External Tuning Research Articles

Nonlinear optical rectification of GaAs/Ga1–x Al x As quantum dots with Hulth-en plus Hellmann confining potential

We investigate the nonlinear optical rectification (NOR) of spherical quantum dots (QDs) under Hulthén plus Hellmann confining potential with the external tuning elements. Energy and wavefunction are determined by using the Nikiforov–Uvarov method. Expression for the NOR coefficient is derived by the density matrix theory. The results show that the applied external elements and internal parameters of this system have a strong influence on intraband nonlinear optical properties. It is hopeful that this tuning of the nonlinear optical properties of GaAs/Ga1–x Al x As QDs can make a greater contribution to preparation of new functional optical devices.

External vs. Internal: An Essay on Machine Learning Agents for Autonomous Database Management Systems

There are many possible ways to configure database management systems (DBMSs) have challenging to manage and set.The problem increased in large-scale deployments with thousands or millions of individual DBMS that each have their setting requirements. Recent research has explored using machine learning-based (ML) agents to overcome this problem's automated tuning of DBMSs. These agents extract performance metrics and behavioral information from the DBMS and then train models with this data to select tuning actions that they predict will have the most benefit. This paper discusses two engineering approaches for integrating ML agents in a DBMS. The first is to build an external tuning controller that treats the DBMS as a black box. The second is to incorporate the ML agents natively in the DBMS's architecture.

Unconventional spin Hall effects in nonmagnetic solids

Direct and inverse spin Hall effects lie at the heart of novel applications that utilize spins of electrons as information carriers, allowing generation of spin currents and detecting them via the electric voltage. In the standard arrangement, applied electric field induces transverse spin current with perpendicular spin polarization. Although conventional spin Hall effects are commonly used in spin-orbit torques or spin Hall magnetoresistance experiments, the possibilities to configure electronic devices according to specific needs are quite limited. Here, we investigate unconventional spin Hall effects that have the same origin as conventional ones, but manifest only in low-symmetry crystals where spin polarization, spin current and charge current are not enforced to be orthogonal. Based on the symmetry analysis for all 230 space groups, we have identified crystal structures that could exhibit unusual configurations of charge-to-spin conversion. The most relevant geometries have been explored in more detail; in particular, we have analyzed the collinear components yielding transverse charge and spin current with spin polarization parallel to one of them, as well as the longitudinal ones, where charge and spin currents are parallel. In addition, we have demonstrated that unconventional spin Hall effect can be induced by controllable breaking the crystal symmetries by an external electric field, which opens a perspective for external tuning of spin injection and detection by electric fields. The results have been confirmed by density functional theory calculations performed for various materials relevant for spintronics. We are convinced that our findings will stimulate further computational and experimental studies of unconventional spin Hall effects.

Employing the Defective Photonic Crystal Composed of Nanocomposite Superconducting Material in Detection of Cancerous Brain Tumors Biosensor: Computational Study

The present research is focused on the externally tunable defect mode properties of a one dimensional (1D) defective photonic crystal (DPhC) for fast detection of cancerous brain tumors. The proposed design has utilized conventional 1D DPhC whose cavity is coated with SiO2 nanoparticles embedded in a superconducting material layer called a nanocomposite layer. The purpose of a nanocomposite superconducting layer is to induce temperature dependent external tuning of the defect mode inside PBG, in addition, to changing in the angle of incidence. The inclusion of a nanocomposite layer also improves the interaction between light and different brain tissue samples under examination. In order to investigate the transmission properties of the proposed structure the transfer matrix formulation in addition to the MATLAB computational tool has been used. First, we have chosen the optimized internal parameters at normal incidence to obtain the maximum performance of the design. Secondly, the effect of change in angle of incidence has been studied to further increase the performance by means of sensitivity, quality factor, the figure of merit and limit of detection to ensure external tuning of defect mode. After achieving a maximum value of sensitivity (4139.24 nm/RIU) corresponding to a sample containing a wall of brain tissues at θ = 63° we have further investigated the effect of change in temperature of nanocomposite layers on the position and intensity both of the defect mode inside PBG. We have found that the increase in temperature results in minute changes in sensitivity but a significant increase in the intensity of defect mode which is highly required in any photonic biosensing design. The findings of this study may be very useful for designing various bio-sensing structures which could have a significant and decisive role in the field of biomedical applications.

Study of diluted kinetic Ising model under sinusoidal external field

The ferromagnet to paramagnet phase transition temperature depends on the dilution concentration in the site diluted Ising ferromagnet. Though this model is a bistable system, few studies reported dynamic phase transition (DPT) in diluted Ising ferromagnet. We study dilution-dependent DPT in diluted Ising ferromagnet via Monte Carlo simulation under a time-varying external field tuning the system’s temperature on several system sizes. The nature of the transition is characterized by employing the finite-size scaling study.

<i>In-situ</i> high pressure polarized Raman spectroscopy of rhenium disulfide

Pressure engineering is known as an efficient, continuous and reversible technique capable of tuning material structure, as well as its electrical, optical, and other physical properties. Raman spectroscopy is used to perform efficient and non-destructive analysis of material structure, and is compatible with the application of external tuning fields. In this work, we combine <i>in-situ</i> pressure engineering and polarized Raman spectroscopy to study the pressure-induced evolution of 18 Raman-active modes in ReS<sub>2</sub> crystal. We find that the ReS<sub>2</sub> undergoes a structural transformation from 1<i>T'</i> to a distorted-1<i>T'</i> phase at 3.04 GPa, followed by an intralayer deformation of Re<sub>4</sub> clusters occurring at 14.24 GPa. Interlayer transitions from disordered to ordered stacking in different in-plane directions are observed at 22.08 GPa and 25.76 GPa when the laser is polarized in different directions, which reflects the pressure-enhanced in-plane anisotropy, i.e. the anisotropy of ReS<sub>2</sub> crystal becomes more prominent under high pressure. Our findings demonstrate the effectiveness of pressure in tuning material properties, and shed light on potential applications of ReS<sub>2</sub> crystals in anisotropic optical and optoelectronic devices.

External tuning of topological phase transitions induced by interaction driven mass renormalization

Electron–electron interactions can be useful for realizing new nontrivial topological phases of matter. Here, we show by means of a tight-binding model and mean field theory how electron–electron interactions can lead to a topological phase transition. By externally adding or removing electrons from the system a band inversion between two bands with different parity is induced. This leads to a topological nontrivial phase if spin–orbit coupling is present. Besides the toy-model illustrating this mechanism, we also propose SmB6 as a possible playground for experimentally realizing a topological phase transition by external tuning.

Oriented External Electric Field Tuning of Unsubstituted Azoheteroarene Thermal Isomerization Half-Lives.

Azoheteroarenes are relatively new photoswitchable compounds, where one of the phenyl rings of an azobenzene molecule is replaced by a heteroaromatic five-membered ring. Recent findings on methylated azoheteroarenes show that these photoswitches have potential in various optically addressable applications. The thermal stability of molecular switches is one of the primary factors considered in the design process. For molecular memory or energy storage devices, long thermal relaxation times are required. However, inducing a short thermal isomerization lifetime is required to release stored energy or as an alternative to photoswitching to avoid overlapping absorption spectra that reduce switching fidelity. In this study, we investigate how oriented external electric fields can be used to tune the thermal isomerization properties of three unsubstituted heteroaryl azo compounds-azoimidazole, azopyrazole, and azopyrrole. We show that favorable electric field orientations can increase the thermal half-life of studied molecules by as much as 60 times or reduce it from tens of days to seconds, compared to their half-life values in the field-free environment. A deeper understanding of the relationship between structure and kinetic properties provides insight as to how molecular switches can be designed for their electric field response in switching applications.

Conditional generative models for sampling and phase transition indication in spin systems

In this work, we study generative adversarial networks (GANs) as a tool to learn the distribution of spin configurations and to generate samples, conditioned on external tuning parameters or other quantities associated with individual configurations. For concreteness, we focus on two examples of conditional variables---the temperature of the system and the energy of the samples. We show that temperature-conditioned models can not only be used to generate samples across thermal phase transitions, but also be employed as unsupervised indicators of transitions. To this end, we introduce a GAN-fidelity measure that captures the model’s susceptibility to external changes of parameters. The proposed energy-conditioned models are integrated with Monte Carlo simulations to perform over-relaxation steps, which break the Markov chain and reduce auto-correlations. We propose ways of efficiently representing the physical states in our network architectures, e.g., by exploiting symmetries, and to minimize the correlations between generated samples. A detailed evaluation, using the two-dimensional XY model as an example, shows that these incorporations bring in considerable improvements over standard machine-learning approaches. We further study the performance of our architectures when no training data is provided near the critical region.

Analysis of Fuzzy and PID control in Pacemaker with cardiovascular model

Cardiac Pacemakers have been effectively implemented to balance the electrical conduction network of the heart; A pacemaker’s primary function is to control an optimal heart rhythm, either because the elementary design of pacemaker of the heart is not quick enough, or because there is a barrier in the electrical conduction mechanism of the heart. Modern pacemakers are capable of external tuning and it enables optimal pacing modes for individual patients to be selected by a cardiologist, especially a cardiac electrophysiologist. In this study, linear and non-linear control logic for the regulation of the electrical response from the cardiac pacemaker has been studied with evaluation. The logic is constructed based on the mathematical model of the heart which has been converted from physiological model to electromechanical model. The pacemaker regulation, based on the control logic is evaluated on the grounds of the PQRST wave complex and the precision of the complex.

Narrow inhomogeneous distribution of spin-active emitters in silicon carbide

Optically active solid-state spin registers have demonstrated their unique potential in quantum computing, communication, and sensing. Realizing scalability and increasing application complexity require entangling multiple individual systems, e.g., via photon interference in an optical network. However, most solid-state emitters show relatively broad spectral distributions, which hinders optical interference experiments. Here, we demonstrate that silicon vacancy centers in semiconductor silicon carbide (SiC) provide a remarkably small natural distribution of their optical absorption/emission lines despite an elevated defect concentration of ≈0.43 μm−3. In particular, without any external tuning mechanism, we show that only 13 defects have to be investigated until at least two optical lines overlap within the lifetime-limited linewidth. Moreover, we identify emitters with overlapping emission profiles within diffraction-limited excitation spots, for which we introduce simplified schemes for the generation of computationally relevant Greenberger–Horne–Zeilinger and cluster states. Our results underline the potential of the CMOS-compatible SiC platform toward realizing networked quantum technology applications.

Strain-Stabilized (π, π) Order at the Surface of Fe1+xTe.

A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces.

Design and Fabrication of Magnetic Loop Antenna for 5MHz

In the short-wave range, size of the antenna become large and therefore is a major constraint. Magnetic Loop Antenna provides an advantage over other antennas in terms of a smaller size, higher quality factor and better signal to noise ratio. It works on the principle of resonance with the inductor provided by loop and external tuning capacitor operating like a tank circuit. The tunable magnetic loop antenna is designed to work in the high frequency range. The antenna consists of a circular hollow copper pipe, an inductive loop feed and a variable tuning capacitor. The antenna is tuned using variable 9-140pF capacitor paralleled with 150pF capacitor. The designed antenna is simulated using 4NEC2 software. The simulated antenna has high efficiency and quality factor of more than 1000. The real time testing show great result at 5.45MHz with bandwidth of 8KHz.

Observation of dynamic screening in the excited exciton states in multilayered MoS2

Excitonic resonance and binding energies can be altered by controlling the environmental screening of the attractive Coulomb potential. Although this screening response is often assumed to be static, the time evolution of the excitonic quasiparticles manifests a frequency dependence in its Coulomb screening efficacy. In this paper, we investigate a ground (1s) and first excited exciton state (2s) in a multilayered transition metal dichalcogenide $({\mathrm{MoS}}_{2})$ upon ultrafast photoexcitation. We explore the dynamic screening effects on the latter and show its resonance frequency is the relevant frequency at which screening from the smaller-sized 1s counterparts is effective. Our finding sheds light on new avenues of external tuning on excitonic properties.

High‐Pressure Structural Investigation of Anomalous Hall Effect Compound Mn3Sn up to 9 GPa

A systematic synchrotron high‐pressure X‐ray powder diffraction study of Mn3Sn up to 9 GPa is reported. In this pressure range, the crystal symmetry remains in the hexagonal P63/mmc space group, with a continuous reduction of the lattice parameters. Analysis indicates that the Mn3Sn system may have a different magnetic ground state above 4 GPa due to varying local atomic interactions. This is brought about by the slightly varying compression rate for the in‐plane a‐ and out‐of‐plane c‐axis, respectively, ≈−0.0235 and ≈−0.0176 Å GPa−1. The results contribute to the understanding of the crystal structure to the external continuous tuning parameter pressure, which modulates the geometrical arrangement of the atomic species in Mn3Sn, thus impacting the interesting physical properties such as the large anomalous Hall effect.

On the inclusion of structural loading and damping in piezoelectric shunt tuning

Expressions for piezoelectric inductive-resistive shunt tuning to a targeted structure mode are obtained by explicit frequency response functions, derived from a combined short- and open-circuit modal representation that accounts for inherent structural damping. The influence of external loading is further included by accounting for the short-circuit dynamic and quasi-static contributions from modes below and above the target mode, respectively. This augmented representation provides accurate frequency response for displacement and electric charge and voltage, and thus be generally used to derive desired shunt tuning strategies. Presently, explicit tuning expressions are based on a flat plateau condition in the voltage amplitude for a parallel inductive-resistive shunt or the charge amplitude for the corresponding series shunt. The tuning robustness with respect to changes in structural damping and external load distributions is verified by a numerical benchmark problem, which further shows that the explicit frequency response functions precisely predict the full structural and electric response around the targeted structure mode. This result can help bypass the need for heavy computations on the full dynamic problem, provide insight into how structural damping and external loading affect shunt tuning and better describe the effective dynamic capacitance, evaluated as the frequency dependent charge-to-voltage ratio.

Adaptive estimation in symmetric location model under log-concavity constraint

We revisit the problem of estimating the center of symmetry θ of an unknown symmetric density f. Although Stone (1975), Van Eeden (1970), and Sacks (1975) constructed adaptive estimators of θ in this model, their estimators depend on external tuning parameters. In an effort to reduce the burden of tuning parameters, we impose an additional restriction of log-concavity on f. We construct truncated one-step estimators which are adaptive under the log-concavity assumption. Our simulations indicate that the untruncated version of the one step estimator, which is tuning parameter free, is also asymptotically efficient. We also study the maximum likelihood estimator (MLE) of θ in the shape-restricted model.

Switching characteristics of periodically multilayered gyromagnetic metamaterials in waveguide structure

This article utilizes the waveguide structure and theory to analyze the electromagnetic wave propagating in periodically multilayered gyromagnetic metamaterials (PMGMs) with the control of external static magnetic field. Based on this, the reflection and transmission characteristics of the PMGMs terahertz (THz) optical switch are further explored. The switchable characteristics shown by the dispersion curves of transverse electric (TE) modes in the three-layered planar gyromagnetic metamaterial waveguides (GMMWs) are analyzed in detail. The numerical results demonstrate that the dispersion curves of GMMWs appear an adjustable bandgap under the external static magnetic field tuning. Furthermore, the full transmission phenomenon appears when the PMGMs THz optical switch presents an ON state, by adjusting the filling ratio and external static magnetic field. The mechanism is analyzed. These properties make it possible for PMGMs to be widely used in the design of photoswitch, phase modulator and so on.

Large valley-polarized state in single-layer NbX2 (X = S, Se): Theoretical prediction

Exploring two-dimensional valleytronic crystals with large valley-polarized state is of considerable importance due to the promising applications in next-generation information related devices. Here, we show first-principles evidence that single-layer NbX2 (X = S, Se) is potentially the long-sought two-dimensional valleytronic crystal. Specifically, the valley-polarized state is found to occur spontaneously in single-layer NbX2, without needing any external tuning, which arises from their intrinsic magnetic exchange interaction and inversion asymmetry. Moreover, the strong spin-orbit coupling strength within Nb-d orbitals renders their valley-polarized states being of remarkably large (NbS2 ∼ 156 meV/NbSe2 ∼ 219 meV), enabling practical utilization of their valley physics accessible. In additional, it is predicted that the valley physics (i.e., anomalous valley Hall effect) in single-layer NbX2 is switchable via applying moderate strain. These findings make single-layer NbX2 tantalizing candidates for realizing high-performance and controllable valleytronic devices.

Intrinsic anomalous valley Hall effect in single-layer Nb3I8

A major obstacle to reaching the wide applications of valleytronics operations is the lack of suitable anomalous valley Hall (AVH) materials, which should be easily fabricated and exhibit spontaneous valley polarization. Here, we identify an excellent two-dimensional AVH material, namely single-layer $\mathrm{N}{\mathrm{b}}_{3}{\mathrm{I}}_{8}$, based on first principles. Single-layer $\mathrm{N}{\mathrm{b}}_{3}{\mathrm{I}}_{8}$ is a robust ferromagnetic semiconductor with a moderate band gap. More importantly, due to the intrinsic breakings of both time-reversal symmetry and inversion symmetry, it exhibits the long-sought spontaneous valley polarization, without requiring any external tuning. Spontaneous valley polarization can reach as large as 107 meV, which is beneficial for practical operations. Furthermore, the experimental exfoliation of single-layer $\mathrm{N}{\mathrm{b}}_{3}{\mathrm{I}}_{8}$ is highly feasible as the stable layered bulk phase exists. Our findings provide an ideal platform for exploring the AVH effect and future valleytronic applications.