Dezheng Yang

Manifestation of unexpected semiconducting properties in few-layer orthorhombic arsenene

In this letter, we demonstrate that few-layer orthorhombic arsenene is an ideal semiconductor. Owing to the layer stacking, multilayer arsenenes always behave as intrinsic direct bandgap semiconductors with gap values of approximately 1 eV. In addition, these bandgaps can be further tuned in its nanoribbons. Based on the so-called acoustic phonon limited approach, the carrier mobilities are predicted to approach as high as several thousand square centimeters per volt–second and to simultaneously exhibit high directional anisotropy. All these characteristics make few-layer arsenene promising for device applications in the semiconducting industry.

A theoretical study of blue phosphorene nanoribbons based on first-principles calculations

Based on first-principles calculations, we present a quantum confinement mechanism for the band gaps of blue phosphorene nanoribbons (BPNRs) as a function of their widths. The BPNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen saturation. Both the two types of nanoribbons are shown to be indirect semiconductors. An enhanced energy gap of around 1 eV can be realized when the ribbons width decreases to ∼10 A. The underlying physics is ascribed to the quantum confinement effect. More importantly, the parameters to describe quantum confinement are obtained by fitting the calculated band gaps with respect to their widths. The results show that the quantum confinement in armchair nanoribbons is stronger than that in zigzag ones. This study provides an efficient approach to tune the band gap in BPNRs.

Negative Poisson’s ratios in few-layer orthorhombic arsenic: First-principles calculations

Using first-principles calculations we demonstrate for the first time that few-layer orthorhombic arsenic possesses a negative Poissons ratio. For a single layer of arsenic, the negative Poissons ratio is predicted to be ~−0.09. As the number of layers increases, the magnitude of the negative Poissons ratio increases and finally approaches a limit at four layers, becoming very close to the bulk value of −0.13. To understand these layer-dependent negative Poissons ratios, we propose a rigid mechanical model in which the intra-layer bond lengths and the normal Poissons ratio of the in-layer plane play key roles.

Theoretical Prediction of Carrier Mobility in Few-Layer BC2N.

An ideal semiconducting material should simultaneously hold a considerable direct band gap and a high carrier mobility. A 2D planar compound consisting of zigzag chains of C-C and B-N atoms, denoted as BC2N, would be a good candidate. It has a direct band gap of 2 eV, which can be further tuned by changing the layer number. At the same time, our first-principles calculations show that few-layer BC2N possesses a high carrier mobility. The carrier mobility of around one million sqaure centimeters per volt-second is obtained at its three-layer. As our study demonstrated, few-layer BC2N has potential applications in nanoelectronics and optoelectronics.

Intrinsic ferromagnetism in hexagonal boron nitride nanosheets

Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

Dirac cone in α-graphdiyne: a first-principles study

We investigate the Dirac cone in α-graphdiyne, which is a predicted flat one-atom-thick allotrope of carbon using first-principles calculations. α-graphdiyne is derived from graphene where two acetylenic linkages (-C ≡C-) are inserted into the single bonds (-C-C-). Thus, α-graphdiyne possesses a larger lattice constant which subsequently affects its electronic properties. Band structures show that α-graphdiyne exhibits similar Dirac points and cone to graphene. Further, the tight-binding method is used to exploit the linear dispersion in the vicinity of Dirac points. Thanks to the larger lattice constant, α-graphdiyne yields a lower Fermi velocity, which might make itself an ideal material to serve the anomalous integer quantum Hall effect.

Effect of inserting a non-metal C layer on the spin-orbit torque induced magnetization switching in Pt/Co/Ta structures with perpendicular magnetic anisotropy

Magnetization switching via charge current induced spin-orbit torques (SOTs) in heavy metal/ferromagnetic metal/heavy metal heterostructures has become an important issue due to its potential applications in high stability and low energy dissipation spintronic devices. In this work, based on a Pt/Co/Ta structure with perpendicular magnetic anisotropy (PMA), we report the effect of inserting a non-metal C interlayer between Co and Ta on the current-induced magnetization switching. A series of measurements based on the extraordinary Hall effect were carried out to investigate the difference of the anisotropy field, switching field, and damping-like and field-like SOT-induced effective fields as well as the current-induced spin Hall effect (SHE) torque after C decoration. The results show that PMA can be reduced by C decoration and the ratio of the effective SHE torque per unit current density and anisotropy field plays an essential role in the switching efficiency. In addition, the obtained switching curren...

Temperature-Dependent Asymmetry of Anisotropic Magnetoresistance in Silicon p-n Junctions

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.

Decoding the mechanism of the mechanical transfer of a GaN-based heterostructure via an h-BN release layer in a device configuration

Mechanical transfer of GaN-based heterostructures using h-BN as the release layer [Y. Kobayashi, K. Kumakura, T. Akasaka, and T. Makimoto, Nature 484, 223 (2012)] has promising applications for the next-generation optoelectronic devices. We investigate such transfer mechanism by mapping out the interlayer sliding energy landscape at each interface of a model heterostructure composed of GaN/BN/substrate together with the reference case of BN/BN interlayer sliding. The calculated results based on density functional theory find a nearly free sliding path for BN/BN, while a slightly higher energy barrier is predicted for hetero-interfaces of strained GaN/BN and BN/graphene substrate. The unstrained GaN/BN interface facilitates an easier peel-off of GaN from the BN layer. Thus, the sliding mechanism, which can also be described by the registry index model, shows dominance of the electrostatic interaction terms associated with the constituent layers of the system, though the van der Waals interaction term seems...

Angular dependence of positive exchange biasing in GdFe∕FeMn bilayers

For Gd45Fe55∕Fe50Mn50 bilayers, both negative and positive exchange biasing have been observed for low and high magnetic cooling field HCF, respectively. These results can be attributed to a competition between antiferromagnetic coupling at GdFe∕FeMn interface and the Zeeman energy of FeMn spins under HCF. In order to reveal the magnetization reversal mechanism, the angular dependence of HE and HC has been investigated. It is found that the negative exchange biasing and the positive one have similar angular dependence that can be described by a magnetization coherent rotation model.