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Abstract

Manipulating valley-dependent Berry phase effects provides remarkable opportunities for both fundamental research and practical applications. Here, by referring to effective model analysis, we propose a general scheme for realizing topological magneto-valley phase transitions. More importantly, by using valley-half-semiconducting VSi2N4 as an outstanding example, we investigate sign change of valley-dependent Berry phase effects which drive the change-in-sign valley anomalous transport characteristics via external means such as biaxial strain, electric field, and correlation effects. As a result, this gives rise to quantized versions of valley anomalous transport phenomena. Our findings not only uncover a general framework to control valley degree of freedom, but also motivate further research in the direction of multifunctional quantum devices in valleytronics and spintronics.

Phosphorene and arsenene are newly discovered two-dimensional semiconductors with many exciting physical properties. Here, using first-principles density functional theory, we investigate the effects of hole doping and strain on the magnetism in phosphorene and arsenene with buckled structures. We show that ferromagnetic ground states originating from Stoner instability can emerge in a broad range of hole concentrations. In addition, strain effectively modulates the magnetism by means of changing the band orders of the top of the valence bands. Magnetoresponses under hole doping and strain in buckled phosphorene and arsenene are distinct due to the contrary band orders at their native states. Furthermore, magnetoelastic coupling effect and magnetostriction in buckled phosphorene and arsenene are discussed. Our results suggest the great potential of buckled phosphorene and arsenene in magnetic device applications.

A 20 V stack of 19 supercapacitors was fabricated from titanium bipolar plates (150 Â 150 Â 0.1 mm 3 ) coated on each side with carbon nanotubes and polypyrrole composite (þ) and pigment carbon black (À), and microporous polymeric separators containing aqueous KCl. Internal sealing of each cell in the stack was achieved by placing a silicone rubber washer between neighboring bipolar plates. The stack was tested by high voltage cyclic voltammetry, galvanostatic charging and discharging, and electrochemical impedance spectroscopy. It approached 25 kW/kg in maximum specific power and 3.64 Wh/kg in specific energy. Performance of the stack through intermittent charging-discharging tests in a storage period of 10 months (still ongoing) remained fairly stable. For example, it exhibited almost zero decay in capacitance after 1000 continuous galvanostatic charging-discharging cycles in the first month of storage (10 V), and less than 6% loss in the seventh month (19 V).

Magneto-optical (MO) effects have been known for more than a century as they reflect the basic interactions between light and magnetism. The origin of MO effects is usually ascribed to the simultaneous presence of band exchange splitting and spin-orbit coupling. Using a tight-binding model and first-principles calculations, we show that topological MO effects, in analogy to the topological Hall effect, can arise in noncoplanar antiferromagnets entirely due to the scalar spin chirality instead of spin-orbit coupling. The band exchange splitting is not required for topological MO effects. Moreover, we discover that the Kerr and Faraday rotation angles in two-dimensional insulating noncoplanar antiferromagnets can be quantized in the low-frequency limit, implying the appearance of quantum topological MO effects, accessible by time-domain THz spectroscopy. arXiv:1811.05803v1 [cond-mat.mtrl-sci]

Portable and wearable electronics require much more flexible graphene-based electrode with high fatigue life, which could repeatedly bend, fold, or stretch without sacrificing its mechanical properties and electrical conductivity. Herein, a kind of ultrahigh fatigue resistant graphene-based nanocomposite via tungsten disulfide (WS) nanosheets is synthesized by introducing a synergistic effect with covalently cross-linking inspired by the orderly layered structure and abundant interfacial interactions of nacre. The fatigue life of resultant graphene-based nanocomposites is more than one million times at the stress level of 270 MPa, and the electrical conductivity can be kept as high as 197.1 S/cm after 1.0 × 10 tensile testing cycles. These outstanding properties are attributed to the synergistic effect from lubrication of WS nanosheets for deflecting crack propagation, and covalent bonding between adjacent GO nanosheets for bridging crack, which is verified by the molecular dynamics (MD) simulations. The WS induced synergistic effect with covalent bonding offers a guidance for constructing graphene-based nanocomposites with high fatigue life, which have great potential for applications in flexible and wearable electronic devices, etc.

The anomalous Hall effect (AHE) and the magneto-optical effect (MOE) are two prominent manifestations of time-reversal symmetry breaking in magnetic materials. Noncollinear antiferromagnets (AFMs) have recently attracted a lot of attention owing to the potential emergence of exotic spin orders on geometrically frustrated lattices, which can be characterized by corresponding spin chiralities. By performing first-principles density functional calculations together with group-theory analysis and tight-binding modelling, here we systematically study the spin-order dependent AHE and MOE in representative noncollinear AFMs Mn3XN (X = Ga, Zn, Ag, and Ni). The symmetry-related tensor shape of the intrinsic anomalous Hall conductivity (IAHC) for different spin orders is determined by analyzing the relevant magnetic point groups. We show that while only the xy component of the IAHC tensor is nonzero for right-handed spin chirality, all other elements − σxy, σyz, and σzx − are nonvanishing for a state with left-handed spin chirality owing to lowering of the symmetry. Our tight-binding arguments reveal that the magnitude of IAHC relies on the details of the band structure and that σxy is periodically modulated as the spin rotates in-plane. The IAHC obtained from first principles is found to be rather large, e.g., it amounts to 359 S/cm in Mn3AgN, which is comparable to other well-known noncollinear AFMs such as Mn3Ir and Mn3Ge. We evaluate also the magnetic anisotropy energy and find that the evolution of spin order is related to the number of valence electrons in the X ion. Interestingly, the left-handed spin chirality could exist in Mn3XN with some particular spin configurations. By extending our analysis to finite frequencies, we calculate the optical isotropy [σxx(ω) ≈ σyy(ω) ≈ σzz(ω)] and the magnetooptical anisotropy [σxy(ω) = σyz(ω) = σzx(ω)] of Mn3XN. Similar to the IAHC, the magneto-optical Kerr and Faraday spectra depend strongly on the spin order. The Kerr rotation angles in Mn3XN are in the range of 0.3 ∼ 0.4 deg, which is large and comparable to other noncollinear AFMs like Mn3Pt and Mn3Sn. Our finding of large AHE and MOE in Mn3XN suggests that these materials present an excellent antiferromagnetic platform for realizing novel spintronics and magneto-optical devices. We argue that the spin-order dependent AHE and MOE are indispensable in detecting complex spin structures in noncollinear AFMs. arXiv:1903.11038v1 [cond-mat.mtrl-sci]