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.

Solid-state qubits have recently advanced to the level that enables them, in principle, to be scaled up into fault-tolerant quantum computers. As these physical qubits continue to advance, meeting the challenge of realizing a quantum machine will also require the development of new supporting devices and control architectures with complexity far beyond the systems used in today’s few-qubit experiments. Here, we report a microarchitecture for controlling and reading out qubits during the execution of a quantum algorithm such as an error-correcting code. We demonstrate the basic principles of this architecture using a cryogenic switch matrix implemented via high-electron-mobility transistors and a new kind of semiconductor device based on gate-switchable capacitance. The switch matrix is used to route microwave waveforms to qubits under the control of a field-programmable gate array, also operating at cryogenic temperatures. Taken together, these results suggest a viable approach for controlling large-scale quantum systems using semiconductor technology.

The formation of conducting channels of ${\mathrm{Ti}}_4{\mathrm{O}}_7$ inside $\mathrm{Ti}{\mathrm{O}}_2$-based memristors is believed to be the origin for the change in electric resistivity of these devices. While the properties of the bulk materials are reasonably well known, the interface between them has not been studied up to now, mostly because of their different crystalline structures. In this work, we present a way to match the interfaces between $\mathrm{Ti}{\mathrm{O}}_2$ and ${\mathrm{Ti}}_4{\mathrm{O}}_7$ and subsequently the band offset between these materials is obtained from density-functional-theory-based calculations. The results show that, while the valence band is located at the ${\mathrm{Ti}}_4{\mathrm{O}}_7$, the conduction band is found at the $\mathrm{Ti}{\mathrm{O}}_2$ structure, resulting in a type-II interface. In this case, the ${\mathrm{Ti}}_4{\mathrm{O}}_7$ acts as a donor to the $\mathrm{Ti}{\mathrm{O}}_2$ matrix.

In this paper, we present a method for a comprehensive analysis of the efficiency roll-off with current density in phosphorescent organic light-emitting diodes (OLEDs). By combining electrical and optical excitation in time-resolved spectroscopic experiments, we are able to measure the excited-state lifetime for different driving conditions. It is, thus, possible to correlate changes of the triplet lifetime with a decrease of the radiative quantum efficiency of the emitting system due to exciton quenching processes. As compared to the conventional analysis of the measured external quantum efficiency (EQE) in dependence of the applied current density, the lifetime analysis is not affected by changes of the charge-carrier balance with current, which can have a significant impact on the interpretation of the results. By performing time-resolved spectroscopy for a series of red phosphorescent OLEDs, triplet-polaron quenching (TPQ) is identified as the dominant mechanism behind the efficiency roll-off up to a current density of $100\mathrm{mA}/{\mathrm{cm}}^2$, while the conventional EQE vs current plot rather suggests triplet-triplet annihilation as the main quenching mechanism. We show that this apparent discrepancy is caused by exciton quenching occurring already at very low current densities, where EQE measurements are not reliable due to significant changes of the charge-carrier balance in this region. In addition, we present evidence that the triplet-polaron quenching rate ${\mathrm{\Gamma }}_{\mathrm{TPQ}}$ is independent of the microcavity so that variations of the triplet lifetimes of a series of devices exhibiting different layer thicknesses can be described with a single parameter set.

By means of experiments in microgravity conditions, we show that granular systems subjected to sinusoidal vibrations respond either by harmonic or gaslike dynamics, depending on the parameters of the vibration, amplitude and frequency, and the container size, while subharmonic response is unstable, except for extreme material properties and particular initial conditions. The absence of subharmonic response in vibrated granular systems implies that granular dampeners cannot reveal higher-order resonances, which makes them even more attractive for technical applications. Extensive molecular dynamics simulations support our findings.

We report on a liquid-nitrogen-cooled compact source for continuous terahertz (THz) emission. The emitter is a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ intrinsic Josephson-junction stack embedded between two gold layers and sandwiched between two MgO substrates. The radiation is emitted to free space through a hollow metallic tube acting as a waveguide. The maximum emission power is $1.17\mu \mathrm{W}$. The tunable emission frequency bandwidth is up to 100 GHz with a maximum emission power at 0.311 THz. Since the operation voltage is about 1 V and the current is less than 30 mA, we are able to drive this terahertz source at 77 K with only one commercial 1.5-V battery, just like a torch. This convenient and economical setup may find applications in fields like tracer-gas detection or nondestructive evaluation.