Opening Of Energy Gap Research Articles

Graphene Spintronics: The Role of Ferromagnetic Electrodes

We report a first principles study of spin transport under finite bias through a graphene-ferromagnet (FM) interface, where FM = Co(111), Ni(111). The use of Co and Ni electrodes achieves spin efficiencies reaching 80% and 60%, respectively. This large spin filtering results from the materials specific interaction between graphene and the FM which destroys the linear dispersion relation of the graphene bands and leads to an opening of spin-dependent energy gaps of ≈0.4-0.5 eV at the K points. The minority spin band gap resides higher in energy than the majority spin band gap located near E(F), a feature that results in large minority spin dominated currents.

Energy-gap opening and quenching in graphene under periodic external potentials

We investigated the effects of periodic external potentials on properties of charge carriers in graphene using both the first-principles method based on density functional theory (DFT) and a theoretical approach based on a generalized effective spinor Hamiltonian. DFT calculations were done in a modified Kohn-Sham procedure that includes the effects of the periodic external potential. Unexpected energy band gap opening and quenching were predicted for the graphene superlattice with two symmetrical sublattices and those with two unsymmetrical sublattices, respectively. Theoretical analysis based on the spinor Hamiltonian showed that the correlations between pseudospins of Dirac fermions in graphene and the applied external potential, and the potential-induced intervalley scattering, play important roles in energy-gap opening and quenching.

Gap opening by asymmetric doping in graphene bilayers

We study the energy gap opening in the electronic spectrum of graphene bilayers caused by asym- metric doping. Both substitutional impurities (boron acceptors and nitrogen donors) and adsorbed potassium donors are considered. The gap evolution with dopant concentration is compared to the situation in which the asymmetry between the layers is induced by an external electric field. The effects of adsorbed potassium are similar to that of an electric field, but substitutional impurities behave quite differently, showing smaller band gaps and a large sensitivity to disorder and sublattice occupation.

Highly anisotropic energy gap in superconductingBa(Fe0.9Co0.1)2As2from optical conductivity measurements

We have measured the complex dynamical conductivity, $\sigma = \sigma_{1} + i\sigma_{2}$, of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ ($T_{c} = 22$ K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency dependence of $\sigma_{1}$ below $T_{c}$, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of $\sigma_{1}$ demonstrates a pronounced coherence peak at frequencies below 15 cm$^{-1}$ (1.8 meV). The temperature dependence of the penetration depth, calculated from $\sigma_{2}$, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of $\Delta_{A} = 3$ meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is $\Delta_{0} = 8$ meV. Overall, our results are consistent with a two-band superconductor with an $s_{\pm}$ gap symmetry.

High‐On/Off‐Ratio Graphene Nanoconstriction Field‐Effect Transistor

A method is reported to pattern monolayer graphene nanoconstriction field-effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback-controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. The opening of a confinement-induced energy gap is observed as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios obtained with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room-temperature on/off ratios for patterned graphene FETs.

Crossover of the three-dimensional topological insulator Bi2Se3 to the two-dimensional limit

The gapless surface states of topological insulators could enable quantitatively different types of electronic device. A study of the topological insulating Bi2Se3 thin films finds that a gap in these states opens up in films below a certain thickness. This in turn suggests that in thicker films, gapless states exist on both upper and lower surfaces. A topological insulator1,2,3,4,5,6,7,8,9 is a new state of quantum matter that is characterized by a finite energy gap in the bulk and gapless modes flowing along the boundaries that are robust against disorder scattering. The topological protection of the surface state could be useful for both low-power electronics10 and error-tolerant quantum computing11,12. For a thin slab of three-dimensional topological insulator, the boundary modes from the opposite surfaces may be coupled by quantum tunnelling, so that a small, thickness-dependent gap is opened up13,14,15. Here we report such results from angle-resolved photoemission spectroscopy on Bi2Se3 films of various thicknesses grown by molecular beam epitaxy. The energy gap opening is clearly seen when the thickness is below six quintuple layers. The gapped surface states also exhibit sizeable Rashba-type spin–orbit splitting because of the substrate-induced potential difference between the two surfaces. The tunable gap and the spin–orbit coupling make these topological thin films ideal for electronic and spintronic device applications.

Spectroscopic evidence of a topological quantum phase transition in ultrathinBi2Se3films

We have measured the band dispersion of ultrathin ${\text{Bi}}_{2}{\text{Se}}_{3}$ films formed on a silicon substrate using angle-resolved photoemission spectroscopy. An energy gap opening in the topological surface-state bands of the three-dimensional topological insulator ${\text{Bi}}_{2}{\text{Se}}_{3}$ is observed due to the hybridization of the top and bottom surfaces. The drastic change in the shape of the band structure and the reversal of the size of the energy gap between 2 and 3 quintuple layer (QL) films indicate that the parity of the conduction and valence bands are exchanged by finite-size effects. The analyses based on an effective four-band model also support the occurrence of a quantum topological phase transition, thus, implying that 2 QL films are trivial while the 3 QL films are in the quantum spin Hall state.

Dynamical Screening and Excitonic Instability in Bilayer Graphene

Electron interactions in undoped bilayer graphene lead to an instability of the gapless state, "which-layer" symmetry breaking, and energy gap opening at the Dirac point. In contrast with single-layer graphene, the bilayer system exhibits instability even for an arbitrarily weak interaction. A controlled theory of this instability for realistic dynamically screened Coulomb interactions is developed, with full account of the dynamically generated ultraviolet cutoff. This leads to an energy gap that scales as a power law of the interaction strength, making the excitonic instability readily observable.

Reconstruction of the Fermi surface and the anisotropic excitation gap ofNa0.5CoO2

We have performed high-resolution angle-resolved photoemission spectroscopy of ${\text{Na}}_{0.5}{\text{CoO}}_{2}$ to elucidate the origin of the two-step phase transition at ${T}_{\text{c}1}=88\text{ }\text{K}$ and ${T}_{\text{c}2}=53\text{ }\text{K}$. At well below ${T}_{\text{c}2}$, we find anisotropic energy-gap opening on the remnant Fermi surface (FS). This gap does not disappear at the metal-insulator transition temperature ${T}_{\text{c}2}$ and survives up to ${T}_{\text{c}1}$. The remnant FS below ${T}_{\text{c}2}$ shows a deviation from the normal-state FS above ${T}_{\text{c}1}$, suggesting a possible reconstruction of FS caused by the magnetic ordering.

Curvature and Hybridization Effects on the Persistent Current of a Carbon Nanotorus

To accurately describe the persistent current for various toroidal carbon nanotubes (TCNs), a semiempirical sp3 tight-binding model is presented, in which the intrinsic curvature and hybridization have been fully taken into account. The calculations show that the curvature and hybridization can induce dramatic changes in the energy spectra of TCNs such as the Fermi energy EF shifting up, an energy gap opening at EF, and the energy spectrum symmetry about EF destroyed, which leads to a decrease of persistent current and changes in the shape of the flux-dependent current. In the presence of curvature and hybridization, the persistent current in non-armchair TCNs is nearly an order of magnitude lower than that obtained by using the Brillouin-zone folding approach, while it is of the same order of magnitude in armchair TCNs.

Energy gap opening in submonolayer lithium on graphene: Local density functional and tight-binding calculations

The adsorption of an alkali-metal submonolayer on graphene occupying every third hexagon of the honeycomb lattice in a commensurate $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ arrangement induces an energy gap in the spectrum of graphene. To exemplify this type of band gap, we present ab initio density functional theory calculations of the electronic band structure of ${\text{C}}_{6}\text{Li}$. An examination of the lattice geometry of the compound system shows the possibility that the nearest-neighbor hopping amplitudes have alternating values constructed in a Kekul\'e-type structure. The band structure of the textured tight-binding model is calculated and shown to reproduce the expected band gap as well as other characteristic degeneracy removals in the spectrum of graphene induced by lithium adsorption. More generally we also deduce the possibility of energy gap opening in periodic metal on graphene compounds ${\text{C}}_{x}M$ if $x$ is a multiple of 3.

Effects of inter-wall interaction and outer-wall disorder on persistent current in a carbon nanotorus

Taken into account inter-wall interaction and outer-wall disorder, we explore the persistent current in a multi-walled carbon nanotorus (MWCNT) within the tight-binding formalism. The calculations show that the inter-wall interaction induces dramatic changes in the energy spectra of both armchair and zigzag MWCNTs such as level splitting, level crossings and/or anticrossings, and even an energy gap opening at the Fermi energy, which leads to a decrease of persistent current and changes in the shape of the flux-dependent current. In the presence of outer-wall disorder, it is found that in the regime of weak disorder, the persistent current decreases with disorder strength, while it increases in the regime of strong disorder. The results show that MWCNTs may be a promising candidate for realizing a new type of surface-doping-like molecule devices.

ARPES and STS investigation of Shockley states in thin metallic films and periodic nanostructures

Due to their extreme surface sensitivity, the Shockley states of (111) noble metal surfaces can be used to study the modifications of atomic and electronic properties of epitaxial ultra thin films and self-organized nanostructures. In metallic interfaces, the different parameters of the Shockley surface state bands (energy, effective mass and eventually spin–orbit splitting) have been shown to be strongly thickness dependent. It was also possible by scanning tunneling spectroscopy to evidence a spectroscopic signature of buried interfaces. Moreover, superperiodic surface structures like the reconstruction on Au(111) vicinal surfaces or self-organized nanodots, lead to spectacular spectroscopic effects. In the vicinal Au(23 23 21) surface, the opening of tiny energy gaps associated with the reconstruction potential of such surfaces has been evidenced. Peculiar growth on these Au vicinal surfaces allows us to obtain high quality self-assembled metallic nanostructures which exhibit homogeneous electronic properties on a large spatial scale resulting from a coherent scattering of the Shockley states.

Superlattice properties of semiconductor nanohelices in a transverse electric field

A charge carrier confined in a quasi-one-dimensional semiconductor helical nanostructure in the presence of an electric field normal to the axis of the helix is subjected to a periodic potential proportional to the strength of the field and the helix radius. As a result, electronic properties of such nanohelices are similar to those of semiconductor superlattices with parameters controlled by the applied field. These properties include Bragg scattering of charge carriers by a periodic potential, which results in energy gap opening at the edge of the superlattice Brillouin zone. This provides an opportunity for creating a new class of tunable high-frequency devices based on semiconductor nanohelices.

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.

Diameter dependent phonon-induced backscattering in semiconducting carbon nanotubes

We report on a many-body theoretical study of the effects of electron-(optical)phonon interaction on transport through zigzag semiconducting carbon nanotubes. We show that the interaction with ${A}_{1}(L)$ phonon mode of energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$ induces the opening of nonequilibrium energy gaps at $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}∕2$ and/or $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$ above (below) the charge neutrality point, depending on the tube diameter and phonon population. These mechanisms, which correspond to inelastic backscattering with phonon emission (absorption), are beyond the scope of the conventional Born-Oppenheimer and Fermi golden rule approximations.

Glass-like and Verwey transitions in magnetite in details

We have measured low field DC and low frequency AC magnetic properties of single crystals of magnetite with continuously reading high-resolution SQUID magnetometer in temperature range from 5 to 150 K . The measurements show hysteresis of Verwey transition in temperature and linear opening of energy gap of about 40 meV above the transition. Below the transition an AC response may be well explained by a thermally activated charge hopping. For frequencies of the driving AC field close to the hopping frequency a spin glass-like behavior is observed. Extracted values of the energy gap, activation energy and attempt period agree with those found from infrared complex conductivity measurements. Characteristic temperature of the glass-like transition coincides with minimum of the spin–spin relaxation time observed by NMR with this sample. Both NMR and our data may be explained consistently on the basis of the fluctuation-dissipation theorem.

Phonon Thermal Transport ofURu2Si2: Broken Translational Symmetry and Strong-Coupling of the “Hidden Order” to the Lattice

A dramatic increase in the total thermal conductivity (kappa) is observed in the hidden order (HO) state of single crystal URu2Si2. Through measurements of the thermal Hall conductivity, we explicitly show that the electronic contribution to kappa is extremely small, so that this large increase in kappa is dominated by phonon conduction. An itinerant BCS or mean-field model describes this behavior well: the increase in kappa is associated with the opening of a large energy gap at the Fermi surface, thereby decreasing electron-phonon scattering. Our analysis implies that the "hidden order" parameter is strongly coupled to the lattice, suggestive of a broken symmetry involving charge degrees of freedom.

Inelastic Quantum Transport and Peierls-Like Mechanism in Carbon Nanotubes

We report on a theoretical study of inelastic quantum transport in (3m,0) carbon nanotubes. By using a many-body description of the electron-phonon interaction in Fock space, a novel mechanism involving optical phonon emission (absorption) is shown to induce an unprecedented energy-gap opening at half the phonon energy, variant Planck's over 2piomega0/2, above (below) the charge neutrality point. This mechanism, which is prevented by Pauli blocking at low bias voltages, is activated at bias voltages on the order of variant Planck's over 2piomega0.

Electronic structure of free-standingGaAs∕AlGaAsnanowire superlattices

Based on an atomistic tight-binding model, extensive calculations for the electronic structure of free-standing $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ nanowire superlattices, oriented along the [100] crystallographic direction, have been performed. Miniband formation and energy gap opening in the conduction and valence bands are studied in detail for different lateral sizes and layer thicknesses of the nanowire superlattice. Generally, the conduction minibands show relatively simple dispersion relations, while in contrast the valence minibands show complicated dispersion relations. For the nanowire superlattice with a GaAs layer thickness $(w)$ and a lateral size $(d)$ of a few nanometers (e.g., $w\ensuremath{\sim}8$ and $d\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$), distinct energy gaps in the order of several tens of meV can appear in the conduction and valence bands. The wave functions of conduction and valence miniband states at the $\ensuremath{\Gamma}$ point have also been studied for the nanowire superlattices. It is shown that at strong lateral confinement situations the wave functions of the conduction miniband states of the nanowire superlattice can be localized in the barrier layers. However, it is generally difficult to observe the same phenomenon from the valence miniband states.