Induced Energy Gap Research Articles

Intrinsic Anomalous Hall Conductivity of Strained Fe: A Combination Approach

In this study, the effect of elastic biaxial strain on electronic structure and the anomalous Hall conductivity (AHC) of α‐Fe has been systematically studied by using the combination of a first principles calculation with the tight binding method through the Wannier function. It is found that applying strain in cubic structure of α‐Fe results in the reduction of crystal symmetry and gives the modification on the bands near Fermi level, which causes a significant change in the AHC of strained systems. The AHC of strained α‐Fe is found to be sensitive with applying strain, which shows the large enhancement of AHC at +2% tensile strain with AHC value is up to 929 Ω−1 cm−1 (increases more than 30%) compared with the unstrained one, while slightly reduces when applying compressive strain. The modification of AHC is found to be related to the change of Berry curvature contributed from the spin orbit coupling (SOC) induced energy gap near Fermi level. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Understanding of isoelectronic alloying induced energy gap variation for a large enhancement of thermoelectric power factor

Understanding of isoelectronic alloying induced energy gap variation for a large enhancement of thermoelectric power factor

Dependence of the Electronic Structure of a Graphene Nanoribbon on the Concentration of Adsorbed Particles

The effect of the indirect exchange of particles adsorbed on a graphene nanoribbon on the width of the induced energy gap and the effective mass of carriers is studied. Analytical dependences of these characteristics on the concentration of adparticles, the adsorbate–substrate binding energy, and the energy level of the adsorbed particle are obtained.

Even-odd chain effect and perpendicular electric field induced spin valve in a zigzag silicene ferromagnetic junction

We study the spin transport in a zigzag silicene nanoribbon (ZSNR) ferromagnetic junction with different even(odd)-chain widths and a perpendicular electric field on its drain electrode. By using the tight-binding lattice model and Keldysh Green’s function method, we calculate the spin-dependent transverse conductance and find that the spin switching effect always exists in the system regardless of the zigzag chain number is an even or an odd. The spin valve phenomena come from the combination of the even-odd chain effect and the perpendicular electric field induced energy gap. When a transmission electron flows from the left lead to the middle scattering region, the transport exhibits a peculiar even-odd chain effect, and then the electron may be prohibited to pass through the middle-right tunneling junction when a perpendicular electron field is applied on the right terminal. The spin transport in the proposed device can be controlled by almost all the system parameters such as Fermi energy, electric field, spin orbit coupling, ferromagnetic magnetization and width of the ZSNR. Furthermore, the proposed device might act as an electrical tunable spin valve to produce and detect a 100% polarized spin current.

Зависимость электронной структуры графеновой наноленты от концентрации адсорбированных частиц

Abstract The effect of the indirect exchange of particles adsorbed on a graphene nanoribbon on the width of the induced energy gap and the effective mass of carriers is studied. Analytical dependences of these characteristics on the concentration of adparticles, the adsorbate–substrate binding energy, and the energy level of the adsorbed particle are obtained.

Induced energy gap in finite-sized superconductor/ferromagnet hybrids

We theoretically study self-consistent proximity effects in finite-sized systems consisting of ferromagnet ($\rm F$) layers coupled to an $s$-wave superconductor ($\rm S$). We consider both $\rm SF_1F_2$ and $\rm SH$ nanostructures, where the $\rm F_1 F_2$ bilayers are uniformly magnetized, and the ferromagnetic $\rm H$ layer possesses a helical magnetization profile. We find that when the $\rm F_1 F_2$ layers are weakly ferromagnetic, a hard gap can emerge when the relative magnetization directions are rotated from parallel to antiparallel. Moreover, the gap is most prominent when the thicknesses of $\rm F_1$ and $\rm F_2$ satisfy $\rm d_{F1}\leq d_{F2}$, respectively. For the $\rm SH$ configuration, increasing the spatial rotation period of the exchange field can enhance the induced hard gap. Our investigations reveal that the origin of these findings can be correlated with the propagation of quasiparticles with wavevectors directed along the interface. To further clarify the source of the induced energy gap, we also examine the spatial and energy resolved density of states, as well as the spin-singlet, and spin-triplet superconducting correlations, using experimentally accessible parameter values. Our findings can be beneficial for designing magnetic hybrid structures where a tunable superconducting hard gap is needed.

The local nature of incompressibility of quantum Hall effect

Since the experimental realization of the integer quantum Hall effect in a two-dimensional electron system, the interrelation between the conductance quantization and the topological properties of the system has been investigated. Assuming that the two-dimensional electron system is described by a Bloch Hamiltonian, system is insulating in the bulk of sample throughout the quantum Hall plateau due to a magnetic field induced energy gap. Meanwhile, the system is conducting at the edges resembling a 2+1 dimensional topological insulator without time-reversal symmetry. Here, by our magneto-transport measurements performed on GaAs/AlGaAs high purity Hall bars with two inner contacts we show that incompressible strips formed at the edges result in Hall quantization, even if the bulk is compressible. Consequently, the relationship between the quantum Hall effect and topological bulk insulator breaks for specific field intervals within the plateaus. The measurement of conducting bulk, strongly challenges all existing single-particle theories.

Superconducting Quantum Interference Single-Electron Transistor

We propose the concept of a quantized single-electron source based on the interplay between Coulomb blockade and magnetic flux-controllable superconducting proximity effect. We show that flux dependence of the induced energy gap in the density of states of a nanosized metallic wire can be exploited as an efficient tunable energy barrier which enables charge pumping configurations with enhanced functionalities. This control parameter strongly affects the charging landscape of a normal metal island with non-negligible Coulombic energy. Under a suitable evolution of a time-dependent magnetic flux the structure behaves likewise a turnstile for single electrons in a fully electrostatic regime.

Valley-dependent electron transport in ferromagnetic/normal/ferromagnetic silicene junction

The electron transport through ferromagnetic/normal/ferromagnetic silicene junction with an induced energy gap is investigated in this work. The energy gap can be tuned by applying electric field or exchange fields due to the buckled structure of silicene. We analyze the local electric field, exchange field, length of normal region-dependence transmission probabilities of four groups and valley conductance. These transmission probabilities and valley conductance can be turned on or off by adjusting the local electric field and exchange field. In particular, a fully valley polarized conductance with 80% transmission is found in this junction, which can be caused by the interplay of valley-dependent massive Dirac electron, the exchange potential and the on-site potential difference of sublattices. Our findings will benefit applications in silicene-based high performance nano-electronics.

Many body effect induced energy gap in an optically pumped graphene system

We develop a simple way to investigate the energy gap induced by many body effect in optically pumped graphene at different carrier densities. The exchange self-energy and energy dispersions are obtained analytically at the long wave limit. An energy gap depending on the carrier density is observed at the Dirac point. The energy gap induced by many body effect lies in the microwave range which is in accordance with the experimental measurements. Our theoretical results indicate that the exchange interaction via Coulomb interaction can be a mechanism to create an energy gap in optically pumped graphene.

Relativistic electron transport through an oscillating barrier: Wave-packet generation and Fano-type resonances

Transport properties of massive Dirac particles are investigated through an oscillating barrier. The Floquet quasienergies related to the time-dependent potential appear both in transmission and reflection as sidebands around the incoming electron's energy. We take all relevant sidebands into account and present time averaged transmission and reflection probabilities in a wide energy range. Most qualitative features of scattering on a static barrier---like Klein paradox---are still visible, but the transmission probability in the evanescent regime observably increases due to the oscillation of the potential. The strongly inelastic scattering process is shown to lead to multiple Fano-type resonances and temporal trapping of the particles inside the oscillating potential. We also present a detailed study of the time evolution of the wave packets generated in the scattering process. Our results can be relevant for graphene with an induced energy gap.

Resonant manifestations of chiral excitons in Faraday and Kerr effects in a topological insulator film

Manifestation of chiral excitons on surfaces of topological insulator thin film with magnetically induced energy gap in the surface spectrum in magnetooptical Kerr and Faraday effects is analyzes. Excitonic contribution to a surface optical conductivity tensor is calculated. Chiral excitons contrary to conventional ones resonantly contribute to Hall conductivity due to lack of the symmetry between the states with opposite angular momentum. They can lead to considerable enhancement of Faraday angle and ellipticity of transmitted electromagnetic wave. Chiral excitons cause decrease of Kerr angle and prominent signatures in ellipticity of reflected electromagnetic wave. Conditions for experimental observation of described effects are discussed.

Effects of spatial dispersion on the Casimir force between graphene sheets

The Casimir force between graphene sheets is investigated with emphasis on the effect from spatial dispersion using a combination of factors, such as a nonzero chemical potential and an induced energy gap. We distinguish between two regimes for the interaction - T=0 $K$ and $T\neq 0$ $K$. It is found that the quantum mechanical interaction (T=0 $K$) retains its distance dependence regardless of the inclusion of dispersion. The spatial dispersion from the finite temperature Casimir force is found to contribute for the most part from $n=0$ Matsubara term. These effects become important as graphene is tailored to become a poor conductor by inducing a band gap.

Giant Goos-Hänchen shift in graphene double-barrier structures

We report giant Goos-H\"{a}nchen shifts [Goos and H\"{a}nchen, Ann. Phys. 436, 333 (1947)] for electron beams tunneling through graphene double barrier structures. We find that inside the transmission gap for the single barrier, the shift displays sharp peaks with magnitudes up to the order of electron beam width and rather small full-widths-at-half-maximum, which may be utilized to design valley and spin beam splitters with wide tunability and high energy resolution. We attribute the giant shifts to quasibound states in the structures. Moreover, an induced energy gap in the dispersion can increase the tunability and resolution of the splitters.

Energy loss spectroscopy of epitaxial versus free-standing multilayer graphene

We present a formalism and numerical results for the energy loss of a charged particle scattered at an arbitrary angle from epitaxially grown multilayer graphene (MLG). It is compared with that of free-standing graphene layers. Specifically, we investigated the effect of the substrate induced energy gap on one of the layers. The gap yields collective plasma oscillations whose characteristics are qualitatively and quantitatively different from those produced by Dirac fermions in gapless graphene. The range of wave numbers for undamped self-sustaining plasmons is increased as the gap is increased, thereby substantially increasing and red-shifting the MLG stopping power for some range of charged particle velocity. We also applied our formalism to interpret several distinct features of experimentally obtained electron energy loss spectroscopy (EELS) data.

Electrical transport in suspended and double gated trilayer graphene

We present a fabrication process for high quality suspended and double gated trilayer graphene devices. The electrical transport measurements in these transistors reveal a high charge carrier mobility (higher than 20 000 cm2/Vs) and ballistic electric transport on a scale larger than 200 nm. We report a particularly large on/off ratio of the current in ABC-stacked trilayers, up to 250 for an average electric displacement of −0.08 V/nm, compatible with an electric field induced energy gap. The high quality of these devices is also demonstrated by the appearance of quantum Hall plateaus at magnetic fields as low as 500 mT.

Massive Dirac electron tunneling through a time-periodic potential in single layer graphene

We explore the transmission through a time-periodic potential barriers in single layer graphene with an induced energy gap in this work. The analytical solutions of the transmission probabilities for the central band and the first sidebands are obtained. The transmission probabilities as the function of the induced energy gap and the incidence electron energy are discussed. We also investigate how critical angles change as the induced energy gap and incidence electron energy. The study of transmission properties will benefit applications in graphene-based nano-electronics.

Colloquium: Topological insulators

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A three-dimensional (3D) topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on $\mathrm{Hg}\mathrm{Te}∕\mathrm{Cd}\mathrm{Te}$ quantum wells are described that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. Experiments on ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.

Nonequilibrium entanglement and noise in coupled qubits

We study charge entanglement in two Coulomb-coupled double quantum dots in thermal equilibrium and under stationary non-equilibrium transport conditions. In the transport regime, the entanglement exhibits a clear switching threshold and various limits due to suppression of tunneling by Quantum Zeno localisation or by an interaction induced energy gap. We also calculate quantum noise spectra and discuss the inter-dot current correlation as an indicator of the entanglement in transport experiments.

Charge and Spin Hall Conductivity in Metallic Graphene

Graphene has an unusual low-energy band structure with four chiral bands and half-quantized and quantized Hall effects that have recently attracted theoretical and experimental attention. We study the Fermi energy and disorder dependence of its spin Hall conductivity sigma(xy)(SH). In the metallic regime we find that vertex corrections enhance the intrinsic spin Hall conductivity and that skew scattering can lead to sigma(xy)(SH) values that exceed the quantized ones expected when the chemical potential is inside the spin-orbit induced energy gap. We predict that large spin Hall conductivities will be observable in graphene even when the spin-orbit gap does not survive disorder.