Science China Physics, Mechanics & Astronomy最新文献

The REAl(Si,Ge) (RE = rare earth) family, known to break both the inversion- and time-reversal symmetries, represents one of the most suitable platforms for investigating the interplay between correlated-electron phenomena and topologically nontrivial bands. Here, we report on systematic magnetic, transport, and muon-spin rotation and relaxation (uSR) measurements on (Nd,Sm)AlGe single crystals, which exhibit antiferromagnetic (AFM) transitions at T_N = 6.1 and 5.9 K, respectively. In addition, NdAlGe undergoes also an incommensurate-to-commensurate ferrimagnetic transition at 4.5 K. Weak transverse-field µSR measurements confirm the AFM transitions, featuring a ∼90% magnetic volume fraction. Zero-field (ZF) µSR measurements reveal a more disordered internal field distribution in NdAlGe than in SmAlGe, reflected in a larger transverse muon-spin relaxation rate λ^T at T ≪ T_N. This may be due to the complex magnetic structure of NdAlGe, which undergoes a series of metamagnetic transitions in an external magnetic field, while SmAlGe shows only a robust AFM order. In NdAlGe, the topological Hall effect (THE) appears between the first and the second metamagnetic transitions for H ∥ c, while it is absent in SmAlGe. Such THE in NdAlGe is most likely attributed to the field-induced topological spin textures. The longitudinal muon-spin relaxation rate λ^L, diverges near the AFM order, followed by a clear drop at T < T_N. In the magnetically ordered state, spin fluctuations are significantly stronger in NdAlGe than in SmAlGe. In general, our longitudinal-field μSR data indicate vigorous spin fluctuations in NdAlGe, thus providing valuable insights into the origin of THE and of the possible topological spin textures in REAl(Si,Ge) Weyl semimetals.

We report on the development of a “two-field-scan” harmonic Hall voltage (HHV) analysis, which collects the second HHV as a function of a swept in-plane magnetic field at 45° and 0° relative to the excitation current, for the determination of the spin-orbit torques of transverse spins in magnetic heterostructures without significant perpendicular spins, longitudinal spins, and longitudinal/perpendicular Oersted fields. We demonstrate that this two-field-scan analysis is as accurate as the well-established but time-consuming angle-scan HHV analysis even in the presence of considerable thermoelectric effects but takes more than a factor of 7 less measurement time. We also show that the fit of the HHV data from a single field scan at 0°, which is commonly employed in the literature, is not reliable because the employment of too many free parameters in the fitting of the very slowly varying HHV signal allows erroneous conclusion about the spin-orbit torque efficiencies.

We explore the possibility that domain wall networks generate the stochastic gravitational wave background (SGWB) observed as a strong common power-law process in the Data Release-2 of Parkes Pulsar Timing Array. We find that a broad range of parameters, specifically wall tension around σ_DW ∼ (29–414 TeV)³ and wall-decay temperature within T_d ∼ 20–257 MeV, can explain this phenomenon at a 68% credible level. Meanwhile, the same parameters could ease the Hubble tension if particles from these domain wall networks decay into dark radiation. We establish a direct analytical relationship, Ω_GW(f_p, T₀)h² ∼ Ω_radh²(Ω_vΔN_eff)², to illustrate this coincidence, underlining its importance in the underlying physics and potential applicability to a wider range of models and data. Conversely, if the common power-law process is not attributed to domain wall networks, our findings impose tight limits on the wall tension and decay temperature.

Dark matter (DM) is a major constituent of the Universe. However, no definite evidence of DM particles (denoted as “χ”) has been found in DM direct detection (DD) experiments to date. There is a novel concept of detecting χ from evaporating primordial black holes (PBHs). We search for χ emitted from PBHs by investigating their interaction with target electrons. The examined PBH masses range from 1 × 10¹⁵ to 7 × 10¹⁶ g under the current limits of PBH abundance f_PBH. Using 205.4 kg·day data obtained from the CDEX-10 experiment conducted in the China Jinping Underground Laboratory, we exclude the χ-electron (χ-e) elastic-scattering cross section (sigma_{{chi}^{e}}) ∼ 5 × 10⁻²⁹ cm² for χ with a mass m_χ ≲ 0.1 keV from our results. With the higher radiation background but lower energy threshold (160 eV), CDEX-10 fills a part of the gap in the previous work. If ((m_{chi}, sigma_{{chi}^{e}})) can be determined in the future, DD experiments are expected to impose strong constraints on f_PBH for large M_PBHs.

While non-Hermiticity provokes intriguing phenomena without Hermitian counterparts, e.g., the skin effect and the breakdown of bulk-boundary correspondence, attracting extensive attention both in fundamental physics and device engineering, the role of finite sizes therein remains elusive. Here, we propose a class of finite-size-induced non-Hermitian phase transitions, relying upon higher-order topological invariants associated with real-space wave functions. The phase diagrams for general non-Hermitian chiral models are further acquired to demonstrate our topological definition. Such phase transitions are elucidated qualitatively by an effective intercell coupling alteration that depends on finite sizes in respective directions. Besides, we mimic these phenomena by analogizing the circuit Laplacian in finite-size electric circuits with nonreciprocal couplings. The resultant admittance spectra agree with our theoretical predictions. Our findings shed light on the finite-size mechanism of non-Hermitian topological phase transitions and pave the way for applications in switching and sensing.

Kármán Vortex Street, a fascinating phenomenon of fluid dynamics, has intrigued the scientific community for a long time. Many researchers have dedicated their efforts to unraveling the essence of this intriguing flow pattern. Here, we apply the lattice Boltzmann method with curved boundary conditions to simulate flows around a circular cylinder and study the emergence of Kármán Vortex Street using the eigen microstate approach, which can identify phase transition and its order-parameter. At low Reynolds number, there is only one dominant eigen microstate W¹ of laminar flow. At Re ¹_c = 53.6, there is a phase transition with the emergence of an eigen microstate pair W^2,3 of pressure and velocity fields. Further at Re ²_c . = 56, there is another phase transition with the emergence of two eigen microstate pairs W^4,5 and W^6,7. Using the renormalization group theory of eigen microstate, both phase transitions are determined to be first-order. The two-dimensional energy spectrum of eigen microstate for W¹, W^2,3 after Re_c¹, W^4–7 after Re ²_c exhibit −5/3 power-law behavior of Kolnogorov’s K41 theory. These results reveal the complexity and provide an analysis of the Kármán Vortex Street from the perspective of phase transitions.

We explore the generation of topological defects in the course of a dynamical phase transition in a ring with a weak link, i.e., a SSS Josephson junction, from the AdS/CFT correspondence. By setting different parameters of the junction (width, steepness, depth) and the final temperature of the quench, the configurations of the charge density and condensate of the order parameters of the dual field theory are presented. Meanwhile, we observe that in the final equilibrium state, variations in parameters of the junctions only affect the configurations of the charge density and condensate of the order parameters, without altering their values outside the junction. However, variations in the final temperature will directly affect the values of the charge density and condensate of the order parameters outside of the junction. Moreover, in the final equilibrium state, we propose an analytic relation between the gauge-invariant velocity in the two superconducting states in the SSS Josephson junction, which agrees well with the numerical results.