Knowledge of the origin of deep levels and their impact on electrical properties is critical for device applications of β-${\mathrm{Ga}}_2{\mathrm{O}}_3$. By annealing under an oxygen (${\mathrm{O}}_2$) atmosphere, the resistivity in shallow-donor (zirconium) doped β-${\mathrm{Ga}}_2{\mathrm{O}}_3$:$\mathrm{Zr}$ single crystals is found to increase by more than 10 orders of magnitude to (7 ± 4) × 10 Ω cm, which is comparable to the resistivity achieved by iron ($\mathrm{Fe}$) acceptor doping of (5 ± 3) × 10 Ω cm. We combine thermoelectric effect spectroscopy and positron annihilation spectroscopy (PAS), which are sensitive to deep levels and concentration of open-volume defects, with modeling of the electrical properties, to study these strongly compensated crystals. We find the compensating level in the ${\mathrm{O}}_2$-annealed β-${\mathrm{Ga}}_2{\mathrm{O}}_3$:$\mathrm{Zr}$ sample to be located at (0.727 ± 0.021) eV (E2*) below the conduction band, which correlates with a vacancy signal from PAS data. The defect is most likely the relaxed split $\mathrm{Ga}$ vacancy $V_{\mathrm{Ga}}^i$, rather than a simple gallium vacancy, considering theoretical predictions of a small energy barrier to relax. We observe that, due to the unique nature of these vacancies and anisotropy in the monoclinic lattice, the Doppler-broadening parameter is rather small compared with other wide-gap compounds, and in such a case the positron diffusion length is a suitable parameter to estimate the open-volume defect concentration.

The nonlinear response of liquids in the terahertz regime has recently attracted significant interest, even though very few measurements have been reported. Here, we report on our measurements based on a $z$-scan technique of the nonlinear refractive-index coefficient $n_2$ at terahertz frequencies for several liquids with noncentrosymmetric molecules, specifically, water, ethanol, and $\alpha$-pinene. We describe how the value of $n_2$ depends on the physical parameters of these molecules. The measured values of $n_2$ of the liquids in the terahertz region are as much as 6 orders of magnitude larger than their corresponding values in the visible or near-IR. Through a simple theoretical model, we confirm that the predominant source of this large third-order nonlinearity is the second-order perturbative component of the vibrational response of these molecules, which have resonances in the mid-IR.

The choice of electrostatic gating over the conventional chemical doping for phase engineering of quantum materials is attributed to the fact that the former can reversibly tune the carrier density without affecting the system’s level of disorder. However, this proposition seems to break down in field-effect transistors involving ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ (STO)-based two-dimensional electron gases. Such peculiar behavior is associated with electron trapping under an external electric field. However, the microscopic nature of the trapping centers remains an open question. In this paper, we investigate electric-field-induced charge-trapping and charge-detrapping phenomena at the conducting interface between the band insulators $\gamma$-${\mathrm{Al}}_2{\mathrm{O}}_3$ and STO. Our transport measurements reveal that the charge trapping under a positive back-gate voltage ($V_g$) above the tetragonal-to-cubic structural transition temperature ($T_c$) of STO has a contribution from electric-field-assisted thermal escape of electrons from the quantum well, and from clustering of oxygen vacancies as well. We observe an additional source of trapping below $T_c$, which arises from the trapping of free carriers at ferroelastic twin walls in the STO. Application of a negative $V_g$ results in charge detrapping, which vanishes above $T_c$. This feature demonstrates the crucial role of structural domain walls in the electrical transport properties of STO-based heterostructures. The number of charges trapped (detrapped) at (from) a twin wall is controlled by the net polarity of the wall and is completely reversible with a sweep of $V_g$.

Nanoparticles with a strong absorption of incident radio frequency (rf) or microwave irradiation are desirable for remote hyperthermia treatments. While controversy has surrounded the absorption properties of spherical metallic nanoparticles, other geometries, such as prolate and oblate spheroids, have not received sufficient attention for application in hyperthermia therapies. Here, we use the electrostatic approximation to calculate the relative-absorption ratio of metallic nanoparticles in various biological tissues. We consider a broad parameter space, sweeping across frequencies from 1 MHz to 10 GHz, while also tuning the nanoparticle dimensions from spheres to high-aspect-ratio spheroids approximating nanowires and nanodisks. We find that, while spherical metallic nanoparticles do not offer differential heating in tissue, large absorption cross sections can be obtained from long prolate spheroids, while thin oblate spheroids offer minor potential for absorption. Our results suggest that metallic nanowires should be considered for rf- and microwave-based wireless hyperthermia treatments in many tissues going forward.

The detection of entangled states is essential in both fundamental and applied quantum physics. However, this task proves to be challenging, especially for general quantum states. One can execute full state tomography but this method is time demanding, especially in complex systems. Other approaches use entanglement witnesses: these methods tend to be less demanding but lack reliability. Here, we demonstrate that artificial neural networks (ANNs) provide a balance between the two approaches. In this paper, we make a comparison of ANN performance with witness-based methods for random general two-qubit quantum states without any prior information on the states. Furthermore, we apply our approach to a real experimental data set.

$\mathrm{Cu}$-deficient phases of chalcopyrite $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ (CIGS), such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_3{\mathrm{Se}}_5$ and $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}{)}_5{\mathrm{Se}}_8$, which are occasionally referred to as ordered vacancy compounds and often unintentionally form on CIGS film surfaces and at grain boundaries, are among the most important subjects in developing chalcopyrite thin-film energy-conversion materials and devices. Here, we revisit the role of the $\mathrm{Cu}$-deficient surface layer (CDL) present at the p-CIGS/n-$\mathrm{Cd}\mathrm{S}$ interface in photovoltaic devices with the effects of alkali-metal doping. The device structure used is alkali-containing soda-lime glass or alkali-free zirconia substrate/$\mathrm{Mo}$/CIGS/${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_2$-based CDL/$\mathrm{Cd}\mathrm{S}$/intrinsic and $\mathrm{Al}$-doped $\mathrm{Zn}\mathrm{O}$. The photovoltaic device performance deteriorates with increasing CDL thickness when no alkali metal is added. However, the CDL thickness significantly affects the results of alkali-halide $\mathrm{Rb}\mathrm{F}$ postdeposition treatment (PDT) and the use of a thick CDL enhances the beneficial effects of $\mathrm{Rb}\mathrm{F}$ PDT, resulting in photovoltaic performance enhancements manifested by improvements in the open-circuit voltage and fill-factor values. On the other hand, $\mathrm{Rb}\mathrm{F}$ PDT leads to degradation of the device performance when the CDL is thin. High $\mathrm{Rb}$ concentration and significant $\mathrm{Ga}$ diffusion from the CIGS layer to the surface CDL with $\mathrm{Rb}\mathrm{F}$ PDT are observed when the CDL is thick. The formation of metastable acceptors, namely, an increase in the nominal carrier density in CIGS, is observed with light-soaking (LS) treatments, regardless of the thickness of CDL. Nevertheless, improvements in photovoltaic efficiency with LS treatments are observed only from devices grown with a thick CDL.

Materials with spin-orbit coupling are of great interest for various spintronics applications due to the efficient electrical generation and detection of spin-polarized electrons. Over the past decade, many materials have been studied, including topological insulators, transition metals, Kondo insulators, semimetals, semiconductors, and oxides; however, there is no unifying physical framework for understanding the physics and therefore designing a material system and devices with the desired properties. We present a model that binds together the experimental data observed on the wide variety of materials in a unified manner. We show that in a material with a given spin-momentum locking, the density of states plays a crucial role in determining the charge-spin interconversion efficiency, and a simple inverse relationship can be obtained. Remarkably, experimental data obtained over the last decade on many different materials closely follow such an inverse relationship. We further deduce two figure of merits of great current interest: the spin-orbit-torque (SOT) efficiency (for the direct effect) and the inverse Rashba-Edelstein effect length (for the inverse effect), which statistically show good agreement with the existing experimental data on wide varieties of materials. Particularly, we identify a scaling law for the SOT efficiency with respect to the carrier concentration in the sample, which agrees with existing data. Such an agreement is intriguing since our transport model includes only Fermi surface contributions and fundamentally different from the conventional views of the SOT efficiency that includes contributions from all the occupied states.

A general and robust Bayesian optimization framework for the extraction of intrinsic physical properties from an integration of pore-scale forward modeling and experimental measurements of macroscopic system responses is developed. The efficiency of the scheme, which utilizes Gaussian process regression, enables the simultaneous extraction of multiple intrinsic physical properties with a minimal number of function evaluations. Here it is applied to nuclear magnetic resonance (NMR) relaxation responses, paving the way for inverse problem approaches to digital rock physics given its general nature. NMR relaxation responses of fluids in porous media may be described by sums of multiexponential decays resulting in a relaxation time distribution. The shape of this distribution is dependent on intrinsic physical system properties, but also effects like diffusion coupling between different relaxation regimes in heterogeneous porous materials. Forward models based on high-resolution images are employed to naturally incorporate structural heterogeneity and diffusive motion without limiting assumptions. Extracting the required multiple intrinsic parameters of the system poses an ill-conditioned multiphysics multiparameter inverse problem where multiple scales are covered by the underlying microstructure. Exploration of the multidimensional search space given an expensive cost function makes an efficient solution strategy mandatory. We propose a workflow to match experimental measurements with simulations via Bayesian optimization, with special attention paid to the multimodal nature of the topography of the objective function using solution space partitioning. A multimodal search strategy using state-of-the-art evolutionary algorithms and gradient-based optimization algorithms guarantees that the multimodal nature is captured. The workflow is demonstrated on $T_2$ relaxation responses of Bentheimer sandstone, extracting three physical parameters simultaneously: the surface relaxivity of quartz grains, the effective transverse relaxation time, and the effective diffusion coefficient in clay regions. Multiple mathematically sound and physically plausible solutions corresponding to global minimum and multiple local minima of the objective function are identified within a limited number of function evaluations. Importantly, the shape of the experimental $T_2$ distribution is recovered almost perfectly, enabling the use of classical interpretation techniques and local analysis of responses based on numerical simulation.

Unlike the electrical conductance that can be widely modulated within the same material even in deep-subwavelength devices, tuning the thermal conductance within a single material system or nanostructure is extremely challenging and requires a large-scale device. This prohibits the realization of robust on/off states in switching the flow of thermal currents. Here, we present the theory of a thermal switch based on resonant coupling of three photonic resonators, in analogy to the field-effect electronic transistor composed of a source, a gate, and a drain. As a material platform, we capitalize on the extreme tunability and low-loss resonances observed in the dielectric function of monolayer hexagonal boron nitride ($h$-BN) under controlled strain. We derive the dielectric function of $h$-BN from first principles, including the phonon-polariton line widths computed by considering phonon-isotope and anharmonic phonon-phonon scattering. Subsequently, we propose a strain-controlled $h$-BN–based thermal switch that modulates the thermal conductance by more than an order of magnitude, corresponding to a contrast ratio in the thermal conductance of $98\mathrm{\%}$, in a deep-subwavelength nanostructure.

The recent development of nonfullerene acceptors (NFAs) has resulted in an abrupt increase in the organic photovoltaic (OPV) efficiency. It remains unclear why charge separation (CS) can occur at a high yield in NFA OPVs despite the small energy-level offset at the donor-acceptor (D-A) interface. $\mathrm{In}$ order to resolve this issue, zinc phtholacyanine (ZnPc) and its fluorinated derivative (${\mathrm{F}}_8\mathrm{Zn}\mathrm{Pc}$) are used to build a model D-A interface for understanding the mechanism underlying the efficient CS in NFA OPVs. At this interface, we find that bound charge-transfer (CT) excitons can undergo a heating process in which the enthalpy increases at a surprisingly fast rate. Specifically, the CT exciton gains 0.3 eV from the environment in merely 10 ps. This fast exciton heating, which eventually leads to CS, resembles the CS behavior observed in NFA bulk heterojunctions (BHJs). Interestingly, when the pair of D-A molecules switches from a face-on to an edge-on orientation, the CT-exciton heating process is reversed to the typical hot CT-exciton cooling process in which the enthalpy decreases with time. By using sub-band-gap excitation, we further find that the exciton heating can occur directly from cold CT excitons without the need of excess energy provided by the energy-level offset. The competition between the exciton heating and cooling is explained by a model that incorporates the effect of entropy, electron delocalization, and structural anisotropy. It is found that for the face-on orientation, the small contact area between the delocalized electron and hole within the CT exciton significantly reduces the number of cold CT states, which increases the entropic driving force and drives the reaction into the heating direction. This finding provides insight on why the entropy-and-delocalization-driven CS would preferentially occur in NFA BHJs. The relative orientation of the NFA molecular stack and the polymer chain, together with bulky side groups, make the delocalized electron and hole in contact with each other via isolated pointlike junctions, which is the key for activating the exciton heating and subsequent CS.

Finding low-cost, highly active, and thermally stable bifunctional electrocatalysts toward oxygen reduction and evolution reactions (ORR-OER) is a key for the development of renewable energy devices, including fuel cells and water splitting. Here, we systematically investigate the physical properties, structural stability, and ORR-OER bifunctional catalytic activity of $\mathrm{Pt}$ supported by a series of MXene substrates using density-functional-theory calculations. Our results indicate that $\mathrm{Pt}$ atoms disperse uniformly on the MXene supports with high structural stability at high temperature and oxidation conditions, because of the strong metal-support interaction. The lattice parameter of MXenes is found to play a crucial role in determining the morphology and stability of these heterostructures and a descriptor is proposed for their design. In particular, $\mathrm{Pt}/{\mathrm{V}}_2\mathrm{C}$ is identified as the stable and most promising bifunctional catalyst with an overpotential comparable to the benchmarks. This work brings up a strategy for the search and design of superb transition metal/MXene catalysts for energy applications.

The tunability of polar and semiconducting properties of low-dimensional transition metal dichalcogenides (TMDs) have propelled them to the forefront of fundamental and applied physical research. These materials can vary their electrophysical properties from nonpolar to ferroelectric, and from direct-band semiconducting to metallic. In addition to classical controlling factors, such as field effect, composition, and doping, new degrees of freedom emerge in TMDs due to the curvature-induced electron redistribution and the associated changes in electronic properties. Here we theoretically explore the elastic and electric fields, flexoelectric polarization and free charge density for a TMD nanoflake placed on a rough substrate with a sinusoidal corrugation profile. Finite element modelling results for different flake thickness and corrugation depth yield insights into the flexoelectric nature of the out-of-plane electric polarization and establish the unambiguous correlation between the polarization and static conductivity modulation. The modulation is caused by the coupling between the deformation potential and inhomogeneous elastic strains, which evolve in the TMD nanoflake due to the adhesion between the flake surface and corrugated substrate. We reveal a pronounced maximum in the thickness dependences of the electron and hole conductivity of ${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{Mo}\mathrm{Te}}_2$ nanoflakes placed on a corrugated substrate, which opens the way for the optimization of their geometry towards significant improvement in their polar and electronic properties, necessary for advanced applications in nanoelectronics and memory devices. Specifically, the obtained results can be useful for the development of nanoscale straintronic devices based on the bended ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{Mo}\mathrm{Te}}_2$, and $\mathrm{Mo}\mathrm{S}\mathrm{Te}$ nanoflakes, such as diodes and bipolar transistors with a bending-controllable sharpness of p-n junctions.

The electronic density of states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors, while its accurate determination is still a challenging task. We apply the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of organic light-emitting diode (OLED) host materials. The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl derivatives, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 3,3′-di(9$H$-carbazol-9-yl)-1,1′-biphenyl (mCBP), and 3′,5-di(9$H$-carbazol-9-yl)-[1,1′-biphenyl]-3-carbonitrile (mCBP-CN), and (ii) carbazole-phenyl (CP) derivatives, 1,3-bis(N-carbazolyl)benzene (mCP) and 9-[3-(9$H$-carbazol-9-yl)phenyl]-9$H$-carbazole-3-carbonitrile (mCP-CN). TSL originates from radiative recombination of charge carriers thermally released from the lower-energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution, with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments, varying from 0 to 6 D, in a similar manner within each series, in line with the dipolar disorder model. The same molecular dipole moment, however, leads to a broader DOS of CP compared with CBP derivatives. Using computer simulations, we attribute the difference between the series to a smaller polarizability of cations in CP derivatives, leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.

Diffraction tomography is a promising label-free three-dimensional (3D) imaging method for transparent samples due to its low phototoxicity. However, it usually requires coherent illuminations and massive measurements, which will, respectively, introduce speckle noises and time-consuming acquisitions, and thus, limit its applications in multicellular dynamic organisms. To address these problems, here we propose a partially coherent and snapshot diffraction tomography method based on light-field microscopy. By incorporating the illumination optical transfer function into the volumetric differential phase contrast model, we obtain a 3D phase stack from the snapshot four-dimensional phase-space measurements. Then a 3D phase-point-spread function is used for 3D deconvolution to achieve computational optical sectioning and enhance the resolution, and finally, we extract 3D refractive-index information from the 3D phase stack. Experiments on freely moving Caenorhabditis elegans are shown accordingly, demonstrating the great potential of the proposed method for label-free 3D imaging of multicellular organisms in vivo at high speed up to the camera frame rate.

Analysis of bipolar thermal conductivity might be very useful in preliminary stages of thermoelectric materials discovery. Using its product—mobility ratio between electrons and holes—it is possible to choose the most promising compound from the series and pave the correct direction of doping. Current work presents positive verification of this approach for $\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}$, which is anticipated to show superior mobility when tuned towards n-type behavior. In agreement with expectation, the mobility increases by an order of magnitude due to rising tellurium content in the ${\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}}_{1-x}{\mathrm{Te}}_x$ series. The effect is most likely driven by change of the dominant charge carriers’ scattering mechanism from ionized impurity influence to point defect and acoustic phonon interaction. Simultaneously, due to a highly anisotropic conduction band, the effective mass of the carriers rises towards the n-type regime. These two effects lead to an improved thermoelectric power factor of electron-doped samples, up to $40\mu {\mathrm{WK}}^{-2}{\mathrm{cm}}^{-1}$ at 740 K for ${\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}}_{0.85}{\mathrm{Te}}_{0.15}$. Based on this result, we suggest n-type doping for other rare-earth-based half-Heusler compounds. Representatives of this group exhibit the smallest lattice thermal conductivity in the pristine form among any half-Heusler thermoelectrics, and are anticipated to show comparably good electrical properties to ScNiSb due to their high mobility ratio in favor of electrons.

High-performance and lightweight waterborne acoustic devices have provided important support for the development of ocean exploration, health monitoring, etc. In this work, we propose a high-efficiency ultrathin nonlocal waterborne acoustic metasurface. Due to strong nonlocal interaction between the waterborne unit cells induced by the fluid-solid interaction, a nonlocal design concept based on diffraction theory and the optimization approach is proposed to design waterborne acoustic metasurfaces, in contrast to the local design based on the generalized Snell’s law. The theoretical efficiency limitation of the generalized Snell’s law can be broken, and unitary efficiency could be obtained even for large-angle anomalous reflection. The high-efficiency modulation is achieved owing to the energy flow along the metasurface resulting from the nonlocal interaction. The thickness of the waterborne acoustic metasurface can be minimized to one tenth or even one sixtieth of the working wavelength. The nonlocal design with fluid-solid interaction paves the way for high-efficiency and deep-subwavelength waterborne acoustic wave manipulation.

As an application in circuit quantum electrodynamics coupled systems, superconducting resonators play an important role in high-sensitivity measurements in a superconducting-semiconductor hybrid architecture. Taking advantage of a high-impedance $\mathrm{Nb}\text{−}\mathrm{Ti}\text{−}\mathrm{N}$ resonator, we perform excited-state spectroscopy on a $\mathrm{Ga}\mathrm{As}$ double quantum dot (DQD) by applying voltage pulses to one gate electrode. The pulse train modulates the DQD energy detuning and gives rise to charge state transitions at zero detuning. Benefiting from the outstanding sensitivity of the resonator, we distinguish different spin-state transitions in the energy spectrum according to the Pauli exclusion principle. Furthermore, we experimentally study how the interdot tunneling rate modifies the resonator response. The experimental results are consistent with the simulated spectra based on our model.