The new dimensional deformation approach is proposed to generate higher-dimensional analogues of integrable systems. An arbitrary ($K$+1)-dimensional integrable Korteweg–de Vries (KdV) system, as an example, exhibiting symmetry, is illustrated to arise from a reconstructed deformation procedure, starting with a general symmetry integrable (1+1)-dimensional dark KdV system and its conservation laws. Physically, the dark equation systems may be related to dark matter physics. To describe nonlinear physics, both linear and nonlinear dispersions should be considered. In the original lower-dimensional integrable systems, only liner or nonlinear dispersion is included. The deformation algorithm naturally makes the model also include the linear dispersion and nonlinear dispersion.

Stochastic gradient descent (SGD), a widely used algorithm in deep-learning neural networks, has attracted continuing research interests for the theoretical principles behind its success. A recent work reported an anomaly (inverse) relation between the variance of neural weights and the landscape flatness of the loss function driven under SGD [Feng Y and Tu Y Proc. Natl. Acad. Sci. USA 118 e2015617118 (2021)}]. To investigate this seeming violation of statistical physics principle, the properties of SGD near fixed points are analyzed with a dynamic decomposition method. Our approach recovers the true “energy” function under which the universal Boltzmann distribution holds. It differs from the cost function in general and resolves the paradox raised by the anomaly. The study bridges the gap between the classical statistical mechanics and the emerging discipline of artificial intelligence, with potential for better algorithms to the latter.

Stochastic resonance is a counterintuitive phenomenon amplifying the weak periodic signal by application of external noise. We demonstrate the enhancement of a weak periodic signal by stochastic resonance in a trapped-ion oscillator when the oscillator is excited to the nonlinear regime and subject to an appropriate noise. Under the full control of the radio-frequency drive voltage, this amplification originates from the nonlinearity due to asymmetry of the trapping potential, which can be described by a forced Duffing oscillator model. Our scheme and results provide an interesting possibility to make use of controllable nonlinearity in the trapped ion, and pave the way toward a practical atomic sensor for sensitively detecting weak periodic signals from real noisy environment.

In practical optical communication systems, there are some factors that can affect transmission quality of optical solitons. The constant coefficient nonlinear Schrödinger (NLS) equation has been unable to meet the actual research needs. We need to use the variable coefficient NLS equation to simulate an actual system, so as to explore its potential application value. Based on the variable coefficient NLS equation, six dispersion decreasing fibers (DDFs) with different dispersion curve functions are used as transmission media to study generation and interaction of two solitons in an optical communication system. The two soliton interaction phenomena, such as the bound state solitons, are theoretically obtained. Moreover, the output characteristics of bound state solitons in different DDFs are discussed, which enriches the transmission phenomenon of two solitons in the optical communication system. This study has great application value in fields such as optical information processing devices, condensed matter physics, and plasma, and provides an indispensable theoretical basis for development of new optical devices.

The Mott insulator and superfluid phase transition is one of the most prominent phenomena in ultracold atoms. We report the observation of a novel 2D quantum phase transition between the Mott insulator and $\pi$ superfluid in a shaking optical lattice. In the deep optical lattice regime, the lowest $S$ band can be tuned to Mott phase, while the higher $P_{x,y}$ bands are itinerant for having larger bandwidth. Through a shaking technique coupling the $s$-orbital to $p_{x,y}$-orbital states, we experimentally observe the transition between the states of the $S$ and $P_{x,y}$ bands, leading to a quantum phase transition from two-dimensional $s$-orbital Mott phase to the $p_{x,y}$-orbital superfluid which condensed at $(\pi,\pi)$ momentum. Using the band-mapping method, we also observe the changes of atomic population in different energy bands during the transition, and the experimental results are well consistent with theoretical expectations.

Metamaterial absorbers with carbon fiber reinforced polymer (CFRP) substrates, which are called meta-CFRPs, have recently gained recognition for their excellent mechanical and electromagnetic performance. Different from traditional metamaterial absorbers with an isotropic substrate, meta-CFRPs with a highly anisotropic CFRP substrate are facing challenges in acquiring polarization-insensitive absorption. Here, a lay-up-oriented structure design method is proposed to solve this problem. Considering the lay-up configuration of CFRP laminates, metallic patterns are designed under corresponding polarization angles and then united together to form an integral structure. A meta-CFRP with a typical CFRP lay-up configuration([0$^{\circ}/45^{\circ}/90^{\circ}/-45^{\circ}$]$_{3s}$) is designed and tested. The experimental results exhibit over 99% microwave absorptivity at 2.44 GHz for all polarization angles. The maximum shift among the resonance peaks of the curves at all polarization angles is only 0.021 GHz. Further studies show that when there are cross-ply laminates in the first few layers of the CFRP substrate, the lay-up-oriented design method can be effectively simplified by ignoring the subsequent lay-up orientations after the first cross-ply layers. Our method can not only provide an effective way for acquiring polarization-insensitive microwave response on meta-CFRPs but also be expected to be promoted to metamaterial absorbers with other anisotropic materials.

Enabling lithium-ion batteries (LIBs) to operate in a wider temperature range, e.g., as low or high as possible or capable of both, is an urgent need and shared goal. Here we report, for the first time, a low-temperature electrolyte consisting of traditional ethylene carbonate, methyl acetate, butyronitrile solvents, and 1 M LiPF$_{6}$ salt, attributed to its very low freezing point ($T_{\rm f} = -126.3\,^{\circ}\!$C) and high ion conductivity at extremely low temperatures (0.21 mS/cm at $-100\,^{\circ}\!$C), successfully extends the service temperature of a practical 9.6 Ah LIB down to $-100\,^{\circ}\!$C (49.6% capacity retention compared to that at room temperature), which is the lowest temperature reported for practical cells so far as we know, and is lower than the lowest natural temperature ($-89.2\,^{\circ}\!$C) recorded on earth. Meanwhile, the high-temperature performance of lithium-ion batteries is not affected. The capacity retention is 88.2% and 83.4% after 800 cycles at 25$\,^{\circ}\!$C and 45$\,^{\circ}\!$C, respectively. The progress also makes LIB a proper power supplier for space vehicles in astronautic explorations.

Cubic gauche polynitrogen (cg-N) is an attractive high-energy density material. However, high-pressure synthesized cg-N will decompose at low pressure and cannot exist under ambient conditions. Here, the stabilities of cg-N surfaces with and without saturations at different pressures and temperatures are systematically investigated based on first-principles calculations and molecular dynamics simulations. Pristine surfaces at 0 GPa are very brittle and will decompose at 300 K, especially (110) surface will collapse completely just after structural relaxation, whereas the decompositions of surfaces can be suppressed by applying pressure, indicating that surface instability causes the cg-N decomposition at low pressure. Due to the saturation of dangling bonds and transferring electrons to the surfaces, saturation with H can stabilize surfaces under ambient conditions, while it is impossible for OH saturation to occur solely from obtaining electrons from surfaces. This suggests that polynitrogen is more stable in an acidic environment or when the surface is saturated with less electronegative adsorbates.

The recent discoveries of near-room-temperature superconductivity in clathrate hydrides present compelling evidence for the reliability of theory-orientated conventional superconductivity. Nevertheless, the harsh pressure conditions required to maintain such high $T_{\rm c}$ limit their practical applications. To address this challenge, we conducted extensive first-principles calculations to investigate the doping effect of the recently synthesized LaB$_{8}$ clathrate, intending to design high-temperature superconductors at ambient pressure. Our results demonstrate that these clathrates are highly promising for high-temperature superconductivity owing to the coexistence of rigid boron covalent networks and the tunable density of states at the Fermi level. Remarkably, the predicted $T_{\rm c}$ of BaB$_{8}$ could reach 62 K at ambient pressure, suggesting a significant improvement over the calculated $T_{\rm c}$ of 14 K in LaB$_{8}$. Moreover, further calculations of the formation enthalpies suggest that BaB$_{8}$ could be potentially synthesized under high-temperature and high-pressure conditions. These findings highlight the potential of doped boron clathrates as promising superconductors and provide valuable insights into the design of light-element clathrate superconductors.

Recent research has focused on using Anderson's localization concept to modulate coherent phonon transport by introducing disorder into periodic structures. However, designing and identifying the disorder's strength remain challenging, and visual evidence characterizing phonon localization is lacking. Here, we investigate the effect of disorder on coherent phonon transport in a two-dimensional Janus MoSSe/WSSe superlattice with a defined disorder strength. Using non-equilibrium molecular dynamics, we demonstrate that strong disorder can lead to strong phonon localization, as evidenced by smaller thermal conductivity and significantly different dependence on defect ratio in strongly disordered structures. Furthermore, we propose a novel defect engineering method to determine whether phonon localization occurs. Our work provides a unique platform for modulating coherent phonon transport and presents visual evidence of the phonon transition from localization to nonlocalization. These findings will contribute to development of phonon transport and even phononics, which are essential for thermoelectric and phononic applications.

Ultracold neutral atoms in higher bands of an optical lattice provide a natural avenue to emulate orbital physics in solid state materials. Here, we report the realization of $^{87}$Rb Bose–Einstein condensates in the fourth and seventh Bloch bands of a hexagonal boron-nitride optical lattice, exhibiting remarkably long coherence time through active cooling. Using band mapping spectroscopy, we observe that atoms condensed at the energy minimum of $\varGamma$ point ($K_{1}$ and $K_{2}$ points) in the fourth (seventh) band as sharp Bragg peaks. The lifetime for the condensate in the fourth (seventh) band is about 57.6 (4.8) ms, and the phase coherence of atoms in the fourth band persists for a long time larger than 110 ms. Our work thus offers great promise for studying unconventional bosonic superfluidity of neutral atoms in higher bands of optical lattices.

We report on ambipolar modulation doping of monolayer FeSe epitaxial films grown by molecular beam epitaxy and in situ spectroscopic measurements via a cryogenic scanning tunneling microscopy. It is found that hole doping kills superconductivity in monolayer FeSe films on metallic Ir(001) substrates, whereas electron doping from polycrystalline IrO$_2$/SrTiO$_3$ substrate enhances significantly the superconductivity with an energy gap of 10.3 meV. By exploring substrate-dependent superconductivity, we elucidate the essential impact of substrate work functions on the superconductivity of monolayer FeSe films. Our results therefore offer a valuable reference guide for further enhancement of the transition temperature $T_{\rm c}$ in FeSe-based superconductors by interface engineering.

We study the motion of a hole with internal degrees of freedom, introduced to the zigzag magnetic ground state of Na$_{2}$IrO$_{3}$, by using the self-consistent Born approximation. We find that the low-, intermediate-, and high-energy spectra are primarily attributed to the singlet, triplet, and quintet hole contributions, respectively. The spectral functions exhibit distinct features such as the electron-like dispersion of low-energy states near the $\varGamma$ point, the maximum $M$-point intensity of mid-energy states, and the hole-like dispersion of high-energy states. These features are robust and almost insensitive to the exchange model and Hund's coupling, and are in qualitative agreement with the angular-resolved photoemission spectra observed in Na$_{2}$IrO$_{3}$. Our results reveal that the interference between internal degrees of freedom in different sublattices plays an important role in inducing the complex dispersions.

The duality between electric and magnetic dipoles inspires recent comparisons between ferronics and magnonics. Here we predict surface polarization waves or “ferrons” in ferroelectric insulators, taking the long-range dipolar interaction into account. We predict properties that are strikingly different from the magnetic counterpart, i.e. the surface Damon–Eshbach magnons in ferromagnets. The dipolar interaction pushes the ferron branch with locked circular polarization and momentum to the ionic plasma frequency. The low-frequency modes are on the other hand in-plane polarized normal to their wave vectors. The strong anisotropy of the lower branch renders directional emissions of electric polarization and chiral near fields when activated by a focused laser beam, allowing optical routing in ferroelectric devices.

Janus WSSe monolayer is a novel two-dimensional (2D) material that breaks the out-of-plane mirror symmetry and has a large built-in electric field. These features lead to sizable Rashba spin-orbit coupling and enhanced nonlinear optical properties, making it a promising material platform for various spintronic and optoelectronic device applications. In recent years, nonlinear photocurrent responses such as shift and injection currents were found to be closely related to the quantum geometry and Berry curvature of materials, indicating that these responses can serve as powerful tools for probing the novel quantum properties of materials. In this work, we investigate the second-order nonlinear photocurrent responses in a Janus WSSe monolayer theoretically based on first-principles calculations and the Wannier interpolation method. It is demonstrated that the Janus WSSe monolayer exhibits significant out-of-plane nonlinear photocurrent coefficients, which is distinct from the non-Janus structures. Our results also suggest that the second-order nonlinear photocurrent response in the Janus WSSe monolayer can be effectively tuned by biaxial strain or an external electric field. Thus, the Janus WSSe monolayer offers a unique opportunity for both exploring nonlinear optical phenomena and realizing flexible 2D optoelectronic nanodevices.

The van der Waals heterojunctions, stacking of different two-dimensional materials, have opened unprecedented opportunities to explore new physics and device concepts. Here, combining the density functional theory with non-equilibrium Green's function technique, we systematically investigate the spin-polarized transport properties of van der Waals magnetic tunnel junctions (MTJs), Cu/MnBi$_{2}$Te$_{4}$/MnBi$_{2}$Te$_{4}$/Cu and Cu/MnBi$_{2}$Te$_{4}$/h-BN/$n\cdot$MnBi$_{2}$Te$_{4}$/Cu ($n=1$, 2, 3). It is found that the maximum tunnel magnetoresistance of Cu/MnBi$_{2}$Te$_{4}$/h-BN/3$\cdot$MnBi$_{2}$Te$_{4}$/Cu MTJs can reach 162.6%, exceeding the system with only a single layer MnBi$_{2}$Te$_{4}$. More interestingly, our results indicate that Cu/MnBi$_{2}$Te$_{4}$/h-BN/$n\cdot$MnBi$_{2}$Te$_{4}$/Cu ($n=2$, 3) MTJs can realize the switching function, while Cu/MnBi$_{2}$Te$_{4}$/h-BN/3$\cdot$MnBi$_{2}$Te$_{4}$/Cu MTJs exhibit the negative differential resistance. The Cu/MnBi$_{2}$Te$_{4}$/h-BN/3$\cdot$MnBi$_{2}$Te$_{4}$/Cu in the parallel state shows a spin injection efficiency of more than 83.3%. Our theoretical findings of the transport properties will shed light on the possible experimental studies of MnBi$_{2}$Te$_{4}$-based van der Waals magnetic tunneling junctions.

Research of spin polarization of magnetic CoFeB thin films is of practical importance in spintronic applications. Here, using a direct characterization technique of spin-resolved photoemission spectroscopy, we obtain the surface spin polarization of amorphous Co$_{40}$Fe$_{40}$B$_{20}$ thin films with different annealing temperatures from 100 ℃ to 500 ℃ prepared by magnetron sputtering. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons' spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing annealing temperature above 100 ℃. These distinct surface properties are attributed to the enhanced Fe–B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for development of high-performance magnetic random-access memories.

The interlayer hybridization (IH) of van der Waals (vdW) materials is thought to be mostly associated with the unignorable interlayer overlaps of wavefunctions ($t$) in real space. Here, we develop a more fundamental understanding of IH by introducing a new physical quantity, the IH admixture ratio $\alpha$. Consequently, an exotic strategy of IH engineering in energy space can be proposed, i.e., instead of changing $t$ as commonly used, $\alpha$ can be effectively tuned in energy space by changing the on-site energy difference (${2\varDelta}$) between neighboring-layer states. In practice, this is feasible via reshaping the electrostatic potential of the surface by deposing a dipolar overlayer, e.g., crystalline ice. Our first-principles calculations unveil that IH engineering via adjusting ${2\varDelta}$ can greatly tune interlayer optical transitions in transition-metal dichalcogenide bilayers, switch different types of Dirac surface states in Bi$_{2}$Se$_{3}$ thin films, and control magnetic phase transition of charge density waves in 1H/1T-TaS$_{2}$ bilayers, opening new opportunities to govern the fundamental optoelectronic, topological, and magnetic properties of vdW systems beyond the traditional interlayer distance or twisting engineering.

With the recent report of near ambient superconductivity at room temperature in the N-doped lutetium hydride (Lu–H–N) system, the understanding of cubic Lu–H compounds has attracted worldwide attention. Generally, compared to polycrystals with non-negligible impurities, the single-crystalline form of materials with high purity can provide an opportunity to show their hidden properties. However, the experimental synthesis of single-crystalline cubic Lu–H compounds has not been reported so far. Here, we develop an easy way to synthesize highly pure LuH$_{2+x}$ single-crystalline films by the post-annealing of Lu single-crystalline films (purity of 99.99%) in H$_2$ atmosphere. The crystal and electronic structures of films were characterized by x-ray diffraction, Raman spectroscopy, and electrical transport. Interestingly, Lu films are silver-white and metallic, whereas their transformed LuH$_{2+x}$ films become purple-red and insulating, indicating the possible formation of an unreported electronic state of Lu–H compounds. Our work provides a novel route to synthesize and explore more single-crystalline Lu–H compounds.

The classic rare-earth tritelluride provides an ideal platform to study the strong correlation state owing to its stable structures and abundance of orders. Here we report the observation of an undiscovered charge density wave (CDW) in LaTe$_{3}$ under 4.2 K, the transition temperature of the CDW states is fitted to be 35 K, and confirmed by the evanishment of this CDW at 77 K via using temperature-dependent scanning tunneling microscope/spectroscopy. The coexistence of these CDWs is confirmed by the atomic resolution and beating pattern simulation. It is the first time to observe the coexistence of unidirectional charge density waves system, providing a new platform to understand the competition and evolution between strong correlation states, and get a deeper sight into the phase lag between different order parameters.

Fe$_{3}$GaTe$_{2}$, a recently discovered van der Waals ferromagnetic crystal with the highest Curie temperature and strong perpendicular magnetic anisotropy among two-dimensional (2D) magnetic materials, has attracted significant attention and makes it a promising candidate for next-generation spintronic applications. Compared with Fe$_{3}$GeTe$_{2}$, which has the similar crystal structure, the mechanism of the enhanced ferromagnetic properties in Fe$_{3}$GaTe$_{2}$ is still unclear and needs to be investigated. Here, by using x-ray magnetic circular dichroism measurements, we find that both Ga and Te atoms contribute to the total magnetic moment of the system with antiferromagnetic coupling to Fe atoms. Our first-principles calculations reveal that Fe$_{3}$GaTe$_{2}$ has van Hove singularities at the Fermi level in nonmagnetic state, resulting in the magnetic instability of the system and susceptibility to magnetic phase transitions. In addition, the calculation results about the density of states in ferromagnetic states of two materials suggest that the exchange interaction between Fe atoms is strengthened by replacing Ge atoms with Ga atoms. These findings indicate the increase of both the itinerate and local moments in Fe$_{3}$GaTe$_{2}$ in view of Stoner and exchange interaction models, which results in the enhancement of the overall magnetism and a higher Curie temperature. Our work provides insight into the underlying mechanism of Fe$_{3}$GaTe$_{2}$'s remarkable magnetic properties and has important implications for searching 2D materials with expected magnetic properties in the future.