JETP Letters最新文献

Tunable linear photonic circuits are important elements in both classical and quantum information technologies. The scaling of such circuits is possible only in the integrated form, which complicates their characterization because of the impossibility of the reconstruction of each element individually. The existing methods of characterization of linear photonic circuits require multiple changes in the phases of transfer matrix elements at various values of control parameters, which provides significant experimental difficulties. In this work, a new approach is proposed: it is demonstrated that the measurement of only transmission coefficients for certain values of the control parameters of the photonic circuit allows the development of a mathematical model that can predicts transmission coefficients for arbitrary values of the parameters. This method has been successfully approved in a numerical experiment for a tunable four-channel interferometer implementing an arbitrary unitary transformation. The proposed method provides new possibilities to efficiently characterize and design tunable linear photonic circuits.

Ratios m_s/m_d and m_u/m_d of the light quark masses have been determined from expressions for squared masses of pseudoscalar mesons (m_{pi }^{2}) and (m_{K}^{2}) obtained with an accuracy of the second order in chiral symmetry breaking. The fit of the theoretical expressions for (m_{pi }^{2}) and (m_{K}^{2}) to their phenomenological values leads to a functional relation between the ratios m_s/m_d and m_u/m_d, which is described by a third-order curve. The application of the lattice calculation result m_s/m_ud = 27.23(10), where m_ud = (m_u + m_d)/2, reported by the flavor lattice averaging group (FLAG) for the case of four quark flavors provides an additional constraint, which significantly reduces the error (~2%) for the ratio m_u/m_d = 0.455(8). The absolute values of the quark masses have been then obtained.

Three samples of a quasi-one-dimensional conductor NbS₃ from the same growth batch are studied at room temperature by transmission electron microscopy. Less defective sample no. 1 and more defective sample no. 2 have been identified as known polytypes. Sample no. 3 is an unknown polytype with the lattice parameter c larger than that in all known polytypes. Electron diffraction patterns of the new polytype demonstrate four systems of satellite reflections, which indicates the formation of four charge density waves in the sample. The possibility of the formation of polytypes with a more complex structure is discussed.

The structural and dynamic properties of a water molecule undergo changes if it is located inside a fullerene (H₂O@C₆₀). In this work, the atomistic simulation method including nuclear quantum effects is used for the first time to describe the low-temperature dynamics and changes in the structure of the water molecule in the fullerene at 5 K. A machine-learning potential on density functional theory trajectories is used to calculate the interactions in this system. Zero-point energy and delocalization of nuclei are taken into account using path integral molecular dynamics.

A relation of the presence of metastable objects with various “lifetimes” in the Universe to both an increase in the entropy and “the arrow of time” is discussed. A finite lifetime of metastable objects makes it possible to assign many of them with their own “local clocks.” The decay of any metastable system in the Universe results in the appearance of more stable objects with the emission of photons and other particles. These photons and particles interact with particles of more stable subsystems in the Universe, leading to their ergodicity. The emission of photons from decaying metastable states in the expanding Universe makes these processes irreversible and specifies the arrow of time although the equations describing these physical processes are reversible.

Type II superconductivity with T_c ∼ 6 K is discovered in LaB₆. The critical fields are determined and the estimates for the coherence length ξ(0) ∼ 240 Å, the Ginzburg−Landau parameter κ_GL ∼ 2, and the electron–phonon coupling constant λ_e−ph ≈ 0.75 are obtained. Precision X-ray diffraction studies at T = 30 K reveal three-dimensional charge-stripe structures in LaB₆. The scenario of superconductivity localized near filamentary channels with a fluctuating electron density arising in the lanthanum hexaboride matrix is discussed.

A simple and popular Bridgman’s formula predicts a linear correlation between the thermal conductivity coefficient and the sound velocity of dense liquids. Unfortunately, it cannot be applied to strongly coupled plasma-related fluids with screened Coulomb interactions, because the sound velocity can greatly increase as screening weakens. We propose a modification of the Bridgman formula by correlating the thermal conductivity coefficient with the transverse (shear) sound velocity. This approach is demonstrated to work reasonably well in screened Coulomb (Yukawa) fluids and can be useful in the context of complex (dusty) plasmas.

The microwave photoconductivity of a system of gapless Dirac fermions in HgTe quantum wells with a critical thickness has been experimentally and theoretically investigated. It has been found that the photoconductivity fluctuates depending on the gate voltage near the Dirac point, and the fluctuation amplitude increases with an increase in the conductor size and a decrease in temperature. A theoretical explanation of the microwave response based on the assumption of the existence of a percolation two-dimensional fractal network of helical edge current states induced by fluctuations in the well thickness near the critical value has been proposed. It has been shown that the microwave photoconductivity of this network fluctuates with a change in the Fermi energy and the behavior of the fluctuation amplitude is in qualitative agreement with the corresponding experimental data.

We present simple qualitative estimates for the maximal superconducting transition temperature, which may be achieved due to electron–phonon coupling in Eliashberg–McMillan theory. It is shown that in the limit of very strong coupling the upper limit for transition temperature is determined in fact by a combination of atomic constants and density of conduction electrons.