Anisotropy-driven quantum capacitance in multi-layered black phosphorus

We report analytic results on quantum capacitance (C$_{q}$) measurements and their optical tuning in dual-gated device with potassium-doped multi-layered black phosphorous (BP) as the channel material. The two-dimensional (2D) layered BP is highly anisotropic with a semi-Dirac dispersion marked by linear and quadratic contributions. The C$_{q}$ calculations mirror this asymmetric arrangement. A further increase to the asymmetry and consequently C$_{q}$ is predicted by photon-dressing the BP dispersion. To achieve this and tune C$_{q}$ in a field-effect transistor (FET), we suggest a configuration wherein a pair of electrostatic (top) and optical (back) gates clamp a BP channel. The back gate shines an optical pulse to rearrange the dispersion of the 2D BP. Analytic calculations are done with Floquet Hamiltonians in the off-resonant regime. The value of such C$_{q}$ calculations, in addition, to its role in adjusting the current drive of an FET is discussed in context of metal-insulator and topological phase transitions and enhancements to the thermoelectric figure of merit.Comment: 4 pages, 3 figure

The tuning of light-matter coupling and dichroism in graphene for enhanced absorption: Implications for graphene-based optical absorption devices

The inter-band optical absorption in graphene characterized by its fine-structure constant has a universal value of 2.3\% independent of the material parameters. However, for several graphene-based photonic applications, enhanced optical absorption in graphene is highly desired. In this work, we quantify the tunability of optical absorption in graphene via the Fermi level in graphene, angle of incidence of the incident polarized light, and the dielectric constant of the surrounding dielectric media in which graphene is embedded. The influence of impurities adsorbed on the surface of graphene on the Lorentzian broadening of the spectral function of the density of states is analytically evaluated within the equilibrium Green's function formalism. Finally, we compute the differential absorption of right and left circularly-polarized light in graphene that is uniaxially and optically strained. The preferential absorption or circular dichroism is investigated for armchair and zigzag strain.Comment: 12 pages, 11 figure

Spin-valley coupled thermoelectric energy converter with strained honeycomb lattices

A caloritronic device setup is proposed that harnesses the intrinsic spin-valley locking of two-dimensional honeycomb lattices with graphene-like valleys, for instance, silicene and stanene. Combining first-principles and analytic calculations, we quantitatively show that when sheets of such materials are placed on a ferromagnetic substrate and held between two contacts at different temperatures, an interplay between the electron degrees-of-freedom of charge, spin, and valley arises. A manifestation of this interplay are finite charge, spin, and valley currents. Uniaxial strain that adjusts the buckling height in silicene-type of lattices, in conjunction with an applied electric field, is shown to further modulate the aforementioned currents. We link these calculations to a Seebeck-like thermopower generator and obtain expressions (and means to optimize them) for two spin-valley polarized performance metrics--the thermodynamic efficiency and thermoelectric figure of merit. A closing summary outlines possible enhancements to presented results through the inherent topological order and substrate-induced external Rashba spin-orbit coupling that exists in silicene-type materials.Comment: 9 pages, 6 figure

Gate-Voltage Tunability of Plasmons in Single and Multi-layer Graphene Structures: Analytical Description and Concepts for Terahertz Devices

The strong light-matter interaction in graphene over a broad frequency range has opened up a plethora of photonics applications of graphene. The goal of this paper is to present the voltage tunability of plasmons in gated single- and multi-layer graphene structures. Device concepts for plasmonic interconnects and antennas and their performance for THz communication are presented. For the first time, the role of gate voltage and the thickness of the gate dielectric on the characteristics of plasmon propagation in graphene are quantified by accounting for both the interface trap capacitance and the quantum capacitance. The gate voltage serves as a powerful knob to tweak the carrier concentration and allows building electrically reconfigurable terahertz devices. By optimizing the gate voltage to maximize the plasmon propagation length in a gated multi-layer graphene geometry, we derive simple scaling trends that give intuitive insight into device modeling and design.Comment: 10 pages, 11 figure

Phenomenological description of the dynamics of bipartite antiferromagnets in the limit of strong exchange

The equation of motion of the staggered order parameter is derived in a step-by-step manner from the coupled Landau-Lifshitz-Gilbert dynamics of bipartite spin moments in the limit of strong antiferromagnetic exchange coupling.Comment: 8 page

Voltage Tunable Plasmon Propagation in Dual Gated Bilayer Graphene

In this paper, we theoretically investigate plasmon propagation characteristics in AB and AA stacked bilayer graphene (BLG) in the presence of energy asymmetry due to an electrostatic field oriented perpendicularly to the plane of the graphene sheet. We first derive the optical conductivity of BLG using the Kubo formalism incorporating energy asymmetry and finite electron scattering. All results are obtained for room temperature (300K) operation. By solving Maxwell's equations in a dual gate device setup, we obtain the wavevector of propagating plasmon modes in the transverse electric (TE) and transverse magnetic (TM) directions at terahertz frequencies. The plasmon wavevector allows us to compare the compression factor, propagation length, and the mode confinement of TE and TM plasmon modes in bilayer and monolayer graphene sheets and also study the impact of material parameters on plasmon characteristics. Our results show that the energy asymmetry can be harnessed to increase the propagation length of TM plasmons in BLG. AA stacked BLG shows a larger increase in propagation length than AB stacked BLG; conversely, it is very insensitive to the Fermi level variations. Additionally, the dual gate structure allows independent modulation of the energy asymmetry and the Fermi level in BLG, which is advantageous for reconfiguring plasmon characteristics post device fabrication.Comment: 19 pages, 13 figure

A Probability-Density Function Approach to Capture the Stochastic Dynamics of the Nanomagnet and Impact on Circuit Performance

In this paper we systematically evaluate the variation in the reversal delay of a nanomagnet driven by a longitudinal spin current while under the influence of thermal noise. We then use the results to evaluate the performance of an All-Spin-Logic (ASL) circuit. First, we review and expand on the physics of previously-published analytical models on stochastic nanomagnet switching. The limits of previously established models are defined and it is shown that these models are valid for nanomagnet reversal times < 200 ps. Second, the insight obtained from previous models allows us to represent the probability density function (PDF) of the nanomagnet switching delay using the double exponential function of the Frechet distribution. The PDF of a single nanomagnet is extended to more complex nanomagnet circuit configurations. It is shown that the delay-variation penalty incurred by nanomagnets arranged in parallel configuration is dwarfed by the average delay increase for nanomagnets arranged in a series configuration. Finally, we demonstrate the impact of device-level performance variation on the circuit behavior using ASL logic gates. While the analysis presented in this paper uses an ASL-AND gate as the prototype switching circuit in the spin domain, the physical concepts are generic and can be extended to any complex spin-based circuit

The optical response of mono-layer transition metal dichalcogenides in a Kerr-type non-linear dielectric environment

We study the optical behaviour of an arrangement in which the interface between a linear and non-linear dielectric media is covered by an embedded mono-layer of transition metal dichalcogenides (TMDC). The optical behaviour is qualitatively obtained through transmission and reflection coefficients which are a function of the third order non-linear susceptibility of the Kerr-type dielectric and the inter-band optical conductivity of the TMDC mono-layer. The inter-band optical conductivity of the TMCD mono-layer is calculated using the Kubo formalism from linear response theory. In particular, we theoretically demonstrate that the optical response of this arrangement can be switched between total internal reflection and a normal transmission regime by controlling the intensity of the incident radiation. The reflection and transmission functions, additionally, are shown to be amenable to further control by altering the inter-band optical conductivity of the embedded TMDC mono-layer. The optical conductivity is an outcome of its energy dispersion; we specifically choose two TMDC mono-layers, MoS$_{2}$ and WSe$_{2}$, which have nearly identical dispersion parameters apart from a much stronger spin-orbit coupling in the latter. The stronger spin-orbit coupling in WSe$_{2}$ does not significantly alter the inter-band optical conductivity to manifest in an enhanced reflection spectrum. However, we find that application of an external perturbation such as strain could be effectively used to modulate the overall optical response. We conclude by offering a brief overview of the applicability of our proposed scheme in devices that employ an all-optical switching mechanism through optical bistability which is the hallmark of a non-linear dielectric.Comment: 12 pages, 8 figure

Voltage-Controlled Topological-Spin Switch for Ultra-Low-Energy Computing--Performance Modeling and Benchmarking

A voltage-controlled topological-spin switch (vTOPSS) that uses a hybrid topological insulator-magnetic insulator multiferroic is presented that can implement Boolean logic operations with sub-10 aJ energy-per-bit and energy-delay product on the order of $10^{-27}$ Js. The device uses a topological insulator (TI), which has the highest efficiency of conversion of electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator (MI) that can respond rapidly to a given spin torque. We present the theory of operation of vTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark vTOPSS against existing spin-based and CMOS devices. Compared to existing spin-based devices, such as all-spin logic and charge-spin logic, vTOPSS offers 100$\times$ lower energy dissipation and (40-100)$\times$ lower energy-delay product. With experimental advances and improved material properties, we show that the energy-delay product of vTOPSS can be lowered to $10^{-29}$ Js, competitive against existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for vTOPSS, implying that highly resistive materials can indeed be used to interconnect vTOPSS devices.Comment: 12 pages, 10 figure

An analytic virtual-source-based current-voltage model for ultra-thin black phosphorus field-effect transistors

In this paper, we develop an analytic physics-based model to describe current conduction in ultra-thin black phosphorus (BP) field-effect transistors (FETs). The model extends the concept of virtual source charge calculation to capture the effect of both hole and electron charges for ambipolar transport characteristics. The model comprehends the in-plane band-structure anisotropy in BP, as well as the asymmetry in electron and hole current conduction characteristics. The model also includes the effect of Schottky-type source/drain contact resistances, which are voltage-dependent and can significantly limit current conduction in the on-state in BP FETs. Model parameters are extracted using measured data of back-gated BP transistors with gate lengths of 1000 nm and 300 nm with BP thickness of 7.3 nm and 8.1 nm, and for the temperature range of 180 K to 298 K. Compared to previous BP models that are validated only for room-temperature and near-equilibrium bias conditions (low drain-source voltage), we demonstrate excellent agreement between the data and model over a broad range of bias and temperature values. The model is also validated against numerical TCAD data of top-gated BP transistors with a channel length of 300 nm. The model is implemented in Verilog-A and the capability of the model to handle both dc and transient circuit simulations is demonstrated using SPECTRE. The model not only provides a physical insight into technology-device interaction in BP transistors, but can also be used to design and optimize BP-based circuits using a standard hierarchical circuit simulator