Thermal runaway occurs when a rise in system temperature results in heat-generation rates exceeding dissipation rates. Here, we demonstrate that thermal runaway occurs in radiative (photon) systems given a sufficient level of negative-differential thermal emission. By exploiting the insulator-to-metal phase transition of vanadium dioxide, we show that a small increase in heat generation (e.g., $10\mathrm{nW}/{\mathrm{mm}}^2$) results in a large change in surface temperature (e.g., ∼35 K), as the thermal emitter switches from high emittance to low emittance. While thermal runaway is typically associated with catastrophic failure mechanisms, detailed understanding and control of this phenomenon may give rise to new opportunities in infrared sensing, camouflage, and rectification.

Boolean algebra, the branch of mathematics in which variables can assume only true or false values, is the theoretical basis of classical computation. The analogy between Boolean operations and electronic switching circuits, highlighted by Shannon in 1938, paved the way for modern computation based on electronic devices. The growth in the computational power of such devices, after an exciting exponential—Moore’s trend—is nowadays blocked by heat dissipation due to computational tasks, which are very demanding due to the miniaturization of chips. Heat is often a detrimental form of energy which increases the system’s entropy, decreasing the efficiency of logic operations. Here, we propose a physical system that is able to perform thermal-logic operations by reversing the old heat-disorder epitome into a heat-order paradigm. We lay the foundations of heat computation by encoding logic state variables in temperature and introducing the thermal counterparts of electronic logic gates. Exploiting quantum effects in thermally biased Josephson junctions (JJs), we propound a possible realization of a functionally complete logic. Our architecture ensures high operational stability and robustness, with switching frequencies reaching the GHz range.

We report the development of an electro-optomechanical system based on surface acoustic waves (SAWs), where a piezoelectric material with a large optoelastic susceptibility is used for the coupling of both radiowaves and optical light to the SAW. In the optical domain, we exploit the tensorial nature of the optoelastic effect to show the polarization dependence of the photon-SAW interaction. We demonstrate increased coupling both by a two-dimensional SAW-focusing circuit and by the optical cavity enhancement of the photon-phonon scattering. We evaluate the optomechanical coupling rate of the system and discuss the future direction for the improvement of the coupling strength.

We demonstrate the self-synchronization of a spin torque nano-oscillator (STNO) using magnetic field feedback. The rf output of the STNO is connected to a waveguide which rests on top of the magnetic tunnel junction (MTJ). The resulting rf current flowing through the waveguide creates a magnetic field on the free layer of the MTJ, which acts as the feedback. We observe a large improvement in the quality factor as the feedback strength is increased. Our results indicate a transition to an oscillatory state dominated by feedback at higher feedback strength. We also explore the effect of feedback phase on magnetization oscillation by inserting an adjustable delay in the feedback path. The experimental results are qualitatively supported by macromagnetic simulation. Our results open an alternative avenue to control the magnetization dynamics of a nanomagnet.

As an attractive degree of freedom in electromagnetic (EM) waves, the orbital angular momentum (OAM) enables infinite communication channels for both classical and quantum communications. The exploration of OAM generation inspires various designs involving spiral phase plates, antenna arrays, metasurfaces, and computer-generated holograms. In this work, we theoretically and experimentally demonstrate an approach to producing OAM carrying EM waves by a point defect in three-dimensional (3D) photonic crystals (PCs). Simultaneous excitation of two vibrational-defect states with an elaborately engineered phase retardation generates a rotational state carrying OAM. Through converting guided waves in a line defect to localized waves in a point defect and then to radiated vortex waves in free space, the lowest four OAM-mode emitters, i.e., OAM indices of $\pm 1$ and $\pm 2$, are successfully realized. This work offers a physical mechanism to generate OAM by PCs, especially when the OAM generation is to be integrated with other designs.

${\mathrm{Ga}}_2{\mathrm{O}}_3$ and $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloys are promising materials for solar-blind UV photodetectors and high-power transistors. Basic key parameters in the device design, such as band-gap variation with alloy composition and band offset between ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$, are yet to be established. Using density-functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we compute formation enthalpies, band gaps, and band-edge positions of $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloys in the monoclinic ($\beta$) and corundum ($\alpha$) phases. We find the formation enthlapies of $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloys are significantly lower than of $({\mathrm{In}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$, and that $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ with $x=0.5$ can be considered as an ordered compound $\mathrm{Al}\mathrm{Ga}{\mathrm{O}}_3$ in the monoclinic phase, with $\mathrm{Al}$ occupying the octahedral sites and $\mathrm{Ga}$occupying the tetrahedral sites. The band gaps of the alloys range from 4.69 to 7.03 eV for $\beta \text{−}({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ and from 5.26 to 8.56 eV for $\alpha \text{−}({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$. Most of the band offset of the $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloy arises from the discontinuity in the conduction band. Our results are used to explain the available experimental data, and consequences for designing MODFETs based on $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3/{\mathrm{Ga}}_2{\mathrm{O}}_3$ are discussed.

The exchange stiffness constant denoted by $A_{\mathrm{ex}}$ is one of the fundamental physical quantities from the Heisenberg exchange Hamiltonian for ferromagnetic materials. A recent theoretical prediction shows the possibility that $A_{\mathrm{ex}}$ in ferromagnetic metal films can be effectively tuned by the application of an electric field at the surface. In this paper, we demonstrate that the thermally excited spin-wave modes in magnetic tunnel junctions contain modulations upon application of an electric field through a $\mathrm{Mg}\mathrm{O}$ tunneling barrier. Different modulations of different spin-wave eigenmodes provide quantitative information regarding the electric field effects on the exchange stiffness, perpendicular anisotropy energy, and saturation magnetization. An electric field of 1 V/nm modulated $A_{\mathrm{ex}}$ by 8.1% at room temperature. The results are significantly higher than those predicted from the Slater-Pauling curve and recent theoretical work.

Zirconia is a promising electrolyte material that has been widely used in solid-oxide fuel cells. In this paper, the effects of defect interaction on the ionic conductivity of scandia- and yttria-doped zirconia are systematically investigated by density-functional-theory calculations and experimental verification. We theoretically predict the doping concentrations of the tetragonal-to-cubic phase transition to be 18 at. % for ${\mathrm{Sc}}^{3+}$ and 9 at. % for ${\mathrm{Y}}^{3+}$, which are in reasonable agreement with the experimental values. Oxygen-vacancy-formation energies, oxygen-vacancy-dopant binding energies, and diffusion barriers are calculated to evaluate ionic conduction properties. Our calculated results show that the binding-energy variances of different defect configurations in scandia-doped zirconia are markedly lower than those in yttria-doped zirconia. Diffusion barriers are calculated using the saddle-point method, and the corresponding experiments are carried out to verify the diffusion-barrier results.

An optical reference based on a two-photon optical transition with ac Stark shift cancellation is proposed. The reference uses two interrogating laser fields at different frequencies. Compared to conventional optical two-photon references, the new approach offers the possibility for improved short-term stability resulting from a higher signal-to-noise ratio and improved long-term stability due to active ac Stark shift cancellation. We demonstrate the ac Stark shift cancellation method on the $5s{}^2{S}_{1/2}\to 5d{}^2{D}_{5/2}$ two-photon transition in ${}^{87}\mathrm{Rb}$.

Effective demagnetizing factors that connect the sample magnetic moment with the applied magnetic field are calculated numerically for perfectly diamagnetic samples of various nonellipsoidal shapes. The procedure is based on calculating the total magnetic moment by integrating the magnetic induction obtained from a full three-dimensional (3D) solution of the Maxwell equations using an adaptive mesh. The results are relevant for superconductors (and conductors in ac fields) when the London penetration depth (or the skin depth) is much smaller than the sample size. Simple but reasonably accurate approximate formulas are given for practical shapes including rectangular cuboids, finite cylinders in axial and transverse fields, as well as infinite rectangular and elliptical cross-section strips.

We quantify the spin Hall angle ${\theta }_{\mathrm{SH}}$ and spin-diffusion length $l_{\mathrm{sd}}$ of $\mathrm{Nb}$ from inverse spin Hall effect (ISHE) measurements in $\mathrm{Nb}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ bilayers under ferromagnetic resonance. By varying the $\mathrm{Nb}$ thickness $t_{\mathrm{Nb}}$ and comparing to a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Pt}$ reference sample, room temperature values of ${\theta }_{\mathrm{SH}}$ and $l_{\mathrm{sd}}$ for $\mathrm{Nb}$ are estimated to be approximately −0.001 and 30 nm, respectively. We also investigate the ISHE as a function of temperature T for different $t_{\mathrm{Nb}}$. Above the superconducting transition temperature $T_c$ of $\mathrm{Nb}$ , a clear $t_{\mathrm{Nb}}$-dependent T evolution of the ISHE is observed whereas below $T_c$, the ISHE voltage drops rapidly and is below the sensitivity of our measurement setup at a lower T. This suggests the strong decay of the quasiparticle (QP) charge-imbalance relaxation length across $T_c$, as supported by an additional investigation of the ISHE in a different sample geometry along with model calculation. Our finding suggests careful consideration should be made when developing superconductor spin Hall devices that intend to utilize QP-mediated spin-to-charge interconversion.

The influence of UV light-induced pitch contraction and dilation on the electroconvection patterns (ECPs) of a chiral nematic liquid crystal containing a photoresponsive chiral dopant is investigated in planar-aligned cells. It is observed that the helical twisting power of the dopant changes (even undergoes handedness inversion) under UV irradiation; consequently, the pitch and the direction of the convection rolls in ECPs (being either parallel with or perpendicular to the surface alignment) could be controlled by the UV intensity and the ac voltage. In contrast to traditional methods, where the pitch is modulated by electric or thermal fields, our method of applying a light field allows a remote and contactless manipulation of the pitch, which is easily detectable via the morphological changes of ECPs. The ability to control the orientation of ECPs by tuning the light intensity can conveniently be utilized as an optical grating, allowing switchable, dual-mode operation.

We propose a device based on a topological Josephson junction where the helical edge states of a two-dimensional (2D) topological insulator are in close proximity to two superconducting leads. The presence of a magnetic flux through the junction leads to a Doppler shift in the spectrum of Andreev bound states, and affects the quantum interference between proximized edge states. We inspect the emergent features, accessing the density of states through a tunnel-coupled metallic probe, thus realizing a topological superconducting quantum-interference-proximity transistor (TSQUIPT). We calculate the expected performance of this device, concluding that it can be used as a sensitive absolute magnetometer due to the voltage drop across the junction decaying to a constant value as a function of the magnetic flux. Contrary to conventional SQUID and SQUIPT designs, no ring structure is needed. The findings pave the way for sensitive hybrid devices that exploit the helical edge states of a 2D topological insulator.

We present a straightforward and computationally cheap method to obtain the phonon-assisted photocurrent in large-scale devices from first-principles transport calculations. The photocurrent is calculated using the nonequilibrium Green’s function with light-matter interaction from the first-order Born approximation, while electron-phonon coupling (EPC) is included through special thermal displacements. We apply the method to a silicon solar-cell device and demonstrate the impact of including EPC in order to properly describe the current due to the indirect band-to-band transitions. The first-principles results are successfully compared to experimental measurements of the temperature and light-intensity dependence of the open-circuit voltage of a silicon photovoltaic module. Our calculations illustrate the pivotal role played by EPC in photocurrent modeling to avoid underestimation of the open-circuit voltage, short-circuit current, and maximum power. This work represents a recipe for computational characterization of future photovoltaic devices including the combined effects of light-matter interaction, phonon-assisted tunneling, and the device potential at finite bias from the level of first-principles simulations.

Antiferromagnetic (AFM) spintronics, where magnetotransport is governed by an antiferromagnet instead of a ferromagnet, opens fascinating new perspectives for both fundamental research and device technology. This is because AFM spintronics have intrinsic appealing properties, such as rigidness to magnetic field, absence of stray field, and ultrafast spin dynamics. One of the urgent challenges, hindering the realization of the full potential of AFM spintronics, is the performance gap between AFM metals and insulators. Here, we demonstrate the insulator-metal transition and persistently large anisotropic magnetoresistance (AMR) in single crystals ${\mathrm{Sr}}_2({\mathrm{Ir}}_{1-x}{\mathrm{Ga}}_x){\mathrm{O}}_4(0\le x\le 0.09)$, which host the same basal-plane AFM lattice with strong spin-orbit coupling. The nondoped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ shows the insulating transport with AMR as large as approximately 16.8% at 50 K. The $\mathrm{Ga}$ substitution of $\mathrm{Ir}$ allows a gradual reduction of electrical resistivity. A clear insulator-to-metal transition is identified in doped samples with x above 0.05, while the AMR can still have approximately 1%, which is sizable in comparison with those in the AFM metals reported so far. Our experiments reveal that all samples have similar fourfold AMR symmetry, which can be well understood in the scenario of magnetocrystalline anisotropy. It is suggested that the spin-orbit-coupled antiferromagnets ${\mathrm{Sr}}_2({\mathrm{Ir}}_{1-x}{\mathrm{Ga}}_x){\mathrm{O}}_4$ are promising candidate materials for AFM spintronics, providing a rare opportunity to integrate the superior spintronic functionalities of AFM metals and insulators.

Electrically active defects have a significant impact on the performance of electronic devices based on wide-band-gap materials. This issue is ubiquitous in diamond science and technology, since the presence of charge traps in the active regions of different classes of diamond-based devices (detectors, power diodes, transistors) can significantly affect their performance, due to the formation of space charge, memory effects, and the degradation of the electronic response associated with radiation-induced damage. Among the most common defects in diamond, the nitrogen-vacancy (N-V) center possesses unique spin properties that enable high-sensitivity field sensing at the nanoscale. Here, we demonstrate that N-V ensembles can be successfully exploited to perform direct local mapping of the internal electric-field distribution of a graphite-diamond-graphite junction exhibiting electrical properties dominated by trap- and space-charge-related conduction mechanisms. By means of optically detected magnetic resonance measurements, we performed both point-by-point readout and spatial mapping of the electric field in the active region at different bias voltages. In this novel “self-diagnostic” approach, defect complexes represent not only the source of detrimental space-charge effects but also a unique tool for their direct investigation, by providing an insight on the conduction mechanisms that could not be inferred in previous studies on the basis of conventional electrical and optical characterization techniques.

Organic light-emitting diodes (OLEDs) are widely studied semiconductor devices for which a simple description by a diode equation typically fails. In particular, a full description of the current-voltage relation, including temperature effects, has to take the low electrical conductivity of organic semiconductors into account. Here, we present a temperature-dependent resistive network, incorporating recombination as well as electron and hole conduction to describe the current-voltage characteristics of an OLED over the entire operation range. The approach also reproduces the measured nonlinear electrothermal feedback upon Joule self-heating in a self-consistent way. Our model further enables us to learn more about internal voltage losses caused by the charge transport from the contacts to the emission layer which is characterized by a strong temperature-activated electrical conductivity, finally determining the strength of the electrothermal feedback. In general, our results provide a comprehensive picture to understand the electrothermal operation of an OLED which will be essential to ensure and predict especially long-term stability and reliability in superbright OLED applications.

In this study, we elaborate on the recent concept of metagratings proposed in Ra’di et al. [Phys. Rev. Lett. 119, 067404 (2017)] for efficient manipulation of reflected waves. Essentially, a metagrating is a set of 1D arrays of polarization line currents which are engineered to cancel scattering in undesirable diffraction orders. We consider a general case of metagratings composed of $N$ polarization electric line currents per supercell. This generalization is a necessary step to totally control diffraction patterns. We show that a metagrating having $N$ equal to the number of plane waves scattered in the far field can be used to control the diffraction pattern. To validate the developed theoretical approach, anomalous and multichannel reflections are demonstrated with 3D full-wave simulations in the microwave regime at $10\mathrm{GHz}$. The results are interesting for the metamaterials community as they allow one to significantly decrease the number of used elements and simplify the design of wavefront manipulation devices, which is very convenient for optical and infrared frequency ranges. Our findings may also provide a way to develop efficient tunable antennas in the microwave domain.

Given the immense attraction of using two-dimensional materials for high-frequency applications, there is a critical need for computational models that aid the production of high-frequency performance projections based on their material properties. In this work, we use the atomistic nonequilibrium Green’s function approach to predict the performance of ballistic radio-frequency transistors based on a broad range of two-dimensional semiconducting materials, including $\mathrm{Mo}{\mathrm{S}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{WSe}}_2$, and phosphorene. Self-consistent ballistic quantum-transport simulations are performed on monolayer transistors with a 15-nm gate length and an effective gate-oxide thickness of 0.44 nm. We show that rather than the bandgap, the effective mass and the dielectric constant significantly influence the unity current gain cutoff frequency ($f_t$), the maximum oscillation frequency ($f_{\text{max}}$), and the intrinsic gain ($G_{\text{int}}$). Phosphorene with a low effective mass and high dielectric constant is found to be the most promising material with $f_t\simeq 10\mathrm{THz}$, $f_{\text{max}}\simeq 6\mathrm{THz}$, and $G_{\text{int}}\simeq 100\text{℧}/\text{℧}$. Results are further corroborated analytically using the Wentzel-Kramers-Brillouin approximation and also benchmarked with previous reports. Furthermore, the effect of extrinsic parameters, such as Schottky barrier height and strain on $f_t$ and $f_{\text{max}}$, is discussed. This study can thus be used as a comprehensive, physics-based search guide of two-dimensional materials for radio-frequency applications.

Due to their topological protection and nanometric size, magnetic skyrmions are anticipated to form components of new high-density memory technologies. In metallic systems, skyrmion manipulation is achieved easily under a low-density electric current flow, although the inevitable thermal dissipation ultimately limits the energy efficacy of potential applications. On the other hand, a near dissipation-free skyrmion and skyrmion phase manipulation is expected by using electric fields; thus meeting better the demands of an energy-conscious society. In this work on an insulating chiral magnet ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$ with magnetoelectric coupling, we use neutron scattering to demonstrate directly (i) the creation of metastable skyrmion states over an extended range in magnetic field and temperature ($T$), and (ii) the in situ electric-field-driven switching between topologically distinct phases; the skyrmion phase and a competing nontopological cone phase. For our accessible electric-field range, the phase switching is achieved in a high-temperature regime, and the remnant ($E=0$) metastable skyrmion state is thermally volatile with an exponential lifetime on hour timescales. Nevertheless, by taking advantage of the demonstrably longer-lived metastable skyrmion states at lower temperatures, a truly nonvolatile and near dissipation-free topological phase change memory function is promised in magnetoelectric chiral magnets.