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Tuning the interaction between the bulk and edge states of topological materials is a powerful tool for manipulating edge transport behavior, opening up exciting opportunities for novel electronic and spintronic applications. This approach is particularly suited to topological crystalline insulators (TCI), a class of topologically nontrivial compounds that are endowed with multiple degrees of topological protection. In this study, we investigate how bulk-edge interactions can influence the edge transport in planar bismuthene, a TCI with metallic edge states protected by in-plane mirror symmetry, using first principles calculations and symmetrized Wannier tight-binding models. By exploring the impact of various perturbation effects, such as device size, substrate potentials, and applied transverse electric field, we examine the evolution of the electronic structure and edge transport in planar bismuthene. Our findings demonstrate that the TCI states of planar bismuthene can be engineered to exhibit either a gapped or conducting unconventional helical spin texture via a combination of substrate and electric field effects. Furthermore, under strong electric fields, the edge states can be stabilized through a delicate control of the bulk-edge interactions. These results open up new directions for discovering novel spin transport patterns in topological materials and provide critical insights for the fabrication of topological spintronic devices., Comment: 23 pages, 8 figures

Here we demonstrate the occurrence of robust itinerant ferromagnetism in Mott-Hubbard systems at both low and high doping concentrations. Specifically, we study the effect of hole doping on the experimentally synthesized LaCrAsO via first-principles calculations and observe that the parent G-type antiferromagnetism vanishes quickly at low doping concentration ($x$ $\sim$ 0.20) and the system becomes ferromagnetic metal due to the canonical double exchange (CDE) mechanism. As $x$ continues to increase, the onsite energy difference between Cr 3$d$ and As 4$p$ orbitals decreases and the system transitions to a ferromagnetic negative charge-transfer energy metal. Therefore, the itinerant ferromagnetism doesn't terminate at intermediate $x$ as CDE mechanism usually predicts. Furthermore, our calculations reveal that both nearest and next-nearest ferromagnetic exchange coupling strengths keep growing with $x$, showing that ferromagnetism caused by negative charge-transfer energy state is "stronger" than that of CDE picture. Our work not only unveils an alternative mechanism of itinerant ferromagnetism, but also has the potential to attract immediate interest among experimentalists., Comment: 6 pages, 3 figures, comments are welcomed

Spin-orbit torque (SOT) is receiving tremendous attention from both fundamental and application-oriented aspects. Co2MnGa, a Weyl ferromagnet that is in a class of topological quantum materials, possesses cubic-based high structural symmetry, the L21 crystal ordering, which should be incapable of hosting anisotropic SOT in conventional understanding. Here we show the discovery of a gigantic anisotropy of self-induced SOT in Co2MnGa. The magnitude of the SOT is comparable to that of heavy metal/ferromagnet bilayer systems despite the high inversion symmetry of the Co2MnGa structure. More surprisingly, a sign inversion of the self-induced SOT is observed for different crystal axes. This finding stems from the interplay of the topological nature of the electronic states and their strong modulation by external strain. Our research enriches the understanding of the physics of self-induced SOT and demonstrates a versatile method for tuning SOT efficiencies in a wide range of materials for topological and spintronic devices., Comment: 15pages, 4figures (To appear Nano Lett.)

The high strength of structural aluminium alloys depends strongly on the controlled precipitation of specific intermetallic phases, whose identity and crystal structure can be difficult to predict. Here, we investigate the Al-Pt system, which shows some similarity to the Al-Cu system as one of their main intermetallic phases, $Al_2Pt$, is nearly isostructural with $\theta^{\prime} (Al_2Cu)$, the metastable phase responsible for the high-strength of Al-Cu alloys. However, phases in Al-Pt alloys are complex and have not been studied in detail. Using a combination of density-functional theory (DFT) calculations and classical nucleation theory (CNT) applied to the Al-Pt system, we design a workflow to predict the thermodynamics of solid solution, intermediate phases such as GP zones, stable and metastable precipitates, and their precipitation sequence. This workflow can be applied to an arbitrary binary alloying system. We confirm the known stable phases $Al_4Pt$, $Al_{21}Pt_8$, $Al_2Pt$, $Al_3Pt_2$, $AlPt (\alpha \& \beta)$, $Al_3Pt_5$, $AlPt_2 (\alpha \& \beta)$ and $AlPt_3 (\alpha \& \beta)$. We also reveal the possible existence of two phases of chemical formulae $Al_5Pt$ and $Al_3Pt$. This large number of intermetallic phases is due to the strong bonding between Al and Pt, which also leads to significant favourable Pt solute formation energy in the Al matrix. Our findings are compared with the known precipitation characteristics of the binary Al-Cu and Al-Au systems. We find that the $\theta^{\prime}$-like $Al_2Pt$ precipitate phase has a lower coherent interfacial energy than $\theta^{\prime}$. Our calculations strongly suggest that $Al_2Pt$ will precipitate first in Al-rich Al-Pt alloys and will form bulk-like interfaces similar to $\eta (Al_2Au)$ rather than like $\theta^{\prime}(Al_2Cu)$., Comment: 32 pages, 9 figures in the main text. 12 pages, 7 figures in the supplementary inforamtion

Intrinsic magnetic topological insulators offer low disorder and large magnetic band gaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultrathin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from a large band gap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultrathin MnBi2Te4. We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a band gap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

Layered indium selenides (In2Se3) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the α- and β′-phases, respectively. In this work, we utilize angle-resolved photoelectron spectroscopy to directly measure the electronic band structure of β′In2Se3 and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Because of n-type doping we can observe the conduction band minima and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the M points, yielding effective masses of 0.21m0 along KM and 0.33m0 along ΓM. The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) band structure with moderate bandgap and small effective mass suggests that β′-In2Se3 is a potentially useful van der Waals semiconductor. This, together with its ferroelectricity makes it a viable material for high-mobility ferroelectric−photovoltaic devices, with applications in nonvolatile memory switching and renewable energy technologies.

Layered indium selenides ($In_{2}Se_{3}$) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the $��$ and $��$' phases, respectively. In this work, we utilise angle-resolved photoelectron spectroscopy to directly measure the electronic bandstructure of $��'-In_{2}Se_{3}$, and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Due to n-type doping we are able to observe the conduction band minima, and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the $\overline{M}$ points, yielding effective masses of 0.21 $m_{0}$ along $\overline{KM}$ and 0.33 $m_{0}$ along $\overline{��M}$. The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) bandstructure with moderate bandgap and small effective mass suggest that $��'-In_{2}Se_{3}$ is a potentially useful new van der Waals semiconductor. This together with its ferroelectricity makes it a viable material for high-mobility ferroelectric-photovoltaic devices, with applications in non-volatile memory switching and renewable energy technologies., 19 pages, 4 + 1 figures; typos corrected;added references; updated figures & discussion to reflect changes in model