It is well known that anisotropy determines the preferred transport direction of carriers. To manipulate the anisotropy is an exciting topic in two-dimensional materials, where the carriers are confined within individual layers. In this work, it is found that uniaxial strain can tune the electronic anisotropy of the 90°-twisted bilayer phosphorene. In this unique bilayer structure, the zigzag direction of one layer corresponds to the armchair one of the other layer and vice versa. Owing to this complementary structure, the directional (zigzag or armchair) deformation response to strain of one layer is opposite to that of the other layer, where the in-plane positive Poisson's ratio plays a key role. As a result, the doubly degenerate highest valence bands split, followed by a recovery of anisotropy. More interestingly, such an anisotropy, namely, the ratio of the effective mass along the direction to that along the direction, reaches as high as 6 under a small strain of 1%, and keeps nearly unchanged up to a strain of 3%. In addition, high anisotropy only holds for hole carriers as the conduction band is insensitive to strain. These findings should shed new light on the design of semiconducting devices, where the hole acts as the transport carrier.
In this letter, we demonstrate a large enhancement of the magnetic moment in zigzag Janus MoSSe nanoribbons arising from the out-of-plane mirror symmetry breaking. Owing to the broken mirror symmetry, the orbital of the edge Mo atom is tilted and largely overlaps the orbital. As a result, a new band which is nearly completely occupied near the Fermi level emerges, leading to a large spin splitting and so a large magnetic moment in zigzag Janus MoSSe nanoribbons. The results shed new light on the understanding of magnetism correlated to the space symmetry breaking in low-dimensional materials.
We report a large but asymmetric magnetoresistance in silicon p-n junctions, which contrasts with the fact of magnetoresistance being symmetric in magnetic metals and semiconductors. With temperature decreasing from 293 K to 100 K, the magnetoresistance sharply increases from 50% to 150% under a magnetic field of 2 T. At the same time, an asymmetric magnetoresistance, which manifests itself as a magnetoresistance voltage offset with respect to the sign of magnetic field, occurs and linearly increases with magnetoresistance. More interestingly, in contrast with other materials, the lineshape of anisotropic magnetoresistance in silicon p-n junctions significantly depends on temperature. As temperature decreases from 293 K to 100 K, the width of peak shrinks from 90° to 70°. We ascribe these novel magnetoresistance to the asymmetric geometry of the space charge region in p-n junction induced by the magnetic field. In the vicinity of the space charge region the current paths are deflected, contributing the Hall field to the asymmetric magnetoresistance. Therefore, the observed temperature-dependent asymmetry of magnetoresistance is proved to be a direct consequence of the spatial configuration evolution of space charge region with temperature.
Galfenol owing excellent deformation due to lattice softening are regarded as a new generation of smart magnetostrictive materials. However, the lack of direct probes of phase transformation and intermediate phase related to lattice softening blocks the comprehensive understanding of their intrinsic magnetostrictive mechanism and further improvement of their performance. In this work, we firstly report an atomic observation of ω phase transformation in Galfenol under low temperature aging by spherical aberration-corrected transmission electron microscopy. The ω precipitates with two variants are directly probed to be decomposed from FeGa bcc matrix with the assistances of both spinodal decomposition and displacive transformation. Their orientation relationships with the long range ordered bcc structure of D03 can be well indexed into (0003)ω1[112¯0]ω1 || (44¯4¯)D03[01¯1]D03 || (4¯401¯)ω2[112¯0]ω2. Density functional theory calculations unveil the precipitate of ω phase in Galfenol is theoretically possible. Further magnetostrictive measurements reveal the ω phase precipitates deteriorate the magnetostriction of Galfenol as empirically expected. Our work is believed to contribute a further insight of precipitate and structural evolution in Galfenol and is significant to maintain the magnetostriction performance of Galfenol in service.
Recently, 27 new half-Heusler compounds XYZ (X = Co, Rh, Fe, Ru, Ni; Y = Sc, Ti, V; Z = P, As, Sb, Si, Ge, Sn, Al, Ga, In) with 18 valence electrons are proposed and their bandgaps span a wide range of 0.10–1.39 eV, which have a great potential of applications in varied areas. Note that the bandgaps are predicted on the gradient-corrected Perdew-Burke-Ernzerhof functional, which underestimates the magnitude of bandgap. To obtain the accurate bandgaps, we recalculate them based on the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. Our results show that the nonlocal correction from the HSE06 functional mainly acts on the two lowest conduction bands. The variation in energy separation between these two bands dominates the relative increment of bandgap. More importantly, the band ordering is distinguished in the presence of HSE06 functional, where the dz2 orbital exhibits. When the lattice constant varies, such a band ordering can be inverted, similar to the case of topological insulators. In addition, we find an abnormal behavior of the bandgap related to the Pauling electronegativity difference between the X- and Z-sites, which arises from the delocalization of charge on the Y-site. We expect that our work can provide guidance to the study of bandgap based on the hybrid density functional theory in the half-Heusler semiconductors.
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