The Dzyaloshinskii-Moriya interaction (DMI), which only exists in noncentrosymmetric systems, is responsible for the formation of exotic chiral magnetic states. The absence of DMI in most two-dimensional (2D) magnetic materials is due to their intrinsic inversion symmetry. Here, using first-principles calculations, we demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY (X/Y = S, Se, Te, X ≠ Y) in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of state-of-the-art ferromagnet/heavy metal (FM/HM) heterostructures. In addition, by performing Monte Carlo simulations, we find that at low temperatures the ground states of the MnSeTe and MnSTe monolayers can transform from ferromagnetic states with worm-like magnetic domains into the skyrmion states by applying external magnetic field. At increasing temperature, the skyrmion states starts fluctuating above 50 K before an evolution to a completely disordered structure at higher temperature. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials.

Very large Dzyaloshinskii-Moriya interaction in two-dimensional Janus manganese dichalcogenides and its application to realize skyrmion states

Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing properties and integration with other 2D family for unique heterostructures In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy (PMA) Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI) Here, we report the experimental observation of Neel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet Fe3GeTe2 (FGT) We demonstrate the ability to drive individual skyrmion by short current pulses along a vdW heterostructure, FGT/h-BN, as highly required for any skyrmion-based spintronic device Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application

/pdf/neel-type-skyrmions-and-their-current-induced-motion-in-van-u46g34si11.pdf

Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

Non-coplanar spin textures with scalar spin chirality can generate effective magnetic field that deflects the motion of charge carriers, resulting in topological Hall effect (THE), a powerful probe of the ground state and low-energy excitations of correlated systems. However, spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy has not been considered in prior studies. Herein, we report direct evidence of universal spin chirality fluctuation by probing the THE above the transition temperatures in two different ferromagnetic ultra-thin films, SrRuO$_3$ and V doped Sb$_2$Te$_3$. The temperature, magnetic field, thickness, and carrier type dependences of the THE signal, along with our Monte-Carlo simulations, unambiguously demonstrate that the spin chirality fluctuation is a universal phenomenon in two-dimensional Ising ferromagnets. Our discovery opens a new paradigm of exploring the spin chirality with topological Hall transport in two-dimensional magnets and beyond

Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy

The quantum anomalous Hall (QAH) effect is a quintessential consequence of non-zero Berry curvature in momentum-space. The QAH insulator harbors dissipation-free chiral edge states in the absence of an external magnetic field. On the other hand, the topological Hall (TH) effect, a transport hallmark of the chiral spin textures, is a consequence of real-space Berry curvature. While both the QAH and TH effects have been reported separately, their coexistence, a manifestation of entangled chiral edge states and chiral spin textures, has not been reported. Here, by inserting a TI layer between two magnetic TI layers to form a sandwich heterostructure, we realized a concurrence of the TH effect and the QAH effect through electric field gating. The TH effect is probed by bulk carriers, while the QAH effect is characterized by chiral edge states. The appearance of TH effect in the QAH insulating regime is the consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii-Moriya interaction and occur during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures potentially provides a unique platform for proof-of-concept dissipationless spin-textured spintronic applications.

Concurrence of Quantum Anomalous Hall and Topological Hall Effects in Magnetic Topological Insulator Sandwich Heterostructures

Non-coplanar spin textures with scalar spin chirality can generate an effective magnetic field that deflects the motion of charge carriers, resulting in a topological Hall effect (THE)1-3. However, spin chirality fluctuations in two-dimensional ferromagnets with perpendicular magnetic anisotropy have not been considered so far. Here, we report evidence of spin chirality fluctuations by probing the THE above the Curie temperature in two different ferromagnetic ultra-thin films, SrRuO3 and V-doped Sb2Te3. The temperature, magnetic field, thickness and carrier-type dependence of the THE signal, along with Monte Carlo simulations, suggest that spin chirality fluctuations are a common phenomenon in two-dimensional ferromagnets with perpendicular magnetic anisotropy. Our results open a path for exploring spin chirality with topological Hall transport in two-dimensional magnets and beyond4-7.

Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy

Automated Endotracheal Tube Placement Check Using Semantically Embedded Deep Neural Networks.

Magnetism is an old field in condensed matter physics,
but it is still vibrant and full of excitement. Regardless of deep
fundamental physics therein, it also has broad application in
engineering technology, modern hard disk drive as an example.
Magnetic skyrmion, a vortex-like structure in two-dimensional
magnetic systems, has been discovered in various magnetic
materials, among which chiral magnets are a family of candidates.
The skyrmions are characterized by nonzero topological charges. The
vortex-like structure of skyrmions makes skyrmion materials good
candidates of new generation of data storage device. So
understanding the transport properties of the skyrmion materials is
important for the possible application in the future. The Hall
effect is a key aspect of electron transports. The topological Hall
effect, which is one component in the total Hall effect, only
depends on the magnetic structures, and the topological Hall
conductivity is proportional to the topological charge. It thus
serves as the transport signature of magnetic skyrmions. The major
mission of this thesis is to investigate the topological charge
distribution in realistic models and uncover the relationship
between the existence of skyrmions and other chiral excitations.
The organization of the thesis is the following. The first chapter
is the introduction. A historical survey about magnetic skyrmions
and chiral magnets is presented firstly. The magnetic skyrmion is
identified by the topological charge. Further, the relationship
between the topological hall effect and topological charge is
described by the emergent electrodynamics. The importance of the
topological charge in chiral magnets is explained in this part.
Following the importance of the topological charge, the
investigation of topological charge in two-dimensional chiral
magnets is presented in the second chapter. The Monte Carlo
simulation is employed to calculate the topological charge on a
square lattice. The results show that the nonzero topological
charge is not necessarily correlated to the existence of skyrmions
in chiral magnets. To understand the numerical results, simple
analysis based on the physical picture of a triangle on the square
lattice is performed. Then we calculate the topological charge in
continuum model of chiral magnets. At the high temperature limit,
the numerical results, picture analysis and the analytic result are
consistent. Then, in this chapter, there is a description of the
recent experimental work on thin film SrRuO3 which confirmed our
theoretical prediction. A discussion on spin chirality, topological
charge and Hall conductivity is presented in the end. However, no
experiment on chiral magnets has been on a perfect monolayer
system. So we extend the investigation of topological charge into
three-dimensional situation. This work is introduced in the third
chapter. The Monte Carlo simulation and the analytical calculation
are presented firstly. A special issue in three-dimensional chiral
magnets is the thickness dependence. The Monte Carlo…

Thermally Driven Topology in Chiral Magnets

Topological spin textures, such as magnetic skyrmions, have been extensively discussed in magnetic materials with broken inversion symmetry Most of these textures are stable at low temperatures and form long-range orders However, the topological nature of these textures, characterized by the index called topological charge, is actually persistent at elevated temperatures In this work, the topological charge of a two-dimensional frustrated spin model with the Dzyaloshinskii-Moriya (DM) interaction was calculated by the Monte Carlo simulations At finite temperatures, nontrivial topology driven by thermal fluctuation was discovered in a spin-disordered phase This thermally driven topology is the dominant topology at low magnetic fields The magnitude of topological charge is quadratic in the DM interaction and linear in the external magnetic field or uniaxial magnetic anisotropy We also propose a real frustrated system, the Mn-Bi monolayer film, with large DM interaction to enable such thermally driven topology A full phase diagram of this system has been explored

Thermally driven topology in frustrated systems

Purpose
To integrate and evaluate an artificial intelligence (AI) system that assists in checking endotracheal tube (ETT) placement on chest x-rays (CXRs) in clinical practice.


Approach
In clinical use over 17 months, 214 CXR images were ordered to check ETT placement with AI assistance by intensive care unit (ICU) physicians. The system was built on the SimpleMind Cognitive AI platform and integrated into a clinical workflow. It automatically identified the ETT and checked its placement relative to the trachea and carina. The ETT overlay and misplacement alert messages generated by the AI system were compared with radiology reports as the reference. A survey study was also conducted to evaluate usefulness of the AI system in clinical practice.


Results
The alert messages indicating that either the ETT was misplaced or not detected had a positive predictive value of 42% (21/50) and negative predictive value of 98% (161/164) based on the radiology reports. In the survey, radiologist and ICU physician users indicated that they agreed with the AI outputs and that they were useful.


Conclusions
The AI system performance in real-world clinical use was comparable to that seen in previous experiments. Based on this and physician survey results, the system can be deployed more widely at our institution, using insights gained from this evaluation to make further algorithm improvements and quality assurance of the AI system.

Integration and evaluation of chest X-ray artificial intelligence in clinical practice.

e13545 Background: Automating RECIST 1.1 would offer benefits in speed, consistency, and cost of clinical trial imaging reads. Detection of new lesions is subjective and a common source of reader adjudication. Current AI lesion detection typically uses single timepoint images. The use of temporal data in AI is a rapidly evolving field in non-medical applications and new lesion detection in response assessment may benefit from longitudinal multi timepoint images. We explore the impact of longitudinal image input to deep neural networks for new lesion detection; as a proof of concept, we compare single vs dual timepoint input in new liver lesion detection in CT. Methods: We utilized CT images from two public datasets: 1) diseased liver (DL), a subset of DeepLesion (doi: 10.1117/1.JMI.5.3.036501), and 2) healthy liver (HL) from potential liver donors (Med Image Anal 69 (2021) 101950).. Due to the lack of a publicly available longitudinal image dataset with new lesions, we simulated (“sim”) a longitudinal study by registering HL scans (sim healthy baseline) with DL scans (sim follow up with new lesion) and selecting well-paired images using mutual information. From each pair of scans, we select one positive (with new lesion on follow up) and one negative (without lesion on follow up) 6x6cm image patch from within the liver, creating a dataset of 3296 patches with a balanced number of pos/neg samples. We trained two deep neural networks to classify the presence of a new liver lesion: 1) a conventional single timepoint model (ST) trained on DL patches only, i.e. using the follow up scan only, and 2) an experimental dual-timepoint model (DT) trained on patch pairs from DL-HL paired scans. We used 5-fold cross validation and paired t tests to compare model AUC and accuracy. Results: Of 10,594 CT studies (4,427 patients, pts) in DeepLesion, we paired 1648 DL scans (946 pts with liver lesions) with one of 20 registered HL scans (20 pts). The DT model outperformed the ST model, increasing AUC from 0.914 ± 0.002 to 0.929 ± 0.005 (p=0.040), with similar trends in accuracy (dataset split, Table 1). Conclusions: The DT model outperformed the ST model in the detection of new liver lesions, suggesting preliminary the benefit of multi-timepoint input to a deep neural network using sim longitudinal data. The use of longitudinal image data to improve interval change detection is a step towards automating radiologic response assessment in clinical trials. We plan to extend this work to automate new lesion detection in real clinical trial longitudinal images. Results (bold) are reported on test set across 5 folds. N image patches (ST) or patch pairs (DT). [Table: see text]

Toward automating RECIST 1.1: Improving AI new lesion detection with longitudinal image data.

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