The mode shape of bending waves in thin silicon and silicon-carbide membranes is measured as a function of space and time, using a phase-shift interferometer with stroboscopic light. The mode shapes hold information about all the relevant mechanical parameters of the samples, including the spatial distribution of static prestress. We present a simple algorithm to obtain a map of the lateral tensor components of the prestress, with a spatial resolution much better than the wavelength of the bending waves. The method is not limited to measuring the stress of bending waves. It is applicable in almost any situation, where the fields determining the state of the system can be measured as a function of space and time.

Spin-orbit coupling in ferromagnets gives rise to the anomalous Hall effect and the anisotropic magnetoresistance, both of which can be used to create spin-transfer torques in a similar manner as the spin Hall effect. In this paper, we show how these effects can be used to reliably switch perpendicularly magnetized layers and to move domain walls. A drift-diffusion treatment of the anomalous Hall effect and the anisotropic magnetoresistance describes the spin currents that flow in directions perpendicular to the electric field. In systems with two ferromagnetic layers separated by a spacer layer, an in-plane electric field causes spin currents to be injected from one layer into the other, creating spin-transfer torques. Unlike the related spin Hall effect in nonmagnetic materials, the anomalous Hall effect and the anisotropic magnetoresistance allow control of the orientation of the injected spins, and hence torques, by changing the direction of the magnetization in the injecting layer. The torques on one layer show a rich angular dependence as a function of the orientation of the magnetization in the other layer. The control of the torques afforded by changing the orientation of the magnetization in a fixed layer makes it possible to reliably switch a perpendicularly magnetized free layer. Our calculated critical current densities for a representative CoFe/Cu/FePt structure show that the switching can be efficient for appropriate material choices. Similarly, control of the magnetization direction can drive domain-wall motion, as shown for NiFe/Cu/NiFe structures.

The two-dimensional electron gas (2DEG) observed at the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ heterointerface has attracted intense research interest in recent years. The high mobility, electric tunability, and giant persistent photoconductivity suggest its potential for electronic and photonic applications. The lack of a $p$-type counterpart as well as a highly spin-polarized carrier in the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ system, however, restricts its widespread application, since both multiple carriers and high spin polarization are very desirable for electronic devices. Here, we report a system of ${\text{LaAlO}}_3/{\text{EuTiO}}_3$ digital heterostructures that may overcome these limitations. Results from first-principles calculations reveal that the 2DEG in the $n$-type ${\text{LaAlO}}_3/{\text{EuTiO}}_3$ is a normal ferromagnet. The $p$-type two-dimensional hole gas, on the other hand, is a 100% spin-polarized half-metal. For digital heterostructures with alternating $n$-type and $p$-type interfaces, a magnetic-field-driven insulator-to-metal transition, together with spatially separated electrons and holes, can be realized by tuning the intrinsic polar field. At low temperatures, the spin-polarized electron-hole pairs may result in spin-triplet exciton condensation, which provides an experimentally accessible system for achieving the theoretically proposed dissipationless spin transport. Our findings open a path for exploring spintronics at the heterointerface of transition-metal oxides.

In this work, we study the folding and unfolding of flowing viscous jets by imposing an electric field. We demonstrate that a folded viscous jet can be induced to unfold through jet widening in a sufficiently strong electric field. The folded jets unfold above a critical slenderness, which increases as the jet capillary number increases. Our systematic elucidation of the mechanisms behind the controlled folding has important implications on processes such as nozzle designs for industrial applications that rely on the manipulation of high-speed viscous jets, including liquid dispensing, printing, and food processing.

We obtain robust perpendicular magnetic anisotropy in a $\beta \text{−}\mathrm{W}/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/\mathrm{MgO}$ structure without the need of any insertion layer between W and ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$. This is achieved within a broad range of W thicknesses (3.0–9.0 nm), using a simple fabrication technique. We determine the spin Hall angle (0.40) and spin-diffusion length for the bulk $\beta$ form of tungsten with a large spin-orbit coupling. As a result of the giant spin Hall effect in $\beta \text{−}\mathrm{W}$ and careful magnetic annealing, we significantly reduce the critical current density for the spin-transfer-torque-induced magnetic switching in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$. The elemental $\beta \text{−}\mathrm{W}$ is a superior candidate for magnetic memory and spin-logic applications.

Using a focused-ion-beam microscope, we create nontopographic features that provide controlled modification of domain-wall structure, size, and pinning strength in 500-nm-wide nanowires composed from $\mathrm{Cr}(3\mathrm{nm})/\text{permalloy}(10\mathrm{nm})/\mathrm{Cr}(5\mathrm{nm})$. The pinning sites consist of linear defects where magnetic properties are modified by a ${\mathrm{Ga}}^{+}$-ion probe of diameter $\sim 10\mathrm{nm}$. Detailed studies of the structural, chemical, and magnetic changes induced by the irradiation, which show the modified region to be $\sim 40–50\text{nm}$ wide, are performed using scanning-transmission-electron-microscopy modes of bright-field imaging, electron-energy-loss spectroscopy, and differential-phase-contrast imaging on an aberration corrected (Cs) instrument. The Fresnel mode of Lorentz-transmission-electron microscopy is used for studies of domain-wall behavior, where we observe changes in depinning strength and structure with irradiation dose and line orientation. We present an understanding of this behavior based upon micromagnetic simulation of the irradiated defects and their effect on the energy terms for the domain walls.

Compositional inhomogeneities in multielemental materials typically form due to lowering of the energy relative to the homogeneous phase. Here, we demonstrate an entropy-driven mechanism in the zinc-blende derived cation-substituted multinary compounds ${\mathrm{Cu}}_2{\mathrm{SnS}}_3$ (CTS) and ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS). Using a motif-based model Hamiltonian and Monte Carlo simulations, we find that disorder leads to a redistribution of the structural motifs in such a way to create cation clustering. The associated formation of (sub-) nanometer-scale compositional inhomogeneities can cause potential fluctuations with detrimental consequences for photovoltaic applications.

With recent advances in nanophotonics and nanofabrication, considerable progress has been achieved in realizations of thin composite layers designed for full absorption of incident electromagnetic radiation, from microwaves to the visible. If the layer is structured at a subwavelength scale, thin perfect absorbers are usually called “metamaterial absorbers,” because these composite structures are designed to emulate some material responses not reachable with any natural material. On the other hand, many thin absorbing composite layers were designed and used already in the time of the introduction of radar technology, predominantly as a means to reduce radar visibility of targets. In view of a wide variety of classical and new topologies of optically thin metamaterial absorbers and plurality of applications, there is a need for a general, conceptual overview of the fundamental mechanisms of full absorption of light or microwave radiation in thin layers. Here, we present such an overview in the form of a general theory of thin perfectly absorbing layers. Possible topologies of perfect metamaterial absorbers are classified based on their fundamental operational principles. For each of the identified classes, we provide design equations and give examples of particular realizations. The concluding section provides a summary and gives an outlook on future developments in this field.

In order to develop a simple method for preventing unwanted thermally driven acoustic gas oscillations, the amplitude-death phenomenon in thermoacoustic oscillators coupled by dissipative and time-delay coupling is experimentally studied. The coupling is designed and built using a needle valve and a half-wavelength tube. Experimental bifurcation diagrams show that oscillations can be completely stopped even in a system comprising identical oscillators when the oscillators are coupled by both dissipative and time-delay coupling. The effectiveness of this type of coupling is confirmed by numerical calculations.

We report experimental and theoretical results on a ballistic energy-conversion method based on a rotary atomizer working with a droplet acceleration-deceleration cycle. In a rotary atomizer, liquid is fed onto the center of a rotating flat surface, where it spreads out under the action of the centrifugal force and creates “atomized” droplets at its edge. The advantage of using a rotary atomizer is that the centrifugal force exerted on the fluid on a smooth, large surface is not only a robust form of acceleration, as it avoids clogging, but also easily allows high throughput, and produces high electrical power. We successfully demonstrate an output power of 4.9 mW and a high voltage up to 3120 V. At present, the efficiency of the system is still low (0.14%). However, the conversion mechanism of the system is fully interpreted in this paper, permitting a conceptual understanding of system operation and providing a roadmap for system optimization. This observation will open up a road for building power-generation systems in the near future.

Detuning a superconducting qubit from its rotating frame is one means to implement a $Z$-gate operation. In this work, we implement a $Z$ gate by pulsing a current through the qubit’s readout dc SQUID. While the dc SQUID acts as a magnetic flux sensor for qubit readout, we in turn may use it as a flux actuator with tunable strength to impose a qubit frequency shift. Using this approach, we demonstrate Ramsey-type free-induction experiments with time constants as long as 280 ns and rotation frequencies as high as 1.4 GHz. We experimentally demonstrate an inferred $Z$-gate fidelity of approximately 90%, limited largely by the bandwidth of our system. In the absence of this limitation, we argue that the inferred fidelity may be improved to as high as 99%.

The advent of parallelized automated methods for rapid whole-genome analysis has led to an exponential drop in costs, thus greatly accelerating biomedical research and discovery. Third-generation sequencing techniques, which would utilize the characteristic electrical conductance of the four different nucleotides, could facilitate longer base read lengths and an even lower price per genome. In this work, we propose and apply a quantum-classical hybrid methodology to quantitatively determine the influence of the solvent on the dynamics of DNA and the resulting electron transport properties of a prototypic sequencing device utilizing a graphene nanopore through which the nucleic-acid chain is threaded. Our results show that charge fluctuations in the nucleotides are responsible for characteristic conductance modulations in this system, which can be regarded as a field-effect transistor tuned by the dynamic aqueous environment.

It is hypothesized that the decreased treatment efficiency in contemporary shock-wave lithotripters is related to tensile wave attenuation due to cavitation in the prefocal beam path. Utilizing high-speed imaging of the beam path and focal pressure waveform measurements, tensile attenuation is associated with bubble proliferation. By systematically testing different combinations of pulse-repetition frequency and gas concentration, we modulate the bubble-dissolution time to identify which conditions lead to bubble proliferation and show that reducing bubble proliferation in the beam path significantly improves acoustic transmission and stone comminution efficiency in vitro. In addition to experiments, a bubble-proliferation model is developed that takes gas diffusion across the bubble wall and bubble fragmentation into account. By aligning the model with experimental observations, the number of daughter bubbles produced after a single lithotripter bubble collapse is estimated to be in the range of $253\sim 510$. This finding is on the same order of magnitude with previous measurements of an isolated bubble collapse in a lithotripter field by Pishchalnikov, McAteer, and Williams [BJU Int. 102, 1681 (2008)], and this estimate improves the general understanding of lithotripsy bubble dynamics in the beam path.

A conceptual paradigm for multiparametric optical biosensing in a lab-on-a-chip, using the cascaded transmission of light through a photonic crystal that contains microfluidic channels and is embedded in a glass slide, is demonstrated. Accurate detection of multiple disease-identifying biomarkers is facilitated by the interaction of surface and photonic-band-gap waveguide modes. Through finite-difference time-domain simulations, levels of light transmission through the device are shown to be simultaneously responsive to analyte bindings and layer thicknesses at different locations along a single optical transmission path through the photonic crystal. Our multiparametric biosensing mechanism supersedes traditional single-resonance-shift-based biosensing and provides a more detailed spectral fingerprint of various diseases or various stages of a given disease. Moreover, the spectral line shape due to the engineered optical modes can logically discriminate between different concentrations of several analytes flowing through the microfluidic channels. The simultaneous detection and differentiation of (combinations of) distinct analytes using a single measurement on one device offers a paradigm for optical biosensing.

The carbon nanotubes’ resilience to mechanical deformation is a potentially important feature for imparting tunable properties at the nanoscale. Using nonequilibrium molecular dynamics and empirical interatomic potentials, we examine the thermal conductivity variations with bending in the thermal transport regime where both ballistic and diffusive effects coexist. These simulations are enabled by the realistic atomic-scale descriptions of uniformly curved and buckled nanotube morphologies obtained by imposing objective boundary conditions. We uncover a contrasting behavior. At shorter lengths, the phonon propagation is affected significantly by the occurrence of localized structural buckling. As the nanotube length becomes comparable with the phonon mean free path, heat transport becomes insensitive to the buckling deformations. Our result settles the controversy around the differences between the current experimental and molecular-dynamics measurements of the thermal transport in bent nanotubes.

Layered $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$ and ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ are of great interest for lithium-ion battery cathodes because of their high theoretical capacities. The practical application of these materials is, however, limited due to poor electrochemical performance. We herein report a comprehensive first-principles study of defect physics in $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$ and ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ using hybrid density-functional calculations. We find that manganese antisites have low formation energies in $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$ and may act as nucleation sites for the formation of impurity phases. The antisites can also occur with high concentrations in ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$; however, unlike in $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$, they can be eliminated by tuning the experimental conditions during preparation. Other intrinsic point defects may also occur and have an impact on the materials’ properties and functioning. An analysis of the formation of lithium vacancies indicates that lithium extraction from $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_2$ is associated with oxidation at the manganese site, resulting in the formation of manganese small hole polarons; whereas in ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ the intrinsic delithiation mechanism involves oxidation at the oxygen site, leading to the formation of bound oxygen hole polarons ${\eta }_{\mathrm{O}}^{+}$. The layered oxides are found to have no or negligible bandlike carriers, and they cannot be doped $n$ or $p$ type. The electronic conduction proceeds through hopping of hole and/or electron polarons; the ionic conduction occurs through lithium monovacancy and/or divacancy migration mechanisms. Since ${\eta }_{\mathrm{O}}^{+}$ is not stable in the absence of negatively charged lithium vacancies in bulk ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$, the electronic conduction near the start of delithiation is likely to be poor. We suggest that the electronic conduction associated with ${\eta }_{\mathrm{O}}^{+}$ and, hence, the electrochemical performance of ${\mathrm{Li}}_2\mathrm{Mn}{\mathrm{O}}_3$ can be improved through nanostructuring and/or ion substitution.

Small-angle x-ray scattering (SAXS) is a powerful technique for the investigation of catalyst materials at the nanoscale. We present results of an anomalous SAXS study on metal-oxide-supported platinum particles used as electrocatalysts for oxygen reduction. The scattering interferences between catalyst particles and support material are taken into account qualitatively and quantitatively by a mathematical model for the data-fitting procedure. Our results clearly demonstrate the fundamental importance of these catalyst-particle–support-material interferences in the analysis of SAXS data from supported catalysts.

ZnS quantum dots (QDs) have limited application potential in QD-sensitized solar cells because of their wide-band-gap, which does not allow absorption of sunlight in the visible and infrared regions. Introducing intermediate-energy levels in the QDs is one way to expand the absorption window into the visible region. We show that this effect is achieved in Mn-doped ZnS QDs. Mn-doped ZnS QDs are synthesized by laser ablation in water and solution-based methods. The structural, optical, and magnetic properties of the $\mathrm{ZnS}:\mathrm{Mn}$ QDs are examined by x-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) emission, photoluminescence excitation (PLE), and magnetic susceptibility measurements. The average particle size of cubic phase $\mathrm{ZnS}:\mathrm{Mn}$ estimated from the XRD and TEM is about 3 nm. The QDs show two PL peaks near 450 and 600 nm, which are attributed to the defect-related emission of ZnS and emission of ${\mathrm{Mn}}^{2+}$ in a ZnS host, respectively. The PLE spectra exhibit near-band-edge absorption of ZnS at 350 nm and the absorption of ${\mathrm{Mn}}^{2+}$ internal-energy levels around 468 nm. The latter absorption is due to the transitions of the $3d^5$ electronic states of ${\mathrm{Mn}}^{2+}$ from the ground state ${}^6{A}_1$ to excited states ${}^4{A}_1$ and ${}^{4}E$ and plays an important role in improving the absorption of the material in the visible region. $\mathrm{ZnS}:\mathrm{Mn}$ QDs coated on ${\mathrm{Zn}}_2{\mathrm{SnO}}_4$ nanowires show greatly improved sensitization in the visible region as demonstrated by incident photon-to-electron conversion efficiency experiments. Our study also shows that the characteristics of solar-cell performance can be tuned with the Mn concentration.