Communications in Theoretical Physics

Understanding the effects of point liquid loading on transversely isotropic poroelastic media is crucial for advancing geomechanics and biomechanics, where precise modeling of fluid-structure interactions is essential. This paper presents a comprehensive analysis of infinite transversely isotropic poroelasticity under a fluid source, based on Biot's theory, aiming to uncover new and previously unexplored insights in the literature. We begin our study by deriving a general solution for fluid-saturated, transversely isotropic poroelastic materials in terms of harmonic functions that satisfy sixth-order homogeneous partial differential equations, using potential theory and Almansi's theorem. Based on these general solutions and potential functions, we construct a Green's function for a point fluid source, introducing three new harmonic functions with undetermined constants. These constants are determined by enforcing continuity and equilibrium conditions. Substituting these into the general solution yields fundamental solutions for poroelasticity that provide crucial support for a wide range of project problems. Numerical results and comparisons with existing literature are provided to illustrate physical mechanisms through contour plots. Our observations reveal that all components tend to zero in the far field and become singular at the concentrated source. Additionally, the contours exhibit rapid changes near the point fluid source but display gradual variations at a distance from it. These findings highlight the intricate behavior of the system under point liquid loading, offering valuable insights for further research and practical applications.

Active matter is a non-equilibrium condensed system consisting of self-propelled particles capable of converting stored or ambient energy into collective motion. Typical active matter systems include cytoskeleton biopolymers, swimming bacteria, artificial swimmers, and animal herds. In contrast to wet active matter, dry active matter is an active system characterized by the absence of significant hydrodynamic interactions and conserved momentum. In dry active matter, the role of surrounding fluids is providing viscous friction at low Reynolds numbers and can be neglected at high Reynolds numbers. This review offers a comprehensive overview of recent experimental, computational, and theoretical advances in understanding phase transitions and critical phenomena in dry aligning active matter, including polar particles, self-propelled rods, active nematics, and their chiral counterparts. Various ways of determining phase transition points as well as non-equilibrium phenomena, such as collective motion, cluster formation, and creation and annihilation of topological defects are reviewed.

This study introduces an innovative dual-tunable absorption film with the capability to switch between ultra-wideband and narrowband absorption. By manipulating the temperature, the film can achieve multi-band absorption within the 30–45 THz range or ultra-wideband absorption spanning 30–130 THz, with an absorption rate exceeding 0.9. Furthermore, the structural parameters of the absorption film are optimized using the particle swarm optimization (PSO) algorithm to ensure the optimal absorption response. The absorption response of the film is primarily attributed to the coupling of guided-mode resonance and local surface plasmon resonance effects. The film's symmetric structure enables polarization incoherence and allows for tuning through various means such as doping/voltage, temperature and structural parameters. In the case of a multi-band absorption response, the film exhibits good sensitivity to refractive index changes in multiple absorption modes. Additionally, the absorption spectrum of the film remains effective even at large incidence angles, making it highly promising for applications in fields such as biosensing and infrared stealth.

In this paper, an active tunable terahertz bandwidth absorber based on single-layer graphene is proposed, which consists of a graphene layer, a photo crystal plate, and a gold substrate. When the Fermi energy (E_f) of graphene is 1.5 eV, the absorber shows high absorption in the range of 3.7 THz–8 THz, and the total absorption rate is 96.8%. By exploring the absorption mechanism of the absorber, the absorber shows excellent physical regulation. The absorber also shows good adjustability by changing the E_f of graphene. This means that the absorber exhibits excellent tunability by adjusting the physical parameters and E_f of the absorber. Meanwhile, the absorber is polarization independent and insensitive to the incident angle. The fine characteristics of the absorber mean that the absorber has superior application value in many fields such as biotechnology and space exploration.

Emergence refers to the existence or formation of collective behaviors in complex systems. Here, we develop a theoretical framework based on the eigen microstate theory to analyze the emerging phenomena and dynamic evolution of complex system. In this framework, the statistical ensemble composed of M microstates of a complex system with N agents is defined by the normalized N × M matrix A, whose columns represent microstates and order of row is consist with the time. The ensemble matrix A can be decomposed as ${\boldsymbol{A}}={\sum }_{I=1}^{r}{\sigma }_{I}{{\boldsymbol{U}}}_{I}\otimes {{\boldsymbol{V}}}_{I}$, where $r={\rm{\min }}(N,M)$, eigenvalue σ_I behaves as the probability amplitude of the eigen microstate U_I so that ${\sum }_{I=1}^{r}{\sigma }_{I}^{2}=1$ and U_I evolves following V_I. In a disorder complex system, there is no dominant eigenvalue and eigen microstate. When a probability amplitude σ_I becomes finite in the thermodynamic limit, there is a condensation of the eigen microstate U_I in analogy to the Bose–Einstein condensation of Bose gases. This indicates the emergence of U_I and a phase transition in complex system. Our framework has been applied successfully to equilibrium three-dimensional Ising model, climate system and stock markets. We anticipate that our eigen microstate method can be used to study non-equilibrium complex systems with unknown order-parameters, such as phase transitions of collective motion and tipping points in climate systems and ecosystems.

In this article, our primary objective is to construct new wormhole solutions by involving a Yukawa-corrected form of Casimir energy density in a well-motivated gravitational theory that allows the coupling of curvature and matter, namely the ${\mathbb{F}}(R,{ \mathcal T })$ theory. To achieve this goal, a wormhole geometry exhibiting a spherically-symmetric nature is taken into account and anisotropic fluid is assumed to be the background ordinary matter source. We first consider the simple linear ${\mathbb{F}}(R,{ \mathcal T })$ theory by assuming ${\mathbb{F}}(R,{ \mathcal T })=R+2\zeta { \mathcal T }$ with ${{\mathbb{L}}}_{m}=-{\mathbb{P}}$ (average pressure). Secondly, we utilize the conformal symmetries of the spherical-symmetric geometry for simplifying the resulting field equations and obtain the corresponding analytical form of the wormhole solution. In both cases, the viability of the proposed solutions is examined by checking the basic features of the wormhole shape model along with the validity of null energy constraints. Further, we study the volume integral quantifier (VIQ), exoticity factor and stability through the Tolman–Oppenheimer–Volkov (TOV) equation as well as the adiabatic index, active gravitational mass and complexity factor graphically. Lastly, we use a newly-proposed wormhole shape function to find the expressions of state variables and discuss the validity of energy bounds. We also explore the significance of this wormhole shape model through different quantities graphically. In all scenarios, the presented solutions are found to be new, promising and viable.

In this paper, the liquid–vapor phase separation under viscous shear is investigated by using a pseudopotential central moment lattice Boltzmann method. Physically, the multiphase shear flow is governed by two competing mechanisms: surface tension and shear force. It is interesting to find that the liquid tends to form a droplet when the surface tension dominates under conditions of low temperature, shear velocity, and viscosity, and in larger domain size. Otherwise, the liquid tends to form a band if shear force dominates. Moreover, the average density gradient is used as a physical criterion to distinguish the spinodal decomposition and domain growth. Both spatial and temporal changes of density are studied during the phase separation under shear.

To the nonlinear Schrödinger–Boussinesq system, with the aid of Adler–Moser polynomials we predict the patterns of higher-order rogue wave solutions containing multiple large parameters. The new interesting rogue wave patterns of a number of true and predicted solutions are graphically illustrated, including fan-, heart-shaped structures and their skewed versions. The results are significant for both experimental and theoretical studies of rogue wave patterns of integrable systems.

Renormalization group analysis has been proposed to eliminate secular terms in perturbation solutions of differential equations and thus expand the domain of their validity. Here we extend the method to treat periodic orbits or limit cycles. Interesting normal forms could be derived through a generalization of the concept 'resonance', which offers nontrivial analytic approximations. Compared with traditional techniques such as multi-scale methods, the current scheme proceeds in a very straightforward and simple way, delivering not only the period and the amplitude but also the transient path to limit cycles. The method is demonstrated with several examples including the Duffing oscillator, van der Pol equation and Lorenz equation. The obtained solutions match well with numerical results and with those derived by traditional analytic methods.

The properties of the non-trivial quantum state in an all-optical environment come mainly from the higher-order quantum electrodynamics effect, which remains one of the few unverified predictions of this theory due to its weak signal. Here, we propose a scheme specifically designed to detect this quantum vacuum, where a tightly focused pump laser interacts with an optical frequency comb (OFC) in its resonant cavity. When the OFC pulse passes through the vacuum polarized by the high-intensity pump laser, its carrier frequency and envelope change. This can be intuitively understood as the asymmetric photon acceleration induced by the ponderomotive force of the pump laser. By leveraging the exceptional ultrahigh frequency and temporal resolution of the OFC, this scheme holds the potential to improve the accuracy of quantum vacuum signal. Combining theoretical and simulation results, we discuss possible experimental conditions, and the detectable OFC signal is shown to be orders of magnitude better than the instrumental detection threshold. This shows our scheme can be verified on the forthcoming laser systems.

The noise feature of a single-mode class-A laser amplifier is investigated by solving the Maxwell–Bloch equations of motion in the presence of the fluctuation force of cavity Langevin. The aim is to calculate the simultaneous fluctuations that are superimposed on the amplitude and phase of the cavity electric field, as well as the atomic population inversion. The correlation function of these fluctuations yields the amplitude, phase, and spontaneous emission noise fluxes, respectively. The amplitude and spontaneous emission noise fluxes exhibit the Lorentzian profiles in both the below-threshold state and the injection-locking region of the above-threshold state. While noise is typically viewed negatively in science and engineering, this research highlights its positive role as a valuable tool for measuring the optical properties of a laser amplifier. For instance, the degree of first-order temporal coherence (DFOTC) is derived by taking the Fourier transform of the amplitude noise flux. The damping rate of DFOTC is associated with the coherence time of the light emitted by the laser amplifier. Furthermore, the uncertainty relation between noise bandwidth and coherence time is confirmed. Finally, it is demonstrated that the input pumping noise flux, together with the output amplitude and spontaneous emission noise fluxes, satisfy the principle of flux conservation.

A mid-infrared femtosecond pulse laser with a single cycle and high intensity is an ideal driving light source for generating isolated attosecond pulses. Due to current experimental limitations, it is difficult to directly achieve this type of laser light source in the laboratory. In this paper, we obtain such an ideal light source by adding a Ti sapphire pulse to the combined pulse laser consisting of two mid-infrared pulses. Specifically, by combining the synthesized pulse consisting of 8 fs/1200 nm/1.62 × 10¹⁴ W cm⁻² and 12 fs/1800 nm/2.71 × 10¹⁴ W cm⁻² with an additional 8 fs/800 nm/1.26 × 10¹⁴ W cm⁻² Ti sapphire pulse, the resulting electric field waveform is very close to that of a 1170 nm femtosecond pulse with an intensity of 1.4 × 10¹⁵ W cm⁻², a single-cycle pulse width, and a carrier-envelope phase of 0.25π. Numerical simulations show that both cases produce high-order harmonic emission spectra with broadband supercontinuum spectra, however, the bandwidth of the supercontinuum spectra and the harmonic intensities in the synthesized pulses are significantly better than those in the single 1170 nm pulse. After inverse Fourier transform, we obtain 66 as a high-intensity isolated attosecond pulse, whose intensity is five orders of magnitude higher than that of a monochromatic field. Here, the phase differences between three combined pulse lasers have little effect on the numerical simulation results when they vary in the range of 0.3π.

We present a calculation by including the relativistic and off-shell contributions to the interaction potentials between two spin-1/2 fermions mediated by the exchange of light spin-0 particles, in both momentum and coordinate spaces. Our calculation is based on the four-point Green's function rather than the scattering amplitude. Among the sixteen potential components, eight that vanish in the non-relativistic limit are shown to acquire nonzero relativistic and off-shell corrections. In addition to providing relativistic and off-shell corrections to the operator basis commonly used in the literature, we introduce an alternative operator basis that facilitates the derivation of interaction potentials in the coordinate space. Furthermore, we calculate both the long-range and short-range components of the potentials, which can be useful for future experimental analyses at both macroscopic and atomic scales.

The QCD axion bubbles can form due to an explicit breaking of the Peccei–Quinn symmetry in the early Universe. In this paper, we investigate the modified formation of a QCD axion bubble in the presence of an axionlike particle (ALP), considering its resonant conversion to a QCD axion. We consider a general scenario where the QCD axion mixes with ALP before the QCD phase transition. In this scenario, the energy density of the ALP can be adiabatically transferred to the QCD axion at a temperature T_R, resulting in the suppression of the cosmic background temperature T_B at which the energy density of the QCD axion equals that of the radiation. The QCD axion bubbles form when the QCD axions arise during the QCD phase transition. Finally, we briefly discuss the impact of the formation of QCD axion bubbles on the formation of primordial black holes.

Active matter is a non-equilibrium condensed system consisting of self-propelled particles capable of converting stored or ambient energy into collective motion. Typical active matter systems include cytoskeleton biopolymers, swimming bacteria, artificial swimmers, and animal herds. In contrast to wet active matter, dry active matter is an active system characterized by the absence of significant hydrodynamic interactions and conserved momentum. In dry active matter, the role of surrounding fluids is providing viscous friction at low Reynolds numbers and can be neglected at high Reynolds numbers. This review offers a comprehensive overview of recent experimental, computational, and theoretical advances in understanding phase transitions and critical phenomena in dry aligning active matter, including polar particles, self-propelled rods, active nematics, and their chiral counterparts. Various ways of determining phase transition points as well as non-equilibrium phenomena, such as collective motion, cluster formation, and creation and annihilation of topological defects are reviewed.

The reason for the present accelerated expansion of the Universe stands as one of the most profound questions in the realm of science, with deep connections to both cosmology and fundamental physics. From a cosmological point of view, physical models aimed at elucidating the observed expansion can be categorized into two major classes: dark energy and modified gravity. We review various major approaches that employ a single scalar field to account for the accelerating phase of our present Universe. Dynamic system analysis was employed in several important models to find cosmological solutions that exhibit an accelerating phase as an attractor. For scalar field models of dark energy, we consistently focused on addressing challenges related to the fine-tuning and coincidence problems in cosmology, as well as exploring potential solutions to them. For scalar–tensor theories and their generalizations, we emphasize the importance of constraints on theoretical parameters to ensure overall consistency with experimental tests. Models or theories that could potentially explain the Hubble tension are also emphasized throughout this review.

We review the theoretical aspects of holographic dark energy (HDE) in this paper. Making use of the holographic principle (HP) and the dimensional analysis, we derive the core formula of the original HDE (OHDE) model, in which the future event horizon is chosen as the characteristic length scale. Then, we describe the basic properties and the corresponding theoretical studies of the OHDE model, as well as the effect of adding dark sector interaction in the OHDE model. Moreover, we introduce all four types of HDE models that originate from HP, including (1) HDE models with the other characteristic length scale; (2) HDE models with extended Hubble scale; (3) HDE models with dark sector interaction; (4) HDE models with modified black hole entropy. Finally, we introduce the well-known Hubble tension problem, as well as the attempts to alleviate this problem under the framework of HDE. From the perspective of theory, the core formula of HDE is obtained by combining the HP and the dimensional analysis, instead of adding a DE term into the Lagrangian. Therefore, HDE remarkably differs from any other theory of DE. From the perspective of observation, HDE can fit various astronomical data well and has the potential to alleviate the Hubble tension problem. These features make HDE a very competitive dark energy scenario.

In this paper, we present an overview on recent progress in studies of QCD at finite temperature and densities within the functional renormalization group (fRG) approach. The fRG is a nonperturbative continuum field approach, in which quantum, thermal and density fluctuations are integrated successively with the evolution of the renormalization group (RG) scale. The fRG results for the QCD phase structure and the location of the critical end point (CEP), the QCD equation of state (EoS), the magnetic EoS, baryon number fluctuations confronted with recent experimental measurements, various critical exponents, spectral functions in the critical region, the dynamical critical exponent, etc, are presented. Recent estimates of the location of the CEP from first-principle QCD calculations within fRG and Dyson–Schwinger equations, which pass through lattice benchmark tests at small baryon chemical potentials, converge in a rather small region at baryon chemical potentials of about 600 MeV. A region of inhomogeneous instability indicated by a negative wave function renormalization is found with μ_B ≳ 420 MeV. It is found that the non-monotonic dependence of the kurtosis of the net-proton number distributions on the beam collision energy observed in experiments could arise from the increasingly sharp crossover in the regime of low collision energy.

Ab initio approaches are among the most advanced models to solve the nuclear many-body problem. In particular, the no-core–shell model and many-body perturbation theory have been recently extended to the Gamow shell model framework, where the harmonic oscillator basis is replaced by a basis bearing bound, resonance and scattering states, i.e. the Berggren basis. As continuum coupling is included at basis level and as configuration mixing takes care of inter-nucleon correlations, halo and resonance nuclei can be properly described with the Gamow shell model. The development of the no-core Gamow shell model and the introduction of the $\hat{\bar{Q}}$-box method in the Gamow shell model, as well as their first ab initio applications, will be reviewed in this paper. Peculiarities compared to models using harmonic oscillator bases will be shortly described. The current power and limitations of ab initio Gamow shell model will also be discussed, as well as its potential for future applications.

This research paper seeks to investigate the characteristics of almost Riemann solitons and almost gradient Riemann solitons within the framework of generalized Robertson Walker (GRW) spacetimes that incorporate imperfect fluids. Our study begins by defining specific properties of the potential vector field linked to these solitons. 
 We examine the potential vector field of an almost Riemann soliton on GRW imperfect fluid spacetimes, establishing that it aligns collinearly with a unit timelike torse-forming vector field. This leads us to express the scalar curvature in relation to the structures of soliton and spacetime. Furthermore, we explore the characteristics of an almost gradient Riemann soliton with a potential function $\psi$ across a range of GRW imperfect fluid spacetimes, deriving a formula for the Laplacian of $\psi$. We also categorize almost Riemann solitons on GRW imperfect fluid spacetimes into three types: shrinking, steady, and expanding, when the potential vector field of the soliton is Killing. We prove that a GRW imperfect fluid spacetime with constant scalar curvature and a Killing vector field admits an almost Riemann soliton. Additionally, we demonstrate that if the potential vector field of the almost Riemann soliton is a $\nu(Ric)$-vector, or if the GRW imperfect fluid spacetime is $\mathcal{W}_{2}$-flat or pseudo-projectively flat, the resulting spacetime is classified as a dark fluid.

A polynomial scheme is proposed here to compute exact solutions of nonlinear partial differential equations (NPDEs) based on series expansions of solutions and a renormalization group (RG) related resummation. The most salient feature of the current approach is that only linear algebraic equations need to be solved to implement the resummation for closed-form exact solution and parameter dependence, which does not require any sophisticated analysis like Cole-Hopf transformation or Painlev'{e} test. New exact solutions of typical NPDEs are computed with this novel method, including one- and two-soliton (solitary wave) solutions, periodic solutions of exponential or elliptic function type. Moreover, reduced equations may also be conveniently computed for further analysis.

This paper focuses on the analytical technique based on nonlocal symmetry and consistent tanh expansion method for constructing abundant analytical solutions of a new extended Kadomtsev-Petviashvili-Benjamin-Ono (eKP-BO) equation in (2+1) dimensions. First, commencing with the Painlevé analysis, the integrability of the (2+1)-dimensional eKP-BO equation and its nonlocal symmetry are discussed. Second, the localization of the nonlocal symmetry of the extended system is determined by means of prolongation method. Furthermore, through this localization process, the initial value problem of the extended system is solved, thereby providing a finite symmetry transformation of the (2+1)-dimensional eKP-BO equation. Finally, we follow the consistent tanh expansion method to unveil the interaction solutions of the soliton-cnoidal type and resonant soliton type to the eKP-BO equation, and we study their dynamic properties in a visual manner.

We consider recently developed black hole in massive
Einstein-dilaton gravity including the coupling of dilaton scalar
field to massive graviton terms. This model has different horizon
structures such as event horizons and inner horizons depending on
the values of certain parameters. These variations influence how the
black hole interacts with its surroundings. We utilize the
well-known Novikov-Thorne model to investigate the thin accretion
disks onto this interesting model. Our research shows a crucial
correlation between the dynamics of the accretion disk and the
parameters of dilatonic black holes in dilaton-massive gravity. We
observe that dilaton-massive gravity leads to significant
contraction and outward expansion in the disks. We provide
comprehensive analysis of the accretion by investigating the direct
and secondary images at different radial distances and observational
angles.

The chiral gravitational wave background (GWB) can be produced by axion-like fields in the early universe. We perform parameter estimation for two types of chiral GWB with the LISA-Taiji network: axion-dark photon coupling and axion-Nieh-Yan coupling. We estimate the spectral parameters of these two mechanisms induced by the axion and determine the normalized model parameters using the Fisher information matrix. For highly chiral GWB signals that we choose to analyze in the mHz band, the normalized model parameters are constrained with a relative error less than $6.7\%$ (dark photon coupling) and $2.2\%$ (Nieh-Yan coupling) at the one-sigma confidence level. The circular polarization parameters are constrained with a relative error around $21\%$ (dark photon coupling) and $6.2\%$ (Nieh-Yan coupling) at the one-sigma confidence level.