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Pseudospin plays a central role in many novel physical properties of graphene and other artificial systemswhich have pseudospins of 1/2. Here we show that in certain photonic crystals (PCs) exhibiting conical dispersions at k = 0, the eigenmodes near the "Dirac-like point" can be described by an effective spin-orbit Hamiltonian with a higher dimension value S = 1, treating the wave propagation in positive index (upper cone), negative index (lower cone), and zero index (flat band) media within a unified framework. The three-component spinor gives rise to boundary conditions distinct from those of pseudospin 1/2, leading to wave transport behaviors as manifested in super Klein tunneling and supercollimation. For example, collimation can be realized more easily with pseudospin 1 than pseudospin 1/2. The effective medium description of the PCs allows us to further understand the physics of pseudospin-1 electromagnetic (EM) waves from the perspective of complementary materials. The special wave scattering properties of pseudospin-1 EM waves, in conjunction with the discovery that the effective photonic potential can be varied by a simple change of length scale, offer ways to control photon transport. As a useful platform to study pseudospin-1 physics, dielectric PCs are much easier to fabricate and characterize than ultracold atom systems proposed previously. The system also provides a platform to realize the concept of "complementary medium" using dielectric materials and has the unique advantage of low loss. [ABSTRACT FROM AUTHOR]

By requiring unambiguous symmetric quantization leading to the Dirac equation in a curved space, we obtain a special representation of the spin connections in terms of the Dirac gamma matrices and their space–time derivatives. We also require that squaring the equation gives the Klein–Gordon equation in a curved space in its canonical from (without spinor components coupling and with no first order derivatives). These requirements result in matrix operator algebra for the Dirac gamma matrices that involves a universal curvature constant. We obtain exact solutions of the Dirac and Klein–Gordon equations in 1 + 1 space–time for a given static metric. [ABSTRACT FROM AUTHOR]

The Ni/SiO 2 / n -type 4H SiC MOS devices have been fabricated for microelectronic device applications. The SiO 2 layer employed in the MOS devices is grown by wet thermal oxidation process. The current–field characteristics of Ni/SiO 2 / n -type 4H SiC MOS devices are quiet interestingly studied by employing Fowler Nordheim (FN) conduction tunneling model, which is verified by theoretical simulation. It is learnt that the tunneling current through the barrier in the MOS devices promptly obeys the FN conduction tunneling mechanism. The simulation results show that the current in the MOS device increases and barrier height decreases with increasing temperature and internal electric field. Therefore, the correction factor for the barrier height of n -type 4H SiC/SiO 2 MOS device due to the influence of both the temperature and internal electric field is employed. The barrier height observed by the experiments is apparently smaller than the simulated one of an ideal MOS device. However, after employing all the correction factors to the barrier height, the simulated current–field curves fairly coincide with the experimental results. The reason for obtaining smaller experimental barrier height for MOS devices is substantially explored with the support of current–field ( J – F ) analysis. On the other hand, this article comprehensively addresses the effects of quantum mechanical, interface trap density and thickness of 4H-SiC on the barrier height. [ABSTRACT FROM AUTHOR]

We propose a first order equation from which the Schrodinger equation can be derived. Matrices that obey certain properties are introduced for this purpose. We start by constructing the solutions of this equation in one dimension and solve the problem of electron scattering from a step potential. We show that the sum of the spin up and down, reflection and transmission coefficients, is equal to the quantum mechanical results for this problem. Furthermore, we present a three dimensional (3D) version of the equation which can be used to derive the Schrodinger equation in 3D. [ABSTRACT FROM AUTHOR]

The light-dressing effect in Xe atoms was identified in laser-assisted elastic electron scattering (LAES) signals. In the angular distribution of LAES signals with energy shifts of ±hw recorded by the scattering of 1 keV electrons by Xe in an intense nonresonant laser field, a peak profile appeared at small scattering angles (<0.5°). This peak was interpreted as evidence of the light dressing of Xe atoms induced by an intense laser field on the basis of a numerical simulation in which the light-dressing effect is included. [ABSTRACT FROM AUTHOR]

A graphene-based barrier can act as an electron waveguide. The characteristics of the guided modes in the waveguide are investigated. The conditions for the incident energy that generates the guide modes are different between the barrier waveguide and the graphene quantum well waveguides. Guided modes only exist in the case of Klein tunneling, and cannot be generated in the case of classical motion in the barrier waveguide, however guided modes can be generated for both the classical motion and Klein tunneling cases in the well waveguides. The fundamental mode is absent in the graphene barrier waveguide, similar to that in the graphene well waveguides in the Klein tunneling case, while the barrier waveguide supports fewer guided modes. In spite of the vanishing effective mass of electrons and holes, the graphene barrier waveguide can guide electronic waves, which differs from the conventional semiconductor homostructure barrier, which cannot form a waveguide. The graphene-based barrier waveguides are potentially useful in future electron guided-wave integrated circuits which could perform optical-like processing. [ABSTRACT FROM AUTHOR]

The nonequilibrium dynamics beyond the linear response of Weyl semimetals is studied after a sudden switching on of a dc electric field. The resulting current is a nonmonotonic function of time with an initial quick increase in polarization current followed by a power-law decay. Particle-hole creation à la Schwinger dominates for long times when the conduction current takes over the leading role with the total current increasing again. The conductivity estimated from a dynamical calculation within a generalized Drude picture agrees with the one obtained from Kubo's formula. The full distribution function of electron-hole pairs changes from Poissonian for short perturbations to a Gaussian in the long perturbation (Landau-Zener) regime. The vacuum persistence probability of high-energy physics manifests itself in a finite probability of no pair creation and no induced current at all times. [ABSTRACT FROM AUTHOR]

We present an exact solution of the one-dimensional modified Klein Gordon and Duffin Kemmer Petiau (for spins 0 and 1) equations with a step potential in the presence of minimal length in the uncertainty relation, where the expressions of the new transmission and reflection coefficients are determined for all cases. As an application, the Klein paradox in the presence of minimal length is discussed for all equations. [ABSTRACT FROM AUTHOR]

Reflection electron-energy loss spectra are obtained for a series of Si samples with different crystallographic orientations, prepared under different technological conditions. Using the experimental spectra, the electron energy loss dependences of the product of the mean inelastic free path and differential inelastic electron scattering cross section are calculated. A new technique is suggested for analyzing the spectra of inelastic electron scattering cross section by simulating experimental spectra with the use of the three-parameter Tougaard universal cross section functions. The results of the simulation are used to determine the nature of loss peaks and to calculate the surface parameter. [ABSTRACT FROM AUTHOR]

We employ the tight-binding propagation method to study Klein tunneling and quantum interference in large graphene systems. With this efficient numerical scheme, we model the propagation of a wave packet through a potential barrier and determine the tunneling probability for different incidence angles. We consider both sharp and smooth potential barriers in n-p-n and n-n' junctions and find good agreement with analytical and semiclassical predictions. When we go outside the Dirac regime, we observe that sharp n-p junctions no longer show Klein tunneling because of intervalley scattering. However, this effect can be suppressed by considering a smooth potential. Klein tunneling holds for potentials changing on the scale much larger than the interatomic distance. When the energies of both the electrons and holes are above the Van Hove singularity, we observe total reflection for both sharp and smooth potential barriers. Furthermore, we consider caustic formation by a two-dimensional Gaussian potential. For sufficiently broad potentials we find a good agreement between the simulated wave density and the classical electron trajectories. [ABSTRACT FROM AUTHOR]

We describe an experimental setup for measuring angle-dependent transmission due to Klein tunneling through quasi-ballistic graphene heterojunctions. Our devices consist of straight and angled leads, in which the barrier height is controlled by a shared gate electrode. Using a balancing technique and a differential measurement, we show how to isolate the angle-dependent contribution to the resistance from other angle-insensitive, gate-dependent, and device-dependent effects. We find that our baseline signal is dominated by mesoscopic conductance fluctuations, but that the increase in the fluctuation amplitude is due to angle-dependent transmission. [ABSTRACT FROM AUTHOR]

The transport property of Dirac electrons through bilayer graphene under a time periodic field is investigated. The effect of external field strength and the system parameters on the transmission probabilities is investigated. The results show that the applied time-periodic potential provides sidebands for Dirac electrons to tunnel through the bilayer graphene, and transport property of system exhibits various kinds of behaviors with the change of external field strength and structure parameters. We also find that, for normal and close to normal incidence, the perfect reflection that was observed for the static barrier is found to persist for the oscillating barrier. [ABSTRACT FROM AUTHOR]

We present a general approach to solve the ( 1 + 1 ) and ( 2 + 1 ) -dimensional Dirac equations in the presence of static scalar, pseudoscalar and gauge potentials, for the case in which the potentials have the same functional form and thus the factorization method can be applied. We show that the presence of electric potentials in the Dirac equation leads to two Klein–Gordon equations including an energy-dependent potential. We then generalize the factorization method for the case of energy-dependent Hamiltonians. Additionally, the shape invariance is generalized for a specific class of energy-dependent Hamiltonians. We also present a condition for the absence of the Klein paradox (stability of the Dirac sea), showing how Dirac particles in low dimensions can be confined for a wide family of potentials. [ABSTRACT FROM AUTHOR]

We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device's electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Perot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene. [ABSTRACT FROM AUTHOR]

The article discusses the parities and spins of the odd-A phosphorus isotopes within elastic magnetic electron-scattering theory. It mentions that spin and parity are being the fundamental properties of nuclei. It presents the pairing off of nucleon spins and orbital momenta in a nucleus to produce zero contribution to the whole nucleus.

An asymmetric quantum well in graphene can act as a slab waveguide for electron waves in a manner analogous to the electromagnetic waves in dielectrics. Guided modes and the probability current density are analyzed in the graphene electron waveguide induced by asymmetric electrostatic potential. The modes in an asymmetric graphene waveguide include guided modes, 'cover modes', 'substrate modes' and 'radiation modes'. The conditions for a guided mode are quantified. It is found that the fundamental mode is absent when both the Klein tunneling and classical motion are present. The confinement of electrons for lower order mode is stronger than for higher order mode. We hope that these characteristics in asymmetric graphene waveguide can provide potential applications in graphene-based waveguide devices. [ABSTRACT FROM AUTHOR]

Abstract: We have analyzed the effect of various system parameters and external time-dependent field on the transport properties of monolayer graphene-based double-barrier(well) structures under a time-periodic potential. Results indicate that the Klein tunneling still exists. Besides, the transmission probability, conductivity, shot noise, and Fano factor exhibit various types of oscillatory behavior with changes in the system parameters, and they are either improved or suppressed in the presence of the time-periodic potential. We have also discussed the reasons underlying these phenomena. The results obtained in this study demonstrate that the transport properties can be controlled by manipulating the structural parameters of the system and the external field strength. [Copyright &y& Elsevier]

Using the coupled series expansion method developed in this paper, we have investigated the quantum behavior of Coulomb impurity states under external electric and magnetic fields in graphene. Under magnetic field, we have obtained an accurate full spectrum of the attractive and repulsive Coulomb impurities for both massless and massive Dirac electrons and have found that bound states can be tuned between in and out of the gap by Coulomb impurities and a magnetic field. When a linear radial electric field in combination with a magnetic field are applied, the energy levels of Coulomb impurity states experience intercrossing, and they disappear because of Klein tunneling as the electric field exceeds the magnetic field. The results are helpful in controlling Coulomb impurity states by external fields in graphene and the method proves to be an efficient way to obtain accurate solutions of the Dirac equation with various fields. [ABSTRACT FROM AUTHOR]

Abstract: A stable high-order accurate finite difference method for the time-dependent Dirac equation is derived. Grid-convergence studies in 1-D and 3-D corroborate the analysis. The method is applied to time-resolved quantum tunneling where a comparison with the solution to the time-dependent Schrödinger equation in 1-D illustrates the differences between the two equations. In contrast to the conventional tunneling probability decay predicted by the Schrödinger equation, the Dirac equation exhibits Klein tunneling. Solving the time-dependent Dirac equation with a step potential in 3-D reveals that particle spin affects the tunneling process. The observed spin-dependent reflection allows for a new type of spin-selective measurements. [Copyright &y& Elsevier]

Abstract: Statistical complexity and Fisher–Shannon information are calculated in a problem of quantum scattering, namely the Klein tunneling across a potential barrier in graphene. The treatment of electron wave functions as masless Dirac fermions allows us to compute these statistical measures. The comparison of these magnitudes with the transmission coefficient through the barrier is performed. We show that these statistical measures take their minimum values in the situations of total transparency through the barrier, a phenomenon highly anisotropic for the Klein tunneling in graphene. [Copyright &y& Elsevier]

Abstract: We study the scattering of massless Dirac particles by oscillating barriers in one dimension. Using the Floquet theory, we find the exact scattering amplitudes for time-harmonic barriers of arbitrary shape. In all cases the scattering amplitudes are found to be independent of the energy of the incoming particle and the transmission coefficient is unity. This is a manifestation of the Klein tunneling in time-harmonic potentials. Remarkably, the transmission amplitudes for arbitrary sharply-peaked potentials also become independent of the driving frequency. Conditions for which barriers of finite width can be replaced by sharply-peaked potentials are discussed. [Copyright &y& Elsevier]

We consider electrostatically coupled quantum dots in topological insulators, otherwise confined and gapped by a magnetic texture. By numerically solving the (2+1) Dirac equation for the wave packet dynamics, we extract the energy spectrum of the coupled dots as a function of bias-controlled coupling and an external perpendicular magnetic field. We show that the tunneling energy can be controlled to a large extent by the electrostatic barrier potential. Particularly interesting is the coupling via Klein tunneling through a resonant valence state of the barrier. The effective three-level system nicely maps to a model Hamiltonian, from which we extract the Klein coupling between the confined conduction and valence dots levels. For large enough magnetic fields Klein tunneling can be completely blocked due to the enhanced localization of the degenerate Landau levels formed in the quantum dots. [ABSTRACT FROM AUTHOR]

We show that the features of Klein tunneling make graphene a unique interface for implementing low control quantum gates between static and mobile qubits. A ballistic electron spin is considered as the mobile qubit, while the static qubit is the electronic spin of a quantum dot fixed in a graphene nanoribbon. Scattering is the low control mechanism of the gate, which in other systems is very difficult to exploit because of both backscattering and the momentum dependence of transmission. We find that the unique features of Klein tunneling enable quasideterministic quantum gates between the spin of a ballistic electron and a static spin held in a dot, regardless of the momenta or the shape of the incident electron wave function. The Dirac equation is used to describe the system in the one particle approximation, with the interaction between the static and the mobile spins modeled by a Heisenberg Hamiltonian. Furthermore, we discuss an application of this model to generate entanglement between two well-separated static qubits. [ABSTRACT FROM AUTHOR]

Quantum resonant tunneling behaviors of double-barrier structures on graphene are investigated under the tight-binding approximation. The Klein tunneling and resonant tunneling are demonstrated for the quasiparticles with energy close to the Dirac points. The Klein tunneling vanishes by increasing the height of the potential barriers to more than 300 meV. The Dirac transport properties continuously change to the Schrödinger ones. It is found that the peaks of resonant tunneling approximate to the eigen-levels of graphene nanoribbons under appropriate boundary conditions. A comparison between the zigzag- and armchair-edge barriers is given. [ABSTRACT FROM AUTHOR]

It is shown that an alternative to the standard scalar quantum electrodynamics (QED) is possible. In this new version, there is only global gauge invariance as far as the charged scalar fields are concerned, although local gauge invariance is kept for the electromagnetic field. The electromagnetic coupling has the form jμ(Aμ +∂μB) where B is an auxiliary field and the current jμ is Aμ independent, so that no "sea gull terms" are introduced. As a consequence of the absence of sea gulls, it is seen that no Klein paradox appears in the presence of a strong square well potential. In a model of this kind, spontaneous breaking of symmetry does not lead to photon mass generation, instead the Goldstone boson becomes a massless source for the electromagnetic field. When spontaneous symmetry breaking takes place infrared questions concerning the theory and generalizations to global vector QED are discussed. In this framework, Q-Balls and other nontopological solitons that owe their existence to a global U(1) symmetry can be coupled to electromagnetism and could represent multiply charged particles now in search in the large hadron collider (LHC). Furthermore, we give an example where an "Emergent" Global Scalar QED can appear from an axion-photon system in an external magnetic field. Finally, formulations of Global Scalar QED that allow perturbative expansions without sea gulls are developed. [ABSTRACT FROM AUTHOR]

We investigate the quantum tunneling of electrons in an AA-stacked bilayer graphene (BLG) n-p junction and n-p-n junction. We show that Klein tunneling of an electron can occur in this system. The quasiparticles are not only chiral but are additionally described by a "cone index." Due to the orthogonality of states with different cone indexes, electron transport across a potential barrier must strictly conserve the cone index, and this leads to the protected cone transport which is unique in AA-stacked BLG. Together with the negative refraction of electrons, electrons residing in different cones can be spatially separated according to their cone index when transmitted across an n-p junction. This suggests the possibility of "cone-tronic" devices based on AA-stacked BLG. Finally, we calculate the junction conductance of the system. [ABSTRACT FROM AUTHOR]

Abstract: In this Letter we have studied the quantum behavior of a spin half neutral fermion interacting with a pseudoscalar potential barrier in ( )-dimensional spacetime. Exact solutions for the corresponding Dirac equation are obtained both for bound and scattering states. The exact energy levels are obtained from the solutions of Dirac equation. The validity of the quasi-classical quantization rule is examined. For the scattering process the transmission and reflection coefficients are exactly calculated. The absence of the Kleinʼs paradox is also discussed. [Copyright &y& Elsevier]

We study the transmission through a step-barrier in gapped graphene and propose a method to enlarge the band gap. The step-barrier structure consists of two or more barriers with different strengths. It is found that the band gap could be effectively enlarged and controlled by adjusting the barrier strengths in the light of the mass term. Klein tunneling at oblique incidence is suppressed due to the asymmetry of step-barrier, contrary to the cases in single-barrier and superlattices. Furthermore, a tunable conductance channel could be opened up in the conductance gap, suggesting an application of the structure as an electron switch. [ABSTRACT FROM AUTHOR]

The next-nearest-neighbor hopping term t′ determines a magnitude, and, hence, the importance of several phenomena in graphene that include self-doping due to broken bonds and the Klein tunneling, which in the presence of t′, is no longer perfect. Theoretical estimates for t′ vary widely, whereas a few existing measurements by using polarization-resolved magnetospectroscopy have found surprisingly large t′, close to or even exceeding the highest theoretical values. Here, we report dedicated measurements of the density of states in graphene by using high-quality capacitance devices. The density of states exhibits a pronounced electron-hole asymmetry that increases linearly with energy. This behavior yields t′ ≈ - 0.3 eV ± 15%, in agreement with the high end of theory estimates. We discuss the role of electron-electron interactions m determining t′ and overview phenomena, which can be influenced by such a large value of t′. [ABSTRACT FROM AUTHOR]

Currently, much effort is being put into detecting unconventional p-wave superconductivity in Josephson junctions based on topological insulators (TIs). For that purpose we propose to use superconducting Klein tunneling, i.e., the reflectionless passage of Cooper pairs through a potential barrier in a gated ballistic junction. This phenomenon occurs due to the fact that the supercurrent is carried by helical Andreev bound states (ABSs) characterized by spin-momentum locking similar to the normal-state carriers. We derive the spectrum of the helical ABSs and the corresponding Josephson current for a junction made on the surface of a three-dimensional TI. The superconducting Klein tunneling is predicted to yield a nonsinusoidal current-phase relation and an anomalous critical current Ic that does not vanish with increasing barrier strength. We also analyze the dependence of the IcRn product (where Rn is the normal-state junction resistance) on the microscopic parameters of the superconductor/TI interface, which leads to lower IcRn values than expected from previous models of the proximity-effect Josephson junctions. [ABSTRACT FROM AUTHOR]

The perfect transmission in a graphene monolayer and the perfect reflection in a Bernal graphene bilayer for electrons incident in the normal direction of a potential barrier are viewed as two incarnations of the Klein paradox. Here we show a new and unique incarnation of the Klein paradox. Owing to the different chiralities of the quasiparticles involved, the chiral fermions in a twisted graphene bilayer show an adjustable probability of chiral tunneling for normal incidence: they can be changed from perfect tunneling to partial or perfect reflection, or vice versa, by controlling either the height of the barrier or the incident energy. As well as addressing basic physics about how the chiral fermions with different chiralities tunnel through a barrier, our results provide a facile route to tune the electronic properties of the twisted graphene bilayer. [ABSTRACT FROM AUTHOR]

The electronic tunneling properties of the two stable forms of trilayer graphene (TLG), rhombohedral ABC and Bernal ABA, are examined for p-n and p-n-p junctions as realized by using a single gate (SG) or a double gate (DG). For the rhombohedral form, due to the chirality of the electrons, the Klein paradox is found at normal incidence for SG devices, while at high-energy interband scattering between additional propagation modes can occur. The electrons in Bernal ABA TLG can have a monolayer- or bilayer-like character when incident on a SG device. Using a DO, however, both propagation modes will couple by breaking the mirror symmetry of the system, which induces intermode scattering and resonances that depend on the width of the DG p-n-p junction. For ABC TLG the DG opens up a band gap which suppresses Klein tunneling. The DG induces also an unexpected asymmetry in the tunneling angle for single-valley electrons. [ABSTRACT FROM AUTHOR]

The role of defect-induced zero-energy modes on charge transport in graphene is investigated using Kubo and Landauer transport calculations. By tuning the density of random distributions of monovacancies either equally populating the two sublattices or exclusively located on a single sublattice, all conduction regimes are covered from direct tunneling through evanescent modes to mesoscopic transport in bulk disordered graphene. Depending on the transport measurement geometry, defect density, and broken sublattice symmetry, the Dirac-point conductivity is either exceptionally robust against disorder (supermetallic state) or suppressed through a gap opening or by algebraic localization of zero-energy modes, whereas weak localization and the Anderson insulating regime are obtained for higher energies. These findings clarify the contribution of zero-energy modes to transport at the Dirac point, hitherto controversial. [ABSTRACT FROM AUTHOR]

We present a scheme for simulating relativistic quantum physics in circuit quantum electrodynamics. By using three classical microwave drives, we show that a superconducting qubit strongly coupled to a resonator field mode can be used to simulate the dynamics of the Dirac equation and Klein paradox in all regimes. Using the same setup we also propose the implementation of the Foldy-Wouthuysen canonical transformation, after which the time derivative of the position operator becomes a constant of the motion. [ABSTRACT FROM AUTHOR]

Guided by the analogy to Mie scattering of light on small particles, we show that the propagation of a Dirac-electron wave in graphene can be manipulated by a circular gated region acting as a quantum dot. Large dots enable electron lensing, while for smaller dots resonant scattering entails electron confinement in quasibound states. Forward scattering and Klein tunneling can be almost switched off for small dots by a Fano resonance arising from the interference between resonant scattering and the background partition. [ABSTRACT FROM AUTHOR]