Thermal counterflow of superfluid ^4He past a cylinder produces quasi-steady eddies not only downstream but also anomalously upstream. However, the mechanism and organizing principles behind the observed multistable wake topologies (0-, 2-, 4-, and 6-vortex states) have remained unclear. We show that the full spectrum of reported normal-fluid wake states is captured numerically with a two-fluid model coupled to Vinen's vortex-line-density equation. Our simulations further reveal that the superfluid component can also develop anomalous upstream eddies, a feature not previously reported. We trace these behaviors to a self-organized zone of enhanced mutual-friction dissipation near the cylinder shoulders that reshapes the effective obstacle, drives upstream eddies in both components, and suppresses intrinsic wake oscillations in the normal fluid. Guided by this mechanism, we perform systematic parameter scans and construct a unified phase diagram in terms of the normal-fluid Reynolds number Re_n and a dimensionless interaction number N, separating inertia- and mutual-friction-controlled transitions and delineating the parameter windows for the discrete wake topologies. These results turn a striking phenomenology into a predictive map and establish mutual-friction feedback as a robust route to unusual wake structures in quantum fluids.

Supported platinum-group-metal single-atom catalysts (SACs) have garnered widespread attention in heterogeneous catalysis due to their theoretical 100% atomic utilization efficiency. Isolated metal atoms usually exhibit high oxidation states rather than metallic states owing to the coordination with nonmetal atoms in the supports. Despite recent advances in modulating the coordination environments of SACs, achieving metallic state-dominated SACs remains a formidable challenge. Herein, a new configuration of metallic Pt single atoms (Pt0) anchored on the CeO2 support is constructed by introducing a localized Pt-Ce metal-metal coordination at the interface in a molten NaAlCl4 environment. The extensive Pt─Ce bonding and electron transfer from the support to the Pt atoms ensure the metallic nature of isolated Pt atoms. This work opens up a new avenue for constructing metallic single atoms and enriches the SACs chemistry.

The minimum separation between reconnecting vortices in fluids and superfluids obeys a universal scaling law with respect to time. The prereconnection and the postreconnection prefactors of this scaling law are different, a property related to irreversibility and to energy transfer and dissipation mechanisms. In the present work, we determine the temperature dependence of these prefactors in superfluid helium from experiments and a numeric model which fully accounts for the coupled dynamics of the superfluid vortex lines and the thermal normal fluid component. At all temperatures, we observe a pre- and postreconnection asymmetry similar to that observed in other superfluids and in classical viscous fluids, indicating that vortex reconnections display a universal behavior independent of the small-scale regularizing dynamics. We also numerically show that each vortex reconnection event represents a sudden injection of energy in the normal fluid. Finally we argue that in a turbulent flow, these punctuated energy injections can sustain the normal fluid in a perturbed state, provided that the density of superfluid vortices is large enough.

Developing low-cost, highly active and robust electrocatalysts for acidic oxygen evolution reaction (OER) is a critical challenge facing the hydrogen-based energy delivery system. As a cheaper alternative to the benchmark IrO2, RuO2 possesses higher OER catalytic activity but suffers from intrinsically low stability arising from Ru dissolution and lattice oxygen overoxidation. Herein, we report a high-vacuum annealing strategy to regulate the oxygen vacancy (Ov) concentration in RuO2, enabling the controllable modulation of the atomic local coordination structures of RuO2. At the optimal Ovlevel, the RuO2-x-2 catalyst forms stable metallic Ru-Ru bonds while maintaining oxidized Ru-O moieties, achieving an ultralow overpotential of 169 mV at 10 mAcm-2 and exceptional stability with 400-hour stable operation in 0.5 M H2SO4. Operando X-ray absorption fine structure (XAFS) and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) results reveal that the Ru-Ru and adjacent Ru-O sites synergistically activate the oxide pathway mechanism, circumventing the scaling relationship and minimizing the structural distortion during the OER. This work provides not only a facile approach to controllable modulate local structure of RuO2 but also insights into the synergistic effects between multiple active centers, which would greatly promote the further development of high-performance RuO2-based OER electrocatalysts.

Strong metal-support interaction (SMSI) is a special form of interaction between metal species and supports in heterogeneous catalysts, and manifests as various types of geometric structure changes. These SMSI behaviors serve as effective strategies to precisely regulate the surface and interface structures of supported metals, and hence has stimulated extensive research interest on the construction and regulation of SMSI. While the nature of SMSI has long been regarded as the strong interfacial metallic bonds between the supported metals and the metal cations in supports, it remains elusive why such a universal principle results in diverse structural changes in different material systems. In this perspective, it is explored and summarized the SMSI behaviors across different material systems, aiming to provide a systematic and unified framework for understanding SMSI. It is suggested that the direct reduction of surface energy and the increase of entropy is the thermodynamic driving force of SMSI, and the kinetic factors, i.e. the diffusion rate of metal atoms, determine the specific form of SMSI. This perspective would provide deeper and more systematic insights into SMSI behaviors and is instrumental to the precise regulation of the surface and interface structures of supported metal catalysts.

The hydrogen oxidation reaction (HOR) shows fast kinetics in proton exchange membrane fuel cells (PEMFCs), and has not drawn intense attention. Here, we propose a tandem electrocatalysis concept, decoupling HOR on two independent active sites for accelerated kinetics. As a proof-of-concept application, a Ru-based tandem HOR catalyst is designed, with Ru nanoclusters decorated with Pt single atoms. Experimental and theoretical studies suggest that H2 dissociation occurs at Ru sites, and then the produced H species migrate to Pt sites followed by the desorption of H+. The strong Ru-H interaction promotes the H2 dissociation step, while the optimum Pt-H interaction ensures the fast desorption, thereby substantially enhancing the HOR kinetics. In H2-O2 fuel cells, this catalyst enables a peak power density of 1.91 W cm-2 and a high anodic mass activity of 23.12 A mg-1 at 0.9 ViR-free with an ultralow noble metal loading of 5 μg cm-2. This work advances the development of low-cost anode catalysts for fuel cells and provides more insight into understanding hydrogen electrocatalysis.

Nonpolar atoms or molecules with light particle mass and weak particle-particle interaction can form quantum liquids and solids (QLS) at low temperatures. Excess electrons can be naturally bound to the surface of a QLS in a vacuum and exhibit unique quantum electronic behaviors in two and lower dimensions. In this article, we review the historical study and recent progress in this area. The main topics covered in this review include the collective and individual electron transport on liquid helium, solid neon, and solid hydrogen, the theoretical proposal and experimental effort toward single electron qubits on superfluid helium, the recent experimental realization of single electron charge qubits on solid neon and the related theoretical calculation. In the end, we review and envision extended exploration of quantum electronics on heterogeneous QLS.

Single electrons trapped on solid neon surfaces (eNe) have recently emerged as a promising platform for charge qubits. Experimental results have revealed their exceptionally long coherence times, yet the actual quantum states of these trapped electrons, presumably on imperfectly flat neon surfaces, remain elusive. In this paper, we examine the electron's interactions with neon surface topography, such as bumps and valleys. By evaluating the surface charges induced by the electron, we demonstrate its strong perpendicular binding to the neon surface. The Schrödinger equation for the electron's lateral motion on the curved 2D surface is then solved for extensive topographical variations. Our results reveal that surface bumps can naturally bind an electron, forming unique quantum ring states that align with experimental observations. We also show that the electron's excitation energy can be tuned using a modest magnetic field to facilitate qubit operation. This study offers a leap in our understanding of eNe qubit properties and provides strategic insights on minimizing charge noise and scaling the system to propel forward quantum computing architectures.

Strong metal-support interaction (SMSI) plays a vital role in tuning the geometric and electronic structures of metal species. Generally, a high-temperature treatment (>500 °C) in reducing atmosphere is required for constructing SMSI, which may induce the sintering of metal species. Herein, we use molten salts as the reaction media to trigger the formation of high-intensity SMSI at reduced temperatures. The strong ionic polarization of the molten salt promotes the breakage of Ti−O bonds in the TiO 2 support, and hence decreases the energy barrier for the formation of interfacial bonds. Consequently, a high-intensity SMSI state is achieved in TiO 2 supported Ir nanoclusters, evidenced by a large number of Ir−Ti bonds at the interface, at a low temperature of 350 °C. Moreover, this method is applicable for triggering SMSI in various supported metal catalysts with different oxide supports including CeO 2 and SnO 2 . This newly developed SMSI construction methodology opens a new avenue and holds significant potential for engineering advanced supported metal catalysts toward a broad range of applications.

The performance of superconducting qubits is degraded by a poorly characterized set of energy sources breaking the Cooper pairs responsible for superconductivity, creating a condition often called "quasiparticle poisoning". Both superconducting qubits and low threshold dark matter calorimeters have observed excess bursts of quasiparticles or phonons that decrease in rate with time. Here, we show that a silicon crystal glued to its holder exhibits a rate of low-energy phonon events that is more than two orders of magnitude larger than in a functionally identical crystal suspended from its holder in a low-stress state. The excess phonon event rate in the glued crystal decreases with time since cooldown, consistent with a source of phonon bursts which contributes to quasiparticle poisoning in quantum circuits and the low-energy events observed in cryogenic calorimeters. We argue that relaxation of thermally induced stress between the glue and crystal is the source of these events.

Developing efficient and robust electrocatalysts toward the oxygen evolution reaction (OER) is critical for proton exchange membrane water electrolysis (PEMWE). RuO2 possesses intrinsically high OER activity, but the concurrent electrochemical dissolution leads to rapid deactivation. Here a unique RuO2 catalyst containing metallic Ru─Ru interactions (m-RuO2) is reported, which maintains stability in practical PEMWE for 100 h at 60 °C and 1 A cm-2. Experimental and theoretical investigations suggest that the presence of Ru─Ru interactions significantly increases the energy barrier for the formation of RuO2(OH)2, which is a key intermediate for Ru dissolution, and hence substantially mitigates the electrochemical corrosion of m-RuO2. Meanwhile, the Ru4d band center downshifts, accordingly, ensuring the high OER activity, and the participation of lattice oxygen in the OER is also suppressed at the Ru─Ru sites, further contributing to the enhanced durability. Interestingly, such enhanced stability is also dependent on the size of metallic Ru─Ru cluster, where the energy barrier is further increased for Ru3, but is decreased for Ru5. These results highlight the significance of local coordination structure modulation on the electrochemical stability of RuO2 and open a feasible avenue toward the development of robust OER electrocatalysts for high-performance PEMWE.

A semiconductor-polymer composite was developed in this work consisting of inorganic halide perovskite (CsPbBr3) crystals embedded in polylactic acid (PLA) matrix. The composite exhibits the essential semiconductor properties of CsPbBr3 and the easy processability of PLA, enabling 3D printed tandem X-ray detectors. The tandem detectors demonstrated a sensitivity of 383 μC Gyair-1 cm-2 when four layers and total 600 µm thick CsPbBr3-PLA were printed. This sensitivity is 23X of single-layer detectors with same absorber thickness. The result indicates that 3D printed tandem detectors can overcome the X-ray absorption vs. charge carrier collection tradeoff in conventional single-layer detectors towards high performance radiation detection.

The completely symmetric states play an essential role in quantum physics. In this paper, we calculate the reduced density matrix (RDM) for a single particle of the completely symmetric system coupled by N spin -21 particles, because it helps to investigate the evolution of expectation value for the observable and to calculate the entanglement between the subsystems. Furthermore, we use Majorana's stellar representation (MSR) to represent the results because it provides an intuitive geometric perspective to comprehend the quantum states in the high-dimensional Hilbert space with distributions and trajectories of the Majorana stars on a Bloch sphere. With the operation properties of the generalized many-body anticommutator, we get the general MSR form of a single-qubit RDM. As the application and verification, we calculate the single-qubit RDM for the Dicke states with the results. Similarly, we further solve the RDM of the spin-N2 state in a uniform magnetic field and study the systems with symmetric structures on the Bloch sphere. The results exhibit the relations between the composite systems and the subsystems, and provide a new idea for the numerical solution of multiqubit systems.

The sluggish kinetics of hydrogen oxidation reaction (HOR) is one of the critical challenges for anion exchange membrane fuel cells. Here, we report epitaxial growth of Ir nanoclusters (<2 nm) on a MoS 2 surface (Ir/MoS 2 ) and optimize the alkaline HOR activity via tailoring interfacial charge transfer between Ir clusters and MoS 2 . The electron transfer from MoS 2 to Ir clusters can effectively prevent the oxidation of Ir clusters, which is not the case for carbon‐supported Ir nanoclusters (Ir/C) synthesized using the same method. Moreover, the HOR performance of the Ir/MoS 2 can be further optimized by tuning the hydrogen binding energy (HBE) via a precise annealing treatment. A substantial exchange current density of 1.28 mA cm ECSA −2 is achieved in the alkaline medium, which is ∼10 times over that of Ir/C. The HOR mass‐specific activity of Ir/MoS 2 heterostructure is as high as 182 mA mg Ir −1 . The experimental results and density functional theory calculations reveal that the significant improved HOR activity is attributed to the decreased HBE, which highlights epitaxial growth is an effective way for boosting catalytic activity of heterostructured catalysts.

Progress towards the realization of quantum computers requires persistent advances in their constituent building blocks—qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems1–3. Electrons, ubiquitous elementary particles of non-zero charge, spin and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits through either motional or spin states depends critically on their material environment3–5. Here we report our experimental realization of a qubit platform based on isolated single electrons trapped on an ultraclean solid neon surface in vacuum6–13. By integrating an electron trap in a circuit quantum electrodynamics architecture14–20, we achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are implemented to measure the energy relaxation time T1 of 15 μs and phase coherence time T2 over 200 ns. These results indicate that the electron-on-solid-neon qubit already performs near the state of the art for a charge qubit21. A solid-state single-electron qubit platform is demonstrated based on trapping and manipulating isolated single electrons on an ultraclean solid neon surface in vacuum, which performs near the state of the art for a charge qubit.

We study intermittency of circulation moments in turbulent superfluid helium by using experimental grid turbulence and numerical simulations of the Hall-Vinen-Bekarevich-Khalatnikov model. More precisely, we compute the velocity circulation $\Gamma_r$ in loops of size $r$ laying in the inertial range. For both, experimental and numerical data, the circulation variance shows a clear Kolmogorov scaling $\langle \Gamma_r^2 \rangle \sim r^{8/3}$ in the inertial range, independently of the temperature. Scaling exponents of high-order moments are comparable, within error bars, to previously reported anomalous circulation exponents in classical turbulence and low-temperature quantum turbulence numerical simulations.

Time dependent observations of point-to-point correlations of the velocity vector field (structure functions) are necessary to fully understand and model fluid flow around complex objects. Using thermal gradients, we induced flow of the normal fluid component of liquid He II and observed the flow by recording fluorescence of He* 2 excimers produced by neutron capture throughout a ~cm 3 volume. We applied an unsupervised machine learning algorithm to identify individual excimer clouds and then track their motion with millimeter and millisecond precision using a purpose-built correlation metric. Owing to the fact excimers are produced over a large region, the data are sparse in comparison to other techniques used to produce excimers. Machine learning is crucially important to track flow represented by sparse data—a situation encountered in other fields of science—and its importance will increase as improvements are made to overcome the sparsity of data.

Satoshi Yui, ∗ Yuan Tang, 3, ∗ Wei Guo, 3, † Hiromichi Kobayashi, 4, ‡ and Makoto Tsubota § Research and Education Center for Natural Sciences, Keio University, 4-1-1 Hiyoshi, Kohoku-ku, Yokohama 223-8521, Japan National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA Mechanical Engineering Department, FAMU-FSU College of Engineering, Florida State University, Tallahassee, Florida 32310, USA Department of Physics, Keio University, 4-1-1 Hiyoshi, Kohoku-ku, Yokohama 223-8521, Japan Department of Physics & Nambu Yoichiro Institute of Theoretical and Experimental Physics (NITEP) & The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan (Dated: March 8, 2022)

For electric double layer supercapacitors, carbon materials originating from the purely physical energy storage mechanism limit the improvement in the capabilities of charge storage. To solve this problem, doping heteroatoms into carbon skeleton is a promising & charming strategy for enhancing electrochemical performance by providing the extra pseudocapacitance. However, the self-discharge behavior of such heteroatom-doped supercapacitors has been a challenging issue for a long time. Here, the porous carbon nanosheets with a tunable total content of heteroatoms are chosen as a demo to systemically decouple the correlation between the total content of heteroatoms and the specific capacitance as well as the self-discharge behavior. The capacitance changes in a range of 164-331 F g(-1)@ 1 A g(-1) with the increased total contents of doped heteroatom, strongly dependent on and sensitive to the total content of heteroatoms. The voltage retention rate and capacitance retention rate for the porous carbon nanosheets with a tunable total content of heteroatoms completely present a quick decline tendency as the increase in the content of heteroatoms, changing from 58% to 34% and 74% to 39%, respectively, indicative of a linear negative relationship. More importantly, the self-discharge mechanisms are elaborately explored and follow the combination of activation-and diffusion-controlled Faradic reactions. This work illustrates the diverse impacts of the doped heteroatoms on the electrochemical performance of supercapacitors, covering specific capacitance and self-discharge behavior, and highlights the importance of balancing the contents of doped heteroatoms in energy storage fields. (C) 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.