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1. Weyl Fermions and broken symmetry phases of laterally confined 3 He films

   https://doi.org/10.1088/1361-648X/acf42b  

   Wu, Hao ; Sauls, J. A. ( September 2023 , Journal of Physics: Condensed Matter)

   Broken symmetries in topological condensed matter systems have implications for the spectrum of Fermionic excitations confined on surfaces or topological defects. The Fermionic spectrum of confined (quasi-2D)³He-A consists of branches of chiral edge states. The negative energy states are related to the ground-state angular momentum,$L_z=(N/2)\hslash$, for$N/2$Cooper pairs. The power law suppression of the angular momentum,$L_z(T)\simeq (N/2)\hslash [1-\frac{2}{3}(\pi T/\mathrm{\Delta }{)}^2]$for$0⩽T\ll T_c$, in the fully gapped 2D chiral A-phase reflects the thermal excitation of the chiral edge Fermions. We discuss the effects of wave function overlap, and hybridization between edge states confined near opposing edge boundaries on the edge currents, ground-state angular momentum and ground-state order parameter of superfluid³He thin films. Under strong lateral confinement, the chiral A phase undergoes a sequence of phase transitions, first to a pair density wave (PDW) phase with broken translational symmetry at$D_{c2}\sim 16{\xi }_0$. The PDW phase is described by a periodic array of chiral domains with alternating chirality, separated by domain walls. The period of PDW phase diverges as the confinement length$D\to D_{c_2}$. The PDW phase breaks time-reversal symmetry, translation invariance, but is invariant under the combination of time-reversal and translation by a one-half period of the PDW. The mass current distribution of the PDW phase reflects this combined symmetry, and originates from the spectra of edge Fermions and the chiral branches bound to the domain walls. Under sufficiently strong confinement a second-order transition occurs to the non-chiral ‘polar phase’ at$D_{c1}\sim 9{\xi }_0$, in which a single p-wave orbital state of Cooper pairs is aligned along the channel.

    

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2. Room‐Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self‐Intercalation

   https://doi.org/10.1002/adma.202205967  

   Zhang, Chenhui ; Liu, Chen ; Zhang, Junwei ; Yuan, Youyou ; Wen, Yan ; Li, Yan ; Zheng, Dongxing ; Zhang, Qiang ; Hou, Zhipeng ; Yin, Gen ; et al ( November 2022 , Advanced Materials)

   Room‐temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano‐spintronic devices. However, such skyrmion‐hosting materials are rare in nature. In this study, a self‐intercalated transition metal dichalcogenide Cr_1+xTe₂with a layered crystal structure that hosts room‐temperature skyrmions and exhibits large THE is reported. By tuning the self‐intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out‐of‐plane to the in‐plane configuration are achieved. Based on the intercalation engineering, room‐temperature skyrmions are successfully created in Cr_1.53Te₂with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion‐induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

    

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3. Topological spin/structure couplings in layered chiral magnet Cr 1/3 TaS 2 : The discovery of spiral magnetic superstructure

   https://doi.org/10.1073/pnas.2023337118  

   Du, Kai ; Huang, Fei-Ting ; Kim, Jaewook ; Lim, Seong Joon ; Gamage, Kasun ; Yang, Junjie ; Mostovoy, Maxim ; Garlow, Joseph ; Han, Myung-Geun ; Zhu, Yimei ; et al ( September 2021 , Proceedings of the National Academy of Sciences)

   Chiral magnets have recently emerged as hosts for topological spin textures and related transport phenomena, which can find use in next-generation spintronic devices. The coupling between structural chirality and noncollinear magnetism is crucial for the stabilization of complex spin structures such as magnetic skyrmions. Most studies have been focused on the physical properties in homochiral states favored by crystal growth and the absence of long-ranged interactions between domains of opposite chirality. Therefore, effects of the high density of chiral domains and domain boundaries on magnetic states have been rarely explored so far. Herein, we report layered heterochiral Cr_1/3TaS₂, exhibiting numerous chiral domains forming topological defects and a nanometer-scale helimagnetic order interlocked with the structural chirality. Tuning the chiral domain density, we discovered a macroscopic topological magnetic texture inside each chiral domain that has an appearance of a spiral magnetic superstructure composed of quasiperiodic Néel domain walls. The spirality of this object can have either sign and is decoupled from the structural chirality. In weak, in-plane magnetic fields, it transforms into a nonspiral array of concentric ring domains. Numerical simulations suggest that this magnetic superstructure is stabilized by strains in the heterochiral state favoring noncollinear spins. Our results unveil topological structure/spin couplings in a wide range of different length scales and highly tunable spin textures in heterochiral magnets.

    

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4. Electrical Detection and Magnetic Imaging of Stabilized Magnetic Skyrmions in Fe 1− x Co x Ge ( x < 0.1) Microplates

   https://doi.org/10.1002/adfm.201805418  

   Stolt, Matthew J. ; Schneider, Sebastian ; Mathur, Nitish ; Shearer, Melinda J. ; Rellinghaus, Bernd ; Nielsch, Kornelius ; Jin, Song ( February 2019 , Advanced Functional Materials)

   Magnetic skyrmions are topologically protected spin textures that are being investigated for their potential use in next generation magnetic storage devices. Here, magnetic skyrmions and other magnetic phases in Fe_1−xCo_xGe (x< 0.1) microplates (MPLs) newly synthesized via chemical vapor deposition are studied using both magnetic imaging and transport measurements. Lorentz transmission electron microscopy reveals a stabilized magnetic skyrmion phase near room temperature (≈280 K) and a quenched metastable skyrmion lattice via field cooling. Magnetoresistance (MR) measurements in three different configurations reveal a unique anomalous MR signal at temperatures below 200 K and two distinct field dependent magnetic transitions. The topological Hall effect (THE), known as the electronic signature of magnetic skyrmion phase, is detected for the first time in a Fe_1−xCo_xGe nanostructure, with a large and positive peak THE resistivity of ≈32 nΩ cm at 260 K. This large magnitude is attributed to both nanostructuring and decreased carrier concentrations due to Co alloying of the Fe_1−xCo_xGe MPL. A consistent magnetic phase diagram summarized from both the magnetic imaging and transport measurements shows that the magnetic skyrmions are stabilized in Fe_1−xCo_xGe MPLs compared to bulk materials. This study lays the foundation for future skyrmion‐based nanodevices in information storage technologies.

    

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5. Emergence of the Vortex State in Confined Ferroelectric Heterostructures

   https://doi.org/10.1002/adma.201901014  

   Hsu, Shang‐Lin ; McCarter, Margaret R. ; Dai, Cheng ; Hong, Zijian ; Chen, Long‐Qing ; Nelson, Christopher T. ; Martin, Lane W. ; Ramesh, Ramamoorthy ( July 2019 , Advanced Materials)

   The manipulation of charge and lattice degrees of freedom in atomically precise, low‐dimensional ferroelectric superlattices can lead to exotic polar structures, such as a vortex state. The role of interfaces in the evolution of the vortex state in these superlattices (and the associated electrostatic and elastic boundary conditions they produce) has remained unclear. Here, the toroidal state, arranged in arrays of alternating clockwise/counterclockwise polar vortices, in a confined SrTiO₃/PbTiO₃/SrTiO₃trilayer is investigated. By utilizing a combination of transmission electron microscopy, synchrotron‐based X‐ray diffraction, and phase‐field modeling, the phase transition as a function of layer thickness (number of unit cells) demonstrates how the vortex state emerges from the ferroelectric state by varying the thickness of the confined PbTiO₃layer. Intriguingly, the vortex state arises at head‐to‐head domain boundaries in ferroelectrica₁/a₂twin structures. In turn, by varying the total number of PbTiO₃layers (moving from trilayer to superlattices), it is possible to manipulate the long‐range interactions among multiple confined PbTiO₃layers to stabilize the vortex state. This work provides a new understanding of how the different energies work together to produce this exciting new state of matter and can contribute to the design of novel states and potential memory applications.

    

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