Finite Carrier Research Articles

Nanoscale Optical Conductivity Imaging of Double-Moiré Twisted Bilayer Graphene.

A central paradigm of moiré materials relies on the formation of superlattices that yield enlarged effective crystal unit cells. While a critical consequence of this phenomenon is the celebrated flat electronic bands that foster strong interaction effects, the presence of superlattices has further implications. Here we explore the advantages of moiré superlattices in twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (hBN) for passively enhancing optical conductivity in the low-energy regime. To probe the local optical response of TBG/hBN double-moiré lattices, we use infrared (IR) nano-imaging in conjunction with nanocurrent imaging to examine local optical conductivity over a wide range of TBG twist angles. We show that interband transitions associated with the multiple moiré flat and dispersive bands produce tunable transparent IR responses even at finite carrier densities, which is in stark contrast to the previously limited metallic near transparency observed only in undoped pristine graphene.

Relationships between two linearizations of the box-ball system: Kerov–Kirillov–Reschetikhin bijection and slot configuration

Abstract The box-ball system (BBS), which was introduced by Takahashi and Satsuma in 1990, is a soliton cellular automaton. Its dynamics can be linearized by a few methods, among which the best known is the Kerov–Kirillov–Reschetikhin (KKR) bijection using rigged partitions. Recently, a new linearization method in terms of ‘slot configurations’ was introduced by Ferrari–Nguyen–Rolla–Wang, but its relations to existing ones have not been clarified. In this paper, we investigate this issue and clarify the relation between the two linearizations. For this, we introduce a novel way of describing the BBS dynamics using a carrier with seat numbers. We show that the seat number configuration also linearizes the BBS and reveals explicit relations between the KKR bijection and the slot configuration. In addition, by using these explicit relations, we also show that even in case of finite carrier capacity the BBS can be linearized via the slot configuration.

Identification and study of the characteristics of friction oscillations in the brake

Purpose.The task of researching modes of established frictional oscillations of the braking mechanism is to find a solution to the initial dynamic problem with friction that satisfies the periodicity conditions. At the same time, the period of motion of the dynamic system is not known in advance. This dynamic system is described by a non-linear dissipative non-autonomous system of differential equations. The methods. The developed technique of spectral analysis of the braking mechanism's oscillations is based on the assumption that its movements are periodic. If deterministic chaos occurs in the analyzed dynamic system, then the autocorrelation function of the time series of movements must have a finite carrier, that is, vanish outside a finite time interval. Findings. In the paper, the method of computational experiment is used to identify and study the characteristics of oscillatory processes in brake mechanisms. At the first stage of the computational experiment, a numerical solution of the considered dynamic problem with friction is carried out using a computational algorithm. As a result, the time series of block movements are calculated. At the second stage of the computational experiment, the obtained time series are studied. The originality. The paper uses phase diagrams in the "displacement-velocity" variables to analyze the process of establishing the oscillations of the brake mechanism and visual detection of attractors. When studying the dependence of amplitudes of displacements, velocities and accelerations of the dynamic system under consideration on changes in its parameters, the method of continuation by parameter was used with a stepwise change in the parameters of the system. Practical implementation. The developed mathematical model of vibrations of the braking mechanism and the computational algorithm for its numerical study are implemented in the form of a computer program for personal computers in the FORTRAN algorithmic language. Almost all available commercial compilers can be used to compile the program, including Compaq Visual Fortran 6.6 and Intel Visual Fortran 10, as well as non-commercial compilers distributed under the GNU license.

Ultrafast and Sensitive Self-Powered Photodetector Based on Graphene/Pentacene Single Crystal Heterostructure with Weak Light Detection Capacity.

Organic materials exhibit efficient light absorption and low-temperature, large-scale processability, and have stimulated enormous research efforts for next-generation optoelectronics. While, high-performance organic devices with fast speed and high responsivity still face intractable challenges, due to their intrinsic limitationsincluding finite carrier mobility and high exciton binding energy. Here an ultrafast and highly sensitive broadband phototransistor is demonstrated by integrating high-quality pentacene single crystal with monolayer graphene. Encouragingly, the -3dB bandwidth can reach up to 26kHz, which is a record-speed for such sensitized organic phototransistors. Enormous absorption, long exciton diffusion length of pentacene crystal, and efficient interfacial charge transfer enable a high responsivity of >105 AW-1 and specific detectivity of >1011 Jones. Moreover, self-powered weak-light detection is realized using a simple asymmetric configuration, and the obvious zero-bias photoresponses can be displayed even under 750nWcm-2 light intensity. Excellent response speed and photoresponsivity enable high-speed image sensor capability in UV-Vis ranges. The results offer a practical strategy for constructing high-performance self-powered organic hybrid photodetectors, with strong applicability in wireless, weak-light detection, and video-frame-rate imaging applications.

Unidirectional valley-contrasting photocurrent in strained transition metal dichalcogenide monolayers

We examine the full static non-linear optical response of uniaxially strained transition metal dichalcogenide monolayers doped with a finite carrier density in the conduction band, in the presence of disorder. We find that the customary shift current is suppressed, yet we identify a strong, valley-dependent non-reciprocal response, which we term a \textit{unidirectional valley-contrasting photo-current} (UVCP). This DC current originates from the combined effect of strain and Kramers symmetry breaking by trigonal warping, while the contributions due to individual valleys can be separated by introducing an energy offset between them by means of a magnetization. This latter fact enables one to monitor inter-valley transitions. The UVCP is proportional to the mobility and is enhanced by the excitonic Coulomb interaction and inter-valley scattering, as well as by a top gate bias. We discuss detection strategies in state-of-the-art experiments.

Advancements in modeling conformally doped X-MSND radiation imagers

A novel bimodal radiation imager (X-ray and thermal neutron) was designed using a fin–trench microstructured silicon diode backfilled with 6LiF, then bonded to a Timepix pixelated imaging ASIC (X-MSNDs). A simulation scheme had been devised which combined finite element analysis and discrete charge carrier transport to simulate the imaging characteristics of such arbitrarily doped 3D silicon Timepix sensors. However, the precise dopant profile of the deeply etched silicon surface that forms the X-MSND diode junction is difficult to measure with conventional optical or chemical staining means. A set of boundary conditions was instead derived from simple particle transport simulations and the constant-surface-concentration silicon diffusion model. Finite element analysis of the microstructured devices using COMSOL Multiphysics and this derived boundary condition revealed that the fin region of the sensors was generally un-depleted under the maximum 300V reverse bias, and the calculated electric field along the thickness of the sensor in the microstructured region is negligible. Charge carrier transport simulations using Allpix2 predicted charge carrier motion in the fin region is dominated by slow diffusion along the fin dimension, resulting in mean charge drift times of 1000ns to collect 95% of charge carriers. Neutron transport simulations indicated that single neutron detection events resulted in highly elongated pixel clusters, typically 1-pixel wide by 6-pixels along the fin dimension. Measured data collected from KSU fabricated X-MSND imagers matched these simulation results.

Coherence and de-coherence in the Time-Resolved ARPES of realistic materials: An ab-initio perspective

Coherence and de-coherence are the most fundamental steps that follow the initial photo-excitation occurring in typical pump-and-probe experiments. Indeed, the initial external laser pulse transfers coherence to the system in terms of creation of multiple electron–hole pairs excitation. The excitation concurs both to the creation of a finite carriers density and to the appearance of induced electromagnetic fields. The two effects, to a very first approximation, can be connected to the simple concepts of populations and oscillations. The dynamics of the system following the initial photo-excitation is, thus, entirely dictated by the interplay between coherence and de-coherence. This interplay and the de-coherence process itself, is due to the correlation effects stimulated by the photo-excitation. Single-particle, like the electron–phonon, and two-particles, like the electron–electron, scattering processes induce a complex dynamics of the electrons that, in turn, makes the description of the correlated and photo-excited system in terms of pure excitonic and/or carriers populations challenging.

Linewidth Broadening in Short-Wavelength Quantum Cascade Lasers

Nonequilibrium Green's function modeling of the structure used for III-V quantum cascade lasers emitting at approximately 5-$\ensuremath{\mu}\mathrm{m}$ wavelength is performed to get insight into and analyze the processes that determine the linewidth of the optical (inter-sub-band) transition. Theoretical results are compared with electroluminescence data collected for real structures. Excellent agreement is found for both the transition energy of approximately $234\phantom{\rule{0.2em}{0ex}}\mathrm{meV}$ and the linewidth of approximately $32\phantom{\rule{0.2em}{0ex}}\mathrm{meV}$, typical for this sort of device. Careful inspection of simulation data shows that, for these structures, the interface-roughness (IR) scattering contributes to the optical linewidth $\ensuremath{\cong}50\mathrm{%}$. The decreased broadening due to IR scattering is not due to very smooth interfaces, but rather due to the finite carrier concentration and the decreased population of final states ${\mathbf{k}}^{{}^{\ensuremath{'}}}$ available for the initial state $\mathbf{k}$ in the limit of vanishing in-plane momentum $\mathbf{k}\ensuremath{\rightarrow}0$. Further line narrowing is caused by nonparabolicity coupled with the highly nonthermal occupation of the lower laser sub-band, which destroys the population inversion for the hot carriers.

Influence of transverse carrier diffusion on two-dimensional optical rogue waves in broad-area semiconductor lasers with a saturable absorber

The statistics and dynamics of two-dimensional rogue waves in a broad-area semiconductor laser with an intracavity saturable absorber are numerically investigated under the effect of transverse carrier diffusion. We show that lateral diffusion of carriers alters the statistics of rogue waves by enhancing their formation in smaller ratios of carrier lifetimes in active and passive materials while suppressing them when the ratio is larger. The temporal dynamics of the emitted rogue waves is also studied and show that the finite nonzero transverse carrier diffusion coefficient gives them a longer duration. To further approach a realistic experimental situation, we also investigate the statistics and dynamics of rogue waves by simulating a circular disk-shape pump which replaces the flat pump profile typically used in numerical simulations of broad-area lasers. We show that a finite pump shape reduces the number of emitted rogue waves per unit area both below and above the laser threshold for every carrier lifetime ratio. The temporal width of the emitted rogue waves is also shown to decrease as a consequence of removing the nonphysical effects of the infinite flat pump on carrier dynamics.

Charge Carriers Density, Temperature, and Electric Field Dependence of the Charge Carrier Mobility in Disordered Organic Semiconductors in Low Density Region

We investigate the transport properties of charge carriers in disordered organic semiconductors using a model that relates a mobility with charge carriers (not with small polarons) hopping by thermal activation. Considering Miller and Abrahams expression for a hopping rate of a charge carrier between localized states of a Gaussian distributed energies, we employ Monte Carlo simulation methods, and calculate the average mobility of finite charge carriers focusing on a lower density region where the mobility was shown experimentally to be independent of the density. There are Monte Carlo simulation results for density dependence of mobility reported for hopping on regularly spaced states neglecting the role of spatial disorder, which does not fully mimic the hopping of charge carriers on randomly distributed states in disordered system as shown in recent publications. In this work we include the spatial disorder and distinguish the effects of electric field and density which are not separable in the experiment, and investigate the influence of density and electric field on mobility at different temperatures comparing with experimental results and that found in the absence of the spatial disorder. Moreover, we analyze the role of density and localization length on temperature and electric field dependence of mobility. Our results also give additional insight regarding the value of localization length that has been widely used as 0.1b where b is a lattice sites spacing.

A Schrödinger–Poisson model for output characteristics of trigate ballistic Si fin field effect transistors (FinFETs)

AbstractThis article presents a Schrödinger–Poisson based technique to predict characteristics of trigate Si fin field effect transistors (FinFETs). The channel characteristics are modeled by a three‐dimensional (3D) Schrödinger wave equation considering quantum‐mechanical ballistic transport. Using a nonequilibrium green function and a 3D Poisson equation, drain current associated with source‐drain Fermi energy difference is evaluated. Contact resistance and its impact on the drain current is adjusted using a modeled parameter in Fermi‐Dirac distribution function. It is observed that the drain current as a function of drain bias saturates because of the finite carrier supply from the source electrode. The model is calibrated using TCAD simulations for quantum transport. To validate the proposed model, output and transfer characteristics of nanoscale FinFETs are compared with experimental data and a good degree of accuracy is observed. It has been demonstrated that the proposed model has the ability to predict FinFETs characteristics having .

Fabrication and regulation of vacancy-mediated bismuth oxyhalide towards photocatalytic application: Development status and tendency

• This review focuses on the vacancy engineering of BiOX. • Popular vacancies creating approaches for BiOX are introduced. • Advanced characterization techniques for determining vacancies are summarized. • Inherent functionality of BiOX vacancies in photocatalysis is clarified. • Photocatalytic applications and behavior of vacancy-mediated BiOX are discussed. Recently, layered bismuth oxyhalides (BiOX) have been a well-deserved hotspot in the field of photocatalysis owning to their fascinating physicochemical properties derived from unique layered structures. Nevertheless, insufficient sunlight absorption, rapid recombination of electron-hole (e - -h + ) pairs, finite carrier concentration, and weak interaction between BiOX surface and reactant molecules inevitably limit the photocatalytic performance of BiOX. Given this, vacancy engineering, which can unleash the great potential to manipulate crystal and electronic structures and surface chemistry of BiOX, is widely applied to improve BiOX to meet the increasingly diverse theoretical and applicable needs. In this review, we focus on recent development in the design of appropriate vacancies on the BiOX for photocatalytic application. The introduction and analysis of popular vacancies creating approaches for BiOX and techniques to distinguish various vacancies are provided. The inherent functionality of BiOX vacancies in photocatalysis at the molecular level is clarified. Then we present representative photocatalytic applications, performance, and corresponding vacancy behavior of vacancy-mediated BiOX. Finally, based on an unambiguous understanding of the vacancy–property relationships and a complete view of the state of the art of vacancy-mediated BiOX, the future directions and possibilities for the rational design of vacancies to acquire ideal properties are explored.

Devising a method for finding a family of membership functions to bifuzzy quantities

This paper has considered a task to expand the scope of application of fuzzy mathematics methods, which is important from a theoretical and practical point of view. A case was examined where the parameters of fuzzy numbers’ membership functions are also fuzzy numbers with their membership functions. The resulting bifuzziness does not make it possible to implement the standard procedure of building a membership function. At the same time, there are difficulties in performing arithmetic and other operations on fuzzy numbers of the second order, which practically excludes the possibility of solving many practical problems. A computational procedure for calculating the membership functions of such bifuzzy numbers has been proposed, based on the universal principle of generalization and rules for operating on fuzzy numbers. A particular case was tackled where the original fuzzy number’s membership function contains a single fuzzy parameter. It is this particular case that more often occurs in practice. It has been shown that the correct description of the original fuzzy number, in this case, involves a family of membership functions, rather than one. The simplicity of the proposed and reported analytical method for calculating a family of membership functions of a bifuzzy quantity significantly expands the range of adequate analytical description of the behavior of systems under the conditions of multi-level uncertainty. A procedure of constructing the membership functions of bifuzzy numbers with the finite and infinite carrier has been considered. The method is illustrated by solving the examples of using the developed method for fuzzy numbers with the finite and infinite carrier. It is clear from these examples that the complexity of analytic description of membership functions with hierarchical uncertainty is growing rapidly with the increasing number of parameters for the original fuzzy number’s membership function, which are also set in a fuzzy fashion. Possible approaches to overcoming emerging difficulties have been described.

Intrinsic losses in photovoltaic laser power converters

Using photovoltaic cells as photovoltaic laser power converters (PLPCs) is a potential technology for long-range wireless power transfer. Intrinsic processes that limit the performance of PLPCs have not been fully investigated. Based on a thermodynamic model, we categorize and calculate the intrinsic losses in PLPCs. We use the experimental data of silicon and gallium arsenide to take into account the unavoidable Auger process. We find that the entropic loss generated during the absorption and emission of radiation is the major loss mechanism. Importantly, we show that in the presence of nonideal absorptivity and volumetric entropy production via Auger recombination, using lasers with photon energy equal to the bandgap of the PLPC can be impractical, e.g., comparable efficiencies can be achieved in much thinner silicon PLPCs illuminated by lasers with higher photon energies. We also investigate the methods of diminishing the intrinsic losses with respect to the Auger process: by intensifying the laser irradiance, the proportion of entropic loss in input power can be arbitrarily reduced; by using spectral and angular filters, the intrinsic losses can be diminished via absorption enhancement or emission restriction. Additionally, we discuss the practical efficiency limit of PLPCs accounting for the entropy production due to finite carrier mobilities. The results in this work estimate the potentials for efficiency improvements, which are fundamental to the design of PLPCs.

Computational intelligence within a resource budget: The case of the honey bee (Apis mellifera) swarm

AbstractThis study investigated the resource budget available to a typical honey bee swarm as it gathered information and decided which was the best of some 13–34 potential nest sites to move into and become its new home. After a brief description of the decision‐making mechanisms involved, the utilization of the swarm's finite energy, memory and carrier resources is described. It was found that each individual scout decision maker only carried enough energy reserves to provide for 2.6 days of visits to evaluate distant sites and, effectively, only enough memory space to remember the location and quality of approximately one site. Scouts, who only formed 4.4% of the swarm bees, consumed 22.8% of the swarm's energy budget, showing that their best‐of‐N computations were energetically very expensive. A typical swarm of 12,000 bees only contained enough energy reserves to last for 13.5 days after which foraging had to restart.

Imaging viscous flow of the Dirac fluid in graphene

The electron-hole plasma in charge-neutral graphene is predicted to realize a quantum critical system in which electrical transport features a universal hydrodynamic description, even at room temperature1,2. This quantum critical 'Dirac fluid' is expected to have a shear viscosity close to a minimum bound3,4, with an interparticle scattering rate saturating1 at the Planckian time,the shortest possible timescale for particles to relax. Although electrical transport measurements at finite carrier density are consistent with hydrodynamic electron flow in graphene5-8, a clear demonstration of viscous flow at the charge-neutrality point remains elusive. Here we directly image viscous Dirac fluid flow in graphene at room temperature by measuring the associated stray magnetic field. Nanoscale magnetic imaging is performed using quantum spin magnetometers realized with nitrogen vacancy centres in diamond. Scanning single-spin and wide-field magnetometry reveal a parabolic Poiseuille profile for electron flow in a high-mobility graphene channel near the charge-neutrality point, establishing the viscous transport of the Dirac fluid. This measurement is in contrast to the conventional uniform flow profile imaged in a metallic conductor and also in a low-mobility graphene channel. Via combined imaging and transport measurements, we obtain viscosity and scattering rates, and observe that these quantities are comparable to the universal values expected at quantum criticality. This finding establishes a nearly ideal electron fluid in charge-neutral, high-mobility graphene at room temperature4. Our results will enable the study of hydrodynamic transport in quantum critical fluids relevant to strongly correlated electrons in high-temperature superconductors9. This work also highlights the capability of quantum spin magnetometers to probe correlated electronic phenomena at the nanoscale.

Multi-photon regime of non-linear Breit-Wheeler and Compton processes in short linearly and circularly polarized laser pulses

Non-linear Breit-Wheeler $e^+e^-$ pair production and its crossing channel - the non-linear Compton process - in the multi-photon regime are analyzed for linearly and circularly polarized short laser pulses. The contribution of multi-photon processes to total cross sections is determined and we also show that (i) the azimuthal angular distributions of outgoing electrons in these processes differ on a qualitative level, and (ii) they depend on the polarization properties of the pulses. A finite carrier envelope phase (CEP) leads to a non-trivial non-monotonic behavior of the azimuthal angle distributions of the considered processes. That effect can be used for the CEP determination.

Shell stress analysis using a variational method based on three-dimensional functions with finite carriers

Shell stress analysis using a variational method based on three-dimensional functions with finite carriers

Investigation of electronic structure of transition metal silicides MnSi1.75 and CoSi for enhanced thermoelectric properties

Transition metal silicides that have low band gap or semimetallic states have gained huge interest due to enhanced thermoelectric efficiency. We present high resolution photoemission studies on two such systems like MnSi1.75 a low band gap semiconductor and CoSi a semimetal to understand the inherent electronic properties. Valence band studies on these systems reveal that there are both localized and delocalized 3d states present where as in the pure transition metals more localized states present near the Fermi level. In both MnSi1.75 and CoSi, the localized 3d states are found to be shifted to higher binding energy due to the strong hybridization with the Si 3s−3p states that gives rise to the hybridization gap in these systems. The delocalized 3d states in both the systems has much lower electronic density near the Fermi level than the pure transition metals that contributes to the conduction. Supporting evidences of delocalized screening in MnSi1.75 and weak correlated satellite in CoSi have been observed in the 2p core-level spectra. We find that the presence of both hybridization gap and the finite carrier concentration at the Fermi level in these transition metal silicides are necessary to have the enhanced thermoelectric properties.

Closed Bernoulli lines with finite buffers: real-time performance analysis, completion time bottleneck and carrier control

Based on the deployment of the new manufacturing strategies (e.g. smart manufacturing), real-time performance analysis, continuous improvement and efficient production management of flexible production systems are with great importance to be investigated. Relevant studies on the serial lines and assembly systems are obtained in recent years. We study real-time performances of closed production lines in this paper. Specifically, the system is assumed to have Bernoulli machines, finite buffers and finite carriers. By using Markovian approach, exact mathematical model for the system under consideration as well as the formulas to calculate the system's transient-based real-time performances are derived. For estimating the performances of a closed line under finite production run-based operation, we propose an approximation method with high accuracy and computational efficiency. Then, completion time bottleneck of the closed line is discussed. Finally, to illustrate the effectiveness of the method, a closed Bernoulli line with carrier control during production is studied.