Narrow Armchair Research Articles

Teleportation of the werner state via graphene-nanoribbon-based quantum channels under the amplitude-damping environment

We study the teleportation of Werner state using electronic spin states at the ends of a narrow armchair graphene nanoribbon as the quantum channels in the amplitude-damping environment. The influences of amplitude damping, temprature T and Coulomb repulsion U on the dynamics of channel state, output state, and the corresponding fidelity between output and input states are discussed in detail. To faithfully characterize the quality of the teleported state, we also calculate the average fidelity of teleportation. The results show that when T or U increases to a certain value, the channel entanglement will suddenly disappear in evolution, thus leading to the decay behaviors of output entanglement and corresponding fidelity. However, if T < 40 K and U < 6eV, the average fidelity of teleportation under the amplitude-damping channel is over 80% at worst. Therefore, such narrow armchair graphene nanoribbon is identified as a very promising solid-state system for quantum teleportation. • Quantum teleportation of Werner state using electronic spin states at the ends of a narrow armchair graphene nanoribbon as the quantum channels in the amplitude-damping environment. • The entanglement dynamics of the amplitude-damping channel strongly depends on the temperature T and Coulomb repulsion U. • The average fidelity of teleportation under the amplitude-damping channel is over 80% at worst in the range of 0 K < T < 40 K and 2eV < U < 6eV considered here. • The fidelity for output state under the amplitude-damping channel is not a monotonic function of the decay probability p, which is different from the monotonically decreasing behavior of the output entanglement.

Quantum teleportation between the narrow armchair graphene nanoribbons with zigzag ends

We study quantum teleportation between the narrow armchair graphene nanoribbons (GNRs) with zigzag ends. Several influences including Coulomb repulsion among electrons, temperature and ribbon length on the output state and teleportation fidelity are discussed in detail. Moreover, we derive the entanglement of the output state and corresponding fidelity as a function of the input and channel entanglement. For practical applications, we also propose a scheme for implementing the single-qubit gates (X, Z and H gates) and two-qubit controlled-NOT gate in GNRs, based on the Bang-Bang control strategy and synchronous step magnetic field.

Topological metal-insulator transition in narrow graphene nanoribbons?

We show that very narrow armchair graphene nanoribbons of length L and width of 2 zigzag-rings undergo a metal-insulator-like transition at a critical length Lc ≈ 10 nm, where the energy gap drops rather abruptly and topological “end” states appear, linked to dramatically transformed aromaticity. At Lc the conductivity, estimated through an invoked computational scheme, also rises almost discontinuously to a value near the nominal minimum conductivity of grapheneσmin=4e2h. The end states (at the zigzag edges) generate sharp peaks in the density of states around the Fermi level at the Dirac points, coinciding with charge-neutrality points, associated with σmin. This suggests metallic-like behaviour, which however is an uncommon combination of interrelated “short-long”, (or “bulk”-“edge”) topological-aromatic transition(s) due to strong quantum confinement, combined with inversion symmetry conflict. This “multi-transition” is rather universal occurring also for wider AGNRs but for a much smaller Lc and in a less sharp way. The assumed lower total energy of such open states is an artefact of the mean-field treatment of the electron-electron interaction. In contrast, the topological “end” states reported here are unique and not spin, but rather pseudospin polarized. Thus, any observed magnetism should be considered non-conventional or of questionable origin.

Towards chirality control of graphene nanoribbons embedded in hexagonal boron nitride.

The integrated in-plane growth of graphene nanoribbons (GNRs) and hexagonal boron nitride (h-BN) could provide a promising route to achieve integrated circuitry of atomic thickness. However, fabrication of edge-specific GNRs in the lattice of h-BN still remains a significant challenge. Here we developed a two-step growth method and successfully achieved sub-5-nm-wide zigzag and armchair GNRs embedded in h-BN. Further transport measurements reveal that the sub-7-nm-wide zigzag GNRs exhibit openings of the bandgap inversely proportional to their width, while narrow armchair GNRs exhibit some fluctuation in the bandgap-width relationship. An obvious conductance peak is observed in the transfer curves of 8- to 10-nm-wide zigzag GNRs, while it is absent in most armchair GNRs. Zigzag GNRs exhibit a small magnetic conductance, while armchair GNRs have much higher magnetic conductance values. This integrated lateral growth of edge-specific GNRs in h-BN provides a promising route to achieve intricate nanoscale circuits.

Exciton absorption spectra in narrow armchair graphene nanoribbons in an electric field

We present an analytical investigation of the exciton optical absorption in a narrow armchair graphene nanoribbon (AGNR) in the presence of a longitudinal external electric field directed parallel to the ribbon axis. The two-body 2D Dirac equation for the massless electron and hole subject to the ribbon confinement, Coulomb interaction and electric field is employed. The ribbon confinement is assumed to be much stronger than the internal exciton electric field which in turn considerably exceeds the external electric field. This justifies an adiabatic approximation implying a slow longitudinal and fast transverse electron-hole relative motion governed by the exciton attraction and accompanied by the external electric field and ribbon confinement effects, respectively. In the single subband approximation of the isolated size-quantized subbands induced by the ribbon confinement the exciton electroabsorption coefficient is determined in an explicit form. The pronounced dependencies of the exciton peak positions, widths and intensities on the ribbon width and electric field strength are traced. The electron-hole exciton attraction modifies remarkably the Franz-Keldysh electroabsorption in the frequency region both below and above the edges determined by the size-quantized energy levels. In the double-subband approximation of the interacting ground and first excited subbands the combined effect of the exciton electro- and autoionization caused by the electric field and intersubband coupling, respectively, are explored. Estimates of the expected experimental values for the typically employed AGNR show that for a weak electric field the exciton quasi-discrete states remain sufficiently stable to be observed in optical experiments, while relatively strong fields free the captured carriers to further restore their contribution to the transport.

Band engineering of B2H2 nanoribbons**Project supported by the National Natural Science Foundation of China (Grant Nos. 61888102, 61390501, and 51872284), the CAS Pioneer Hundred Talents Program, the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB30000000 and XDB28000000), the Beijing Nova Program, China (Grant No. Z181100006218023), and the University of Chinese Academy of Sciences.

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B2H2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B2H2 nanoribbons. Based on first-principles calculations, we have found that all narrow armchair nanoribbons with and without mirror symmetry (ANR-s and ANR-as, respectively) are semiconducting. The energy gap has a relation with the width of the ribbon. When the ribbon is getting wider, the gap disappears. The zigzag ribbons without mirror symmetry (ZNR-as) have the same trend. But the zigzag ribbons with mirror symmetry (ZNR-s) are always metallic. We have also found that the metallic ANR-as and ZNR-s can be switched to semiconducting by applying a tensile strain along the nanoribbon. A gap of 1.10 eV is opened under 16% strain for the 11.0-Å ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B2H2 nanoribbon increases its possibility of potential applications in nanodevices.

Thermoelectric properties of nanostructured systems based on narrow armchair graphene nanoribbons

Thermoelectric properties of hybrid systems composed of graphene nanoribbons (GNRs) coupled to rectangular rings or functionalized with aromatic carbon molecules are theoretically addressed here. Graphene-based nanostructures are designed with the purpose of enhancing thermopower responses compared to the thermal performance of pristine GNRs. The electronic transport is calculated using standard tight binding models and the Landauer transport formalism. We found that both semiconducting and metallic armchair nanoribbons coupled to rings exhibit a pronounced enhancement of the thermoelectric responses with comparable intensities, due to Fano antiresonance and Breit–Wigner-like resonances in the electronic transport. As expected, details of the ring geometry and ribbons are important in determining the precise chemical potential values for optimal performance. Different configurations of attached aromatic molecules (single and double molecules) at the graphene nanoribbon edges are addressed. Our findings show that the presence of a molecule induces a gap formation in the metallic pristine GNRs, and a pronounced peak of the Seebeck coefficient is revealed for low chemical potential values, independent of the molecule length. Other features on the Seebeck spectra are found to depend on the electronic nature of the GNRs and on the molecule length and distribution. We have shown that by playing with them, it is possible to design better thermoelectric devices based on GNRs.

Probing plasmons on the edge of graphene nanoribbons under electric field

By using first principle method, we address the plasmon excitation in narrow zigzag and armchair graphene nanoribbons under the effect of the perpendicular electric field. We observed that the edge plasmons with very high one-dimensional confinement mainly originated from π transition peaks and along with increasing the ribbon width, edges contribute much less than in narrow ribbons. Applying electric field excites and hybrids distinct plasmon modes which indicate, the magnitude, intensity and type of plasmon modes within various momentum transfer range are dependent differently on the gating. The key role of charge polarization at the edge and surface plasmons is discussed in the presence of electric field based on the evolution of the dielectric function. Confined edge and surface plasmon modes change under electric field makes graphene nanoribbons a suitable candidate for new terahertz plasmonic applications.

Excitonic States in Narrow Armchair Graphene Nanoribbons on Gold Surfaces

Narrow graphene nanoribbons (GNRs) exhibit electronic and optical properties that are not present in extended graphene. Most importantly, they possess band gaps in the order of a few electron volts, which has been subject to numerous studies. Here we report on the experimental observation of exctionic states in the band gap of N = 7 armchair GNRs (7-GNR) on Au(111) and Au(788) using energy- and angle-resolved two-photon photoemission spectroscopy. Thereby, an exciton binding energy in the 7-GNR on Au(111) of 160 ± 60 meV has been determined. On the stepped Au(788) surface, the exciton binding energy is in the same range.

Electro-optical properties of new structure photodetectors based on graphene nanoribbons: an ab initio study

In this paper, we propose photodetectors based on sandwiched graphene nanoribbons and investigate their optical and electrical characteristics. Here we combine three narrow armchair graphene (AGNRs) in a way that two AGNRs with the same width sandwich another AGNR with a different width and smaller length. Some of these structures produce sharp peaks in their absorption spectrum that increase selectivity in some frequencies. Some of the peaks shift to mid-infrared, and so are useful in various optical communication windows. These structures also increase the absorption coefficient at some wavelengths up to 90-fold. The responsivity peak shows a maximum of about a 52% increase. We also show that some of these structures can reduce dark current by about three times. Finally, we demonstrate the effect of channel length on dark current in these devices.

Exciton absorption in narrow armchair graphene nanoribbons

We develop an analytical approach to the exciton optical absorption for narrow gap armchair graphene nanoribbons (AGNR). We focus on the regime of dominant size quantization in combination with the attractive electron–hole interaction. An adiabatic separation of slow and fast motions leads via the two-body Dirac equation to the isolated and coupled subband approximations. Discrete and continuous exciton states are in general coupled and form quasi-Rydberg series of purely discrete and resonance type character. The corresponding oscillator strengths and widths are derived. We show that the exciton peaks are blue-shifted, become broader and increase in magnitude upon narrowing the ribbon. At the edge of a subband the singularity related to the 1D density of states is transformed into finite absorption via the presence of the exciton. Our analytical results are in good agreement with those obtained by other methods including numerical approaches. Estimates of the expected experimental values are provided for realistic AGNR.

Scaling Effects on Static Metrics and Switching Attributes of Graphene Nanoribbon FET for Emerging Technology

In this paper, we have investigated the static metrics and switching attributes of graphene nanoribbon field-effect transistors (GNR FETs) for scaling the channel length from 15 nm down to 2.5 nm and GNR width by approaching the ultimate vertical scaling of oxide thickness. We have simulated the double-gate GNR FET by solving a numerical quantum transport model based on selfconsistent solution of the 3D Poisson equation and 1D Schrödinger equation within the non-equilibrium Green's function formulism. The narrow armchair GNR, e.g. (7,0), improved the device robustness to shortchannel effects, leading to better OFF-state performance considering OFF-current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ratio, subthreshold swing, and drain-induced barrier-lowering. The wider armchair GNRs allow the scaling of channel length and supply voltage, resulting in better ON-state performance, such as the larger intrinsic cut-off frequency for the channel length below 7.5 nm at smaller gate voltage as well as smaller intrinsic gate-delay time with the constant slope for scaling the channel length and supply voltage. The wider armchair GNRs, e.g. (13,0), have smaller power-delay product for scaling the channel length and supply voltage, reaching to ~0.18 (fJ/μm).

Orientation and strain modulated electronic structures in puckered arsenene nanoribbons

Orthorhombic arsenene was recently predicted as an indirect bandgap semiconductor. Here, we demonstrate that nanostructuring arsenene into nanoribbons successfully transform the bandgap to be direct. It is found that direct bandgaps hold for narrow armchair but wide zigzag nanoribbons, which is dominated by the competition between the in-plane and out-of-plane bondings. Moreover, straining the nanoribbons also induces a direct bandgap and simultaneously modulates effectively the transport property. The gap energy is largely enhanced by applying tensile strains to the armchair structures. In the zigzag ones, a tensile strain makes the effective mass of holes much higher while a compressive strain cause it much lower than that of electrons. Our results are crucial to understand and engineer the electronic properties of two dimensional materials beyond the planar ones like graphene.

Bending and buckling of narrow armchair graphene nanoribbons via STM manipulation

Semiconducting graphene nanoribbons (GNRs) are envisioned to play an important role in future electronics. This requires the GNRs to be placed on a surface where they may become strained. Theory predicts that axial strain, i.e. in-plane bending of the GNR, will cause a change in the band gap of the GNR. This may negatively affect device performance. Using the tip of a scanning tunneling microscope we controllably bent and buckled atomically well-defined narrow armchair GNR and subsequently probed the changes in the local density of states. These experiments show that the band gap of 7-ac-GNR is very robust to in-plane bending and out-of-plane buckling.

Transport, Aharonov–Bohm, and Topological Effects in Graphene Molecular Junctions and Graphene Nanorings

The unique ultrarelativistic, massless, nature of electron states in two-dimensional extended graphene sheets, brought about by the honeycomb lattice arrangement of carbon atoms in two dimensions, provides ingress to explorations of fundamental physical phenomena in graphene nanostructures. Here, we explore the emergence of new behavior of electrons in atomically precise segmented graphene nanoribbons (GNRs) and graphene rings with the use of tight-binding calculations, nonequilibrium Green’s function transport theory, and a newly developed Dirac continuum model that absorbs the valence-to-conductance energy gaps as position-dependent masses, including topological-in-origin mass barriers at the contacts between segments. Through transport investigations in variable-width segmented GNRs with armchair, zigzag, and mixed edge terminations, we uncover development of new Fabry–Pérot-like interference patterns in segmented GNRs, a crossover from the ultrarelativistic massless regime, characteristic of extended graphene systems, to a massive relativistic behavior in narrow armchair GNRs, and the emergence of nonrelativistic behavior in zigzag-terminated GNRs. Evaluation of the electronic states in a polygonal graphene nanoring under the influence of an applied magnetic field in the Aharonov–Bohm regime and their analysis with the use of a relativistic quantum field theoretical model unveils development of a topological-in-origin zero-energy soliton state and charge fractionization. These results provide a unifying framework for analysis of electronic states, coherent transport phenomena, and the interpretation of forthcoming experiments in segmented GNRs and polygonal rings.

Impurity electrons in narrow electric-field-biased armchair graphene nanoribbons

We present an analytical investigation of the quasi-Coulomb impurity states in a narrow gapped armchair graphene nanoribbon (GNR) in the presence of a uniform external electric field directed parallel to the ribbon axis. The effect of the ribbon confinement is taken to be much greater than that of the impurity electric field, which in turn considerably exceeds the external electric field. Under these conditions we employ the adiabatic approximation assuming that the motion parallel (``slow'') and perpendicular (``fast'') to the ribbon axis are separated adiabatically. In the approximation of the isolated size-quantized subbands induced by the ``fast'' motion, the complex energies of the impurity electron are calculated in explicit form. The real and imaginary parts of these energies determine the binding energy and width of the quasidiscrete state, respectively. The energy width increases with increasing the electric field and ribbon width. The latter forms the background of the mechanism of dimensional ionization. The $S$ matrix---the basic tool of study of the transport problems---can be trivially derived from the phases of the wave functions of the continuous spectrum presented in explicit form. In the double-subband approximation, we calculate the complete widths of the impurity states caused by the combined effect of the electric field and the Fano resonant coupling between the impurity states of the discrete and continuous spectra associated with the ground and first excited size-quantized subbands. Our analytical results are shown to be in agreement with those obtained by other theoretical approaches. Estimates of the expected experimental values for the typically employed GNRs show that for weak electric field the impurity quasidiscrete states remain sufficiently stable to be observed in the corresponding experiment, while a relatively strong field unlocks the captured electrons to further restore their contribution to the transport.

Effects of Stone-Wales Defect Symmetry on the Electronic Structure and Transport Properties of Narrow Armchair Graphene Nanoribbon

We report a first principles calculation to investigate the electron transport properties of defected armchair graphene nanoribbon (AGNR) influenced by Stone-Wales (SW) defect. The SW defect is found to be able to effectively influence the electronic structure of the defected AGNRs, and their electron transport behaviors can exhibit prominent differences depending on the symmetry of the nanostructured morphology. Moreover, our simulations have revealed that the introducing of the SW defect could be favorable for the electron transport of the defective AGNR. Our investigation has confirmed the possibility of tuning the electron transport of graphene nanoribbon by introducing a topological defect, which could be helpful to extending the field of applications for graphene nanoribbon-based nanodevices.

Electronic Properties of Au-Doped Narrow Armchair Graphene Nanoribbons: A First Principle Calculations

The centre Au-doped armchair graphene nanoribbons (AGNRs) are investigated using the local density approximation based on density function theory. The charge density, electronic energy band and project density of states of centre Au-doped AGNRs are calculated. Our results indicate the charge density is transferred between C and Au atoms and mainly located on the Au atoms. The centre Au-doped AGNRs are an indirect band gap semiconductor with an energy gap of 1.046 eV. The Fermi level is located on valence band so that the AGNRs of doping Au become into degenerate semiconductor. The project density of states is calculated to reveal localization and hybridization between C-2pand Au-6s, 5delectronic states. The localization and hybridization are much stronger in the valence band. The hybridization between C-2pand Au-6pelectronic states are strongly in the conduction band.

Ab initio systematic study of chirality effects on phonon spectra, mechanical and thermal properties of narrow single walled carbon nanotubes

Pseudo-potential method under frame work of density functional perturbation theory (DFPT) has been utilized to calculate the phonon spectrum, phonon DOS, specific heat capacity and mechanical properties of narrow armchair and zigzag single walled carbon nanotubes (SWCNTs). Our calculations show that although their Young modulus are about 1TPa, but armchair SWCNTs have greater compressive modulus than the zigzag tubes while the situation would be vice versa for tensile modulus. On the other hand, it was found out that specific heat capacity of narrow tubes shows chirality independence and decreases by increasing the radius of nanotube whereas, this is not the case for the wider ones. In addition, chirality dependence of the radial breadth mode (RBM) frequency of narrow SWCNTs has been shown.

Magnetic Correlations in Short and Narrow Graphene Armchair Nanoribbons

Electronic states at the ends of a narrow armchair nanoribbon give rise to a pair of nonlocally entangled spins. We propose two experiments to probe these magnetic states, based on magnetometry and tunneling spectroscopy, in which correlation effects lead to a striking, nonlinear response to external magnetic fields. On the basis of low-energy theories that we derive here, it is remarkably simple to assess these nonlinear signatures for magnetic edge states. The effective theories are especially suitable in parameter regimes where other methods such as quantum Monte Carlo simulations are exceedingly difficult due to exponentially small energy scales. The armchair ribbon setup discussed here provides a promisingly well-controlled (both experimentally and theoretically) environment for studying the principles behind edge magnetism in graphene-based nanostructures.