Theoretical study on martensitic-type transformation path from rutile phase to α-PbO2 phase of TiO2

The martensitic-type phase transformation paths from the rutile to theα-PbO2 phase of TiO2 are studied with linear interpolation and NEB/G-SSNEB methods based on first-principles calculations.Its potential energy surface and the lowest energy path are revealed.Our results indicate that the titanium atoms of the rutile phase shuffle along the[0-11]rut crystal direction to form theα-PbO2 phase.During the phase transition,the oxygen atoms are dragged by the heavier titanium atoms and then reach their new equilibrium positions.The barrier of phase transition from nudged elastic band theory is about 231 meV,which is qualitatively consistent with previous theoretical calculations from the monoclinic phase to the tetragonal phase for ZrO2 and HfO2.Debye model can also be successfully used to predict the pressure and temperature of the phase transformation.

Parametric instability in the pure-quartic nonlinear Schrödinger equation

We study the nonlinear stage of modulation instability(MI)in the non-intergrable pure-quartic nonlinear Schrödinger equation where the fourth-order dispersion is modulated periodically.Using the three-mode truncation,we reveal the complex recurrence of parametric resonance(PR)breathers,where each recurrence is associated with two oscillation periods(PR period and internal oscillation period).The nonlinear stage of parametric instability admits the maximum energy exchange between the spectrum sidebands and central mode occurring outside the MI gain band.

Dual-wavelength pumped latticed Fermi-Pasta-Ulam recurrences in nonlinear Schrödinger equation

We show that the nonlinear stage of the dual-wavelength pumped modulation instability(MI)in nonlinear Schrödinger equation(NLSE)can be effectively analyzed by mode truncation methods.The resulting complicated heteroclinic structure of instability unveils all possible dynamic trajectories of nonlinear waves.Significantly,the latticed-Fermi-Pasta-Ulam recurrences on the modulated-wave background in NLSE are also investigated and their dynamic trajectories run along the Hamiltonian contours of the heteroclinic structure.It is demonstrated that there has much richer dynamic behavior,in contrast to the nonlinear waves reported before.This novel nonlinear wave promises to inject new vitality into the study of MI.

Thermal-contact capacity of one-dimensional attractive Gaudin-Yang model

Tan's contact C is an important quantity measuring the two-body correlations at short distances in a dilute system.Here we make use of the technique of exactly solved models to study the thermal-contact capacity K_(T),i.e.,the derivative of C with respect to temperature in the attractive Gaudin-Yang model.It is found that K_(T) is useful in identifying the low temperature phase diagram,and using the obtained analytical expression of K_(T),we study its critical behavior and the scaling law.Especially,we show K_(T) versus temperature and thus the non-monotonic tendency of C in a tiny interval,for both spin-balanced and imbalanced phases.Such a phenomenon is merely observed in multi-component systems such as SU(2)Fermi gases and spinor bosons,indicating the crossover from the Tomonaga-Luttinger liquid to the spin-coherent liquid.

Regulating the dopant clustering in LiZnAs-based diluted magnetic semiconductor

Tuning of the magnetic interaction plays the vital role in reducing the clustering of magnetic dopant in diluted magnetic semiconductors(DMS).Due to the not well understood magnetic mechanism and the interplay between different magnetic mechanisms,no efficient and universal tuning strategy is proposed at present.Here,the magnetic interactions and formation energies of isovalent-doped(Mn) and aliovalent(Cr)-doped LiZnAs are studied based on density functional theory(DFT).It is found that the dopant–dopant distance-dependent magnetic interaction is highly sensitive to the carrier concentration and carrier type and can only be explained by the interplay between two magnetic mechanisms,i.e.,superexchange and Zener’s p–d exchange model.Thus,the magnetic behavior and clustering of magnetic dopant can be tuned by the interplay between two magnetic mechanisms.The insensitivity of the tuning effect to U parameter suggests that our strategy could be universal to other DMS.

Inelastic Interaction of Double-Valley Dark Solitons for the Hirota Equation

For a scalar integrable model,it is generally believed that the solitons interact with each other elastically,for instance,multi-bright solitons from the nonlinear Schrodinger equation and the Korteweg-de Vries equation,etc.We obtain double-valley dark solitons from the defocusing Hirota equation by the Darboux transformation.Particularly,we report a remarkable phenomenon for the inelastic interaction of the double-valley dark solitons,in contrast to the solitons interacting with each other elastically for a scalar integrable model in previous works.Furthermore,we give the explicit conditions for the elastic collision based on the asymptotic analysis results.It is shown that the double-valley dark solitons could also admit elastic interaction under the special parameters settings.

Theoretical study on order–disorder phase transition of CH3NH3PbCl3

Order–disorder phase transitions for CH3NH3PbCl3 are studied with density functional theory. Our calculations show that the disorder is manifested in two aspects in the cubic phase, namely, the disorder of orientation and rotation of organic groups. Organic groups of [CH3] and [NH3] in cubic crystals can easily rotate around its C3 axis. At the same time,[CH3NH3]^+ organic groups can also orient to different spatial directions due to the weak interactions between organic group and inorganic frame. Our results show that its possible phase transition path starts from the deviation of organic groups from the crystal c-axis. Its structural transition changes from disordered cubic phase to hydrogen-only disordered tetragonal structure in the process of decreasing symmetry. The disordered high temperature cubic phase can be expressed as a statistical average of substructures we rebuilt. The electrostatic repulsive force between adjacent organic groups triggers out the formation of low temperature phase on cooling.

Straight and twisted Weyl nodal line phonons in Ho2CF2 material

Based on first-principles calculations, symmetry analysis and model construction, we predict that Ho2CF2hosts both straight and twisted Weyl nodal lines in its bulk phonon spectrum. We identify that the top two phonon bands entangle with each other, forming two straight Weyl nodal lines on the K–H and K′–H′paths at the Brillouin zone(BZ) boundary,and six twisted Weyl nodal lines within the BZ. All the Weyl nodal lines along the kz direction and across the entire BZ.The symmetry analysis indicates that these Weyl nodal lines are protected by the PT symmetry and crystal symmetry. The Berry phase and drumhead-like nontrivial surface states are calculated. We also construct a tight-binding model to describe these nodal lines. Our work provides an excellent material platform for exploring the fascinating physics associated with straight and twisted Weyl nodal line phonons.

Low-lying electronic states of osmium monoxide OsO

The ground state of osmium monoxide(OsO) has long been controversial. In this paper, the low-lying Λ–S and ? electronic states of OsO have been comprehensively studied by the high-precision multi-reference calculations. The ground state of OsO is unexpectedly the closed-shell1Σ+state with a double bond instead of the previously reported3Φ or5Σ+state;after including the spin–orbit coupling effects, the ground state becomes3Π2. With the help of the theoretical spectroscopic constants and transition dipole moments, the emission spectra in the region of 405 nm–875 nm are assigned.Our results will facilitate the future studies of absorption and emission spectra of OsO.

Thermodynamic limit of the XXZ central spin model with an arbitrary central magnetic field

The U(1)symmetry of the X X Z central spin model with an arbitrary central magnetic field B is broken,since its total spin in the z-direction is not conserved.We obtain the exact solutions of the system by using the off-diagonal Bethe ansatz method.The thermodynamic limit is investigated based on the solutions.We find that the contribution of the inhomogeneous term in the associated T-Q relation to the ground state energy satisfies an N^(-1)scaling law,where N is the total number of spins.This result makes it possible to investigate the properties of the system in the thermodynamic limit.By assuming the structural form of the Bethe roots in the thermodynamic limit,we obtain the contribution of the direction of B to the ground state energy.It is shown that the contribution of the direction of the central magnetic field is a finite value in the thermodynamic limit.This is the phenomenon caused by the U(1)symmetry breaking of the system.

The application of quantum coherence as a resource

Quantum coherence is a basic concept in quantum mechanics, representing one of the most fundamental characteristics that distinguishes quantum mechanics from classical physics. Quantum coherence is the basis for multi-particle interference and quantum entanglement. It is also the essential ingredient for various physical phenomena in quantum optics, quantum information, etc. In recent years, with the proposal of a quantum coherence measurement scheme based on a resource theory framework, quantum coherence as a quantum resource has been extensively investigated. This article reviews the resource theories of quantum coherence and introduces the important applications of quantum coherence in quantum computing,quantum information, and interdisciplinary fields, particularly in quantum thermodynamics and quantum biology. Quantum coherence and its applications are still being explored and developed. We hope this review can provide inspiration for relevant research.

Special breathing structures induced by bright solitons collision in a binary dipolar Bose–Einstein condensates

We numerically investigate the breathing dynamics induced by collision between bright solitons in a binary dipolar Bose–Einstein condensates, whose dipole–dipole interaction and contact interaction are attractive. We identify three special breathing structures, such as snakelike special breathing structure, mixed breathing structure, and divide breathing structure.The characteristics of these breathing structures can be described by breathing frequency ?, maximum breathing amplitude A and lifetime τ, which can be manipulated by atomic number Ni and interspecies scattering length a12. Meanwhile, the above breathing structures can realize the process of quasi-transition with a reasonable Ni and a12. Additionally, the collision of two special breathing structures also can bring more abundant breathing dynamics. Our results provide a reference for the study of soliton interactions and deepen the understanding of soliton properties in a binary dipolar Bose–Einstein condensates.

Off-diagonal approach to the exact solution of quantum integrable systems

We investigate the t–W scheme for the anti-ferromagnetic X X X spin chain under both periodic and open boundary conditions. We propose a new parametrization of the eigenvalues of the transfer matrix. Based on it, we obtain the exact solution of the system. By analyzing the distribution of zero roots at the ground state, we obtain the explicit expressions of the eigenfunctions of the transfer matrix and the associated W operator(see Eqs.(10) and(70)) in the thermodynamic limit. We find that the ratio of the quantum determinant with the eigenvalue of W operator for the ground state exhibits exponential decay behavior. Thus this fact ensures that the so-called inversion relation(the t–W relation without the W-term) can be used to study the ground state properties of quantum integrable systems with/without U(1)-symmetry in the thermodynamic limit.

Exact surface energy and elementary excitations of the XXX spin-1/2 chain with arbitrary non-diagonal boundary fields

We study the physical properties of the XXX spin-1/2 chain with arbitrary non-diagonal boundary fields. By using a combination of numerical analysis and analytical method, we obtain the surface energy and elementary excitations of the model. It shows that the contributions of the two boundary fields to the surface energy are additive. We also find that there exists a kind of excitations related to the boundary string.

Modulation Instability and Non-Degenerate Akhmediev Breathers of Manakov Equations

We reveal a special subset of non-degenerate Akhmediev breather(AB)solutions of Manakov equations that only exist in the focusing case.Based on exact solutions,we present the existence diagram of such excitations on the frequency-wavenumber plane.Conventional single-frequency modulation instability leads to simultaneous excitation of three ABs with two of them being non-degenerate.

Quench Dynamics of Bose–Einstein Condensates in Boxlike Traps

By quenching the interatomic interactions, we investigate the nonequilibrium dynamics of two-dimensional Bose–Einstein condensates in boxlike traps with power-law potential boundaries. We show that ring dark solitons can be excited during the quench dynamics for both concave and convex potentials. The quench's modulation strength and the steepness of the boundary are two major factors influencing the system's evolution. In terms of the number of ring dark solitons excited in the condensate, five dynamic regimes have been identified. The condensate undergoes damped radius oscillation in the absence of ring dark soliton excitations. When it comes to the appearance of ring dark solitons, their decay produces interesting structures. The excitation patterns for the concave potential show a nested structure of vortex-antivortex pairs. The dynamic excitation patterns for the convex potential, on the other hand, show richer structures with multiple transport behaviors.

Breather Interaction Properties Induced by Self-Steepening and Space-Time Correction

We study the properties of breather interactions in nonlinear Kerr media with self-steepening and space-time correction and with either self-focusing or self-defocusing nonlinearity,and present a new family of exact breather solutions via the Darboux transformation with a special-designed quadratic spectral parameter.In contrast to the previous results of the nonlinear Schrodinger equation(NLSE)hierarchy,we show that the relative phase of colliding breathers has a significant effect on the collision manifestation.In particular,only the out-of-phase interactions can generate small amplitude waves at the collision center,which are analogous to the NLSE superregular breathers.Our results will deepen our understanding of the properties of breather interactions and they will offer the possibility of experimental observations of super-regular breather dynamics in systems with self-steepening and space-time correction.

Quantum reflection of a Bose-Einstein condensate with a dark soliton from a step potential

We study dynamical behaviors of a Bose-Einstein condensate(BEC)containing a dark soliton reflected from potential wells and potential barriers,respectively.The orientation angle of the dark soliton and the width of the potential change play key roles on the reflection probability Rs.Variation of the reflection probability with respect to the orientation angleθof the dark soliton can be well described by a cosine function Rs~cos[λ(θ-π/2)],whereλis a parameter determined by the width of the potential change.There are two characteristic lengths which determine the reflection properties.The dependence of the reflection probability on the width of the potential change shows distinct characters for potential wells and potential barriers.The length of the dark soliton determines the sensitive width of potential wells,whereas for potential barriers,the decay length of the matter wave in the region of the barrier qualifies the sensitive width of the barrier.The time evolution of the density profiles of the system during the reflection process is studied to disclose the different behaviors of matter waves in the region of the potential variation.

First-principles investigations on the mechanical, thermal,electronic, and optical properties of the defect perovskites Cs_2SnX_6(X= Cl, Br, I)

The mechanical properties, thermal properties, electronic structures, and optical properties of the defect perovskites Cs2SnX6（X = Cl, Br, I） were investigated by first-principles calculation using PBE and HSE06 hybrid functional. The optic band gaps based on HSE06 are 3.83 eV for Cs2SnCl6, 2.36 eV for Cs2SnBr6, and 0.92 eV for Cs2SnI6, which agree with the experimental results. The Cs2SnCl6, Cs2SnBr6, and Cs2SnI6 are mechanically stable and they are all anisotropic and ductile in nature. Electronic structures calculations show that the conduction band consists mainly of hybridization between the halogen p orbitals and Sn 5s orbitals, whereas the valence band is composed of the halogen p orbitals. Optic properties indicate that these three compounds exhibit good optical absorption in the ultraviolet region, and the absorption spectra red shift with the increase in the number of halogen atoms. The defect perovskites are good candidates for probing the lead-free and high power conversion efficiency of solar cells.

Destroying a Peldan Electrostatic Solution Black Hole

By throwing a particle with electric charge and angular momentum into the black holes, much evidence shows that the naked singularity of some （3＋1）-dimensional black holes might be seen, which is not allowed in the weak cosmic censorship conjecture. In this study, we consider a （2＋1）-dimensional Peldan black hole and find that it could be destroyed under certain conditions in both extreme and near-extreme cases.