bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photon ring structure of rotating regular black holes and no-horizon spacetimes
Authors: Kumar Rahul , Ghosh Sushant G 
Published: January 2021

Abstract
The Kerr black holes possess a photon region with prograde and retrograde orbits radii, respectively,

M
⩽

r

p

−

⩽
3
M

and

3
M
⩽

r

p

+

⩽
4
M

, and thereby always cast a closed photon ring or a shadow silhouette for a ⩽ M. For a > M, it is a no-horizon spacetime (naked singularity) wherein prograde orbits spiral into the central singularity, and retrograde orbits produce an arc-like shadow with a dark spot at the center. We compare Kerr black holes’ photon ring structure with those produced by three rotating regular spacetimes, viz Bardeen, Hayward, and nonsingular. These are non-Kerr black hole metrics with an additional deviation parameter of g related to the nonlinear electrodynamics charge. It turns out that for a given a, there exists a critical value of g, g
E such that Δ = 0 has no zeros for g > g
E, one double zero at r = r
E for g = g
E, respectively, corresponding to a no-horizon regular spacetime and extremal black hole with degenerate horizon. We demonstrate that, unlike the Kerr naked singularity, no-horizon regular spacetimes can possess closed photon ring when g
E < g ⩽ g
c, e.g. for a = 0.10M, Bardeen (g
E = 0.763 332M < g ⩽ g
c = 0.816 792M), Hayward (g
E = 1.052 97M < g ⩽ g
c = 1.164 846M) and nonsingular (g
E = 1.2020M < g ⩽ g
c = 1.222 461M) no-horizon spacetimes have closed photon ring. These results confirm that the mere existence of a closed photon ring does not prove that the compact object is necessarily a black hole. The ring circularity deviation observable ΔC for the three no-horizon rotating spacetimes satisfy ΔC ⩽ 0.10 as per the M87* black hole shadow observations. We have also appended the case of Kerr–Newman no-horizon spacetimes (naked singularities) with similar features.
Parity violating metric-affine gravity theories
Authors: Iosifidis Damianos , Ravera Lucrezia 
Published: January 2021

Abstract
We study a metric-affine gravitational theory given by the Einstein–Hilbert (EH) action plus a parity violating contribution (which we will refer to as the Hojman term, also known as Holst term) in vacuum. We find out that for a certain value of the Barbero–Immirzi (BI) parameter the total action possesses a remarkable invariance under particular transformations of the affine connection. We prove that in all cases, with appropriate gauge choices, the connection reduces to the Levi-Civita one and that the theory turns out to be equivalent to general relativity (GR) in vacuum. Subsequently, we generalize our discussion and analyze the case of metric-affine f(R) gravity plus the Hojman term. In particular, we show that for f′(R) ≠ constant the theory results to be on-shell equivalent to a metric-compatible torsionless scalar–tensor model with a propagating pseudo-scalar. Matter coupling of the aforementioned models is also discussed, together with explicit examples and applications.
New sum rules for Wigner 3jm symbols: application to expectation values of hydrogenic ions
Authors: Pain Jean-Christophe , Gilleron Franck 
Published: January 2021

Abstract
We present new sum rules for 3jm coefficients, which involve, in addition to the usual weighting factor (2j + 1) where j is an angular momentum, the quantity [j(j + 1)]
k
with k ⩾ 1. The sum rules appear for instance in the statistical modeling of rotational spectra within the theory of moments, and enable one to deduce the expectation values of r

k
(used in the theory of Stark effect for hydrogenic ions) in parabolic coordinates from the expectation values of r

k
in spherical coordinates.
CH3NH3PbI3 as a radio frequency decoupling capacitor: interplay between Maxwell–Wagner polarization and a pseudo inductive response
Authors: Besra Nripen , Sardar Kausik , Mazumder Nilesh , Bhattacharjee Souvik , Das Anjan , Das Bikram , Sarkar Saikat , Chattopadhyay Kalyan Kumar 
Published: January 2021

Abstract
Frequency-dispersive impedance analysis of CH3NH3PbI3 perovskite is carried out under the external Direct current (DC) field to investigate the interplay of dielectric polarization and delocalized carrier transport. Switching of capacitance from positive to negative values is observed in the radio frequency range (42.1–42.5 MHz) for the external bias ranging from 0–4 V. The switching frequency outlined a decreasing trend with an increase in bias. Upon fitting the experimentally obtained dispersions, a bi-relaxation mechanism is unveiled. One of its constituents arises due to the typical Maxwell–Wagner interfacial polarization between the grain cores and boundaries and acts at the lower frequencies. The other one is manifested via hopping of delocalized carriers, resulting in a high frequency degenerative pseudo inductive response. The interference of these two mechanisms is manifested into an asymmetric Breit–Wigner–Fano profile of the dielectric susceptance spectra. The results are further elaborated from a theoretical point of view involving the energy band structure, electron localization function, and Mulliken charge distribution.
Tunable polarization-independent MoS2-based coherent perfect absorber within visible region
Authors: Zamharir Sara Ghayeb , Karimzadeh Rouhollah , Luo Xin 
Published: January 2021

Abstract
The paper suggests a new perfect absorber design based on the coherent perfect absorption (CPA) phenomenon of two-dimensional MoS2 monolayer in the visible region. Our proposed structure consists of a single MoS2 layer that is sandwiched between a SiO2 layer and a silver film with the nanocube dielectric array. According to the finite difference time domain simulations, CPA occurs at 722.2 nm in the certain structural configuration. Our computational results confirm the tunability of CPA resonant wavelength as a function of geometric and optical parameters of the nanocube dielectric array. Therefore, our CPA design may offer a solution to the light absorption enhancement of MoS2 monolayer in the visible region for optoelectronics applications.
In situ and operando electron microscopy in heterogeneous catalysis—insights into multi-scale chemical dynamics
Authors: Chee See Wee , Lunkenbein Thomas , Schlögl Robert , Cuenya Beatriz Roldan 
Published: January 2021

Abstract
This review features state-of-the-art in situ and operando electron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolved operando measurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field’s future directions will also be discussed.
Tailoring the electronic structure and magnetic properties of pyrochlore Co2Ti1−x
Ge
x
O4: a GGA + U
ab initio study
Authors: Ghosh Sayandeep , Singh Sobhit , Das Debashish , Ghosh Subhradip , Mishra Pankaj Kumar , Thota Subhash 
Published: January 2021

Abstract
We report the electronic structure and magnetic properties of Co2Ti1−x
Ge
x
O4 (0 ⩽ x ⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U
eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a ≠ 1) due to the non-degenerate

d

z

2

and

d

x

2

−

y

2

states (e

g
orbitals) of the B-site Co. We observe large exchange splitting (ΔEX ∼ 9 eV) between the up and down spin bands of t
2g
and e

g
states, respectively, of tetrahedral and octahedral Co2+ (4A2(g)(F)) and moderate crystal-field splitting (ΔCF ∼ 4 eV) and the Jahn–Teller distortion (ΔJT ∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽ E

g
⩽ 3.3 eV). The exchange interaction (J
BB ∼ −4.8 meV, for (x, y) = (0.25, 0)) between the Co2+ dominates the overall antiferromagnetic behaviour of the system for all ‘x’ as compared to J
AA (∼−2.2 meV, for (x, y) = (0.25, 0)) and J
AB (∼−1.8 meV, for (x, y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasing x due to the imbalance of Co spins between the A- and B-sites.
Laser ray bundle: a thermometry method for HIFU heating
Authors: Daoud Hussein , Oweis Ghanem F 
Published: January 2021

Abstract
High-intensity focused ultrasound (HIFU) is a therapeutic modality that heats tissue by focusing a sound beam into a narrow, localized region. This paper is concerned with the thermo-optic lensing effect observed by Al-Qraini et al (Ultrasound Med. Biol.
39 647–59) when visible light was shone at a transparent tissue-mimicking phantom heated by HIFU. Thermal lensing was caused by light refraction as it passed through the thermal gradient region. HIFU thermal lensing was reproduced experimentally and studied analytically using the geometric (eikonal) ray equation to devise a method for temperature measurement. Under the Gaussian thermal field assumption, explicit closed form solutions were found for the forward thermo-optic ray deflection problem, and for the inverse opto-thermal conversion problem that yielded the underlying temperature field from measured ray deflections. Neither temperature calibration nor iterative computations were needed. In the experimental setup, a bundle of thin laser rays illuminated the HIFU-heated spot in a clear tissue-mimicking phantom. The ray deflections were recorded and directly converted into temperature with very good temporal and spatial resolution. These compared reasonably well against the output of an embedded thermocouple (TC)—considering the absorptive and viscous heating artifacts which were also investigated. The TC metallic tip junction was carefully positioned to coincide with the axis of peak heating. It was found that the TC stem (wire) transferred heat by conduction from the tip into the tissue located off the HIFU heating axis. The proposed thermometry method can be useful in elucidating heating from focused ultrasound sources, or from other sources of localized heating such as lasers.
Accurate and dynamic 3D shape measurement with digital image correlation-assisted phase shifting
Authors: Hu Pengyu , Yang Shuming , Zheng Fenghe , Yuan Ye , Wang Teng , Li Shusheng , Liu Haibao , Dear John P 
Published: January 2021

Abstract
Phase-shifting profilometry (PSP) has been widely used in structured-light (SL) systems for three-dimensional (3D) shape measurements, but the speed of the PSP technique is limited by increased phase-shifting patterns. This paper proposes an accurate and dynamic 3D shape measurement method by projecting only four patterns, including three-step phase-shifting patterns and one speckle pattern. Three-step phase-shifting images are used to obtain the initial unwrapped phase map with phase ambiguity. Based on the principle of digital image correlation and multi-view geometry, the absolute phase can be recovered reliably without requiring any embedded features or pre-defined information of the object. To improve the measurement accuracy, the projector coordinate is used as the measuring coordinate to establish a novel stereo SL system model. By solving a least square solution using the triple-view information, accurate 3D surface data can be reconstructed. The experimental results indicate that the proposed method can perform high-speed and accurate 3D shape measurements with an accuracy of 10.64 μm, which is superior to conventional methods and has certain instructive significance for 3D profilometry and measurement engineering.
Improved sodium storage properties of nickel sulfide nanoparticles decorated on reduced graphene oxide nanosheets as an advanced anode material
Authors: Li Kai , Yu Jianhua , Wang Qianyu , Li Lizhi , Zhang Wendi , Ma Jianru , Zhang Jianmin , Liu Pu , Li Dan 
Published: January 2021

Abstract
For sodium ion batteries, the fabrication of nanocrystal anode materials has been identified as a satisfactory strategy to improve electrochemical performance and maintain the structural integrity of electrodes. However, the issues of agglomeration and serious volume variation have always existed within the process of charging/discharging in anode materials. In this work, a series of composites of nickel sulfide nanoparticles decorated on reduced graphene oxide nanosheets (denoted as NiS2@rGO) were successfully synthesized via a simple one-step hydrothermal method under different temperatures. The strategy of confining nickel sulfide nanoparticles within the interlayer of graphene nanosheets can not only avoid the agglomeration, but also alleviate the volume change to some extent in electrode materials. For sodium ion storage, the NiS2@rGO synthesized at 160 °C exhibited a higher reversible capacity and better rate capability.
A novel hybrid 3D dose reconstruction approach for pre-treatment verification of intensity modulated proton therapy plans
Authors: Arjunan Manikandan , Sharma Dayananda Shamurailatpam , Kaushik Suryakant , Krishnan Ganapathy , Patro Kartikeshwar C , Padanthaiyil Noufal Mandala , Rajesh T , Jalali R 
Published: January 2021

Abstract

Aim. A novel hybrid three-dimensional (3D) dose reconstruction method, based on planar dose measured at a single shallower depth, was developed for use as patient-specific quality assurance (PSQA) of intensity modulated proton therapy (IMPT) plans. The accuracy, robustness and sensitivity of the presented method were validated for multiple IMPT plans of varying complexities. Methods and Materials. An in-house MATLAB program was developed to reconstruct 3D dose distribution from the planar dose (GyRBE) measured at 3 g cm−2 depth in water or solid phantom using a MatriXX PT ion chamber array. The presented method was validated extensively for 11 single-field optimization (SFO) and multi-field optimization (MFO) plans on Proteus Plus. A total of 47 reconstructed planar doses at different depths were compared against the corresponding RayStation treatment planning system (TPS) and MatriXX PT measurement using a gamma passing rate (γ%) evaluated for 3%/3 mm. The robustness of the reconstruction method with respect to depth, energy layers, field dimensions and complexities in the spot intensity map (SIM) were analysed and compared against the standard PSQA. The sensitivity of the reconstruction method was tested for plans with intentional errors. Results. The presented reconstruction method showed excellent agreement (mean γ% > 98%) and robustness with both TPS-calculated and measured dose planes at all depths (2.97–30 g cm−2), energy layers (82.1–225.5 MeV), field dimensions, target volume (17.7–1000 cm3) and SIMs from both SFO and MFO plans. In comparison to the overall mean ± SD γ% from standard PSQA, the reconstruction method showed reductions in mean γ% within 1% for both standard cubes and clinical plans. The reconstruction method was sensitive enough to detect intentional spot positional errors in a selected energy layer of a plan. Conclusion. The presented hybrid reconstruction method is sufficiently accurate, robust and sensitive to estimate planar dose at any user-defined depth. It simplifies the measurement setup and eliminates multiple depth measurements.
Low-frequency conductivity tensor imaging with a single current injection using DT-MREIT
Authors: Sadighi Mehdi , Şişman Mert , Açıkgöz Berk C , Eroğlu Hasan H , Eyüboğlu B Murat 
Published: January 2021

Abstract
Diffusion tensor-magnetic resonance electrical impedance tomography (DT-MREIT) is an imaging modality to obtain low-frequency anisotropic conductivity distribution employing diffusion tensor imaging and MREIT techniques. DT-MREIT is based on the linear relationship between the conductivity and water self-diffusion tensors in a porous medium, like the brain white matter. Several DT-MREIT studies in the literature provide cross-sectional anisotropic conductivity images of tissue phantoms, canine brain, and the human brain. In these studies, the conductivity tensor images are reconstructed using the diffusion tensor and current density data acquired by injecting two linearly independent current patterns. In this study, a novel reconstruction algorithm is devised for DT-MREIT to reconstruct the conductivity tensor images using a single current injection. Therefore, the clinical applicability of DT-MREIT can be improved by reducing the total acquisition time, the number of current injection cables, and contact electrodes to half by decreasing the number of current injection patterns to one. The proposed method is evaluated utilizing simulated measurements and physical experiments. The results obtained show the successful reconstruction of the anisotropic conductivity distribution using the proposed single current DT-MREIT.
An analytical model of full-field displacement and strain induced by amplitude-modulated focused ultrasound in harmonic motion imaging
Authors: McGarry Matthew D J , Campo Adriaan , Payen Thomas , Han Yang , Konofagou Elisa E 
Published: January 2021

Abstract
The majority of disease processes involves changes in the micro-structure of the affected tissue, which can translate to changes in the mechanical properties of the corresponding tissue. Harmonic motion imaging (HMI) is an elasticity imaging technique that allows the study of the mechanical parameters of tissue by detecting the tissue response by a harmonic motion field, which is generated by oscillatory acoustic radiation force. HMI has been demonstrated in tumor detection and characterization as well as monitoring of ablation procedures. In this study, an analytical HMI model is demonstrated and compared with a finite element model (FEM), allowing rapid and accurate computation of the displacement, strain, and shear wave velocity (SWV) at any location in a homogenous linear elastic material. Average absolute differences between the analytical model and the FEM were respectively 1.2% for the displacements and 0.5% for the strains for 41 940 force voxels at 0.22 s per displacement evaluation. A convergence study showed that the average difference could be further decreased to 1.0% and 0.15% for the displacements and strains, respectively, if force resolution is increased. SWV fields, as calculated with the FEM and the analytical model, have regional differences in velocities up to 0.57 m s−1 with an average absolute difference of 0.11 ± 0.07 m s−1, primarily due to imperfections in the non-reflecting FEM boundary conditions. The apparent SWV differed from the commonly used plane-wave approximation by up to 1.2 m s−1 due to near and intermediate field effects. Maximum displacement amplitudes for a model with an inclusion stabilize within 10% of the homogenous model at an inclusion radius of 10 mm while the maximum strain reacts faster, stabilizing at an inclusion radius of 3 mm. In conclusion, an analytical model for HMI stiffness estimation is presented in this paper. The analytical model has advantages over FEM as the full-field displacements do not need to be calculated to evaluate the model at a single measurement point. This advantage, together with the computational speed, makes the analytical model useful for real-time imaging applications. However, the analytical model was found to have restrictive assumptions on tissue homogeneity and infinite dimensions, while the FEM approaches were shown adaptable to variable geometry and non-homogenous properties.
First measurements of line-integrated electron density in an ITER-like configuration using the JET far infrared polarimeter diagnostic
Authors: Rossi Riccardo , Boboc Alexandru , Orsitto Francesco Paolo , Gelfusa Michela , Gaudio Pasquale 
Published: January 2021

Abstract
Polarimetry exploits the optical activity and birefringent properties of thermonuclear plasmas to calculate some important quantities for their control like the line-integrated electron density and magnetic field distribution. The Joint European Torus (JET) far infrared polarimeter shares the same probing laser beams of the interferometer, with eight channels, four vertical and four lateral. While the vertical channels were already optimised to provide accurate measurements of the Faraday rotation angle, Cotton–Mouton (CM) phase shift and ellipticity, the lateral channels had been only optimised for Faraday rotation angle alone. By setting the initial polarisation angle of the lateral channel at zero degrees the CM effect is minimised, the ellipticity is almost zero, and the CM phase shift angle is impossible to measure. During the recent JET experimental campaign (C38 in 2019–2020), the input polarisation for the lateral channels was altered for a class of pulses to analyse the possibility to measure the CM phase shift angle and the ellipticity, and, more important, to assess if it is possible to provide information of line-integrated electron density using the lateral channels of JET polarimetry in an ITER-like configuration. As a note, ITER will have only tangential channels with reflectors buried deep inside the first wall, so this setup was truly an ITER-like configuration. The results clearly show huge improvements, which can be achieved by just changing the input polarisation. Moreover, the analysis of the measurements shows that the polarimetric measurements have a systematic error, which is probably due to the effect of refraction and to the in-vessel mirrors, which was only partially taken into account during the calibration phase before the plasma. Thus, a new calibration method was developed and the results presented on a statistical basis. It has been demonstrated that, varying the input polarisation of the polarimeter and using the new calibration method, it is possible to measure the line-integrated electron density, using the CM phase shift (or the ellipticity) of the lateral channels, with good accuracy with respect to the electron density measured by the interferometer that was considered the reference.