Quantum Transport Calculations Research Articles

Symmetry-controlled SrRuO3/SrTiO3/SrRuO3 magnetic tunnel junctions: spin polarization and its relevance to tunneling magnetoresistance

Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO3/SrTiO3/SrRuO3(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO3(001) electrodes and evanescent states in the SrTiO3(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO3/SrTiO3/SrRuO3(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs.

Giant resistance switch in twisted transition metal dichalcogenide tunnel junctions

Abstract Resistance switching in multilayer structures are typically based on materials possessing ferroic orders. Here we predict an extremely large resistance switching based on the relative spin-orbit splitting in twisted transition metal dichalcogenide (TMD) monolayers tunnel junctions. Because of the valence band spin splitting which depends on the valley index in the Brillouin zone, the perpendicular electronic transport through the junction depends on the relative reciprocal space overlap of the spin-dependent Fermi surfaces of both layers, which can be tuned by twisting one layer. Our quantum transport calculations reveal a switching resistance larger than 106% when the relative alignment of TMDs goes from 0º to 60º and when the angle is kept fixed at 60º and the Fermi level is varied. By creating vacancies, we evaluate how inter-valley scattering affects the efficiency and find that the resistance switching remains large (104%) for typical values of vacancy concentration. Not only should this resistance switching be observed at room temperature due to the large spin splitting, but our results also show how twist angle engineering and control of van der Waals heterostructures could be used for next-generation memory and electronic applications.

Giant tunneling resistance and robust switching behavior in ferroelectric tunnel junctions of WS2/Ga2O3 heterostructures: The influence of metal–semiconductor contacts

The ferroelectric tunneling junctions (FTJs) are widely recognized as one of the non-volatile memories with significant potential. Ferroelectricity usually fades away as materials are thinned down below a critical value, and this problem is particularly acute in the case of shrinking device sizes, thus attracting attention to two-dimensional ferroelectric materials (2DFEMs). In this work, we designed 2D ferroelectric Ga2O3-based FTJs with out-of-plane polarization, and the influence of metal–semiconductor contact in the electrode region on the system is considered. Here, using density functional theory combined with the non-equilibrium Green's function approach to quantum transport calculations, we demonstrate robust ferroelectric polarization-controlled switching behavior between metallic and semiconducting states in Ga2O3/WS2 ferroelectric heterostructures. The potential barrier of the metal–semiconductor contact in the electrode region is lower than that of the intrinsic material, thereby resulting in an increased probability of electron tunneling. Our results reveal the crucial role of 2DFEMs in the construction of FTJs and highlight the significant impact of electrode contact types on performance. This provides a promising approach for developing high-density ferroelectric memories based on 2D ferroelectric semiconductor heterostructures.

Exploring Aromaticity Effects on Electronic Transport in Cyclo[n]carbon Single-Molecule Junctions.

Cyclo[n]carbon (Cn) is one member of the all-carbon allotrope family with potential applications in next-generation electronic devices. By employing first-principles quantum transport calculations, we have investigated the electronic transport properties of single-molecule junctions of Cn, with n = 14, 16, 18, and 20, connected to two bulk gold electrodes, uncovering notable distinctions arising from the varying aromaticities. For the doubly aromatic C14 and C18 molecules, slightly deformed complexes at the singlet state arise after bonding with one Au atom at each side; in contrast, the reduced energy gaps between the highest occupied and the lowest unoccupied molecular orbitals due to the orbital reordering observed in the doubly anti-aromatic C16 and C20 molecules lead to heavily deformed asymmetric complexes at the triplet state. Consequently, spin-unpolarized transmission functions are obtained for the Au-C14/18-Au junctions, while spin-polarized transmission appears in the Au-C16/20-Au junctions. Furthermore, the asymmetric in-plane π-type hybrid molecular orbitals of the Au-C16/20-Au junctions contribute to two broad but low transmission peaks far away from the Fermi level (Ef), while the out-of-plane π-type hybrid molecular orbitals dominate two sharp transmission peaks that are adjacent to Ef, thus resulting in much lower transmission coefficients at Ef compared to those of the Au-C14/18-Au junctions. Our findings are helpful for the design and application of future cyclo[n]carbon-based molecular electronic devices.

High-performance transistors based on monolayer all-inorganic two-dimensional halide perovskite Cs2PbI2Cl2 and its optoelectronic application

The two-dimensional (2D) Ruddlesden−Popper phase all-inorganic halide perovskite Cs2PbI2Cl2 has attracted tremendous attention because of its high carrier mobility, strong light-absorbing ability, and excellent environmental stability, possessing enormous potential for use as a high-performance optoelectronic device. Here, we present two transistor devices based on monolayer (ML) Cs2PbI2Cl2 and reveal their performance and current transport mechanisms through ab initio quantum transport calculations. The n-type 10 nm metal-oxide-semiconductor field-effect transistor shows a high on-state current Ion of 3160 μA/μm, and excellent performance regarding subthreshold swing, intrinsic delay time (τ), and power consumption, which completely match the International Roadmap for Device and Systems (2021 version) targets for high-performance devices and low-power devices in 2028. Moreover, phototransistors based on monolayer Cs2PbI2Cl2 also display a strong response to UV light, with a light-to-dark current ratio reaching a maximum of 104. Meanwhile, a high specific detectivity of 2.45 × 1011 Jones is obtained. The results of our study suggest that ML Cs2PbI2Cl2 is a promising candidate for high-performance transistors.

Order-N calculations for thermoelectric power factor based on linear response theory

We present an order-N quantum transport calculation methodology to evaluate thermoelectric transport coefficients, such as electric conductivity and Seebeck coefficient. Different from a conventional method using the electric conductivity spectrum, it obtains the coefficients directly from the correlation function between heat and electric current based on linear response theory. As an example, we apply the methodology to a two-dimensional square-lattice model with static disorder and confirm that the calculated results are consistent with those obtained by the conventional method. The proposed methodology provides an effective approach to evaluate the thermoelectric performance of micron-scale materials based on quantum mechanics from an atomistic viewpoint.

Two-dimensional heterostars from nanographene and group-III nitrides for efficient spin filters: Spintronic and quantum transport properties

Two-dimensional (2D) heterostructures open the door toward novel applications, such as multiterminal transistors, by integrating the properties of the participant materials. Here we construct 2D heterostars from zigzag-triangular graphene (ZTG) and group-III nitrides. They are investigated using density functional theory and non-equilibrium Green’s function approaches. These heterostars are ferromagnetically spin-ordered devices that originate and distribute on ZTG. Further control over the net spin can be achieved by forming bilayer heterostars where spin can be increased, decreased, or removed. Single-layer heterostars are wide energy gap semiconductors of ∼3.5 eV that decrease to ∼1 eV in bilayers or are kept unchanged in other bilayers. Quantum transport calculations show that different spintronic devices can be constructed by connecting the source/drain voltage to different terminals of the nanostars. These devices are characterized by a low tunneling current through the potential barrier of group-III nitrides that significantly increased by applying gate voltage. A spin filter based on graphene-AlN nanostar with efficient separation of spin-up current is demonstrated. Also, a significant improvement in both the spin-up and spin-down currents can be achieved in the same nanostar but using other terminals. 2D heterostars are then potential candidates for building multi-terminal spintronic devices.

Edge-dependent electronic transport and diode effect in C3N nanoribbons

Quantum transport calculations are conducted for C3N nanoribbons with different edge terminations within the nonequilibrium Green’s function formalism in combination with density functional theory. The electronic transport in the system strongly depends on the properties of the edge states. For example, the current in metallic carbon-terminated nanoribbons is several orders of magnitude higher than the current in nitrogen-terminated nanoribbons with a semiconducting nature. In addition, C-terminated nanoribbons show very pronounced negative differential resistance. A diode structure with a rectification ratio of 10 is proposed, consisting of a heterojunction of C- and N-terminated nanoribbons. These findings can be of practical importance in creating functional device structures from this 2D material.

Enantioselectivity from inelastic electron tunnelling through a chiral sensor

The vibration theory of olfaction, which explains it as the sensing of odorant molecules by their vibrational energies through inelastic electron tunnelling spectroscopy (IETS) has inspired olfactory sensor ideas. However, this theory has been presumed inadequate to explain the difference in smell between enantiomers (chiral molecules, which are mirror images of each other), since these have identical vibrational spectra. Going beyond phenomenological assumptions of enantioselective tunnelling, we show on the basis of ab initio modelling of real chiral molecules, that this drawback is indeed obviated for IETS-based olfactory sensors if they are chiral. Our treatment unifies IETS with chirality induced spin selectivity, which explains that charge polarization in chiral molecules by accompanied by spin polarization. First, we apply ab initio symmetry adapted perturbation theory to explain and illustrate enantioselective coupling of chiral odorant molecules and chiral olfactory sensors. This naturally leads to enantioselective coupling of the vibrational mode of an odorant to electron transport (electron-vibron coupling) in an IETS-based sensor when both odorant and sensor are chiral. Finally, we show, from phenomenological quantum transport calculations, that that in turn results in enantioselective IET spectra. Thus, we have demonstrated the feasibility of enantioselective sensing within a vibration framework. Our work also limns the possibility of quantum biomimetic electronic nose sensors that are enantioselective, a feature which could open up new sensing applications.

A first-principles study of bilayer 1T'-WTe2/CrI3: a candidate topological spin filter

The ability to manipulate electronic spin channels in 2D materials is crucial for realizing next-generation spintronics. Spin filters are spintronic components that polarize spins using external electromagnetic fields or intrinsic material properties like magnetism. Recently, topological protection from backscattering has emerged as an enticing feature that can be leveraged to enhance the robustness of 2D spin filters. In this work, we propose and then characterize one of the first 2D topological spin filters: bilayer CrI3/1T’-WTe2. To do so, we use a combination of density functional theory, maximally localized Wannier functions, and quantum transport calculations to demonstrate that a terraced bilayer satisfies the principal criteria for being a topological spin filter: namely, that it is gapless, exhibits spin-polarized charge transfer from WTe2 to CrI3 that renders the bilayer metallic, and has a topological boundary which retains the edge conductance of monolayer 1T’-WTe2. In particular, we observe that small negative ferromagnetic moments are induced on the W atoms in the bilayer, and the atomic magnetic moments on the Cr are approximately 3.2 μB/Cr compared to 2.9 μB/Cr in freestanding monolayer CrI3. Subtracting the charge and spin densities of the constituent monolayers from those of the bilayer further reveals spin-orbit coupling-enhanced spin-polarized charge transfer from WTe2 to CrI3. We demonstrate that the bilayer is topologically trivial by showing that its Chern number is zero. Lastly, we show that interfacial scattering at the boundary between the terraced materials does not remove WTe2’s edge conductance. Altogether, this evidence indicates that BL 1T’-WTe2/CrI3 is gapless, magnetic, and topologically trivial, meaning that a terraced WTe2/CrI3 bilayer heterostructure in which only a portion of a WTe2 monolayer is topped with CrI3 is a promising candidate for a 2D topological spin filter. Our results further suggest that 1D chiral edge states may be realized by stacking strongly ferromagnetic monolayers, like CrI3, atop 2D nonmagnetic Weyl semimetals like 1T’-WTe2.

Combining linear-scaling quantum transport and machine-learning molecular dynamics to study thermal and electronic transports in complex materials

We propose an efficient approach for simultaneous prediction of thermal and electronic transport properties in complex materials. Firstly, a highly efficient machine-learned neuroevolution potential (NEP) is trained using reference data from quantum-mechanical density-functional theory calculations. This trained potential is then applied in large-scale molecular dynamics simulations, enabling the generation of realistic structures and accurate characterization of thermal transport properties. In addition, molecular dynamics simulations of atoms and linear-scaling quantum transport calculations of electrons are coupled to account for the electron-phonon scattering and other disorders that affect the charge carriers governing the electronic transport properties. We demonstrate the usefulness of this unified approach by studying electronic transport in pristine graphene and thermoelectric transport properties of a graphene antidot lattice, with a general-purpose NEP developed for carbon systems based on an extensive dataset.

Ultrashort channel MoSe2 transistors with selenium atoms replaced at the interface: first-principles quantum-transport study

Realizing n- and p-type transition metal dichalcogenide (TMD)-based field-effect transistors for nanoscale complementary metal oxide semiconductor (CMOS) applications remains challenging owing to undesirable contact resistance. Quantumtransport calculations were performed by replacing single-sided Se atoms of TMD near the interface with As or Br atoms to further improve the contact resistance. Here, partial selenium replacement produced a novel interface with a segment of metamaterial MoSeX (Pt/MoSeX/MoSe2; X = As, Br). Such stable metamaterials exhibit semi-metallicity, and the contact resistance can be thus lowered. Our findings provide insights into the potential of MoSe2-based nano-CMOS logic devices.

Nanoribbons of large-gap quantum spin Hall insulator: electronic structures and transport properties

Two-dimensional Bi grown on semiconductor substrate, a large-gap quantum spin Hall insulator characterized by a (p x , p y )-orbital hexagonal lattice, has been theoretically proposed and experimentally confirmed. Here, by combining tight-binding modeling with first-principles calculations, we investigate the electronic structures and quantum transport properties of Bi nanoribbons (NRs), focusing on the topological edge states for nanoelectronics. We reveal that band gap emerges due to the quantum confinement, and the gaps size depends crucially on the width and edge shape: for zigzag NRs, the gap decreases monotonically with the increase of width; while for armchair NRs, it can be categorized into three subgroups with band-gap hierarchies of Eg(3p−1)>Eg(3p)>Eg(3p+1) , so that the overall relation is an oscillating dependence dumped by 1/width decay. Quantum transport calculations demonstrate that the conductance is quantized to 2 e2/h , and an applied gate voltage can efficiently regulate the conductance plateau, originating from the interplay between gate voltage and topological gaps. Furthermore, the quantized conductance remains robust against strong disorder, suggesting the unique advantage of topological states for electronic transport. This work not only provides fundamental insights into the electronic properties of topological insulator nanostructures, but also sheds light on the potential applications of exotic states for quantum devices compatible with semiconductor technology.

Thermopower in Underpotential Deposition-Based Molecular Junctions.

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule-electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.

Role of fluorination in the high-frequency response of azobenzene-based molecular junction: First-principles calculations

Photochromic molecules such as azobenzene present a great potential to tune the device functionality under external stimuli. Fluorination is known to further enhance the functionality of azobenzene due to significant contributions to the structural and electronic properties. Here, we conduct quantum capacitance and quantum transport calculations to study the effect of fluorination on the high-frequency response of azobenzene-based molecular junctions. We found that fluorination results in smaller capacitance in both trans and cis conformations. However, this processes significantly reduces the cutoff frequency of the molecular junctions due to the reduced conductivity. cis isomers of both pristine and fluorinated azobenzene molecular structures show smaller cutoff frequencies as compared to trans configurations due to the enhanced capacitance. These findings can be useful in developing azobenzene-based functional device structures.

Magnetic-ferroelectric synergic control of multilevel conducting states in van der Waals multiferroic tunnel junctions towards in-memory computing.

van der Waals (vdW) multiferroic tunnel junctions (MFTJs) based on two-dimensional materials have gained significant interest due to their potential applications in next-generation data storage and in-memory computing devices. In this study, we construct vdW MFTJs by employing monolayer Mn2Se3 as the spin-filter tunnel barrier, TiTe2 as the electrodes and In2S3 as the tunnel barrier to investigate the spin transport properties based on first-principles quantum transport calculations. It is highlighted that apparent tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects with a maximum TMR ratio of 6237% and TER ratio of 1771% can be realized by using bilayer In2S3 as the tunnel barrier under finite bias. Furthermore, the physical origin of the distinguished TMR and TER effects is unraveled from the k||-resolved transmission spectra and spin-dependent projected local density of states analysis. Interestingly, four distinguishable conductance states reveal the implementation of four-state nonvolatile data storage using one MFTJ unit. More importantly, in-memory logic computing and multilevel data storage can be achieved at the same time by magnetic switching and electrical control, respectively. These results shed light on vdW MFTJs in the applications of in-memory computing as well as multilevel data storage devices.

First-principles studies on the electronic and contact properties of monolayer Ga2STe-metal contacts.

Monolayer (ML) Janus III-VI compounds have attracted the use of multiple competitive platforms for future-generation functional electronics, including non-volatile memories, field effect transistors, and sensors. In this work, the electronic and interfacial properties of ML Ga2STe-metal (Au, Ag, Cu, and Al) contacts are systematically investigated using first-principles calculations combined with the non-equilibrium Green's function method. The ML Ga2STe-Au/Ag/Al contacts exhibit weak electronic orbital hybridization at the interface, while the ML Ga2STe-Cu contact exhibits strong electronic orbital hybridization. The Te surface is more conducive to electron injection than the S surface in ML Ga2STe-metal contact. Quantum transport calculations revealed that when the Te side of the ML Ga2STe is in contact with Au, Ag and Cu electrodes, p-type Schottky contacts are formed. When in contact with the Al electrode, an n-type Schottky contact is formed with an electron SBH of 0.079 eV. When the S side of ML Ga2STe is in contact with Au and Al electrodes, p-type Schottky contacts are formed, and when it is in contact with Ag and Cu electrodes, n-type Schottky contacts are formed. Our study will guide the selection of appropriate metal electrodes for constructing ML Ga2STe devices.

Lead Green's functions from quadratic eigenvalue problems without mode velocity calculations.

In quantum transport calculations, the proper handling of incoming and outgoing modes for retarded Green's functions is achieved via the lead self-energies. Computationally efficient and accurate methods to calculate the self-energies are thus very important. Here we present an alternative method for calculating lead self-energies which improves on a standard approach to solving quadratic eigenvalue problems that arise in quantum transport modeling. The method is based on a perturbative analysis of the generalized Schur decomposition to determine the relevant set of eigenvalues for transmitting modes. This allows us to circumvent finding the velocities of the modes (left- or right-moving) that are needed in order to calculate the lead Green's function from translationally invariant Green's functions. This saves computational time irrespective of the value of the imaginary part added to the energy. We compare our method with two existing methods-a popular iterative method and a standard eigenvalue method that explicitly calculates the velocities of the propagating modes. Our comparison shows that both eigenvalue methods are more robust than the iterative method. Furthermore, the comparison also shows that above a small threshold of propagating modes, the standard eigenvalue method requires extra computation time over our perturbation method. This excess of computation time grows linearly with the number of propagating modes.

Nonequilibrium Magneto-Conductance as a Manifestation of Spin Filtering in Chiral Nanojunctions.

It is generally accepted that spin-dependent electron transmission may appear in chiral systems, even without magnetic components, as long as significant spin-orbit coupling is present in some of its elements. However, how this chirality-induced spin selectivity (CISS) manifests in experiments, where the system is taken out of equilibrium, is still debated. Aided by group theoretical considerations and nonequilibrium DFT-based quantum transport calculations, here we show that when spatial symmetries that forbid a finite spin polarization in equilibrium are broken, a net spin accumulation appears at finite bias in an arbitrary two-terminal nanojunction. Furthermore, when a suitably magnetized detector is introduced into the system, the net spin accumulation, in turn, translates into a finite magneto-conductance. The symmetry prerequisites are mostly analogous to those for the spin polarization at any bias with the vectorial nature given by the direction of magnetization, hence establishing an interconnection between these quantities.

Topological field-effect transistor with quantized on/off conductance of helical/chiral dislocation states

Topology is a key ingredient driving the emergence of quantum devices. The topological field-effect transistor (TFET) has been proposed to outperform the conventional field-effect transistor by replacing the on state with topology-protected quantized conductance, while the off state has the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET, having both on and off quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured, with one acting as a channel and the other as a gate controlled by a local magnetic field. Reversible field switching is achieved with the on and off conductance of $2{e}^{2}/h$ and ${e}^{2}/h$, respectively, as shown by tight-binding quantum transport calculations. Furthermore, $\mathrm{BaBi}{\mathrm{O}}_{3}$ is shown as one candidate material that has the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices.