Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2.

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.

Binding structure, breaking forces and conductance of Au-Octanedithiol-Au molecular junction under stretching processes: a DFT-NEGF study.

Au-n-octanedithiol-Au molecular junction (Au-SC8S-Au) has been investigated using density functional theory combined with the nonequilibrium Green's function approach. Theoretically calculated results are used to build the relationship between the interface binding structures and single-molecule quantum conductance of n-octanedithiol (SC8S) embodied in a gold nanogap with or without stretching forces. To understand the electron transport mechanism in the single molecular nanojunction, we designed three types of Au-SC8S-Au nanogaps, including flat electrode through an Au atom connecting (Model I), top-pyramidal or flat electrodes with the molecule adsorbing directly (Model II), and top-pyramidal Au electrodes with Au atomic chains (Model III). We first determined the optimized structures of different Au-SC8S-Au nanogaps, and then predicted the distance-dependent stretching force and conductance in each case. Our calculated results show that in the Model I with an Au atom bridging the flat Au (111) gold electrodes and the SC8S molecule, the conductance decreases exponentially before the fracture of Au-Au bond, in a good agreement with the experimental conductance in the literature. For the top-pyramidal electrode Models II and III, the magnitudes of molecular conductance are larger than that in Model I. Our theoretical calculations also show that the Au-Au bond fracture takes place in Models I and III, while the Au-S bond fracture appears in Model II. This is explained due to the total strength of three synergetic Au-Au bonds stronger than an Au-S bond in Model II. This is supported from the broken force about 2 nN for the Au-Au bond and 3 nN for the Au-S bond.

Magnetoresistance of Ni/WSe2/Ni junctions: robustness against the thickness of WSe2.

Magnetoresistive materials are vital for the development of storage devices. Using the first-principles transport simulations with nonequilibrium Green's function calculation, we investigate the magnetoresistive properties of Ni/WSe2/Ni junctions withm-layers of WSe2(m= 1, 2, ⋯ ,6). Form≤ 2, the junctions are metallic inspite of the semiconducting nature of few-layer WSe2. However, the junctions exhibit transport gaps form> 2. Interestingly, magnetoresistance of the junctions stays around 6% when there are more than one layer of WSe2in the center, which is closely related to the robust spacial variation of interfacial properties and can be attributed to no spin flipping in tunneling regions. Our results suggest that Ni/WSe2/Ni junctions have a robust magnetoresistance which is insensitive to the thickness of WSe2.

Carrier-Envelope-Phase Modulated Currents in Scanning Tunneling Microscopy.

Carrier-envelope-phase (CEP) stable optical pulses combined with state-of-the-art scanning tunneling microscopy (STM) can track and control ultrafast electronic tunneling currents. On the basis of nonequilibrium Green's function formalism, we present a time and frequency domain theoretical study of CEP-stable pulse-induced tunneling currents between an STM tip and a metal substrate. It is revealed that the experimentally observed phase shift between the maximum tunneling current and maximum electric field is caused by the third-order response to the electric field. The shift is also found to be sensitive to the duration of pulses. The tunneling process can thus be precisely manipulated by varying the phase and duration of these pulses.

Nonequilibrium Bond Forces in Single-Molecule Junctions.

Passing a current across two touching C60 molecules imposes a nonequilibrium population of bonding and antibonding molecular orbitals, which changes the equilibrium bond character and strength. A current-induced bond force therefore contributes to the total force at chemical-bond distances. The combination of first-principles calculations with scanning probe experiments exploring currents and forces in a wide C60-C60 distance range consistently evidences the presence of current-induced attraction that occurs when the two molecules are on the verge of forming a chemical bond. The unique opportunity to arrange matter at the atomic scale with the atomic force and scanning tunneling microscope tip has enabled closely matching molecular junctions in theory and experiment. The findings consequently represent the first report of current-induced bond forces at the single-molecule level and further elucidate the intimate relation between charge transport and force. The results are relevant to molecular electronics and chemical reactions in the presence of a current.

Understanding and Engineering Phonon-Mediated Tunneling into Graphene on Metal Surfaces.

Metal-intercalated graphene on Ir(111) exhibits phonon signatures in inelastic electron tunneling spectroscopy with strengths that depend on the intercalant. Extraordinarily strong graphene phonon signals are observed for Cs intercalation. Li intercalation likewise induces clearly discriminable phonon signatures, albeit less pronounced than observed for Cs. The signal can be finely tuned by the alkali metal coverage and gradually disappears upon increasing the junction conductance from tunneling to contact ranges. In contrast to Cs and Li, for Ni-intercalated graphene the phonon signals stay below the detection limit in all transport ranges. Going beyond the conventional two-terminal approach, transport calculations provide a comprehensive understanding of the subtle interplay between the graphene-electrode coupling and the observation of graphene phonon spectroscopic signatures.

Charge Separation in Donor-C60 Complexes with Real-Time Green Functions: The Importance of Nonlocal Correlations.

We use the nonequilibrium Green function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-Parr-Pople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent density matrix renormalization group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical nonlocal correlation corrections are included. This CS process occurs in ∼10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicates that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.

Electron Traversal Times in Disordered Graphene Nanoribbons.

Using the partition-free time-dependent Landauer-Büttiker formalism for transient current correlations, we study the traversal times taken for electrons to cross graphene nanoribbon (GNR) molecular junctions. We demonstrate electron traversal signatures that vary with disorder and orientation of the GNR. These findings can be related to operational frequencies of GNR-based devices and their consequent rational design.

Field Effect in Graphene-Based van der Waals Heterostructures: Stacking Sequence Matters.

Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultrathin devices based on 2D atomic crystals.

Microscopic Theory of Resistive Switching in Ordered Insulators: Electronic versus Thermal Mechanisms.

We investigate the dramatic switch of resistance in ordered correlated insulators when they are driven out of equilibrium by a strong voltage bias. Microscopic calculations on a driven-dissipative lattice of interacting electrons explain the main experimental features of resistive switching (RS), such as the hysteretic I-V curves and the formation of hot conductive filaments. The energy-resolved electron distribution at the RS reveals the underlying nonequilibrium electronic mechanism, namely Landau-Zener tunneling, and also justifies a thermal description in which the hot-electron temperature, estimated from the first moment of the distribution, matches the equilibrium-phase transition temperature. We discuss the tangled relationship between filament growth and negative differential resistance and the influence of crystallographic structure and disorder in the RS.

Shuttlecock-Shaped Molecular Rectifier: Asymmetric Electron Transport Coupled with Controlled Molecular Motion.

A fullerene derivative with five hydroxyphenyl groups attached around a pentagon, (4-HOC6H4)5HC60 (1), has shown an asymmetric current-voltage (I-V) curve in a conducting atomic force microscopy experiment on gold. Such molecular rectification has been ascribed to the asymmetric distribution of frontier molecular orbitals over its shuttlecock-shaped structure. Our nonequilibrium Green's function (NEGF) calculations based on density functional theory (DFT) indeed exhibit an asymmetric I-V curve for 1 standing up between two Au(111) electrodes, but the resulting rectification ratio (RR ∼ 3) is insufficient to explain the wide range of RR observed in experiments performed under a high bias voltage. Therefore, we formulate a hypothesis that high RR (>10) may come from molecular orientation switching induced by a strong electric field applied between two electrodes. Indeed, molecular dynamics simulations of a self-assembled monolayer of 1 on Au(111) show that the orientation of 1 can be switched between standing-up and lying-on-the-side configurations in a manner to align its molecular dipole moment with the direction of the applied electric field. The DFT-NEGF calculations taking into account such field-induced reorientation between up and side configurations indeed yield RR of ∼13, which agrees well with the experimental value obtained under a high bias voltage.

Control of single-molecule junction conductance of porphyrins via a transition-metal center.

Using scanning tunneling microscope break-junction experiments and a new first-principles approach to conductance calculations, we report and explain low-bias charge transport behavior of four types of metal-porphyrin-gold molecular junctions. A nonequilibrium Green's function approach based on self-energy corrected density functional theory and optimally tuned range-separated hybrid functionals is developed and used to understand experimental trends quantitatively. Importantly, due to the localized d states of the porphyrin molecules, hybrid functionals are essential for explaining measurements; standard semilocal functionals yield qualitatively incorrect results. Comparing directly with experiments, we show that the conductance can change by nearly a factor of 2 when different metal cations are used, counter to trends expected from gas-phase ionization energies which are relatively unchanged with the metal center. Our work explains the sensitivity of the porphyrin conductance with the metal center via a detailed and quantitative portrait of the interface electronic structure and provides a new framework for understanding transport quantitatively in complex junctions involving molecules with localized d states of relevance to light harvesting and energy conversion.

Continuous-wave GaAs/AlGaAs quantum cascade laser at 5.7 THz.

Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.

Quantum Interference Enhancement of the Spin-Dependent Thermoelectric Response.

We investigate the influence of quantum interference (QI) and broken spin-symmetry on the thermoelectric response of node-possessing junctions, finding a dramatic enhancement of the spin-thermopower (Ss), figure-of-merit (ZsT), and maximum thermodynamic efficiency (ηsmax) caused by destructive QI. Using many-body and single-particle methods, we calculate the response of 1,3-benzenedithiol and cross-conjugated molecule-based junctions subject to an applied magnetic field, finding nearly universal behavior over a range of junction parameters with Ss, ZsT, and reaching peak values of 2π/3(k/e), 1.51, and 28% of Carnot efficiency, respectively. We also find that the quantum-enhanced spin-response is spectrally broad, and the field required to achieve peak efficiency scales with temperature. The influence of off-resonant thermal channels (e.g., phonon heat transport) on this effect is also investigated.

Quasi-periodic scattering of topological edge states induced by the vacancies in chloridized gallium bismuthide nanoribbons.

The chloridized gallium bismuthide was predicted to be a two-dimensional topological insulator with large topological band gap. It may be beneficial for achieving the quantum spin Hall effect and its related applications at high temperatures. To better understand the quantum transport in topological nanoribbons, we investigated the effect of vacancy on the quantum transport of topological edge states in the armchair chloridized gallium bismuthide nanoribbons by combining density functional theory and nonequilibrium Green's function. The results suggest the vacancies at center are more likely to cause the scattering of topological edge states. The average scattering is insensitive to the enlargement of vacancy along the transport direction. More interestingly, the obvious scattering of topological edge states can only be found at some special energies, and these special energies are distributed quasi-periodically. The quasi-periodic scattering may be used as a kind of fingerprint of vacancies. Our studies may be helpful for the application of topological nanoribbons.

Enhanced tunneling electroresistance through interfacial charge-modulated barrier inα-In2Se3-based ferroelectric tunnel junction.

The manipulation of tunneling resistance is critical for ferroelectric tunnel junction (FTJ) devices. In this work, we propose a mechanism to manipulate tunneling resistance through interfacial charge-modulated barrier in two-dimensional (2D)n-type semiconductor/ferroelectric FTJs. Driven by ferroelectric reversal, different effective tunneling barriers are realized by the depletion or accumulation of electrons near then-type semiconductor surface in such devices. Thus, the tunneling resistance in FTJs undergoes significant changes for different polarization orientations, resulting in a giant tunneling electroresistance (TER) effect. To illustrate this idea, we construct 2D FTJs based onn-InSe/α-In2Se3van der Waals (vdW) heterostructures. Based on the electronic transport calculations, it is found that TER ratio can reach 4.20 × 103% in the designed FTJs. The physical origin of the giant TER effect is verified through analysis of the effective potential energy of then-InSe/α-In2Se3vdW heterostructures and the real-space transmission eigenstates of the designed FTJs. This work contributes to the knowledge of carrier tunneling mechanisms at the interface of semiconductor/In2Se3vdW heterostructures, and providing a significant insight into the TER effect of this FTJ systems, also presenting an alternative approach for the design of FTJ-based devices.

Nernst effect in anisotropic four-terminal topological nodal-line semimetals.

The Nernst effect is the transverse mode of thermoelectric transport, in which a longitudinal thermal gradient induces a transverse current in the conductor while under a perpendicular magnetic field. Here the Nernst effect in a mesoscopic topological nodal-line semimetals (TNLSMs) system of four-terminal cross-bar with the spin-orbit coupling under a perpendicular magnetic field is studied. The Nernst coefficientNcin two non-equivalen connection modes (kz-ymode andkx-ymode) is calculated based on the tight-binding Hamiltonian combined with the nonequilibrium Green's function method. When the magnetic field is absent withφ = 0.0, the Nernst coefficientNc=0is exactly regardless of the temperature. When the magnetic field is not zero, the Nernst coefficient exhibits a series of densely oscillating peaks. The height of peak strongly depends on the magnetic field, and the Nernst coefficient is an even function of the Fermi energyEFsatisfying the symmetrical propertyNc(-EF)=Nc(EF). The Nernst coefficient is also closely related to the temperatureT. When the temperature is very low (orT→0), the Nernst coefficient depends linearly on temperature. In the presence of a strong magnetic field, the Nernst coefficient shows peaks when the Fermi energy crosses the Landau levels. Under the weak magnetic field, the influence of spin-orbit coupling in TNLSMs materials on Nernst effect is very obvious. In the presence of the mass term, thePT-symmetry of the system is destroyed, the nodal ring of TNLSMs is broken and an energy gap will be opened. The Nernst coefficientNchas a large value in the energy gap, which is very promising for the application of the transverse thermoelectric transport.

Thermoelectric transport in two-terminal topological nodal-line semimetals nanowires.

Recently discovered topological nodal-line semimetals (TNLSMs) have received considerable research interest due to their rich physical properties and potential applications. TNLSMs have the particular band structure to lead to many novel properties. Here we theoretically study the thermoelectric transport of a two-terminal pristine TNLSM nanowires and TNLSMsp-n-pjunctions. The Seebeck coefficientsScand the thermoelectrical figure of meritZTare calculated based on the Landauer-Büttiker formula combined with the nonequilibrium Green's function method. In pristine TNLSM nanowires, we discuss the effect of the magnetic fieldsφ, the disorderD, the on-site energyµz, and the mass termmon the thermoelectric coefficient and find that the transport gap can lead to a largeScandZT. When transmission coefficient jumps from one integer plateau to another,ScandZTshow a series of peaks. The peaks ofScandZTare determined by the jump of the transmission coefficient plateau and are not associated with the plateau itself. For TNLSMsp-n-pjunctions,ScandZTstrongly depend on the parameterξof potential well. We can get a largeZTby adjusting the parameterξand magnetic fieldφ. In TNLSMsp-n-pjunctions,ZThas the large value and is easily regulated. This setup has promising application prospects as a thermoelectric device.

Nonequilibrium Green's function method for phonon heat transport in quantum system.

Phonon heat transport property in quantum devices is of great interesting since it presents significant quantum behaviors. In the past few decades, great efforts have been devoted to establish the theoretical method for phonon heat transport simulation in nanostructures. However, modeling phonon heat transport from wavelike coherent regime to particlelike incoherent regime remains a challenging task. The widely adopted theoretical approach, such as molecular dynamics, semiclassical Boltzmann transport equation, captures quantum mechanical effects within different degrees of approximation. Among them, Non-equilibrium Green's function (NEGF) method has attracted wide attention, as its ability to perform full quantum simulation including many-body interactions. In this review, we summarized recent theoretical advances of phonon NEGF method and the applications on the numerical simulation for phonon heat transport in nanostructures. At last, the challenges of numerical simulation are discussed.

To inquire the effects of different geometric electrodes on current characteristics of C24 fullerene molecular junction.

Two different types of geometric electrode configurations were utilized to form C24 fullerene based molecular junction. The C24 molecule was intercalated in-between gold electrodes with two different shapes viz. knife edge and flat edge and the resultant molecular junctions (MJs) were simulated using nonequilibrium Green's function combined with semiempirical extended Huckel theory (EHT). Different transport parameters, namely projected device density of states, current-voltage curve, differential conductance curve, molecular orbitals, and transmission spectra were investigated at discrete bias voltages to gain insight about the various transport phenomena occurring in these molecular junctions. The results show that when the C24 fullerene is placed in between the flat-edged electrodes, current and conductance are higher in magnitude in comparison to the knife-edged configuration. These deductions give us a new perspective to design advanced molecular devices for electronics applications in the future.