Journal of Experimental and Theoretical Physics

Phân tích lý thuyết tuần tự được thực hiện về hiện tượng đường hầm trong các cấu trúc kim loại thường/siêu dẫn loại d, trong đó có các trung tâm tán xạ nằm ở lớp giữa giữa kim loại thường và siêu dẫn. Kết quả cho thấy, sự hiện diện của một trung tâm tán xạ bên trong lớp cách điện dẫn đến việc giảm một phần các giá trị dẫn điện bất thường cao đã được dự đoán trong vùng điện áp thấp (nỗi bất thường trong điện áp bằng 0 - zero bias anomaly (ZBA)). Trong quá trình này, việc đưa vào thuật ngữ "can thiệp" trong toán tử dòng điện (can thiệp do hiện tượng đường hầm qua một trung tâm tán xạ kết hợp với hiện tượng đường hầm tiềm năng trực tiếp) làm giảm ZBA. Hiệu ứng được dự đoán gần như độc lập với vị trí của trung tâm tán xạ trong lớp giữa và hình dạng của đường cong cộng hưởng của sự tán xạ (được coi là Lorentz trong trường hợp hiện tượng đường hầm cộng hưởng qua trung tâm tán xạ).

The radial component of the Coulomb Green’s function (CGF) is written in the form of a double series in Laguerre polynomials (Sturm’s functions in the Coulomb problem), which contains two free parameters α and α′. The obtained result is applicable both in the nonrelativistic case and for the CGF of the squared Dirac equation with a Coulomb potential. The CGF is decomposed into the resonance and potential components (the latter is a smooth function of energy) for α = α′. In the momentum representation, the CGF with the free parameters is written in the form of an expansion in four-dimensional spherical functions. The choice of the parameters α and α ′ in accordance with the specific features of the given problem radically simplifies the calculation of the composite matrix elements for electromagnetic transitions. Closed analytic expressions (in terms of hypergeometric functions) are obtained for the amplitudes of bound-bound and bound-free two-photon transitions in the hydrogen atom from an arbitrary initial state ¦nl〉, which generalize the known (one-photon) Gordon formulas. The dynamic polarizability tensor components αnlm(ω) for an arbitrary n are expressed in terms of the hypergeometric function 2 F 1 depending only on l and $$\tilde \omega $$ and through the polynomial functions $$f_{nl} (\tilde \omega )$$ of frequency $$\tilde \omega = {{\hbar \omega } \mathord{\left/ {\vphantom {{\hbar \omega } {\left| {E_n } \right|}}} \right. \kern-\nulldelimiterspace} {\left| {E_n } \right|}}$$ . The Rydberg (n ≫ 1) and threshold (ℏω ∼ ¦ E n¦) asymptotic forms of polarizabilities are investigated.

Experimental results on temperature dependences of the low-frequency dielectric permittivity of the group of magnetic fluoroperovskites with different crystal and magnetic structures are presented. Orthorhombic NaCoF3 and NaNiF3, cubic RbFeF3, hexagonal RbNiF3 and tetragonal K2CoF4 and K2NiF4 were investigated. The analysis of experimental results in combination with those of our previous studies of other fluoroperovskites was carried out taking into account the influence of the spontaneous magnetodielectric effect on lattice dynamics. It revealed the role of the spin-phonon coupling and the anharmonic contribution, which leads an increase of dielectric permittivity at heating, and contribution of the hidden structural instability manifested as an increase of the dielectric permittivity at cooling. It was established that the relative contributions of these three main mechanisms to the temperature dependence of dielectric permittivity are significantly different in all fluoroperovskites under study but they are well correlated with the tolerance factor t, which characterizes a relation between ionic radii and is a measure of stability of AMF3 perovskite crystal structure. The obtained results and their analysis reliably demonstrate that the low-frequency dielectric spectroscopy is a highly sensitive method to study particular features of the lattice dynamics of fluoroperovskites at magnetic and structural phase transitions.

The complex permittivity tensor of a magnetically active isotropic medium is expanded, to third-order accuracy, in terms of a small parameter equal to the ratio of the mean distance between the atoms of the medium to the wavelength of an electromagnetic wave. A dispersion equation is constructed. On the basis of this equation, the refractive indices of the medium for normal waves are obtained when a longitudinal magnetic field is applied to the medium. It is shown that calculations to third-order accuracy yield different values for the velocities of all four normal waves that propagate in the medium in forward and backward directions. Calculations are carried out for the experiment conducted with the use of a ring laser for measuring the expansion coefficients of the complex permittivity tensor that are responsible for the small difference between the velocities of the normal waves propagating in forward and backward directions. It is shown that, in the case of an isotropic optically nonactive medium, the third-order expansion coefficients can be measured by means of a ring laser with an absolute accuracy on the order of 10−14.

The specific heat and resistive upper critical magnetic field of the single crystal La1.85Sr0.15CuO4 are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated.

First-principles calculations are performed relating to the stability of a series of perovskite crystals with respect to transition to the ferroelectric and the antiferroelectric state. The calculations employ the generalized Gordon-Kim method, in which the total charge density of an ionic crystal is represented as a superposition of the densities of the individual ions. In the spirit of the nonequilibrium thermodynamics of Leontovich the charge density of an individual ion is calculated in the presence of external auxiliary fields which deform this density. Multipole deformations up to quadrupole are taken into account. The actual magnitude of the deformation is found by minimizing the total energy of the crystal in the Thomas-Fermi-Dirac approximation. The calculated values of the ion shifts in the ferroelectric phase for BaTiO3, and also the electron contribution to the dielectric constant ε ∞ and the dynamic Born effective charges Z eff are found to be in good agreement with the experimental data. The proposed method allows one to obtain an analytical expression for ε ∞, Z eff, and the dynamic vibration matrix. It is shown that these expressions formally coincide with the expressions arising in the phenomenological models of the polarized and deformed ion. Analysis of the expressions obtained confirms the validity of the classical theory of ferroelectrics of displacement type for perovskite crystals.

The effect of the magnetic field on the properties of a massive neutrino is analyzed. A general expression is derived in terms of the self-energy operator of the neutrino in an external magnetic field of arbitrary strength. This expression is valid for any relationship between the masses of the neutrino, a charged lepton, and a W-boson. An anomalous magnetic moment of a standard neutrino is investigated. The probability of massive neutrino decay into a W-boson and a charged lepton is calculated for various values of the magnetic field strength.

Một hệ thống ngẫu nhiên phi tuyến đơn giản, đó là dao động Kramers quá mức, với nhiễu màu nhân đôi, được nghiên cứu bằng phương pháp phân tích và mô phỏng số. Kết quả cho thấy trong vùng mà hiện tượng gián đoạn on-off xảy ra, hệ thống trở nên nhạy cảm quá mức với các tín hiệu định kỳ bên ngoài yếu.