Conventional impurity doping of ultrasmall nanoscale silicon ($\mathrm{Si}$) currently used in very-large-scale integration faces serious miniaturization challenges below the 14-nm technology node such as dopant out-diffusion and inactivation by clustering in $\mathrm{Si}$-based field-effect transistors (FETs). For $\mathrm{Si}$ nanocrystals showing quantum confinement, self-purification and massively increased ionization energies cause doping to fail. Modulation doping is widely used in group III–group V materials in particular for optoelectronic applications, where high carrier densities at low scattering and nonradiative recombination rates are crucial. Setting out from our recent success to introduce modulation acceptors into ${\mathrm{Si}\mathrm{O}}_2$ with aluminum to provide holes to $\mathrm{Si}$ [König et al., Sci. Rep. 7, 46703 (2017)], we follow our atomistic concept to investigate other possible modulation acceptors in ${\mathrm{Si}\mathrm{O}}_2$. Using density functional theory and experimental verification of key candidates by capacitance-voltage and deep-level-transient-spectroscopy measurements, we elucidate the role of atomistic parameters that determine the ability of the dopant species to provide modulation-acceptor states to ${\mathrm{Si}\mathrm{O}}_2$ and thus holes to $\mathrm{Si}$. Modulation-doped ${\mathrm{Si}\mathrm{O}}_2$ can replace conventional doping of ultrasmall nanoscale $\mathrm{Si}$ from the ${\mathrm{Si}\mathrm{O}}_2$ coating or trench of a FET and have a high potential for carrier-selective tunneling contacts in $\mathrm{Si}$-based heterojunction solar cells and tunnel FETs.

Nanocatalysts with their unique properties play an increasingly crucial role in areas ranging from energy and the environment to medical sciences. Despite numerous experimental studies on the utilization of nanocatalysts, there is a lack of theoretical understanding about the effectiveness of nanocatalyst application in porous media. In this study, we conduct both linear stability analysis (based on Quasi-Steady-State approximation) and nonlinear numerical simulations (based on a pseudospectral technique) to investigate mixing and in situ conversion of a reactant in the presence of nanocatalysts. Using the linear stability analysis, we parameterize the role of the in situ conversion on the stability of the system. Characterization of the nanocatalyst efficiency in terms of the degree of mixing and in situ conversion rate of the reactant is also performed through nonlinear numerical simulations. Our results show that while the increase of the nanocatalyst log mobility ratio attenuates the instabilities at the displacement front, the progress of the nanocatalytic reaction demonstrates a destabilizing effect. It is also seen that the nanocatalyst deposition leads to the rise of two competing mechanisms, the interplay of which controls the dynamics of miscible nanocatalytic reactive flows in porous media. Moreover, we introduce the effective penetration depth of nanocatalysts in porous media. Results reveal the crucial role of viscous fingering for increasing the reaction rate and indicate how the injected nanocatalysts lose their effectiveness due to their deposition in porous media. This study provides new insight into the application of nanocatalysts in porous media.

Optical and thermal metamaterials-based devices have been extensively explored under the guidance of transformation optics/thermotics theories to tune light and heat propagation to realize unprecedented functions, such as cloaking, concentrating, camouflaging, etc. Other than these functions, what else can they do? In this paper, we demonstrate another interesting and potential application, thermal encoding, based on the thermal energy shielding and harvesting units. The proposed thermal encoding protocol takes advantage of the binary (contrary) states of heat flux through two units, which can be encoded as binary digits of 1 and 0, respectively. Steady and transient simulations and experiments validate the feasibility of thermal encoding, which can be further effectively encrypted depending on the encoding or decoding protocol. Such thermal encoding supplements the existing implementations of thermal memory or computing. The proposed encoding protocol may open an alternative avenue for triggering counterpart developments by utilizing the binary states of electromagnetic and optical metamaterials for information encoding, decoding, and storage.

We present a rigorous analytical model for understanding the multipole emission enhancement of sound using acoustic metamaterials. A two-dimensional cylindrical hyperlens-like structure is utilized as a demonstration structure. An analytical expression for the enhanced efficiency is obtained via radiation impedance theory. The analytical results show that enhanced multipole radiation can be achieved by using only one monopole source encircled by the structure. The good agreement between the analytical and the simulation results and the experimental results verifies the applicability of our analytical model. Consequently, our analytical model provides an efficient and intuitive tool for synthesis and analysis of enhanced emission systems.

Magnetoelectric interfaces provide efficient pathways for manipulating the magnetic order with low-power-consuming electric fields. A prime example of this is the voltage-controlled domain-pattern transfer from a multiferroic ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ film into an amorphous ${\mathrm{Co}}_{0.9}{\mathrm{Fe}}_{0.1}$ ferromagnetic layer. Here we use a combination of optical second-harmonic generation and magnetic force microscopy to scrutinize the evolution of this transfer under voltage application. The nondestructive nature of our analysis permits the in operando examination of the magnetoelectric poling. We trigger ferroelectric switching events in ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ and track the evolution of their magnetoelectric transfer to the ferromagnetic domains of ${\mathrm{Co}}_{0.9}{\mathrm{Fe}}_{0.1}$ with spatial resolution. This reveals a wake-up effect in the magnetoelectric coupling across the interface. It is followed by simultaneous 90° rotations of the in-plane net polarization and net magnetization that progress with the increase of the voltage applied to the device.

Based on standard silicon-on-insulator platforms, we propose an interferometer structure operating at telecom wavelengths that is driven by pump-induced nonlinear Kerr-effect. Special attention is given to the waveguide design in achieving the smallest full-vectorial effective mode area corresponding to the largest nonlinear phase shift. We present that this work is so far, in theory, the most promising approach for high-speed optical-driven quantum interference, whose realizability is reflected in two advantages: (1) the proposed strategy mitigates the pump-induced noise from the sideband, spontaneous four-wave mixing, and spontaneous Raman scattering and (2) the pulsed pump at $2.2\mu \mathrm{m}$ facilitates a relatively higher nonlinear response and negligible free-carrier absorption.

Through recent advances, the relevance of magnetic tunnel junctions (MTJs) to the microelectronics industry continues to rise. However, their reversal speed still suffers from incubation delay, a consequence of the collinear magnetization equilibrium states in perpendicularly magnetized MTJs (p-MTJs). We propose to tune the free layer in a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ stack close to the reorientation transition, aiming to inducing a noncollinearity between the free layer and the reference layer to improve the magnetization reversal speed. Upon varying the layer thickness, the introduction of an in-plane magnetic component is observed in the free layer. Through time-resolved step-response measurements of the conductance, the dynamics are observed to consist of a stochastic delay followed by a repeatable magnetization-reversal pathway. We explore the separation of the measured response into these components and find the power requirements and switching speed to improve over the p-MTJ reference. This is a direct demonstration of the noncollinear improvement pathway, which is appealing for applications where the switching speed and a low energy cost of operation prevail over long-term stability.

Photophoretic force due to the optically-induced thermal effect provides an effective way to manipulate the light-absorbing particles suspended in ambient gases. However, how this force temporally responds to the intensity modulation of the illumination light is unclear. Here, by vertically trapping a micron-sized absorbing particle with a negative photophoretic force generated by a focused Gaussian beam, we demonstrate that the temporal change in the photophoretic force in response to the intensity modulation is remarkably slow (with a time constant up to approximately 1 s) due to the slow change in the particle’s temperature. When the trapping beam is turned off for a few tens or hundreds of milliseconds, the trapped particle is pulled up toward the light source by the remaining photophoretic force, whereas when the intensity of the trapping beam is increased for a short duration, the particle is pushed off by the radiation pressure. The instantaneous position of the trapped particles following the intensity modulation of the trapping laser is tracked and theoretically modelled. The understanding of the temporal behavior of the photophoretic force will be useful for the control of photophoretic-based optical pulling, transportation, and manipulation of atmospheric particles.

Microcapillaries are inherently fluidic sensing platforms that can, under appropriate conditions, support lasing. We propose and demonstrate a lasing microcapillary sensor based on “star” and “triangle” interferences arising from reflection and refraction at the inner and outer microcapillary walls. These interferences lead to distinct modal trajectories that sample the channel medium inside the microcavity structure. Both star and triangle resonances are therefore sensitive to the nature of the channel medium, yielding theoretical and experimental sensitivities significantly exceeding those found for sensors based on conventional whispering-gallery modes. Additional enhancement can be achieved because the two main resonance families shift in an opposite sense, yielding a differential sensitivity that can reach several thousand nanometers per refractive index “unit.” These devices represent a robust, yet easily fabricated optical sensor and capillary-based lasing system for microfluidic sensing.

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are developed for double quantum dots in $\mathrm{Ga}\mathrm{As}$ heterostructures, but they can easily be adapted to other quantum-dot-based qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot’s parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference.

We investigate the hopping transport of positively charged mobile oxygen vacancies $V_{\mathrm{o}}^{+}$ in electroforming-free bipolar memristive ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ switches by conducting impedance spectroscopy and quasistatic state-test measurements. We demonstrate that ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ switches with mobile oxygen vacancies ($V_{\mathrm{o}}^{+}$) and fixed substitutional ${\mathrm{Ti}}^{\mathrm{4+}}$ donors on ${\mathrm{Fe}}^{\mathrm{3+}}$ lattice sites close to the bottom electrode have a rectifying top electrode with an unflexible barrier height and a rectifying and/or nonrectifying bottom electrode with a flexible barrier height. The field-driven hopping transport of the oxygen vacancies determines the reconfiguration of the flexible barrier and the dynamics of the resistive switching. Average activation energies of 0.53 eV for trapping and of 0.31 eV for the release of oxygen vacancies by the ${\mathrm{Ti}}^{\mathrm{4+}}$ donors during application of the SET and RESET excitation pulses are extracted, respectively. The larger activation energy during SET is experimentally verified by impedance spectroscopy measurements and evidences the local enhancement of the electrostatic potential profile at the bottom electrode due to the ${\mathrm{Ti}}^{\mathrm{4+}}$ donors on ${\mathrm{Fe}}^{\mathrm{3+}}$ lattice sites.

In field-effect transistors (FETs) that typically have three electrodes, the charge density and electrical conductance of semiconductor films are tuned by an electric field provided by the gate electrode. However, the other two electrodes (source and drain) are inevitably applied with different voltages to generate a source-drain current to make the FET work, which causes a continuous evolution of charge accumulation (or depletion) in the three-dimensional semiconducting films during gate-voltage scanning and thus leads to an intrinsic delay between on and off states. To avoid the delay and accelerate the switching speed, here we propose that transconductance of semiconducting films can be significantly increased (denoted as “giant transconductance” here) when the transport channel is within extra static compensation electric fields and experimentally verify it in organic FETs. The giant transconductance is theoretically derived as super transconductance, corresponding to an abrupt current variation at a fixed gate voltage and thus ideally zero subthreshold swing as well as infinite effective field-effect mobility $({\mu }_{\mathrm{FET}})$. Giant transconductance is compatible with high temperature (up to 400 K), but both the doping concentration and the film thickness should be synergically optimized to warrant the super-rapid switching. For practical applications, we choose poly(3-hexylthiophene) and 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene to experimentally demonstrate the giant transconductance and its capability of current modulation. Giant transconductance could be potentially used in oscillator circuits, sensors, and high-performance circuits.

A recently proposed method for the extraction of electrical energy from a circuit that is static with respect to the surface of Earth is investigated experimentally. In this method an electrical circuit moves due to Earth’s rotation in Earth’s magnetic field, and thus is supposed to generate an electromotive force. Our experimental results demonstrate that the proposed method yields voltages more than 3 orders of magnitude smaller than the predicted signal. This discrepancy is explained, and constraints on alternative schemes are briefly considered.

We report a detailed theoretical and experimental study of the effect of $\mathrm{Cr}$ substitution in place of $\mathrm{Fe}$ on the structural, magnetic, transport, and electronic properties of ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ alloys. A comprehensive structural analysis is performed using anomalous XRD and EXAFS spectroscopy. Quaternary Heusler compounds ${\mathrm{Co}}_2{\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x\mathrm{Si}$ with $\mathrm{Cr}$ content ($x=0.1$, 0.3, 0.5) are found to crystallize in cubic structure. The synchrotron-based EXAFS studies reveal that the antisite disorder increases with an increase in $\mathrm{Cr}$ concentration. The saturation magnetization values in all alloys are found to be less than those expected from the Slater-Pauling rule, which may be due to some inherent disorder. Theoretical calculations successfully verify the experimental findings and reveal $\mathrm{Si}$ at $\mathrm{Fe}$/$\mathrm{Cr}$ antisite disorder to be energetically the most favorable. A detailed resistivity analysis in the temperature range of 5–300 K is performed, taking into account different scattering mechanisms. The residual resistivity ratio is found to decrease with increasing $\mathrm{Cr}$ concentration. A disorder-induced resistivity minimum due to a weak localization effect is seen for $x=0.5$. The resistivity measurements also indirectly support the half-metallic nature in these alloys. A detailed analysis shows that the half-metallic character survives up to 100 K for $x=0.1$, whereas the alloys with $x=0.3$ and 0.5 show the signature of the half-metallic nature even at higher temperatures. First-principles calculation carried out with a more robust exchange correlation functional (namely HSE-06) confirms the half metallicity in the entire concentration range. Unlike the generalized gradient approximation and the local-density approximation , this clearly confirms a full band gap in the minority spin channel and hence confirms the robust half-metallic character of ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$- and $\mathrm{Cr}$-substituted alloys. Theoretically simulated band gap and magnetic moments compliment the experimental findings and are compared wherever possible. All these properties make ${\mathrm{Co}}_2{\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x\mathrm{Si}$ a promising material for spintronics application.

In optical metatronics, where properly designed nanostructures function as lumped optical circuit elements, the capacitive and inductive optical circuit elements are nanostructures with positive and negative real parts of the permittivity of their materials, respectively. Aside from this difference in material properties, other features such as shapes and dimensions of these two elements may be similar. In the rf and microwave domains, however, conventional capacitors and inductors have vastly different features, i.e., one is made of parallel plates and the other is formed by wound wires. Here, inspired by optical metatronics, we propose to bring the notion of optical metatronics back into the low-frequency domain, by suggesting a capacitorlike structure that behaves as an inductive circuit element at microwave frequencies. This simple design may be achieved by adding certain sidewalls in the capacitor structure, and thus its shape is different from conventional inductors made of wires and spirals. The approximate range of inductance can be conceptually evaluated using relations analogous to those of the capacitance. Using numerical and analytical methods, we determine its complex impedance and quality factor as a function of frequency and all the dimensional parameters involved. From this analysis, we find that as compared with a conventional inductor this structure may have certain advantages, which include a higher quality factor due to its approximately closed structure and strong dependence of its inductance value on its height with weaker dependence on its cross section. We also study a subwavelength resonator using our proposed structure as its inductive element, confirming its performance numerically.

We present a quantum-limited Josephson-junction-based three-wave-mixing parametric amplifier, the superconducting nonlinear asymmetric inductive element (SNAIL) parametric amplifier (SPA), which uses an array of SNAILs as the source of tunable nonlinearity. We show how to engineer the nonlinearity over multiple orders of magnitude by varying the physical design of the device. As a function of design parameters, we systematically explore two important amplifier nonidealities that limit dynamic range: the phenomena of gain compression and intermodulation distortion, whose minimization are crucial for high-fidelity multiqubit readout. Through a comparison with first-principles theory across multiple devices, we demonstrate how to optimize both the nonlinearity and the input-output port coupling of these SNAIL-based parametric amplifiers to achieve higher saturation power, without sacrificing any other desirable characteristics. The method elaborated in our work can be extended to improve all forms of parametrically induced mixing that can be employed for quantum-information applications.

We have extended the Charge Extraction by a Linearly Increasing Voltage (CELIV) technique for determination of the built-in potential and the charge-carrier mobility in thin-film metal-insulator-metal (M-I-M) diodes. The validity of the presented analytical theory is verified by drift-diffusion simulations and experimentally demonstrated on organic solar cells. In contrast to the original CELIV theory, which assumes a uniform charge-carrier distribution in the active layer of the device, here we derive an analytical expression for determining the built-in potential and mobility in the case of a nonuniform charge-carrier distribution where charges have diffused into the active layer from the contacts. The extended CELIV theory is applicable on all thin-film M-I-M diodes, e.g., organic solar cells. Drift-diffusion simulations show that the error for mobility estimation can be an order of magnitude if not correcting for the carrier profile.

The Dzyaloshinskii-Moriya interaction (DMI) has attracted considerable recent attention owing to the intriguing physics behind it and the fundamental role it plays in stabilizing magnetic solitons, such as magnetic skyrmions and chiral domain walls. A number of experimental efforts have been devoted to probe the DMI, among which the most popular method is the Brillouin light scattering spectroscopy (BLS) to measure the frequency difference of spin waves with opposite wave vectors $\pm \mathbf{k}$ perpendicular to the in-plane magnetization $\mathbf{m}$. Such a technique, however, is not applicable to the cases of $\mathbf{k}\parallel \mathbf{m}$, since the spin-wave reciprocity is recovered then. For a narrow magnetic strip, it is also difficult to measure the DMI strength using BLS because of the spatial resolution limit of lights. To fill these gaps, we propose to probe the DMI via the propagation of spin waves in ferromagnetic films. We show that the DMI can cause the noncollinearity of the group velocities of spin waves with $\pm \mathbf{k}\parallel \mathbf{m}$. In heterogeneous magnetic thin films with different DMIs, negative refractions of spin waves emerge at the interface under proper conditions. These findings enable us to quantify the DMI strength by measuring the angle between the two spin-wave beams with $\pm \mathbf{k}\parallel \mathbf{m}$ in homogeneous film and by measuring the incident and negative refraction angles in heterogeneous films. For a narrow magnetic strip, we propose a nonlocal scheme to determine the DMI strength via nonlinear three-magnon processes. We implement theoretical calculations and micromagnetic simulations to verify our ideas. The results presented here are helpful for future measurement of the DMI and for designing novel spin-wave spintronic devices.

We experimentally study the auto-oscillating spin-wave modes in ${\text{Ni}}_{80}{\text{Fe}}_{20}/\beta \text{−}\mathrm{W}$ constriction-based spin Hall nano-oscillators as a function of bias current, strength of the in-plane applied field, and azimuthal field angle in the low-field range of 40–80 mT. We observe two different spin-wave modes: (i) a linearlike mode confined to the internal field minima near the edges of the nanoconstriction, and only weakly dependent on the bias current and the applied-field angle, and (ii) a second, lower-frequency mode with significantly higher threshold current and stronger dependence on both the bias current and the external-field angle. Micromagnetic modeling qualitatively reproduces the experimental data and reveals that the second mode is a spin-wave bullet and that the spin Hall nano-oscillator mode hops between the two modes, resulting in a substantial increase in linewidths. In contrast to the linearlike mode, the bullet is localized in the middle of the constriction and shrinks with increasing bias current. Using intrinsic frequency doubling at zero field angle, we can reach frequencies above 9 GHz in fields as low as 40 mT, which is important for the development of low-field spintronic oscillators with applications in microwave-signal generation and neuromorphic computing.