The dark energy concept in the standard cosmological model can explain the expansion of the universe. However, the mysteries surrounding dark energy remain, such as its source, its unusual negative pressure, its long-range force, and its unchanged density as the universe expands. We propose a graviton momentum hypothesis, develop a semiclassical model of gravitons, and explain the pervasive dark matter and accelerating universe. The graviton momentum hypothesis is incorporated into the standard model and explains well the mysteries related to dark energy.

Thermal-electric conversion is crucial for smart energy control and harvesting, such as thermal sensing and waste heat recovering. So far, researchers are aware of two main ways of direct thermal-electric conversion, Seebeck and pyroelectric effects, each with different working mechanisms, conditions and limitations. Here, we report the concept of Geometric Thermoelectric Pump (GTEP), as the third way of thermal-electric conversion beyond Seebeck and pyroelectric effects. In contrast to Seebeck effect that requires spatial temperature difference, GTEP converts the time-dependent ambient temperature fluctuation into electricity. Moreover, GTEP does not require polar materials but applies to general conducting systems, and thus is also distinct from pyroelectric effect. We demonstrate that GTEP results from the temperature-fluctuation-induced charge redistribution, which has a deep connection to the topological geometric phase in non-Hermitian dynamics, as a consequence of the fundamental nonequilibrium thermodynamic geometry. The findings advance our understanding of geometric phase induced multiple-physics-coupled pump effect and provide new means of thermal-electric energy harvesting.

We propose a general approach based on the gradient descent method to study the inverse problem, making it possible to reversely engineer the microscopic configurations of materials that exhibit desired macroscopic properties. Particularly, we demonstrate its application by identifying the microscopic configurations within any given frequency range to achieve transparent phonon transport through one-dimensional harmonic lattices. Furthermore, we obtain the phonon transmission in terms of normal modes and find that the key to achieving phonon transparency or phonon blocking state lies in the ratio of the mode amplitudes at ends.

We theoretically demonstrate a rich and significant new families of exact spatially localized and periodic wave solutions for a modified Korteweg–de Vries equation. The model applies for the description of different nonlinear structures which include breathers, interacting solitons and interacting periodic wave solutions. A joint parameter which can take both positive and negative values of unity appeared in the functional forms of those closed form solutions, thus implying that every solution is determined for each value of this parameter. The results indicate that the existence of newly derived structures depend on whether the type of nonlinearity of the medium should be considered self-focusing or defocusing. The obtained nonlinear waveforms show interesting properties that may find practical applications.

Parton distribution functions (PDFs) are defining expressions of hadron structure. Exploiting the role of effective charges in quantum chromodynamics, an algebraic scheme is described which, given any hadron's valence parton PDFs at the hadron scale, delivers predictions for all its PDFs (unpolarized and polarized) at any higher scale. The scheme delivers results that are largely independent of both the value of the hadron scale and the pointwise form of the charge; and, inter alia, enables derivation of a model-independent identity that relates the strength of the proton's gluon helicity PDF, $\Delta G_p^\zeta$, to that of the analogous singlet polarized quark PDF and valence quark momentum fraction. Using available data fits and theory predictions, the identity yields $\Delta G_p(\zeta_{_{\scriptstyle \rm C}}=\sqrt{3}\,{\rm GeV})=1.48(10)$. It furthermore entails that the measurable quark helicity contribution to the proton spin is $\tilde a_{0p}^{\zeta_{_{\scriptstyle \rm C}}}=0.32(3)$, thereby reconciling contemporary experiment and theory.

High-resolution photoelectron energy spectra of osmium anions are obtained using the slow-electron velocity-map imaging method. The energy levels of excited states $^{4}\!F_{7/2}$, $^{4}\!F_{5/2}$ and $^{4}\!F_{3/2}$ of Os$^{-}$ are determined to be 148.730(13), 155.69(15), and 176.76(13) THz [or 4961.09(41), 5193.4(49), and 5896.1(42) cm$^{-1}$], respectively. The lifetime of the opposite-parity excited state $^{6}\!D_{9/2}^{\rm o}$ is determined to be 201(10) µs using a cold ion trap, about 15 times shorter than the previous result 3(1) ms. Our high-level multi-configuration Dirac–Hartree–Fock calculations yield a theoretical lifetime 527 µs. Our work shows that the laser cooling rate of Os$^{-}$ is as fast as that of Th$^{-}$. The advantages of Os$^{-}$ are its near-IR range cooling transition and simple electronic structure, which make Os$^{-}$ a promising candidate for laser cooling of negative ions. We propose a general approach to produce cold atoms and molecules based on the sympathetic cooling of negative ions in combination with a threshold photodetachment.

As x-ray probe pulses approach the subfemtosecond range, conventional x-ray photoelectron spectroscopy (XPS) is expected to experience a reduction in spectral resolution due to the effects of the pulse broadening. However, in the case of resonant x-ray photoemission, also known as resonant Auger scattering (RAS), the spectroscopic technique maintains spectral resolution when an x-ray pulse is precisely tuned to a core-excited state. We present theoretical simulations of XPS and RAS spectra on a showcased CO molecule using ultrashort x-ray pulses, revealing significantly enhanced resolution in the RAS spectra compared to XPS, even in the sub-femtosecond regime. These findings provide a novel perspective on potential utilization of attosecond x-ray pulses, capitalizing on the well-established advantages of detecting electron signals for tracking electronic and molecular dynamics.

We present a method that the atomic transition frequency measurement relies on the accurate wavemeter, optical frequency comb and stable Fabry–Pérot cavity to precise determination of stable even isotope shift on single Yb$^{+}$ ion ($A=168$, 170, 172, 174, 176). The $6s$ ${}^{2}\!S_{1/2} \leftrightarrow 6p\,{}^{2}\!P_{1/2}$ and $5d\,{}^{2}\!D_{3/2} \leftrightarrow 6s\,{}^{3}[3/2]_{1/2}$ resonance dipole transition frequencies are preliminarily measured by using a wavemeter which is calibrated by the 729 nm clock laser of ${}^{40}$Ca$^{+}$. Meanwhile, those frequencies are double checked by using optical frequency comb for correction of deviation. Ultimately, by changing frequency locking points at an ultralow expansion cavity more slightly and monitoring the corresponding atomic fluorescence changing with 17%, we finally improve the resonant frequency uncertainty to $\pm 6$ MHz, which is one order of improvement in precision higher than previously published measurements on the same transitions. A King-plot analysis with sensitivity to coupling between electrons and neutrons is carried out to determine the field and mass shift constants. Our measurement combined with existing or future isotope shift measurements can be used to determine basic properties of atomic nuclei, and to test new forces beyond the Standard Model.

Trapped atoms on photonic structures inspire many novel quantum devices for quantum information processing and quantum sensing. Here, we demonstrate a hybrid photonic-atom chip platform based on a GaN-on-sapphire chip and the transport of an ensemble of atoms from free space towards the chip with an optical conveyor belts. Due to our platform's complete optical accessibility and careful control of atomic motion near the chip with a conveyor belt, successful atomic transport towards the chip is made possible. The maximum transport efficiency of atoms is about $50\%$ with a transport distance of $500\,\mathrm{µ m}$. Our results open up a new route toward the efficient loading of cold atoms into the evanescent-field trap formed by the photonic integrated circuits, which promises strong and controllable interactions between single atoms and single photons.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

Thermal cloaks offer the potential to conceal internal objects from detection or to prevent thermal shock by controlling external heat flow. However, most conventional natural materials lack the desired flexibility and versatility required for on-demand thermal manipulation. We propose a solution in the form of homogeneous multilayer thermodynamic cloaks. Through an ingenious design, these cloaks achieve exceptional and extreme parameters, enabling the distribution of multiple materials in space. We first investigate the effects of important design parameters on the thermal shielding effectiveness of conventional thermal cloaks. Subsequently, we introduce an autonomous tuning function for the thermodynamic cloak, accomplished by leveraging two phase transition materials as thermal conductive layers. Remarkably, this tuning function does not require any energy input. Finite element analysis results demonstrate a significant reduction in the temperature gradient inside the thermal cloak compared to the surrounding background. This reduction indicates the cloak's remarkable ability to manipulate the spatial thermal field. Furthermore, the utilization of materials undergoing phase transition leads to an increase in thermal conductivity, enabling the cloak to achieve the opposite variation of the temperature field between the object region and the background. This means that, while the temperature gradient within the cloak decreases, the temperature gradient in the background increases. This work addresses a compelling and crucial challenge in the realm of thermal metamaterials, i.e., autonomous tuning of the thermal field without energy input. Such an achievement is currently unattainable with existing natural materials. This study establishes the groundwork for the application of thermal metamaterials in thermodynamic cloaks, with potential extensions into thermal energy harvesting, thermal camouflage, and thermoelectric conversion devices. By harnessing phonons, our findings provide an unprecedented and practical approach to flexibly implementing thermal cloaks and manipulating heat flow.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

The rise of artificial microstructures has made it possible to modulate propagation of various kinds of waves, such as light, sound and heat. Among them, the focusing effect is a modulation function of particular interest. We propose an atomic level triangular structure to realize the phonon focusing effect in single-layer graphene. In the positive incident direction, our phonon wave packet simulation results confirm that multiple features related to the phonon focusing effect can be controlled by adjusting the height of the triangular structure. More interestingly, a completed different focusing pattern and an enhanced energy transmission coefficient are found in the reverse incident direction. The detailed mode conversion physics is discussed based on the Fourier transform analysis on the spatial distribution of the phonon wave packet. Our study provides physical insights to achieving phonon focusing effect by designing atomic level microstructures.

Owing to the bipartite nature of honeycomb lattice, the electrons in graphene host valley degree of freedom, which gives rise to a rich set of unique physical phenomena including chiral tunneling, Klein paradox, and quantum Hall ferromagnetism. Atomic defects in graphene can efficiently break the local sublattice symmetry, and hence, have significant effects on the valley-based electronic behaviors. Here we demonstrate that an individual flower defect in graphene has the ability of valley filter at the atomic scale. With the combination of scanning tunneling microscopy and Landau level measurements, we observe two valley-polarized density-of-states peaks near the outside of the flower defects, implying the symmetry breaking of the $K$ and $K'$ valleys in graphene. Moreover, the electrons in the $K$ valley can highly penetrate inside the flower defects. In contrast, the electrons in the $K'$ valley cannot directly penetrate, instead, they should be assisted by the valley switch from the $K'$ to K. Our results demonstrate that an individual flower defect in graphene can be regarded as a nanoscale valley filter, providing insight into the practical valleytronics.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

One hallmark of Weyl semimetals is the emergence of Fermi arcs (FAs) in surface Brillouin zones, where FAs connect the projected Weyl nodes of opposite chiralities. Unclosed FAs can give rise to various exotic effects that have attracted tremendous research interest. Configurations of FAs are usually thought to be determined fully by the band topology of the bulk states, which seems impossible to manipulate. Here, we show that FAs can be simply modified by a surface gate voltage. Because the penetration length of the surface states depends on the in-plane momentum, a surface gate voltage induces an effective energy dispersion. As a result, a continuous deformation of the surface band can be implemented by tuning the surface gate voltage. In particular, as the saddle point of the surface band meets the Fermi energy, the topological Lifshitz transition takes place for the FAs, during which the Weyl nodes switch their partners connected by the FAs. Accordingly, the magnetic Weyl orbits composed of the FAs on opposite surfaces and chiral Landau bands inside the bulk change their configurations. We show that such an effect can be probed by the transport measurements in a magnetic field, in which the switch-on and switch-off conductances by the surface gate voltage signal the Lifshitz transition. Our work opens a new route for manipulating the FAs by surface gates and exploring novel transport phenomena associated with the topological Lifshitz transition.

We report the observation of a magnetic transition at the temperature about 56 K, through the high-pressure heat capacity and magnetic susceptibility measurements on the samples that have been claimed to be a near-room-temperature superconductor [Dasenbrock-Gammon et al.Nature615, 244 (2023)]. Our results show that this magnetic phase is robust against pressure up to 4.3 GPa, which covers the critical pressure of boosting the claimed superconductivity.

High-pressure structural search was performed on the hydrogen-rich compound LuBeH$_8$ at pressures up to 200 GPa. We found an $Fm\bar{3}m$ structure that exhibits stability and superconductivity above 100 GPa. Our phonon dispersion, electronic band structure, and superconductivity analyses in the 100–200 GPa pressure range reveal a strong electron–phonon coupling in LuBeH$_8$, while the superconducting critical temperature $T_{\rm c}$ shows a decreasing trend as the pressure increases, with $T_{\rm c}=255$ K at 200 GPa and maximal $T_{\rm c}=355$ K at 100 GPa. This study demonstrated the room-temperature superconductivity in $Fm\bar{3}m$-LuBeH$_8$, thus enriching the family of ternary hydrides. These findings provide valuable guidance for identifying new high-temperature superconducting hydrides.

Recently, superconductors with higher-order topology have stimulated extensive attention and research interest. Higher-order topological superconductors exhibit unconventional bulk-boundary correspondence, thus allow exotic lower-dimensional boundary modes, such as Majorana corner and hinge modes. However, higher-order topological superconductivity has yet to be found in naturally occurring materials. We investigate higher-order topology in a two-dimensional Josephson junction comprised of two s-wave superconductors separated by a topological insulator thin film. We find that zero-energy Majorana corner modes, a boundary fingerprint of higher-order topological superconductivity, can be achieved by applying magnetic field. When an in-plane Zeeman field is applied to the system, two corner modes appear in the superconducting junction. Furthermore, we also discover a two-dimensional nodal superconducting phase which supports flat-band Majorana edge modes connecting the bulk nodes. Importantly, we demonstrate that zero-energy Majorana corner modes are stable when increasing the thickness of topological insulator thin film.

The superconductivity of two-dimensional (2D) materials has extremely important research significance. To date, superconducting transition temperatures ($T_{\rm c}$) of 2D superconductors are still far from practical applications. Previously, 2D MXene Mo$_2$N has been successfully synthesized [Urbankowski et al.Nanoscale9 17722, (2017)]. We systematically investigate the effects of carbonization and further hydrogenation on the stability, electronic property and superconductivity of 1T- and 2H-$M_{2}$N ($M$ = Mo, W) based on first-principles calculations. The results show that the 1T-$M_{2}$N and 2H-$M_{2}$N ($M$ = Mo, W) are all dynamically and thermodynamically stable after carbonization and further hydrogenation. After carbonization, $T_{\rm c}$'s of 1T-$M_{2}$NC$_{2}$ ($M$ = Mo, W) are all increased, while $T_{\rm c}$'s of 2H-$M_{2}$NC$_{2}$ ($M$ = Mo, W) are all decreased. By further hydrogenation, the $T_{\rm c}$'s of 1T- and 2H-$M_{2}$NC$_{2}$H$_{2}$ are all increased. Among all of these structures, $T_{\rm c}$ of 1T-Mo$_2$NC$_2$H$_2$ is the highest one, reaching 42.7 K, and the corresponding electron-phonon coupling strength $\lambda$ is 2.27. Therefore, hydrogenation is an effective method to modulate $T_{\rm c}$'s of 2D $M_{2}$NC$_{2}$ ($M$ = Mo, W) materials.

We focus on the effect of ionic radius of lanthanides and the number of electrons in $4f$ orbitals on the superconducting temperature in 12442-type iron-based superconductors Rb$Ln_{2}$Fe$_{4}$As$_{4}$O$_{2}$ (Ln = Sm and Ho). Electronic properties of RbSm$_{2}$Fe$_{4}$As$_{4}$O$_{2}$ and RbHo$_{2}$Fe$_{4}$As$_{4}$O$_{2}$ with the largest differences of ionic radii and numbers of electrons in $4f$ orbital, and the largest difference of superconducting temperatures by using first-principles calculations. We predict that the ground state of Rb$Ln_{2}$Fe$_{4}$As$_{4}$O$_{2}$ is spin-density-wave-type in-plane striped antiferromagnet, and the magnetic moment around each Fe atom is about $2\mu_{\scriptscriptstyle{\rm B}}$. RbSm$_{2}$Fe$_{4}$As$_{4}$O$_{2}$ has a great influence on the energy band near the $\varGamma$ point, and a Dirac-like dispersion energy band appears. This band is mainly contributed by the $d_{z^2}$ orbital of Fe, which proves that RbSm$_{2}$Fe$_{4}$As$_{4}$O$_{2}$ has a stronger three-dimensionality. At the same time, this extra Fermi surface appears at the $\varGamma$ point, which also shows that Sm can effectively enhance the coupling strength within Fe$_{2}$As$_{2}$ bilayers. This is also confirmed by the charge density difference $\rho$(RbHo$_{2}$Fe$_{4}$As$_{4}$O$_{2}$)$\,-\rho$(RbSm$_{2}$Fe$_{4}$As$_{4}$O$_{2}$). It increases the internal coupling strength of the bilayer Fe$_{2}$As$_{2}$ layers, which in turn leads to a higher $T_{\rm c}$ of RbSm$_{2}$Fe$_{4}$As$_{4}$O$_{2}$ than RbHo$_{2}$Fe$_{4}$As$_{4}$O$_{2}$. Determining the details of their electronic structure, which may be closely related to superconductivity, is crucial to understanding the underlying mechanism. Such microscopic studies provide useful clues for our further research of other high-temperature superconductors.

Magnetic skyrmions are topological quasiparticles with nanoscale size and high mobility, which have potential applications in information storage and spintronic devices. The manipulation of skyrmion's dynamics in the track is an important topic due to the skyrmion Hall effect, which can deviate the skyrmions from the preferred direction. We propose a new model based on the ferromagnetic skyrmion, where the skyrmion velocity can be well controlled by adjusting the direction of the current. Using this design, we can avoid the annihilation of the skyrmion induced by the skyrmion Hall effect, which is confirmed by our micromagnetic simulation based on Mumax$^{3}$. In the meantime, we increase the average velocity of the skyrmion by varying the intrinsic material parameters in the track, where the simulations agree well with our analytical results based on the Thiele equation. Finally, we give a phase diagram of the output of the skyrmion in the T-type track, which provides some practical ways for design of logic gates by manipulating crystalline anisotropy through the electrical control.