bluuebunny/crossref_metadata_2025_split huggingface dataset:
Canonical Noether and the energy–momentum non-uniqueness problem in linearized gravity
Authors: Baker Mark Robert 
Published: March 2021

Abstract
Recent research has highlighted the non-uniqueness problem of energy–momentum tensors in linearized gravity; many different tensors are published in the literature, yet for particular calculations a unique expression is required. It has been shown that (A) none of these spin-2 energy–momentum tensors are gauge invariant and (B) the Noether and Hilbert energy–momentum tensors are not, in general, equivalent; therefore uniqueness criteria is difficult to specify. Conventional wisdom states that the various published energy–momentum tensors for linearized gravity can be derived from the canonical Noether energy–momentum tensor of spin-2 Fierz-Pauli theory by adding ad-hoc ‘improvement’ terms (the divergence of a superpotential and terms proportional to the equations of motion), that these superpotentials are in some way unique or physically significant, and that this implies some meaningful connection to the Noether procedure. To explore this question of uniqueness, we consider the most general possible energy–momentum tensor for linearized gravity with free coefficients using the Fock method. We express this most general energy–momentum tensor as the canonical Noether tensor, supplemented by the divergence of a general superpotential plus all possible terms proportional to the equations of motion. We then derive systems of equations which we solve in order to prove several key results for spin-2 Fierz–Pauli theory, most notably that there are infinitely many conserved energy–momentum tensors derivable from the ‘improvement’ method, and there are infinitely many conserved symmetric energy–momentum tensors that follow from specifying the Belinfante superpotential alone. This disproves several recent claims that the Belinfante tensor is uniquely associated to the Hilbert tensor in spin-2 Fierz–Pauli theory. We give two new energy–momentum tensors of this form. Most importantly, since there are infinitely many energy–momentum tensors of this form, no meaningful or unique connection to Noether’s first theorem can be claimed by application of the canonical Noether ‘improvement’ method.
Electric-dipole forbidden transitions for probing atomic state preparation: the case of the Autler–Townes effect
Authors: Ramírez-Martínez F , Ponciano-Ojeda F , Hernández-Gómez S , Del Ángel A , Mojica-Casique C , Hoyos-Campo L M , Flores-Mijangos J , Sahagún D , Jáuregui R , Jiménez-Mier J 
Published: March 2021

Abstract
This work demonstrates an scheme in which a weak electric quadrupole (E2) transition with a controlled frequency detuning in a ladder configuration is used as an extremely non-perturbing probe for precisely identifying subtle details of the effects driven by AC fields coupling atomic states and the preparation of quantum states. For this purpose, we provide a precise experimental and theoretical study of the Autler Townes (AT) effect derived from a strong electric dipole transition stimulated by a resonant laser beam and probed by the E2 interaction. The experiment was carried out for a 87Rb atomic gas at room temperature in a velocity-selective scheme. The AT effect was monitored via the splitting of the fluorescence spectra associated with the spontaneous decay to the ground state. The theoretical description incorporates the modification of standard few-level schemes introduced by forbidden electric-dipole transitions selection rules. We develop an analytic ladder three-level scheme to approximate the cyclic 5S1/2
F = 2 → 5P3/2
F = 3 → 6P3/2
F = 1, 2, 3 → 5S1/2
F = 2 path. Other levels that could have effects on the fluorescence are included via a fourth level with effective parameters. Doppler effects and finite bandwidths of the laser beams are included in the theoretical model to closely reproduce the experimental results.
Effect of gaps on magnetic noise of cylindrical ferrite shield
Authors: Lu Jixi , Sun Chang , Ma Danyue , Yang Ke , Zhao Junpeng , Han Bangcheng , Quan Wei , Zhang Ning , Ding Ming 
Published: March 2021

Abstract
Cylindrical ferrite shields consisting of multiple annuli can provide low-magnetic-noise environments. However, the gaps between different parts of the shield adversely affect the shielding performance. In this study, we numerically study the influence of the gaps on both the axial and radial magnetic noise of the multi-annular ferrite shield with different gap widths, permeabilities, and aspect ratios using the generalized Nyquist relation and the finite element method. Our study indicates that as the gap width increases, the axial magnetic noise decreases, while the radial magnetic noise increases. For the shield with a gap width of 0.10 mm and an aspect ratio of one in our study, the axial magnetic noise decreases by 5.2%, and the radial magnetic noise increases by 8.3%–33.5%. The difference in the increase is related to the permeability, and gaps have smaller effect on the radial magnetic noise of the shield with lower permeabilities. Therefore, compared with the no-gap shield, the multi-annular ferrite shield can provide an even better low-magnetic-noise environment along the axial direction. Although the gaps can induce an increase of the radial magnetic noise, this detrimental effect can be suppressed significantly by using a ferrite material with lower permeability and reducing the gap width. Our findings can aid in the application and optimization of low-noise ferrite shields.
Mathematical modelling of the conductivity in CZTiS-CZSnS as a function of synthesis temperature
Authors: Mesa M Patarroyo , Báez Y T Castellanos , Cerón-Achicanoy M A , Gómez-Cuaspud J , Chaparro W Aperador , López E Vera 
Published: March 2021

AbstractThe electrical behavior of photovoltaic materials related with Cu2ZnTiS4and Cu2ZnSnS4materials were analyzed as function of synthesis temperature in accordance with a new mathematical model based on the Kramers–Kronig equations with a high reliability. The samples were obtained through a hydrothermal route and a subsequent thermal treatment of solids at 550 °C for 1 h under nitrogen flow (50 ml min−1). The characterization was done by x-ray diffraction, ultraviolet spectroscopy (UV), Raman spectroscopy, atomic force microscopy (AFM) and solid state impedance spectroscopy (IS) techniques. The structural characterization, confirm the obtention of a tetragonal material with spatial groupI-42m, oriented along (1 1 2) facet, with nanometric crystal sizes (5–6 nm). The AFM and Raman analysis confirm a high level of chemical homogeneity and correlation with the synthesis temperature, associated with the roughness of the samples. The UV spectroscopy confirm a band gap around 1.4–1.5 eV, evidencing the effectiveness of the synthesis process. The IS results at room temperature with a probability of 95%, confirm a high consistency of data with respect to values of real and imaginary impedance, allowing to obtain information of the conductance, reactance and inductance, achieving conductivity values around 10−5and 10−3Ω−1 m−1in comparison with traditional mathematical models used for this purpose.
Simplified chemical processed Cd1−x
Al
x
S thin films for high-performance photodetector applications
Authors: Deva Arun Kumar Karuppiah , Mele Paolo , Anitha M , Varadharajaperumal S , Alagarasan Devarajan , Alhokbany Norah S , Ahamad Tansir , Alshehri Saad M 
Published: March 2021

Abstract
In this present investigation, we report the effect of aluminum (Al) doping on the photoelectric performance of cadmium sulfide (CdS) thin films prepared by cost-effective automatic nebulizer spray method. The doping of Al concentrations varied from 1 at.% to 9 at.% in the steps of 3 at.%. X-ray diffraction (XRD) patterns show hexagonal crystal structure with polycrystalline nature and the enrichment of crystallite sizes as a function of Al doping concentrations. The formed impurity phase i.e. CdO might be helpful in enhancing the photoelectric performance by its additional photo-generated charge carriers. The optical studies confirm the maximum absorption showed in the visible spectral range with the corresponding minimum bandgap of 2.28 eV for 6 at.% of Al. The room temperature photoluminescence studies show an increase of near-band-edge (NBE) emission as a function of Al doping concentration and this NBE is close to the obtained bandgap in terms of wavelength. In addition, the observed red emission at 635 nm is due to the surface-related impurities or native defect states. From the present work, the observed responsivity (R), external quantum efficiency (EQE) and detectivity (D*) of the CdS:Al detectors are 8.64 AW−1, ∼2018% and 9.29 × 1011 jones, respectively for the optimum 6 at.% of CdS:Al film. The performance of CdS:Al films reported in this work are significantly improved when compared with literature reports. The present investigation, therefore offers a potential material, CdS:Al, as a photodetector for various scientific and industrial applications.
Structure-property correlations and scaling in the magnetic and magnetocaloric properties of GdCrO3 particles
Authors: Shi Jianhang , Sauyet Theodore , Dang Yanliu , Suib Steven L , Seehra Mohindar S , Jain Menka 
Published: March 2021

Abstract
The structure, magnetic, and magnetocaloric (MC) properties of orthorhombic nanocrystalline GdCrO3 with six particle sizes: ⟨d⟩ = 87, 103, 145, 224, 318, and 352 nm are reported. The particle size was tailored by annealing under different temperatures and estimated by scanning electron microscopy. With increase in ⟨d⟩, Goldschmidt tolerance factor t, orthorhombic strain s, and out-of-plane Cr–O1–Cr bond angle first decrease, reaching minimum values for ⟨d⟩ = 224 nm, and then increase for sample with ⟨d⟩ = 318 nm and 352 nm, thus showing a V-shaped variation. Temperature dependence of the magnetization (M) reveals an antiferromagnetic transition at

T

N

Cr

∼
168

K for ⟨d⟩ ⩾ 224 nm and

T

N

Cr

∼
167

K for ⟨d⟩ < 224 nm and an essentially d-independent spin-reorientation at T
SR = 9 K. M measured at 5 K and 7 T first increases with increase in ⟨d⟩, reaching maximum value for sample with ⟨d⟩ = 224 nm, and then decreases for samples with ⟨d⟩ = 318 nm and 352 nm, showing an inverted-V variation with ⟨d⟩. Similar ⟨d⟩-dependence is observed for the magnetic entropy change (MEC) and relative cooling power (RCP) showing a close relationship between the structural and magnetic properties of GdCrO3 nanoparticles investigated here. The 224 nm sample with the minimum values of t, s, and Cr–O1–Cr bond angle exhibits the maximum value of MEC (−ΔS) = 37.8 J kg−1 K−1 at 5 K under a field variation (ΔH) of 7 T and its large estimated RCP of 623.6 J Kg−1 is comparable with those of typical MC materials. Both (−ΔS) and RCP are shown to scale with the saturation magnetization M
S, suggesting that M
S is the crucial factor controlling their magnitudes. Assuming (−ΔS) ∼ (ΔH)
n
, the temperature dependence of n for the six samples are determined, n varying between 1.3 at 5 K to n = 2.2 at 130 K in line with its expected magnitudes based on mean-field theory. These results on structure-property correlations and scaling in GdCrO3 suggest that its MC properties are tunable for potential low-temperature magnetic refrigeration applications.
Growth of h-BN/graphene heterostructure using proximity catalysis
Authors: Yang Hui , Wang Bojun , Niu Xiaobin , Guo Guo-Ping 
Published: March 2021

Abstract
In this study, a proximity catalysis route was developed for the fast growth of graphene/h-BN vertical heterostructures on Cu foils, which shows much improved synthesis efficiency (500 times faster than other routes) and good crystalline quality graphene (large single crystalline length up to 10 μm). The key advantage of our synthesis route is the introduction of fresh Cu foil (or Cu foam) into the high-temperature zone using a turntable. At high temperatures, Cu vapor acts as a gaseous catalyst, which can reduce the energy barrier of graphene growth and promote the decomposition of carbon sources. Therefore, after the first layer of hexagonal boron nitride is grown on the Cu substrate, another layer of graphene can be grown by introducing a fresh catalyst. Our calculations have revealed the catalytic effect and graphene growth contribution of Cu vapor evaporated by the suspended catalyst. We also investigated the growth sequence of graphene from 1 to 24 carbon atoms on h-BN/Cu and determined the morphology evolution of these carbon clusters. In this regard, multilayer stacked heterogeneous structures can be synthesized, thus increasing their potential applications in high performance electronic devices and energy harvesting/transition directions.
Transfer learning from ECG to PPG for improved sleep staging from wrist-worn wearables
Authors: Li Qiao , Li Qichen , Cakmak Ayse S , Da Poian Giulia , Bliwise Donald L , Vaccarino Viola , Shah Amit J , Clifford Gari D 
Published: March 2021

Abstract

Objective. To develop a sleep staging method from wrist-worn accelerometry and the photoplethysmogram (PPG) by leveraging transfer learning from a large electrocardiogram (ECG) database. Approach. In previous work, we developed a deep convolutional neural network for sleep staging from ECG using the cross-spectrogram of ECG-derived respiration and instantaneous beat intervals, heart rate variability metrics, spectral characteristics, and signal quality measures derived from 5793 subjects in Sleep Heart Health Study (SHHS). We updated the weights of this model by transfer learning using PPG data derived from the Empatica E4 wristwatch worn by 105 subjects in the ‘Emory Twin Study Follow-up’ (ETSF) database, for whom overnight polysomnographic (PSG) scoring was available. The relative performance of PPG, and actigraphy (Act), plus combinations of these two signals, with and without transfer learning was assessed. Main results. The performance of our model with transfer learning showed higher accuracy (1–9 percentage points) and Cohen’s Kappa (0.01–0.13) than those without transfer learning for every classification category. Statistically significant, though relatively small, incremental differences in accuracy occurred for every classification category as tested with the McNemar test. The out-of-sample classification performance using features from PPG and actigraphy for four-class classification was Accuracy (Acc) = 68.62% and Kappa = 0.44. For two-class classification, the performance was Acc = 81.49% and Kappa = 0.58. Significance. We proposed a combined PPG and actigraphy-based sleep stage classification approach using transfer learning from a large ECG sleep database. Results demonstrate that the transfer learning approach improves estimates of sleep state. The use of automated beat detectors and quality metrics means human over-reading is not required, and the approach can be scaled for large cross-sectional or longitudinal studies using wrist-worn devices for sleep staging.
Plasma plume expansion with pulsed electron neutralization
Authors: Habl L , Lafleur T , Rafalskyi D , Chabert P 
Published: March 2021

Abstract
Electrons neutralizing the ion beam from a gridded ion source are typically provided by an external cathode. This cathode emits a continuous current that ensures quasi-neutrality of the plume, and current balance of the ion source. A new type of neutralization scheme has recently been identified in the context of radio-frequency (RF) biased ion sources, where instead of a continuous electron current, the plume is neutralized by electron pulses emitted from the same plasma source as the ion beam itself. In contrast to conventional gridded ion sources, experiments have shown that pulsed neutralization produces hot electrons with a strongly anisotropic energy distribution in the plume. By making use of a two-dimensional particle-in-cell (PIC) simulation, we analyze the pulsed neutralization and plasma expansion to understand the fundamental plume physics in these systems, and perform a direct comparison with the expansion observed in typical DC systems. Electron trapping in the near-field plume region is found to be critical for ensuring quasi-neutrality, and the plume potential is observed to be higher than the downstream acceleration grid potential to prevent excessive electron backstreaming into the plasma source. This potential difference results in the formation of high-energy electron beams that generate collective plume oscillations with frequencies above the applied RF frequency. A detailed parametric study is performed to investigate the influence of the pulse frequency, emission current, and capacitance between the source and outer surrounding boundaries. In particular, the pulse frequency and emission current have a significant effect on the resulting plume potential, and the effectiveness of the resulting ion beam neutralization.
Storing short single-photon-level optical pulses in Bose–Einstein condensates for high-performance quantum memory
Authors: Saglamyurek Erhan , Hrushevskyi Taras , Rastogi Anindya , Cooke Logan W , Smith Benjamin D , LeBlanc Lindsay J 
Published: March 2021

Abstract
Large-scale quantum networks require quantum memories featuring long-lived storage of non-classical light together with efficient, high-speed and reliable operation. The concurrent realization of these features is challenging due to inherent limitations of matter platforms and light–matter interaction protocols. Here, we propose an approach to overcome this obstacle, based on the implementation of the Autler–Townes-splitting (ATS) quantum-memory protocol on Bose–Einstein condensate (BEC) platform. We demonstrate a proof-of-principle of this approach by storing short pulses of single-photon-level light as a collective spin-excitation in a rubidium-BEC. For 20 ns long-pulses, we achieve an ultra-low-noise memory with an efficiency of 30% and lifetime of 15 μs. The non-adiabatic character of the ATS protocol (leading to high-speed and low-noise operation) in combination with the intrinsically large atomic densities and ultra-low temperatures of the BEC platform (offering highly efficient and long-lived storage) opens up a new avenue toward high-performance quantum memories.
First-principles study of mechanical and thermodynamic properties of W-V alloys
Authors: Cui Zilin , Zhang Xin , Xu Yuhong , Lei Guangjiu , Tang Changjian , Chen Xi , Li Heng , Liu Yangyang , Zhu Yiqin , Li Ming , Geng Shaofei , Liu Qijun , Ni Yuxiang , Liu Haifeng , Wang Xianqu , Huang Jie , Liu Hai , Cheng Jun 
Published: March 2021

Abstract
The lattice structure, mechanical properties, electronic structure and thermodynamic properties of W1−xVx alloys (x = 0, 0.0625, 0.125, 0.25, 0.3125, 0.5 and 1) were calculated and simulated by first-principles method based on DFT. The mechanical properties including Poisson’s ratio, elastic moduli and B/G ratio were analysed via V concentration. The simulated results showed the ductility of alloys getting improved while the strength decreased with the addition of vanadium. And we calculated the anisotropy of Young’s modules and ideal tensile strength. The projected density of states is calculated to analyse the electronic structure. Furthermore, thermodynamic properties like Debye temperature and low temperature heat capacity were also calculated in this work. All the results can provide reference for selecting plasma facing materials in fusion reactor design.
Double photoionization of atomic oxygen: Feshbach resonances in the two-electron continuum
Authors: Gorczyca T W , P Ballance C , Manson S T , Kilcoyne D , Stolte W C 
Published: March 2021

Abstract
We describe a joint experimental and theoretical investigation on oxygen double photoionization—the emission of two electrons from atomic oxygen following single photon absorption. High-resolution experimental measurements were performed at the Advanced Light Source, revealing sharp resonance structure superimposed on the more familiar Wannier-like, nearly-linear background. These resonance features are attributed to ionization-plus-excitation Feshbach resonances embedded in the double ionization continuum, doubly-excited states that lie above the double-ionization threshold. Such features are absent in the double photoionization cross section of He, or other quasi-two-electron systems, for which the doubly-ionized atomic core remains inert. For a corresponding theoretical analysis, the R-matrix with pseudostates (RMPS) method was invoked by calculating final-state, two-electron resonances-plus-continua wavefunctions and corresponding single-photon absorption cross sections. Overall agreement is found in the direct, background double photoionization cross section. However, the RMPS method, using a small basis due to practical computational limitations, was unable to reproduce quantitatively the smooth background or the sharper resonance features observed in the measurements, showing instead large-scale oscillations about the experimental background, and characteristic pseudoresonance jitter, associated with an insufficient convergence of the pseudostate representation to the true two-electron infinite series of Feshbach resonances embedded in the two-electron continuum. The prominent resonance structure observed highlights the need to consider multiple excitation processes in atoms more complex than He or quasi-two-electron systems.
Orbital and Stellar Parameters for 2M06464003+0109157: A Double-lined Eclipsing Binary of Spotted, Sub-solar Twins
Authors: Miller Annaliese , Kounkel Marina , Boggio Chase , Covey Kevin , Price-Whelan Adrian M. 
Published: March 2021

Abstract
We calculated physical and orbital properties for 2M06464003+0109157, a 1.06 day eclipsing double lined spectroscopic binary. We modeled the system’s ASAS-SN and TESS light curves, measuring the system’s inclination and radii of each component. Extracting radial velocities for each component from 6 SDSS/APOGEE spectra, we measured the system’s mass ratio and performed a full orbital fit. Our analysis indicates that 2M06464003+0109157 has components with nearly equal masses (m
1/m
2 = 0.99 ± 0.01; M
1,2 = 0.57 ± 0.015 M
☉) and comparable radii (R
1 = 0.66 ± 0.05 R
☉, R
2 = 0.57 ± 0.06 R
☉). The solution required two star spots to incorporate the out of eclipse variation that is seen in the light curve. We report our full characterization of this system, and prospects for similar analyses using survey data to measure precise physical and orbital properties for EBs.
Beryllium-driven structural evolution at the divertor surface
Authors: Cusentino M.A. , Wood M.A. , Thompson A.P. 
Published: March 2021

Abstract
Erosion of the beryllium first wall material in tokamak reactors has been shown to result in transport and deposition on the tungsten divertor. Experimental studies of beryllium implantation in tungsten indicate that mixed W–Be intermetallic deposits can form, which have lower melting temperatures than tungsten and can trap tritium at higher rates. To better understand the formation and growth rate of these intermetallics, cumulative molecular dynamics (MD) simulations of both high and low energy beryllium deposition in tungsten were performed. In both cases, a W–Be mixed material layer (MML) emerged at the surface within several nanoseconds, either through energetic implantation or a thermally-activated exchange mechanism, respectively. While some ordering of the material into intermetallics occurred, fully ordered structures did not emerge from the deposition simulations. Targeted MD simulations of the MML to further study the rate of Be diffusion and intermetallic growth rates indicate that for both cases, the gradual re-structuring of the material into an ordered intermetallic layer is beyond accessible MD time scales(⩽1 μs). However, the rapid formation of the MML within nanoseconds indicates that beryllium deposition can influence other plasma species interactions at the surface and begin to alter the tungsten material properties. Therefore, beryllium deposition on the divertor surface, even in small amounts, is likely to cause significant changes in plasma-surface interactions and will need to be considered in future studies.