Mechanism behind the switching of current induced by a gate field in a semiconducting nanowire junction

We propose an orbital-controlled model to explain the gate field induced switching of current in a semiconducting PbS-nanowire junction. A single-particle scattering formalism in conjunction with a posteriori density-functional approach involving a hybrid functional is used to study the electronic current; both first- and higher-order Stark effects are explicitly treated in our model. Our calculation reveals that after a threshold gate voltage, orbital mixing produces p components at the S atoms in the participating orbitals. This results in an interlayer orbital interaction that allows electrons to delocalize along the channel axis. As a consequence, a higher conductance state is found. A similar feature is also found in a PbSe nanowire junction, which suggests that this model can be used universally to explain the gate field induced switching of current in lead-chalcogenide nanowire junctions

Tuning the ferromagnetism of one-dimensional Fe ∕ Pt ∕ Fe multilayer barcode nanowires via the barcode layer effect

Using first-principles density functional theory, we have predicted equilibrium structures and electronic and magnetic properties of one-dimensional ferromagnetic Fe∕Pt∕Fe multilayer barcode nanowires. By increasing the thickness of the Pt layer and consequently reducing the thickness of the Fe layer in the nanowire in the ferromagnetic configuration, we found that the average magnetic moment per iron atom, μav, increases monotonically with an ∼1∕N(Fe) dependence, where N(Fe) is the number of Fe layers. The monotonic increase in average magnetic moment is attributed to the change in magnetic moment at the Fe-Pt interface, and a simple model is proposed to explain this ∼1∕N(Fe) variation of μav in the barcode wires. Modulation of the ferromagnetism based on the number of ferromagnetic and nonmagnetic layer sequences in the nanowire suggests the possible application of these nanowires in nanometer scale magnetic barcodes. Furthermore, analysis of the Kohn-Sham energy bands in barcode nanowires suggests strong dependence of spin-polarized conductance on the nonmagnetic Pt spacer layer thickness, opening up the possibility for their application in magnetoelectronics or spintronics

Controlling interlayer exchange coupling in one-dimensional Fe/Pt multilayered nanowire

We report a first-principles density-functional study of interlayer exchange coupling (IEC) in one-dimensional Fe/Pt multilayered nanowire. The magnetic moment of the interfacial Fe atom in the Fe/Pt multilayered nanowire is found to be higher than that of the Fe atom away from the interface. A mechanism based on multistep electron transfer between the layers and spin flip within the layer is proposed to explain the magnetic-moment enhancement at the interface. The calculated IEC and magnetoresistance are found to switch signs as the width of the nonmagnetic Pt spacer varies. Depending on the width of the Pt spacer, the competition among short- and long-range direct exchanges, indirect Ruderman-Kittel-Kasuya-Yosida exchange, and superexchange is found to be responsible for the nonmonotonous feature in IEC

First-principles study of the variation of electron transport in a single molecular junction with the length of the molecular wire

We report a first-principles study of quantum transport in a prototype two-terminal device consisting of a molecular nanowire acting as an inter-connect between two gold electrodes. The wire is composed of a series of bicyclo[1.1.1]pentane (BCP) cage-units. The length of the wire (L) is increased by sequentially increasing the number of BCP cage units in the wire from 1 to 3. A two terminal model device is made out of each of the three wires. A parameter free, nonequilibrium Green’s function approach, in which the bias effect is explicitly included within a many body framework, is used to calculate the current-voltage characteristics of each of the devices. In the low bias regime that is considered in our study, the molecular devices are found to exhibit Ohmic behavior with resistances of 0.12, 1.4, and 6.5 μΩ for the wires containing one, two, and three cages respectively. Thus the conductance value, Gc, which is the reciprocal of resistance, decreases as e−βL with a decay constant (β) of 0.59 Å−1. This observed variation of conductance with the length of the wire is in excellent agreement with the earlier reported exponential decay feature of the electron transfer rate predicted from the electron transfer coupling matrix values obtained using the two-state Marcus-Hush model and the Koopman’s theorem approximation. The downright suppression of the computed electrical current for a bias up to 0.4 V in the longest wire can be exploited in designing a three terminal molecular transistor; this molecular wire could potentially be used as a throttle to avoid leakage gate current

Codoping in a single molecular junction from first principles

Using a codoping model, where a cation and an anion are introduced simultaneously into the host to maintain charge neutrality, we probed the electron transport characteristics in a single molecular junction. We used the 1, 12-dicarba-closo-dodecaborane inorganic molecule as a precursor, replaced one of the vertex carbon atoms by a boron atom, and simultaneously decorated it with an endohedrally doped alkali atom (Li or Na) to look into the role of dopant atoms in the conductivity. The commonly used thiolate anchoring groups are used to attach the molecule between two gold electrodes, and a parameter-free, first-principles, single-particle Green’s function approach is used to calculate the current-voltage characteristics. When compared to the undoped system, a significant increase in current is observed for the system codoped with Na and B; about an order of magnitude increase in the observed current is found at an applied bias of ~2 V. Charge transfer from the alkali atom to the host is found to have a profound effect on the electronic structure, causing a dramatic change in the conductivity. Since the single alkali atom controls the behavior of electron flow in this circuit, we call this device a “single-atom-controlled” device

Giant amplification of tunnel magnetoresistance in a molecular junction: Molecularspin-valve transistor

Amplification of tunnel magnetoresistance by gate field in a molecular junction is the most important requirement for the development of a molecular spin valve transistor. Herein, we predict a giant amplification of tunnel magnetoresistance in a single molecular spin valve junction, which consists of Ru-bis-terpyridine molecule as a spacer between two ferromagnetic nickelcontacts. Based on the first-principles quantum transport approach, we show that a modest change in the gate field that is experimentally accessible can lead to a substantial amplification (320%) of tunnel magnetoresistance. The origin of such large amplification is attributed to the spin dependent modification of orbitals at the molecule-lead interface and the resultant Stark effect induced shift in channel position with respect to the Fermi energ

Magnetic properties of one-dimensional Ni/Cu and Ni/Al multilayered nanowires: Role of nonmagnetic spacers

We have used density functional theory within spin-polarized local density approximation to investigate the equilibrium structure, electronic, and magnetic properties of one-dimensional Ni/Cu and Ni/Al multilayered nanowires. In particular, we look into the subtle changes in the magnetic properties of the nanowires with the change in the width of the nonmagnetic spacer. Our calculations yield the magnitude of cohesive energy in both the systems to decrease with the increase in the concentration of the nonmagnetic spacer, suggesting that Ni rich nanowires are more stable. Analysis of the magnetic moment per Ni atom (μav) in the Ni/Cu hybrid multilayered nanowire suggests that there is a steady decrease in μav with the increase in the number of Cu layers. In contrast, in Ni/Al multilayered nanowire, there is a nonmonotonic decrease in μav with the increase in Al layers. The observed difference in magnetic property between Ni/Cu and Ni/Al multilayered nanowires is attributed to the dissimilar interfacial bonding in the two cases. In the case of Ni/Al nanowire, the nonmonotonic variation in μav is due to the strong directional nature of the Ni d and Al p hybridization, which favors Ni to have higher coordination number. Higher coordination for Ni leads to smaller μav in the Ni/Al multilayered nanowire. However, the hybridization between Ni d and Cu s states is predominantly responsible for the smaller μav in the Ni/Cu nanowire. Furthermore, we found that in Ni/Al multilayered nanowire with two Al spacer layer, the antiferromagnetic configuration is favored over ferromagnetic configuration. In Ni/Cu multilayered nanowire, ferromagnetic configuration is favored over antiferromagnetic configuration for the same spacer lengt

Boron nitride nanotubes for spintronics

With the end of Moore\u27s law in sight, researchers are in search of an alternative approach to manipulate information. Spintronics or spin-based electronics, which uses the spin state of electrons to store, process and communicate information, offers exciting opportunities to sustain the current growth in the information industry. For example, the discovery of the giant magneto resistance (GMR) effect, which provides the foundation behind modern high density data storage devices, is an important success story of spintronics; GMR-based sensors have wide applications, ranging from automotive industry to biology. In recent years, with the tremendous progress in nanotechnology, spintronics has crossed the boundary of conventional, all metallic, solid state multi-layered structures to reach a new frontier, where nanostructures provide a pathway for the spin-carriers. Different materials such as organic and inorganic nanostructures are explored for possible applications in spintronics. In this short review, we focus on the boron nitride nanotube (BNNT), which has recently been explored for possible applications in spintronics. Unlike many organic materials, BNNTs offer higher thermal stability and higher resistance to oxidation. It has been reported that the metal-free fluorinated BNNT exhibits long range ferromagnetic spin ordering, which is stable at a temperature much higher than room temperature. Due to their large band gap, BNNTs are also explored as a tunnel magneto resistance device. In addition, the F-BNNT has recently been predicted as an ideal spin-filter. The purpose of this review is to highlight these recent progresses so that a concerted effort by both experimentalists and theorists can be carried out in the future to realize the true potential of BNNT-based spintronics