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.

The ability of high-temperature superconductors (HTSs) to carry very large currents with almost no dissipation makes them irreplaceable for high-power applications. The development and further improvement of HTS-based cables require an in-depth understanding of the superconducting vortex dynamics in the presence of complex pinning landscapes. We present a critical current analysis of a real HTS sample in a magnetic field by combining state-of-the-art large-scale Ginzburg-Landau simulations with reconstructive three-dimensional scanning-transmission-electron-microscopy tomography of the pinning landscape in Dy-doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. This methodology provides a unique look at the vortex dynamics in the presence of a complex pinning landscape responsible for the high-current-carrying-capacity characteristic of commercial HTS wires. Our method demonstrates very good functional and quantitative agreement of the critical current between simulation and experiment, providing a new predictive tool for HTS wire designs.

Thin films of functional materials, from graphene to semiconductor heterostructures, and from nanomembranes to Langmuir-Blodgett films play key roles in modern technologies. For such films optical interrogation is the main and often the only practical method of characterization. Here, we show that characterization of the optical response of thin films can be greatly improved with a type of coherent spectroscopy using two counterpropagating beams of light. The spectroscopy is selective to particular types of multipole resonances that form the absorption spectrum of the film, and therefore can reveal lines that are hidden in conventional absorption spectroscopy. We explicitly demonstrate selectivity of this spectroscopy in a series of proof-of-principle experiments with plasmonic metamaterial arrays designed to exhibit different multipole resonances. We further demonstrate the analytic potential of this spectroscopy by extracting the hidden resonance from the spectrum of a complex nanostructure.

Using a circuit QED device, we demonstrate a simple qubit-measurement pulse shape that yields fast ring-up and ring-down of the readout resonator regardless of the qubit state. The pulse differs from a square pulse only by the inclusion of additional constant-amplitude segments designed to effect a rapid transition from one steady-state population to another. Using a Ramsey experiment performed shortly after the measurement pulse to quantify the residual population, we find that compared to a square pulse followed by a delay, this pulse shape reduces the time scale for cavity ring-down by more than twice the cavity time constant. At low drive powers, this performance is achieved using pulse parameters calculated from a linear cavity model; at higher powers, empirical optimization of the pulse parameters leads to similar performance.

We show that the use of subnanometer adhesion layers significantly enhances the thermal interface conductance at metal-dielectric interfaces. A metal-dielectric interface between Au and sapphire (${\text{Al}}_2{\mathrm{O}}_3$) is considered using Cu (low optical loss) and Cr (high optical loss) as adhesion layers. To enable high throughput measurements, each adhesion layer is deposited as a wedge such that a continuous range of thicknesses could be sampled. Our measurements of thermal interface conductance at the metal-${\text{Al}}_2{\mathrm{O}}_3$ interface made using frequency-domain thermoreflectance show that a 1-nm-thick adhesion layer of Cu or Cr is sufficient to enhance the thermal interface conductance by more than a factor of 2 or 4, respectively, relative to the pure $\text{Au}/{\text{Al}}_2{\mathrm{O}}_3$ interface. The enhancement agrees with the diffuse-mismatch-model-based predictions of accumulated thermal conductance versus adhesion-layer thickness assuming that it contributes phonons with wavelengths less than its thickness, while those with longer wavelengths transmit directly from the Au.

Hydrogen pickup and embrittlement pose a challenging safety limit for structural alloys used in a wide range of infrastructure applications, including zirconium alloys in nuclear reactors. Previous experimental observations guide the empirical design of hydrogen-resistant zirconium alloys, but the underlying mechanisms remain undecipherable. Here, we assess two critical prongs of hydrogen pickup through the ${\mathrm{ZrO}}_2$ passive film that serves as a surface barrier of zirconium alloys; the solubility of hydrogen in it—a detrimental process—and the ease of ${\mathrm{H}}_2$ gas evolution from its surface—a desirable process. By combining statistical thermodynamics and density-functional-theory calculations, we show that hydrogen solubility in ${\mathrm{ZrO}}_2$ exhibits a valley shape as a function of the chemical potential of electrons, ${\mu }_e$. Here, ${\mu }_e$, which is tunable by doping, serves as a physical descriptor of hydrogen resistance based on the electronic structure of ${\mathrm{ZrO}}_2$. For designing zirconium alloys resistant against hydrogen pickup, we target either a dopant that thermodynamically minimizes the solubility of hydrogen in ${\mathrm{ZrO}}_2$ at the bottom of this valley (such as Cr) or a dopant that maximizes ${\mu }_e$ and kinetically accelerates proton reduction and ${\mathrm{H}}_2$ evolution at the surface of ${\mathrm{ZrO}}_2$ (such as Nb, Ta, Mo, W, or P). Maximizing ${\mu }_e$ also promotes the predomination of a less-mobile form of hydrogen defect, which can reduce the flux of hydrogen uptake. The analysis presented here for the case of ${\mathrm{ZrO}}_2$ passive film on Zr alloys serves as a broadly applicable and physically informed framework to uncover doping strategies to mitigate hydrogen embrittlement also in other alloys, such as austenitic steels or nickel alloys, which absorb hydrogen through their surface oxide films.

We explore the ability of a qubit probe to characterize unknown parameters of its environment. By resorting to the quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises (“baths”) which the qubit may probe. These include the probe-bath coupling strength, the correlation time of generic types of bath spectra, and the power laws governing these spectra, as well as their dephasing times $T_2$. Our central result is that by optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible. Applications of this protocol that combines dynamical control and estimation theory tools to quantum sensing are illustrated for a nitrogen-vacancy center in diamond used as a probe.

The switching of magnetization by ultrafast lasers alone in FePt could open a technological perspective for magnetic recording technology. Recent experimental results [D. Lambert et al., Science 345, 1337 (2014)] indicate a dynamical magnetization response in FePt under circularly polarized laser pulses. Using high-temperature micromagnetic modeling, based on the stochastic Landau-Lifshitz-Bloch equation, we investigate the possibility of magnetization switching in FePt under the action of an ultrafast heat pulse assisted by either a constant or optomagnetic field. We evaluate the necessary magnitude and duration of the inverse Faraday field to produce a reliable switching. Our results also reproduce experimentally observed magnetization patterns originated from the nonhomogeneous temperature distribution.

We propose and demonstrate a new autolocking scheme using a three-mirror ring cavity consisting of a linear quadratic regulator and a time-varying Kalman filter. Our technique does not require a frequency scan to acquire resonance. We utilize the singular perturbation method to simplify our system dynamics and to permit the application of linear control techniques. The error signal combined with the transmitted power is used to estimate the cavity detuning. This estimate is used by a linear time-varying Kalman filter which enables the implementation of an optimal controller. The experimental results validate the controller design, and we demonstrate improved robustness to disturbances and a faster locking time than a traditional proportional-integral controller. More important, the time-varying Kalman filtering approach automatically reacquires lock for large detunings, where the error signal leaves its linear capture range, a feat which linear time-invariant controllers cannot achieve.

We study the scattering of electromagnetic waves in anisotropic energetic materials. Nonlinear light-matter interactions in molecular crystals result in frequency-conversion and polarization changes. Applied electromagnetic fields of moderate intensity can induce these nonlinear effects without triggering chemical decomposition, offering a mechanism for the nonionizing identification of explosives. We use molecular-dynamics simulations to compute such two-dimensional THz spectra for planar slabs made of pentaerythritol tetranitrate and ammonium nitrate. We discuss third-harmonic generation and polarization-conversion processes in such materials. These observed far-field spectral features of the reflected or transmitted light may serve as an alternative tool for standoff explosive detection.

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.

Self-shaping of curved structures, especially those involving flexible thin layers, is attracting increasing attention because of their broad potential applications in, e.g., nanoelectromechanical andmicroelectromechanical systems, sensors, artificial skins, stretchable electronics, robotics, and drug delivery. Here, we provide an overview of recent experimental, theoretical, and computational studies on the mechanical self-assembly of strain-engineered thin layers, with an emphasis on systems in which the competition between bending and stretching energy gives rise to a variety of deformations, such as wrinkling, rolling, and twisting. We address the principle of mechanical instabilities, which is often manifested in wrinkling or multistability of strain-engineered thin layers. The principles of shape selection and transition in helical ribbons are also systematically examined. We hope that a more comprehensive understanding of the mechanical principles underlying these rich phenomena can foster the development of techniques for manufacturing functional three-dimensional structures on demand for a broad spectrum of engineering applications.

All-spin-logic devices are promising candidates to augment and complement beyond-CMOS integrated circuit computing due to nonvolatility, ultralow operating voltages, higher logical efficiency, and high density integration. However, the path to reach lower energy-delay product performance compared to CMOS transistors currently is not clear. We show that scaling and engineering the nanoscale magnetic materials and interfaces is the key to realizing spin-logic devices that can surpass the energy-delay performance of CMOS transistors. With validated stochastic nanomagnetic and vector spin-transport numerical models, we derive the target material and interface properties for the nanomagnets and channels. We identify promising directions for material engineering and discovery focusing on the systematic scaling of magnetic anisotropy ($H_k$) and saturation magnetization ($M_s$), the use of perpendicular magnetic anisotropy, and the interface spin-mixing conductance of the ferromagnet–spin-channel interface ($G_{\text{mix}}$). We provide systematic targets for scaling a spin-logic energy-delay product toward 2 aJ ns, comprehending the stochastic noise for nanomagnets.

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.

Acoustic microswimmers present a great potential for microfluidic applications and targeted drug delivery. Here, we introduce armored microbubbles (size range, $10–20\mu \mathrm{m}$) made by three-dimensional microfabrication, which allows the bubbles to last for hours even under forced oscillations. The acoustic resonance of the armored microbubbles is found to be dictated by capillary forces and not by gas volume, and its measurements agree with a theoretical calculation. We further measure experimentally and predict theoretically the net propulsive flow generated by the bubble vibration. This flow, due to steady streaming in the fluid, can reach $100\mathrm{mm}/\mathrm{s}$, and is affected by the presence of nearby walls. Finally, microswimmers in motion are shown, either as spinning devices or free swimmers.

In a hybrid quantum network, the linking of two types of quantum nodes through photonic channels requires excellent matching of the central frequency and bandwidth between both nodes and their interfacing photons. However, preexisting photon sources cannot fulfill this requirement. Using a novel conjoined double-cavity strategy, we report the generation of nondegenerate narrow-band photon pairs by cavity-enhanced spontaneous parametric down-conversion. The central frequencies and bandwidths of the signal and idler photons are independently set to match with trapped ions and solid-state quantum memories. With this source we achieve the bandwidths and central wavelengths of 4 MHz at 935 nm and 5 MHz at 880 nm for the signal and idler photons, respectively, with a normalized spectral brightness of $4.9\mathrm{photon}\mathrm{pairs}/(\mathrm{s}\mathrm{MHz}\mathrm{mW})$. Because of its ability to be independently locked to two different wavelengths, the conjoined double cavity is universally suitable for a hybrid quantum network consisting of various quantum nodes.

The roughness of Pt electrodes is shown to have a direct impact on the electroforming characteristics of $\mathrm{Pt}/\mathrm{Ti}/{\mathrm{HfO}}_2/\mathrm{Pt}$ device structures. Specifically, an increase in roughness leads to a reduction in the electroforming voltage of ${\mathrm{HfO}}_2$, an increase in the failure rate of devices, and a corresponding reduction in resistive switching reliability. A finite-element model is used to investigate the significance of local electric-field enhancement on the breakdown process. This simulation shows that high-aspect-ratio asperities can produce field enhancements of more than an order of magnitude but that the generation and redistribution of defects moderate this effect prior to dielectric breakdown. As a consequence, the effect of field enhancement is less than anticipated from the initial electric-field distribution alone. Finally, it is argued that the increase in the device failure rate with increasing electrode roughness derives partly from an increase in the film defect density and effective device area and that these effects contribute to the reduction in breakdown voltage.

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.

Understanding the mechanisms of generation of oxygen vacancies in monoclinic $(m)\text{−}{\mathrm{HfO}}_2$ is important for improving and controlling its performance as an oxide layer in transistor gate stacks and in resistive random-access memory (RRAM) devices. We use ab initio calculations to investigate the mechanism of formation of Frenkel pairs of oxygen vacancies and interstitial ions in $m\text{−}{\mathrm{HfO}}_2$ under electron-injection conditions. The results demonstrate that the formation of stable pairs of neutral oxygen vacancies and interstitial oxygen ions assisted by extra electrons is thermodynamically feasible and requires overcoming activation barriers of less than 1.3 eV at preexisting O vacancies. A preexisting oxygen vacancy can act as an electron trap and facilitate the formation of an O vacancy and O interstitial ion pair nearby. The resulting O divacancy is stabilized by weak attraction between neutral vacancies, further lowering the formation energy of the defect pair. The binding energy per vacancy in larger oxygen-vacancy aggregates increases as the aggregate grows, facilitating the formation of defect pairs next to larger vacancy aggregates. These results are useful for understanding the mechanisms of oxide degradation and electroforming in RRAM cells, which can proceed through creation of new O vacancies in the vicinity of preexisting vacancies complementing vacancy aggregation via diffusion processes.

A type of gear is proposed based on the interaction of individual multipoles. The underlying principle relies on previously unknown continuous degenerate ground states for pairs of interacting multipoles which are free to rotate around specific axes. These special rotation axes, in turn, form a one-parameter family of possible configurations. This allows for the construction of magnetic bevel gears with any desired inclination angle between the in- and output axes. Further, the design of gear systems with more than two multipoles is possible and facilitates tailored applications. Ultimately, an analogy between multipoles and mechanical gears is revealed. In contrast to the mechanical case, the multipole “teeth” mesh smoothly. As an illustrative application, the example of a quadrupole-dipole interaction is then used to construct a $1:2$ gear ratio.