Ab?initio characterization of graphene nanoribbons and their polymer precursors

Bottom-up fabrication of graphene nanoribbons (GNRs) from halogen-terminated aromatic precursors is a promising method for achieving atomically precise nanoribbons at competitive yields. GNR fabrication proceeds via the polymerization of the precursors and successive dehydrogenation. By first principles density functional theory calculations, we perform a systematic characterization of the polymeric precursors and the corresponding graphene nanoribbons in terms of structural and electronic properties, and we compute the Raman and infrared spectra. The band structure properties are examined by considering the bonding features and the partial charge densities of the structures. The physical origin of the infrared and Raman peaks is investigated in terms of the morphology and vibrational properties of the precursors and products. We show that light spectroscopy provides a unique fingerprint for each type of GNR, which may be used to monitor the quality of the final products and the kinetics of the synthesis process.     

Topological edge states and electronic structures of a 2D topological insulator： Single-bilayer Bi （111）

Providing the strong spin-orbital interaction, Bismuth is the key element in the family of three-dimensional topological insulators. At the same time, Bismuth itself also has very unusual behavior, existing from the thinnest unit to bulk crystals. Ultrathin Bi （111） bilayers have been theoretically proposed as a two-dimensional topological insulator. The related experimental realization achieved only recently, by growing Bi （111） ultrathin bilayers on topological insulator Bi2Te3 or Bi2Se3 substrates. In this review, we started from the growth mode of Bi （111） bilayers and reviewed our recent progress in the studies of the electronic structures and the one-dimensional topological edge states using scanning tunneling microscopy/spectroscopy （STM/STS）, angle-resolved photoemission spectroscopy （ARPES）, and first principles calculations.     

Thermoelectric transport and spin density of graphene nanoribbons with Rashba spin–orbit interaction

In the present paper, we have theoretically investigated thermoelectric transport properties of armchair and zigzag graphene nanoribbons with Rashba spin–orbit interaction, as well as dephasing scattering processes by applying the nonequilibrium Green function method. Behaviors of electronic and thermal currents, as well as thermoelectric coefficients are studied. It is found that both electronic and thermal currents decrease, and thermoelectric properties been suppressed, with increasing strength of Rashba spin–orbit interaction. We have also studied spin split and spin density induced by Rashba spin–orbit interaction in the graphene nanoribbons.     

Quantum transport signatures of non-trivial topological edge states in a ring-shaped Su–Schrieffer–Heeger double-chain system

In the ring-shaped Su–Schrieffer–Heeger(SSH)double-chain,the quantum interference between the two different electron tunneling paths of the upper and lower chains has an important influence on the electron transport properties of non-trivial topological edge states.Here,we have studied the electron transport signatures of non-trivial topological edge states in a ring-shaped SSH double-chain system based on the wave-guide theory and transfer-matrix method.In the ringshaped SSH double-chain with the upper chain being different from the lower one,it is demonstrated that the electron transmission probability displays the four and two resonance peaks associated with the non-trivial topological edge states in the weak and strong coupling regimes,respectively.Whereas in the case of the upper chain being the same as the lower one,the two transmission resonance peaks associated with the non-trivial topological edge states in the weak coupling regime are only found,and that in the strong coupling regime disappear that originated from the destructive interference between the two different electron tunneling paths of the upper and lower chains.Consequently,the variation of the number of transmission resonance peaks associated with the non-trivial topological edge states in the weak and strong coupling regimes suggests that an alternative scheme for detecting non-trivial topological edge states in the ring-shaped SSH doublechain system.     

Bloch–Floquet waves and controlled stop bands in periodic thermo-elastic structures

An algorithm for controlling the stop bands for elastic Bloch–Floquet waves within a periodic structure is proposed. Explicit asymptotic estimates of frequencies of translational and rotational standing waves, together with the numerical estimates of the stop band frequencies, are given. Thermal pre-stress is introduced and used to control the position of the stop bands on the dispersion diagram.     

Role of 3D-paired pentagon–heptagon defects in electronic and transport properties of zigzag graphene nanoribbons

Electronic and transport properties of 11 zigzag graphene nanoribbons (11-z-GNRs) with two types of 3D-paired pentagon–heptagon defects (3D-PPHDs) are studied using density functional theory combined with non-equilibrium Green’s function method. The C ad-dimers that have been introduced to z-GNRs to form these 3D-PPHDs have induced local strains forcing the C-bonds in the ad-dimers to hybridize in sp³-like rather than sp²-like orbitals. Such transformations that cause extra electrons to accumulate around the 3D-PPHDs are responsible for the variations in the electronic and transport properties of the defected z-GNRs. Density of states (DOS) for 11-z-GNRs containing either type of 3D-PPHDs, compared with that of the pristine 11-z-GNR, exhibits a peak at a particular energy level below the Fermi level, within its first plateau. Each peak, corresponding to a dip in the related zero-bias conductance profile, is attributed to electrons trapped in the localized states around the same energy level. The current–voltage (I–V) characteristics are almost linear for 11-z-GNR defected by either type of 3D-PPHD. Presence of a 3D-PPHD, made by addition of a single C ad-dimer, above an 11-z-GNR super-cell decreases its average conductance by 10.7 %, while a 3D-PPHD made by addition of two C ad-dimers to the opposite sides of the same 11-z-GNR super-cell decreases its conductance by 23 %.     

Effects of edge hydrogenation and Si doping on spin-dependent electronic transport properties of armchair boron–phosphorous nanoribbons

In this article, the spin-dependent electronic and transport properties of the armchair boron–phosphorous nanoribbons(ABPNRs) are mainly studied by using the non-equilibrium Green function method combined with the spin-polarized density function theory. Our calculated electronic structures indicate that the edge hydrogenated ABPNRs exhibit a ferromagnetic bipolar magnetic semiconductor property, and that the Si atom doping can make ABPNRs convert into up-spin dominated half metal. The spin-resolved transport property results show that the doped devices can realize 100% spinfiltering function, and that the interesting negative differential resistance phenomenon can be observed. Our calculations suggest that the ABPNRs can be constructed as a spin heterojunction by introducing Si doping partially, and it would be used as a spin-diode for nano-spintronics in future.     

Erratum to: “Emission and Absorption of Neutrinos by Nonrelativistic Neutrons”

Journal of Experimental and Theoretical Physics -     

“Soft” edge states in inhomogeneous 2D electron systems

The structural details of an edge electronic state in external-field-bounded 2D charged (electron or hole) systems at the zero boundary density of the mobile charge carriers are discussed. It is shown that the so-called “soft” edge electronic states (SESs) appear along these boundaries. The details of their structure and spectrum are analyzed, and the possible effects with the SESs are outlined.     

Topological quantum phase transitions and topological flat bands on the kagomé lattice

We investigate the topological properties of the tight-binding electrons on the two-dimensional kagomé lattice with two kinds of short-range hopping integral and two kinds of staggered magnetic flux. Considering the nearest-neighbor hopping (t(1)) with the staggered flux parameter φ(1) and the next nearest-neighbor hopping (t(2)) with the staggered flux parameter φ(2), we demonstrate a series of topological quantum phase transitions and find some topological bands with high Chern numbers, when tuning one parameter (t(2) or φ(2)) while the others are fixed. We have also found that, in some parameter regions, the system exhibits interesting topological flat bands with Chern number C =± 1 and a large gap above them, and the flatness ratio can reach a high value of about 170.     

Even and odd coherent states are “shadowed-like” states

Shadowed states were introduced recently in the literature. It was shown that these states always exhibit nonclassical effects [R. Srinivasan and C.T. Lee, Phys. Lett. A 218 (1996) 151]. Here we show that the well known even and odd coherent states are shadowed-like states, whose differences and similarities are discussed.