JETP Letters最新文献

Quantum key distribution systems with an untrusted intermediate node described by the so-called measurement device independent (MDI) protocol have been actively studied in the last decade. In early works, it was only argued why such a quantum key distribution system ensures the security of distributed keys mentioning that the security proof of the MDI protocol, which was not presented, is similar to that for the basic Bennett‒Brassard 84 (BB84) protocol. For this reason, despite the existing experimental implementations of the MDI quantum key distribution system, physical reasons for the protocol security are still questionable. Such quantum key distribution systems provide a common key between two network nodes connected through the intermediate untrusted node, which does not require protection of the equipment on it, and an eavesdropper sees the entire operation of the equipment, including the results of the operation of photodetectors. In this work, the MDI protocol has been analyzed. It has been shown that the physical reasons for the protocol security are based on fundamental properties such as the interference of photons from different sources, monogamy of entanglement, and nonorthogonality of states. A simple and explicit derivation is given showing the equivalence of the MDI and BB84 protocols and physical reasons for the identity of the corresponding expressions for the length of the final key.

The grazing X-ray diffraction study of phase-II NbS₃ whiskers with charge density waves has shown that they exhibit pairs of broadened reflections from two close phases with the monoclinic angles β₁ and β₂ instead of single ((h0l)) peaks. The phase with the smaller monoclinic angle β₁ = 95.9°–97.2° is close in lattice parameters to phase I without charge density waves. Phase II with β₂ = 99.9°–100.0° has a reduced b lattice parameter along the strongest bond, which implies an increased concentration of sulfur vacancies in this phase compared to phase I. It has been concluded that the studied whiskers have underwent a spinodal decomposition with the redistribution of sulfur vacancies resulting in the alternation of phases in the (001) direction perpendicular to the layers. A satellite structure indicating the formation of a superlattice with a period of 35.2 nm in this direction is observed in the most structurally perfect whisker of phase II. The spinodal decomposition is combined with high internal stresses present in whiskers with a monoclinic lattice. The partial spinodal decomposition is also observed in some phase-I whiskers without charge density waves with the appearance of thin layers of phase IV and phase II with the monoclinic angle β = 104°. The existence of thin layers of phase II in the bulk of phase I in such samples exposed to a high-frequency voltage has been confirmed by the presence of Shapiro steps on current–voltage characteristics. Spinodal decomposition should be taken into account to describe the properties of NbS₃ whiskers nominally belonging to a certain polytype.

Previously [A.O. Savostianov, I.Yu. Eremchev, T. Plakhotnik, and A.V. Naumov, Phys. Rev. B 110, 045430 (2024)], we have shown that the zero-phonon line broadening of single tetra-tert-butylterrylene molecules embedded in the matrix of amorphous polyisobutylene is a result of coupling with resonant vibrational modes arising due to the impurity effect on the host normal modes. However, the question remained whether this model would be compatible with the extensive single-molecule spectroscopy and photon echo data previously obtained for the same host/guest system. In the present work we demonstrate the high predictive power of the resonant mode model treated in the framework of the non-perturbative theory of electron–phonon coupling, and good agreement with all experimental data. Furthermore, for single-molecule data we find an unexpectedly large dispersion of the force constants for impurity molecules. To explain this dispersion, we propose a simple microscopic model that assumes fluctuations in the distance between the impurity molecule and its nearest surroundings.

We develop a theory of composite Ramsey sequences of rf pulses interacting with the Zeeman structure at the long-lived atomic level, beyond the rotating wave approximation. Such sequences are proposed in experiments to detect the violation of local Lorentz invariance [R. Shaniv, R. Ozeri, M.S. Safronova, S.G. Porsev, V.A. Dzuba, V.V. Flambaum, and H. Häffner, Phys. Rev. Lett. 120, 103202 (2018)]. Based on Fourier analysis, we have shown that taking into account non-resonant contributions leads to a radical change in the dynamics of the quantum system (with respect to the rotating wave approximation) in the case when the number of Ramsey pulses exceeds several tens. As a result, the effectiveness of using such rf pulses sequences to test local Lorentz invariance has not yet been fully determined and requires additional research.

Si_{1 – x}Ge_x alloy nanoparticles with controlled composition have been obtained by nanosecond laser ablation of silicon–germanium targets in isopropanol. The synthesized product exhibits a polycrystalline structure and a unimodal size distribution with a predominant content of microparticles and retains the stoichiometry of the composition of the targets used for the synthesis. Nanothermometry with the detection and analysis of a Raman signal from single alloy nanoparticles with a size of ~200 nm demonstrates a threefold increase (in comparison with nanoparticles of pure silicon) in the heating efficiency of a nanomaterial with the composition Si_0.45Ge_0.55 by 785-nm laser radiation falling into the first “transparency window” of biological tissues. Stimulated by continuous infrared radiation, the diffusion of silicon atoms to the surface (when heated to 650 K) and their oxidation lead to the gradual transformation of alloy nanoparticles into germanium clusters encapsulated in a SiO_x matrix.

A theory of superconducting states in metals with toroidal symmetry is presented.

The quantum weight is a new concept to describe gap electronic states of matter. This quantity is obtained by integrating the quantum metric (the real part of the quantum metric tensor) in the same way as the Berry phase is obtained by integrating the Berry curvature (the imaginary part of the quantum metric tensor). The quantum weight determines a number of kinetic quantities such as the nonlinear anomalous Hall effect, optical conductivity, and photovoltaic effect. In this work, it is shown that nonmagnetic disorder in topological insulators can induce a singularity in the quantum metric and quantum weight.

We report a synthesis of two members of recently discovered high-temperature superconductors of the 12442 family, with formula MCa₂Fe₄As₄F₂ (M = Rb, K) and transition temperatures of 32.7 and 34.6 K, respectively. Quality of the samples was assessed using X-ray powder diffraction, superconducting transitions were identified through transport and magnetic experiments. The temperature dependence of the upper critical field and vortex activation energy was investigated under magnetic fields up to 19 T. Two distinct thermally activated flux flow regimes were observed in both systems. Field dependences of activation energy ({{U}_{0}}(H)) indicate a change in the properties of vortex matter in these regimes and distinctly different dissipation mechanisms, reminiscent of cuprate HTSC.

Due to sufficiently high lasing thresholds, stimulated emission in relatively small ZnO microcrystal lasers is often considered to be fed by an inverted electron–hole plasma (EHP). In this study, the nature of optical gain in such emitters is investigated using ZnO microrods 1–6 µm in diameter synthesized by a modified thermal evaporation method and exhibiting whispering-gallery mode (WGM) lasing in the near ultraviolet range. It is demonstrated that optical gain in these objects is not a consequence of population inversion of the EHP at either low or room temperatures. Instead, the primary gain mechanism is the process of scattering of electron–hole pairs by free electrons. Unlike the case of large ZnO WGM microcavities, in small-diameter microrods this process turns out to be dominant over a wide temperature range.

A formula for the single-particle current–voltage characteristic (J(V,T,c,beta )) of “dirty” (with low concentrations of identical nonmagnetic impurities in the disordered insulator layer) SIN junctions (S is a superconductor, I is a disordered insulator, and N is a normal metal) has been obtained in the region of low temperatures and voltages 0 ≤ |eV| ( ll ) T ( ll ) Δ₀, where e is the elementary charge, Δ₀ is the superconducting gap in the S edge of a junction at the temperature T = 0, V is the voltage across the junction, c ( ll ) 1 is the dimensionless concentration of impurities in the I layer, β = (ε₀ – μ)/Δ₀ is the dimensionless deviation of the single-impurity electron energy level ε₀ relevant for this problem (on identical impurities in the I layer) from the electron chemical potential μ of the junction. It has been shown that an anomalously large sign-alternating deviation of the single-particle current–voltage characteristic of the dirty SIN junction from the value calculated by the existing theoretical expression is observed in a certain limited region Ω_d(c, β) on the (c, β) parameter plane in the presence of random narrow-band quantum jumpers in the disordered I layer. A numerical example has demonstrated that the relative deviation of the single-particle current–voltage characteristic at typical values of the parameters of the dirty SIN junction in the region Ω_d(c, β) can reach several orders of magnitude, which ensures the possibility of the experimental observation of this effect. The conditions for the applicability of the considered model of the dirty SIN junction have been discussed and the scheme of the corresponding experiment has been proposed.