JETP Letters最新文献

Tunneling between two sheets of bilayer graphene, the crystal lattices of which are rotated relative to each other by a small angle, has been studied. An anomalous behavior of the tunneling conductivity caused by van Hove singularities at the edges of the conduction and valence bands spatially localized in different sheets of bilayer graphene has been found.

The spectral power of primordial gravitational waves is calculated in the quantum version of conformal general relativity. The fundamental variables of quantum gravity in the used approach are special variables, which constitute the dynamic part of the spin connection, rather than components of the metric tensor. It has been shown that the proposed model in the Born approximation reproduces the standard spectral power of primordial gravitational waves generated in the canonical inflation process. This has made it possible to test the quantum version of the conformal theory of gravity in a specific phenomenological problem.

Recently (2024), the NA61/SHINE collaboration has presented new experimental data on ({{pi }^{ pm }}), ({{K}^{ pm }}), proton, and antiproton productions in central ⁴⁰Ar + ⁴⁵Sc collisions at the laboratory momenta P_lab = 13–150 A GeV/c and has compared these data with predictions of popular theoretical models. It turned out that the models poorly describe the data in the entire energy range. In this work, it has been suggested for the first time that nucleons participating in non-diffractive collisions cannot diffractively dissociate in subsequent nucleon–nucleon collisions. This idea has been implemented in the Geant4 FTF model. Good description of the data, including the first NICA BM@N data on ({{pi }^{ + }}) meson production in the collisions of ⁴⁰Ar nuclei with various nuclei at an energy of 3.2 A GeV, has been achieved.

The cross sections for charge exchange and ionization induced by collisions of protons with the energies from 0.01 keV to 10 MeV with atoms of the superheavy elements Nh (Z = 113), Fl (Z = 114), Mc (Z = 115), Lv (Z = 116), Ts (Z = 117), and Og (Z = 118) have been calculated. These cross sections at the energies E ≈ 0.01−100 keV and E > 100 keV have been calculated in the adiabatic approximation and in the Born approximation with the cross section normalization, respectively. Since data on the cross sections for superheavy elements under consideration are absent, cross sections for Xe (Z = 54) and U (Z = 92) atoms have also been calculated and compared with the available experimental data and theoretical calculations. The distinctive properties of the charge exchange and ionization cross sections for atoms of superheavy elements have been revealed using the results of this work.

The quantum properties of superradiant pulses generated at the radiative recombination of an electron–hole condensate in semiconductor heterostructures at room temperature have been studied using optical homodyne tomography. Signatures of the quantum entanglement of superradiant states, which are superpositions of two coherent states, have been revealed. It has been demonstrated that reconstructed Wigner functions under certain conditions are very similar to the Wigner functions of the Schrödinger cat states.

The effect of the variation of the spin current on the magnetic susceptibility of a magnonic waveguide in the form of a “ferromagnet–normal metal” heterostructure is investigated. Based on the Landau–Lifshitz–Gilbert model with the current term in the Slonczewski–Berger form, which describes the magnetization dynamics including the spin moment transfer, expressions are obtained for the real and imaginary parts of the magnetic susceptibility in the geometry of surface spin waves in the damping mode. The resulting model correctly approximates experimental data demonstrating an increase in the amplitude of spin waves propagating in a YIG/Pt heterostructure. It is shown that an increase in the spin current leads to an increase in the resonance frequency of spin waves and in the magnetic susceptibility tensor components in resonance. The results of this study can be used to design waveguides for spin waves with controllable losses and high-sensitivity magnetic field sensors.

Dynamic disorder in metal halide perovskites is driven by thermal structural fluctuations of cross-linked octahedra leading to a broadband central peak in the low-frequency region of electronic Raman scattering spectrum. In this work, a temperature dependence of the Raman central peak in metal halide perovskite CsPbBr₃ is experimentally demonstrated. The results of this study hold promise for the development of edge-cutting remote temperature sensors.

Within the Rabi quantum model, it has been shown theoretically that a two-level atom ultrastrongly coupled to an electromagnetic field generates or absorbs photons. Photons can be generated if the field is initially in the vacuum state. Under certain initial states of the atom + field system, the absorption of photons is possible in the field mode in the ultrastrong atom–field coupling regime. If the atom is initially in the ground (unexcited) state and the field is in the vacuum state, photons can be generated under resonance condition ω_a ≈ ω_f or ξ ≡ ω_a/ω_f ≈ 1, where ω_a and ω_f are the atomic transition and field frequencies, respectively, in the ultrastrong coupling regime. At the negative detuning when ξ ( ll ) 1 or ω_a ( ll ) ω_f, Rabi oscillations of the average number of photons ({{langle hat {n}rangle }_{t}}) with (0 leqslant {{langle hat {n}rangle }_{t}} leqslant {{n}_{{max }}} gg 1) are observed in the case of the ultrastrong coupling with the atom–field coupling constant (tilde {g} equiv {text{|}}g{text{|/}}{{omega }_{{text{f}}}} sim 1); in this case, the population of the excited atomic state is P_e(t) ≈ 0.5. At a large positive detuning when ξ ( gg ) 1, photons are not generated, i.e., (langle hat {n}rangle approx 0), and the atom remains in the initial state, i.e., P_e(t) ≈ 0. The statistics of photons in the generation regime is nearly chaotic: the variance of photons is much larger than the level of the coherent field state (i.e., is super-Poisson). Field photons are absorbed without the excitation of the atom in the ultrastrong coupling regime in the case of the coherent initial state of the field ((langle hat {n}(t = 0rangle > 0)) for certain positive detuning values. In this case, the field becomes sub-Poisson.

A system of equations has been proposed for a monochromatic weakly nonlinear light wave in a Kerr medium. This system is equivalent up to the third order in electric field to the known equation ({kern 1pt} {text{curl}},{text{curl}}{mathbf{E}} = k_{0}^{2}[{mathbf{E}} + alpha {text{|}}{mathbf{E}}{{{text{|}}}^{2}}{mathbf{E}} + beta ({mathbf{E}} cdot {mathbf{E}}){mathbf{E}}{text{*}}]), but the new equations are much more convenient for numerical computation. Optical fields with small structures of two or three wavelengths have been simulated using this system. It has been found that a stable self-focused light beam (a two-dimensional vector soliton) in some parametric domain is possible even without modification of nonlinearity. “Inelastic” collisions between two such narrow beams with opposite circular polarizations have been calculated. Furthermore, examples of interacting optical vortices, spatial separation of the circular polarizations, and the Kelvin–Helmholtz instability have been given for defocusing nonlinearity.

The paper presents the results of studying the vibrational spectrum of the PbNi_1/3Nb_2/3O₃ (PNN) crystal using Raman scattering in the temperature range from 100 to 650 K, including the region of the “diffuse phase transition” with a maximum permittivity at ({{T}_{m}} = 150{kern 1pt} ) K at a frequency of 10 kHz. Polarized light scattering spectra in PNN are similar to those observed in a number of studied relaxor ferroelectrics with a perovskite structure (AB{kern 1pt} '{kern 1pt} B{kern 1pt} ''{kern 1pt} {{O}_{3}}). Analysis of the temperature evolution of phonon modes allowed us to identify special points in the dynamics of the PNN crystal lattice at ({{T}_{1}} = 280{kern 1pt} ) K and ({{T}_{m}} = 150{kern 1pt} ) K. It is shown that anomalies in the temperature behavior of optical phonons in the vicinity of ({{T}_{1}}) can be associated with the dynamics of polar nanoregions, the nature of which changes from dynamic to static. In the studied temperature range, a sequence of phase transformations is proposed for PNN crystals.