We demonstrate electrical magnetization switching and spin pumping in magnetically doped Weyl semimetals. The Weyl semimetal is a three-dimensional gapless topological material, known to have nontrivial coupling between the charge and the magnetization due to the chiral anomaly. By solving the Landau-Lifshitz-Gilbert equation for a multilayer structure of a Weyl semimetal, an insulator and a metal while taking the charge-magnetization coupling into account, magnetization dynamics is analyzed. It is shown that the magnetization dynamics can be driven by the electric voltage. Consequently, switching of the magnetization with a pulsed electric voltage can be achieved, as well as precession motion with an applied oscillating electric voltage. The effect requires only a short voltage pulse and may therefore be energetically favorable for us in spintronics devices compared to conventional spin-transfer torque switching.

In amorphous oxide semiconductors (AOSs), defects induced by disorder and stoichiometric imbalance significantly affect the electrical conduction and thereby threshold stability of AOS-based devices. While band tails and deep subgap states near the valence-band edge stem from the disordered coordination of anions, the role of disordered cations is poorly understood. Here, based on first-principles calculations, we report the formation of undercoordinated cations in AOSs, such as amorphous In-Ga-Zn-O and Zn-Sn-O semiconductors, which are classified into two groups: undercoordinated cation pairs and undercoordinated single cations. Cation-pair defects belong to the category of O-vacancy defects and exhibit deep subgap states formed by the interaction between cations, whereas single-cation defects act as electron traps, similar to dangling bonds. The undercoordinated single cations have a tendency that the defect level near the conduction-band edge becomes deep as the coordination number decreases. Our results provide not only explanations for the origin of unresolved subgap states near the conduction band but have implications to control defect states and thereby improve the stability of AOS-based devices.

Quantum computing architectures are on the verge of scalability, a key requirement for the implementation of a universal quantum computer. The next stage in this quest is the realization of quantum error-correction codes, which will mitigate the impact of faulty quantum information on a quantum computer. Architectures with ten or more quantum bits (qubits) have been realized using trapped ions and superconducting circuits. While these implementations are potentially scalable, true scalability will require systems engineering to combine quantum and classical hardware. One technology demanding imminent efforts is the realization of a suitable wiring method for the control and the measurement of a large number of qubits. In this work, we introduce an interconnect solution for solid-state qubits: the quantum socket. The quantum socket fully exploits the third dimension to connect classical electronics to qubits with higher density and better performance than two-dimensional methods based on wire bonding. The quantum socket is based on spring-mounted microwires—the three-dimensional wires—that push directly on a microfabricated chip, making electrical contact. A small wire cross section (approximately 1 mm), nearly nonmagnetic components, and functionality at low temperatures make the quantum socket ideal for operating solid-state qubits. The wires have a coaxial geometry and operate over a frequency range from dc to 8 GHz, with a contact resistance of approximately $150\mathrm{m}\mathrm{\Omega }$, an impedance mismatch of approximately $10\mathrm{\Omega }$, and minimal cross talk. As a proof of principle, we fabricate and use a quantum socket to measure high-quality superconducting resonators at a temperature of approximately 10 mK. Quantum error-correction codes such as the surface code will largely benefit from the quantum socket, which will make it possible to address qubits located on a two-dimensional lattice. The present implementation of the socket could be readily extended to accommodate a quantum processor with a ($10\times 10$)-qubit lattice, which would allow for the realization of a simple quantum memory.

Magnetic flux noise is a dominant source of dephasing and energy relaxation in superconducting qubits. The noise power spectral density varies with frequency as $1/f^{\alpha }$, with $\alpha \lesssim 1$, and spans 13 orders of magnitude. Recent work indicates that the noise is from unpaired magnetic defects on the surfaces of the superconducting devices. Here, we demonstrate that adsorbed molecular ${\mathrm{O}}_2$ is the dominant contributor to magnetism in superconducting thin films. We show that this magnetism can be reduced by appropriate surface treatment or improvement in the sample vacuum environment. We observe a suppression of static spin susceptibility by more than an order of magnitude and a suppression of $1/f$ magnetic flux noise power spectral density of up to a factor of 5. These advances open the door to the realization of superconducting qubits with improved quantum coherence.

We systematically investigate visible defect-related photoluminescence (PL) properties of ZnO films with Ag nanostructures by changing the size of Ag and the power densities of excitation-laser light. We find that the PL intensity and the recombination rate are enhanced and peak at a particular size of a Ag nanostructure where its surface-plasmon resonance corresponds to the defect-emission band. The enhancements are due to the excitation of surface plasmons in Ag nanostructures and resultant local-field enhancement in the recombination energy of the defect-emission band. Furthermore, the saturation intensity of the defect-related emission in the higher-excitation power-density regime ($>\text{approximately}$ $10\mathrm{MW}/{\mathrm{cm}}^2$) is found to be strongly enhanced in the presence of Ag nanostructures, and the enhancement degree depends on the size of the nanostructures. The enhanced saturation behavior in the excitation power dependence of the emission intensity is well reproduced by the theoretical calculation based on a simple rate-equation model considering the surface-plasmon-induced enhancement of the radiative recombination rate. This result indicates that the saturation-intensity enhancement originates from the inhibition of the filling effect of defect states with excited carriers via plasmon-induced modification of their radiative processes.

We study a range of radio-frequency ion-trap geometries and investigate the effect of integrating dielectric cavity mirrors on their trapping potential using numerical modeling. We compare five different ion-trap geometries with the aim to identify ion-trap and cavity configurations that are best suited for achieving small cavity volumes and thus large ion-photon coupling as required for scalable quantum-information networks. In particular, we investigate the trapping-potential distortions caused by the dielectric material of the cavity mirrors in all three dimensions for different mirror orientations with respect to the trapping electrodes. We also analyze the effect of the mirror material properties such as dielectric constants and surface conductivity, and study the effect of surface charges on the mirrors. As well as perfectly symmetric systems, we also consider traps with optical cavities that are not centrally aligned where we find a spatial displacement of the trap center and asymmetry of the resulting trap only at certain cavity orientations. The best trap-cavity configurations with the smallest trapping-potential distortions are those where the cavities are aligned along the major symmetry axis of the electrode geometries. These cavity configurations also appear to be the most stable with respect to any mirror misalignment. Although we consider particular trap sizes in our study, the presented results can be easily generalized and scaled to different trap dimensions.

The dielectric constant of polymer-ferroelectric dielectrics may be tuned by changing the temperature, offering a platform for monitoring changes in interfacial transport with the polarization strength in organic field-effect transistors (FETs). Temperature-dependent transport studies of FETs are carried out from a solution-processed organic semiconductor, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), using both ferroelectric- and nonferroelectric-gate insulators. Nonferroelectric dielectric-based TIPS-pentacene FETs show a clear activated transport, in contrast to the ferroelectric dielectric polymer, poly(vinylidene fluoride-trifluoroethylene), where a negative temperature coefficient of the mobility is observed in the ferroelectric temperature range. The current-voltage ($I\text{−}V$) characteristics from TIPS-pentacene diodes signal a space-charge-limited conduction (SCLC) for a discrete set of trap levels, suggesting that charge injection and transport occurs through regions of ordering in the semiconductor. The carrier mobility extracted from temperature-dependent $I\text{−}V$ characteristics from the trap-free SCLC region shows a negative coefficient beyond 200 K, similar to the trend observed in FETs with the ferroelectric dielectric. At moderate temperatures, the polarization-fluctuation-dominant transport inherent in a ferroelectric dielectric, in conjunction with the nature of traps, results in an effective detrapping of the shallow-trap states into more mobile states in TIPS-pentacene.

We experimentally demonstrate a negative Kerr nonlinearity for quasiundoped graphene. Hereto, we introduce the method of chirped-pulse-pumped self-phase modulation and apply it to graphene-covered silicon waveguides at telecom wavelengths. The extracted Kerr-nonlinear index for graphene equals $n_{2,\mathrm{gr}}=-{10}^{-13}{\mathrm{m}}^2/\mathrm{W}$. Whereas the sign of $n_{2,\mathrm{gr}}$ turns out to be negative in contrast to what has been assumed so far, its magnitude is in correspondence with that observed in earlier experiments. Graphene’s negative Kerr nonlinearity strongly impacts how graphene should be exploited for enhancing the nonlinear response of photonic (integrated) devices exhibiting a positive nonlinearity. It also opens up the possibility of using graphene to annihilate unwanted nonlinear effects in such devices, to develop unexplored approaches for establishing Kerr processes, and to extend the scope of the “periodic poling” method often used for second-order nonlinearities towards third-order Kerr processes. Because of the generic nature of the chirped-pulse-pumped self-phase modulation method, it will allow fully characterizing the Kerr nonlinearity of essentially any novel (2D) material.

Biosensing with ferromagnet-based magnetoresistive devices has been dominated by electrical detection of particle-induced changes to a device’s (quasi-)static magnetic configuration. There are however potential advantages to be gained from using field dependent, high frequency resonant magnetization dynamics for magnetic particle detection. Here, we demonstrate the use of nanoconfined ferromagnetic resonances in periodically nanopatterned magnetic films for the detection of adsorbed magnetic particles having diameters ranging from 6 nm to $4\mu \mathrm{m}$. The nanopatterned films contain arrays of holes which appear to act as preferential adsorption sites for small particles. Hole-localized particles act in unison to shift the frequencies of the patterned layer’s ferromagnetic-resonance modes, with shift polarities determined by the localization of each mode within the nanopattern’s repeating unit cell. The same polarity shifts are observed for a large range of coverages, even when quasicontinuous particle sheets form above the hole-localized particles. For large particles, preferential adsorption no longer occurs, leading to resonance shifts with polarities that are independent of the mode localization, and amplitudes that are comparable to those seen in continuous layers. Indeed, for nanoparticles adsorbed onto a continuous layer, the particle-induced shift of the layer’s fundamental mode is up to 10 times less than that observed for nanoconfined modes in the nanopatterned systems, the low shift being induced by relatively weak fields emanating beyond the particle in the direction of the static applied field. This result highlights the importance of having particles consistently positioned in the close vicinity of confined modes.

Direct imaging of cavitation from solid nanoparticles has been a challenge due to the combined nanosized length and time scales involved. We report on high-speed microscopic imaging of inertial cavitation from gas trapped on nanoparticles with a tunable hemispherical depression (nanocups) at nanosecond time scales. The high-speed recordings establish that nanocups facilitate bubble growth followed by inertial collapse. Nanoparticle size, acoustic pressure amplitude, and frequency influence bubble dynamics and are compared to model predictions. Understanding these cavitation dynamics is critical for applications enhanced by acoustic cavitation.

High-density InAs quantum-dot (QD) layers are extensively used for testing intermediate-band solar cells based on two-step two-photon-absorption (TS-TPA) up-conversion processes. Here, photocurrent (PC), photoluminescence, and atomic-force microscopy are used to clarify the limitation of the high-density InAs QD approach. We measure two-color-beam PC spectra on InAs QD layers with QD densities ranging from 0.5 to $4.6\times 10^{10}/{\mathrm{cm}}^2$. It is shown that the high-density QD layer has a reduced PC-generation efficiency, and that the TS-TPA PC is reduced by trapping, rather than recombination.

We present a fully semiconductor-based magnetic tunnel junction that uses spin-orbit coupled materials made of intrinsic ferromagnetic semiconductors. Unlike more common approaches, one of the electrodes consists of a near-zero magnetic-moment ferromagnetic semiconductor, samarium nitride, with the other electrode composed of the more conventional ferromagnetic semiconductor gadolinium nitride. Fabricated tunnel junctions show a magnetoresistance as high as 200%, implying strong spin polarization in both electrodes. In contrast to conventional tunnel junctions, the resistance is largest at high fields, a direct result of the orbital-dominant magnetization in samarium nitride that requires that the spin in this electrode must align opposite to that in the gadolinium nitride when the magnetization is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetization phase in the samarium nitride, a direct result of the orbital-dominant ferromagnetism. Thus, an alternative type of functionality can be brought to magnetic tunnel junctions by the use of different electrode materials, in contrast to the usual focus on tuning the barrier properties.

Electron spins in solids have a central role in many current and future spin-based devices, ranging from sensitive sensors to quantum computers. Many of these apparatuses rely on the formation of well-defined spin structures (e.g., a 2D array) with controlled and well-characterized spin-spin interactions. While being essential for device operation, these interactions can also result in undesirable effects, such as decoherence. Arguably, the most important pure quantum interaction that causes decoherence is known as the “flip-flop” process, where two interacting spins interchange their quantum state. Currently, for electron spins, the rate of this process can only be estimated theoretically, or measured indirectly, under limiting assumptions and approximations, via spin-relaxation data. This work experimentally demonstrates how the flip-flop rate can be directly and accurately measured by examining spin-diffusion processes in the solid state for physically fixed spins. Under such terms, diffusion can occur only through this flip-flop-mediated quantum-state exchange and not via actual spatial motion. Our approach is implemented on two types of samples, phosphorus-doped ${}^{28}\mathrm{Si}$ and nitrogen vacancies in diamond, both of which are significantly relevant to quantum sensors and information processing. However, while the results for the former sample are conclusive and reveal a flip-flop rate of approximately 12.3 Hz, for the latter sample only an upper limit of approximately 0.2 Hz for this rate can be estimated.

The generation of propagating Bessel beams is typically limited to optical frequencies with bulky experimental setups. Recent works have demonstrated Bessel-beam generation at microwave and millimeter-wave frequencies utilizing low-profile, planar, leaky-wave antennas. These studies have assumed a single leaky mode in the antenna. In this work, the rigorous analysis of a planar Bessel-beam launcher supporting multiple modes is presented. By employing the mode-matching technique, a complete electromagnetic solution of the structure, its supported modes, and radiated fields is obtained. Additionally, a coupled system of two planar Bessel launchers is analyzed, and it is shown that the system can both transmit and receive Bessel beams. The energy-transfer characteristics of the coupled system are analyzed and discussed. An analysis of the coupled system’s even and odd modes of operation show that efficient power transfer is possible, and that an odd mode is preferred since it yields higher field confinement and power-transfer efficiency.

We investigate the luminescence of Ga- and N-polar ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/{\mathrm{In}}_y{\mathrm{Ga}}_{1-y}\mathrm{N}$ quantum wells grown by plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6$H$-SiC substrates. In striking contrast to their Ga-polar counterparts, the N-polar quantum wells prepared on freestanding GaN do not exhibit any detectable photoluminescence even at 10 K. Theoretical simulations of the band profiles combined with resonant excitation of the quantum wells allow us to rule out carrier escape and subsequent surface recombination as the reason for this absence of luminescence. To explore the hypothesis of a high concentration of nonradiative defects at the interfaces between wells and barriers, we analyze the photoluminescence of Ga- and N-polar quantum wells prepared on 6$H$-SiC as a function of the well width. Intense luminescence is observed for both Ga- and N-polar samples. As expected, the luminescence of the Ga-polar quantum wells quenches and redshifts with increasing well width due to the quantum confined Stark effect. In contrast, both the intensity and the energy of the luminescence from the N-polar samples are essentially independent of the well width. Transmission electron microscopy reveals that the N-polar quantum wells exhibit abrupt interfaces and homogeneous composition, excluding emission from In-rich clusters as the reason for this anomalous behavior. The microscopic origin of the luminescence in the N-polar samples is elucidated using spatially resolved cathodoluminescence spectroscopy. Regardless of well width, the luminescence is found to not originate from the N-polar quantum wells but from the semipolar facets of $\vee$-pit defects. These results cast serious doubts on the potential of N-polar ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/{\mathrm{In}}_y{\mathrm{Ga}}_{1-y}\mathrm{N}$ quantum wells grown by plasma-assisted molecular beam epitaxy for the development of long-wavelength light-emitting diodes.

We use electronic transport and atom-probe tomography to study $\mathrm{Zn}\mathrm{O}:\mathrm{Al}/{\mathrm{Si}\mathrm{O}}_2/\mathrm{Si}$ Schottky diodes on lightly doped $n$- and $p$-type Si. We vary the carrier concentration in the $\mathrm{ZnO}:\mathrm{Al}$ films by 2 orders of magnitude, but the Schottky barrier height remains nearly constant. Atom-probe tomography shows that Al segregates to the interface, so that the $\mathrm{ZnO}:\mathrm{Al}$ at the junction is likely to be metallic even when the bulk of the $\mathrm{ZnO}:\mathrm{Al}$ film is semiconducting. We hypothesize that the observed Fermi-level pinning is connected to the insulator-metal transition in doped ZnO. This implies that tuning the band alignment at oxide/Si interfaces may be achieved by controlling the transition between localized and extended states in the oxide, thereby changing the orbital hybridization across the interface.

One important attribute of graphene that makes it attractive for high-performance electronics is its inherently large thermal conductivity ($\kappa$) for the purposes of thermal management. Using a combined density-functional theory and classical molecular-dynamics approach, we predict that the $\kappa$ of graphene supported on hexagonal boron nitride ($h$-BN) can be as large as 90% of the $\kappa$ of suspended graphene, in contrast to the significant suppression of $\kappa$ (more than 70% reduction) on amorphous silica. Interestingly, we find that this enhanced thermal transport is largely attributed to increased lifetimes of the in-plane acoustic phonon modes, which is a notable contrast from the dominant contribution of out-of-plane acoustic modes in suspended graphene. This behavior is possible due to the charge polarization throughout graphene that induces strong interlayer adhesion between graphene and $h$-BN. These findings highlight the potential benefit of layered dielectric substrates such as $h$-BN for graphene-based thermal management, in addition to their electronic advantages. Furthermore, our study brings attention to the importance of understanding the interlayer interactions of graphene with layered dielectric materials which may offer an alternative technological platform for substrates in electronics.

A powerful technique is introduced for simulating mechanical and electromechanical properties of one-dimensional nanostructures under arbitrary combinations of bending, twisting, and stretching. The technique is based on an unconventional control of periodic symmetry which eliminates artifacts due to deformation constraints and quantum finite-size effects and allows transparent electronic-structure analysis. Via density-functional tight-binding implementation, the technique demonstrates its utility by predicting nonlinear electromechanical properties in carbon nanotubes and abrupt behavior in the structural yielding of ${\mathrm{Au}}_7$ and ${\mathrm{Mo}}_6{\mathrm{S}}_6$ nanowires. The technique drives simulations markedly closer to the realistic modeling of these slender nanostructures under experimental conditions.

In this paper, we study the Schottky transport in a narrow-gap semiconductor and few-layer graphene in which the energy dispersions are highly nonparabolic. We propose that the contrasting current-temperature scaling relation of $J\propto T^2$ in the conventional Schottky interface and $J\propto T^3$ in graphene-based Schottky interface can be reconciled under Kane’s $\mathbf{k}·\mathbf{p}$ nonparabolic band model for narrow-gap semiconductors. Our model suggests a more general form of $J\propto (T^2+\gamma k_B{T}^3)$, where the nonparabolicty parameter $\gamma$ provides a smooth transition from $T^2$ to $T^3$ scaling. For few-layer graphene, we find that $N$-layer graphene with $ABC$ stacking follows $J\propto T^{2/N+1}$, while $ABA$ stacking follows a universal form of $J\propto T^3$ regardless of the number of layers. Intriguingly, the Richardson constant extracted from the Arrhenius plot using an incorrect scaling relation disagrees with the actual value by 2 orders of magnitude, suggesting that correct models must be used in order to extract important properties for many Schottky devices.