Ultrafast switching in spin field-effect transistors based on borophene nanoribbons.

Borophene, owing to the high mobility and long spin coherent length of its carriers, presents significant opportunities in ultrafast spintronics. In this research, we investigate the spin-dependent conductance of a Datta-Das field-effect transistor (FET) based on an armchair ß12-borophene nanoribbon (BNR) using the tight-binding (TB) Hamiltonian in combination with the non-equilibrium Green's function (NEGF) method. The spin FET electrodes are magnetized by ferromagnetic (FM) insulators arranged in both parallel and anti-parallel configurations. This device acts as a controllable spin filter in the presence of Rashba spin-orbit coupling (SOC) for both configurations and its spin current is well modulated by a gate voltage and the strength of the Rashba SOC. For anti-parallel configurations, an energy gap emerges within a certain range of incoming electron energy which can disappear for electrons with flipped spin under the Rashba SOC. Furthermore, our findings indicate that the electron-electron (e-e) interaction helps the spin precession of electrons injected into the spin FET channel, thereby strengthening the Rashba SOC effect. Notably, a gate voltage can adjust the current-voltage (I-V) characteristics of this device. Finally, our calculations demonstrate that under the same conditions, the current magnitude and Ion/Ioff ratio of borophene spin FETs are several times higher than those of graphene and silicene spin FETs.

Transition metal dichalcogenide coated gold nanoshells for highly effective photothermal therapy.

Transition metal dichalcogenides (TMD) coated gold nanoshells (GNSs), in addition to having low cytotoxicity and a biocompatibility value greater than graphene, exhibit strong light absorption in the near-infrared (NIR) region and high photothermal conversion efficiency. Using a quasi-static approach and bioheat equations, the optical and photothermal properties of GNSs coated with various TMDs are studied for treatment of skin cancer. Our findings show that the intensity of localized surface plasmon resonance (LSPR) peaks and their position in the extinction spectrum of nanoparticles (NPs) can be easily tuned within biological windows by varying the core radius, the gold shell thickness and the number of coating layers of the different TMDs. In order to engineer heat production at designated spatial locations of NPs, near electric field (NEF) enhancement is investigated. Moreover, the effect of laser intensity and the number of TMD layers on the temperature rise and the amount of thermal damage in skin tumor tissue and its surroundings are studied. Our results introduce GNSs with various TMD coatings as superlative nanoagents for photothermal therapy (PTT) applications.

Topological spintronics in a polyacetylene molecule device.

Using the Su-Schrieffer-Heeger Hamiltonian and exploiting the Green's function method in the framework of the Landauer-Büttiker formalism, the topological and spin dependent electron transport properties of a trans polyacetylene molecule are studied. It is found that molecules with the intracell single carbon-carbon bonding and the even number of monomers in their chains have two edge states and possess topological properties though their Hamiltonians do not respect the chiral symmetry. A perpendicular exchange magnetic field and two perpendicular and transverse electric fields are used to induce and manipulate the quantum spin dependent electron transport properties. The exchange field induces the spin polarization in different electron energy regions which are expanded by stronger exchange fields. Therefore this proposed device works as a perfect spin filter. The spin polarization can be manipulated by applying the perpendicular electric field and remains robust against the transverse electric field variations.

Tight-binding Hamiltonian considering up to the third nearest neighbours for trans polyacetylene.

Utilizing the linear combination of atomic orbitals in the Slater-Koster approach in combination with the density functional theory band structure data, a new tight-binding Hamiltonian up to the third nearest neighbours for the dimerized trans polyacetylene is proposed. The effects of strain are also considered in the Hamiltonian by varying the distance between two successive CH groups along the molecular symmetry axis. Using this new Hamiltonian and exploiting the Green's function method in the framework of the Landauer-Büttiker formalism, the electronic transport properties in a trans polyacetylene chain in the presence and absence of strain are studied. It is shown that at a peculiar value of compression strain, the electron conductance shifts 0.27 eV in energy which is an exploitable magnitude for straintronic applications of the trans polyacetylene specially as strain sensors and strain switches.

Graphene coated gold nanoparticles: an emerging class of nanoagents for photothermal therapy applications.

Graphene coated gold nanoparticles (GGNPs) have attracted great attention in recent years because of their high thermal stability and unique optical properties. In this paper, we study photothermal properties of GGNPs using the Mie and Gans theories combined with the Pennes bioheat equation. The effect of various sizes and different shapes of GGNPs such as nanosphere, nanorod and nanodisc are taken into account. The extinction efficiency and temperature distribution in tumor tissue show that graphene coated gold nanorods, because of the high temperature rise during laser irradiation, are more suitable candidates for photothermal therapy (PTT) applications. Also, we show that the extinction peak of graphene coated gold nanorods can be adjusted in the biological windows by increasing the graphene shell thickness and/or by changing their aspect ratio. Finally, we investigated the effect of the number of graphene layers upon the temperature rise in the tumor and found that the temperature rise increases with increasing number of graphene layers. Our findings introduce a new class of nanoagents which can be used in PTT applications.

Extrinsic Rashba spin-orbit coupling effect on silicene spin polarized field effect transistors.

Regarding the spin field effect transistor (spin FET) challenges such as mismatch effect in spin injection and insufficient spin life time, we propose a silicene based device which can be a promising candidate to overcome some of those problems. Using non-equilibrium Green's function method, we investigate the spin-dependent conductance in a zigzag silicene nanoribbon connected to two magnetized leads which are supposed to be either in parallel or anti-parallel configurations. For both configurations, a controllable spin current can be obtained when the Rashba effect is present; thus, we can have a spin filter device. In addition, for anti-parallel configuration, in the absence of Rashba effect, there is an intrinsic energy gap in the system (OFF-state); while, in the presence of Rashba effect, electrons with flipped spin can pass through the channel and make the ON-state. The current voltage (I-V) characteristics which can be tuned by changing the gate voltage or Rashba strength, are studied. More importantly, reducing the mismatch conductivity as well as energy consumption make the silicene based spin FET more efficient relative to the spin FET based on two-dimensional electron gas proposed by Datta and Das. Also, we show that, at the same conditions, the current and [Formula: see text] ratio of silicene based spin FET are significantly greater than that of the graphene based one.

Gain equation for a free-electron laser with a helical wiggler and ion-channel guiding.

The theory of ion-channel guiding in a helical wiggler is presented. Electron motion in the combined ion electrostatic and wiggler magnetostatic fields is analyzed in the absence of the radiation field. The Phi function that determines the rate of change of axial velocity with energy is derived and studied numerically. A detailed analysis of the pendulum equation and the gain equation in the low-gain-per-pass limit are presented. It is shown that the gain for stable group I orbits is positive, while for group II orbits the gain is negative in the negative mass regime and positive in the positive mass regime.