Light-induced photocarrier generation is an essential process in all solar cells, including organic-inorganic hybrid (${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$) solar cells, which exhibit a high short-circuit current density ($J_{\mathrm{sc}}$) of approximately $20\mathrm{mA}/{\mathrm{cm}}^2$. Although the high $J_{\mathrm{sc}}$ observed in the hybrid solar cells relies on strong electron-photon interaction, the optical transitions in the perovskite material remain unclear. Here, we report artifact-free ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ optical constants extracted from ultrasmooth perovskite layers without air exposure and assign all of the optical transitions in the visible and ultraviolet region unambiguously, based on density-functional theory (DFT) analysis that assumes a simple pseudocubic crystal structure. From the self-consistent spectroscopic ellipsometry analysis of the ultrasmooth ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ layers, we find that the absorption coefficients of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ ($\alpha =3.8\times {10}^4{\mathrm{cm}}^{-1}$ at 2.0 eV) are comparable to those of ${\mathrm{CuInGaSe}}_2$ and CdTe, and high $\alpha$ values reported in earlier studies are overestimated seriously by the extensive surface roughness of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ layers. The polarization-dependent DFT calculations show that ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ interacts strongly with the ${{\mathrm{PbI}}_3}^{-}$ cage, modifying the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ dielectric function in the visible region rather significantly. In particular, the transition matrix element of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ varies, depending on the position of ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ within the Pb—I network. When the effect of ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ on the optical transition is eliminated in the DFT calculation, the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ dielectric function deduced from DFT shows an excellent agreement with the experimental result. As a result, distinct optical transitions observed at $E_0(E_g)=1.61\mathrm{eV}$, $E_1=2.53\mathrm{eV}$, and $E_2=3.24\mathrm{eV}$ in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ are attributed to the direct semiconductor-type transitions at the $R$, $M$, and $X$ points in the pseudocubic Brillouin zone, respectively. We further perform the quantum efficiency (QE) analysis for a standard hybrid-perovskite solar cell incorporating a mesoporous ${\mathrm{TiO}}_2$ layer and demonstrate that the QE spectrum can be reproduced almost perfectly when the revised ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ optical constants are employed. Depth-resolved QE simulations confirm that $J_{\mathrm{sc}}$ is limited by the material’s longer wavelength response and indicate the importance of optical confinement and long carrier-diffusion lengths in hybrid perovskite solar cells.

Layer-selective manipulation of the magnetization direction in a multilayer magnetic structure offers a way to develop a large-capacity magnetic recording device with multiple recording layers. Here, we use a double-layer perpendicular magnetic nanodot consisting of two layers with low and high perpendicular magnetic anisotropy (PMA) and show that layer-selective magnetization switching can be accomplished by utilizing a microwave-assisted magnetization switching technique. Since the low- and high-PMA magnetic layers have different ferromagnetic resonance frequencies, their magnetization excitation that reduces the switching field can be individually induced by adjusting the frequency of the applied microwave field ($f_{\mathrm{rf}}$), and this principle allows $f_{\mathrm{rf}}$ to select the layer to be switched. In addition, by measuring thermally excited ferromagnetic-resonance signals from the nanodot, we find that the magnetization dynamics of the two PMA layers couple through interactions, and we identify the excitation modes responsible for the layer-selective magnetization switching.

Technical means for identifying when tampering occurs is a critical part of many containment and surveillance technologies. Conventional fiber-optic seals provide methods for monitoring enclosed inventories, but they are vulnerable to spoofing attacks based on classical physics. We address these vulnerabilities with the development of a quantum seal that offers the ability to detect the intercept-resend attack using quantum integrity verification. Our approach represents an application of entanglement to provide guarantees in the authenticity of the seal state by verifying it is transmitted coherently. We implement these ideas using polarization-entangled photon pairs that are verified after passing through a fiber-optic-channel test bed. Using binary-detection theory, we find the probability of detecting inauthentic signals is greater than 0.9999 with a false-alarm chance of $10^{-9}$ for a 10-s sampling interval. In addition, we show how the Hong-Ou-Mandel effect concurrently provides a tight bound on redirection attack, in which tampering modifies the shape of the seal. Our measurements limit the tolerable path-length change to submillimeter disturbances. These tamper-indicating features of the quantum seal offer unprecedented security for unattended monitoring systems.

Linear and nonlinear vibration energy harvesting has been the focus of considerable research in recent years. However, fundamental limits on the harvestable energy of a harvester subjected to an arbitrary excitation force and different constraints is not yet fully understood. Understanding these limits is not only essential for an assessment of the technology potential, but it also provides a broader perspective on the current harvesting mechanisms and guidance in their improvement. Here, we derive the fundamental limits on the output power of an ideal energy harvester for arbitrary excitation waveforms and build on the current analysis framework for the simple computation of this limit for more sophisticated setups. We show that the optimal harvester maximizes the harvested energy through a mechanical analog of a buy-low-sell-high strategy. We also propose a nonresonant passive latch-assisted harvester to realize this strategy for an effective harvesting. It is shown that the proposed harvester harvests energy more effectively than its linear and bistable counterparts over a wider range of excitation frequencies and amplitudes. The buy-low-sell-high strategy also reveals why the conventional bistable harvester works well at low-frequency excitation.

Optical trapping of light-absorbing particles in a gaseous environment is governed by a laser-induced photophoretic force, which can be orders of magnitude stronger than the force of radiation pressure induced by the same light intensity. In spite of many experimental studies, the exact theoretical background underlying the photophoretic force and the prediction of its influence on the particle motion is still in its infancy. Here, we report the results of a quantitative analysis of the photophoretic force and the stiffness of trapping achieved by levitating graphite and graphite-coated glass shells of calibrated sizes in an upright diverging hollow-core vortex beam, which we refer to as an “optical funnel”. The measurements of forces are conducted in air at various gas pressures in the range from 5 mbar to 2 bar. The results of these measurements lay the foundation for mapping the optically induced force to the intensity distribution in the trap. The mapping, in turn, provides the necessary information to model flight trajectories of particles of various sizes entering the beam at given initial speed and position relative to the beam axis. Finally, we determine the limits of the parameter space for the particle speed, size, and radial offset to the beam axis, all linked to the laser power and the particular laser-beam structure. These results establish the grounds for developing a touch-free optical system for precisely positioning submicrometer bioparticles at the focal spot of an x-ray free-electron laser, which will significantly enhance the efficiency of studying nanoscale morphology of proteins and biomolecules in femtosecond coherent diffractive imaging experiments.

${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal attracts much attention as a solid electrolyte for all-solid-state Li-ion batteries (LIBs) because of its extremely high ionic conductivity at room temperature. Knowing the relationship between the structural framework and dynamics of Li ions will deepen the understanding of lithium-superionic conductors as solid electrolytes in LIBs. However, clarifying the dynamics of Li ions in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal is hindered by the lack of adequate techniques. In this study, we directly observe the fast Li-ion diffusion in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal by using state-of-the-art quasielastic neutron scattering. A signal for the self-diffusion of Li ions is clearly observed at 473 K. The jump diffusion model is used to determine the self-diffusion constant $D$, mean residence time ${\tau }_0$, and jump length $⟨l⟩$ of the Li ions. We use crystal-structure analysis to successfully identify $⟨l⟩$ as the average bond length ${l^{\mathrm{av}}}_{\mathrm{Li}─\mathrm{Li}}$ for the nearest-neighbor $\mathrm{Li}─\mathrm{Li}$ bonds. The Li ions are located within stable regions in the conduction pathways, and the potential barrier is low between them. Consequently, we suggest that the Li ions migrate between stable regions within $⟨l⟩=4.3\AA$ in ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ metastable crystal.

Measuring local magnetization dynamics and its spatial variation is essential for advancements in spintronics and relevant applications. Here we demonstrate a phase-sensitive imaging technique for studying patterned magnetic structures based on picosecond laser heating. With the time-resolved anomalous Nernst effect (TRANE) and extensions, we simultaneously image the dynamic magnetization and rf driving current density. The stroboscopic detection implemented in TRANE microscopy provides access to both amplitude and phase information of the ferromagnetic resonance (FMR) and rf current. Using this approach, we measure the spatial variation of the Oersted driving field angle across a uniform channel. In a spatially nonuniform sample with a cross shape, a strong spatial variation for the rf current as well as FMR precession is observed. We find that both the amplitude and the phase of local FMR precession are closely related to those of the rf current.

We propose and analyze a hybrid device by integrating a microscale diamond beam with a single built-in nitrogen-vacancy ($NV$) center spin to a superconducting coplanar waveguide (CPW) cavity. We find that under an ac electric field the quantized motion of the diamond beam can strongly couple to the single cavity photons via a dielectric interaction. Together with the strong spin-motion interaction via a large magnetic-field gradient, it provides a hybrid quantum device where the diamond resonator can strongly couple both to the single microwave-cavity photons and to the single $NV$ center spin. This enables coherent information transfer and effective coupling between the $NV$ spin and the CPW cavity via mechanically dark polaritons. This hybrid spin-electromechanical device, with tunable couplings by external fields, offers a realistic platform for implementing quantum information with single $NV$ spins, diamond mechanical resonators, and single microwave photons.

Néel walls in soft magnetic NiFe/NiFeGa hybrid stripe structures surrounded by a NiFe film are investigated by high-resolution Lorentz transmission-electron-microscopic imaging. An antiparallel orientation of magnetization in 1000-nm-wide neighboring unirradiated-irradiated stripes is observed by forming high-angle domain walls during magnetization reversal. Upon downscaling the stripe structure size from 1000 to 200 nm, a transition from a discrete domain pattern to an effective magnetic medium is observed for external magnetic-field reversal. This transition is associated with the vanishing ability of hosting high-angle domain walls between adjacent stripes.

We present a simple on-chip electronic thermometer with the potential to operate down to 1 mK. It is based on transport through a single normal-metal–superconductor tunnel junction with rapidly widening leads. The current through the junction is determined by the temperature of the normal electrode that is efficiently thermalized to the phonon bath, and it is virtually insensitive to the temperature of the superconductor, even when the latter is relatively far from equilibrium. We demonstrate here the operation of the device down to 7 mK and present a systematic thermal analysis.

In field-effect transistors, the inherent randomness of dopants and other charges is a major cause of device-to-device variability. For a quasi-one-dimensional device such as carbon nanotube transistors, even a single charge can drastically change the performance, making this a critical issue for their adoption as a practical technology. Here we calculate the effect of the random charges at the gate-oxide surface in ballistic carbon nanotube transistors, finding good agreement with the variability statistics in recent experiments. A combination of experimental and simulation results further reveals that these random charges are also a major factor limiting the subthreshold swing for nanotube transistors fabricated on thin gate dielectrics. We then establish that the scaling of the nanotube device uniformity with the gate dielectric, fixed-charge density, and device dimension is qualitatively different from conventional silicon transistors, reflecting the very different device physics of a ballistic transistor with a quasi-one-dimensional channel. The combination of gate-oxide scaling and improved control of fixed-charge density should provide the uniformity needed for large-scale integration of such novel one-dimensional transistors even at extremely scaled device dimensions.

Conventional phased arrays require a large number of sources in forming a complex wave front, resulting in complexity and a high cost to operate the individual sources. We present a passive phased array using an acoustic metascreen that transmits sound energy from a single source and steers the transmitted wave front to form the desired fields. The metascreen is composed of elements that have a discrete resolution along the screen at an order smaller than the wavelength, allowing for fine wave-front shaping beyond the paraxial approximation. The performance is verified in experiment by forming a self-bending beam. Our metascreen-based passive array with its simplicity and capability has applications in places where conventional active arrays are complex and have limitations.

Optical or electrical stimulation of neural circuits in mice during natural behavior is an important paradigm for studying brain function. Conventional systems for optogenetics and electrical microstimulation require tethers or large head-mounted devices that disrupt animal behavior. We report a method for wireless powering of small-scale implanted devices based on the strong localization of energy that occurs during resonant interaction between a radio-frequency cavity and intrinsic modes in mice. The system features self-tracking over a wide (16-cm diameter) operational area, and is used to demonstrate wireless activation of cortical neurons with miniaturized stimulators ($10{\mathrm{mm}}^3$, 20 mg) fully implanted under the skin.

Near-field heat engines are devices that convert the evanescent thermal field supported by a primary source into usable mechanical energy. By analyzing the thermodynamic performance of three-body near-field heat engines, we demonstrate that the power they supply can be substantially larger than that of two-body systems, showing their strong potential for energy harvesting. Theoretical limits for energy and entropy fluxes in three-body systems are discussed and compared with their corresponding two-body counterparts. Such considerations confirm that the thermodynamic availability in energy-conversion processes driven by three-body photon tunneling can exceed the thermodynamic availability in two-body systems.

Despite the ever-increasing interest in multiscale porous materials, the chemophysical origin of their thermal properties at the nanoscale and its connection to the macroscale properties still remain rather obscure. In this paper, we link the atomic- and macroscopic-level thermal properties by combining tools of statistical physics and mean-field homogenization theory. We begin with analyzing the vibrational density of states of several calcium-silicate materials in the cement paste. Unlike crystalline phases, we indicate that calcium silicate hydrates (CSH) exhibit extra vibrational states at low frequencies ($<2\mathrm{THz}$) compared to the vibrational states predicted by the Debye model. This anomaly is commonly referred to as the boson peak in glass physics. In addition, the specific-heat capacity of CSH in both dry and saturated states scales linearly with the calcium-to-silicon ratio. We show that the nanoscale-confining environment of CSH decreases the apparent heat capacity of water by a factor of 4. Furthermore, full thermal conductivity tensors for all phases are calculated via the Green-Kubo formalism. We estimate the mean free path of phonons in calcium silicates to be on the order of interatomic bonds. This satisfies the scale separability condition and justifies the use of mean-field homogenization theories for upscaling purposes. Upscaling schemes yield a good estimate of the macroscopic specific-heat capacity and thermal conductivity of cement paste during the hydration process, independent of fitting parameters.

Cement paste has a complex distribution of pores and molecular-scale spaces. This distribution controls the hysteresis of water sorption isotherms and associated bulk dimensional changes (shrinkage). We focus on two locations of evaporable water within the fine structure of pastes, each having unique properties, and we present applied physics models that capture the hysteresis by dividing drying and rewetting into two related regimes based on relative humidity (RH). We show that a continuum model, incorporating a pore-blocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above approximately 20% RH. In addition, we show with molecular models and experiments that water in spaces of $\lesssim 1\mathrm{nm}$ width evaporates below approximately 20% RH but reenters throughout the entire RH range. This water is responsible for a drying shrinkage hysteresis similar to that of clays but opposite in direction to typical mesoporous glass. Combining the models of these two regimes allows the entire drying and rewetting hysteresis to be reproduced accurately and provides parameters to predict the corresponding dimensional changes. The resulting model can improve the engineering predictions of long-term drying shrinkage accounting also for the history dependence of strain induced by hysteresis. Alternative strategies for quantitative analyses of the microstructure of cement paste based on this mesoscale physical model of water content within porous spaces are discussed.

We numerically demonstrate a practical means of systematically controlling topological transport on the surface of a three-dimensional topological insulator, by introducing strong disorder in a layer of depth $d$ extending inward from the surface of the topological insulator. The dependence on $d$ of the density of states, conductance, scattering time, scattering length, diffusion constant, and mean Fermi velocity are investigated. The proposed control via disorder depth $d$ requires that the disorder strength be near the large value which is necessary to drive the topological insulator into the nontopological phase. If $d$ is patterned using masks, gates, ion implantation, etc., then integrated circuits may be fabricated. This technique will be useful for experiments and for device engineering.

We demonstrate a general method to assemble and propel highly maneuverable colloidal carpets which can be steered via remote control in any direction of the plane. These colloidal micropropellers are composed by an ensemble of spinning rotors and can be readily used to entrap, transport, and release biological cargos on command via a hydrodynamic conveyor-belt effect. An efficient control of the cargo transportation combined with remarkable “healing” ability to surpass obstacles demonstrate a great potential towards development of multifunctional smart devices at the microscale.

We report on highly tunable radio-frequency (rf) characteristics of a low-loss and compact three-dimensional (3D) metamaterial made of superconducting thin-film spiral resonators. The rf transmission spectrum of a single element of the metamaterial shows a fundamental resonance peak at $\sim 24.95\mathrm{MHz}$ that shifts to a 25% smaller frequency and becomes degenerate when a 3D array of such elements is created. The metamaterial shows an in situ tunable narrow frequency band in which the real part of the effective permeability is negative over a wide range of temperature. This narrow frequency band gradually possesses near-zero and positive values for the real part of permeability as the superconducting critical temperature is approached. The studied 3D metamaterial can be used for increasing power-transfer efficiency and tunability of electrically small rf antennas.

We study microfluidic self-digitization in Hele-Shaw cells using pancake droplets anchored to surface-tension traps. We show that above a critical flow rate, large anchored droplets break up to form two daughter droplets, one of which remains in the anchor. Below the critical flow velocity for breakup, the shape of the anchored drop is given by an elastica equation that depends on the capillary number of the outer fluid. As the velocity crosses the critical value, the equation stops admitting a solution that satisfies the boundary conditions; the drop breaks up in spite of the neck still having finite width. A similar breaking event also takes place between the holes of an array of anchors, which we use to produce a 2D array of stationary drops in situ.