Based on the nonlinear propagation of thermal diffusion waves, we demonstrate the existence of a net heat current even in the absence of a mean temperature gradient. Unlike the steady-state heat current, this current of thermal waves is not driven by the material thermal conductivity, but rather by its temperature derivative. The heat current propagates outward or inward of its excitation source when the thermal conductivity increases or decreases with temperature, respectively. The modulation of the heat flux direction can thus be achieved by varying the material temperature around the thermal conductivity peak exhibited by various dielectrics. For a silicon plate with a thermal conductivity peak at 40 K that supports the propagation of thermal waves with an excitation amplitude of 10 K, we observe a net heat flux exceeding $60\text{W}{\mathrm{cm}}^{-2}$ at 20 K and falling below $-20$ W ${\mathrm{cm}}^{-2}$ at 65 K. Larger excitation amplitudes allow higher or lower heat fluxes to be achieved, which makes them easy to observe and potentially apply for harvesting or evacuating thermal energy in systems undergoing natural temperature fluctuations on earth and in outer space.

In the synthesis of two-dimensional (2D) materials and the fabrication of 2D devices, the strain introduced by lattice mismatch is inevitable. Meanwhile, strain is also a frequently used strategy to improve the Curie temperature of 2D magnets. However, the impact of strain on the magnon relaxation time is unclear. In this work, we investigated the impacts of strain on the magnon-magnon interaction and magnon relaxation time in the ferromagnetic ${\mathrm{Cr}\mathrm{Ge}\mathrm{Te}}_3$ monolayer by combining first-principles calculations and theoretical analysis. We find the magnon relaxation times in strained and unstrained ${\mathrm{Cr}\mathrm{Ge}\mathrm{Te}}_3$ monolayers have similar dependences on temperature, wave vectors, and magnetic fields. However, the magnon relaxation time is remarkably reduced by tensile strain. When the external magnetic field is 0.1 T, the maximum shortened ratio of the magnon relaxation time can reach 49.3%, revealing an enhancement of the magnon-magnon interaction. With the magnetic field strengthening, the shortened ratio of the magnon relaxation time decreases, but it still reaches up to 19.4% under a 5-T magnetic field. This shortening of magnon relaxation time is undesired for applications in spintronics. Our work provides physical insights into the impacts of strain on 2D magnets.

Using Landau-Ginzburg-Devonshire (LGD) approach, we proposed the analytical description of the influence of chemical strains on spontaneous polarization and the electrocaloric response in ferroelectric core-shell nanorods. We postulate that the nanorod core presents a defect-free single-crystalline ferroelectric material, and elastic defects are accumulated in the ultrathin shell, where they can induce tensile or compressive chemical strains. Finite-element modeling (FEM) based on the LGD approach reveals transitions of domain-structure morphology induced by chemical strains in the ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ nanorods. Namely, tensile chemical strains induce and support the single-domain state in the central part of the nanorod, while the curled domain structures appear near the unscreened or partially screened ends of the rod. The vortexlike domains propagate toward the central part of the rod and fill it entirely, when the rod is covered by a shell with compressive chemical strains above some critical value. The critical value depends on the nanorod sizes, aspect ratio, and screening conditions at its ends. Both analytical theory and FEM predict that the tensile chemical strains in the shell increase the nanorod polarization, lattice tetragonality, and electrocaloric response well above the values corresponding to the bulk material. The physical reason for the increase is strong electrostriction coupling between the mismatch-type elastic strains induced in the core by chemical strains in the shell. Comparison with earlier XRD data confirmed an increase of the tetragonality ratio in tensile ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ nanorods compared to the bulk material. Obtained analytical expressions, which are suitable for the description of strain-induced changes in a wide range of multiaxial ferroelectric core-shell nanorods and nanowires, can be useful for strain engineering of advanced ferroelectric nanomaterials for energy storage, harvesting, electrocaloric applications, and negative capacitance elements.

The atomic orbital, as an intrinsic degree of freedom, is critical in improving band-tailoring tactics in condensed-matter physics. In photonics, analogously to atomic orbitals, multiple Mie resonances provide an effective way for photonic band engineering. In this work, we demonstrate the topological phase transitions in photonic Chern insulators imposed by band topology through multiple Mie resonances. By continuously adjusting geometric parameters and the external magnetic field in a two-dimensional gyromagnetic photonic crystal, we present the inversion of Mie resonances together with alterations in the Chern number for the corresponding photonic band. Furthermore, we propose a hetero–photonic structure that combines photonic lattices with distinct Chern numbers and exhibits chiral edge states at the heterointerface. A simple magnetic regulation approach is also provided for effective control of the edge state’s on–off switching. Our work establishes a correlation between the band topology of photonic Chern insulators and Mie resonances, contributing to advancements in higher-band-topology research.

We present rapid and robust protocols for stimulated rapid adiabatic passage and quantum logic gates. Our gates are based on geometric phases acquired by instantaneous eigenstates of a slowly accelerating “inertial” Hamiltonian. To begin, we establish the criteria for inertial evolution and subsequently engineer pulse shapes that fulfill these conditions. These tailored pulses are then used to optimize geometric logic gates. We analyze a realization of our protocols with ${}^{87}\mathrm{Rb}$ atoms, resulting in gate fidelity that approaches the current state of the art, with marked improvements in robustness.

A printed metasurface on a grounded, thin cylindrical-shell dielectric substrate is introduced, capable of concealing large objects under both transverse electric and magnetic polarizations. When a plane wave illuminates the cylinder, the metasurface effectively absorbs the incident power on the lit side by inducing surface waves, and transports it to the shadow side before reconstructing the incident wavefront and thereby achieving cloaking. For each polarization, the surface waves are built and meticulously optimized to meet local and global lossless and gainless conditions, ensuring a passive surface characterization. Retrieved from the optimized complete tangential fields, a spatially modulated tensor surface reactance is realized using an array of printed Jerusalem-cross-shaped conductor patterns. At a microwave frequency, a prototype cloak is designed, fabricated, and experimentally characterized for its scattering properties. Measurement results confirm the effectiveness of the cloaking approach and its printed metasurface realization.

This study examines the effects of negative voltage pulses (NVPs) on emission from $(\mathrm{In},\mathrm{Ga})\mathrm{N}/\mathrm{Ga}\mathrm{N}$ light-emitting diodes (LEDs) with a wide quantum well in the active layer. For low dc forward voltages (1–3 V), the diode emits light only during the periodic application of the NVPs, provided that the separation of NVPs is sufficiently large (from a few microseconds to milliseconds). These effects are due to a slow buildup of “dark charge,” i.e., charge in the ground state of the wide well. The NVPs reduce the electric field in the well and deplete dark charge from the well (through radiative and nonradiative recombination). After the NVP, the concentration of dark charge once again slowly increases and saturates. For low forward voltages (or currents), the saturation occurs after milliseconds, which is caused by the long buildup times of dark charge. By varying the duration of NVPs, we can estimate the decay time of dark charge for a given negative voltage. This decay time decreases rapidly with increasing (absolute value of) negative voltage. We present a numerical model of LED emission, which is in qualitative agreement with experimental results. Short pulses of light generated by NVPs may create new applications for wide-well LEDs and laser diodes.

Conventional wireless-power-transfer (WPT) systems are linear and time invariant, which sets fundamental limitations on their performance, including a trade-off between transfer efficiency and the level of transferred power. In this paper, we introduce and study a possibility of temporal modulation for inductive wireless-power-transfer systems and uncover that this trade-off is avoided as a consequence of varying the inductive coupling strength in time. Our theoretical analysis reveals that under the optimal modulation depth and phase, the time modulation can yield a substantial improvement in the WPT efficiency, while the received power at the load is also improved compared to the static WPT reference system. We experimentally demonstrate the concept with a low-frequency system and observe a threefold improvement in efficiency over the reference static counterpart. This technical capability reconciles the inherent trade-off between the WPT efficiency and transferred power, paving the way for simultaneous advancements in both efficiency and delivered power.

Thermoelectric conversion technology is increasingly important for sustainable energy, particularly in transforming waste heat into electricity. In this study, we explore the thermoelectric properties of three novel two-dimensional materials, monolayer $R\text{TeCl}$ (with $R=\text{La},\text{Pr},\text{Nd}$), using density-functional theory and semiclassical Boltzmann transport theory. These materials are stable and can be easily exfoliated from bulk structures due to their low cleavage energy. Notably, monolayer $R\text{TeCl}$ materials have high power factors ($(2.04\text{--}4.48)\times {10}^4\text{μ}\text{W}{\text{m}}^{-1}{\text{K}}^{-2}$), attributed to multiple valleys in their valence-band maximum. Their complex crystal structures and the presence of two heavy elements contribute to an ultralow lattice thermal conductivity ($(2.42\text{--}4.58)\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$) at room temperature. Impressively, the figure of merit ($ZT$) values of monolayer $R\text{TeCl}$ range from 0.4 to 1.2 at 300 K. Across the temperature range of 300 to 700 K, the average $ZT$ value and thermoelectric conversion efficiency reach $1.32\text{--}2.75$ and $15.3\mathrm{\%}\text{--}22.6\mathrm{\%}$, respectively. Our results indicate that monolayer $R\text{TeCl}$ is a promising candidate for nanoelectronics and low-dimensional thermoelectric devices, offering a new avenue for high-performance thermoelectric materials.

${\mathrm{Fe}}_2({\mathrm{Mo}\mathrm{O}}_4{)}_3$ is a well-established L-type ferrimagnetic (L-FIM) material with field-induced magnetic ordering ($T_{\mathrm{N}2}$) and multiferroic properties below $T_{\mathrm{N}1}$ (around 12 K). In this study, we investigate the magnetic properties of ${\mathrm{Fe}}_2({\mathrm{Mo}\mathrm{O}}_4{)}_3$ through temperature- and field-dependent ac and dc magnetic susceptibility $\chi (T)$ measurements. Isothermal magnetization data reveal an additional metamagnetic transition ($H_{\mathrm{C}2}$) beyond the existing boundary between L-FIM and multiferroic phases ($H_{\mathrm{C}1}$). Frequency-dependent ac magnetic susceptibility data demonstrate reentrant-spin-glass-like behavior below $T_{\mathrm{N}1}$, with a critical temperature ($T_{\mathrm{g}0}$) of 6.2 K. Notably, a nonlinear magnetodielectric response and concurrent anomalies in M(H) at the two metamagnetic transitions ($H_{\mathrm{C}1}$ and $H_{\mathrm{C}2}$) allude to a profoundly intertwined magnetoelectric (ME) nature. A finite nonlinear ME effect (${\alpha }_{\mathrm{ME}}$) of about 0.56 ps/m is comparable to that of ME materials such as ${\mathrm{Nd}\mathrm{Cr}\mathrm{Ti}\mathrm{O}}_5$ and ${\mathrm{Mn}\mathrm{Ga}}_2{\mathrm{O}}_4$. The temperature-dependent adiabatic temperature change (Δ$T_{\mathrm{m}}$) due to the contribution of magnetic spin entropy exhibits a small value (approximately 0.8 K) with an oscillatory-like magnetocaloric effect. Remarkably, the adiabatic temperature change (Δ$T_{\mathrm{ME}}$) owing to magnetoelectric coupling is quite large (5.2 K under a 7 T magnetic field) near $T_{\mathrm{N}2}$. The tunability of $T_{\mathrm{N}2}$ with temperature and magnetic field strength represents a unique multicaloric medium whose temperature and field parameters can be easily adjusted for potential cryogenic applications near liquid-helium temperatures.

Superlattices composed of ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ (HZO/LSMO), with nominally 10-nm-thick HZO and 1-nm-thick LSMO in a period, were deposited epitaxially on (110)-oriented $({\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7})({\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}){\mathrm{O}}_3$ (LSAT) substrates with thick LSMO electrodes. These 1-nm-thick LSMO spacer layers interrupt the growth of HZO grains, keeping the superlattice free from a paraelectric monoclinic phase. This makes it possible to achieve a total superlattice thickness above 50 nm in the pure ferroelectric orthorhombic phase. The strain gradient due to the strain relaxation in thick superlattices induces a rhombohedral distortion with the help of the flexoelectric field. The distorted structure is more stable than the metastable orthorhombic ferroelectric phase, making it more resistant to ferroelectric fatigue. The $(\mathrm{HZO}/\mathrm{LSMO}{)}_5$ superlattice exhibits a large remanent polarization of about 19.3 µC/${\mathrm{cm}}^2$ and a greatly enhanced fatigue resistance, by keeping more than 90% of its polarization after ${10}^9$ bipolar switching cycles. Our results provide additional opportunities for the design and optimization of ${\mathrm{Hf}\mathrm{O}}_2$-based ferroelectric materials to optimize the structure and properties for integrated ferroelectric applications.

Tunable acoustic metasurfaces have garnered substantial attention in recent years, offering an approach to manipulate acoustic waves efficiently. Despite the rapid progress in this field, obtaining a dynamic acoustic metasurface with a noncontact design remains a challenging goal. Here, we theoretically propose and experimentally demonstrate dynamic acoustic metasurfaces designed using magnetic levitation. The meta-atom consists of an acoustic cavity with a floating magnet. By varying the magnitude of the current in a coil located at the bottom of the cavity, the position of the magnet can be dynamically adjusted, effectively changing the height of the cavity. The reflection phase can thus be continuously modulated in real time, enabling the switching of versatile functionalities. Our work opens up a magnetic levitation approach towards intelligent acoustic metasurfaces with noncontact control.

Damping typically results in attenuation of vibrations and elastic wave propagation in mechanical systems. Contrary to this conventional understanding, we demonstrate experimentally and explain theoretically the revival of an elastic wave transmitted through a periodic metastructure when a weak non-Hermitian defect (damping mechanism) induces violation of time-reversal symmetry. Damping alters the nature of the system’s resonant modes, instigating interference in the scattering field. This leads to transmission revival, revealing the presence of hidden modes, which are otherwise masked by the symmetry. Our findings offer an approach for designing dissipation-driven switches and controllers and nondestructive structural health-monitoring systems.

Refrigeration systems based on electrocaloric materials are considered as potential alternatives for the current vapor compression–based devices. These systems offer efficiencies closer to the Carnot limits, while also being compatible with miniaturization, compactness, and integration into electronics and wearables. Several prototypes have been proposed that mostly rely on mechanical and fluid motion for heat transfer, which prevent these systems from reaching higher operation frequency, good thermal contact, and low loss. Motionless electrocaloric solid-state devices have so far been conceptualized, but their relative complexity has hampered prototyping. In this work, we study the performance of a solid-state electrocaloric cooler that relies on thermoelectric switches to control heat flow. Our device operation mode minimizes energy consumption by absorbing heat passively through the heat switches. The applied currents, absorbed heat, power consumption, and coefficient of performance are assessed for a wide set of parameters following a steady-state heat propagation model, covering operation temperature, material properties, geometric features, frequency of operation, and material polarization losses. We show that a coefficient of performance (COP) above 1 can be obtained for temperature spans up to 6 K (for different material properties, geometry factors, or electrocaloric loss) and adiabatic temperature changes around 1 K higher than the applied temperature span. Higher temperature spans between 6 to 10 K yield a COP of the order of 0.1, leading to a significant increase in power consumption. These results are intended to guide research into these solid-state devices, in terms of choice of materials, temperatures, and geometry.

Standard terahertz time-domain spectroscopy uses a relatively slow multidata acquisition process that has hindered the technique’s ability to resolve “fast” dynamics occurring on the microsecond timescale. This timescale, inaccessible to most ultrafast pump-probe techniques, hosts a range of phenomena that has been left unexplored due to a lack of proper real-time monitoring techniques. In this work, chirped-pulse spectral encoding, a photonic time-stretch technique, and high-speed electronics are used to demonstrate time-resolved terahertz detection at a rate up to 1.1 MHz. This configuration relies on a tabletop optical source and a setup able to resolve every terahertz transient generated by the same source. We investigate the performance of this single-pulse terahertz detection system at different acquisition rates in terms of experimental noise, dynamic range, and signal-to-noise ratio. Our results pave the way towards single-pulse terahertz time-domain spectroscopy at arbitrarily fast rates to monitor complex dynamics in real time.