Spin Hall effect from bipolaron dynamics in organics.

Using an extended Su-Schrieffer-Heeger model and a nonadiabatic dynamics method, we investigate the dynamics of bipolarons in coupled nondegenerate organic chains including the spin-orbit coupling and interchain coupling. By tracing the time-dependent evolution of the charges and spins in each chain, an obvious oscillating spin Hall effect (SHE) from the bipolaron transport is revealed. The results are compared with that from polaron-dominated transport. A reduction of amplitude and an increase of oscillating frequency are observed for the SHE from the bipolaron transport. The mechanism is attributed to the enhanced skew scattering off the larger transient deformations of the chains in the case of the bipolaron. Spectrum analysis by fast Fourier transform of the SHE signal demonstrates a distinct shift of two characteristic peaks to a higher onset frequency compared to the polaron transport. The charge-spin conversion efficiency is also compared, where a larger conversion efficiency is obtained from the bipolaron transport due to the lower saturated velocity. The effects of the strength of the electric field and the interactions are discussed. This work reveals the role of the bipolaron in organic SHE and provides a feasible way to achieve larger conversion efficiency by controlling the species of carriers with the concentration of the dopant.

A theoretical study of novel orthorhombic group-IVB nitride halide monolayers for photocatalytic overall water splitting.

Hydrogen energy is very important as a new clean energy source to combat the growing environmental problems. In this regard, novel photocatalyst materials for water splitting have a wide range of applications. Using first principles calculations, we theoretically studied three orthorhombic group-IVB nitride halide monolayers, Hf2N2Br2, Janus HfZrN2Br2 and Janus Hf2N2ClBr. The energy, dynamic and thermal stabilities are demonstrated for all three monolayers. Using the HSE hybrid functional, the calculations reveal that they are direct band gap semiconductors with suitable band edge positions, good optical absorptions, and anisotropic carrier mobilities, which makes them promising for water splitting applications. Importantly, the photogenerated carriers provide enough driving force to trigger the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) within wide pH ranges, and then overall water splitting can be achieved spontaneously. We conclude that orthorhombic group-IVB nitride halide monolayers have potential applications in photocatalytic nanodevices.

Weak-field polaron dynamics in organic ferromagnets.

With a nonadiabatic dynamical method the polaron dynamics in organic ferromagnets with spin radicals is investigated under weak electric fields. The results reveal two novel phenomena different from those in normal polymers due to the existence of spin radicals. One is that the velocity of the polaron is asymmetric upon the reversal of the applied electric field, which is explained from the asymmetric polarity of the polaron charge density in different directions of the field, and hence its effect on the lattice distortion. The other is the 'intermittent rebound' of the polaron, where the polaron intermittently moves against the electric field force during a short interval behaving like a negative current. The details of lattice distortion and charge distribution of the polaron during the process have been revealed. We further found that there exist different critical fields for the above two phenomena. With an increase of the electric field, the 'intermittent rebound' of the polaron vanishes first and subsequently the asymmetric polaron velocity. This work demonstrates the unique properties of polaron transport in organic ferromagnets, and will be helpful in the future design of organic ferromagnetic devices.

Stretch or contraction induced inversion of rectification in diblock molecular junctions.

Based on ab initio theory and nonequilibrium Green's function method, the effect of stretch or contraction on the rectification in diblock co-oligomer molecular diodes is investigated theoretically. Interestingly, an inversion of rectifying direction induced by stretching or contracting the molecular junctions, which is closely related to the number of the pyrimidinyl-phenyl units, is proposed. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals as well as transmission coefficients under external biases gives an inside view of the observed results. It reveals that the asymmetric molecular level shift and asymmetric evolution of orbital wave functions under biases are competitive mechanisms for rectification. The stretching or contracting induced inversion of the rectification is due to the conversion of the dominant mechanism. This work suggests a feasible technique to manipulate the rectification performance in molecular diodes by use of the mechanically controllable method.

Prediction ofπ-electrons mediated high-temperature superconductivity in monolayer LiC12.

Prediction and synthesis of two-dimensional high transition temperature (TC) superconductors is an area of extensive research. Based on calculations of the electronic structures and lattice dynamics, we predict that graphene-like layered monolayer LiC12is aπ-electrons mediated Bardeen-Cooper-Schrieffer-type superconductor. Monolayer LiC12is theoretically stable and expected to be synthesized experimentally. From the band structures and the phonon dispersion spectrum, it is found that the saddle point ofπ-bonding bands induces large density of states at the Fermi energy level. There is strongly coupled between the vibration mode in the in-plane direction of the lithium atoms and theπ-electrons of carbon atoms, which induces the high-TCsuperconductivity in LiC12. TheTCcan reach to 41 K under an applied 10% biaxial tensile strain based on the anisotropic Eliashberg equation. Our results show that monolayer LiC12is a good candidate asπ-electrons mediated electron-phonon coupling high-TCsuperconductor.

Intrinsic Valley-Polarized Quantum Anomalous Hall Effect and Controllable Topological Phase Transition in Janus Fe2SSe.

The valley-polarized quantum anomalous Hall effect (VP-QAHE) in topological materials, which usually is induced by applying external manipulations, has attracted intensive attention. Here, we predict the formation and regulation of the intrinsic VP-QAHE in ferromagnetic Janus monolayer Fe2SSe. Spontaneous valley polarization (VP) appears without external manipulations due to the Janus structure in monolayer Fe2SSe. The spontaneous VP in addition to the nonzero Chern number in Fe2SSe confirm the intrinsic VP-QAHE. Besides, the topologically protected chiral-spin-valley locking edge states can be regulated by reversing the magnetization. Topological phase transitions between metal, half-metal, topological insulator, and ferrovalley phases can be obtained by applying biaxial strains in Fe2SSe, and the nontrivial band gap reaches up to 441 meV. Also, the topological phase with the VP-QAHE is robust under certain conditions. Both the intrinsic VP-QAHE and controllable topological phase transitions can be achieved in Janus monolayer Fe2SSe, which provides an avenue for the applications of dissipationless valleytronic devices.

Spin-Dependent Polaron Dynamics in Organic Ferromagnets.

The spin-dependent polaron dynamics in organic ferromagnets under driven electric fields are investigated by using the extended Su-Schrieffer-Heeger (SSH) model coupled with a nonadiabatic dynamics method. It is found that the spin-down polaron with the same spin orientation as the radicals drifts faster than the spin-up one under the same driven electric field. In an applicable range of driven electric fields, the velocity of the spin-down polaron is about 3.4 times that of the spin-up one. The dynamical property of the polaron with each spin (up or down) is asymmetric upon the reversal of the driven electric fields. The diverse dynamical properties of polarons with specific spins can be attributed to the spin nondegenerate polaron energy levels, the dipole moment generated by the asymmetrical polaron charge distributions and the strong electron-lattice coupling in organic ferromagnets. Our findings are expected to be useful for improving organic ferromagnet based spintronic devices.

Manipulable Electronic and Optical Properties of Two-Dimensional MoSTe/MoGe2N4 van der Waals Heterostructures.

van der Waals heterostructures (vdWHs) can exhibit novel physical properties and a wide range of applications compared with monolayer two-dimensional (2D) materials. In this work, we investigate the electronic and optical properties of MoSTe/MoGe2N4 vdWH under two different configurations using the VASP software package based on density functional theory. The results show that Te4-MoSTe/MoGe2N4 vdWH is a semimetal, while S4-MoSTe/MoGe2N4 vdWH is a direct band gap semiconductor. Compared with the two monolayers, the absorption coefficient of MoSTe/MoGe2N4 vdWH increases significantly. In addition, the electronic structure and the absorption coefficient can be manipulated by applying biaxial strains and changing interlayer distances. These studies show that MoSTe/MoGe2N4 vdWH is an excellent candidate for high-performance optoelectronic devices.

Large valley polarization in a novel two-dimensional semiconductor H-ZrX2(X=Cl, Br, I).

Inspired by the new progress in the research field of two-dimensional valleytronics materials, we propose a new class of transition metal halides, i.e. H-ZrX2(X = Cl, Br, I), and investigated their valleytronics properties under the first-principles calculations. It harbors the spin-valley coupling at K and K' points in the top of valence band, in which the valley spin splitting of ZrI2can reach up to 115 meV. By carrying out the strain engineering, the valley spin splitting and Berry curvature can be effectively tuned. The long-sought valley polarization reaches up to 108 meV by doping Cr atom, which corresponds to the large Zeeman magnetic field of 778 T. Furthermore, the valley polarization in ZrX2can be lineally adjusted or flipped by manipulating the magnetization orientation of the doped magnetic atoms. All the results demonstrate the well-founded application prospects of single-layer ZrX2, which can be considered as great candidate for the development of valleytronics and spintronics.

Strain forces tuned the electronic and optical properties in GaTe/MoS2 van der Waals heterostructures.

Two-dimensional (2D) van der Waals heterostructures (vdWHs) have attracted widespread attention in fundamental materials science and device physics. In this work, we report a novel GaTe/MoS2 vdWH and theoretically investigate the electronic and optical properties based on first-principles calculations. GaTe/MoS2 vdWH possesses an indirect band gap with type-II band alignment. Meanwhile, the interfacial charge transfer from MoS2 to GaTe can effectively separate electrons and holes. Also, this vdWH shows improved visible-ultraviolet optical absorption properties compared with those of the isolated GaTe or MoS2 monolayers. More remarkably, the biaxial strain can not only modulate the band gap but also enhance the optical performance in GaTe/MoS2 vdWH. In particular, the tensile strain is more effective for improving the optical absorption in the visible light region. These findings indicate that GaTe/MoS2 vdWH is a promising candidate for nanoelectronics and optoelectronic devices.

Spin-current rectification in an organic magnetic/nonmagnetic device.

We propose a spin diode based on an organic magnetic co-oligomer or a magnetic/nonmagnetic heterojunction structure. The current and its spin polarization in the device are calculated with the spin-dependent Landauer-Büttiker formula. It is found that, by reversing the applied bias, the charge current and the spin current (SC) may be rectified at the same time or separately. A normal charge-current rectification usually takes place if the spatial electric structure is asymmetric. While a spin-current rectification may appear in two forms or their combination: one is that the spin-polarized orientation keeps unchanged but the magnitude of the SC is asymmetric with the bias; another is that only the spin orientation of the SC flips when the bias is reversed. By designing a suitable organic spin device, either of the two kinds of spin-current rectifications is obtained in our calculations. Finally, the effects of the properties of the organic interlayer and the structural asymmetry on the rectification are discussed.

Spin Polarization Properties of Pentagonal PdSe2 Induced by 3D Transition-Metal Doping: First-Principles Calculations.

The electronic structure and spin polarization properties of pentagonal structure PdSe2 doped with transition metal atoms are studied through first- principles calculations. The theoretical investigations show that the band gap of the PdSe2 monolayer decreases after introducing Cr, Mn, Fe and Co dopants. The projected densities of states show that p-d orbital couplings between the transition metal atoms and PdSe2 generate new spin nondegenerate states near the Fermi level which make the system spin polarized. The calculated magnetic moments, spin density distributions and charge transfer of the systems suggest that the spin polarization in Cr-doped PdSe2 will be the biggest. Our work shows that the properties of PdSe2 can be modified by doping transition metal atoms, which provides opportunity for the applications of PdSe2 in electronics and spintronics.

Optical Properties of Graphene/MoS2 Heterostructure: First Principles Calculations.

The electronic structure and the optical properties of Graphene/MoS2 heterostructure (GM) are studied based on density functional theory. Compared with single-layer graphene, the bandgap will be opened; however, the bandgap will be reduced significantly when compared with single-layer MoS2. Redshifts of the absorption coefficient, refractive index, and the reflectance appear in the GM system; however, blueshift is found for the energy loss spectrum. Electronic structure and optical properties of single-layer graphene and MoS2 are changed after they are combined to form the heterostructure, which broadens the extensive developments of two-dimensional materials.

Spin-dependent transport and functional design in organic ferromagnetic devices.

Organic ferromagnets are intriguing materials in that they combine ferromagnetic and organic properties. Although challenges in their synthesis still remain, the development of organic spintronics has triggered strong interest in high-performance organic ferromagnetic devices. This review first introduces our theory for spin-dependent electron transport through organic ferromagnetic devices, which combines an extended Su-Schrieffer-Heeger model with the Green's function method. The effects of the intrinsic interactions in the organic ferromagnets, including strong electron-lattice interaction and spin-spin correlation between π-electrons and radicals, are highlighted. Several interesting functional designs of organic ferromagnetic devices are discussed, specifically the concepts of a spin filter, multi-state magnetoresistance, and spin-current rectification. The mechanism of each phenomenon is explained by transmission and orbital analysis. These works show that organic ferromagnets are promising components for spintronic devices that deserve to be designed and examined in future experiments.

Towards Rectifying Performance at the Molecular Scale.

Molecular diode, proposed by Mark Ratner and Arieh Aviram in 1974, is the first single-molecule device investigated in molecular electronics. As a fundamental device in an electric circuit, molecular diode has attracted an enduring and extensive focus during the past decades. In this review, the theoretical and experimental progresses of both charge-based and spin-based molecular diodes are summarized. For the charge-based molecular diodes, the rectifying properties originated from asymmetric molecules including D-σ-A, D-π-A, D-A, and σ-π type compounds, asymmetric electrodes, asymmetric nanoribbons, and their combination are analyzed. Correspondingly, the rectification mechanisms are discussed in detail. Furthermore, a series of strategies for modulating rectification performance is figured out. Discussion on concept of molecular spin diode is also involved based on a magnetic co-oligomer. At the same time, the intrinsic mechanism as well as the modulation of the spin-current rectification performance is introduced. Finally, several crucial issues that need to be addressed in the future are given.