This capability is demonstrated by using Rhodamine 6G as the gain medium to excite single fluorescent microspheres at 575 nm. Next, we present an optofluidic PDMS platform combining a microvalve network (automaton) for sample preparation of nanoliter volumes, on-chip distributed feedback dye laser for target excitation, and optical detection. We conduct concurrent capture and fluorescence tagging of Zika virus nucleic acid on magnetic beads in 30 min. Target-carrying beads are then optically excited using the on-chip laser as they flow through an analysis channel, followed by highly specific fluorescence detection. This demonstration of a complete all-in-one biosensor is a tangible step in the development of a rapid, point-of-care device that can assist in limiting the severity of future outbreaks.

Gravitational dynamics in Higgs inflation: Preinflation and preheating with an auxiliary field

The dynamics of both the preinflationary and the preheating epochs for a model consisting of a Higgs inflaton plus an additional auxiliary field are studied in full General Relativity. The minimally coupled auxiliary field allows for parametric-type resonances that successfully transfer energy from the inflaton condensate to particle excitations in both fields. Depending on the interaction strengths of the fields, the broad resonance periods lead to structure formation consisting of large under/over-densities, and possibly the formation of compact objects. Moreover, when confronting the same model to multi-field inhomogeneous preinflation, the onset of inflation is shown to be a robust outcome. At relatively large Higgs values, the non-minimal coupling acts as a stabilizer, protecting the dynamics of the inflaton, and significantly reducing the impact of perturbations in other fields and matter sectors. These investigations further confirm the robustness of Higgs inflation to multi-field inhomogeneous initial conditions, while putting in evidence the formation of complex structures during the reheating.

Missing final state puzzle in the monopole-fermion scattering

It has been known that when a charged fermion scatters off a monopole, the fermion in the s-wave component must flip its chirality, i.e., fermion number violation must happen. This fact has led to a puzzle; if there are two or more flavors of massless fermions, any superposition of the fermion states cannot be the final state of the s-wave scattering as it is forbidden by conservation of the electric and flavor charges. The unitary evolution of the state vector, on the other hand, requires some interpretation of the final states. We solve the puzzle by finding new particle excitations in the monopole background, where multi-fermion operators exhibit condensation. The particles are described as excitations of closed-string configurations of the operators.

Relativistic quantum field theory of stochastic dynamics in the Hilbert space

We develop an action formulation of stochastic dynamics in the Hilbert space. By generalizing the Wiener process into 1+3-dimensional spacetime, we define a Lorentz-invariant random field. By coupling the random to quantum fields, we obtain a random-number action which has the statistical spacetime translation and Lorentz symmetries. The canonical quantization of the theory results in a Lorentz-invariant equation of motion for the state vector or density matrix. We derive the path integral formula of $S$-matrix and the diagrammatic rules for both the stochastic free field theory and stochastic $\phi^4$-theory. The Lorentz invariance of the random $S$-matrix is strictly proved. We then develop a diagrammatic technique for calculating the density matrix. Without interaction, we obtain the exact $S$-matrix and density matrix. With interaction, we prove a simple relation between the density matrices of stochastic and conventional $\phi^4$-theory. Our formalism leads to an ultraviolet divergence which has the similar origin as that in QFT. The divergence is canceled by renormalizing the coupling strength to random field. We prove that the stochastic QFT is renormalizable even in the presence of interaction. In the models with a linear coupling between random and quantum fields, the random field excites particles out of the vacuum, driving the universe towards an infinite-temperature state. The number of excited particles follows the Poisson distribution. The collision between particles is not affected by the random field. But the signals of colliding particles are gradually covered by the background excitations caused by random field.

Manipulating Goldstone modes via superradiant light in a bosonic lattice gas inside a cavity

We study the low-energy excitations of a bosonic lattice gas with cavity-mediated interactions. By performing two successive Hubbard-Stratonovich transformations, we derive an effective field theory to study the strongly-coupling regime. Taking into account the quantum fluctuation, we report the unusual effect of the superradiant cavity light induced density imbalance, which has been shown to have a negligible effect on the single particle excitation in the previous studies. Instead, we show that such negligible fluctuation of density imbalance dramatically changes the behavior of the low-energy excitation and results in a free switching between two types of Goldstone modes in its single particle excitation, i.e., type I and type II with odd and even power energy-momentum dispersion, respectively. Our proposal would open a new horizon for manipulating Goldstone modes from bridging the cavity light and strongly interacting quantum matters.

Second-quantized Unruh-DeWitt detectors and their quantum reference frame transformations

We generalize the Unruh-DeWitt detector model to second quantization. We illustrate this model by applying it to an excited particle in a superposition of relativistic velocities. We calculate, to first order, how its decay depends on whether its superposition of velocities is coherent or incoherent. Further, we generalize the framework of quantum reference frames to allow transformations to the rest frames of second-quantized Unruh DeWitt detectors. As an application, we show how to transform into the rest frame of a decaying particle that, in the laboratory frame, is in a linear superposition of relativistically differing velocities.

Fractionalizing global symmetry on looplike topological excitations

Symmetry fractionalization (SF) on topological excitations is one of the most remarkable quantum phenomena in topological orders with symmetry, i.e., symmetry-enriched topological phases. While much progress has been theoretically and experimentally made in 2D, the understanding on SF in 3D is far from complete. A long-standing challenge is to understand SF on looplike topological excitations which are spatially extended objects. In this work, we construct a powerful topological-field-theoretical framework approach for 3D topological orders, which leads to a systematic characterization and classification of SF. For systems with Abelian gauge groups ($G_g$) and Abelian symmetry groups ($G_s$), we successfully establish equivalence classes that lead to a finite number of patterns of SF, although there are notoriously infinite number of Lagrangian-descriptions of the system. We compute topologically distinct types of fractional symmetry charges carried by particles. Then, for each type, we compute topologically distinct statistical phases of braiding processes among loop excitations and external symmetry fluxes. As a result, we are able to unambiguously list all physical observables for each pattern of SF. We present detailed calculations on many concrete examples. As an example, we find that the SF in an untwisted $\mathbb{Z}_2\times \mathbb{Z}_2$ topological order with $\mathbb{Z}_2$ symmetry is classified by $ (\mathbb{Z}_2)^6\oplus (\mathbb{Z}_2)^2\oplus (\mathbb{Z}_2)^2\oplus (\mathbb{Z}_2)^2$. If the topological order is twisted, the classification reduces to $(\mathbb{Z}_2)^6$ in which particle excitations always carry integer charge. Pure algebraic formalism of the classification is given by: $ \bigoplus_{\nu_i} \mathcal H^4 ( G_g\leftthreetimes_{\nu_i}G_s, U (1)) /\Gamma_\omega ({\nu_i}) \,$. Several future directions are proposed.

Inhomogeneous quantum quenches in the sine-Gordon theory

We study inhomogeneous quantum quenches in the attractive regime of the sine--Gordon model. In our protocol, the system is prepared in an inhomogeneous initial state in finite volume by coupling the topological charge density operator to a Gaussian external field. After switching off the external field, the subsequent time evolution is governed by the homogeneous sine--Gordon Hamiltonian. Varying either the interaction strength of the sine--Gordon model or the amplitude of the external source field, an interesting transition is observed in the expectation value of the soliton density. This affects both the initial profile of the density and its time evolution and can be summarised as a steep transition between behaviours reminiscent of the Klein--Gordon, and the free massive Dirac fermion theory with initial external fields of high enough magnitude. The transition in the initial state is also displayed by the classical sine--Gordon theory and hence can be understood by semi-classical considerations in terms of the presence of small amplitude field configurations and the appearance of soliton excitations, which are naturally associated with bosonic and fermionic excitations on the quantum level, respectively. Features of the quantum dynamics are also consistent with this correspondence and comparing them to the classical evolution of the density profile reveals that quantum effects become markedly pronounced during the time evolution. These results suggest a crossover between the dominance of bosonic and fermionic degrees of freedom whose precise identification in terms of the fundamental particle excitations can be rather non-trivial. Nevertheless, their interplay is expected to influence the sine--Gordon dynamics in arbitrary inhomogeneous settings.

Effect of Impurity on the Doping-Induced in-Gap States in a Mott Insulator

Motivated by the recent measurements of the spatial distribution of single particle excitation states in a hole-doped Mott insulator, we study the effects of impurity on the in-gap states, induced by the doped holes, in the Hubbard model on the square lattice by the cluster perturbation theory. We find that a repulsive impurity potential can move the in-gap state from the lower Hubbard band towards the upper Hubbard band, providing a good account for the experimental observation. The distribution of the spectral function in the momentum space can be used to discriminate the in-gap state induced by doped holes and that by the impurity. The spatial characters of the in-gap states in the presence of two impurities are also discussed and compared to the experiment.