Anomalous spin current anisotropy in a noncollinear antiferromagnet

Cubic materials host high crystal symmetry and hence are not expected to support anisotropy in transport phenomena. In contrast to this common expectation, here we report an anomalous anisotropy of spin current can emerge in the (001) film of Mn3Pt, a noncollinear antiferromagnetic spin source with face-centered cubic structure. Such spin current anisotropy originates from the intertwined time reversal-odd ([Formula: see text]-odd) and time reversal-even ([Formula: see text]-even) spin Hall effects. Based on symmetry analyses and experimental characterizations of the current-induced spin torques in Mn3Pt-based heterostructures, we find that the spin current generated by Mn3Pt (001) exhibits exotic dependences on the current direction for all the spin components, deviating from that in conventional cubic systems. We also demonstrate that such an anisotropic spin current can be used to realize low-power spintronic applications such as the efficient field-free switching of the perpendicular magnetizations.

Néel Spin Currents in Antiferromagnets

Ferromagnets are known to support spin-polarized currents that control various spin-dependent transport phenomena useful for spintronics. On the contrary, fully compensated antiferromagnets are expected to support only globally spin-neutral currents. Here, we demonstrate that these globally spin-neutral currents can represent the Néel spin currents, i.e., staggered spin currents flowing through different magnetic sublattices. The Néel spin currents emerge in antiferromagnets with strong intrasublattice coupling (hopping) and drive the spin-dependent transport phenomena such as tunneling magnetoresistance (TMR) and spin-transfer torque (STT) in antiferromagnetic tunnel junctions (AFMTJs). Using RuO_{2} and Fe_{4}GeTe_{2} as representative antiferromagnets, we predict that the Néel spin currents with a strong staggered spin polarization produce a sizable fieldlike STT capable of the deterministic switching of the Néel vector in the associated AFMTJs. Our work uncovers the previously unexplored potential of fully compensated antiferromagnets and paves a new route to realize the efficient writing and reading of information for antiferromagnetic spintronics.

Room-temperature third-order nonlinear Hall effect in Weyl semimetal TaIrTe4

The second-order nonlinear Hall effect observed in the time-reversal symmetric system has not only shown abundant physical content, but also exhibited potential application prospects. Recently, a third-order nonlinear Hall effect has been observed in MoTe₂ and WTe₂. However, few-layer MoTe₂ and WTe₂ are usually unstable in air and the observed third-order nonlinear Hall effect can be measured only at low temperature, which hinders further investigation as well as potential application. Thus, exploring new air-stable material systems with a sizable third-order nonlinear Hall effect at room temperature is an urgent task. Here, in type-II Weyl semimetal TaIrTe₄, we observed a pronounced third-order nonlinear Hall effect, which can exist at room temperature and remain stable for months. The third-order nonlinear Hall effect is connected to the Berry-connection polarizability tensor instead of the Berry curvature. The possible mechanism of the observation of the third-order nonlinear Hall effect in TaIrTe₄ at room temperature has been discussed. Our findings will open an avenue towards exploring room-temperature nonlinear devices in new quantum materials.

Gapped topological kink states and topological corner states in honeycomb lattice

Based on the tight-binding calculations on honeycomb lattice and photonic experimental visualization on artificial graphene (AG), we report the domain-wall-induced gapped topological kink states and topological corner states. In honeycomb lattice, domain walls (DWs) with gapless topological kink states could be induced either by sublattice symmetry breaking or by lattice deformation. We find that the coexistence of these two mechanisms will induce DWs with gapped topological kink states. Significantly, the intersection of these two types of DWs gives rise to topological corner state localized at the crossing point. Through the manipulation of the DWs, we show AG with honeycomb lattice structure not only a versatile platform supporting multiple topological corner modes in a controlled manner, but also possessing promising applications such as fabricating topological quantum dots composed of gapped topological kink states and topological corner states.

Multifunctional two-dimensional semiconductors SnP3: universal mechanism of layer-dependent electronic phase transition

Two-dimensional (2D) semiconductors SnP₃ are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (∼10⁴ cm² V^-1 s^-1 for bilayer), strong absorption coefficients (∼10⁵ cm^-1) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP₃ are smaller than those of the common arsenic-type honeycomb structures GeP₃ and InP₃. More importantly, a semiconductor-to-metal transition is discovered with the layer number N  >  2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP₃, InP₃ and SnP₃).

Inversion symmetry breaking induced triply degenerate points in orderly arranged PtSeTe family materials

k paths exactly with [Formula: see text] symmetry allow to find triply degenerate points (TDPs) in band structures. The paths that host the type-II Dirac points in PtSe₂ family materials also have the [Formula: see text] spatial symmetry. However, due to Kramers degeneracy (the systems have both inversion symmetry and time reversal symmetry), the crossing points in them are Dirac ones. In this work, based on symmetry analysis, first-principles calculations, and [Formula: see text] method, we predict that PtSe₂ family materials should undergo topological transitions if the inversion symmetry is broken, i.e. the Dirac fermions in PtSe₂ family materials split into TDPs in PtSeTe family materials (PtSSe, PtSeTe, and PdSeTe) with orderly arranged S/Se (Se/Te). It is different from the case in high-energy physics that breaking inversion symmetry I leads to the splitting of Dirac fermion into Weyl fermions. We also address a possible method to achieve the orderly arranged in PtSeTe family materials in experiments. Our study provides a real example that Dirac points transform into TDPs, and is helpful to investigate the topological transition between Dirac fermions and TDP fermions.

Large Positive Thermal Expansion and Small Band Gap in Double-ReO3-Type Compound NaSbF6

Double-ReO₃-type structure compound NaSbF₆ undergoes a low-temperature rhombohedral to high-temperature cubic phase between 303 and 323 K, as revealed by temperature-dependent X-ray diffractions. Although many double-ReO₃-type fluorides exhibit either low thermal expansion or negative thermal expansion (NTE), NaSbF₆ exhibits positive thermal expansion (PTE) with a large volumetric coefficient of thermal expansion, α_v = 62 ppm/K, in its cubic phase. Raman spectroscopy reveals that the low-frequency transverse vibration of fluorine atoms is stiffened in NaSbF₆, compared with the typical NTE compound CaZrF₆ with the same structure. The related weak contraction associated with the polyhedral rocking would be overcome by the notable elongation of the Na-F bond length on heating, thus leading to the large volumetric PTE. Unlike ScF₃ and CaZrF₆ which are insulators with a wide band gap, a relative small band gap of 3.76 eV was observed in NaSbF₆. The small band gap can be attributed to the hybridization between the Sb 5s and F 2p orbitals.