Tunable multiple nonvolatile resistance states in a MnSe-based van der Waals multiferroic tunnel junction.

Two-dimensional (2D) van der Waals (vdW) multiferroic tunnel junctions (MFTJs) composed of a ferromagnetic metal and a ferroelectric barrier have controllable thickness and clean interface and can realize the coexistence of tunneling magnetoresistance (TMR) and tunneling electroresistance (TER). Therefore, they have enormous potential application in nonvolatile multistate memories. Here, using first principles combined with non-equilibrium Green's function method, we have systematically investigated the spin-dependent transport properties of Fe3GeTe2/MnSe/Fe3GeTe2 vdW MFTJs with various numbers of barrier layers. By controlling the polarization orientation of the ferroelectric barrier MnSe and the magnetization alignment of the ferromagnetic electrodes Fe3GeTe2, the MnSe-based MFTJs exhibit four nonvolatile resistance states, with the TMR (TER) becoming higher and reaching a maximum of 1.4 × 106% (4114%) as the MnSe layers increase from a bilayer to a tetralayer. Using asymmetric Cu and Fe3GeTe2 as the electrodes, the TER can be further improved from 349% to 618%. Moreover, there is a perfect spin filtering effect in these MFTJs. This work demonstrates the potential applications of MnSe-based devices in multistate nonvolatile memories and spin filters, which will stimulate experimental studies on layer-controllable spintronic devices.

Negative differential resistance, perfect spin-filtering effect and tunnel magnetoresistance in vanadium-doped zigzag blue phosphorus nanoribbons.

We investigate the electronic and transport properties of vanadium-doped zigzag blue phosphorus nanoribbons by first-principles quantum transport calculations. We study the spin-dependent transport properties and obtain current-voltage curves showing obvious spin polarization and negative differential behaviors. These interesting transport behaviors can be explained by the band structure of the vanadium-doped zigzag blue phosphorus nanoribbons. The tunnel magnetoresistance and spin-filtering effects under different magnetic configurations originate predominately from the symmetry matching between the band structures of the electrodes. According to our results, vanadium-doped zigzag blue phosphorus nanoribbons can be used as a perfect spin filter with a large tunnel magnetoresistance. This also indicates that blue phosphorus nanoribbons are a promising candidate for their future application in spintronics.

Half-metallic and magnetic semiconducting behaviors of metal-doped blue phosphorus nanoribbons from first-principles calculations.

We investigate the electronic and magnetic properties of substitutional metal atom impurities in two-dimensional (2D) blue phosphorene nanoribbons using first-principles calculations. In impure zigzag blue phosphorene nanoribbons (zBPNRs), a metal atom substitutes for a P atom at position "A/B". The V-"B"structure shows half-metallic properties, while the Mn-"A/B", V-"A", Fe-"B", and Cr-"A/B" structures show magnetic semiconductor properties. In addition, the Fe-"A" system shows magnetic metallic properties. On the other hand, for metal-doped armchair blue phosphorene nanoribbons (aBPNRs), the Mn-"A/B", V-"A", Fe-"A/B", and Cr-"A/B" structures show magnetic semiconductor properties, while the V-"B" structure shows nonmagnetic properties. We find that the magnetic properties of such substitutional impurities can be understood by regarding the exchange splitting of the metal 3d orbitals. And from analyzing the electron orbitals, we conclude that the main contribution of the DOS for every system comes from the d and p orbitals. These results suggest excellent candidates for new magnetic semiconductors and half-metals for spintronic devices based on blue phosphorenes.

Enhancing the Thermoelectric Figure of Merit by Low-Dimensional Electrical Transport in Phonon-Glass Crystals.

Low-dimensional electronic and glassy phononic transport are two important ingredients of highly efficient thermoelectric materials, from which two branches of thermoelectric research have emerged. One focuses on controlling electronic transport in the low dimension, while the other focuses on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystals as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculations and classical molecular dynamics simulations, we show that the π-π-stacking bis(dithienothiophene) molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameters, we obtained a maximum room-temperature figure of merit, ZT, of 1.48 at optimal doping, thus validating our idea.

Topological field-effect quantum transistors in HgTe nanoribbons.

We propose practical designs to realize topological field-effect quantum transistors in an HgTe nanoribbon with an inverted band structure. Our theoretical calculations show that, as a strip-shape top gate is placed on the HgTe nanoribbon and with an increasing gate voltage, two new conductance channels develop in the HgTe nanoribbon and are localized to the lattice sites neighboring the boundaries of the gate, leading to an additional quantization of the conductance of 2e(2)/h. The quantum states in the new channels are not only robust against a short-range Anderson disorder, but can also couple with the intrinsic helical edge states in the boundaries of the HgTe nanoribbon to open a gap in the energy spectrum, indicating their topological characteristics. More importantly, the newly developed conductance channels can be turned on or off easily by adjusting the gate voltage. The proposal of controllable topological edge states produced by the gate voltage opens a new route for future topological field-effect quantum transistors in nanoelectronics and spintronics.

Thermal spin filtering, thermal spin switching and negative-differential-resistance in thermal spin currents in zigzag SiC nanoribbons.

Spin caloritronics with a combination of spintronics and thermoelectrics has potential applications in future information science and opens a new direction in the development of multi-functional materials. Based on density functional theory and the nonequilibrium Green's function method, we calculate thermal spin-dependent transport through a zigzag silicon carbide nanoribbon (ZSiCNR), which is a heterojunction consisting of a left electrode (ZSiC-2H1H) and right electrode terminated (ZSiC-1H1H) by hydrogen. Our results show that when the temperature in the left contact increases over a critical value, the thermal spin-down current increases remarkably from zero, while the thermal spin-up current remains zero in the total-temperature region, indicating that a perfect thermal spin filter together with a perfect spin switcher is obtained. Furthermore, the thermal spin current shows a negative differential resistance effect and quantum oscillation behaviors. These results suggest that the zigzag SiC nanoribbon proposed by us can be designed as a highly-efficient spin caloritronics device with multiple functionalities.

Large negative differential resistance and rectifying behaviors in isolated thiophene nanowire devices.

We design isolated molecular nanowires composed of thiophene oligomers sandwiched between two one-dimensional gold electrodes. Electronic transport through the molecular junctions with two interface geometries is studied by performing the first principles calculations based on density functional theory and nonequilibrium Green's function. The current-voltage (I-V) curves of the molecular wires display an unexpected negative differential resistance and rectifying behaviors along with the oscillation effects, different from other theoretical and experimental studies about the analogous thiophene devices. The significant difference is attributed to the design of the one-dimensional gold electrodes with large enough vacuum layer in transverse direction in order to suppress the interaction between wires. Such transport behaviors indicate that the thiophene molecular device would be an important candidate in future molecular electronics.

Sub-9 nm high-performance and low-power transistors based on an in-plane NbSe2/MoSe2/NbSe2 heterojunction.

Due to the ability to reduce the gate length of field-effect transistors (FETs) down to sub-10 nm without obviously affecting the performance of the device, the utilization of two-dimensional (2D) semiconductor materials as channel materials for FETs is of great interest. However, in-plane 2D/2D heterojunction FETs have received less attention in previous studies than vertical van der Waals heterojunction devices. Based on the above reasons, this study has investigated the transport properties of an in-plane NbSe2/MoSe2/NbSe2 heterojunction FET with different gate lengths by using ab initio quantum transport simulation. The results reveal that a gate length of sub-9 nm gives the device a low subthreshold swing down to 62 mV dec-1 and a high on-state current up to 1040 µA µm-1. Most importantly, the on-state current, delay time, and power dissipation of the FET with the optimized channel length can nearly meet or even exceed the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors. The findings for this FET can provide the design and development guidance for other in-plane heterojunction electrical devices in the post-Moore era.

Spin-filter and Fano antiresonant effect in conductance through a zigzaglike polymer device: nonequilibrium Green's function approach.

Electronic transport through a quasi-one-dimensional zigzaglike polymer device is theoretically studied by means of the nonequilibrium Green's function approach. In this system, the main zigzag chain consists of carbon atoms and is attached with side radicals at its next-near-neighbor sites. The results show that a zero point of the linear conductance spectrum occurs due to the Fano antiresonance induced by the electron hoppings between the main chain and the side radicals, which leads to the fact that the linear conductance spectrum displays an insulating band around the antiresonant point. The increase in the polymer size makes both edges of the insulating band to become steep rapidly, which makes the insulating band approach to be a well-defined one. The formation of the dimer along the main chain enhances the insulating band width but much suppresses the electronic transport through the device. Moreover, if the many-body effect due to the electron interaction is taken into account, the antiresonance effect and the well-defined insulating band remain. As a result, a well spin-polarized window appears as the spin splitting occurred by applying an external magnetic field. These results strongly propose that there is a new way for the organic polymer to be applied as a spin-filter.

Field-controlled Luttinger liquid and possible crossover into spin liquid in strong-rail ladder systems.

The thermodynamics and transport properties of strong-rail ladder systems are investigated by means of Green's function theory. It is shown that the magnetic behavior clearly manifests a typical antiferromagnetism with gapped or gapless low-lying excitations, which is in agreement with the experimental results. In addition, the temperature-field-induced phase diagram is explored, and we demonstrate a Luttinger liquid behavior in the window h(c) (marking the ending of the M=0 plateau)

Nature of the ferromagnetic behavior and possible occurrence of the ferrimagnetic phase transition in genuinely organic molecule-based assemblages with an S = 1 and S = 1/2 antiferromagnetic alternating spin chain: a Green's function approach.

The temperature dependence of magnetic susceptibility and sublattice magnetizations were calculated for a Heisenberg Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic alternating spin chain by means of the many-body Green's function theory to show the possible occurrence of a ferrimagnetic phase transition for genuinely organic molecule-based magnets. The S = 1 site in the chain is composed of two S = 1/2 spins coupled by a finite ferromagnetic interaction. From the calculated results, it is found that the sublattice magnetization at low-spin S = 1/2 sites changes its sign from negative to positive with increasing temperature, giving rise to the spin alignments along the chain changing from antiferromagnetic to ferromagnetic ones, which indicates that there is a magnetic phase transition occurring. Because of the weak intermolecular antiferromagnetic interactions, the curves of the magnetic susceptibility multiplied by temperature (chiT) against temperature show a round peak at low temperatures, which is well consistent with recent experimental observations, and the ferrimagnetic phase transition could only be detected at an ultralow-temperature region and under very weak external magnetic fields in practical organic materials. From the analysis of the sublattice magnetizations, it is uncovered that the appearance of the low-temperature peak in the curves of the chiT originates from the ferromagnetic spin alignments for all the spins along the chain, and the intermolecular antiferromagnetic interactions play a pivotal role in ferrimagnetic spin alignments of the magnetic systems. It is also found that the higher spatial symmetry of the intermolecular antiferromagnetic interactions have contributions to stabilize the ferrimagnetic ordering state in the molecule-based magnetic materials.

Magnetic properties of very-high-spin organic pi-conjugated polymers based on Green's function theory.

Magnetic properties of two very-high-spin organic pi-conjugated polymers have been investigated theoretically by means of the many-body Green's function method with random phase approximation. The polymers are designed with a large density of cross-links and alternating connectivity of radical modules with unequal spin quantum numbers (S), macrocyclic S=2 or 3, and cross-linking S=1/2 modules, which permit large net S values for either antiferromagnetic or ferromagnetic exchange coupling between the modules. The numerical results reveal that, ascribing to the zero-temperature spin fluctuations, the sublattice magnetizations of the two polymers are both smaller than their classical spin values and the ground-state magnetizations of them are also smaller than their predicted values in the antiferromagnetic exchange coupling case. However, these magnetic behaviors do not occur in the ferromagnetic exchange coupling case. On the basis of our synthesis of the temperature dependence of the magnetic susceptibility multiplied by temperature, and through comparing the theoretical results with the experimental measurements, it is concluded that the magnetic exchange couplings between the modules within the two high-spin polymers should be ferromagnetic exchange couplings, which are consistent with other theoretical results drawn from the investigations into the ground-state properties of the two organic polymers.

4-Carboxy-pyridinium 3-carb-oxy-4-hydroxy-benzene-sulfonate.

Cocrystallization of 4-carboxy-pyridine (4-CPY) and 5-sulfosalicylic acid (5-H(2)SSA) yields the title salt, C(6)H(6)NO(2) (+)·C(7)H(5)O(6)S(-). In the crystal structure, the components of the salt are linked by a combination of inter-molecular O-H⋯O and N-H⋯O, and weak C-H⋯O hydrogen bonds, forming a three-dimensional framework.

Half-metallicity of the (001), (111) and (110) surfaces of CoRuMnSi and interface half-metallicity of CoRuMnSi/CdS.

Recent studies have indicated that the quaternary Heusler alloy CoRuMnSi shows a half-metallic ferromagnetism (Kundu et al., Sci. Rep., 7, (2017), 1803). The (111), (110), and (001) surfaces and the interfaces with CdS (111) substrate of the quaternary Heusler alloy CoRuMnSi were explored by carrying out a first-principles investigation based on a density functional theory. Calculations showed that the half metallicity can be preserved for the Si-terminated (111) surface and subsurface while the half-metallicity approved in the bulk CoRuMnSi is destroyed at Co, Ru, and Mn-terminations (111) surfaces and subsurfaces. Regrettably, the surface states ruin the gap in the spin-down channel at both MnSi- and CoRu-terminated (001) surfaces and subsurfaces. Remarkably, the (110) surfaces and subsurfaces have a nearly half-metallicity property with a high spin polarization. Based on spin magnetic character calculations, the spin magnetic moments of surface and subsurface atoms are larger and smaller than those in the bulk quaternary Heusler alloy CoRuMnSi. For the interface of CoRuMnSi/CdS (111), the bulk half-metallicity is destroyed at Si-Cd and Si-S configurations.

The spin-dependent transport properties of zigzag α-graphyne nanoribbons and new device design.

By performing first-principle quantum transport calculations, we studied the electronic and transport properties of zigzag α-graphyne nanoribbons in different magnetic configurations. We designed the device based on zigzag α-graphyne nanoribbon and studied the spin-dependent transport properties, whose current-voltage curves show obvious spin-polarization and conductance plateaus. The interesting transport behaviours can be explained by the transport spectra under different magnetic configurations, which basically depends on the symmetry matching of the electrodes' bandstructures. Simultaneously, spin Seebeck effect is also found in the device. Thus, according to the transport behaviours, zigzag α-graphyne nanoribbons can be used as a dual spin filter diode, a molecule signal converter and a spin caloritronics device, which indicates that α-graphyne is a promising candidate for the future application in spintronics.

Ultrahigh spin thermopower and pure spin current in a single-molecule magnet.

Using the non-equilibrium Green's function (NEGF) formalism within the sequential regime, we studied ultrahigh spin thermopower and pure spin current in single-molecule magnet(SMM), which is attached to nonmagnetic metal wires with spin bias and angle (θ) between the easy axis of SMM and the spin orientation in the electrodes. A pure spin current can be generated by tuning the gate voltage and temperature difference with finite spin bias and the arbitrary angle except of θ=1/2π, 2/3π. In the linear regime, large thermopower can be obtained by modifying V(g) and the angles (θ). These results are useful in fabricating and advantaging SMM devices based on spin caloritronics.

The transport properties and new device design: the case of 6,6,12-graphyne nanoribbons.

By performing first-principle quantum transport calculations, we studied the transport properties of three kinds of 6,6,12-graphyne nanoribbons with different edges and different cutting directions. The nanoribbon with zigzag edges shows metallic properties and the spin-polarized currents show an obvious negative differential resistance effect, the other two nanoribbons terminated by a phenyl ring are semiconductors and spin-unpolarized. We also designed several nanowire devices based on these 6,6,12-graphyne nanoribbons, such as rectifier, spin filter diode, spin FET and spin caloritronics devices. These results indicate that 6,6,12-graphyne is a potential candidate for spintronics and spin caloritronics.

Magnetic properties and quantum phase transitions of purely organic molecule-based ferrimagnets based on Green's function theory.

Magnetic properties of a Heisenberg diamondlike spin chain model for purely organic molecule-based ferrimagnets are investigated by means of the many-body Green's function method within random phase approximation. The molecule-based ferrimagnet is composed of S=1 biradical and S=1/2 monoradical molecules alternating with intermolecular antiferromagnetic (AF) interactions, and the S=1 site is composed of two S=1/2 spins by a finite intramolecular ferromagnetic (F) interaction. The numerical results reveal that occurrence of ferrimagnetic spin alignments along the chain is determined by the intermolecular AF interactions. Owing to the very small intermolecular AF interactions, the curves of the product of magnetic susceptibility and temperature (chiT) against temperature display as a round peak at low temperatures, and the ferrimagnetic phase transition could only be detected at ultralow temperatures in practical organic compounds. Temperature- and magnetic-field-induced magnetic phase transitions are discussed, which are consistent with the experimental findings. The lower spatial symmetry of intermolecular interactions makes it easy to form spin pairs with a singlet (S=0) ground state along the chain and to reduce Curie temperature. The formations of molecular dimers and trimers along the chain have contributions to bring about F alignments with effective S=1/2 magnetic supramolecules and to enhance Curie temperature. In addition, the experimental data of the magnetic susceptibility measurements for a molecule-based ferrimagnet are also fairly compared with our theoretical results.

Magnetic studies of a syn-anti triatomic carboxylate-bridging chainlike copper(II) complex exhibiting ferromagnetic exchange.

The magnetic properties of triatomic syn-anti carboxylate bridging copper(II) complex, {[Cu(2,2'-bipydine)(maleate)].2H2O}infinity (complex 1), were investigated experimentally and theoretically, suggesting weak ferromagnetic intrachain interaction. The magnetic data were analyzed and interpreted in terms of Heisenberg chain model corrected by a mean molecular field. Fitting parameters obtained for J, g, and zJ' are 3.14 cm(-1), 2.08, and -0.13, respectively. Density functional theory with generalized gradient approximation was applied to calculate the electronic structure and spin distribution of the present complex. The structural and electronic factors controlling the magnetic interactions were also determined. Ferromagnetic intrachain interactions through triatomic syn-anti carboxylate bridge result from nonplanarity of the bridging network, the exchange pathway involving both the sigma and pi orbitals of the carboxylate bridge and the spin delocalization of each magnetic orbital on the atoms of the carboxylate bridge from the copper(II) centers.