Diamond, a wide band-gap semiconductor, can be engineered to exhibit superconductivity when doped heavily with boron. The phenomena has been demonstrated in samples grown by chemical vapor deposition where the boron concentration exceeds the critical concentration for the metal-to-insulator transition of $n_{\mathrm{MIT}}\gtrsim 4\times {10}^{20}/{\mathrm{cm}}^3$. While the threshold carrier concentration for superconductivity is generally well established in the literature, it is unclear how well correlated higher critical temperatures are with increased boron concentration. Previous studies have generally compared several samples grown under different plasma conditions, or on substrates having different crystallographic orientations, in order to vary the incorporation of boron into the lattice. Here, we present a study of a single sample with unchanging boron concentration, and instead modify the charge-carrier concentration by introducing compensating defects via high-energy ion irradiation. Superconductivity is completely suppressed when the number of defects is sufficient to compensate the hole concentration to below threshold. Furthermore, we show it is possible to recover the superconductivity by annealing the sample in vacuum to remove the compensating defects.

Submonolayer quantum dots (SML QDs) combine the large gain cross section of quantum wells (QWs) with the potential benefits of a stronger confinement. We demonstrate here an optically pumped vertical external-cavity surface emitting laser (VECSEL) based on active SML QDs. The ultrafast SML QD VECSEL is optimized for passive modelocking with an intracavity semiconductor saturable absorber mirror (SESAM) around 1030 nm. We have achieved the highest cw output power of 11.2 W from a QD-based VECSEL to date. With a birefringent filter inside the VECSEL cavity, we obtain a tuning range of 47 nm centered at 1028 nm. Any attempt to passively modelock the VECSEL with a QW SESAM reveals fundamental limitations. The intrinsically higher linewidth enhancement factor of SML QDs compared to QWs or self-assembled Stranski-Krastanov QDs is further increased at higher pump powers. Our experiments confirm prior theoretical predictions that a strong amplitude-phase coupling can destabilize cw modelocking and introduce chaotic multiple pulse fluctuations.

The rotational Doppler effect associated with light’s orbital angular momentum has been found to be a powerful tool to detect rotating bodies. However, this method has only been demonstrated experimentally on the laboratory scale under well-controlled conditions so far. However, its real potential lies in practical applications in the field of remote sensing. We establish a 120-m-long free-space link between the rooftops of two buildings and show that both the rotation speed and the rotational symmetry of objects can be identified from the detected rotational Doppler frequency shift signal at photon-count level. Effects of possible slight misalignments and atmospheric turbulence are quantitatively analyzed in terms of mode power spreading to the adjacent modes as well as the transfer of rotational frequency shifts. Moreover, our results demonstrate that with the preknowledge of the object’s rotational symmetry one may always deduce the rotation speed no matter how strong the coupling to neighboring modes is. Without any information of the rotating object, the deduction of the object’s symmetry and rotational speed may still be obtained as long as the mode-spreading ratio does not exceed $100\mathrm{\%}$. Our work supports the feasibility of a practical sensor to remotely detect both the speed and symmetry of rotating bodies.

Inorganic cesium-lead-halide perovskites ($\mathrm{Cs}\mathrm{Pb}{X}_3$, where $X=\mathrm{Cl}$, $\mathrm{Br}$, and $\mathrm{I}$) have recently emerged as promising semiconductors for photovoltaic and light-emitting devices. In this study, we investigate electronic and vibrational properties of $\mathrm{Cs}\mathrm{Pb}{X}_3$ using the density-functional-theory calculations. We explicitly evaluate k-dependent electron-phonon scattering rates of the polar longitudinal-optical phonon modes. The transport property at room temperature is then computed based on Boltzmann transport theory within the relaxation-time approximation. The computational results identify the fundamental limit of the carrier mobility and its dependence on the halide species. Our results show that different choices of $X$ lead to the variation in the mobility by a factor of 3 to 5 depending on the carrier concentration between ${10}^{15}$ and ${10}^{18}{\mathrm{cm}}^{-3}$. The preferred carrier type (electron or hole) in terms of the mobility also varies with $X$. Through the detailed analysis on the band structures and scattering rates, we provide insights into the role of halide species in the transport properties.

We report a single-stage bidirectional interface capable of linking ${\mathrm{Sr}}^{+}$ trapped-ion qubits in a long-distance quantum network. Our interface converts photons between the ${\mathrm{Sr}}^{+}$ emission wavelength at 422 nm and the telecom $C$ band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single-photon level with efficiencies of 9.4% and 1.1%, respectively. Furthermore, we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.

Improving the write efficiency of magnetic tunnel junctions (MTJs) by using spin-orbit torque (SOT) is essential for realizing high-density spintronic memory. Here, we investigate a voltage-controlled spintronic memory (VoCSM) with a $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode. The magnetic properties of MTJs with a storage layer such as $\mathrm{Fe}\text{−}\mathrm{B}$ or $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ on an amorphous boride spin Hall electrode are found to exhibit an extremely small magnetic dead layer, a small saturation magnetization, and a large magnetic anisotropy. The spin Hall angle estimated by spin Hall magnetoresistance is −0.18 for the amorphous $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode, which is a magnitude twice that for a $\mathrm{Ta}$ spin Hall electrode. The write current density using SOT from the $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode is small compared with that from the $\mathrm{\beta }\text{−}\mathrm{Ta}$ spin Hall electrode. By combining the self-aligned fabrication technique with the $\mathrm{Ta}\text{−}\mathrm{B}$ electrode, a small value of critical switching current $(I_c\approx 79\mu \mathrm{A})$ is achieved despite a large MTJ size $(60\times 150{\mathrm{nm}}^2)$. We successfully reduce the write current to 48 µA, utilizing a voltage-controlled magnetic anisotropy by applying the voltage to a MTJ: $V_{\mathrm{MTJ}}=-1\mathrm{V}$. Moreover, the device also exhibits a low write error rate $(<1\times {10}^{-8})$, high endurance ($>1\times {10}^{12}$ cycles), and large break-down voltage (>2.5 V). These results indicate that VoCSM with the $\mathrm{Ta}\text{−}\mathrm{B}$ spin Hall electrode could open a path to realizing high-density nonvolatile memories with low power consumption and high-speed read and write operations.

We demonstrate a large angular dependence of magnetization dynamics in ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanocross arrays of varying sizes. By subtle variation of the azimuthal angle (ϕ) of an in-plane bias magnetic field, the spin configuration and the ensuing spin-wave dynamics, including mode softening, mode splitting, mode crossover, and mode merging, can be drastically varied to the extent that a frequency minimum corresponding to mode softening converts to a mode crossover, various mode splitting and mode crossover disappear, and additional mode splitting appears. Numerically simulated spin-wave spectra and phase profiles reveal the nature of various spin-wave modes and the origin of the above variation of the dynamics with a bias-field angle. All of these above observations are further modified with the variation of the dimensions of the nanocross. The numerically calculated magnetostatic field distributions further support the variation of dynamics with a bias-field angle. These results open an avenue for engineering nanocross-based magnetic devices such as magnetic storage, spin-wave logic, and on-chip data communication devices.

Self-assembled quantum dots (QDs) provide an ideal platform for realization of on-demand maximally entangled photon pairs, which is essential for quantum-information technologies. When the two intermediate exciton states are indistinguishable in energy, the biexciton cascade process naturally yields the maximally entangled photon pairs. However, because of the low symmetry of QDs, the two bright states should belong to different irreducible representations in realistic experiments, and their energy difference, called the “fine-structure splitting” (FSS), is much larger than their homogeneous broadening, giving rise to only classically correlated photon pairs by erasing the “which-way” information. In this work, we show that since these two states belong to different representations, their lifetimes should also be slightly different, which is termed “exciton lifetime asymmetry.” In contrast to the extensively studied FSS, investigation of this lifetime asymmetry is missing in the literature. Here we conduct an investigation of the exciton lifetime asymmetry in self-assembled QDs and present a theory to deduce this asymmetry indirectly from measurable qualities of QDs. We further reveal that the intrinsic lifetimes and their asymmetries are more fundamental quantities of QDs, which determine the upper bound of the extrinsic lifetime asymmetries. Exact relations between lifetime asymmetries, FSS, and the polarization angle are also derived. These relations may be measured in experiments by studying the degree of polarization, as well its evolution under external stress. Our findings provide a complete description of the symmetry of the optical properties of QDs, which can give an important basis to deepen our understanding of QDs.

A stable lattice of topological defects occurring at the electric reorientation of a nematic fluid is studied by quantitative polarimetry and laser diffraction. Generation of optical vortices by topological defects is demonstrated in the case of two distinct mechanisms. First, individual defects convert circularly polarized light partially into a vortex beam with opposite handedness, while beams diffracted on the defect lattice do not carry vorticity. Second, dislocation of the lattice structure is a topological defect on a larger length scale; then beams diffracted on a single dislocation possess optical vortex character. The vortex-generation efficiency is tunable by the applied voltage.

We present a method to measure quadratic terahertz optical nonlinearities in terahertz time-domain spectroscopy. We use a rotating linear polarizer (a polarizing chopper) to modulate the amplitude of the incident terahertz pulse train. We use phase-sensitive lock-in detection at the fundamental and the second harmonic of the modulation frequency to separate the materials’ responses that are linear and quadratic in a terahertz electric field. We demonstrate this method by measuring the quadratic terahertz Kerr effect in the presence of the much stronger linear electro-optic effect in the (110) $\mathrm{GaP}$ crystal. We propose that the method can be used to detect terahertz second-harmonic generation in noncentrosymmetric media in time-domain spectroscopy, with broad potential applications in nonlinear terahertz photonics and related technology.

We present a multiband double-negative acoustic metamaterial (AM) based on coupled Helmholtz resonators (HRs). By assembly of the detuned HR pairs with the optimized coupling, the combined structure exhibits multiple alternatively double-positive and double-negative passbands around $1000$ Hz. Theoretical analysis, numerical simulations, and experimental measurements demonstrate the feasibility of our method to achieve multiband double negativity. The proposed design advances the practicality of multiband double-negative acoustic metamaterial devices.

A temperature gradient in a continuous fluid induces the motion of dispersed micro- and nanoparticles even when the fluid is motionless. This phenomenon is known as thermophoresis, and it is expected to be the basis for techniques to control particle motion. In this study, we use the thermophoresis of micro- and nanoparticles in a microchannel filled with an aqueous solution to control the particle motion near the inlet of a sudden contraction, which is a narrower channel connecting two wider channels. Microfluidic devices with sudden contractions are widely used to develop various devices with micro- and nanometer dimensions, such as nanopore sensors. A near-infrared laser is used to create a strong temperature gradient of $O({10}^6)\mathrm{K}{\mathrm{m}}^{-1}$ and induce thermophoresis of micro- and nanoparticles. Because the heating by the laser irradiation is localized near the inlet of the contraction, this configuration is useful for controlling particle translocation into and through the contraction. We characterize our experimental setup by quantifying flow and temperature fields near the contraction channel using particle image velocimetry and laser-induced fluorescence, respectively. Then, we observe the obstruction of the particle translocation into the contraction channel induced by the laser-induced thermophoresis for various parameters such as channel dimensions, flow speeds, particle sizes, and laser powers. Near the inlet of the contraction channel, the counterbalance of thermophoretic force and flow drag leads to the ringlike pattern formation of the particle distribution. Moreover, we carry out some demonstrations using the proposed system to selectively translocate particles and enhance the sensing performance due to increased particle density. Thanks to the noncontact nature of laser-induced thermophoresis, the integration of our method into existing microfluidic devices is feasible and expected to improve technologies for manipulating particles in fluids.

Ferroelectric $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ is promising for photovoltaic applications, especially in regard to the exploitation of ferroelectric photovoltaic effects for charge separation. However, its large band gap limits efficient sunlight absorption. Here, we demonstrate a new strategy to effectively tune the band gap of tetragonal $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ via superlattice structuring with the ferroelectric $\mathrm{Bi}\mathrm{Cr}{\mathrm{O}}_3$. The $(\mathrm{Bi}\mathrm{Cr}{\mathrm{O}}_3{)}_m/(\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3{)}_n$ superlattices are found to exhibit conventional ferroelectric properties, but low fundamental band gaps, smaller than either of the parent materials. First-principles calculations reveal that the unexpected band-gap reduction is induced by charge reconstruction due to lattice strain, octahedral distortion, and polarization discontinuity at the $\mathrm{Bi}\mathrm{Cr}{\mathrm{O}}_3$-$\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ interfaces. Ultimately, these results provide a new strategy, in the form of superlattice structuring, which could open the door to the creation of efficient ferroelectric photovoltaics.

We aim at providing a global perspective on electromagnetic nonreciprocity and clarifying confusions that arose in recent developments of the field. We provide a general definition of nonreciprocity and classify nonreciprocal systems according to their linear time-invariant (LTI), linear time-variant (LTV), or nonlinear natures. The theory of nonreciprocal systems is established on the foundation formed by the concepts of time reversal, time-reversal symmetry, time-reversal symmetry breaking, and related Onsager-Casimir relations. Special attention is given to LTI systems, the most-common nonreciprocal systems, for which a generalized form of the Lorentz reciprocity theorem is derived. The delicate issue of loss in nonreciprocal systems is demystified and the so-called thermodynamics paradox is resolved from energy-conservation considerations. An overview of the fundamental characteristics and applications of LTI, LTV, and nonlinear nonreciprocal systems is given with the help of pedagogical examples. Finally, asymmetric structures with fallacious nonreciprocal appearances are debunked.

Despite their ubiquity in nanoscale electronic devices, the physics of tunnel barriers has not been developed to the extent necessary for the engineering of devices in the few-electron regime. This problem is of urgent interest, as this is the specific regime into which current extreme-scale electronics fall. Here, we propose theoretically and validate experimentally a compact model for multielectrode tunnel barriers, suitable for design-rules-based engineering of tunnel junctions in quantum devices. We perform transport spectroscopy at approximately $T=4$ K, extracting effective barrier heights and widths for a wide range of biases, using an efficient Landauer-Büttiker tunneling model to perform the analysis. We find that the barrier height shows several regimes of voltage dependence, either linear or approximately exponential. Effects on threshold, such as metal-insulator transition and lateral confinement, are included because they influence parameters that determine barrier height and width (e.g., the Fermi energy and local electric fields). We compare these results to semiclassical solutions of Poisson’s equation and find them to agree qualitatively. Finally, this characterization technique is applied to an efficient lateral tunnel barrier design that does not require an electrode directly above the barrier region in order to estimate barrier heights and widths.

The exploration of the binary valley degree of freedom in topological photonic systems has inspired many intriguing optical phenomena such as the photonic Hall effect, robust delay lines, and perfect out-coupling refraction. In this work, we experimentally demonstrate the tunability of electromagnetic flow in a valley photonic crystal waveguide. By continuously controlling the phase differences of a microwave monopolar antenna array, the flow of electromagnetic waves can split into different directions according to the chirality of the phase vortex, and the splitting ratio varies from 0.9 to 0.1. Topological valley transport of edge states is also observed at the photonic domain wall. Tunable edge state dispersion, i.e., from gapless valley-dependent modes to gapped flat bands, is found at the photonic boundary between a valley photonic crystal waveguide and a perfect electric conductor, leading to the tunable frequency bandwidth of high transmission. Our work paves the way to the controllable and dynamic modulation of electromagnetic flow in topological photonic systems.

The alloying of wide-band-gap $\mathrm{Zn}\mathrm{O}$ and $\mathrm{Ga}\mathrm{N}$ causes band-gap reduction, enabling visible-light-driven photocatalysis for high-efficiency water splitting. However, challenges in single-crystal $(\mathrm{Ga}\mathrm{N}{)}_{1-x}(\mathrm{Zn}\mathrm{O}{)}_x$ solid-solution synthesis prevent a better understanding of the optical properties and electronic structures. Here, low-temperature epitaxial growth of $(\mathrm{Ga}\mathrm{N}{)}_{1-x}(\mathrm{Zn}\mathrm{O}{)}_x$ thin films with a wide tunability of chemical composition is demonstrated by using a multitarget pulsed-laser-deposition (PLD) system. The phase pure $(\mathrm{Ga}\mathrm{N}{)}_{1-x}(\mathrm{Zn}\mathrm{O}{)}_x$ solid solution is obtained by alternately depositing $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Zn}\mathrm{O}$ with the thickness of each $\mathrm{Ga}\mathrm{N}/\mathrm{Zn}\mathrm{O}$ pair set within one or two unit cells. The band gap of the solid-solution thin films as a function of systematically controlled chemical composition shows asymmetric bowing with a minimum at approximately 2.0 eV for x = 0.65. Furthermore, a large absorption coefficient ($>{10}^4{\mathrm{cm}}^{-1}$) in the visible-light region is observed. The shape of the absorption edge is not consistent with that of a direct-transition semiconductor. First-principles calculation suggests that this inconsistency originates from localization of the valence-band maximum on $\mathrm{N}$ atoms bonded with $\mathrm{Zn}$. A technique for fabricating high-quality epitaxial $(\mathrm{Ga}\mathrm{N}{)}_{1-x}(\mathrm{Zn}\mathrm{O}{)}_x$ solid solutions is essential for acquiring a deep understanding of the fundamental properties of this system.

Vertical stacks of two-dimensional (2D) materials separated by a van der Waals gap and held together by van der Waals forces are immensely promising for a plethora of nanotechnological applications. Charge control in these stacks may be modeled with use of either a simple electrostatics approach or a detailed atomistic one. In this paper, we compare these approaches for a gated 2D transition-metal dichalcogenide bilayer and show that recently reported electrostatics-based models of this system give large errors in band energy compared with atomistic (density-functional-theory) simulations. These errors are due to the tails of the ionic potentials that reduce the electrical-equivalent van der Waals gap between the 2D layers, and can be corrected by use of the reduced gap in the electrostatic model. For a physical van der Waals gap (defined as the chalcogen-to-chalcogen distance) of 3 Å in a 2D bilayer, the electrical-equivalent gap is less than 1 Å. For the example of band-to-band-tunneling-based ultra-low-power transistors, this is seen to lead to errors of several hundred millivolts or more in the threshold voltage estimated from electrostatics.

High-performance organic field-effect transistors (OFETs) are an essential building block for future flexible electronics. Although there has been steady progress in the development of high-mobility organic semiconductors, the performance of lateral state-of-the-art OFETs still falls short, especially with regard to the transition frequency. One candidate to overcome the shortcomings of the lateral OFET is its vertical embodiment, the vertical organic field-effect transistor (VOFET). However, the detailed mechanism of VOFET operation is poorly understood and a matter of discussion. Proposed descriptions of the formation and geometry of the vertical channel vary significantly. In particular, values for lateral depth of the vertical channel reported so far show a large variation. This is an important question for the transistor integration, though, since a channel depth in the micrometer range would severely limit the possible integration density. Here, we investigate charge transport in such VOFETs via drift-diffusion simulations and experimental measurements. We use a (vertical) organic light-emitting transistor [(V)OLET] as a means to map the spatial distribution of charge transport within the vertical channel. Comparing simulation and experiment, we can conclusively describe the operation mechanism which is mainly governed by an accumulation of charges at the dielectric interface and the channel formation directly at the edge of the source electrode. In particular, we quantitatively describe how the channel depth depends on parameters such as gate-source voltage, drain-source voltage, and lateral and vertical mobility. Based on the proposed operation mechanism, we derive an analytical estimation for the lateral dimensions of the channel, helping to predict an upper limit for the integration density of VOFETs.

We explore the feasibility of exciting localized spin-wave modes in ferromagnetic nanostructures using surface acoustic waves. The time-resolved Faraday effect is used to probe the magnetization dynamics of an array of nickel nanowires. The optical-pump pulse excites both spin-wave modes of the nanowires and acoustic modes of the substrate and we observe that, when the frequencies of these modes coincide, the amplitude of magnetization dynamics is substantially enhanced due to magnetoelastic coupling between the two. Notably, by tuning the magnitude of an externally applied magnetic field, optically excited surface acoustic waves can selectively excite either the upper or lower branches of a splitting in the nanowire’s spin-wave spectrum, which micromagnetic simulations indicate is caused by localization of spin waves in different parts of the nanowire. Thus, our results indicate the feasibility of using acoustic waves to selectively excite spatially confined spin waves, an approach that may find utility in future magnonic devices where coherent structural deformations could be used as coherent sources of propagating spin waves.