To date, for the known exchange-bias (EB) systems there has been one pinning phase and one pinned phase, and the pinning and pinned phases are inherent to the material and do not mutually transform into each other. Interestingly, we show here that EB is observed in a special system $[\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3{]}_{0.1}[\mathrm{Fe}\mathrm{Ti}{\mathrm{O}}_3{]}_{0.9}$ (HI-9). Neutron powder diffraction and magnetic measurement confirm the following for HI-9: (i) two types of short-range antiferromagnetic (AFM) orderings coexist and serve as two pinning phases; (ii) the pinned phase is not intrinsic to the structure but can be dynamically produced from the pinning phase with the help of an external magnetic field. Consequently, two anomalous EB behaviors are observed: (i) both the coercivity ($H_C$) and the exchange-bias field ($H_E$) simultaneously decrease to zero at 30 K; (ii) for a high cooling field ($H_{\mathrm{cool}}$), $H_E$ decreases logarithmically with increasing $H_{\mathrm{cool}}$. Using Arrott plots it is confirmed that the first-order magnetic phase transformation (FOMPT) from the AFM ${\mathrm{Fe}}^{2+}$ to ferromagnetic (FM) ${\mathrm{Fe}}^{2+}$ and the second-order magnetic phase transformation for the process whereby the FM ${\mathrm{Fe}}^{2+}$ align with the external-field direction coexist in HI-9. The Morin transition and FOMPT cause anomalous EB behaviors. This work may provide fresh ideas in the application of the EB effect.

Transistor structures comprising graphene and subwavelength metal gratings hold great promise for plasmon-enhanced terahertz detection. Despite considerable theoretical effort, little experimental evidence for terahertz plasmons in such structures has been found so far. Here we report an experimental study of plasmons in graphene-insulator-grating structures using Fourier-transform spectroscopy in the 5–10-THz range. The plasmon resonance is clearly visible above the Drude absorption background even in chemical-vapor-deposited graphene with low carrier mobility of approximately ${10}^3$ ${\mathrm{cm}}^2/$V s. We show that the plasmon lifetime exceeds the transport relaxation time extracted from dc mobility, and argue that the former is weakly sensitive to scattering by grain boundaries and macroscopic defects inherent in chemical-vapor-deposited samples. We find that a grating coupler close to graphene strongly modifies the plasmon spectrum, which is determined by metal stripe width but not by grating period. We present a simple theory of grating-coupled two-dimensional plasmons, akin to the tight-binding theory of electrons in solids, that reproduces the observed resonant frequencies without fitting parameters. Our results demonstrate the prospect of large-area commercially available graphene for resonant terahertz detectors.

We describe a method for the active control of terahertz (THz) waves using vanadium dioxide (${\mathrm{VO}}_2$)-embedded hybrid metamaterials (MMs). ${\mathrm{VO}}_2$ undergoes an insulator–metal phase transition at around 68 °C, causing its conductivity to change by five orders of magnitude. Our MM consists of a periodic array of two metallic structures connected with ${\mathrm{VO}}_2$ pads. Simulations show that this MM can realize mode switching via the ${\mathrm{VO}}_2$ pads with high conductivity in the conduction state and high dynamic conductivity across the phase transition. Such material characterizations have rarely been observed in other materials, such as semiconductors, superconductors, and graphene. More interestingly, this mode switch can be realized experimentally using not only a thermal stimulus, but also electrical and optical stimuli. The phase transition mechanisms are the same for thermal and electrical stimuli, but different for the optical stimulus. This diverse range of stimuli is very useful for manipulating the THz waves in practical applications.

Second-order nonlinear interactions in disordered materials based on random phase matching suggest intriguing opportunities for extremely broadband frequency conversion. Here, we present a quantitative realistic model for random phase matching in zinc-blende polycrystals ($\mathrm{Zn}\mathrm{Se}$, $\mathrm{Zn}\mathrm{S}$, $\mathrm{Ga}\mathrm{As}$, $\mathrm{Ga}\mathrm{P}$, etc.) that takes into account effects of random crystal orientation and grain-size fluctuations, and includes polarization analysis of the generated output. Our simulations are based on rigorous transformation of the second-order susceptibility tensor in randomly rotated coordinates—to account for random orientation of crystalline domains, and demonstrate a good agreement with our experimental data for $\mathrm{Zn}\mathrm{Se}$ using a nanosecond $\lambda =4.7\mu \mathrm{m}$ source—in terms of variations of the strength and polarizations of the output fields. Also, it is revealed that random phase matching is most suitable for ultrafast (sub-100-fs) interactions with focused beams, e.g., second-harmonic generation, sum- and difference-frequency generation, and optical parametric oscillation, that typically require short ($<1\mathrm{mm}$) interaction lengths, where disordered polycrystals can be on a par, in terms of conversion yield, with ideal quasi-phase-matched crystals.

Coherence times of electron spins bound to phosphorus donors have been measured, using a standard Hahn echo technique, to be up to 20 ms in isotopically pure silicon with $[\text{P}]={10}^{14}{\text{cm}}^{-3}$ and at temperatures $\le 4\text{K}$. Although such times are exceptionally long for electron spins in the solid state, they are nevertheless limited by donor electron spin-spin interactions. Suppressing such interactions requires even lower donor concentrations, which lie below the detection limit for typical ESR spectrometers. Here we describe an alternative method for phosphorus donor ESR detection, exploiting the spin-to-charge conversion provided by the optical donor-bound-exciton transition. We characterize the method and its dependence on laser power and use it to measure a coherence time of $T_2=130\text{ms}$ for one of the purest silicon samples grown to date $([\text{P}]=5\times {10}^{11}{\text{cm}}^{-3})$. We then benchmark this result using an alternative application of the donor-bound-exciton transition: optically polarizing the donor spins before using conventional ESR detection at 1.7 K for a sample with $[\text{P}]=4\times {10}^{12}{\text{cm}}^{-3}$, and measuring in this case a $T_2$ of 350 ms. In both cases, $T_2$ is obtained after accounting for the effects of magnetic field noise, and the use of more stable (e.g., permanent) magnets could yield even longer coherence times.

In this paper we present a consistent theoretical approach and an extensive experimental study of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$- (CIGS-)based solar cells to investigate the influence of fluctuating potentials on the limitations of solar-cell performance. The absorptance is calculated for extensions to the Shockley-Queisser model involving the description of tail states under the Urbach-rule, optimal-fluctuation-theory, and band-gap-fluctuation models, as well as the expected values for the saturation current density, short-circuit current density, and open-circuit voltage ($V_{\mathrm{OC}}$). Three CIGS-based solar cells with $[\mathrm{Cu}]/([\mathrm{Ga}]+[\mathrm{In}])$ ratios of 0.53, 0.71, and 0.84 are grown to intentionally have sufficiently different amplitudes of fluctuating potentials. We show both theoretically and experimentally the role played by fluctuating potentials, in particular in the $V_{\mathrm{OC}}$ losses. We provide evidence for a higher degree of correlation of electrostatic fluctuating potentials with $V_{\mathrm{OC}}$ losses in comparison with band-gap fluctuations. Additionally, our results show the influence of fluctuating potentials not just at low temperature but also at room temperature.

Metasurfaces have shown great potential to generate Airy beams, but the available schemes with dielectric and plasmonic metasurfaces suffer from a lack of amplitude modulation and/or low transmission efficiency. Huygens’ metasurfaces have the capability of manipulating electromagnetic wave fronts without reflection, and hence have offered a robust approach to exploit empowered functionalities, such as reflecting, refracting, and focusing plane waves. Here we propose a Huygens’ metasurface structure comprised of a metal capacitor sandwiched between double-pair U-shaped ring resonators, which is capable of independently controlling the electric and magnetic dipole responses. The electric admittance and magnetic impedance can be tuned within a wide range, which enables us to achieve nearly arbitrary transmission amplitude and phase for the transmitted waves. The meta-atom of Huygens’ metasurface can be designed to have ultrahigh transmission amplitude (nearly 100% for the maximum value), and the resultant Airy-beam generators assembled with the Huygens’ metasurfaces present high transmission efficiency. We theoretically design transmissive Airy-beam generators that require polynary amplitude and binary phase distributions, and experimentally verify their excellent performances in terms of transmission efficiency. Our results, based on the independent control of electric admittance and magnetic impedance, can stimulate making high-efficiency wave-front manipulation devices with diversified functionalities in different frequency domains.

By means of the plane-wave method we study spin-wave dynamics in two-dimensional bicomponent magnonic crystals based on a squeezed hexagonal lattice and consist of a permalloy thin film with cobalt inclusions. We explore the dependence of a spin-wave frequency on the external magnetic field, especially in weak fields where the mode softening takes place. For considered structures, the mode softening proves to be highly nonuniform on both the mode number and the wave vector. We find this effect to be responsible for the omnidirectional band gap opening. Moreover, we show that the enhancement of the demagnetizing field caused by the squeezing of the structure is of crucial importance for the nonuniform mode softening. This allows us to employ this mechanism to design magnonic gaps with different sensitivity for the tiny change of the external field. The effects we find should be useful in designing and optimization of spin-wave filters highly tunable by an external magnetic field.

Quantum sensors, such as the nitrogen-vacancy (N-V) color center in diamond, are known for their exquisite sensitivity but their performance over time is subject to degradation by environmental noise. To improve the long-term robustness of a quantum sensor, here we realize an integrated combinatorial spin sensor in the same micrometer-scale footprint, which exploits two different spin sensitivities to distinct physical quantities to stabilize one spin sensor with local information collected in real time via the second sensor. We show that we can use the electronic spins of a large ensemble of N-V centers as sensors of the local magnetic field fluctuations, affecting both spin sensors, in order to stabilize the output signal of interleaved Ramsey sequences performed on the ${}^{14}\mathrm{N}$ nuclear spin. An envisioned application of such a device is to sense rotation rates with a stability of several days, allowing navigation with limited or no requirement for geolocalization. Our results would enable stable rotation sensing for over several hours, which already reflects better performance than microelectromechanical systems (MEMS) gyroscopes of comparable sensitivity and size.

A voltage-controlled topological spin switch (VTOPSS) that uses a hybrid topological insulator–magnetic insulator multiferroic material is presented that can implement Boolean logic operations with sub-10-aJ energy per bit and an energy-delay product on the order of ${10}^{-26}$ J s. The device uses a topological insulator, which has the highest efficiency of conversion of the electric field to spin torque yet observed at room temperature, and a low-moment magnetic insulator that can respond rapidly to a given spin torque. We present the theory of operation of the VTOPSS, develop analytic models of its performance metrics, elucidate performance scaling with dimensions and voltage, and benchmark the VTOPSS against existing spin-based and CMOS devices. Compared with existing spin-based devices, such as all-spin logic and charge-spin logic devices, the VTOPSS offers 10–70 times lower energy dissipation and 70–1700 times lower energy-delay product. With experimental advances and improved material properties, we show that the energy and energy-delay product of the VTOPSS can be lowered to a few attojoules per bit and ${10}^{-28}$ J s, respectively. As such, the VTOPSS technology offers competitive metrics compared with existing CMOS technology. Finally, we establish that interconnect issues that dominate the performance in CMOS logic are relatively less significant for the VTOPSS, implying that highly resistive materials can indeed be used to interconnect VTOPSS devices.

Active cooling systems use electrical work to bring a hot device (such as an integrated circuit) down to near ambient temperature by draining heat from the hot area of the device to a passive heat sink. Commercial thermoelectric modules are optimized for refrigeration, and are not ideal for active cooling. Refrigeration maintains a temperature that is below the ambient temperature in a device (such as a kitchen refrigerator) by pumping heat from the cold area of the device to a heat sink. The thermoelectric figure of merit zT is used traditionally to evaluate the performance of thermoelectric modules, including refrigeration modules. But it is not a good indicator of the performance of active cooling materials. Here, we describe an efficient, all-solid-state active cooler based on the Peltier effect in metals with high thermoelectric power factor due to electron correlation effects (${\mathrm{Ce}\mathrm{Pd}}_3$) or magnon drag ($\mathrm{Co}$) and high passive thermal conductivity. We show theoretically and experimentally that the effective thermal conductance under applied current can exceed the limits imposed by Fourier’s heat conduction law. The designed device measures an effective thermal conductance that is an order of magnitude larger than the passive thermal conductance at ΔT = 1 K with the dynamic response of 4 s.

The use of ultrasoft cantilevers gives dynamic cantilever magnetometry (DCM) unique capabilities and sensitivity to probe magnetization processes in individual microscopic samples. However, to date, DCM has been applied mostly to wire-shaped samples, because no universal method is yet available for assembling nanosamples and a cantilever. Here we report a sample-transfer method that allows DCM to be applied to individual nanostructures without imposing any particular shape requirements. The method uses a dual-beam system of a focused ion beam and a scanning electron microscope and a nanomanipulator. We demonstrate the proposed method by measuring the magnetization process of a cobalt microdisk, a ${\mathrm{Co}\mathrm{Fe}}_2{\mathrm{O}}_4$ nanopyramid, and a nonmagnetic sample. The use of the proposed method allows the application of DCM to be extended to the study of the magnetic properties of individual nanosamples of arbitrary shape.

$\mathrm{Ni}$-$\mathrm{Co}$-$\mathrm{Mn}$-$\mathrm{Sb}$-based Heusler shape-memory alloys that undergo a martensitic-structural transition around room temperature are well known for exhibiting large magnetic entropy change and elastocaloric effect. Here, we report the observation of a large adiabatic temperature change of $-11\mathrm{K}$ in a $\mathrm{Ni}$-$\mathrm{Co}$-$\mathrm{Mn}$-$\mathrm{Sb}$ system by using direct adiabatic temperature-change measurements in pulsed magnetic fields. We show that a large magnetic cooling can be achieved in a wide temperature range spanning from 120 to 270 K by purposefully varying the chemical composition. The temperature- and field-dependent irreversibility of the effect is analyzed through a detailed experimental study of the protocol-dependent magnetocaloric effect. The present study is an important contribution towards the understanding of irreversible magnetocaloric effects in materials with magnetostructural transition.

Nonlinear electrical transport studies at high pulsed magnetic fields, above the range accessible by dc magnets, are of direct fundamental relevance to the physics of superconductors, domain-wall, charge-density waves, and topological semimetal. All-superconducting very high field magnets also make it technologically relevant to study vortex matter in this regime. However, pulsed magnetic fields reaching 100 T in milliseconds impose technical and fundamental challenges that have prevented the realization of these studies. Here, we present a technique for sub-microsecond smart current-voltage measurements, which enables the determination of the superconducting critical current in pulsed magnetic fields, beyond the reach of any dc magnet. We demonstrate the excellent agreement of this technique with low dc field measurements on ${\mathrm{Y}}_{0.77}{\mathrm{Gd}}_{0.23}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ coated conductors with and without ${\mathrm{Ba}\mathrm{Hf}\mathrm{O}}_3$ nanoparticles. Exploring the uncharted high-magnetic-field region, we discover a characteristic influence of the magnetic field rate of change ($dH/dt$) on the current-voltage curves in a superconductor. We fully capture this unexplored vortex physics through a theoretical model based on the asymmetry of the vortex-velocity profile produced by the applied current.

Fluid flow along microchannels can be induced by keeping opposite walls at different temperatures and placing elongated tilted pillars inside the channel. The driving force for this fluid motion arises from the anisotropic thermophoretic effect of the elongated pillars that generates a force parallel to the walls and perpendicular to the temperature gradient. The force is not determined by the thermophilic or thermophobic character of the obstacle surface, but by the geometry and the thermophoretic anisotropy of the obstacle. Via mesoscale hydrodynamic simulations, we investigate the pumping properties of the device as a function of the channel geometry and pillar surface properties. Applications as fluidic mixers and fluid alternators are also outlined, together with the potential use of all these devices to harvest waste heat energy. Furthermore, similar devices can also be built employing diffusiophoresis or electrophoresis.

Magnonic crystals are systems that can be used to design and tune the dynamic properties of magnetization. Here, we focus on one-dimensional Fibonacci magnonic quasicrystals. We confirm the existence of collective spin waves propagating through the structure as well as dispersionless modes; the reprogammability of the resonance frequencies, dependent on the magnetization order; and dynamic spin-wave interactions. With the fundamental understanding of these properties, we lay a foundation for the scalable and advanced design of spin-wave band structures for spintronic, microwave, and magnonic applications.

This work shows that, counterintuitively, adding extremely small ($<10$ nm in dimension) fully absorbing metal nanoparticles to a material can notably enhance its light emission. It also shows that the mystery behind the unexpected large enhancements in surface-enhanced Raman spectroscopy, even greater than a factor of ${10}^{10}$, is the tunneling out of photons from the strongly absorbing metal surface. Dynamics in a strongly coupled emitter-matter quantum system demands such behavior in the case of limiting small metal nanoparticles and proximal metal surfaces. This effect can be exploited further in light generation, optical sensing, and radiative heat transfer.

Deterministic switching of polarization vortices in ferroelectrics by a uniaxial mechanical load is challenging. Here, we demonstrate, by using phase field simulations and ferroelectric instability analyses, a deterministic switching of polarization vortices in a compositionally graded ferroelectric (CGFE) nanoplate under a compressive stress. The obtained results suggest that the rational distribution of material constituents in the CGFE nanoplate tailors the distributions of electromechanical coupling and total energy densities of the nanoplate, which modifies the stress-induced vortex multiplication and annihilation behaviors, distinguished from those in a homogeneous ferroelectric nanoplate. The switching of polarization vortices by compressive stress is verified through instability analyses for the CGFE nanoplate, which not only confirms when the polarization switching occurs, but also demonstrates how the polarization responds at the onset of the switching. In addition, the effects of various factors on the feasibility of stress-induced vortex switching, including material gradient, temperature, and width-to-height ratio of the CGFE nanoplate, are investigated and summarized in phase diagrams.

Quantum dots attached to BCS superconducting leads exhibit a $0-\pi$ impurity quantum phase transition, which can be experimentally controlled either by the gate voltage or by the superconducting phase difference. For the pertinent superconducting single-impurity Anderson model, we present two simple analytical formulae describing the position of the phase boundary in parameter space for the weakly correlated and Kondo regime, respectively. Furthermore, we show that the two-level approximation provides an excellent description of the low-temperature physics of superconducting quantum dots near the phase transition. We discuss reliability and mutual agreement of available finite-temperature numerical methods (numerical renormalization group and quantum Monte Carlo) and suggest an alternative approach for efficient determination of the quantum phase boundary from measured finite-temperature data. Our results enable fast and efficient, yet reliable characterization and design of such nanoscopic tunable Josephson-junction devices.

Spintronic neurons that emit sharp voltage spikes are required for the realization of hardware neural networks enabling fast data processing with low power consumption. In many neuroscience and computer science models, neurons are abstracted as nonlinear oscillators. Magnetic nano-oscillators called spin-torque nano-oscillators are interesting candidates for imitating neurons at the nanoscale. These oscillators, however, emit sinusoidal waveforms without spiking while biological neurons are relaxation oscillators that emit sharp voltage spikes. Here, we propose a simple way to imitate neuron spiking in high-magnetoresistance nanoscale spin valves, where both magnetic layers are free and thin enough to be switched by spin torque. Our numerical-simulation results show that the windmill motion induced by spin torque in the proposed spintronic relaxation oscillator gives rise to spikes whose shape and frequency, set by the charging and discharging times, can be tuned through the amplitude of injected dc current. We also find that these devices can exhibit chaotic oscillations. Chaoticlike neuron dynamics has been observed in the brain, and it is desirable in some neuromorphic computing applications, whereas it should be avoided in others. We demonstrate that the degree of chaos can be tuned in a wide range by engineering the magnetic stack and anisotropies and by changing the dc current. The proposed spintronic relaxation oscillator is a promising building block for hardware neuromorphic chips, leveraging nonlinear dynamics for computing.