Science China Physics, Mechanics & Astronomy最新文献

The spin-orbit assisted Mott insulator α-RuCl₃ is a prime candidate for material realization of the Kitaev quantum spin liquid. While little attention has been paid to charge degrees of freedom, charge effects, such as electric polarization, may arise in this system. Here, we report distortion-induced local electric polarization in α-RuCl₃ as evidenced by single-crystal X-ray diffraction, second harmonic generation (SHG) and dielectric measurements. The SHG signal appears at room temperature and develops substantially in the Kitaev paramagnetic state when short-range spin correlations come into play. Despite sizable pyroelectric currents in the Kitaev paramagnetic state, the absence of hysteresis in the polarization-electric field (P-E) points to the short-range nature of electric polarization. This localized electric polarization is likely the result of distortion-induced charge dimerization, achieved through virtual hopping-induced charge redistribution. In addition, the electric polarization is boosted by short-range spin correlations via spin-phonon coupling in the Kitaev paramagnetic state. Our results emphasize the importance of charge degrees of freedom in α-RuCl₃, which establish a novel platform to investigate charge effects in Kitaev materials.

We extend our research on the energy flux and waveform characteristics of gravitational waves generated by merging nonspinning binary black holes through self-consistent effective one-body theory to include binary systems with slowly spinning black holes. Initially, we decompose the equation for the null tetrad component of the gravitationally perturbed Weyl tensor ψ ^B₄ into radial and angular parts, leveraging the second-order approximation of the rotation parameter a. Subsequently, we derive an analytical solution for the radial equation and observe that our results are contingent upon the parameters a₂, a₃, and a, which represent the second- and third-order correction parameters, respectively. Ultimately, we calculate the energy flux, the radiation-reaction force and the waveform for the “plus” and “cross” modes of the gravitational waves generated by merging slowly spinning binary black holes.

We perform a unified description of the experimental data of the π⁺π⁻ and J/ψπ^± invariant mass spectra for e⁺e⁻ → J/ψπ⁺π⁻ and the D⁰D^*− mass spectrum for e⁺e⁻ → D⁰D^*−π⁺ at e⁺e⁻ center-of-mass energies 4.23 and 4.26 GeV. The analysis takes into account open-charm meson loops that contain triangle singularities, the (J/psipi-D{bar D}^*) coupled-channel interaction respecting unitarity, and the strong (pipi-K{bar K}) final state interaction using dispersion relations. The analysis leads to a precise determination of the Z_c(3900) pole with the pole mass and width (3880.7 ± 1.7_stat ± 22.4_syst) MeV and (35.9 ± 1.4_stat ± 15.3_syst) MeV, respectively, and hints at that the (D{bar D}^*) molecular and non-molecular components are of similar importance in the Z_c(3900) formation.

At magic twisted angles, Dirac cones in twisted bilayer graphene (TBG) can evolve into flat bands, serving as a critical playground for the study of strongly correlated physics. When chiral symmetry is introduced, rigorous mathematical proof confirms that the flat bands are locked at zero energy in the entire Moiré Brillouin zone (BZ). Yet, TBG is not the sole platform that exhibits this absolute band flatness. Central to this flatness phenomenon are topological nodes and their specific locations in the BZ. In this study, considering twisted bilayer systems that preserve chiral symmetry, we classify various ordered topological nodes in base layers and all possible node locations across different BZs. Specifically, we constrain the node locations to rotational centers, such as Γ and M points, to ensure the interlayer coupling retains equal strength in all directions. Using this classification as a foundation, we systematically identify the conditions under which Moiré flat bands emerge. Additionally, through the extension of holomorphic functions, we provide proof that flat bands are locked at zero energy, shedding light on the origin of the band flatness. Remarkably, beyond Dirac cones, numerous twisted bilayer nodal platforms can host flat bands with a degeneracy number of more than two, such as four-fold, six-fold, and eight-fold. This multiplicity of degeneracy in flat bands might unveil more complex and enriched correlation physics.

Using the unrestricted Hartree-Fock variational method, we study the extended Hubbard model in the triangular lattice at 3/4 filling. Due to the nesting instability of the Fermi surface, various density wave states compatible with the 2 × 2 supercell are found as the ground state by tuning the onsite and nearest neighbor repulsions U and V. Surprisingly, the flux phases with complex bond order parameters are realized in a wide range of the U-V phase diagram. They also acquire nontrivial topological properties due to the spontaneous breaking of the time-reversal symmetry. Our study suggests that the flux phases can be stabilized by the interplay between short-range repulsive interactions in correlated electron systems.