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We theoretically study the quantum scattering and transport of electrons with Mexican-hat dispersion through both step and rectangular potential barriers by using the transfer matrix method. Owing to the torus-like iso-energy lines of the Mexican-hat dispersion, we observe the presence of double reflections and double transmissions in both two different barrier scenarios, i.e., the normal reflection (NR), retro-reflection (RR), normal transmission (NT), and specular transmission (ST).For the step potential with electrons incident from the large wavevector, the transmission is primarily governed by NT with nearly negligible ST, while the reflection is dominant by RR (NR) within (outside) the critical angle. Additionally, for electrons incident from the small wavevector, the NT can be reduced to zero by adjusting the barrier, resulting in a significant enhancement of ST and RR. For the rectangular barrier, the transmission and reflection spectra resemble those of the step barrier, but there are two kinds of resonant tunneling which can lead to perfect NT or ST. There exists a negative differential conductance (NDC) effect in the conductance spectrum. The conductance and the peak-to-valley ratio of the NDC effect can be effectively controlled by adjusting the height and width of the barrier as well as the incident energy. Our results provide a deeper understanding of the electron states governed by the Mexican-hat dispersion., Comment: 8 pages, 5 figures

Two-dimensional polyaniline with C$_3$N stoichiometry, is a newly fabricated layered material that has been expected to possess fascinating electronic, thermal, mechanical and chemical properties. The nature of its counterpart nano-ribbons/structures offering even more tunability in property because of the unique quantum confinement and edge effect, however, has not been revealed sufficiently. Here, using the first-principles calculation based on density functional theory and nonequilibrium Green's function technique, we first perform a study on the electron band structure of armchair C$_3$N nanoribbons (AC$_3$NNRs) without and with H-passivation. The calculated results show that the pristine AC$_3$NNRs are metal, while the H-passivated ones are either direct or indirect band gap semiconductors depending on the detailed edge atomic configurations. Then we propose a lateral planar homogenous junction with an interface between the pristine and H-passivated AC$_3$NNRs, in which forms a Schottky-like barrier. Interestingly, our further transport calculation demonstrates that this AC$_3$NNRs-based heterojunction exhibits a good rectification behavior. In specification, the average rectification ratio (RR) can reach up to $10^3$ in the bias regime from 0.2 to 0.4 V. Particularly, extending the length of semiconductor part in the heterojunction leads to the decrease of the current through the junction, but the RR can be enlarged obviously. The average RR increases to the order of $10^4$ in the bias from 0.25 to 0.40 V, with the boosted maximum up to $10^5$ at 0.35 V. The findings of this work may be serviceable for the design of functional nanodevices based on AC$_3$NNRs in the future., Comment: 9 pages, 8 figures

Multiferroic van der Waals (vdW) heterostrucutres offers an exciting route towards novel nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructure composed of ferromagnetic FeCl$_2$ monolayer and ferroelectric Sc$_2$CO$_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that FeCl$_2$/Sc$_2$CO$_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of Sc$_2$CO$_2$. Intriguingly, the half-metallic phase exhibits a Type-III broken gap band alignment, which can be beneficial for tunnelling field-effect transistor application. We perform a quantum transport simulation, based on a \emph{proof-of-concept} two-terminal nanodevice, to demonstrate all-electric-controlled valving effects uniquely enabled by the nonvolatile ferroelectric switching of the heterostructure. These findings unravels the potential of FeCl$_2$/Sc$_2$CO$_2$ vdW heterostructures as a building block for designing a next generation of ultimately compact information processing, data storage and spintronics devices., Comment: 9 pages, 5 figures, accepted for publication in Physical Review B (2022)

We theoretically study the power-law decay behavior of the local density of states (LDOS) oscillations near a line defect in system with semi-Dirac points by using a low-energy k.p Hamiltonian. We find that the LDOS oscillations are strongly anisotropic and sensitively depend on the orientation of the line defect. We analytically obtain the decay indexes of the LDOS oscillations near a line defect running along different directions by using the stationary phase approximation. Specifically, when the line defect is perpendicular to the linear dispersion direction, the decay index is -5/4 whereas it becomes -1/4 if the system is gapped, both of which are different from the decay index -3/2 in isotropic Dirac systems. In contrast, when the line defect is perpendicular to the parabolic dispersion direction, the decay index is always -1/2 regardless of whether the system is gapped or not, which is the same as that in a conventional semimetal. In general, when the defect runs along an arbitrary direction, the decay index sensitively depends on the incident energy for a certain orientation of the line defect. It varies from -5/4 to -1/2 due to the absence of strict stationary phase point. Our results indicate that the decay index -5/4 provides a fingerprint to identify semi-Dirac points in 2D electron systems., Comment: Published version: 9 Pages, 6 Figures

For the pristine phosphorene nanoribbons (PNRs) with edge states, there exist two categories of edge bands near the Fermi energy (EF), i.e., the shuttle-shaped twofold-degenerate and the near-flat simple degenerate edge bands. However, the usual experimental measurement may not distinguish the difference between the two categories of edge bands. Here we study the varying rule for the edge bands of PNRs under an external electrostatic field. By using the KWANT code based on the tight-binding approach, we find that the twofold-degenerate edge bands can be divided into two separated shuttles until the degeneracy is completely removed and a gap near EFis opened under a sufficiently strong in-plane electric field. Importantly, each shuttle from the ribbon upper or lower edge outmost atoms is identified according to the local density of states. However, under a small off-plane field the shuttle-shaped bands are easily induced into two near-flat bands contributed from the edge atoms of the top and bottom sublayers, respectively. The evidence provides the edge and sublayer degrees of freedom (DOF) for the PNRs with shuttle-shaped edge bands, of which is obviously different from another category PNRs intrinsically with near-flat edge bands. This is because that the former category of ribbons solely have four zigzag-like atomic configurations at the edges in each unit cell, which also results in that the property is robust against the point defect in the ribbon center area. As an application, furthermore, based on this issue we propose a homogenous junction of a shuttle-edge-band PNR attached by two electric gates. Interestingly, the transport property of the junction with field manipulation well reflects the characteristics of the two DOFs. These findings may provide a further understanding on PNRs and initiate new developments in PNR-based electronics.

Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostructures composed of MoSi$_2$N$_4$ contacted by graphene and NbS$_2$ monolayers using first-principle density functional theory calculations. We show that the MoSi$_2$N$_4$/NbS$_2$ contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For MoSi$_2$N$_4$/graphene contact, the SBH can be modulated via interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights on the physics of 2D electrical contact to MoSi$_2$N$_4$, and shall offer a critical first step towards the design of high-performance electrical contacts to MoSi$_2$N$_4$-based 2D nanodevices., Comment: 5 pages, 5 figures (accepted version, to appear in Applied Physics Letters)

Ultrathin films of topological Dirac semimetal, Na$_3$Bi, has recently been revealed as an unusual electronic materials with field-tunable topological phases. Here we investigate the electronic and transport properties of ultrathin Na$_3$Bi as an electrical contact to two-dimensional (2D) metal, i.e. graphene, and 2D semiconductor, i.e. MoS$_2$ and WS$_2$ monolayers. Using combined first-principle density functional theory and nonequilibrium Green's function simulation, we show that the electrical coupling between Na$_3$Bi bilayer thin film and graphene results in a notable interlayer charge transfer, thus inducing sizable $n$-type doping in the Na$_3$Bi/graphene heterostructures. In the case of MoS$_2$ and WS$_2$ monolayers, the lateral Schottky transport barrier is significantly lower than many commonly studied bulk metals, thus unraveling Na$_3$Bi bilayer as a high-efficiency electrical contact material for 2D semiconductors. These findings opens up an avenue of utilizing topological semimetal thin film as electrical contact to 2D materials, and further expands the family of 2D heterostructure devices into the realm of topological materials., Comment: 8 pages, 6 figures. Authors' accepted version

The central question in the field of 2D materials is how a material behaves when it is patterned at nanometer scale with different edge geometries. Due to the anisotropy inherent in the puckered structure, black phosphorene nanostructures may have more varieties of edge geometries. Here, we present a comprehensive 2D planar crystallographic characterization of phosphorene uniformly by a chiral vector (angle), from which a new type of edge atomic configuration, the slope edge geometry is discovered. The phosphorene nanoribbons (PNRs) with slope edges, like previously noticed zigzag and skewed-armchair PNRs, also own interesting twofold-degenerate edge states. These three marginal directions, together with the skewed-zigzag and armchair directions without edge states, divide a phosphorene into four regions among which the electronic property is different from each other. Further, for a PNR cutting along any possible direction, by taking into account the z-direction in structure, whether or not it owns edge states depends on the periodic zigzag-like morphology along the edges. Moreover, we propose a PNR-based z-shaped homogenous junction with scale $\sim$100 nm, where the central scattering region between two PNR electrodes is a phosphorene quantum dot (PQD) of PNR segment with various edge morphologies. Interestingly, the calculated conductance of the junction relies sensitively on the central PQD edge states, which exhibits a terrace with resonant oscillations near Fermi energy, otherwise the electron transport is blocked due to the absence of edge states. Remarkably, the number of oscillating peaks exactly matches to the number of sawtooth along an edge of PQD, since the discrete energy levels of the zigzag-like edge provide the transporting channels for electrons. The results here may be extended to the group-VA 2D materials for observing the electronic property of nanostructures.

We analytically study the electronic structures and optical properties of zigzag-edged black phosphorene nanoribbons (ZPNRs) utilizing the tight-binding (TB) Hamiltonian and Kubo formula. By solving the discrete Schordinger equation directly, we obtain the energy spectra and wavefunctions for a $N$-ZPNR with $N$ number of transverse zigzag atomic chains, and classify the eigenstates according to the lattice symmetry. We then obtain the optical transition selection rule of ZPNRs based on the symmetry analysis and the analytical expressions of the optical transition matrix elements. Under an incident light linearly-polarized along the ribbon, importantly, we find that the optical transition selection rule for the $N$-ZPNR with even- or odd-$N$ is qualitatively different. In specification, for even-$N$ ZPNRs the inter- (intra-) band selection rule is $\Delta n=$odd (even), since the parity of the wavefunction corresponding to the $n$th subband in the conduction (valence) band is $(-1)^{n}[(-1)^{(n+1)}]$ due to the presence of the $C_{2x}$ symmetry. In contrast, all optical transitions are possible among all subbands due to the absence of the $C_{2x}$ symmetry. Our findings provide a further understanding on the electronic states and optical properties of ZPNRs, which are useful in the explanation of the optical experiment data on ZPNR samples., Comment: 13 pages, 9 figures

We propose and investigate a spin transistor device consisting of two ferromagnetic leads connected by a two-dimensional topological insulator as the channel material. It exploits the unique features of the topological spin-helical edge states, such that the injected carriers with a non-collinear spin-direction would travel through both edges and show interference effect. The conductance of the device can be controlled in a simple and allelectric manner by a side-gate voltage, which effectively rotates the spin-polarization of the carrier. At low voltages, the rotation angle is linear in the gate voltage, and the device can function as a good spin-polarization rotator by replacing the drain electrode with a paramagnetic material., Comment: 5 pages, 5 figures

The recent discovery of Dirac semimetals represents a new achievement in our fundamental understanding of topological states of matter. Due to their topological surface states, high mobility, and exotic properties associated with bulk Dirac points, these new materials have attracted significant attention and are believed to hold great promise for fabricating novel topological devices. For nanoscale device applications, effects from finite size usually play an important role. In this report, we theoretically investigate the electronic properties of Dirac semimetal nanostructures. Quantum confinement generally opens a bulk band gap at the Dirac points. We find that confinement along different directions shows strong anisotropiceffects. In particular, the gap due to confinement along vertical c-axis shows a periodic modulation, which is absent for confinement along horizontal directions. We demonstrate that the topological surface states could be controlled by lateral electrostatic gating. It is possible to generate Rashba-like spin splitting for the surface states and to shift them relative to the confinement-induced bulk gap. These results will not only facilitate our fundamental understanding of Dirac semimetal nanostructures, but also provide useful guidance for designing all-electrical topological spintronics devices., Comment: 23 pages, 5 figures

We perform a numerical study on the spin-resolved transport in a quantum wire (QW) under the modulation of both Rashba spin-orbit coupling (SOC) and a perpendicular magnetic field (MF) by adopting the developed Usuki transfer-matrix method in combination with the Landauer-Buttiker formalism. Wide spin filtering energy windows can be achieved in this system for a spin-unpolarized injection. In addition, both the width of these energy windows and the magnitude of the spin conductance within these energy widows can be tuned by varying the Rashba SOC strength, which can be apprehended by analyzing the energy dispersions and the spin-polarized density distributions inside the QW, respectively. Further study also demonstrates that these Rashba-SOC-controlled spin filtering energy windows show a strong robustness against disorders. These findings may not only benefit to further understand the spin-dependent transport properties of the QW in the presence of external fields but also provide a theoretical instruction to design a spin filter device., Comment: 21 pages, 5 figures

Spin-dependent electron transport in a periodically stubbed quantum wire in the presence of Rashba spin-orbit interaction (SOI) is studied via the nonequilibrium Green's function method combined with the Landauer-Buttiker formalism. The coexistence of spin filtering, charge and spin switching are found in the considered system. The mechanism of these transport properties is revealed by analyzing the total charge density and spin-polarized density distributions in the stubbed quantum wire. Furthermore, periodic spin-density islands with high polarization are also found inside the stubs, owing to the interaction between the charge density islands and the Rashba SOI-induced effective magnetic field. The proposed nanostructure may be utilized to devise an all-electrical multifunctional spintronic device., Comment: 4 pages, 4 figures

The counterpropagating edge states of a two-dimensional topological insulator (TI) carry electrons of opposite spins. We investigate the transport properties of edge states in a two-dimensional TI which is contacted to ferromagnetic leads. The application of a side-gate voltage induces a constriction or quantum point contact (QPC) which couples the two edge channels. The transport properties of the system is calculated via the Keldysh nonequilibrium Green's function method. We found that inter-edge spin-flip coupling can significantly enhance (suppress) the charge current when the magnetization of the leads are anti-parallel (parallel) to one another. On the other hand, spin-conserving inter-edge coupling generally reduces the current by backscattering regardless of the magnetization configuration. The charge current and the conductance as a function of the bias voltage, also exhibit similar trends with respect to spin-flip coupling strength, for both parallel and anti-parallel configurations. Hence, gate voltage modulation of edge states via a QPC can provide a means of modulating the spin or charge current flow in TI-based spintronics devices., Comment: 6 pages, 3 figures, submitted to J. Appl. Phys

We investigate theoretically the spin-dependent electron transport in a Rashba quantum wire with rough edges. The charge and spin conductances are calculated as function of the electron energy or the wire length by adopting the spinresolved lattice Green function method. For a single disordered Rashba wire, it is found that the charge conductance quantization is destroyed by the edge disorder. However, a nonzero spin conductance can be generated and its amplitude can be manipulated by the wire length, which is attributed to the broken structure symmetries and the spin-dependent quantum interference induced by the rough boundaries. For a large ensemble of disordered Rashba wires, the average charge conductance decreases monotonically, however, the average spin conductance increases to a maximum value and then decreases, with increasing wire length. Further study shows that the influence of the rough edges on the charge and spin conductances can be eliminated by applying a perpendicular magnetic field to the wire. In addition, a very large magnitude of the spin conductance can be achieved when the electron energy lies between the two thresholds of each pair of subbands. These findings may not only benefit to further apprehend the transport properties of the Rashba low-dimensional systems but also provide some theoretical instructions to the application of spintronics devices., Comment: 14 pages, 6 figures

We theoretically investigate the spin-dependent transport for the system of an armchair-edge graphene nanoribbon (AGNR) between two ferromagnetic (FM) leads with arbitrary polarization directions at low temperatures, where a magnetic insulator is deposited on the AGNR to induce an exchange splitting between spin-up and -down carriers. By using the standard nonequilibrium Green's function (NGF) technique, it is demonstrated that, the spin-resolved transport property for the system depends sensitively on both the width of AGNR and the polarization strength of FM leads. The tunneling magnetoresistance (TMR) around zero bias voltage possesses a pronounced plateau structure for system with semiconducting 7-AGNR or metallic 8-AGNR in the absence of exchange splitting, but this plateau structure for 8-AGNR system is remarkably broader than that for 7-AGNR one. Interestingly, the increase of exchange splitting $\Delta$ suppresses the amplitude of the structure for 7-AGNR system. However, the TMR is enhanced much for 8-AGNR system under the bias amplitude comparable to splitting strength. Further, the current-induced spin transfer torque (STT) for 7-AGNR system is systematically larger than that for 8-AGNR one. The findings here suggest the design of GNR-based spintronic devices by using a metallic AGNR, but it is more favorable to fabricate a current-controlled magnetic memory element by using a semiconducting AGNR., Comment: 8 pages, 8 figures

We study the transport property for armchair-edge graphene nanoribbons (AGNRs) with an adatom coupling to a semi-infinite quantum wire. Using the nonequilibrium Green's function approach with tight-binding approximation, we demonstrate that the tunneling current through the system is sensitively dependent on both the AGNR width and adatom location. Interestingly, when the adatom locates onto a carbon atom in the 3$j$th chain from the edge of a metallic AGNR, the system shows a transmission gap accompanied by a threshold voltage in $I-V$ curve like a semiconducting AGNR. This effect may be useful in scanning tunneling microscopy experimental characterization on graphene samples., Comment: 3 pages, 4 figures

We study theoretically the electronic structure, transport and optical properties for a zigzag single-wall carbon nanotube connected to two normal conductor leads under the irradiation of an external electromagnetic field at low temperatures, with particular emphasis on the features of high-frequency response. Using the standard nonequilibrium Green's function techniques, we examine the time-averaged density of states, the conductivity, the dielectric function and the electron energy loss spectra for the system with photon polarization parallel with the tunneling current direction, respectively. Through some numerical examples, it is shown that the density of states is strongly dependent on the incident electron energy, the strength and frequency of the applied field. For higher electron energies in comparison with lead-nanotube coupling energy, the system conductance decreases with increasing the field strength and increases with increasing the field frequency respectively, and shows some oscillation structures. Moreover, the optical functions for the system have also a rich structure with the variation of field frequency. It may demonstrate that this transport dependence on the external field parameters can be used to give the energy spectra information of carbon nanotubes and to detect the high-frequency microwave irradiation., Comment: 6 Revtex pages, 4 figures. to be appeared in JAP

We study theoretically the quantum transport properties of the Dirac fermions with spin-orbit interactions (SOIs) in graphene by using the method of Schwinger proper time together with decomposition over Landau level poles and Kubo formula. The analytical expressions for both longitudinal and Hall conductivities are derived explicitly. It is found that, from some numerical examples, when the Rashba SOI is taken into account the Shubnikov-de Haas (SdH) oscillation peaks of the longitudinal conductivity versus the chemical potential are split, while the SdH oscillation of the longitudinal conductivity versus a external magnetic field exhibits a beating pattern. Furthermore, the Rashba SOI tends to suppress the quantum Hall effect in graphene., Comment: Revtex 12 pages with 5 figures

We investigate theoretically the electronic structure and transport for a two-level quantum wire with Rashba spin-orbit coupling (SOC) under the irradiation of an external laser field at low temperatures. The photon-induced transitions between SOC-splitted subbands with the same lateral confinement quantum numbers and between subbands with different confinement quantum number are expected. Using the method of equation of motion (EOM) for Keldysh nonequilibrium Green's functions (NGF), we examine the time-averaged density of states (DOS) and the spin polarized conductance for the system with photon polarization perpendicular to the wire direction. Through the analytical analysis and some numerical examples, the interplay effects of the external laser field and the Rashba SOC on both the DOS and the conductance of the system are demonstrated and discussed. It is found that the external laser field can adjust the spin polarization rate and the transport of the quantum wire system with some proper Rashba SOC strengths., Comment: 7 pages, 6 figures

We study the transport properties for a Luttinger-liquid (LL) quantum wire in the presence of both Rashba spin-orbit coupling (SOC) and a weak external in-plane magnetic field. The bosonized Hamiltonian of the system with an externally applied longitudinal electric field is established. And then the equations of motion for the bosonic phase fields are solved in the Fourier space, with which the both charge and spin conductivities for the system are calculated analytically based on the linear response theory. Generally, the ac conductivity is an oscillation function of the strengths of electron-electron interaction, Rashba SOC and magnetic field, as well as the driving frequency and the measurement position in the wire. Through analysis with some examples it is demonstrated that the modification on the conductivity due to electron-electron interactions is more remarkable than that due to SOC, while the effects of SOC and Zeeman splitting on the conductivity are very similar. The spin-polarized conductivities for the system in the absence of Zeeman effect or SOC are also discussed, respectively. The ratio of the spin-polarized conductivities $\sigma_\uparrow/\sigma_\downarrow$ is dependent of the electron-electron interactions for the system without SOC, while it is independent of the electron-electron interactions for the system without Zeeman splitting., Comment: 10 pages, 8 figures

We theoretically investigate the electron transport properties for a semiconductor quantum wire containing a single finite-size attractive impurity under an external terahertz electromagnetic field illumination in the ballistic limit. Within the effective mass free-electron approximation, the scattering matrix for the system has been formulated by means of a time-dependent mode matching method. Some interesting properties of the electron transmission for the system have been shown through a few groups of numerical examples. It is found that in the case of the comparative stronger field amplitude and the frequency resonant with the two lowest lateral energy levels in the impurity region, the field-induced intersubband transition dominates the process as if without the impurity. And there is a step-arising on the transmission as a function of the incident electron energy. However, in the case of lower field amplitude and the non-resonant frequencies both multiple symmetry Breit-type resonance peaks and asymmetry Fano-type dip lines appear in the electron transmission dependence on the incident energy due to the presence of the impurity and the external field. Therefore, within certain energy range the transmission as a function of the field frequency and/or field amplitude shows a rich structure. Moreover, the transmission dependence on the strength and size of the impurity is also discussed. It is suggested that these results mostly arise from the interplay effects between the impurity in a quantum wire and the applied field.