Metastable Excitation Research Articles

Non-Nucleon Metastable Excitations in Nuclear Matter and e– Catalysis As a Quark-Cumulative Mechanism for Initiating Low-Energy Nuclear Chemical Processes: Phenomenology

Non-Nucleon Metastable Excitations in Nuclear Matter and e– Catalysis As a Quark-Cumulative Mechanism for Initiating Low-Energy Nuclear Chemical Processes: Phenomenology

Skyrmion lattice in centrosymmetric magnets with local Dzyaloshinsky–Moriya interaction

It is common for the local inversion symmetry to break in crystals, even though the whole crystal has global inversion symmetry. This local inversion symmetry breaking allows for a local Dzyaloshinsky–Moriya interaction (DMI) in magnetic crystals. Here we show that the local DMI can stabilize a skyrmion as a metastable excitation or as a skyrmion crystal in equilibrium. We consider the crystal structure with layered structure as an example, where local inversion is violated in each layer but a global inversion center exists in the middle of the two layers. These skyrmions come in pairs that are related by inversion symmetry. The two skyrmions with opposite helicity in a pair form a bound state. We study the properties of a skyrmion pair in the ferromagnetic background and determine the equilibrium phase diagram, where a robust lattice of skyrmion pairs is stabilized. Our results point to a new direction to search for the skyrmion lattice in centrosymmetric magnets.

Initiation of Artificial Radioactivity of Impurity Elements in a Lead Cathode under Conditions of a Glow Discharge

The possibility of initiation of nuclear chemical processes in the Pb cathode during a glow discharge in a low-temperature deuterium-containing nonequilibrium plasma leading to a significant (at times) decrease in the isotope content of some impurity elements (specifically, Zn) and increasing others (specifically, W, Fe, Mn, and Al). Such processes can be understood by introducing the existence in nuclear matter of metastable non-nucleon excitations of internal shaking (isu-states) formed by initiating actions on the nuclei of electrons with high (on chemical scales) kinetic energy Ee ~ 3–5 eV.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Current-Induced Nucleation and Motion of Skyrmions in Zero Magnetic Field

We study the stabilization and electrical manipulation of skyrmions in magnetic ultrathin films in the absence of an applied magnetic field. We show that this requires an increased magnetic anisotropy, controlled by the sample thickness, as compared to usual skyrmionic samples, so that the uniform state corresponds to the zero-field ground state and the skyrmions to metastable excitations. Although skyrmion stabilization at zero field is demonstrated over a broad range of thicknesses, electrical control appears to be more demanding to avoid skyrmion deformation. In the thinnest samples, the large magnetic anisotropy prevents skyrmion deformations and we show that they can be nucleated progressively by current pulses, which underlines that the only possible transition occurs between uniform and skyrmion states. The solitonic skyrmions in zero applied magnetic field have the same properties as compared to field-stabilized ones, with a long-term stability and high mobility when excited by a spin-orbit torque.

Probabilistic effect of metastable excitation on lock-in amplified plasma laser-induced fluorescence diagnostics at modulation frequencies comparable to the fluorescence frequency

In this paper, effects of discharge parameters and modulation frequency on the signal of laser-induced fluorescence measurements of ion velocity distribution functions are investigated in the LIF Test Source. A maximum modulation frequency is found for each given set of parameters, beyond which the signal gradually declines. Meanwhile, this maximum modulation frequency occurred consistently at ~1/10 of the theoretical frequency limit and photon counts received by a photomultiplier tube, which indicates that as modulation frequency and the associated per-pulse-excitation-event count decrease, the transition from the macroscopic statistical signal to the microscopic probabilistic signal is a gradual process.

Controlled creation of quantum skyrmions

We study the creation of quantum skyrmions in quadratic nanoscopic lattices of quantum spins coupled by Dzyaloshinkii-Moriya and exchange interactions. We numerically show that different kinds of quantum skyrmions, characterized by the magnitude of their spin expectation values and strong differences in their stability, can appear as ground state and as metastable excitations. In dependence on the coupling strengths and the lattice size, an adiabatic rotation scheme of the magnetization at the lattice boundaries allows for a controlled creation of quantum skyrmions.

High sensitivity hydrogen analysis in zircaloy-4 using helium-assisted excitation laser-induced breakdown spectroscopy

High-sensitivity detection of hydrogen (H) contained in zircaloy-4, a commonly used material for nuclear fuel containers, is crucial in a nuclear power plant. Currently, H detection is performed via gas chromatography, which is an offline and destructive method. In this study, we developed a technique based on metastable excited-state He-assisted excitation to achieve excellent quality of H emission spectra in double-pulse orthogonal laser-induced breakdown spectroscopy (LIBS). The production of metastable excited-state He atoms is optimized by using LiF as sub-target material. The results show a narrow full-width-at-half-maximum of 0.5 Å for the H I 656.2 nm emission line, with a detection limit as low as 0.51 mg/kg. Thus, using this novel online method, H in zircaloy-4 can be detected efficiently, even at very low concentrations.

A combined actinometry approach for medium pressure N2–O2 plasmas

An argon actinometry approach which uses the 777 nm and 844 nm atomic oxygen lines along with the common N2/ line ratio method is used to experimentally investigate the excitation mechanisms of the O(3p5P) state in medium pressure (0.75 Torr) N2–O2 plasmas. The method provides strong evidence that metastable excitation from the O(3s5S◦) state is an important excitation mechanism for O(3p5P) state for these conditions. The strong dependence of this additional mechanism on electron density along with the use of the N2/ line ratio method allow for the simultaneous determination of effective electron temperature, dissociation fraction, metastable O(3s5S◦) density and electron density. Predicted electron temperatures, electron densities and dissociation fractions are consistent with those found for similar discharges in prior studies. Additionally, we investigate the efficacy of using the observed 616 nm atomic oxygen line from the O(4d5D◦) state for actinometry, which could open the door to a novel and comprehensive actinometry technique.

Resonant electron impact excitation of highly charged Fe ions studied with a compact electron beam ion trap

AbstractWe present extreme ultraviolet spectra of 3s3p–3s3d transitions in Fe14+ observed with a compact electron beam ion trap. The contributions of indirect excitation via a metastable state and resonant excitation are studied by observing the electron energy dependence of the spectra for the energy range of 60–210 eV. The results indicate that the 3s3d 1D2 level is directly excited from the 3s2 ground state whereas the 3s3d 3D3 level has a large contribution of the indirect excitation via the 3s3p 3P2 metastable state. Comparisons with the theoretical excitation cross sections including MNn resonant excitations show good qualitative agreement with the experimental results for the electron energy dependent features.

The importance of cascade emission and metastable excitation in modeling strong atomic oxygen lines in laboratory plasmas

A collisional-radiative model for optically accessible atomic oxygen lines in laboratory plasmas has been developed to identify dominant mechanisms and assess the validity and utility for plasma diagnostics. A rate sensitivity analysis of the common 777 nm/844 nm line ratio was carried out and used to determine the importance of often ignored mechanisms including cascade emission and metastable excitation. Commonly used basic models for emission of these lines is compared to the model developed here and the mechanisms responsible for disagreement are identified. At low pressures metastable excitation from the state becomes important and at high pressures collisional relaxation of to is also non-negligible. At all pressures cascade emission from higher lying states was found to be crucial in accurately estimating the 777 nm/844 nm line ratio. Effective cascade excitation cross sections were calculated and used to include a cascade rate in the basic analytic models, which improves overall agreement with the collisional radiative model, particularly in the medium pressure range. Finally, the 777 nm/844 nm line ratio is calculated and presented for a variety of common laboratory plasma parameters.

Skyrmions in the Moiré of van der Waals 2D Magnets.

We explore the skyrmion formation and control possibilities in two-dimensional (2D) magnets from the ubiquitous moiré pattern in vdW heterostructures. Using the example of a ferromagnetic monolayer on an antiferromagnetic substrate, we demonstrate a new origin of skyrmions in the 2D magnets from the lateral modulation of interlayer magnetic coupling by the locally different atomic registries in moiré. The moiré skyrmions are doubly degenerate with opposite topological charge and trapped at an ordered array of sites with the moiré periodicity that can be dramatically tuned by strain and interlayer translation. At relatively strong interlayer coupling, the ground states are skyrmion lattices, where magnetic field can switch the skyrmion vorticity and location in the moiré. At weak interlayer coupling limit, we find metastable skyrmion excitations on the ferromagnetic ground state that can be deterministically moved between the ordered moiré trapping sites by current pulses. Our results point to potential uses of moiré skyrmions both as information carriers and as drastically tunable topological background of electron transport.

Repulsive polarons in alkaline-earth-metal-like atoms across an orbital Feshbach resonance

We characterize properties of the so-called repulsive polaron across the recently discovered orbital Feshbach resonance in alkaline-earth(-like) atoms. Being a metastable quasiparticle excitation at the positive energy, the repulsive polaron is induced by the interaction between an impurity atom and a Fermi sea. By analyzing in detail the energy, the polaron residue, the effective mass, and the decay rate of the repulsive polaron, we reveal interesting features that are intimately related to the two-channel nature of the orbital Feshbach resonance. In particular, we find that the life time of the repulsive polaron is non-monotonic in the Zeeman-field detuning bewteen the two channels, and has a maximum on the BEC-side of the resonance. Further, by considering the stability of a mixture of the impurity and the majority atoms against phase separation, we show that the itin- erant ferromagnetism may exist near the orbital Feshbach resonance at appropriate densities. Our results can be readily probed experimentally, and have interesting implications for the observation of itinerant ferromagnetism near an orbital Feshbach resonance.

Development of a hybrid model to quantify the single and double strand breaks following the iron 57 de-excitation

The therapeutic utility of certain Auger emitters such as iodine-125 depends on their position within the cell nucleus. Or diagnostically, and to maintain as low as possible cell damage, it is preferable to have radionuclide localized outside the cell or at least the core. One solution to this problem is to consider markers capable of conveying anticancer drugs to the tumor site regardless of their location within the human body. The objective of this study is to simulate the impact of a complex such as bleomycin on single and double strand breaks in the DNA molecule. Indeed, this simulation consists of the following transactions: Construction of BLM -Fe- DNA complex, simulation of the electron’s transport from the metastable state excitation of Fe 57 by the Monte Carlo method, treatment of chemical reactions in the considered environment by the diffusion equation. For physical, physico-chemical and finally chemical steps, the geometry of the complex is considered as a sphere of 50 nm centered on the binding site , and the mathematical method used is called step by step based on Monte Carlo codes.

Continuous local model for two-dimensional spin ice

We propose a classical continuous model Hamiltonian with a ground state presenting a spin ice structure. We analyze the introduction of metastable excitations on this ground state, showing the emergence of pairs of magnetic monopoles. The interaction between monopoles and dipoles in the system is studied. As a consequence, we obtain an effective nonlocal interaction between monopoles and dipoles from a local classical spin model.

Excitation of 11/2− isomeric state in 137Ce in (γ,n) reaction within 10–22MeV energy range

Isomeric yields ratios and energy differential cross-section of metastable state excitation in the [Formula: see text] reaction has been studied in the 10–22[Formula: see text]MeV end-point energy range of bremsstrahlung beams. Experimental data are compared with Hauser–Feshbach calculations performed with TALYS-1.6 statistical model code.

In-situ study of light production and transport in phonon/light detector modules for dark matter search

The CRESST experiment (Cryogenic Rare Event Search with Superconducting Thermometers) searches for dark matter via the phonon and light signals of elastic scattering processes in scintillating crystals. The discrimination between a possible dark matter signal and background is based on the light yield.We present a new method for evaluating the two characteristics of a phonon/light detector module that determine how much of the deposited energy is converted to scintillation light and how efficiently a module detects the produced light. In contrast to former approaches with dedicated setups, we developed a method which allows us to use data taken with the cryogenic setup, during a dark matter search phase. In this way, we accounted for the entire process that occurs in a detector module, and obtained information on the light emission of the crystal as well as information on the performance of the module (light transport and detection).We found that with the detectors operated in CRESST-II phase 1, about 20% of the produced scintillation light is detected. A part of the light is likely absorbed by creating meta-stable excitations in the scintillating crystals. The light not detected is not absorbed entirely, as an additional light detector can help to increase the fraction of detected light.

Linear response theory of uniformly complex‐scaled many‐body systems

AbstractLinear response functions for a uniformly complex‐scaled many‐body system are derived. Differences between these functions and their Hermitian analogs are emphasized including additional poles at metastable excitations. Energy absorption is investigated and it is shown how a complex‐scaled external perturbation and its complex‐conjugate couples to the system's energy and lifetime.

Observation and Identification of Metastable Excited States in Ultrafast Laser-Ionized Pyridine

We report on the fragmentation of ionized pyridine (C(5)H(5)N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron-type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. A previously-developed method is used to identify the precursor and final masses of these ions. The metastable process that creates the most prevalent peak is shown to be C(5)H(5)N(+) → C(4)H(4)(+) + HCN. Simulations confirm this result and place restrictions on the processes for several other observed metastable reactions.

Relaxation of the cyclotron spin-flip excitation in a spin-unpolarized quantum Hall system

Cyclotron spin-flip excitation in an even-integer quantum Hall system is the lowest-energy excitation separated from the ground state by the gap slightly smaller than the cyclotron energy, and from the upper magnetoplasma excitation by the Coulomb gap [1, 2]. At the filling factor ν = 2 in a real system these gaps are in the vicinity of 10 meV and 0.5 meV and thus are much larger than the Zeeman gap (∼ 0.1 meV) and temperature (< 0.01 meV) respectively. Under these conditions the cyclotron spin flip excitation can not relax in a purely electronic way but only with emission of a short-wave phonon (k ∼ 3 107 /cm). As a result the relaxation in a modern wide-thickness quantum well occurs very slowly, the characteristic relaxation time being ∼ 1 s. The cyclotron spin-flip excitation in the narrow vicinity of filling factors ν = 2, 4, ... should therefore be considered as a collective metastable excitation in the quantum Hall system.