Science China Physics, Mechanics & Astronomy最新文献

We report the discovery of a peculiar X-ray transient, EP240408a, by Einstein Probe (EP) and follow-up studies made with EP, Swift, NICER, GROND, ATCA and other ground-based multiwavelength telescopes. The new transient was first detected with Wide-field X-ray Telescope (WXT) on board EP on April 8th, 2024, manifested in an intense yet brief X-ray flare lasting for 12 s. The flare reached a peak flux of 3.9 × 10⁻⁹ erg cm⁻² s⁻¹ in 0.5–4 keV, ∼300 times brighter than the underlying X-ray emission detected throughout the observation. Rapid and more precise follow-up observations by EP/FXT, Swift and NICER confirmed the finding of this new transient. Its X-ray spectrum is non-thermal in 0.5–10 keV, with apower-law photon index varying within 1.8–2.5. The X-ray light curve shows a plateau lasting for ∼4 d, followed by a steep decay till becoming undetectable ∼10 d after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7–23 d is found for EP240408a, which is intermediate between the commonly found fast and long-term transients. No counterparts have been found in optical and near-infrared, with the earliest observation at 17 h after the initial X-ray detection, suggestive of intrinsically weak emission in these bands. We demonstrate that the remarkable properties of EP240408a are inconsistent with any of the transient types known so far, by comparison with, in particular, jetted tidal disruption events, gamma-ray bursts, X-ray binaries and fast blue optical transients. The nature of EP240408a thus remains an enigma. We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of ∼10 d. The detection and follow-ups of more of such objects are essential for revealing their origin.

Alloying/doping is a widely used technique for improving the electrical, mechanical, and optical properties of materials. However, this technology induces significant distortions in the lattice structure, mass distribution, and potential field, greatly enhancing phonon scattering. Here, we introduce the concept of alloying/doping path and employ crystal symmetry, lattice deformation, and electron distribution to characterize it. Based on this new concept, the phonon thermal transport behavior in alloyed/doped materials can be well designed, and along different alloying/doping paths, the difference in thermal conductivity can be up to 45 times. On one hand, strategic alloying/doping that combines high crystal symmetry, large lattice contraction, and the same electron distribution suppresses phonon-phonon scattering phase space, induces phonon stiffening, and bolsters electronic structure symmetry, respectively. These synergistic effects significantly improve thermal conductivity. On the other hand, random alloying/doping has a low symmetry, leading to the typical “U” shape of alloying/doping level-dependent thermal conductivity. Our theory is corroborated in three-dimensional (3D) Si, 2D MoS₂, and quasi-1D TiS₃, affirming its efficacy and broad applicability in controlling phonon transport.

Comprehension of photon-triggered molecular processes is essential in the study of various important topics in physics, chemistry, and biology. Here we propose a correlated tunneling picture to understand the dissociative ionization process of molecules in intense laser fields based on a quantum model developed in the framework of many-body S-matrix theory including nuclear vibrational motion. In this quantum correlation picture, the single ionization of H₂ and the subsequent electron-ion recollision-induced dissociation are considered as an entangled correlated process. It enables us to attribute the interference pattern in the joint-energy spectra to combined effects of single-slit diffraction and multi-slit interference of correlated electron-nuclear wave packets in the time domain. Our work opens a new avenue to understanding molecular dissociative ionization processes in external fields.

From the perspective of resource-theoretic approach, this study explores the quantification of imaginary in quantum physics. We propose a well defined measure of imaginarity, the geometric-like measure of imaginarity. Compared with the usual geometric imaginarity measure, this geometric-like measure of imaginarity exhibits smaller decay difference under quantum noisy channels and higher stability. As applications, we show that both the optimal probability of state transformations from a pure state to an arbitrary mixed state via real operations, and the maximal probability of stochastic-approximate state transformations from a pure state to an arbitrary mixed state via real operations with a given fidelity f, are given by the geometric-like measure of imaginarity.

A multi-relaxation-time discrete Boltzmann model for compressible non-ideal gases with adjustable specific heat ratio is proposed, and the impact of surface tension on Rayleigh-Taylor instability (RTI) is investigated from two perspectives: macroscopic and non-equilibrium characteristics. In terms of physical cognition, (1) it is found that there are two critical surface tensions: the upper critical value and the lower critical value. When the surface tension is below the lower critical value, the RTI evolution aligns qualitatively with the case without surface tension. As the surface tension coefficient increases, the inhibitory effect on RTI evolution gradually strengthens. When the surface tension is greater than the upper critical value, the disturbance interface tends to be stable after multiple oscillations. When the surface tension is in between, the perturbation interface first reverses, or even multiple reverses, and then destabilizes and develops rapidly. (2) A series of new kinetic behavior characteristics are given, and it is found that some behavior characteristics have important reference value for the control of system behavior. For example, the first peak in the growth rate of the global average non-organized momentum flux strength D₂ corresponds to the onset of the regular nonlinear stage. The peak in the growth rate of the global average non-organized energy flux strength D_3,1 marks the beginning of the re-acceleration stage. The onset of the uniform acceleration stage in the growth rate of the global average non-equilibrium strength D_TNE corresponds to the system transitioning into the regular nonlinear stage, while its terminal point (also the peak) corresponds to the system entering the re-acceleration stage. These insights enhance understanding of RTI mechanisms in complex fluids kinetically.

The underlying physics of QCD phase transition in the early Universe remains largely unknown due to its strong-coupling nature during the quark-gluon plasma/hadron gas transition, yet a holographic model has been proposed to quantitatively fit the lattice QCD data while with its duration of the first-order phase transition (FoPT) left undetermined. At specific baryon chemical potential, the first-order QCD phase transition agrees with the observational constraint of baryon asymmetry. It, therefore, provides a scenario for phase transition gravitational waves (GWs) within the standard model of particle physics. If these background GWs could contribute dominantly to the recently claimed common-spectrum red noise from pulsar timing array (PTA) observations, the duration of this FoPT can be well constrained, and the associated primordial black holes are still allowed by current observations.

Type-II Dirac semimetals exhibit a unique Fermi surface topology, which allows them to host novel topological superconductivity (TSC). We reveal a novel inter-orbital superconducting state, corresponding to the B_1u and B_2u pairings under the D_4h point group. Intriguingly, we find that both first- and second-order TSC coexist in this novel state. It is induced by a dominant inter-orbital attraction and possesses surface helical Majorana cones and hinge Majorana flat bands, spanning the entire z-directed hinge Brillouin zone. Further investigation uncovers that these higher-order hinge modes are robust against the C_4z symmetry-breaking perturbation.

Ta₂NiSe₅ is a promising candidate for hosting an excitonic insulator (EI) phase, a novel electronic state driven by electron-hole Coulomb attraction. However, the role of electron-lattice coupling in the formation of the EI phase remains controversial. Here, we use angle-resolved photoemission spectroscopy (ARPES) to study the band structure evolution of Ta₂Ni(Se_1−xS_x)₅ with sulfur substitution and potassium deposition, which modulate the band gap and the carrier concentration, respectively. We find that the Ta 5d states originating from the bottom of the conduction band persist at the top of the valence band in the low-temperature monoclinic phase, indicating the importance of exciton condensation in opening the gap in the semi-metallic band structure. We also observe that the characteristic overlap between the conduction and valence bands can be restored in the monoclinic lattice by mild carrier injection, suggesting that the lattice distortion in the monoclinic phase is not the main factor for producing the insulating gap, but rather the exciton condensation in the electronic system is the dominant driving force. Our results shed light on the electron-lattice decoupling and the origin of the EI phase in Ta₂Ni(Se_1−xS_x)₅.