The injection of a nondissolvable gas into a saturated liquid results in subcooling of the liquid due to forced evaporation into the bubble. Previous studies have assumed the rate of evaporation of liquid into the bubble to be independent of the degree of subcooling. In our study, we quantify the bubble growth by direct observation using high-speed imaging and prove that this hypothesis is not true. A phenomenological model of the bubble growth as a function of the degree of subcooling is developed and we find excellent agreement between the measurements and theory. This bubble-cooling process is employed in cooling a liquid. By identification of all heat flows, we can describe the cool-down curve well using bubble cooling, which provides an alternative cooling method for liquids without the use of complicated techniques.

We perform electromechanical imaging with improved resolution by piezoresponse force microscopy on a biocompatible relaxor ferroelectric crystal in conductive liquid environments. The nanoscale domain wall and characteristic domain pattern in $\mathrm{Na}\mathrm{Cl}$ solutions of up to almost 0.1M can be clearly detected. The unique insulated nanoelectrode tip of scanning probe microscopy has been applied to overcome traditional limitations of electrical investigation in aqueous environments by efficiently sensing local electrical signals. Unlike the good insulation of air or vacuum environments, the electromigration of ions in electrolytes corresponding to the applied electric field forms a double-layer structure, which screens the electric potential distributions near the biased tip and sample surface. Systematical numerical simulations of the Debye screening effect near the tip-electrolyte and sample-electrolyte interfaces give intuitive clarifications on the physical natures and help formulate strategies to improve electromechanical probing in a liquid environment with high conductivity.

We measure the dynamic properties of ${\mathrm{Co}}_{25}{\mathrm{Fe}}_{75}$ thin films grown by dc magnetron sputtering. Using ferromagnetic resonance spectroscopy, we demonstrate an ultralow total damping parameter of <0.0013 in the out-of-plane configuration, whereas for the in-plane configuration, the minimum total damping is <0.0020. In both cases, low inhomogeneous linewidth broadening occurs in macroscopic films. The minimum full-width at half-maximum linewidth is 1 mT at a 10-GHz resonance frequency for a 12-nm thick film. We characterize the morphology and structure of these films as a function of seed layer combinations, which indicate a large variation in the qualitative behavior of the in-plane linewidth vs resonance frequency. Finally, we use wavevector-dependent Brillouin light scattering spectroscopy to characterize the spin-wave dispersion at wave vectors up to $23\mu {\mathrm{m}}^{-1}$.

We demonstrate that a photonic gauge potential, which arises from the phase degree of freedom in dynamic refractive-index modulation and hence is achormatic, can be used to achieve a broadband optical switch using the configuration of a photonic Aharonov-Bohm interferometer (ABI). The resulting ABI switch has a far larger bandwidth and lower cross talk than the conventional Mach-Zehnder interferometer. Using coupled-mode theory and full-wave numerical modeling, we compare the response of the two interferometers in the presence of nonidealities. Our results indicate the importance of the photonic gauge potential for broadband optical signal processing.

We propose a solid-state refrigeration technique based on repeated adiabatic magnetization and demagnetization cycles of a superconductor, which acts as the working substance. The gradual cooling down of a substrate (normal metal) in contact with the working substance is demonstrated for different initial temperatures of the substrate. Excess heat is given to a hot large-gap superconductor. The on-chip refrigerator works in a cyclic manner because of an effective thermal switching mechanism: heat transport between $N$-$N$ versus $N$-$S$ junctions is asymmetric because of the appearance of the energy gap. This switch permits selective cooling of the metal. We find that this refrigeration technique can cool down a 0.3 ${\mathrm{cm}}^3$ block of Cu by almost 2 orders of magnitude starting from 200 mK, and down to about 1 mK starting from the base temperature of a dilution fridge (10 mK). The corresponding cooling power at 200 and 10 mK for a $1\times 1{\mathrm{cm}}^2$ interface are 25 and 0.06 nW respectively, which scales with the area of the interface.

Spatial differentiation is a fundamental mathematical operation used in many fields of science or engineering. Recently, optical analog computing of spatial differentiation, which can overcome the speed and energy limitations of digital differentiation techniques, has been realized in some nano systems such as the excitation of surface plasmon polaritons (SPPs). However, the inevitable backscattering of SPP propagation around defects or discontinuities may bring some undesired noises to the output signal. In this work, we firstly design a backscattering-immune spatial differentiator with the utilization of the nonreciprocal plasmonic platform, where the topologically protected one-way SPP leaky mode exists, in the terahertz (THz) region. Guided by the nonreciprocal spatial coupled-mode theory, we show that the ideal transfer function of the first-order spatial differentiation is able to be realized by subtly tuning the balance between the asymmetric leaky rate stemming from the nonreciprocal nature and intrinsic absorption rate of the system. Full wave simulations show that the first-order differentiation without backscattering around defects can be precisely implemented by using the designed nonreciprocal differentiator. Our proposed backscattering-immune spatial differentiator can find widespread applications for robust edge detection and image processing in the THz frequency range.

Engineering Gilbert damping of ferromagnetic metal films is of great importance to exploit and design spintronic devices that are operated with an ultrahigh speed. Based on the scattering theory of Gilbert damping, we extend the torque method originally used in studies of magnetocrystalline anisotropy to theoretically determine Gilbert dampings of ferromagnetic metals. This method is utilized to investigate Gilbert dampings of 3d transition-metal ferromagnet iron, cobalt, and nickel monolayers that are contacted by the prototypical topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Amazingly, we find that their Gilbert dampings are strongly enhanced by about one order of magnitude compared with dampings of their bulk forms and free-standing monolayers due to the strong spin-orbit coupling of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our work provides an attractive route for tailoring Gilbert damping of ferromagnetic metallic films by putting them in contact with topological insulators.

We demonstrate an absolute frequency measurement using the ultrabroadband comb generated in a periodically poled lithium niobate waveguide (PPLN WG). Based on the measured frequency information, we determine that the spectral broadening in the PPLN WG is due to quadratic nonlinearity. We also show that the self-referenced carrier-envelope offset beat signal observed in the ultrabroadband comb is generated based on the 2f–3f interference of different comb series. This study is not only useful for the practical application of frequency measurement but is also suitable for basic research of nonlinear optics.

Efficient electrical switching of antiferromagnets is at the center of their application in high-density and ultrafast nonvolatile memories. Antiferromagnetic (AFM) ${\mathrm{Mn}}_2\mathrm{Au}$ with opposite-spin sublattices is a unique metallic material in which the Edelstein effect can induce fieldlike torque switching of the AFM moments. However, the antidamping-torque-induced AFM moment switching remains to be proved in metallic AFM materials. Here, we demonstrate current-induced AFM moment switching in both a (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$ single layer and ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{Pt}$ heterojunction, detected by the spin Hall magnetoresistance and anisotropic magnetoresistance at room temperature. In (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$, fieldlike torque induces the alignment of AFM moments perpendicular to the writing current direction. Once the (103)-oriented ${\mathrm{Mn}}_2\mathrm{Au}$ film is covered by a thin $\mathrm{Pt}$ layer, the antidamping torque induced by the spin current from the $\mathrm{Pt}$ layer drives the AFM moments of ${\mathrm{Mn}}_2\mathrm{Au}$ switching toward the direction parallel to the writing current, which is in contrast to the case without $\mathrm{Pt}$. The simultaneous realization of fieldlike torque and antidamping torque in metallic ${\mathrm{Mn}}_2\mathrm{Au}$ makes it a promising candidate in AFM spintronics.

Understanding gas transport in liquid-saturated porous media is crucial for reducing mass transport-related inefficiencies in polymer-electrolyte-membrane (PEM) electrolyzers. While incompressible fluid-fluid displacement in porous media has been studied extensively, transport behavior with high compressibility effects remains poorly understood. Here, we investigate the impact of compressibility on gas transport in porous media via experiments in patterned micromodels. Macroscopically, we find that the displacement pattern follows the classical transition from capillary to viscous fingering as capillary number increases, despite the compressed state of the injected gas. Microscopically (i.e., pore scale), we find that the displacement occurs via discrete bursts in the form of Haines jumps. We demonstrate that in the presence of compressibility, the pore throat size exerts fundamental control over the burst velocity. Furthermore, we show that the inclusion of a thin, low-porosity region with small pore throats at the inlet of the micromodel increases the burst velocity of gas into the bulk of the micromodel, leading to significantly reduced gas saturation in the bulk. Our work provides a mechanistic explanation of the previously reported performance improvement due to the addition of microporous layers in PEM electrolyzers.

Nanoscale oscillators based on the spin-transfer torque effect are attractive candidates for the hardware implementation of neural networks using an array of coupled oscillators. Here, we demonstrate that the mutual coupling through rf strip lines can be used for synchronizing two spin-torque nano-oscillators (STNOs). Using the current nanotechnology, it is feasible to design asymmetric coupling between two STNOs via this scheme, which would mimic many biological neural networks. A unique feature of our experiment is that we can characterize the synchronized state by varying the phase as well as the strength of the coupling, going into the nonlinear regime.

We present a powerful method to implement a metasurface with the ability to convert a wave with linear polarization into multiple beams with orbital angular momentum (OAM) such that each beam possesses a flexible topological charge, independent direction, and polarization state. As opposed to conventional geometric-phase-based metasurfaces, the proposed method provides highly dense scalar and full tensor impedance textures governed by holography theory, thereby continuously controlling the main beam directions and broadening the operational bandwidth. In order to demonstrate the validity of this approach, we design two metasurfaces converting a linearly polarized beam emitted from an ordinary horn into OAM-carrying waves that have linear and circular polarizations with diverse topological charges. The theoretical and numerical results show that the two metasurfaces can generate OAM beams from 14 to 16 GHz with stable phase and intensity profiles of the near-field wave front over the entire band.

We theoretically and experimentally examine the spectroscopy of warm atoms traversing the evanescent field of a guided mode in silica glass. The subwavelength scale of the evanescent field leads to a large spectral broadening dominated by transit-time dephasing, creating a lineshape quite different from that of atoms in free space, and also distinct from that arising from transit-time-limited interactions in a field with a Gaussian intensity profile. We show reasonable agreement between our modeling of this scenario and experimental $D_2$-line spectra obtained from an exposed-core fiber immersed in rubidium vapor.

Laguerre-Gaussian (LG) beams and their orbital angular momentum (OAM) have found a plethora of applications—from quantum information to gravitational-wave detection. In this work, we probe the OAM of LG beams by using a digital micromirror device programmed as a single slit. We observe the diffraction patterns of the beams through the slit and we show that we can determine the OAM of the incoming beams from the changes in the diffraction pattern as the slit is moved transversely. Moreover, we demonstrate that we can access higher OAM modes compared to previous works involving slit diffraction. Our results are valuable in the study of OAM especially for beams where the current technology for detection is not yet mature, for example, beams whose wavelengths are in the terahertz regime or beams of higher energy such as, electron vortex beams.

Nongeminate recombination in organic solar cells is one of the main causes of reduced device performances. The quantification, and the subsequent reduction, of nongeminate recombination losses requires the development of methods, which allow an accurate and reliable determination of the charge-carrier density, mobility, and lifetime. Here, we present a method, based on the charge-density dependence of the recombination current, to determine the recombination parameters from impedance measurements of organic solar cells. We show that the use of the difference between the capacitance under illumination and the capacitance in the dark for the calculation of the charge density present in the active layer, allows the identification of the dominant recombination mechanism and the subsequent determination of either the charge-carrier lifetime or the bimolecular recombination coefficient.

A resistive coupling circuit is used to model the recently discovered dissipative coupling in a hybridized cavity photon-magnon system. With this model as a basis we design a planar cavity in which a controllable transition between level attraction and level repulsion can be achieved. This behavior can be quantitatively understood using an LCR-circuit model with a complex coupling strength. Our work therefore develops and verifies a circuit method to model level repulsion and level attraction and confirms the universality of dissipative coupling in the cavity photon-magnon system. The realization of both coherent coupling and dissipative coupling in a planar cavity may provide new avenues for the design and adaptation of dissipatively coupled systems for practical applications in information processing.

Flow-focusing is used in microfluidics to generate droplets that are smaller than the characteristic length scale of the flow geometry. Conventionally, flow-focusing takes place inside micrometer-sized channels due to capillary effects. In this study, we demonstrate that the transient meniscus profile created with Faraday waves on liquid films can enable flow-focusing. Using a magnetic shaker, we generate Faraday waves on a liquid film leading to flow-focusing that increases the resolution of a nozzleless, jet-based printing technique called blister-actuated laser-induced forward transfer (BALIFT). We perform experiments to demonstrate how transient meniscus formation enables jetting at lower laser-pulse energies than the threshold, and use numerical modeling to examine this process at smaller length scales relevant to printing applications.

Doping fluorine (F) into the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$ layer is critical to the performance of enhancement-mode ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ high-electron-mobility transistors (HEMTs). However, the understanding of the role of F in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ HEMTs is rather limited. Using the first-principles-calculated defect formation energies and transition energy levels, combined with the special quasirandom structure approach and the detailed balance theory, we investigate the interaction between F and native defects and impurities, as well as its effect on the Fermi energy of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$ alloy. Our results suggest that F is incorporated as ${\mathrm{F}}_i^{-}$ in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$ layer, which exhibits auto n-type conductivity because of unintentionally induced oxygen (O). $\mathrm{F}$ doping causes the redistribution of the charge states of intrinsic defects and impurities, and thus the Fermi energy of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$ layer. The charge-redistribution depends on the difference between the concentrations of F and O. Finally, we reveal the mechanism for the change of the electronical performance of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$$/\mathrm{Ga}\mathrm{N}$ HEMTs after $\mathrm{F}$ doping. The positive shift of the threshold voltage is related to the negatively charged F _i. Only when the concentration of F is higher than that of unintentionally induced O in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}$, F begins to increase the surface potential and the Schottky barrier height of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{N}/\mathrm{Ga}\mathrm{N}$ HEMTs.

In this paper, we investigate active control of near-field radiative heat transfer between monolayer graphene sheets by a magnetic field. We predict the existence of the giant thermal magnetoresistance (e.g., approximately 2.0 for $B=1\mathrm{T}$ and 50.0 for $B=20\mathrm{T}$) and negative thermal magnetoresistance of near-field radiative heat flux for different Fermi energies. Moreover, we observe Shubnikov–de Haas-like oscillations in the spectral heat flux, originating from the intraband or interband transitions between various Landau levels. Our findings provide alternative insights into the active control of near-field thermal radiation and may be leveraged for potential applications in noncontact thermal management.

There is a wide range of science and applications accessible via the strain engineering of quantum transport in 2D materials. We propose a realistic experimental platform for uniaxial strain engineering of ballistic charge transport in graphene. We focus on high-aspect-ratio mesoscopic devices ($W=1000$ nm, $L=100$ nm) whose conductivity is well defined even without atomically precise crystal edges. We develop an applied theoretical model, based on this platform, to calculate charge conductivity and demonstrate realistic graphene quantum strain transistors (GQSTs). We define GQSTs as mechanically strained ballistic graphene transistors with on:off conductivity ratios $>{10}^4$, which can be operated via modest gate voltages. Such devices would permit excellent transistor operations in pristine graphene, where there is no band gap. We consider all dominant uniaxial strain effects on conductivity, while including experimental considerations to guide the realization of the proposal. We predict multiple strain-tunable transport signatures, and demonstrate that a broad range of experimentally accessible device parameters lead to robust GQSTs. These devices could find applications in flexible electronic transistors, strain sensors, and valleytronics.