We use a model of vortex dynamics and collective weak-pinning theory to study the residual dissipation due to trapped magnetic flux in a dirty superconductor. Using simple estimates, approximate analytical calculations, and numerical simulations, we make predictions and comparisons with experiments performed in CERN and Cornell on resonant superconducting radio-frequency $\mathrm{Nb}\mathrm{Cu}$, doped-$\mathrm{Nb}$ and ${\mathrm{Nb}}_3\mathrm{Sn}$ cavities. We invoke hysteretic losses originating in a rugged pinning potential landscape to explain the linear behavior of the sensitivity of the residual resistance to trapped magnetic flux as a function of the amplitude of the radio-frequency field. Our calculations also predict and describe the crossover from hysteretic-dominated to viscous-dominated regimes of dissipation. We propose simple formulas describing power losses and crossover behavior, which can be used to guide the tuning of material parameters to optimize cavity performance.

Magnetoresistive spin-valve sensors based on the giant-magnetoresisitive (GMR) and tunneling-magnetoresisitive (TMR) effect with a flux-closed vortex-state free-layer design are compared by means of sensitivity and low-frequency noise. The vortex-state free layer allows high saturation fields up to 80 mT with negligible hysteresis, making it attractive for applications with a high dynamic range. The measured GMR devices comprise pink noise lower by a factor of 300 and better linearity in resistance but are less sensitive to external magnetic fields than TMR sensors. The results show comparable detectivity at low frequencies of about $2\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ for 1-$\mu \mathrm{m}$-diameter devices and $0.8\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ for 2-$\mu \mathrm{m}$-diameter devices at 1 Hz with ten active elements connected in series. The performance of the TMR minimum detectable field at frequencies in the white-noise limit is better by a factor of about 20 than that of the GMR devices.

The realization of high-$\beta$ lasers is one of the prime applications of cavity QED promising ultra-low thresholds, integrability, and reduced power consumption in the field of green photonics. In such nanolasers, spontaneous emission can play a central role even above the threshold. By going beyond rate-equation approaches, we revisit the definition of a laser threshold in terms of the input-output characteristics and the degree of coherence of the emission. We demonstrate that there are new regimes of cavity-QED lasing, realized, e.g., in high-$Q$ nanolasers with extended gain material, for which the two can differ significantly such that coherence is reached at much higher pump powers than required to observe the thresholdlike intensity jump. Against the common perception, such devices do not benefit from high-$\beta$ factors in terms of power reduction, as a significant amount of stimulated emission is required to quieten the spontaneous emission noise.

Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have been developed for decades for spin-transfer-torque magnetic random-access memory. Common stack designs use a hard layer (HL) with strong PMA to pin the reference layer (RL) by forming a synthetic antiferromagnet through a thin nonmagnetic coupling layer. Compared to bottom-pinned MTJs, very limited progress has been made to top-pinned MTJs, especially on the RL pinning due to its inferior thermal robustness. Herein, an alternative stack design is proposed for top-pinned MTJs, i.e., a synthetic ferromagnet (SFM). In the SFM, the RL is coupled with the HL ferromagnetically through a coupling layer. Micromagnetic simulations predict the advantage of the SFM design to stabilize the RL at scaled critical dimension (CD), which is experimentally proven by the observation of an increased RL pinning field on the device level. Because of the RL stray field acting on the free layer (FL), a compensation magnet (CM) is inserted below the FL to form a top-pinned MTJ stack without compromising magnetotransport properties. Devices with centered FL switching loops can be obtained after a two-step field setting. The stray field of CM has limited impact on the RL due to the large distance in between, thus keeping the RL’s pinning field larger than 150 mT down to devices with 20-nm CD. Finally, current switching is realized in devices with SFM and CM, showing critical current density around 5–8 ${\mathrm{MA}/\mathrm{cm}}^2$ and an averaged thermal stability as high as 50. Thus, the SFM pinning layer design shows great potential in stabilizing top-pinned devices and paves the way for multiple future spintronic applications requiring a top-pinned stack design.

We theoretically demonstrate the nontrivial transmission properties of a graphene-insulator-metal waveguide segment of deeply subwavelength scale. We show that, at midinfrared frequencies, the graphene-covered segment allows for the resonant transmission through the graphene-plasmon modes as well as the nonresonant transmission through background modes, and that these two pathways can lead to a strong Fano interference effect. The Fano interference enables a strong modulation of the overall optical transmission with a very small change in graphene Fermi level. By engineering the waveguide junction, it is possible that the two transmission pathways perfectly cancel each other out, resulting in a zero transmittance. We theoretically demonstrate the transmission modulation from 0% to 25% at 7.5-µm wavelength by shifting the Fermi level of graphene by a mere 15 meV. In addition, the active region of the device is more than 50 times shorter than the free-space wavelength. Thus, the reported phenomenon is of great advantage to the development of on-chip plasmonic devices.

For decades, thin-film interconnects have been designed considering that the electric-current-induced mass transport or electromigration in thin films decreases with the reduction in the length of the film. This phenomenon is often called the Blech length effect, which dictates the seizure of electromigration if the product of the current density and the film length is smaller than a critical value. We report a phenomenon in $\mathrm{Cu}$ film stripes fabricated as per Blech configuration, where we observe that the electric-current-driven mass transport at the cathode increases as the sample length is reduced. We call this phenomenon the “inverse Blech length effect.” Furthermore, the mass transport at the cathode in $\mathrm{Cu}$ film increases linearly with the inverse of the sample length. Finite-element analysis reveals an increase in the self-induced temperature gradients, which are very large to induce thermomigration in $\mathrm{Cu}$, at the ends of $\mathrm{Cu}$ film with a decrease in the sample length. Therefore, the contribution of thermomigration increases in the overall mass transport at the cathode as the sample length is decreased. The ensuing electromigration-thermomigration coupling is used to qualitatively explain the observation of the inverse Blech length phenomenon. The findings in this work open avenues for the design of device-level interconnects in microelectronic devices.

Nonadiabatic geometric quantum computation is promising as it is robust against certain types of local noises. However, its experimental implementation is challenging due to the need for complex control on multilevel and/or multiple quantum systems. Here, we propose to implement it on a two-dimensional square superconducting qubit lattice. In the construction of our geometric quantum gates, we only use the simplest and experimentally accessible control over the qubit states of the involved quantum systems, without introducing any auxiliary state. Specifically, our scheme is achieved by parametrically tunable all-resonant interaction, which leads to high-fidelity quantum gates. Meanwhile, this simple implementation can be conveniently generalized to a composite scenario, which can further suppress the systematic error during the gate operations. In addition, universal nonadiabatic geometric quantum gates in decoherence-free subspace can also be realized based on the tunable coupling between only two transmon qubits, without consulting multiple qubits and only using two physical qubits to encode a logical qubit. Therefore, our proposal provides a promising way to achieve high-fidelity geometric manipulation for robust solid-state quantum computation.

Edge-illumination tomography is a modality for performing x-ray phase-contrast imaging in 3D, based on structuring the primary beam into an array of beamlets and employing a dedicated incoherent sensing mechanism. In this article, we provide a theoretical framework for the spatial resolution of this method, based on the concept of the bowtie-shaped essential support of the Radon transform in frequency space, which is well known in conventional tomography. The additional complexity caused by the use of beamlets is added to the model, and its validity is confirmed by means of simulated and experimental results. In essence, we show that the precise location of non-negligible entries in the two-dimensional frequency spectrum of an edge-illumination sinogram can be predicted by only two parameters, the beamlet width and the sample thickness. This enables the derivation of sampling schemes tailored to avoid aliasing, thus maximizing spatial resolution.

Nonlinear acoustic metamaterials (NAMs) provide possibilities for exploiting the latest technologies for wave manipulations. Recently, the desired ultralow- and ultrabroadband wave suppressions have been achieved by chaotic bands in NAMs [Nat. Commun. 8, 1288 (2017)]. This work describes a remote interaction mechanism in NAMs: bridging coupling of nonlinear locally resonant band gaps. Theoretical studies on a triatomic model containing two nonlinearly coupled resonances clarify the principle of bridging band gaps. Bridging band gaps greatly change the nonlinear effective mass density and generate chaotic bands between nonlinear local resonances. The bandwidth and the efficiency of the wave reduction in chaotic bands can be effectively manipulated by modulating the frequency distance between the bridging pair. NAM beams are created to experimentally demonstrate this mechanism by including the bifurcations of periodic solutions. Our study extends the knowledge of NAMs, and more nonlinear effects are anticipated based on this mechanism.

We present radiofrequency thermometry based on a tunnel junction between a superconductor and proximitized normal metal. It allows operation in a wide range of biasing conditions. We demonstrate that the standard finite-bias quasiparticle tunneling thermometer suffers from large dissipation and loss of sensitivity at low temperatures, whereas thermometry based on a zero-bias anomaly avoids both of these problems. For these reasons, the latter method is suitable down to lower temperatures, here to about 25 mK. Both thermometers are shown to measure the same local temperature of the electrons in the normal metal in the range of their applicability.

The physical origin of the spectral shift of a thermally generated surface phonon-polariton (SPhP) resonance of a silicon carbide ($\mathrm{Si}\mathrm{C}$) bulk mediated by a nonresonant film is explained. The local density of electromagnetic states (LDOS) in a nonresonant intrinsic silicon ($\mathrm{Si}$) film due to thermal emission by $\mathrm{Si}\mathrm{C}$, derived using fluctuational electrodynamics, exhibits a local maximum near the SPhP-resonant frequency in addition to a lower frequency resonance generated by gap modes emerging in the vacuum gap separating the $\mathrm{Si}\mathrm{C}$ and $\mathrm{Si}$ layers. Multiple reflections within the vacuum gap also induce a LDOS drop around the SPhP-resonant frequency. As a result, depending on the film thickness to vacuum gap ratio and the location where the LDOS is calculated in the film, the low-frequency resonance can dominate the LDOS, such that the SPhP resonance appears to be redshifted. A similar spectral behavior is observed on the monochromatic radiative heat flux absorbed by the $\mathrm{Si}$ film. It is shown that apparent spectral (red and blue) shifts of the SPhP resonance mediated by a nonresonant film are bounded by the transverse and longitudinal optical phonon frequencies of $\mathrm{Si}\mathrm{C}$. This work is of importance in applications involving dissimilar materials, such as thermophotovoltaics and thermal rectification, where gap modes may significantly disrupt flux resonance. Gap modes may also be at the origin of the resonance redshift systematically observed in near-field thermal spectroscopy.

High-temperature ferromagnetic resonance (FMR) in $\mathrm{FePt}$-based media materials is studied for the first time. The FMR linewidth (ΔH) as a function of temperature (T), field angle (${\theta }_H$), and the volume fraction (x) of carbon in the material is determined, and the effective Gilbert damping constant and the Bloch−Bloembergen relaxation time are estimated. The data suggest that at temperatures 10–45 K below the Curie temperature, two-magnon scattering and spin-flip magnon-electron scattering make comparable contributions to ΔH. With a decrease in T, ΔH increases due to enhancement of the two-magnon scattering. ΔH can be tuned via varying x and shows a maximum at ${\theta }_H\approx {45}^{\circ }$ when varying ${\theta }_H$.

We demonstrate the complete filling of both deionized water (DI water) and liquid metal (eutectic gallium-indium, $\mathrm{E}\mathrm{Ga}\mathrm{In}$) into closed-end microchannels driven by a constant pressure at the inlet. A mathematical model based on gas diffusion through a porous polydimethylsiloxane (PDMS) wall is developed to unveil the physical mechanism in the filling process. The proposed theoretical analysis based on our model agrees well with the experimental observations. We also successfully generate traveling surface acoustic waves by actuating interdigitated microchannels filled with $\mathrm{E}\mathrm{Ga}\mathrm{In}$. Our work provides significant insights into the fabrication of liquid electrodes that can be used for various acustofluidics applications.

We provide a transmission-line representation for channels exhibiting spin-momentum locking (SML) that can be used for both time-dependent and steady-state transport analysis on a wide variety of materials with spin-orbit coupling such as topological insulators, Kondo insulators, transition metals, semimetals, oxide interfaces, and narrow band-gap semiconductors. This model is based on a time-dependent four-component diffusion equation obtained from the Boltzmann transport equation assuming linear response and elastic scattering in the channel. We classify all electronic states in the channel into four groups ($U^{+}$, $D^{+}$, $U^{-}$, and $D^{-}$) depending on the spin index [up ($U$), down ($D$)] and the sign of the $x$ component of the group velocity ($+,-$) and assign an average electrochemical potential to each of the four groups to obtain the four-component diffusion equation. For normal metal channels, the model decouples into the well-known transmission-line model for charge and a time-dependent version of the Valet-Fert equation for spin. We first show that, in the steady-state limit, our model leads to simple expressions for charge-spin interconversion in SML channels in good agreement with existing experimental data on diverse materials. We then use the full time-dependent model to study spin-charge separation in the presence of SML, a subject that has been controversial in the past. Our model shows that the charge and spin signals travel with two distinct velocities, resulting in well-known spin-charge separation, which is expected to persist even in the presence of SML. However, our model predicts that the lower velocity signal is purely spin, while the higher velocity signal is largely charge with an additional spin component proportional to the degree of SML, which has not been noted before. Finally, we note that our model can be used within standard circuit simulators such as SPICE to obtain numerical results for complex geometries.

The magnetic properties of graphene have attracted much attention for more than a decade. Recent studies have shown that adatoms or atomic vacancies in graphene could exhibit localized magnetic moments. However, a macroscopic spin-polarized semiconducting band structure has never been experimentally realized in graphene. Here, we demonstrate that a graphene monolayer, hybridized with an underlying $\mathrm{Ni}(111)$ substrate, exhibits a spin-polarized semiconducting state even at room temperature. Our spin-polarized scanning tunneling microscopy (STM) experiments, complemented by first-principles calculations, explicitly demonstrate that the interaction between graphene and the $\mathrm{Ni}$ substrate generates a large gap in graphene and simultaneously leads to a relative shift between majority- and minority-spin bands. Consequently, the graphene sheet on the $\mathrm{Ni}$ substrate exhibits a spin-polarized gap with an energy of several tens of meV even at room temperature.

The formation of instabilities linked with plasma flares of several centimeters (ionization zone) and driven by low to moderate current in three-dimensionally confined magnetron plasma is experimentally observed in a side-on view near the vicinity of the substrate. The flare velocity on account of cross-field drift is found to be 20.5 km/s with an estimated local tangential field $E_t\sim 205\mathrm{V}/\mathrm{m}$. The optical and Langmuir-probe measurements unambiguously demonstrate that an approximately 5.3-cm-wavelength wave of plasma particles propagating along the approximately $E_t\times B_0$ drift is accountable for the excitation and ionization of atoms. The observed instability is associated with a characteristic double layer with a potential drop of approximately 6.1 V.

We demonstrate spectroscopic microtomography in the visible wavelength range (450–700 nm) using digital holographic tomography, a wavelength-scanning source, and a variable-optical-path-length strategy. This multidimensional imaging system can directly measure the absorption-coefficient and refractive-index spectra of a three-dimensional specimen. We apply the system developed to characterize three types of polystyrene beads (undyed, blue dyed, and red dyed) as well as oxygenated and deoxygenated single red blood cells.

In this paper, we present and study the dynamics of an active microwave cloaking technique that alleviates the trade-offs between loss, bandwidth, and size inherent in passive approaches. The presented cloak consists of a finite number of satellite antennas loaded with parity-time-symmetric lumped admittances, which, without external control, can track in real time the amplitude, frequency, and phase variations of the impinging signals and generate the required current distributions at the antennas to totally suppress the scattered waves, including reflections and shadows. We analytically derive the optimal density and frequency dispersion of the antenna satellites surrounding the object in order to achieve substantial reduction in the scattering crosssection over a wide bandwidth. We also investigate the conditions necessary to maintain the cloak stability and study its transient response for excitation with pulsed signals.

Based on micromagnetic simulations, magnetization switching in a triangle magnetic element by voltage control of the magnetic anisotropy and Dzyaloshinskii-Moriya interaction under an in-plane magnetic field is proposed. The proposed switching scheme is not toggle switching but deterministic switching, in which the magnetic state is determined by the polarity of the applied voltage pulse. The mechanism and conditions for the switching are clarified. The results provide a fast and low-power writing method for magnetoresistive random-access memories.