Phase-change alloys have seen widespread use, from rewritable optical disks to current interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of the carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase-change material, $\mathrm{Ge}\mathrm{Te}$. We study this material using a time-resolved optical technique called the picosecond acoustic method to investigate the in situ thermally assisted amorphous-to-crystalline phase transformation in $\mathrm{Ge}\mathrm{Te}$. Our work reveals a clear evolution of electron-phonon coupling during the phase transformation, as the spectra of photoexcited acoustic phonons in the amorphous ($a$-$\mathrm{Ge}\mathrm{Te}$) and crystalline ($\alpha$-$\mathrm{Ge}\mathrm{Te}$) phases are different. In particular, and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline $\mathrm{Ge}\mathrm{Te}$ suggests that part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the skin depth of the pump light. Alternatively, the photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electron-hole plasma in the crystalline state. Consequently, these findings prove the existence of the nonthermal transport of energy, which is much faster than lattice heat diffusion.

Scanning diamond magnetometers based on the optically detected magnetic resonance of the nitrogen-vacancy center offer very high sensitivity and noninvasive imaging capabilities when the stray fields emanating from ultrathin magnetic materials are sufficiently low (less than $10\mathrm{mT}$). Beyond this low-field regime, the optical signal quenches and a quantitative measurement is challenging. While the field-dependent photoluminescence from the nitrogen-vacancy center can still provide qualitative information on magnetic morphology, this operation regime remains unexplored, particularly for surface magnetization larger than approximately $3\mathrm{mA}$. Here, we introduce a multiangle reconstruction (MARE) that captures the full nanoscale domain morphology in all magnetic field regimes leading to photoluminescence quench. To demonstrate this, we use ${\mathrm{[Ir/Co/Pt]}}_{14}$ multilayer films with surface magnetization an order of magnitude larger than previous reports. Our approach brings noninvasive nanoscale magnetic field imaging capability of the nitrogen-vacancy center to the study of a wider pool of magnetic materials and phenomena.

We develop an analytical framework for self-stabilizing optical manipulation of freestanding metasurfaces in three dimensions. Our framework reveals that the challenging problem of stabilization against translational and rotational perturbations in three dimensions is reduced to a simpler scattering analysis of the metasurface unit cell in two dimensions. We derive universal analytical stiffness coefficients applicable to arbitrary three-dimensional radial metasurfaces and radial beam-intensity profiles. The analytical nature of our framework facilitates highly efficient discovery of optimal optomechanical metasurfaces. Such use of metasurfaces for mechanical stabilization enables macroscale and long-range control in collimated, but otherwise unfocused light beams, and could open up avenues for manipulation beyond traditional optical tweezing and transport.

Magnetic flux quantization in superconductors allows the implementation of fast and energy-efficient digital superconducting circuits. However, information representation in magnetic flux severely limits the functional density and is a long-standing problem. Here, we introduce the concept of superconducting digital circuits that do not utilize magnetic flux and have no inductors. We argue that neither the use of geometric nor kinetic inductance is promising for the scaling down of superconducting circuits. The key idea of our approach is the utilization of bistable Josephson junctions, allowing the representation of information through the Josephson energy. Since the proposed circuits are composed only of Josephson junctions, they can be called all-Josephson junction (all-JJ) circuits. We present a methodology for the design of circuits consisting of conventional and bistable junctions. We analyze the principles of the circuit’s functioning, ranging from simple logic cells to an 8-bit parallel adder. The utilization of bistable junctions in the all-JJ circuits is promising for the simplification of schematics and a decrease of the JJ count, leading to space efficiency.

We report experiments on the control of intrinsic magnetic damping by thermal torque effects produced by the spin Seebeck effect and the anomalous Nernst effect in a thin layer of a ferromagnetic metal (Permalloy (Py), ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$). Damping is measured in ferromagnetic resonance (FMR) experiments on a sample excited by microwave radiation and detected by the dc-voltage-generated spin rectification and magnonic charge pumping in the Py film. Application of a temperature gradient in the longitudinal configuration increases or decreases the dc-voltage line width, depending on the sign of the thermal gradient, demonstrating that the magnetic damping in Py is controlled by currents generated by thermal effects. The absolute values of the line-width changes in Py may reach 1 order of magnitude larger than in the ferrimagnetic insulator yttrium iron garnet. The large change in magnetic damping is interpreted as a superposition of different phenomena in the same metallic ferromagnet.

While the modification of skyrmion motion by edges and defects has been widely discussed, how the motion of skyrmions would be affected by a surface terrace remains unclear. In this work, micromagnetic simulations are performed to reveal the skyrmion transport behaviors in multilayers modified by a surface terrace. It is found that skyrmions can be partially nucleated or annihilated at the surface terrace, and the nucleation or annihilation process is related to the competition between the nucleation or annihilation barrier and the binding energy of skyrmions. Interestingly, such processes can effectively modulate the transport characteristics of the skyrmion. The skyrmion decelerates or is even blocked in the annihilation process, whereas it accelerates in the nucleation process. As a result, the skyrmion mobility can be very different between upward and downward motion, and the surface terrace can even be viewed as a surface diode for skyrmions. Meanwhile, a transverse shift of the skyrmion along the terrace occurs in the nucleation process and the shifting distance depends on the applied current. In this way, the skyrmion can be shifted to a specific trajectory by controlling the current. Based on these effects, we propose two prototypal skyrmionic devices, the skyrmion surface diode and skyrmion track selector, which are potentially useful in skyrmionics.

Large-area single-crystal “mosaic” diamond wafers, produced by homoepitaxial diamond chemical vapor deposition (CVD) on seed crystals aligned in close proximity, are of increasing interest for high-power electronic and optical applications, not least due to their extraordinary thermal properties. However, thermal conductivity might be reduced if a significant thermal-barrier resistance (TBR) forms on junctions between the single-crystal blocks of the mosaic. Here, using a steady-state longitudinal heat-flow method, we measure with a high accuracy the in-plane thermal conductivity $\kappa (T)$ in the broad temperature range of 6–410 K for a diamond mosaic crystal grown by microwave plasma CVD. At room temperature, the conductivity as high as $24.0\pm 0.5\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-1}$ is determined within a single block, reducing by less than 2% only for $\kappa (T)$ measured across the junction due to low enough TBR of approximately ${10}^{-4}{\mathrm{cm}}^2\mathrm{K}{\mathrm{W}}^{-1}$. However, below 100 K the TBR strongly increases with the temperature decrease, resulting in a dramatic reduction of the mosaic thermal conductivity compared to that for the single-crystal block. We associate the appearance of TBR with a layer (approximately equal to $20\mu \mathrm{m}$ thick) of defected and stress material near the junction, as revealed with a confocal Raman mapping and transmission electron microscopy. The phonon scattering from defects in the zone around the junction strongly reduces the local thermal conductivity, as confirmed by modeling of heat transport. The observed temperature dependence $\kappa (T)\sim T^2$ of the local conductivity at $T<83$ K suggests the dominance of the phonon-dislocation scattering among other scattering processes. Our results show that the diamond mosaics preserve excellent thermal properties of the constituent single-crystal blocks at room and higher temperatures, and can be effectively used in the applications where the thermal conductivity is of primary relevance.

Optogenetics is a revolutionary method for studying neural activity by exploiting the optical stimulation of neurons made possible by artificial genetic expression of light-sensitive transmembrane “gate” proteins. Evidence of functional conformational changes of the gate proteins can be obtained by difference infrared (IR) absorption spectroscopy triggered by light stimuli. Here we investigate the effect on the photocycle of the prototype optogenetic protein channelrhodopsin (ChR2) of gold surfaces placed in close proximity to the protein molecules. In order to do this, we bring difference IR spectroscopy to the nanoscale, using a platform based on the coupling of a tunable mid-IR quantum cascade laser and an atomic force microscope (AFM). Sensitivity to individual subwavelength-sized membrane patches, embedding fewer than a hundred ChR2 molecules, is achieved by taking advantage of a plasmonic field enhancement in the nanogap between the gold-coated AFM tip and an ultraflat gold surface used as a sample support. We obtain relative difference absorption variations smaller than ${10}^{-2}$ and benchmark nanospectroscopy difference spectra against those taken with the Fourier transform IR (FTIR) spectroscopic technique on the same sample. We identify distinct simultaneous processes in the ChR2 photocycle by performing singular value decomposition of the FTIR difference spectra, and use this procedure to compare IR nanospectroscopy data on individual membrane patches in contact with gold surfaces with those obtained on thick stacks of membrane patches unexposed to metal surfaces. ChR2 proteins maintain their gate function when placed in a 14-nm-wide gap between two gold surfaces, apart from minor modifications to their kinetics. Our results are relevant to optogenetic applications that require physical contact between nanoscale metallic probes and electrodes and the membrane of single neuronal cells. More generally, our work paves the way towards the spectroscopic study of transmembrane proteins at the nanoscale.

Ferrimagnetic ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) is the prototypical material for studying magnonic properties due to its exceptionally low damping. By substituting the yttrium with rare earth elements that have a net magnetic moment, we can introduce an additional spin degree of freedom. Here, we study the magnetic coupling in epitaxial ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG/GIG) heterostructures grown by pulsed laser deposition. From bulk sensitive magnetometry and surface sensitive spin Seebeck effect and spin Hall magnetoresistance measurements, we determine the alignment of the heterostructure magnetization as a function temperature and external magnetic field. The ferromagnetic coupling between the $\mathrm{Fe}$ sublattices of YIG and GIG dominates the overall behavior of the heterostructures. Because of the temperature-dependent gadolinium moment, a magnetic compensation point of the total bilayer system can be identified. This compensation point shifts to lower temperatures with increasing YIG thickness due the parallel alignment of the iron moments. We show that we can control the magnetic properties of the heterostructures by tuning the thickness of the individual layers, opening up a large playground for magnonic devices based on coupled magnetic insulators. These devices could potentially control the magnon transport analogously to electron transport in giant magnetoresistive devices.

Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.

In recent years, unshielded atomic systems have been attracting researchers’ attention, in which decoherence is one of the major problems, especially for high-precision measurements. The nonlinear Zeeman effect and magnetic field gradient are the main decoherence sources of atomic electron spin in the Earth-field range. Here, we propose a method to cancel out the two dominant broadening effects simultaneously by an all-optical dynamic decoupling approach based on Raman scattering in the ${}^{87}\mathrm{Rb}$ Zeeman sublevels. By adjusting the parameters of the Raman lasers, we realize spin control along an arbitrary direction. We analyze the state evolution of atomic spin under the Raman light-control sequence in detail. The results show that both the nonlinear Zeeman effect and magnetic field gradient can be significantly suppressed.

We show that large magneto-optic Kerr effects (MOKEs) emerge when an antiferromagnet (AFM) is proximately coupled to a topological insulator (TI) film—where neither the perfect collinear Néel ordered single-domain AFM nor the unmagnetized TI individually shows any MOKE. Because of the lack of macroscopic magnetization, the AFM only couples to the spin of one of the TI’s surfaces breaking time-reversal and inversion symmetry—which leads to a small microdegree MOKE signal. This small MOKE can be easily enhanced by 5 orders of magnitude, via cavity resonance, by optimizing the AFM and TI film thicknesses on the substrate. For slightly off-resonant structures, a $6^{\circ }$ Kerr rotation can be electrically switched on by varying the Fermi energy. This requires less than 20 meV, which is encouraging for low-power spintronics and magneto-optic devices. We further show that this simple structure is easily resilient to 5% material growth error.

An integrated hydrodynamic model that involves a laser heat source, surface evaporation, and interfacial driving forces is developed to simulate the complex physical process of a nanosecond pulsed laser impacting on a water droplet. Droplet deformation tracked by the coupled level set and volume of fluid method is contrasted with the experimental results, and it is shown that the coupled model can be used to simulate the droplet expansion dynamics. From the simulation of droplet deformation, it can be found that, with increasing laser energy, the droplet becomes thinner. With the highest laser energy, the jet on the opposite side to laser impact is interpreted as the flow of high-speed fluid near the axis, which is also confirmed by the velocity field. The simulated temperature reaches or even exceeds the critical value, and the recoil pressure does not completely cover the whole hemispherical surface of the droplet. The liquid-removal depth, which represents the distance that the liquid-phase interface retreats during evaporation, is much shallower than the absorption depth. It can be concluded that propulsion is gained from the intense evaporation of the superheated layer.

The main losses in perovskite solar cells (PSCs) are often related to their charge-transport layers (CTLs). While it has been shown how losses related to charge extraction and recombination at the CTL interfaces can be minimized individually, a comprehensive picture is still lacking. In this work, we describe how recombination at interfaces is related to the distribution of electric potential within the device based on drift-diffusion simulations. Our results show that interface recombination in PSCs can only be fully understood when the whole device is considered. We find that devices with poor conductivity in the CTLs or small built-in potential will suffer from fast recombination at interfaces and that the interface recombination can be avoided by improving these. To illustrate our results, we present a framework that provides an intuitive way to understand electrical potential and compare losses in PSCs. We show that the simulation data can be well understood within the framework for the whole range of parameters studied.

Technologies based on magnetic skyrmions, such as computational devices that can operate at high speed or with low energy consumption, have been proposed by many researchers. Recently, synthetic antiferromagnetic (SAF) structures have been proposed to increase the stability and mobility of skyrmions by reducing or eliminating the skyrmion Hall effect. Here, we numerically study the current-induced dynamics of skyrmions on SAF bilayer structures. We demonstrate the effective control and manipulation of SAF skyrmions, including directional displacement and alignment. Furthermore, we design SAF-skyrmion-based logic gates, such as the and, or, xor, and not gates. Our design provides guidance for future development of spintronic computing devices that use topological nanoscale spin textures as information carriers.

Comprehensive control of the domain-wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories through domain-wall logic concepts to nanomagnetic logic. In this work, focused ${\mathrm{Ga}}^{+}$ ion irradiation is investigated as an effective means to control domain-wall nucleation in $\mathrm{Ta}$/$\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$/$\mathrm{Mg}\mathrm{O}$ nanostructures. We show that, analogously to ${\mathrm{He}}^{+}$ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling bidirectional manipulation schemes. First, the irradiation effects are assessed at the film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain-wall nucleation point is achieved by employing focused irradiation of preirradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from the coherent radiation of a Stoner-Wohlfarth particle to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.