JETP Letters最新文献

A new superconductor BaAg_1.8Bi₂ with a previously unknown variant of the monoclinically distorted CaBe₂Ge₂ structure (space group C2/m) has been crystallized in the form of single crystals from a bismuth flux. The temperature and magnetic field dependences of the magnetic susceptibility and resistance have shown that this compound transits to a superconducting state at the temperature T_c = 5.4 K. According to the found Ginzburg–Landau parameter κ = 27, this compound is a type-II superconductor with the first and second critical fields H_c1(0) = 53 Oe and H_c2(0) = 2.1 × 10⁴ Oe and the critical current density reaching 4.4 kA/cm² at 2.5 K. It can be assumed that, similar to some superconducting 112 type bismuthides, superconductivity in this compound is due to the planar square bismuth sublattice contained in the fluorite-like [BiAg_0.8] layer rather than to the [AgBi] layers where Ag atoms are locally disordered. This could explain the unusually high T_c value for bismuthides belonging to the CaBe₂Ge₂ structural type and its derivatives.

Heterostructures with InAs/InGaAs quantum dots and In_xGa_1–xAs/GaAs(001) metamorphic buffer layers are grown by molecular-beam epitaxy. The structures are designed to obtain single-photon emission in the telecommunication C-band wavelength range. The possibility of reducing the thickness of the In_xGa_1–xAs graded layer in order to form efficient microcavity structures with a cavity length as small as two wavelengths is examined. The structures with metamorphic buffer layers grown on top of an Al_0.9Ga_0.1As/GaAs distributed Bragg reflector are grown and characterized by cross-sectional transmission electron microscopy and photoluminescence spectroscopy.

Hexagonal boron nitride is distinguished among solid-state materials with luminescent properties as a material to create single-photon sources efficiently emitting at room temperature. In this work, it is demonstrated that helium ion irradiation with fluences of (1–5) × 10¹⁴ ion/cm² increases the ultraviolet radiation intensity with a maximum at a wavelength of 320 nm due to the formation of new luminescent centers. The subsequent electron irradiation further increases the intensity of 320 nm luminescence apparently due to the formation of carbon-containing defects in the volume of hBN through recombination-enhanced migration. On the contrary, the intense helium ion irradiation stimulates the formation of nonradiative recombination centers, which reduce the lifetime of nonequilibrium charge carriers.

Unusual resonance lines have been observed for a dimer associate of impurity ⁶³Cu²⁺ ions in a BaF₂ single crystal in continuous-wave electron paramagnetic resonance spectra recorded using a dielectric resonator. The phase of these lines is orthogonal to the phase of magnetic field modulation. The appearance of such lines is attributed to the features of the spin dynamics of high-spin electron systems resonantly interacting with a microwave electromagnetic field. The magnetic dipole moment of the spin system is transformed into the electric quadrupole moment under the sufficiently intense resonant excitation of magnetic dipole transitions. In this case, the interaction of the magnetic dipole and electric quadrupole oscillators occurs in the spin system. The simultaneous excitation of magnetic dipole and electric quadrupole resonant transitions in the dielectric resonator of a spectrometer leads to the appearance of phase-shifted resonance lines.

The possibility of the femtosecond laser structural micromodification (microlabeling) of diamond through a solid immersion layer made of Ge₇Se₉₃ chalcogenide glass at a wavelength of 1.55 μm has been investigated. The two-photon absorption coefficient of Ge₇Se₉₃ has been measured to be β₂ = (0.09 ± 0.01) cm/GW, which allows the propagation of intense femtosecond laser radiation in this spectral range through realistically thin (<0.1–1 mm) immersion layers. Despite the nonlinear absorption and optical damage in the immersion volume, photoluminescent microlabels have been observed in the test writing modes in the volume of diamond, which become more uniform with decreasing exposure time and pulse energy.

The development of the two-photon laser lithography technique for the fabrication of optical elements with characteristic dimensions of a few microns is an important goal. Here, two-photon laser lithography is used to produce micro-optical waveguides from OrmoComp® hybrid photoresist. The waveguides are optically isolated from a substrate and are connected to total internal reflection prism adapters for coupling optical radiation in and out of them. The transmission spectra of the entire input adapter–waveguide–output adapter structure are calculated and measured and it is shown that the transmission coefficient in the few-mode regime is 20–40% in the spectral range of 700–1650 nm. According to calculations, the main mechanism of losses in such a structure is determined by strong scattering in the region of joint between the conical part of the adapter and the waveguide caused by the complex structure of the optical field, as well as by the violation of the total internal reflection regime in the prisms due to the large angular aperture of the focused radiation beam. It is shown that the Goos–Hänchen effect has to be taken into account in the design of the coupling elements.

The simulation of the motion of ultracold neutrons is important for estimating their losses, accurately measuring their lifetimes, and describing other experiments. In material traps, it is necessary to take into account not only the specular reflection, but also the diffuse elastic reflection of ultracold neutrons from the walls of a trap. The angular distribution of diffusely reflected neutrons is usually described by Lambert’s cosine law, which does not have a strict theoretical justification and is often violated. In this work, an experiment has been proposed to measure the deviation of the angular distribution of diffusely reflected ultracold neutrons from Lambert’s cosine law. This deviation can be determined by the difference in the number of neutrons leaving a long, narrow trap for ultracold neutrons through its central and end windows. The performed Monte Carlo calculations corresponding to the proposed experiment have shown a significant effect for different shapes of the trap.

The superconducting order parameter of the RbCa₂Fe₄As₄F₂ compound belonging to the new 12442 family of iron-based superconductors with a critical temperature of T_c ~ 32 K has been studied. Two superconducting condensates with the order parameters Δ_L ~ 6.3 meV and Δ_S ~ 2.8 meV have been detected for the first time by multiple Andreev reflection spectroscopy. The temperature dependence of the superconducting critical current density J_c(T) in the intrinsic field has been measured. The approximation of the dependence J_c(T) demonstrates the correspondence of the experimental data to the double-gap model with the s-wave order parameter and the gaps of Δ_L ~ 6 meV and Δ_S ~ 2 meV. The superconducting order parameters obtained by two different methods are in good agreement with each other.

A theory has been developed to describe the magnetization curves of an ensemble of single-domain particles. This theory includes their temperature-induced excitations and is in fact a generalization of the two-level Stoner–Wohlfarth model and its relaxation version, as well as a multilevel relaxation model for describing Mössbauer spectra. A basically new aspect of this theory is the solution of a system of differential equations for nonequilibrium populations of stochastic states. The resultant model includes the physical mechanisms for the formation of magnetization curves of nanoparticles in real situations and describes self-consistently the qualitative features of the transformation of these curves under the variation of the temperature and the strength and direction of an external field.

The electric and magnetic frequency rearrangement of a Bragg resonance in the spectrum of spin waves in a magnonic crystal in the form of 100-nm-thick yttrium iron garnet film with attached 10-nm-thick platinum strips is reported. The effect of the spin current on the position of the Bragg band gap depends on the polarity of the voltage applied to the platinum strips. The positive voltage does not affect the band gap position, while the applied negative voltage lowers the band gap by about of 5 MHz. At frequencies outside the band gap, depending on the polarity of the voltage applied to platinum, either the enhancement or suppression of the spin wave is observed.