We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, between the T d and 1T' phases in type-II Weyl semimetal MoTe 2 . Thinning down the MoTe 2 crystal from bulk material to about 25nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from ~40K to more than 300K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning defects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent α = 1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe 2 crystals of various thicknesses shows results consistent with our transport measurements.zone grows approximately linearly with reducing sample thickness as determined by four-probe resistivity measurement. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent = 1.27, consistent with theoretical expectations as explained later in this letter. Temperature-dependent Raman spectroscopy is also performed on MoTe 2 crystals of various thicknesses, showing results consistent with our transport measurements.
The quantum spin Hall (QSH) effect describes the state of matter in certain 2D electron systems, in which an insulating bulk state arises together with helical states at the edge of the sample. In stark contrast to its closest kin, the integer quantum Hall state, the QSH state exists only in time‐reversal symmetric system (e.g., in non‐magnetic materials and without the application of external magnetic field). This article reviews the development of the understanding and construction of the QSH states after their first theoretical proposal, with an emphasis on the materials perspective. Certain semiconductor quantum wells and 2D materials with strong spin–orbit coupling have been found to support QSH states.
The cover image shows a semi‐classical model of quantum spin Hall (QSH) effect, where the bulk of a two‐dimensional system is insulating and counter‐propagating spin‐polarized electrons move along its edges without dissipation. The edge states are protected by time‐reversal symmetry and insensitive to disorders. The unique edges of QSH materials provide a route to generate and detect Majorana fermions, which is a possible solution on quantum computing. In article number 1900026, Chuanwu Cao and Jian‐Hao Chen review the development of the understanding and construction of the QSH states with an emphasis on the materials perspective.
We report the continuous argon ions irradiation of itinerant Fe3GeTe2, a two-dimensional ferromagnetic metal, with the modification to its transport properties measured in situ. Our results show that defects generated by argon ions irradiation can significantly weaken the magnetization (M) and coercive field (Hc) of Fe3GeTe2, demonstrating the tunable magnetism of this material. Specifically, at base temperature, we observed a reduction of M and Hc by up to 40% and 62.4%, respectively. After separating the contribution from different mechanisms based on the Tian-Ye-Jin (TYJ) scaling relation, it’s the skew scattering that dominates the contribution to anomalous Hall effect in argon ions irradiated Fe3GeTe2. These findings highlight the potential of in situ transport modification as an effective method for tailoring the magnetic properties of two-dimensional magnetic materials, and provides new insights into the mechanisms underlying the tunable magnetism in Fe3GeTe2.
A lot of efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with carbon atom vacancy, or light adatoms like hydrogen or fluorine. At the meantime, the large negative magnetoresistance (MR) widely observed in these systems is not well understood, nor had it been associated with the presence of magnetic moments. In this paper, we study the systematic evolution of the large negative MR of in-situ hydrogenated graphene in ultra-high vacuum (UHV) environment. We find for most combination of electron density (ne) and hydrogen density (nH), MR at different temperature can be scaled to 𝛼𝛼 = 𝜇𝜇 𝐵𝐵 𝐵𝐵 𝑘𝑘 𝐵𝐵 (𝑇𝑇 − 𝑇𝑇 * ) ⁄ , where T * is the Curie-Weiss temperature. The sign of T * indicates the existence of tunable ferromagnetic-like (T * >0) and anti-ferromagnetic-like (T * <0) coupling in hydrogenated graphene. However, the lack of hysteresis of MR or anomalous Hall effect below |T * | points to the fact that long-range magnetic order did not emerge, which we attribute to the competition of different magnetic orders and disordered arrangement of magnetic moments on graphene. We also find that localized impurity states introduced by H adatoms could modify the capacitance of hydrogenated graphene. This work provides a new way to extract information from large negative MR behavior and can be a key to help understanding interactions of magnetic moments in graphene.
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