Green Function Technique Research Articles

Thermally driven spin-Seebeck transport in chiral dsDNA-based molecular devices

By employing the nonequilibrium Green's function technique, we study the thermal-induced spin-Seebeck transport through a chiral double-stranded DNA (dsDNA) connected to a normal-metal and a ferromagnetic lead. How the main parameters of the dsDNA-based system influence the spin-Seebeck transport is analyzed at length, and the thermally created charge (spin-related) current displays the rectification effect and the negative differential thermal conductance feature. More importantly, the spin current exhibits the rectification behavior of the spin-Seebeck effect; even the perfect spin-Seebeck effect can be obtained with the null charge current. Thus, the chiral dsDNA-based system can act as a spin(charge)-Seebeck diode, spin(charge)-Seebeck switch, and spin(charge)-Seebeck transistor. Our results provide new ways to design spin caloritronic devices based on dsDNA or other organic molecules.

Low temperature anomaly of light stimulated magnetization and heat capacity of the 1D diluted magnetic semiconductors

This article reports magnetization and specific heat capacity anomalies in one dimensional diluted magnetic semiconductors observed at very low temperatures. Based on quantum field theory double time temperature dependent Green function technique is employed to evaluate magnon dispersion and the time correlation function. It is understood that magnon-photon coupling and magnetic impurity concentration controls both, such that near absolute temperature magnetization is nearly zero and abruptly increase to saturation level with decreasing magnon-photon coupling strength. We also found out dropping of magnetic specific heat capacity as a result of increase in magnetic impurity concentration x, perhaps because of inter-band disorder that would suppress the enhancement of density of spin waves.

The modified Green function technique for the exterior Dirichlet problem in linear thermoelasticity

The modified Green function technique for the exterior Dirichlet problem in linear thermoelasticity

Dissipative Intraplate Faulting During the 2016 Mw6.2 Tottori, Japan Earthquake

AbstractThe 2016 Mw6.2 Tottori earthquake occurred on 21 October 2016 and produced thousands of aftershocks. Here we analyze high‐resolution‐relocated seismicity together with source properties of the mainshock to better understand the rupture process and energy budget. We use a matched‐filter algorithm to detect and precisely locate >10,000 previously unidentified aftershocks, which delineate a network of sharp subparallel lineations exhibiting significant branching and segmentation. Seismicity below 8 km depth forms highly localized fault structures subparallel to the mainshock strike. Shallow seismicity near the main rupture plane forms more diffuse clusters and lineations that often are at a high angle (in map view) to the mainshock strike. An empirical Green's function technique is used to derive apparent source time functions for the mainshock, which show a large amplitude pulse 2–4 s long. We invert the apparent source time functions for a slip distribution and observe a ~16 km2patch with average slip ~3.2 m. 93% of the seismic moment is below 8 km depth, which is approximately the depth below which the seismicity becomes very localized. These observations suggest that the mainshock rupture area was entirely within the lower half of the seismogenic zone. The radiated seismic energy is estimated to be 5.7 × 1013J, while the static stress drop is estimated to be 18–27 MPa. These values yield a radiation efficiency of 5–7%, which indicates that the Tottori mainshock was extremely dissipative. We conclude that this inefficiency in energy radiation is likely a product of the immature intraplate environment and the underlying geometric complexity.

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn2O5 (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature TC. This can be explained as an effect of vanishing ferroelectric ordering above TC. It was found that the reduced sound velocity increases with increasing V(3) (the third-order atomic force constants of the anharmonic phonons) in the interval T < TC, whereas the reduced sound velocity remains unchanged in the interval TC < T < TN. It was also found that the reduced sound velocity increases with the increase of V(4) (the fourth-order atomic force constants of the anharmonic phonons) in the interval T < TN. In addition, the ferroelectric phase transition temperature TC decreases when V(4) increases in the interval T < TN. Those theoretical results are in agreement with the experimental data.

La1−xSrxMnO3 Nanoparticles for Magnetic Hyperthermia

Using a modified Heisenberg model and a Green's function technique we have studied the influence of size d and concentration x of Sr dopants on the Curie temperature TC, saturation magnetization MS, coercivity HC and specific absorption rate (SAR) of single‐domain La1−xSrxMnO3 nanoparticles. Their magnetic properties are explained based on the “magnetically death” surface layer and the competition between the ferromagnetic double‐exchange and antiferromagnetic super‐exchange interaction. We calculated the specific absorption rate which characterizes the efficiency of the absorption energy when the magnetic hyperthermia is applied as a therapeutic method for the treatment of tumors. A set of nanoparticles appropriate for in vivo and in vitro medical applications, such as magnetic hyperthermia and drug delivery to cancer cells, is determined. The established methodology and microscopic model are suitable to study the magnetic properties of low‐dimensional systems (thin films and nanoparticles) such as La1−xAxMnO3 with A = Ca, Ag, Ba, Na for medical applications.

Dispersion and Attenuation of Love-Type Waves Due to a Point Source in Magneto-Viscoelastic Layer

AbstractThis paper intends to study the dispersion and attenuation characteristics of Love-type wave propagation due to a disturbance point source in a hydrostatic stressed magneto-viscoelastic layer, lying over a heterogeneous fibre-reinforced elastic half-space. The shear elastic moduli and mass density of half-space are the functions of depth and heterogeneity parameters. The coupled field equations are solved with the aid of Green's function technique and Fourier transform. The dispersion and damping equations have been obtained for the wave. The deduced equations coincide with the classical Love-wave condition for the uniform homogeneous isotropic structure. Numerical computations are carried out for involved parameters and demonstrated with the help of graphs. The effects of hydrostatic stress, magnetic coupling parameter, dissipation factor, attenuation coefficient, reinforcement parameters, heterogeneity parameters and order of the depth variation on the dispersion and damping curves are highlighted.

First-Principle-Based Phonon Transport Properties of Nanoscale Graphene Grain Boundaries.

The integrity of phonon transport properties of large graphene (linear and curved) grain boundaries (GBs) is investigated under the influence of structural and dynamical disorder. To do this, density functional tight‐binding (DFTB) method is combined with atomistic Green's function technique. The results show that curved GBs have lower thermal conductance than linear GBs. Its magnitude depends on the length of the curvature and out‐of‐plane structural distortions at the boundary, having stronger influence the latter one. Moreover, it is found that by increasing the defects at the boundary, the transport properties can strongly be reduced in comparison to the effect produced by heating up the boundary region. This is due to the large reduction of the phonon transmission for in‐plane and out‐of‐plane vibrational modes after increasing the structural disorder in the GBs.

Polarization Photoelectric Conversion in Layered GeS

AbstractPolarization photovoltaic effect is a unique character for an energy material with specific in‐plane anisotropy. Especially, if the energy material has a direct bandgap close to 1.6 eV, it will efficiently absorb full sunlight spectrum with specific axial polarization. In this study, polarized microtransmittance measurements of GeS multilayer with polarization angles ranging from θ = 0° (E || a) [through 90° (E || b)] to θ = 180° (E || a) have been studied near band edge. The polarized absorption edge follows a sinusoidal variation of E(θ) = 1.6 + 0.05⋅|sin(θ)| eV with respect to the angle change of the polarized absorption spectra. This anisotropic optical response is well reproduced by first‐principles calculations based on a combined Green's function technique, the GW–Bethe–Salpeter equation (BSE) approach. To characterize highly anisotropic band structure of layered GeS, polarized thermoreflectance measurement and first‐principles quasiparticle band‐structure calculations are also carried out. The interband transitions belonging to E ||a and E ||b polarizations are respectively identified. The polarized surface photovoltaic effects of a GeS Schottky solar cell are also tested. The special in‐plane optical anisotropy (along a and perpendicular to the a axis) renders GeS owning highly bi‐axial responsivity with respect to the c‐plane photoelectric conversion.

Electronic correlation effects on the Curie temperature in double perovskites

In order to understand the role of electronic parameters controlling the half-metallic ground state and high Curie temperature in double perovskites we study the ferromagnetic compound Sr2FeMO6, where M is a transition metal. Here, we present a theoretical electronic approach to study the thermodynamic properties of these perovskites using the Green function technique and the renormalization perturbation expansion method. Our approach is based on an interacting picture of localized Fe-electrons and conduction M-electrons via a double-exchange-type mechanism and taking into account electronic correlation. Our results show an increase of the Curie temperature together with a shift to high band filling in good agreement with experimental results.

Microscopic tight-binding study of the interplay between on-site and inter-site Coulomb interactions in Graphene

It is realized that the Coulomb interaction plays a vital role in describing the magnetic properties of graphene-on-substrate. We attempt here to study the competition between on-site and inter-site repulsive Coulomb interactions, with impurities at both the sub-lattices in graphene-on-substrate .The Hamiltonian describes the electron hopping up to third-nearest-neighbors within tight-binding approximation. Due to substrate effect, A-sub lattice is raised by energy and B-sub lattice acquired energy , besides impurity effects at both the sub lattices. The model consists of on-site repulsive Coulomb interaction at two sublattices with same on-site Coulomb energy U and nearest-neighbor inter-site Coulomb interactions between A and B sub lattice with inter site Coulomb energy . The Coulomb interaction is considered within Hartree-Fock type mean-field approximation. The Hamiltonian is solved by Zubarev's Green's function technique and an expression for temperature dependent difference in electron occupancies between two sub-lattices is calculated and solved self-consistency. Finally the charge gap and hence modified substrate induced gap is computed numerically by varying different physical parameters of the system like on-site and inter-site Coulomb interactions and substrate induced gap, impurity concentration and electron occupancy. It is observed that inter-site Coulomb interaction is smaller than on-site Coulomb interaction and plays an important role in describing the properties of graphene.

Charged impurity-tuning of midgap states in biased Bernal bilayer black phosphorus: an anisotropic electronic phase transition

In this paper, we analytically investigate the electronic density of states (DOS) of bilayer Bernal black phosphorus (BBP) in order to study the anisotropic electronic phase transition. We employ the Green's function approach, the Born approximation, and the tight-binding Hamiltonian model including ten intra-layer and four inter-layer hopping energies. BBP consisting of two coupled layers of black phosphorus is a suitable candidate for studying the layer-dependent electronic properties of few-layer black phosphorus. We examine the electronic properties of BBP under conditions in which only one layer and both layers are subjected to a dilute charged impurity and a perpendicular electric field. Our findings show that there is no phase transition when the impurity is doped on only one layer, whereas in the case of both layers, BBP suffers a phase transition from semiconductor to semimetal at strong impurity scattering potentials. Also, applying the electric field on one layer of BBP leads to an increase in the band gap, whereas in the case of both layers, the band gap decreases with the electric field and eventually, a phase transition appears at a bias voltage of more than 1.8 eV. Consequently, the band gap of BBP can be tuned by applying an electric field and a charged impurity, and thereby these findings provide insights for future experimental research on black phosphorus.

Adsorption of gas molecules on a manganese phthalocyanine molecular device and its possibility as a gas sensor.

A theoretical investigation of the gas detection performance of manganese(ii) phthalocyanine (MnPc) molecular junctions for six different gases (NO, CO, O2, CO2, NO2, and NH3) is executed through a non-equilibrium Green's function technique in combination with spin density functional theory. Herein, we systematically studied the adsorption structural configurations, the adsorption energy, the charge transfer, and the spin transport properties of the MnPc molecular junctions with these gas adsorbates. Remarkably, NO adsorption can achieve an off-state of the Mn spin; this may be an effective measure to switch the molecular spin. In addition, our results indicate that by measuring spin filter efficiency and the changes in total current through the molecular junctions, the CO, NO, O2, and NO2 gas molecules can be detected selectively. However, the CO2 and NH3 gas adsorptions are difficult to be detected due to weak van der Waals interaction between these two gases and central Mn atom. Our findings provide important clues to the application of nanosensors for highly sensitive and selective based on MnPc molecular junction systems.

Tunable- φ Josephson junction with a quantum anomalous Hall insulator

We theoretically study the Josephson current in a superconductor/quantum anomalous Hall insulator/superconductor junction by using the lattice Green function technique. When an in-plane external Zeeman field is applied to the quantum anomalous Hall insulator, the Josephson current $J$ flows without a phase difference across the junction $\theta$. The phase shift $\varphi$ appealing in the current-phase relationship $J\propto \sin(\theta-\varphi$) is proportional to the amplitude of Zeeman fields and depends on the direction of Zeeman fields. A phenomenological analysis of the Andreev reflection processes explains the physical origin of $\varphi$. A quantum anomalous Hall insulator breaks time-reversal symmetry and mirror reflection symmetry simultaneously. However it preserves magnetic mirror reflection symmetry. Such characteristic symmetry property enable us to have a tunable $\varphi$-junction with a quantum Hall insulator.

Coherent control of the route of magnetic phases in quasi-1D armchair graphene nanoribbons via doping in the presence of electronic correlations

In this work, we show that the magnetic phase transition in both semiconducting and metallic armchair graphene nanoribbons would be observed in the presence of electronic dopant. However, the mutual interactions between electrons are also considered based on theoretically tight-binding and Hubbard model calculations considering nearest neighbors within the framework of Green's function technique. This work showed that charge concentration of dopant in such system depending on the weak and strong mutual repulsions plays a crucial role in determining the magnetic phase. It follows from the obtained results that the ground state turns paramagnetic in a range of carrier concentrations by neglecting the electronic correlations. The inclusion of a Coulombic repulsion between electrons stops the phase transition and system remains in its ground state antiferromagnetic phase. Furthermore, we concluded that magnetic phases are insensitive to the electron-electron interaction at all weak and strong concentrations of dopant. In addition, this paper provides a controllable gap engineering by doping and inclusion of electron-electron repulsions for further studies on such system as a new potential nanomaterial for magnetic graphene nanoribbon-based applications.

Electroelastic ellipsoidal inclusion with imperfect interface and its application to piezoelectric composite materials

In this work, a new micromechanical model is presented to determine the effective behavior of piezoelectric composite materials with ellipsoidal reinforcements and imperfect interfaces. The integral equation is obtained by the Green's function technique applied to the electroelastic heterogeneous problem. This integral equation is further solved in the case of an ellipsoidal inclusion embedded in an infinite matrix with imperfect interface. By introducing the concept of interior and exterior-point Eshelby tensors, the solution of the inclusion problem is given in the case of displacement and electric fields discontinuities across the interface between inclusion and matrix. The concentration equation is expressed in anisotropic electroelasticity for ellipsoidal inclusions with imperfect interface. This equation is exact in the case of linear spring interface model for spherical and cylindrical inclusions. We compare the concentration tensors to the results given by exact model for cylindrical inclusion and in-plane loading. The Mori-Tanaka homogenization scheme is used to determine the effective electroelastic moduli of piezoelectric composite materials. In the case of spherical and cylindrical inclusions, the effective electroelastic moduli are compared to other models and experimental results. The combined influence of shape of inclusions and interface parameters is analyzed on the effective electroelastic behavior.

Thermomechanical modelling of dry friction at high velocity applied to a Ti6Al4V-Ti6Al4V tribopair

Abstract A semi-analytical model is developed to predict thermal and mechanical responses of the contact surface during the dry friction of Ti6Al4V tribopair at high velocities. This model is achieved by solving thermal and mechanical problems separately, before coupling both interdependent solutions using the dissipation of mechanical work into heat. The thermal problem is solved by Green's function techniques, while the mechanical one is treated by considering the adiabatic shearing process of contact asperities. The accuracy of this new semi-analytical model is first evaluated by comparison with the results of a two-dimensional numerical finite element simulation. Then, the proposed model is adapted to the specific dry friction process considered in this study by means of experimental testing with post-mortem microstructural analysis.

On the intra- and interband plasmon modes in doped armchair graphene nanoribbons

With the help of the simple tight-binding Hamiltonian and Green's function technique, we study how intraband and interband plasmon modes of both semiconducting and metallic armchair graphene nanoribbons are influenced by the width, chemical doping, and incident momentum direction. In particular, we investigate the behavior of the frequency-dependent susceptibility when the system is exposed to photons or electrons. Injecting electrons by doping creates a new collective mode due to new states between the valence and conduction bands corresponding to intraband transition for which the effect of ribbon width on these transitions in the semiconducting case is much more sensitive than metallic ones. Furthermore, some critical chemical potential and momentum values for both intraband and interband modes lead to different behaviors for resonant peaks. Another remarkable point is the high sensitivity of intraband plasmons to the direction of incident momentum. In particular, the susceptibility of doped nanoribbons vanishes at perpendicular directions, i.e., the intraband plasmons disappear.

Simulations of Coulomb systems confined by polarizable surfaces using periodic Green functions.

We present an efficient approach for simulating Coulomb systems confined by planar polarizable surfaces. The method is based on the solution of the Poisson equation using periodic Green functions. It is shown that the electrostatic energy arising from the surface polarization can be decoupled from the energy due to the direct Coulomb interaction between the ions. This allows us to combine an efficient Ewald summation method, or any other fast method for summing over the replicas, with the polarization contribution calculated using Green function techniques. We apply the method to calculate density profiles of ions confined between the charged dielectric and metal surfaces.

Electronic and transport properties of BCN alloy nanoribbons

The dependence of the carbon (C) concentration on the electronic and transport properties of boron carbonitride (BCN) alloy nanoribbons have been investigated using surface Green's functions technique and random Hamiltonian model by considering random hopping parameters including first and second nearest neighbors. Our calculations indicate that substituting boron (nitrogen) sites with carbon atoms induces a new band close to conduction (valence) band and carbon atoms behave like a donor (acceptor) dopants. Also, while both nitrogen and boron sites are substituted randomly by carbon atoms, new bands are induced close to both valence and conduction bands. The band gap decreases with C substituting and the number of charge carriers increases in low bias voltage. Far from Fermi level in the higher range of energy, transmission coefficient and current of the system are reduced by increasing the C concentration. Based on our results, tuning the electronic and transport properties of BCN alloy nanoribbons by random carbon dopants could be applicable to design nanoelectronics devices.