Spin-dependent polarization and quantum Hall conductivity in decorated graphene: influence of locally induced spin-orbit-couplings and impurities.

We study spin transport through graphene-like substrates in the presence of one or several, locally induced spin-orbit coupling (SOC) terms resulting from periodically placed strips, on their top and decorated with a random distribution of impurities. Intrinsic SOC, Rashba SOC and/or pseudo-spin-inversion-asymmetry coupling are considered. A systematic investigation of the spin conductance identifies the main SOC terms which lead to its energy dependence as well as the extent to which the impurity concentration and each SOC term can affect or tune it, In addition, the spin current flow is considered in the presence of different SOC impurities and their related group symmetry suchC6v,C3v,D6handD3h. Further, we show that the quantum spin-Hall effect (QSHE) related to the spin edge states depends only on the spin character when the PIA and ISO terms are not sublattice resolved, and on both the spin and sublattice character when they are. In addition, we show that the RSO term plays a major role in obtaining edge states that are either protected on both edges or only on one edge against backscattering. This Rashba term creates an anticrosing gap that affects the symmetry in the edge localizations and leads to half-topological states. The results can facilitate the experimental choice of appropriately decorated strips to (i) develop spin-transistor devices by tuning the Fermi energy, (ii) control the robustness of the QSHE against backscattering even in the presence of on-site sublattice asymmetry induced by a transverse electric field or functionalizations, and (iii) provide a strong theoretical support for spintronic quantum devices.

Commensurability oscillations in a periodically modulated phosphorene.

The recent experimental realization of high-quality phosphorene leads to novel electronic and optical properties with possible new device applications due to its huge direct band gap. We study the commensurability or Weiss oscillations in monolayer phosphorene in the presence of a weak perpendicular magnetic field B and a weak and periodic, electric or magnetic one-dimensional modulation. Either modulation broadens the Landau levels into bands, whose width oscillates with B, and the oscillations appear in the electrical conductivity perpendicular to the modulation taken along the direction (x) of the smaller effective mass. Compared with the oscillations of the diffusive conductivity in a two-dimensional electron gas (2DEG) for typical electron densities [Formula: see text], the ones in phosphorene, with typical [Formula: see text], have approximately similar height but a period significantly smaller when plotted versus [Formula: see text] while plotted versus B they occur at significantly higher fields. The Shubnikov-de Haas oscillations exhibit a similar behaviour. When the modulation is taken along the direction (y) of the larger effective mass, the oscillation period is close to that of a 2DEG. For equal modulation strengths the bandwidth due to a magnetic modulation is one order of magnitude larger than that due to an electric one and the amplitude of the oscillations in the diffusive conductivity about 50 times larger. Numerical results are presented for experimentally relevant parameters.

Exchange, correlation, and scattering effects on surface plasmons in arm-chair graphene nanoribbons.

Using Maxwell's equations for the incoming and outgoing electromagnetic field, in interaction with a metallic arm-chair graphene nanoribbon (AGNR), and the relationship between the density-density response function and the conductivity, we study surface plasmons (SPs) in a AGNR following the Lindhard, random-phase approximation (RPA), and Hubbard approaches. For transverse magnetic (TM) modes we obtain analytical dispersion relations (DRs) valid for q ≤ kF and assess their width dependence. In all approaches we include screening. In the long-wavelength limit q → 0 there is a small but noticeable difference between the DRs of the three approaches. In this limit the respective, scattering-free conductivities differ drastically from those obtained when scattering by impurities is included. We demonstrate that the SP field is proportional to the square of the quality factor Q. The reflection amplitude shows that metallic AGNRs do not support Brewster angles. In addition, AGNRs do not support transverse electric (TE) SPs.

Unconventional quantum Hall effect in Floquet topological insulators.

We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity [Formula: see text] at zero Fermi energy, to a Hall insulator state with [Formula: see text]. These findings open new possibilities for experimentally realizing nontrivial quantum states and unusual quantum Hall plateaus at [Formula: see text].

Electrically tunable magnetoplasmons in a monolayer of silicene or germanene.

We theoretically study electrically tunable magnetoplasmons in a monolayer of silicene or germanene. We derive the dynamical response function and take into account the effects of strong spin-orbit coupling (SOC) and of an external electric filed E(z) perpendicular to the plane of the buckled silicene/germanene. Employing the random-phase approximation we analyze the magnetoplasmon spectrum. The dispersion relation has the same form as in a two-dimensional electron gas with the cyclotron and plasma frequencies modified due to the SOC and the field E(z). In the absence of SOC and E(z), our results agree well with recent experiments on graphene. The predicted effects could be tested by experiments similar to those on graphene and would be useful for future spintronics and optoelectronic devices.

Giant magnetoresistance and spin Seebeck coefficient in zigzag α-graphyne nanoribbons.

We investigate the spin-dependent electric and thermoelectric properties of ferromagnetic zigzag α-graphyne nanoribbons (ZαGNRs) using density-functional theory combined with non-equilibrium Green's function method. A giant magnetoresistance is obtained in the pristine even-width ZαGNRs and can be as high as 10(6)%. However, for the doped systems, a large magnetoresistance behavior may appear in the odd-width ZαGNRs rather than the even-width ones. This suggests that the magnetoresistance can be manipulated in a wide range by the dopants on the edges of ZαGNRs. Another interesting phenomenon is that in the B- and N-doped even-width ZαGNRs the spin Seebeck coefficient is always larger than the charge Seebeck coefficient, and a pure-spin-current thermospin device can be achieved at specific temperatures.

Dc and ac transport in silicene.

We investigate dc and ac transport in silicene in the presence of a perpendicular electric field E(z) that tunes its band gap, finite temperatures, and level broadening. The interplay of silicene's strong spin-orbit interaction and the field E(z) gives rise to topological phase transitions. We show that at a critical value of E(z) the dc spin-Hall conductivity undergoes a transition from a topological insulator phase to a band insulator one. We also show that the spin- and valley-Hall conductivities exhibit a strong temperature dependence. In addition, the longitudinal conductivity is examined as a function of the carrier density n(e), for screened Coulomb impurities of density n(i), and found to scale linearly with n(e)/n(i). It also exhibits an upward jump at a critical value of ne that is associated with the opening of a new spin subband. Furthermore, the contributions of the spin-up and spin-down carriers to the power absorption spectrum depend sensitively on the topological phase and valley index. Analytical results are presented for both dc and ac conductivities in the framework of linear response theory.

Spin-dependent ballistic transport properties and electronic structures of pristine and edge-doped zigzag silicene nanoribbons: large magnetoresistance.

The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. By switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In the ferromagnetic state, even- or odd-width ZSiNRs exhibit a drastically different magnetoresistance. In an odd-width edge-doped ZSiNR a large magnetoresistance occurs compared to that in a pristine ZSiNR. The situation is reversed in even-width ZSiNRs. These phenomena result from the drastic change in the conductance in the antiparallel configuration.

Graphene in inhomogeneous magnetic fields: bound, quasi-bound and scattering states.

The electron states in graphene-based magnetic dot and magnetic ring structures and combinations of both are investigated. The corresponding spectra are studied as a function of the radii, the strengths of the inhomogeneous magnetic field and of a uniform background field, the strength of an electrostatic barrier and the angular momentum quantum number. In the absence of an external magnetic field we have only long-lived quasi-bound and scattering states and we assess their influence on the density of states. In addition, we consider elastic electron scattering by a magnetic dot, whose average B vanishes, and show that the Hall and longitudinal resistivities, as a function of the Fermi energy, exhibit a pronounced oscillatory structure due to the presence of quasi-bound states. Depending on the dot parameters this oscillatory structure differs substantially for energies below and above the first Landau level.

Ballistic transport through graphene nanostructures of velocity and potential barriers.

We investigate the electronic properties of graphene nanostructures when the Fermi velocity and the electrostatic potential vary in space. First, we consider the transmission T and conductance G through single and double barriers. We show that G for velocity barriers differs markedly from that for potential barriers for energies below the height of the latter and it exhibits periodic oscillations as a function of the energy for strong velocity modulation. Special attention is given to superlattices (SLs). It is shown that an applied bias can efficiently widen or shrink the allowed minibands of velocity-modulated SLs. The spectrum in the Kronig-Penney limit is periodic in the strength of the barriers. Collimation of an electron beam incident on an SL with velocity and potential barriers is present but it disappears when the potential barriers are absent. The number of additional Dirac points may change considerably if barriers and wells have sufficiently different Fermi velocities.

Kronig-Penney model of scalar and vector potentials in graphene.

We consider a one-dimensional (1D) superlattice (SL) on graphene consisting of very high and very thin (δ-function) magnetic and potential barriers with zero average potential and zero magnetic field. We calculate the energy spectrum analytically, study it in different limiting cases, and determine the condition under which an electron beam incident on an SL is highly collimated along its direction. In the absence of the magnetic SL the collimation is very sensitive to the value of W/W(s) and is optimal for W/W(s) = 1, where W is the distance between the positive and negative barriers and L = W + W(s) is the size of the unit cell. In the presence of only the magnetic SL the collimation decreases and the symmetry of the spectrum around k(y) is broken for W/W(s) ≠ 1. In addition, a gap opens which depends on the strength of the magnetic field. We also investigate the effect of spatially separated potential and magnetic δ-function barriers and predict a better collimation in specific cases.

Tunable quantum dots in bilayer graphene.

We demonstrate theoretically that quantum dots in bilayers of graphene can be realized. A position-dependent doping breaks the equivalence between the upper and lower layer and lifts the degeneracy of the positive and negative momentum states of the dot. Numerical results show the simultaneous presence of electron and hole confined states for certain doping profiles and a remarkable angular momentum dependence of the quantum dot spectrum, which is in sharp contrast with that for conventional semiconductor quantum dots. We predict that the optical spectrum will consist of a series of nonequidistant peaks.

Electric-field manipulation of spin states in confined non-magnetic/magnetic heterostructures.

The energy spectrum and states of an electron in a non-magnetic/magnetic heterostructure placed between two materials (e.g. oxides) acting as barriers is studied in the presence of a magnetic field perpendicular or parallel to the well. A potential step is formed at the interface between the non-magnetic and magnetic material in the presence of a magnetic field since spin-up electrons see a barrier whereas the spin-down ones see a well. A rich band structure is obtained which can be tuned by a perpendicular electric field. Numerical results are presented for a ZnSe/Zn(1-x)Mn(x)Se heterostructure and their pertinence to spin-polarized transport is pointed out.

Palliative treatment of advanced esophageal cancer with metal-covered expandable stents. A cost-effectiveness and quality of life study.

PURPOSE: The aim of this retrospective study was to evaluate the efficacy and safety of endoscopic therapy with self-expanding metallic endoprostheses in the management of inoperable primary malignant esophageal obstruction or stenosis and the cost-effectiveness of the method. PATIENTS AND METHODS: Between 5/1997-12/2002, obstruction of the esophagus was diagnosed in 78 patients (52 males, 26 females, age range 53-102, mean 72.3 years). The etiology was squamous cell carcinoma (n=42) and adenocarcinoma of the oesophagus (n=36). In total, 89 ultraflex metal stents were introduced endoscopically. In 46 patients dilation with Savary dilators prior to stent placement was required. A cost-effective analysis was performed, comparing oesophageal stenting with laser therapy. RESULTS: Stents were placed successfully in all patients. After 48 h, all patients were able to tolerate solid or semi-solid food. During the follow-up period 8 patients developed dysphagia due to food impaction (treated successfully endoscopically). Eleven patients developed recurrent dysphagia 4-16 weeks after stenting due to tumor overgrowth and were treated with placement of a second stent. The median survival time was 18 weeks. There was no survival difference between squamous cell and adenocarcinoma of the esophagus. A similar cost was calculated for both procedures. A significant improvement in quality of life was noted in patients undergoing stenting (96% and 75% vs. 71% and 57% for the first two months). CONCLUSION: Placement of self-expanding metal stents is a safe and cost-effective treatment modality that improves the quality of life, compared with laser therapy, for patients with inoperable malignant esophageal obstruction.