Based on full quantum-transport simulations, we report a comprehensive study of the evaporative cooling process in a double-barrier semiconductor heterostructure thermionic refrigerator. Our model, which self-consistently solves the nonequilibrium Green’s function framework and the heat equation, is capable of calculating the electron temperature and electrochemical potential inside the device. By investigating the dependence of those thermodynamic parameters as a function of the barrier thickness and height, we answer open questions on evaporative cooling in solid-state systems, and give a clear recipe to reach high electron refrigeration. In particular, simulation results demonstrate that the best cooling is obtained when (i) the device operates at the maximum resonant condition; (ii) the quantum well state is symmetrically coupled with the contacts. The present results then shed light on physical properties of evaporative cooling in semiconductor heterostructures and will allow the development of thermionic cooling devices towards unprecedented performances to be sped up.

We measure the terahertz conductivity of doped ceria by terahertz time-domain spectroscopy at different temperatures to characterize the shape of the potential associated with microscopic ion hopping. The experimental results for $\mathrm{Gd}$- and $\mathrm{La}$-doped ceria show that the activation energy in doped ceria is larger than that in stabilized zirconia. This result can be explained by a simple rigid-body model with the assumption of doping-induced lattice expansion, i.e., by considering the gap between cations. Our results for conventional fluorite-type solid-oxide electrolytes show that terahertz spectroscopy can be used to investigate fast microscopic ionic conduction in various electrolytes, which cannot be accessed through conventional impedance measurements.

In this study, we theoretically explore the probability distribution of the write-error rate (WER) of a voltage-controlled magnetoresistive random-access memory (VC MRAM). A probability density function (PDF) of the WER is analytically obtained using the change-of-variable technique with the assumption that the anisotropy constant of the VC MRAM follows the normal distribution. Furthermore, we assume that the WER of a single memory cell forms a parabola as a function of the anisotropy constant, resulting from numerical simulations by solving the Landau-Lifshitz-Gilbert equation based on the macrospin model. Theoretical analysis facilitates in identifying types of the PDF: a monotonic decreasing function with a peak at the smallest WER, and a function with local maximum.

Colloid separation from dilute solutions is of significant relevance in a multitude of technological applications. Here we use numerical simulations to investigate a recently proposed particle separation technique based on diffusiophoresis. Diffusiophoresis refers to the chemotactic migration of particles induced by local chemical gradients that allows for efficient particle manipulation. It can act in a cooperating or competing manner with respect to fluid advection, and is usually accompanied by diffusioosmosis that refers to the bulk flow adjacent to a solid surface. We consider a microfluidic setup with cocurrent flow in two horizontal channels connected by vertical diverging nanochannels, where the top flow contains suspended colloidal particles and solute at a low concentration and the bottom flow contains solute at a higher concentration. We analyze the fluid, solute, and particle dynamics to show that the particle motion is driven into the diverging nanochannels under the combined influence of fluid advection and diffusiophoresis, the latter arising due to the imposed solute gradient. Selective separation and characterization of the particles based on their size and surface charge can be achieved, whose efficiency can be improved by controlling the diffusiophoretic mobility of the particle, solute Péclet number, and imposed solute gradient. Diffusioosmosis can enhance or inhibit the diffusiophoretic separation, depending on the surface charge of the solid surfaces.

Except for a few cases to which sophisticated techniques are applicable, it is difficult to observe experimentally local electric fields or near fields associated with excitation of electromagnetic modes supported by building blocks of nanostructures. Although elucidating the behavior of the local electric fields or near fields is very useful to develop potential applications of the nanostructures, in many cases, their behaviors have been only speculated from simulations based on the electromagnetic theory. Here, we develop a simple technique that allows us to study experimentally the average behavior of the local electric fields inside two planar waveguide layers interacting with each other in all-dielectric multilayer structures. The study is made possible by doping the two waveguide layers with two different fluorophores. Fluorescence intensities detected at two different wavelengths allow us to study separately the behaviors of the local electric fields inside each waveguide layer. Angle-scan fluorescence excitation spectra are measured for two different series of multilayer samples by exciting the samples in an attenuated total reflection geometry. The line shapes observed are in good agreement with theoretical ones calculated based on the electromagnetic theory, confirming that the experimental fluorescence excitation spectra reflect well the average behaviors of the local electric fields. Applying the developed technique, we investigate systematically the changes in the local electric fields inside the waveguide layers caused by the change in the strength of coupling between the two waveguide modes. In one series of the samples, a gradual transition in the behavior of the local electric fields, from a Fano-resonant behavior to a double-peak behavior, is demonstrated with increasing the coupling strength. In another series of the samples, the coupling strength is widely changed and the formation of the coupled modes in a strong coupling regime is clearly observed. The observed changes in the local electric fields can be reproduced very well by simple analytical calculations based on a coupled oscillator model. The model is helpful to gain physical insights into the coupling of the waveguide modes, which cannot be obtained by the electromagnetic calculations.

In this work, we study the Schottky-barrier height (SBH) at the junction between $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ and platinum, a system of great importance for the next generation of high-power and high-temperature electronic devices. Specifically, we obtain interfacial atomic structures at different orientations using our structure-matching algorithm and compute their SBH using electronic structure calculations based on hybrid density-functional theory. The orientation and strain of platinum are found to have little impact on the barrier height. In contrast, we find that decomposed water molecules ($\mathrm{H}$.$\mathrm{OH}$), which could be present at the interface from ${\mathrm{Ga}}_2{\mathrm{O}}_3$ substrate preparation, has a strong influence on the SBH, in particular in the ($\overline{2}01$) orientation. The SBH can range from approximately $2$ eV for a pristine interface to nearly zero for a full $\mathrm{H}$.$\mathrm{OH}$ coverage. This result suggests that a SBH of approximately $2$ eV can be achieved for the ${\mathrm{Ga}}_2{\mathrm{O}}_3(\overline{2}01)/\mathrm{Pt}$ junction using the substrate-preparation methods that can reduce the amount of adsorbed water at the interface.

In this work, we demonstrate nonreciprocal acoustic wave transmission in two-cascaded Fabry-Perot-like slab resonators undergoing time modulation of their effective density. A phase difference is introduced into the time modulation between the two coupled resonators to produce spatial bias to access unidirectional wave propagation. A theoretical model based on plane-wave expansion and the transfer-matrix method is developed to study the Fabry-Perot-based system. The theoretical model allows rapid and precise characterization of the acoustic dispersion, with results in excellent agreement with finite-element-based numerical simulations. We demonstrate acoustic nonreciprocity behavior for the fundamental acoustic mode and further show that, with proper optimization, the coupled Fabry-Perot-based device can achieve nonreciprocal acoustic frequency conversion. This work could inspire the design of simple and compact nonreciprocal acoustic devices for efficient wave control.

Crystal-damage events such as tracks and point defects have been used to record and detect radiation for a long time and recently they have been proposed as a means for dark-matter detection. Color centers can be read out optically and we propose a scheme based on selective plane illumination microscopy for micrometer-scale imaging of large volumes corresponding to kilogram mass detectors. This class of detectors would be passive and would operate at room temperature and we call this the passive low-energy optical color-center nuclear-recoil (PALEOCCENE) detection method. We apply these concepts to the detection of reactor neutrinos using coherent elastic neutrino nucleus scattering (CE$\nu$NS). Crystal-damage formation energies are intrinsically on the order of 25 eV, resulting in similarly low nuclear-recoil thresholds. This would enable the observation of reactor-neutrino CE$\nu$NS with detectors as small as 10 g. Additionally, a competitive search for spin-dependent dark-matter scattering down to a dark-matter mass of $0.3$ GeV could be possible. Passive crystal detectors might also be attractive for nuclear-nonproliferation safeguards if used to monitor reactor power and to put limits on plutonium production. The passive nature and small footprint of the proposed detectors implies that these might fit well within accepted reactor-safeguards operations.

Superconducting nanowire single-photon detectors (SNSPDs) offer high-quantum-efficiency and low-dark-count-rate single-photon detection. In a growing number of cases, large magnetic fields are being incorporated into quantum microscopes, nanophotonic devices, and sensors for nuclear and high-energy physics that rely on SNSPDs, but superconducting devices generally perform poorly in large magnetic fields. Here, we demonstrate robust performance of amorphous SNSPDs in magnetic fields of up to $\pm 6$ T with a negligible dark-count rate and unchanged quantum efficiency at typical bias currents. Critically, we also show that the SNSPD can be used as a magnetometer with a sensitivity of better than $100\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ and as a thermometer with a sensitivity of $20\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$ at 1 K. Thus, a single-photon detector integrated into a quantum device can be used as a multifunctional quantum sensor capable of describing the temperature and magnetic field on chip simply by varying the bias current to change the operating modality from single-photon detection to thermometry or magnetometry.

Currently, thermally excited magnons are being intensively investigated, owing to their potential in computing devices and thermoelectric conversion technologies. We report the detection of a thermal magnon current propagating in a magnetic insulator yttrium iron garnet under a temperature gradient using a quantum sensor: electron spins associated with nitrogen-vacancy (N-V) centers in diamond. A thermal magnon current is observed as modified Rabi-oscillation frequencies of N-V spins hosted in a beam-shaped bulk diamond that is resonantly coupled with coherent magnon propagating over a long distance. Additionally, using a nanodiamond, alteration in N-V spin-relaxation rates, depending on the applied temperature gradient, are observed under nonresonant N-V excitation conditions. The demonstration of probing a thermal magnon current mediated by coherent magnons via N-V spin states serves as a basis for creating a device platform that hybridizes spin caloritronics and spin qubits.

In this research, we propose and design an acoustic metamuffler (AMM) by coupling a microperforated plate and a composite waveguide formed by a main waveguide and a Helmholtz resonator. The proposed mechanism and deliberately designed structure are conducive to generating multimode resonances that help to improve the coupling absorption effect and lead to broadband (4 octaves) sound insulation. We develop an effective circuit model to analytically predict the insulation bandwidth and put forward numerical and experimental measurements that demonstrate the effectiveness of the proposed concept. The designed AMM produces sound insulation with an average of 20 dB of sound-transmission loss at a low-frequency range, extending from 100 to 1600 Hz, while having an ultrathin thickness of 6.2 cm (1/55λ for the lowest working frequency). Our findings could have pragmatic applications for acoustic insulators or absorbers.

High-precision frequency estimation is an ubiquitous issue in fundamental physics and a critical task in spectroscopy. Here, we propose a quantum Ramsey interferometry to realize high-precision frequency estimation in a spin-$1$ Bose-Einstein condensate via driving the system through quantum phase transitions (QPTs). In our scheme, we combine adiabatically driving the system through QPTs with $\pi /2$ pulse to realize the initialization and recombination. Through adjusting the laser frequency under fixed evolution time, one can extract the transition frequency via the lock-in point. The lock-in point can be determined from the pattern of the population measurement. In particular, we find the measurement precision can approach the Heisenberg-limited scaling. Moreover, the scheme is robust against detection noise and nonadiabatic effect. Our proposed scheme does not require single-particle resolved detection and is within the reach of current experiment techniques, which may point out an alternative way for high-precision frequency estimation.

We report on a detailed study of low-temperature photoluminescence (PL) and magnetophotoluminescence under perpendicular magnetic fields (up to 30 T) and circularly polarized excitation on high-quality monolayer (ML) ${\mathrm{WS}}_2$ on a thick layer (120 nm) of talc dielectric. Remarkably, we obtained high-quality samples without any evidence of localized exciton emission at low temperature and a PL linewidth comparable to that of ${\mathrm{WS}}_2/h$-$\mathrm{BN}$ samples. As a consequence, we observe well-resolved emission peaks at low temperature due to the formation of excitonic complexes, including a dark-trion (DT) state and phonon replicas of the DT without the application of an in-plane magnetic field. The nature of the emission peaks, the magnetic field dependence of the degree of polarization, and g factors are discussed in detail and compared with the corresponding results obtained for $h$-$\mathrm{BN}$ encapsulated transition-metal dichalcogenide (TMD) samples. We observe that under ${\sigma }^{+}$-polarized excitation the sign of the circular polarization of biexcitons is reversed under higher magnetic fields. In addition, the dark-trion polarization increases considerably with increasing perpendicular magnetic field, demonstrating different behavior compared with previous studies of dark trions on monolayer ${\mathrm{WSe}}_2$. Our results suggest that talc is indeed a promising layered material for the surface protection of ML TMDs and to explore fundamental physics in view of applications in optoelectronic devices.

We evaluate the influence of Coulomb drag of electrons and holes in gated $n$ and $p$ regions caused by ballistic electrons and holes generated in a depleted $i$ region due to interband tunneling on the current-voltage characteristics and impedance of $p^{+}$-$p$-$i$-$n$-$n^{+}$ graphene tunneling transistor structures (GTTSs). This drag leads to current amplification in the gated $n$ and $p$ regions and positive feedback between the amplified dragged current and the injected tunneling current. A sufficiently strong drag can result in a negative real part of the GTTS impedance, enabling a plasma instability and self-excitation of plasma oscillations in the terahertz (THz) frequency range. This effect might be used for the generation of THz radiation.

Based on the self-consistent calculations of the eight-band $\mathbf{k}\cdot \mathbf{p}$ model and Poisson equation, we theoretically demonstrate that the intersubband optical absorption of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ type-II quantum wells (QWs) is effectively tuned by the electric-field-driven band-inversion transition. By utilizing this property, $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs can be made into highly efficient electroabsorption modulators (EAMs) for TM-polarized mid-infrared radiation (MIR) (approximately $4.2$–$12\mu \mathrm{m}$) and TE-polarized far-infrared radiation (FIR) (approximately $12$–$100\mu \mathrm{m}$). At low temperature, an example $p$-$i$-$n$ waveguide EAM composed of six-period $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs achieves a drive voltage of $<0.5$ V, a 3-dB cutoff bandwidth of approximately $24.6$ GHz, and a dynamic power consumption of $\le 0.06$ mW/GHz for a 20-dB on:off ratio modulation of $4.46$-$\mu \mathrm{m}$ TM-polarized radiation. Although the modulation efficiencies drop by 70%–80% when the temperature increases, the performance of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs is (at least) comparable with conventional $1.55$-$\mu \mathrm{m}$ EAMs at 300 K. These results suggest that $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs are promising devices for high-speed and low-power-consumption MIR (or FIR) free-space optical communications.

Phonon drag due to momentum exchange between electrons and phonons can lead to a substantially increased Seebeck coefficient desirable for thermoelectric energy conversion. However, this effect is only usually observable at low temperatures when the phonon mean free path is long, and it is thought to become negligible above room temperature or in heavily doped materials due to strong phonon scattering. Here, we present first-principles calculations of all thermoelectric transport properties of silicon-germanium alloys from 150 to 1100 K. Results show that phonon drag is dominant at low temperatures. At 1100 K, phonon drag still contributes to 10%–20% of the thermoelectric figure of merit, zT, and its relative contribution increases with higher carrier concentration. The favorable comparison between our calculations and reported experiments brings us closer to predicting the thermoelectric transport properties of alloys using first-principles simulations. The surprising insights in phonon drag could stimulate the search for better thermoelectric materials.

Quantum key distribution (QKD), which enables the secure distribution of symmetric keys between two legitimate parties, is highly useful in future network security. An access network that connects multiple end users with one network backbone can be combined with QKD to build security for end users in a scalable and cost-effective way. In this paper, we show that a downstream-access network can be implemented using continuous-variable (CV) QKD. The security analysis is conducted independently with other network nodes, which greatly relax the trustfulness requirement on the network nodes. The security is then obtained against other network parties even when the quantum signals are broadcast throughout the network. The passive beam splitter is sufficient to deterministically distribute the quantum signals, and standard CV QKD systems can be directly fitted in the downstream-access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV QKD downstream-access network, where up to 64 end users are shown to be feasible to access the network. We provide the security-analysis framework for realizing CV QKD in the downstream-access network, which will boost the diversity for constructing practical CV QKD networks.