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Since the invention of transistors, the flow of electrons has become controllable in solid-state electronics. The flow of energy, however, remains elusive, and energy is readily dissipated to lattice via electron-phonon interactions. Hence, minimizing the energy dissipation has long been sought by eliminating phonon-emission process. Here, we report a different scenario for facilitating energy transmission at room temperature that electrons exert diffusive but quasiadiabatic transport, free from substantial energy loss. Direct nanothermometric mapping of electrons and lattice in current-carrying GaAs/AlGaAs devices exhibit remarkable discrepancies, indicating unexpected thermal isolation between the two subsystems. This surprising effect arises from the overpopulated hot longitudinal-optical (LO) phonons generated through frequent emission by hot electrons, which induce equally frequent LO-phonon reabsorption (“hot-phonon bottleneck”) cancelling the net energy loss. Our work sheds light on energy manipulation in nanoelectronics and power-electronics and provides important hints to energy-harvesting in optoelectronics (such as hot-carrier solar-cells). Minimizing the energy dissipation is usually sought by eliminating phonon-emission process. Here, the authors find a different approach for facilitating energy transmission at room temperature that electrons exert diffusive but quasiadiabatic transport, free from substantial energy loss.

As electronic chips become smaller, efficient heat dissipation becomes a greater challenge. Electrons in such devices quickly accelerate over small distances, becoming “hot”—that is, out of equilibrium with the rest of the system. Weng et al. designed a thermometry probe that measures the effective temperature of hot electrons with a spatial resolution of about 50 nanometers. The method is based on the optical measurement of current noise and provides a glimpse into where heat is naturally dissipated in a working device. Science , this issue p. 775 A scanning probe maps the temperature of electrons going through a constriction in a GaAs/AlGaAs device. In modern microelectronic devices, hot electrons accelerate, scatter, and dissipate energy in nanoscale dimensions. Despite recent progress in nanothermometry, direct real-space mapping of hot-electron energy dissipation is challenging because existing techniques are restricted to probing the lattice rather than the electrons. We realize electronic nanothermometry by measuring local current fluctuations, or shot noise, associated with ultrafast hot-electron kinetic processes (~21 terahertz). Exploiting a scanning and contact-free tungsten tip as a local noise probe, we directly visualize hot-electron distributions before their thermal equilibration with the host gallium arsenide/aluminium gallium arsenide crystal lattice. With nanoconstriction devices, we reveal unexpected nonlocal energy dissipation at room temperature, which is reminiscent of ballistic transport of low-temperature quantum conductors.

As the basis of a diverse set of photonic applications, such as hologram imaging, polarization, and wave front manipulation, the local phase control of electromagnetic waves is fundamental in photonic research. However, currently available bulky, passive, range-limited phase modulators pose an obstacle in photonic applications. Here, we propose a new mechanism to achieve a wide phase modulation range, with graphene used as a tunable loss to drive an underdamped to overdamped resonator transition. Based on this mechanism, we present widely tunable phase modulation in the terahertz regime, realized in gate-tuned ultrathin reflective graphene metasurfaces. A one-port resonator model, supported by full-wave simulations, explains the underlying physics of the discovered extreme phase modulation and indicates general strategies for designing tunable photonic devices. As an example, we demonstrate a gate-tunable terahertz (THz) polarization modulator with a graphene metasurface. Our findings establish the possibility for photonic applications based on active phase manipulation.

Recent works in metamaterials and transformation optics have demonstrated exotic properties in a number of open systems, including perfect absorption/transmission, electromagnetically induced transparency, cloaking or invisibility, etc. Meanwhile, non-Hermitian physics framework has been developed to describe the properties of open systems, however, most works related to this focus on the eigenstate properties with less attention paid to the reflection characteristics in complex frequency plane, despite the usefulness of zero-reflection (ZR) for applications. Here we demonstrate that the indirectly coupled two-magnon system not only exhibits non-Hermitian eigenmode hybridization, but also ZR states in complex frequency plane. The observed perfect-ZR (PZR) state, i.e., ZR with pure real frequency, is manifested as infinitely narrow reflection dips (~67 dB) with infinite group delay discontinuity. This reflection singularity of PZR distinguishes from the resonant eigenstates but can be adjusted on or off resonance with the eigenstates. Accordingly, the absorption and transmission can be flexibly tuned from nearly full absorption (NFA) to nearly full transmission (NFT) regions. Absorption, transmission and reflection are three processes characterizing optical devices. Absorption allows for signal conversion and transmission is important for signal transfer, however, reflection is frequently detrimental to device performance. Here, Qian et al demonstrate a magnonic device with controllable absorption and transmission while maintain zero reflection.

The study of thermoelectric behaviors in miniatured transistors is of fundamental importance for developing bottom-level thermal management. Recent experimental progress in nanothermetry has enabled studies of the microscopic temperature profiles of nanostructured metals, semiconductors, two-dimensional material, and molecular junctions. However, observations of thermoelectric (such as nonequilibrium Peltier and Thomson) effect in prevailing silicon (Si)—a critical step for on-chip refrigeration using Si itself—have not been addressed so far. Here, we carry out nanothermometric imaging of both electron temperature ( T e ) and lattice temperature ( T L ) of a Si nanoconstriction device and find obvious thermoelectric effect in the vicinity of the electron hotspots: When the electrical current passes through the nanoconstriction channel generating electron hotspots (with T e ~1500 K being much higher than T L ~320 K), prominent thermoelectric effect is directly visualized attributable to the extremely large electron temperature gradient (~1 K/nm). The quantitative measurement shows a distinctive third-power dependence of the observed thermoelectric on the electrical current, which is consistent with the theoretically predicted nonequilibrium thermoelectric effects. Our work suggests that the nonequilibrium hot carriers may be potentially utilized for enhancing the thermoelectric performance and therefore sheds new light on the nanoscale thermal management of post-Moore nanoelectronics. Thermoelectric property of silicon itself is important for the thermal management of post-Moore nanoelectronics. Here, Xue et al directly observe unconventional thermoelectric cooling/heating effects enhanced by hot electrons in silicon nanodevices.

Ferroelectric semiconductors have the advantages of switchable polarization ferroelectric field regulation and semiconductor transport characteristics, which are highly promising in ferroelectric transistors and nonvolatile memory. However, it is difficult to prepare a Sn-based perovskite film with both robust ferroelectric and semiconductor properties. Here, by doping with 2-methylbenzimidazole, Sn-based perovskite [93.3 mol% (FA 0.86 Cs 0.14 )SnI 3 and 6.7 mol% PEA 2 SnI 4 ] semiconductor films are transformed into ferroelectric semiconductor films, owing to molecular reconfiguration. The reconfigured ferroelectric semiconductors exhibit a high remanent polarization ( P r ) of 23.2 μC/cm 2 . The emergence of ferroelectricity can be ascribed to the hydrogen bond enhancement after imidazole molecular doping, and then the spatial symmetry breaks causing the positive and negative charge centers to become non-coincident. Remarkably, the transistors based on perovskite ferroelectric semiconductors have a low subthreshold swing of 67 mv/dec, which further substantiates the superiority of introducing ferroelectricity. This work has developed a method to realize Sn-based ferroelectric semiconductor films for electronic device applications. The authors observe the emergence of ferroelectricity in Sn-based perovskite [93.3 mol% (FA0.86Cs0.14)SnI3 and 6.7 mol% PEA2SnI4] semiconductor films doped with 2-methylbenzimidazole, ascribing to the hydrogen bond enhancement after imidazole molecular doping.

Broken symmetries play a fundamental role in superconductivity and influence many of its properties in a profound way. Understanding these symmetry breaking states is essential to elucidate the various exotic quantum behaviors in non-trivial superconductors. Here, we report an experimental observation of spontaneous rotational symmetry breaking of superconductivity at the heterointerface of amorphous (a)-YAlO 3 /KTaO 3 (111) with a superconducting transition temperature of 1.86 K. Both the magnetoresistance and superconducting critical field in an in-plane field manifest striking twofold symmetric oscillations deep inside the superconducting state, whereas the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute this behavior to the mixed-parity superconducting state, which is an admixture of s -wave and p -wave pairing components induced by strong spin-orbit coupling inherent to inversion symmetry breaking at the heterointerface of a-YAlO 3 /KTaO 3 . Our work suggests an unconventional nature of the underlying pairing interaction in the KTaO 3 heterointerface superconductors, and brings a new broad of perspective on understanding non-trivial superconducting properties at the artificial heterointerfaces. Superconducting interfaces involving KTaO3 have recently attracted attention due to their relatively high transition temperature. Here, the authors study amorphous-YAlO 3 /KTaO 3 interfaces and find two-fold symmetry in the superconducting regime, possibly due to a mixed-parity superconducting state.

The recently emerged remote epitaxy technique, utilizing 2D materials (mostly graphene) as interlayers between the epilayer and the substrate, enables the exfoliation of crystalline nanomembranes from the substrate, expanding the range of potential device applications. However, remote epitaxy has been so far applied to a limited range of material systems, owing to the need of stringent growth conditions to avoid graphene damaging, and has therefore remained challenging for the synthesis of oxide nanomembranes. Here, we demonstrate the remote epitaxial growth of an oxide nanomembrane (vanadium dioxide, VO 2 ) with a sub-nanometer thick amorphous interlayer, which can withstand potential sputtering-induced damage and oxidation. By removing the amorphous interlayer, a 4-inch wafer-scale freestanding VO 2 nanomembrane can be obtained, exhibiting intact crystalline structure and physical properties. In addition, multi-shaped freestanding infrared bolometers are fabricated based on the epitaxial VO 2 nanomembranes, showing high detectivity and low current noise. Our strategy provides a promising way to explore various freestanding heteroepitaxial oxide materials for future large-scale integrated circuits and functional devices. Remote epitaxy is a technique enabling the growth of exfoliable crystalline membranes, but its application to oxide materials is challenging. Here, the authors report the remote epitaxial growth of wafer-scale VO 2 nanomembranes, showing their application for the fabrication of multi-shaped freestanding infrared bolometers.

Metamaterials provide compelling capabilities to manipulate electromagnetic waves beyond the natural materials and can dramatically enhance both their electric and magnetic fields. The enhanced magnetic fields, however, are far less utilized than the electric counterparts, despite their great potential in spintronics. In this work, we propose and experimentally demonstrate a hybrid perfect metamaterial absorbers which combine the artificial metal/insulator/metal (MIM) metamaterial with the natural ferromagnetic material permalloy (Py) and realize remarkably larger spin rectification effect. Magnetic hot spot of the MIM metamaterial improves considerably electromagnetic coupling with spins in the embedded Py stripes. With the whole hybridized structure being optimized based on coupled-mode theory, perfect absorption condition is approached and an approximately 190-fold enhancement of spin-rectifying photovoltage is experimentally demonstrated at the ferromagnetic resonance at 7.1 GHz. Our work provides an innovative solution to harvest microwave energy for spintronic applications, and opens the door to hybridized magnetism from artificial and natural magnetic materials for emergent applications such as efficient optospintronics, magnonic metamaterials and wireless energy transfer.

Among the coronal mass ejections (CMEs) and solar proton events (SPEs) frequently observed by near-Earth spacecraft, the SPE that occurred on 28 October 2021 stands out as a remarkable research event. This is due to the infrequency of reported ground-level enhancements it induced. The CSES (China seismo-electromagnetic satellite) is equipped with high-energy particle detectors, namely, HEPP and HEPD, capable of measuring protons within an energy range of 2 MeV to 143 MeV. These detectors provide valuable opportunities for studying solar activity. By utilizing the Monte Carlo method to simulate the pile-up effect and accounting for the detector’s dead time, with the assistance of real-time incident counting rates, we successfully corrected the spectra in the 10–50 MeV range. The energy spectrum is important for understanding solar proton events. We used the data from the HEPP (high-energy particle package) and HEPD (high-energy particle detector) to obtain the total event-integrated spectrum, which possessed good continuity. Additionally, we compared the observations from the CSES with those from the NOAA satellite and achieved reasonable agreement. We also searched for ground-based responses to this solar activity in China and discovered Forbush decreases detected by the Yang Ba Jing Muon Telescope experiment. In conclusion, the HEPP and HEPD can effectively combine to study solar activity and obtain a smooth and consistent energy spectrum of protons across a very wide energy range.