Transparent semiconducting oxides (TSO) are promising candidates for the fabrication of flexible and low-cost electronic devices, as they contain only abundant materials, are nontoxic, and exhibit high carrier mobilities. The formation of rectifying Schottky-barrier contacts is a prerequisite for devices, such as rectifiers, photodetectors, and metal-semiconductor field-effect transistors, and it was found that the presence of oxygen plays an essential role during the formation of the Schottky contacts. With electrical measurements on Pt/zinc-tin-oxide (ZTO) and ${\mathrm{PtO}}_x/\mathrm{ZTO}$ Schottky-barrier contacts and depth-resolved x-ray photoelectron spectroscopy measurements we demonstrate the important role of oxygen at the interface between TSOs and the metal contact for the rectifying behavior of diodes. In the vicinity of the interface, ${\mathrm{PtO}}_x$ is reduced to Pt in a two-step process. $\mathrm{Pt}{(\mathrm{OH})}_4$ is reduced within one day, whereas the reduction of PtO takes place over a time period of several weeks. The reduction results in improved rectification compared to $\mathrm{Pt}/\mathrm{ZTO}$, due to a filling of oxygen vacancies, which leads to a reduction of the free-carrier concentration in the vicinity of the ${\mathrm{PtO}}_x/\mathrm{ZTO}$ interface. This increases the depletion layer width and subsequently reduces the tunneling current, resulting in a higher rectification ratio. The time scale of the permanent performance improvement can be shortened significantly by applying a reverse bias to the diode. The described mechanism is most likely also present at other transparent-semiconducting-oxide–metal interfaces.

Material electromagnetic duality symmetry requires a system to have equal electric and magnetic responses. Intrinsically dual materials that meet the duality conditions at the level of the constitutive relations do not exist in many frequency bands. Nevertheless, discrete objects like metallic helices and homogeneous dielectric spheres can be engineered to approximate the dual behavior. We exploit the extra degrees of freedom of a core-shell dielectric sphere in a particle optimization procedure. The duality symmetry of the resulting particle is more than 1 order of magnitude better than previously reported nonmagnetic objects. We use $T$-matrix-based multiscattering techniques to show that the improvement is transferred onto the duality symmetry of composite objects when the core-shell particle is used as a building block instead of homogeneous spheres. These results are relevant for the fashioning of systems with high duality symmetry, which are required for some technologically important effects.

Angular dispersion—the response of a metasurface strongly depending on the impinging angle—is an intrinsic property of metasurfaces, but its physical origin remains obscure, which hinders its applications in metasurface design. We establish a theory to quantitatively describe such intriguing effects in metasurfaces, and we verify it by both experiments and numerical simulations on a typical terahertz metasurface. The physical understanding gained motivates us to propose an alternative strategy to design metadevices exhibiting impinging-angle-dependent multifunctionalities. As an illustration, we design a polarization-control metadevice that can behave as a half- or quarter-wave plate under different excitation angles. Our results not only reveal the physical origin of the angular dispersion but also point out an additional degree of freedom to manipulate light, both of which are important for designing metadevices facing versatile application requests.

The electrical transport behaviors of novel materials under the external magnetic field $B$, especially the large or tunable magnetoresistance (MR) effect, are of broad importance in both fundamental science and applications. Here three kinds of ${\mathrm{HfTe}}_5$ crystals with varied Te-deficiency concentrations are synthesized under different growth conditions, and they demonstrate distinct electrical and magnetotransport properties. The temperatures of the resistivity cusp or MR cusp of the as-grown ${\mathrm{HfTe}}_{5-\delta }$ ($\delta =0.02$, 0.08, 0.13) crystals are tuned from 25 to 90 K. The maximum MR of these three ${\mathrm{HfTe}}_{5-\delta }$ crystals, under 2 K and 9 T $B$, are $1.52\times {10}^3\%$, $2.63\times {10}^4\%$, and $6.91\times {10}^3\%$ for sample SI (${\mathrm{HfTe}}_{4.98}$), SII (${\mathrm{HfTe}}_{4.92}$), and SIII (${\mathrm{HfTe}}_{4.87}$), respectively. The fitting of Hall data by the two-carrier model suggests that the extremely large MR effect of sample ${\mathrm{HfTe}}_{4.92}$ measured at 2 K is attributed to the cooperative action of the high mobility and the coexistence of the electron and hole carriers. Our work provides a viable route to tune superior MR properties in similar compounds through defect engineering, which may be promising to develop magnetic memory sensor devices.

Solar panels easily heat up upon intense solar radiation due to excess energy dissipation of the absorbed photons or by nonradiative recombination of charge carriers. Still, photoinduced self-heating is often ignored when characterizing lab-sized samples. For light-intensity-dependent measurements of the open-circuit voltage (Suns-$V_{\mathrm{O}\mathrm{C}}$), allowing us to characterize the recombination mechanism, sample heating is often not considered, although almost 100% of the absorbed energy is converted into heat. Here, we show that the frequently observed stagnation or even decrease in $V_{\text{OC}}$ at increasingly high light intensities can be explained by considering an effective electrothermal feedback between the recombination current and the open-circuit voltage. Our analytical model fully explains the experimental data for various solar-cell technologies, comprising conventional inorganic semiconductors as well as organic and perovskite materials. Furthermore, the model can be exploited to determine the ideality factor, the effective gap, and the temperature rise from a single Suns-$V_{\text{OC}}$ measurement at ambient conditions.

$\mathrm{Fe}/\mathrm{MgO}$-based magnetic tunnel junctions are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite-size effects and temperature-dependent magnetic properties which must be carefully controlled for practical applications. In this article, we investigate the electronic, structural, and magnetic properties of $\mathrm{MgO}/\mathrm{Fe}/\mathrm{MgO}$ sandwiches using first-principles calculations and atomistic spin modeling based on a fully parametrized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin-spiral ground-state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layerwise temperature dependence of the magnetization. The different temperature dependences of the interface and bulklike layers results in an unexpected nonmonotonic temperature variation of the effective magnetic-anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure $\mathrm{Fe}/\mathrm{MgO}$ interface and the role of elevated temperatures providing insight when interpreting experimental data of nanoscale magnetic tunnel junctions.

Defects in complex multilayered thin-film solar cells are often analyzed by capacitance-based techniques, which originally were developed for simple homojunctions in single-crystal bulk semiconductors. We discuss the impact of a capacitive and conductive buffer layer on the impedance and capacitance spectra of thin-film solar cells. While the resulting capacitance spectra are indistinguishable from those caused by a deep defect level, the buffer layer and $p\text{−}n$ junction can clearly be distinguished in the experimental impedance spectra. Exploiting bias voltage and illumination as additional experimental parameters allows us to test a given hypothesis—deep defect or buffer layer—to explain characteristic capacitance steps in thermal admittance spectroscopy of thin-film solar cells. We address the controversial origin of the $N1$ signature commonly observed in admittance spectroscopy of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ solar cells. The circuit element dominantly defining the main capacitance step in our devices is unaffected by applied bias voltage, and its conductivity increases linearly with illumination intensity. We conclude that the main capacitance step in our devices is most plausibly explained by the presence of a buffer layer connected in series to the $p\text{−}n$ junction of the device and is not related to any deep defects or mobility freeze-out.

Atomic systems display a rich variety of quantum dynamics due to the different possible symmetries obeyed by the atoms. These symmetries result in selection rules that have been essential for the quantum control of atomic systems. Superconducting artificial atoms are mainly governed by parity symmetry. Its corresponding selection rule limits the types of quantum systems that can be built using electromagnetic circuits at their optimal coherence operation points (“sweet spots”). We use third-order nonlinear coupling between the artificial atom and its readout resonator to engineer the selection rules of our atom, allowing us to drive transitions forbidden by the parity selection rule for linear coupling to microwave radiation. A $\mathrm{\Lambda }$-type system emerges from these newly accessible transitions, implemented here in the fluxonium artificial atom coupled to its “antenna” resonator. We demonstrate coherent manipulation of the fluxonium artificial atom at its sweet spot by stimulated Raman transitions. This type of transition enables the creation of previously inaccessible quantum operations, such as the control and readout of physically protected artificial atoms.

Classically coherent dynamics analogous to those of quantum two-level systems are studied in the setting of force sensing. We demonstrate quantitative control over the coupling between two orthogonal mechanical modes of a nanowire cantilever through measurement of avoided crossings as we deterministically position the nanowire inside an electric field. Furthermore, we demonstrate Rabi oscillations between the two mechanical modes in the strong-coupling regime. These results give prospects of implementing coherent two-mode control techniques for force-sensing signal enhancement.

Ge-Sb-Te (GST) compounds exhibit substantial electrical and optical contrast between the amorphous and crystalline phases. Despite extensive studies of GST compounds, the underlying mechanism for fast transitions between the amorphous and crystalline phases is yet to be revealed. In this paper, we study the properties of phonons and a long-ranged $p$-orbital network of hexagonal GST compounds using first-principles calculations. By investigating volume-dependent phonon dispersions, we observe the structural instability at elevated temperature due to the spontaneous softening of a specific in-plane vibrational mode ($E_u$). We find that the atomic distortion by the $E_u$ mode is associated with weakening of delocalized $p$ bonding inducing large structural and electrical changes. We also discuss how to manipulate the $E_u$ mode to control the device performance. Our finding helps deepen the understanding of the phase-change mechanism and improve the device performance, especially the switching power and operating temperature.

Real-time and accurate measurements of gas properties are highly desirable for numerous real-world applications. Here, we use an optical-frequency comb to demonstrate absolute number-density and temperature measurements of a sample gas with state-of-the-art precision and accuracy. The technique is demonstrated by measuring the number density of ${}^{12}\mathrm{C}{}^{16}{\mathrm{O}}_2$ with an accuracy of better than 1% and a precision of 0.04% in a measurement and analysis cycle of less than 1 s. This technique is transferable to numerous molecular species, thus offering an avenue for near-universal gas concentration measurements.

We present the impact of growth conditions on the magnetic and magnetocaloric properties of ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ thin films. The magnetocaloric effect can be controlled to obtain, for example, flat temperature-independent magnetic entropy variations $-\mathrm{\Delta }{S}_M(T)$ over an unusually large temperature range extending from 100 to 300 K. This tabletop shape makes it of great interest from a practical point of view and should inspire alternative ways of tailoring magnetocaloric materials in terms of their implementation in magnetocaloric devices. We show also that the magnetocaloric effect is a great tool to better characterize magnetic materials with multiple magnetic phase transitions.

Similar to its electromagnetic counterpart, magnetic cloaking also has very important technological applications. However, the traditional method to build a static magnetic cloak requires the use of superconducting materials as the diamagnetic component, which seriously limits the practical potential because of the cryogenic condition. We show that a diamagnetic active current boundary combined with a high-permeability magnetic inner shell (MIS) can be designed to solve this problem, rendering an ideal magnetic cloaking effect at zero frequency. We first theoretically prove that a current boundary could magnetically behave as a superconductor to external observers. Based on this phenomena, we introduce a high-permeability MIS made of magnetically ultrasoft metallic sheets (permeability $\mu >{10}^3$) and experimentally prove that the bilayer combination can exactly balance out the disturbance to the external probing field and, meanwhile, have a large invisible inner space. We also show that the active boundary currents can be accordingly configured to overcome the permeability and frequency band limits, leading to a robust cloak over the entire quasistatic frequency region. Our work creates an efficient way to circumvent the traditional limits of metamaterials to build magnetic cloaks for ultralow frequencies. The active-passive hybrid approach could be generally extended to yield other artificial magnetic devices or systems as well.

Electrical power generation from a moderate-temperature thermal source by means of direct conversion of infrared radiation is important and highly desirable for energy harvesting from waste heat and micropower applications. Here, we demonstrate direct rectified power generation from an unbiased large-area nanoantenna-coupled tunnel diode rectifier called a rectenna. Using a vacuum radiometric measurement technique with irradiation from a temperature-stabilized thermal source, a generated power density of $8\mathrm{nW}/{\mathrm{cm}}^2$ is observed at a source temperature of $450°\mathrm{C}$ for the unbiased rectenna across an optimized load resistance. The optimized load resistance for the peak power generation for each temperature coincides with the tunnel diode resistance at zero bias and corresponds to the impedance matching condition for a rectifying antenna. Current-voltage measurements of a thermally illuminated large-area rectenna show current zero crossing shifts into the second quadrant indicating rectification. Photon-assisted tunneling in the unbiased rectenna is modeled as the mechanism for the large short-circuit photocurrents observed where the photon energy serves as an effective bias across the tunnel junction. The measured current and voltage across the load resistor as a function of the thermal source temperature represents direct current electrical power generation.

Based on a rational classification of defects in amorphous materials, we propose a simplified model to describe intrinsic defects and hydrogen impurities in amorphous indium gallium zinc oxide ($a$-IGZO). The proposed approach consists of organizing defects into two categories: point defects, generating structural anomalies such as metal—metal or oxygen—oxygen bonds, and defects emerging from changes in the material stoichiometry, such as vacancies and interstitial atoms. Based on first-principles simulations, it is argued that the defects originating from the second group always act as perfect donors or perfect acceptors. This classification simplifies and rationalizes the nature of defects in amorphous phases. In $a$-IGZO, the most important point defects are metal—metal bonds (or small metal clusters) and peroxides ($\mathrm{O}─\mathrm{O}$ single bonds). Electrons are captured by metal—metal bonds and released by the formation of peroxides. The presence of hydrogen can lead to two additional types of defects: metal-hydrogen defects, acting as acceptors, and oxygen-hydrogen defects, acting as donors. The impact of these defects is linked to different instabilities observed in $a$-IGZO. Specifically, the diffusion of hydrogen and oxygen is connected to positive- and negative-bias stresses, while negative-bias illumination stress originates from the formation of peroxides.

We demonstrate that a simple phenazine derivative can serve as a dual emitter for organic light-emitting diodes, showing simultaneous luminescence from the singlet and triplet excited states at room temperature without the need of heavy-atom substituents. Although devices made with this emitter achieve only low quantum efficiencies of $<0.2\%$, changes in fluorescence and phosphorescence intensity on the subpercent scale caused by an external magnetic field of up to 30 mT are clearly resolved with an ultra-low-noise optical imaging technique. The results demonstrate the concept of using simple reporter molecules, available commercially, to optically detect the spin of excited states formed in an organic light-emitting diode and thereby probe the underlying spin statistics of recombining electron-hole pairs. A clear anticorrelation of the magnetic-field dependence of singlet and triplet emission shows that it is the spin interconversion between singlet and triplet which dominates the magnetoluminescence response: the phosphorescence intensity decreases by the same amount as the fluorescence intensity increases. The concurrent detection of singlet and triplet emission as well as device resistance at cryogenic and room temperature constitute a useful tool to disentangle the effects of spin-dependent recombination from spin-dependent transport mechanisms.

We explore robust magnetization-dynamic behaviors in soft magnetic nanoparticles in single-domain states and find their related high-efficiency energy-dissipation mechanism using finite-element micromagnetic simulations. We also make analytical derivations that provide deeper physical insights into the magnetization dynamics associated with Gilbert damping parameters under applications of time-varying rotating magnetic fields of different strengths and frequencies and static magnetic fields. Furthermore, we find that the mass-specific energy-dissipation rate at resonance in the steady-state regime changes remarkably with the strength of rotating fields and static fields for given damping constants. The associated magnetization dynamics are well interpreted with the help of the numerical calculation of analytically derived explicit forms. The high-efficiency energy-loss power can be obtained using soft magnetic nanoparticles in the single-domain state by tuning the frequency of rotating fields to the resonance frequency; what is more, it is controllable via the rotating and static field strengths for a given intrinsic damping constant. We provide a better and more efficient means of achieving specific loss power that can be implemented in magnetic hyperthermia applications.

We present an approach where point defects and defect complexes are identified using power-dependent photoluminescence excitation spectroscopy, impurity data from SIMS, and density-functional-theory (DFT)-based calculations accounting for the total charge balance in the crystal. Employing the capabilities of such an experimental computational approach, in this work, the ultraviolet-C absorption band at 4.7 eV, as well as the 2.7- and 3.9-eV luminescence bands in AlN single crystals grown via physical vapor transport (PVT) are studied in detail. Photoluminescence excitation spectroscopy measurements demonstrate the relationship between the defect luminescent bands centered at 3.9 and 2.7 eV to the commonly observed absorption band centered at 4.7 eV. Accordingly, the thermodynamic transition energy for the absorption band at 4.7 eV and the luminescence band at 3.9 eV is estimated at 4.2 eV, in agreement with the thermodynamic transition energy for the ${\mathrm{C}}_{\mathrm{N}}^{-}$ point defect. Finally, the 2.7-eV PL band is the result of a donor-acceptor pair transition between the $V_{\mathrm{N}}$ and ${\mathrm{C}}_{\mathrm{N}}$ point defects since nitrogen vacancies are predicted to be present in the crystal in concentrations similar to carbon-employing charge-balance-constrained DFT calculations. Power-dependent photoluminescence measurements reveal the presence of the deep donor state with a thermodynamic transition energy of 5.0 eV, which we hypothesize to be nitrogen vacancies in agreement with predictions based on theory. The charge state, concentration, and type of impurities in the crystal are calculated considering a fixed amount of impurities and using a DFT-based defect solver, which considers their respective formation energies and the total charge balance in the crystal. The presented results show that nitrogen vacancies are the most likely candidate for the deep donor state involved in the donor-acceptor pair transition with peak emission at 2.7 eV for the conditions relevant to PVT growth.

The spin Hall effect has attracted a lot of interest in spintronics because it offers the possibility of a faster switching route with an electric current than with a spin-transfer-torque device. Recently, fieldlike spin-orbit torque has been shown to play an important role in the magnetization switching mechanism. However, there is no simple method for observing the fieldlike spin-orbit torque efficiency. We suggest a method for measuring fieldlike spin-orbit torque using a linear change in the resonance field in spectra of direct-current (dc)-tuned spin-torque ferromagnetic resonance. The fieldlike spin-orbit torque efficiency can be obtained in both a macrospin simulation and in experiments by simply subtracting the Oersted field from the shifted amount of resonance field. This method analyzes the effect of fieldlike torque using dc in a normal metal; therefore, only the dc resistivity and the dimensions of each layer are considered in estimating the fieldlike spin-torque efficiency. The evaluation of fieldlike-torque efficiency of a newly emerging material by modulation of the resonance field provides a shortcut in the development of an alternative magnetization switching device.