The skyrmion annihilations induced by local reversal of background field in skyrmion lattice

The understanding of the creation and annihilation dynamics of a magnetic skyrmion is significant due to its potential applications in information storage and spintronics. Although there have been extensive investigations on the annihilation of isolated skyrmion, topological annihilation in periodic skyrmion lattice is a more complex process. We report a micromagnetic simulation study about the annihilation process of a two-dimensional skyrmion triangular lattice triggered by a uniform field HREV of comparable size to the skyrmion, which is opposite to the direction of the background field, revealing two annihilation modes. When the HREV center is within the range of a skyrmion, the neighboring skyrmions annihilate in situ, while the center is between adjacent skyrmions, anti-skyrmion is induced in the interstitial region. Both mechanisms tend to experience the intermediate topological vortex or antivortex structure, and the spin system undergoes a long period of relaxation to reach a stable state after the topological charge is stabilized. Our results present a local annihilation scheme that is easy to achieve in a 2D skyrmion lattice and highlight the role of interaction between skyrmions in the transformation between different kinds of topological defects.

Modeling and validation of nonlinear voltage-current characteristics of ITER PF joint sample tested in SULTAN facility

The ITER Poloidal Field (PF) coils are wound into double pancakes with NbTi cable-in-conduit conductors, which are connected by joints in shaking hands lap-type configuration. The coils are operating in pulsed mode with a maximum operating current of 55 kA and peak magnetic field of 6.4 T, utilizing electromagnetic load on the conductors and joints. A series of PF qualification joint samples modified in praying hands configuration is measured in the SULTAN facility. For some samples, a nonlinear voltage-current (VI) characteristic is observed during the assessment of joint resistance. The growth of joint resistance versus the B×I product is larger than what is expected from the magneto-resistant copper contribution. Two non-homogeneous contact resistance models are developed and combined to quantitatively evaluate the reason for the nonlinear VI behavior in combination with the relevant power dissipation and current redistribution in the joint. The simulations reveal that, for the particular pre-qualification PFJEU2 sample with resistance variation up to 3.5 nΩ, the most probable reason for the nonlinear VI characteristic is a widely spread defective connection between copper sole and shim. The electromagnetic force involves a separation effect on the mechanically and electrically weakly connected parts, resulting into a varying resistance depending on transport current and background field. The hypothesis and models are validated by an experiment on a similar sample PFJEU3 and a post-mortem examination of the PFJEU2 sample.

A new and gauge-invariant littlest Higgs model with T-parity

We inspect the Littlest Higgs model with T-parity, based on a global symmetry SU(5) spontaneously broken to SO(5), in order to elucidate the pathologies it presents due to the non-trivial interplay between the gauge invariance associated to the heavy modes and the discrete T-parity symmetry. In particular, the usual Yukawa Lagrangian responsible for providing masses to the heavy ‘mirror’ fermions is not gauge invariant. This is because it contains an SO(5) quintuplet of right-handed fermions that transforms nonlinearly under SU(5), hence involving in general all SO(5) generators when a gauge transformation is performed and not only those associated to its gauge subgroup. Part of the solution to this problem consists of completing the right-handed fermion quintuplet with T-odd ‘mirror partners’ and a gauge singlet, what has been previously suggested for other purposes. Furthermore, we find that the singlet must be T-even, the global symmetry group must be enlarged, an additional nonlinear sigma field should be introduced to parametrize the spontaneous symmetry breaking and new extra fermionic degrees of freedom are required to give a mass to all fermions in an economic way while preserving gauge invariance. Finally, we derive the Coleman–Weinberg potential for the Goldstone fields using the background field method.

The Role of Longitudinal Polarizations in Horndeski and Macroscopic Gravity: Introducing Gravitational Plasmas

We discuss some general and relevant features of longitudinal gravitational modes in Horndeski gravity and their interaction with matter media. Adopting a gauge-invariant formulation, we clarify how massive scalar and vector fields can induce additional transverse and longitudinal excitations, resulting in breathing, vector, and longitudinal polarizations. We review, then, the interaction of standard gravitational waves with a molecular medium, outlining the emergence of effective massive gravitons, induced by the net quadrupole moment due to molecule deformation. Finally, we investigate the interaction of the massive mode in Horndeski gravity with a noncollisional medium, showing that Landau damping phenomenon can occur in the gravitational sector as well. That allows us to introduce the concept of “gravitational plasma”, where inertial forces associated with the background field play the role of cold ions in electromagnetic plasma.

Simulation study on parametric dependence of whistler-mode hiss generation in the plasmasphere

We conduct electromagnetic particle simulations to examine the applicability of nonlinear wave growth theory to the generation process of plasmaspheric hiss. We firstly vary the gradient of the background magnetic field from a realistic model to a rather steep gradient model. Under such variation, the threshold amplitude in the nonlinear theory increases quickly and the overlap between threshold and optimum amplitude disappears correspondingly, the nonlinear process is suppressed. In the simulations, as we enlarge the gradient coefficient of the background magnetic field, waves generated near the equator do not grow through propagation. By examining the range of suitable values of inhomogeneity factor S (i.e., $$|S|<2$$ | S | < 2 ), we find the generation of wave packets is limited to the equatorial region when the background field is steep, showing a good agreement with what is indicated by critical distance in the theory. We then check the dependence of generation of hiss emissions on different hot electron densities. Since the overlap between threshold and optimum amplitude vanishes, the nonlinear process is weakened when hot electron density becomes smaller. In the simulation results, we find similar wave structures in all density cases, yet with different magnitudes. The existence of suitable S values implies that the nonlinear process occurs even at a low level of hot electron density. However, by examining $$J_E$$ J E that closely relates to the wave growth, we find energy conveyed from particles to waves is much limited in small density cases. Therefore, the nonlinear process is suppressed when hot electron density is small, which agrees with the theoretical analysis. Graphical Abstract

Consistent higher order $$ \sigma \left(\mathcal{GG}\to h\right) $$, $$ \Gamma \left(h\to \mathcal{GG}\right) $$ and Γ(h → γγ) in geoSMEFT

We report consistent results for Γ(h → γγ), $$ \sigma \left(\mathcal{GG}\to h\right) $$ σ GG → h and $$ \Gamma \left(h\to \mathcal{GG}\right) $$ Γ h → GG in the Standard Model Effective Field Theory (SMEFT) perturbing the SM by corrections $$ \mathcal{O}\left({\overline{\upsilon}}_T^2/16{\pi}^2{\Lambda}^2\right) $$ O υ ¯ T 2 / 16 π 2 Λ 2 in the Background Field Method (BFM) approach to gauge fixing, and to $$ \mathcal{O}\left({\overline{\upsilon}}_T^4/{\Lambda}^4\right) $$ O υ ¯ T 4 / Λ 4 using the geometric formulation of the SMEFT. We combine and modify recent results in the literature into a complete set of consistent results, uniforming conventions, and simultaneously complete the one loop results for these processes in the BFM. We emphasize calculational scheme dependence present across these processes, and how the operator and loop expansions are not independent beyond leading order. We illustrate several cross checks of consistency in the results.

The Effect of Oblique Image Acquisition on the Accuracy of Quantitative Susceptibility Mapping and a Robust Tilt Correction Method

Purpose: Quantitative susceptibility mapping (QSM) is increasingly used for clinical research where oblique image acquisition is commonplace but its effects on QSM accuracy are not well understood. Theory and Methods: The QSM processing pipeline involves defining the unit magnetic dipole kernel, which requires knowledge of the direction of the main magnetic field B0 with respect to the acquired image volume axes. The direction of B0 is dependent upon the axis and angle of rotation in oblique acquisition. Using both a numerical brain phantom and in-vivo acquisitions, we analysed the effects of oblique acquisition on magnetic susceptibility maps. We compared three tilt correction schemes at each step in the QSM pipeline: phase unwrapping, background field removal and susceptibility calculation, using the root-mean-squared error and QSM-tuned structural similarity index (XSIM). Results: Rotation of wrapped phase images gave severe artefacts. Background field removal with projection onto dipole fields gave the most accurate susceptibilities when the field map was first rotated into alignment with B0. LBV and VSHARP background field removal methods gave accurate results without tilt correction. For susceptibility calculation, thresholded k-space division, iterative Tikhonov regularisation and weighted linear total variation regularisation all performed most accurately when local field maps were rotated into alignment with B0 before susceptibility calculation. Conclusion: For accurate QSM, oblique acquisition must be taken into account. Rotation of images into alignment with B0 should be carried out after phase unwrapping and before background field removal. We provide open-source tilt-correction code to incorporate easily into existing pipelines: https://github.com/o-snow/QSM_TiltCorrection.git.

Injury incidence and burden during senior inter-provincial field hockey tournaments

Background: Field hockey is an Olympic sport played internationally and in which South Africa is a participating nation. It also has its own world cup. Few injury studies have been published on South African field hockey. Research efforts should increase within the sport to ensure safe participation and mitigate the inherent injury risks. Objectives: The objective of the study was to attend the male and female inter-provincial field hockey tournaments in South Africa and determine the incidence of injury and burden of acquired sport injuries (time-loss and medical attention). Methods: A quantitative, descriptive, longitudinal study, including 133 females and 139 males, was conducted. Participants completed baseline questionnaires prior to the tournament and post-match questionnaires detailing injuries during the tournament. Results: The recorded injuries were 77.9 (females) and 99.5 (males) per 1 000 player match hours. Medical attention was 51.9 (females) and 70.3 (males) injuries per 1 000 player match hours. The result for time-loss injuries was 4.3 (females) and 7.5 (males) injuries per 1 000 player match hours. Discussion: The study found high incidence rates of all injuries and medical attention injuries; however, the incidence of time-loss injuries was low in comparison to existing literature. Comparing current results to existing literature is challenging due to the heterogeneity of methodologies and injury definitions in field hockey research. Conclusion: This was the largest observational study in field hockey conducted in South Africa. The international sporting body should establish a consensus for future research and the South African Hockey Association explore long-term surveillance in South Africa to mimic similar national codes.

The Role of a Writing Center in Academic Writing Socialization of Second Language Graduate Students

Introduction: The present study aims to investigate second language graduate students’ academic writing socialization in relation to their experiences at writing center in North American higher educational context. The study documents how graduate students are socialized to use academic language in order to participate effectively within their academic communities by employing Weidman, Twale, and Stein’s (2001) framework for Graduate and Professional Student Socialization. Methods: The data is collected through semi-structured interviews with five graduate students who had experience visiting writing center to receive support for their academic writing. The data was analyzed based on the tenets of thematic analysis, which followed an iterative process. Results: It was revealed that second language graduate students’ reasons for visiting the Campus Writing Center included their educational background, field of study, and their first language(s). It was also shown that all participants expected revision on their grammatical errors as well as feedback on global areas such as idea development and organization during their visits. Furthermore, the analysis indicated that the participants gained both positive and negative experiences from the tutoring sessions, while it was found that writing center was not the only resource our participants relied on for the development of their academic writing. Discussion: There are various factors influencing and contributing to second language graduate students’ development of writing socialization within academic community. It is a challenging task for students from other educational and cultural backgrounds to adapt and socialize into new environments, especially in the academic community of higher education. Therefore, the support from writing service and writing development programs/workshops that are tailored to the specific needs of second language graduate students would be one helpful resource to help them go smoothly through the process of second language academic writing socialization. Given that second language graduate students generally benefited from a strong supervision and supportive feedback, and appreciated them as reported in the literature, it is also important to survey international students’ academic enculturation experiences periodically in terms of areas such as writing, speaking and participation in scholarly activities, faculty mentorship and offer feedback-support to overcome issues reported by students. Limitations: The number of participants and the lack of students’ academic text investigation were noted as limitations of the study. It is suggested that further research incorporates various sources of data collection such as tutor’s perspectives and the analysis of participants’ texts. Conclusions: It was concluded that writing center played an important role in academic writing socialization experiences of the participants, and there were various factors influencing and contributing to their academic writing socialization. Overall, it was concluded that the developmental processes into academic writing in second language were non-linear, dynamic, and multimodal.