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Register a new userFerromagnetism with in-plane magnetization, Dirac spin-gapless semiconducting property, and tunable topological states in two-dimensional rare-earth-metal dinitrides
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As the bulk single-crystal MoN2/ReN2 with a layered structure was successfully synthesized in experiment, transition-metal dinitrides have attracted considerable attention in recent years. Here, we focus on rare-earth-metal (Rem) elements and propose seven stable Rem dinitride monolayers with a 1T structure, namely 1T-RemN2. These monolayers have a ferromagnetic ground state with in-plane magnetization. Without spin-orbit coupling (SOC) effect, the band structures are spin-polarized with Dirac points at the Fermi level. Remarkably, the 1T-LuN2 monolayer shows an isotropic magnetic anisotropy energy in the xy-plane with in-plane magnetization, indicating easy tunability of the magnetization direction. When rotating the magnetization vector in the xy-plane, our proposed model can accurately describe the variety of the SOC band gap and two topological states (Weyl-like semimetal and Chern insulator states) appear with tunable properties. The Weyl-like semimetal state is a critical point between the two Chern insulator states with opposite sign of the Chern numbers. The large nontrivial band gap (up to 60.3 meV) and the Weyl-like semimetal state are promising for applications in spintronic devices.
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We design a multiferroic metal that combines seemingly incompatible ferromagnetism, ferroelectricity, and metallicity by hole doping a two-dimensional (2D) ferroelectric with high density of states near the Fermi level. The strong magnetoelectric effect is demonstrated in hole-doped and arsenic-doped monolayer {alpha}-In2Se3 using first-principles calculations. Taking advantage of the oppositely charged surfaces created by an out-of-plane polarization, the 2D magnetization and metallicity can be electrically switched on and off in an asymmetrically doped monolayer. The substitutional arsenic defect pair exhibits an intriguing electric field-tunable charge disproportionation process accompanied with an on-off switch of local magnetic moments. The charge ordering process can be controlled by tuning the relative strength of on-site Coulomb repulsion and defect dipole-polarization coupling via strain engineering. Our design principle relying on no transition metal broadens the materials design space for 2D multiferroic metals.
Since the discovery of graphene, two-dimensional materials with atomic level thickness have rapidly grown to be a prosperous field of physical science with interdisciplinary interests, for their fascinating properties and broad applications. Very recently, the experimental observation of ferromagnetism in Cr$_2$Ge$_2$Te$_6$ bilayer and CrI$_3$ monolayer opened a door to pursuit long-absent intrinsic magnetic orders in two-dimensional materials. Meanwhile, the ferroelectricity was also experimentally found in SnTe monolayer and CuInP$_2$S$_6$ few layers. The emergence of these ferroic orders in the two-dimensional limit not only brings new challenges to our physical knowledge, but also provides more functionalities for potential applications. Among various two-dimensional ferroic ordered materials, transition/rare-earth metal halides and their derivants are very common. In this Research Update, based on transition/rare-earth metal halides, the physics of various ferroic orders in two-dimensional will be illustrated. The potential applications based on their magnetic and polar properties will also be discussed.
We use high-resolution, tunable angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic properties of single crystals of MnBi2Te4, a material that was predicted to be the first intrinsic antiferromagnetic (AFM) topological insulator. We observe both bulk and surface bands in the electronic spectra, in reasonable agreement with the DFT calculations results. In striking contrast to the earlier literatures showing a full gap opening between two surface band manifolds along (0001) direction, we observed a gapless Dirac cone remain protected in MnBi2Te4 across the AFM transition (TN = 24 K). Our data also reveal the existence of a second Dirac cone closer to the Fermi level, predicted by band structure calculations. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering, we do observe splitting of the bulk band that develops below the TN . Having a moderately high ordering temperature, MnBi2Te4 provides a unique platform for studying the interplay between topology and magnetic ordering.
In the newly discovered magnetic topological insulator MnBi$_2$Te$_4$, both axion insulator state and quantized anomalous Hall effect (QAHE) have been observed by tuning the magnetic structure. The related (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures with increased tuning knobs, are predicted to be a more versatile platform for exotic topological states. Here, we report angle-resolved photoemission spectroscopy (ARPES) studies on a series of the heterostructures (MnBi$_2$Te$_4$, MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$). A universal gapless Dirac cone is observed at the MnBi$_2$Te$_4$ terminated (0001) surfaces in all systems. This is in sharp contrast to the expected gap from the original antiferromagnetic ground state, indicating an altered magnetic structure near the surface, possibly due to the surface termination. In the meantime, the electron band dispersion of the surface states, presumably dominated by the top surface, is found to be sensitive to different stackings of the underlying MnBi$_2$Te$_4$ and Bi$_2$Te$_3$ layers. Our results suggest the high tunability of both magnetic and electronic structures of the topological surface states in (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures, which is essential in realizing various novel topological states.
The rare-earth nickelates are a rich playground for transport properties, known to host non-Fermi liquid character, resistance saturation and metal-insulator transitions. We report a study of transport in LaNiO3 in the presence of tunable disorder induced by irradiation. While pristine LaNiO3 samples are metallic, highly irradiated samples show insulating behaviour at all temperatures. Using irradiation fluence as a tuning handle, we uncover an intermediate region hosting a metal-insulator transition. This transition falls within the Mott-Ioffe-Regel regime wherein the mean free path is comparable to lattice spacing. In the high temperature metallic regime, we find a transition from non-Fermi liquid to a Fermi-liquid-like character. On the insulating side of the metal-insulator transition, we find behaviour that is consistent with weak localization. This is reflected in magnetoresistance that scales with the square of the field and in resistivity. In the highly irradiated insulating samples, we find good agreement with variable range hopping, consistent with Anderson localization. We find qualitatively similar behaviour in thick PrNiO3 films as well. Our results demonstrate that ion irradiation can be used to tailor transport, serving as an excellent tool to study the physics of localization.
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