We report on the development of a heterodyne receiver at mid-infrared wavelength for high-resolution spectroscopy applications. The receiver employs a superconducting NbN hot electron bolometer as a mixer and a room temperature distributed feedback quantum cascade laser operating at 10.6 μm (28.2 THz) as a local oscillator. The stabilization of the heterodyne receiver has been achieved using a feedback loop controlling the output power of the laser. Improved Allan variance times as well as a double sideband receiver noise temperature of 5000 K and a noise bandwidth of 2.8 GHz of the receiver system are demonstrated.
We investigate the conductance of a 1D disordered conducting loop with two contacts, immersed in a magnetic flux. We show the appearance in this model of the Al'tshuler-Aronov-Spivak behaviour. We also investigate the case of a chain of loops distributed with finite density: in this case we show that the interference effects due to the presence of the loops can lead to the delocalization of the wave function.
Recent multispacecraft observations in the Earth’s magnetosphere have revealed an abundance of magnetic holes—localized magnetic field depressions. These magnetic holes are characterized by the plasma pressure enhancement and strongly localized currents flowing around the hole boundaries. There are several numerical and analytical models describing 2D configurations of magnetic holes, but the 3D distribution of magnetic fields and electric currents is studied poorly. Such a 3D magnetic field configuration is important for accurate investigation of charged particle dynamics within magnetic holes. Moreover, the 3D distribution of currents can be used for distant probing of magnetic holes in the magnetosphere. In this study, a 3D magnetic hole model using the single-fluid approximation and a spatial scale hierarchy with the distinct separation of gradients is developed. It is shown that such 3D holes can be obtained as a generalization of 1D models with the plasma pressure distribution adopted from the kinetic approach. The proposed model contains two magnetic field components and field-aligned currents. The magnetic field line configuration resembles the magnetic trap where hot charged particles bounce between mirror points. However, the approximation of isotropic pressure results in a constant plasma pressure along magnetic field lines, and the proposed magnetic hole model does not confine plasma along the field direction.
We present the results of 3D-hydrodynamical simulations of accretion flow in the eclipsing dwarf nova V1239 Her in quiescence. The model includes the optical star filling its Roche lobe, a gas stream emanating from the inner Lagrangian point of the binary system, and the accretion disc structure. A cold hydrogen gas stream is initially emitted towards a point-like gravitational centre. A stationary accretion disc is formed in about 15 orbital periods after the beginning of accretion. The model takes into account partial ionization of hydrogen and uses realistic cooling function for hydrogen. The light curve of the system is calculated as the volume emission of optically thin layers along the line of sight up to the optical depth τ = 2/3 calculated using Planck-averaged opacities. The calculated eclipse light curves show good agreement with observations, with the changing shape of pre-eclipse and post-eclipse light curves being explained entirely due to ˜50 per cent variations in the mass accretion rate through the gas stream.
3D problem of the formation of smallscale density caverns with a nonstationary electric field in the region of auroral electric currents and kinetic Alfvén wave currents is considered. It is shown that an excess of the electron current velocity over a certain critical value of their thermal velocity is a probable cause of cavern formation. Linear and nonlinear stages of the density cavern formation are considered, and their main parameters are estimated. In the case of comparatively strong magnetic fields, caverns can be formed with comparable longitudinal and transverse (with respect to the magnetic field) scales. The properties of parameters of smallscale density caverns and nonstationary electric field agree with wellknown experimen tal data.
We propose a 3D model of small-scale density cavities stimulated by an auroral field-aligned current and an oscillating field-aligned current of kinetic Alfvén waves. It is shown that when the field-aligned current increases so that the electron drift velocity exceeds a value of the order of the electron thermal velocity, the plasma becomes unstable to the formation of cavities with low density and strong electric field. The condition of instability is associated with the value of the background magnetic field. In the case of a relatively weak magnetic field (where the electron gyro-radius is greater than the ion acoustic wavelength), the current instability can lead to the formation of one-dimensional cavities along the magnetic field. In the case of a stronger magnetic field (where the ion acoustic wavelength is greater than the electron gyro-radius, but still is less than the ion gyro-radius), the instability can lead to the formation of 3D density cavities. In this case, the spatial scales of the cavity, both along and across the background magnetic field, can be comparable, and at the earlier stage of the cavity formation they are of the order of the ion acoustic wavelength. Rarefactions of the cavity density are accompanied by an increase in the electric field and are limited by the pressure of bipolar electric fields that occur within them. The estimates of typical density cavity characteristics and the results of numerical solutions agree with known experimental data: small-scale structures with a sufficiently strong electric field are observed in the auroral regions with strong field-aligned current.
Reflectivity of shocked compressed xenon plasma is calculated within the framework of the density functional theory approach. Dependencies on the frequency of incident radiation and on the plasma density are analyzed. The Fresnel formula for the reflectivity is used. The longitudinal expression in the long-wavelength limit is applied for the calculation of the imaginary part of the dielectric function. The real part of the dielectric function is calculated by means of the Kramers-Kronig transformation. The results are compared with experimental data. The approach for the calculation of plasma frequency is developed.
Warm dense matter (WDM) is a state of a substance with a solid-state density and temperature from 1 to 100 eV. Researchers believe that such a state exists in the cores of giant planets. Investigation of WDM is important for some applications, such as surface treatment on the nanometer scale, laser ablation, and the formation of the plasma sources of the X-ray radiation into the inertial synthesis. In this study, the conductivity and the thermal conductivity are calculated based on density functional theory and the Kubo-Greenwood theory. This approach was already used to simulate the transport properties in a broad range of densities and temperatures, and its efficiency has been demonstrated. The conductivity and the thermal conductivity of aluminum and gold are investigated. Both the isothermal state, when the electron temperature equals the ion temperature, and the two-temperature state, when the electron temperature exceeds the ion temperature, are considered. The calculations were performed for a solid body and liquid in the range of electron temperatures from 0 to 6 eV.
Ab initio calculations of the intermolecular potential energy surface (PES) of CO-N-2 have been carried out using the closed-shell single-and double-excitation coupled cluster approach with a non-iterative perturbative treatment of triple excitations method and the augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set supplemented with midbond functions. The global minimum (D-e = 117.35 cm(-1)) of the four-dimensional PES corresponds to an approximately T-shaped structure with the N-2 subunit forming the leg and CO the top. The bound rovibrational levels of the CO-N-2 complex were calculated for total angular momenta J = 0-8 on this intermolecular potential surface. The calculated dissociation energies D-0 are 75.60 and 76.79 cm(-1) for the ortho-N-2 (A-symmetry) and para-N-2 (B-symmetry) nuclear spin modifications of CO-N-2, respectively. Guided by these bound state calculations, a new millimeter-wave survey for the CO-N-2 complex in the frequency range of 110-145 GHz was performed using the intracavity OROTRON jet spectrometer. Transitions not previously observed were detected and assigned to the subbands connecting the K = 0 and 1, (j(CO), j(N2)) = (1, 0) states with a new K = 1, (j(CO), j(N2)) = (2, 0) state. Finally, the measured rotational energy levels of the CO-N-2 complex were compared to the theoretical bound state results, thus providing a critical test of the quality of the PES presented. The computed rovibrational wave functions were analyzed to characterize the nature of the different bound states observed for the two nuclear spin species of CO-N2.
The homoepitaxial growth of Si on Si(100) covered by a resist mask is a necessary technological step for the fabrication of donor-based quantum devices with scanning tunneling microscope lithography. In the present work, the chlorine monolayer is selected as the resist. Using density functional theory, we investigated the adsorption of a single silicon atom on Si(100)-2 × 1-Cl as the starting process of Si epitaxy. The incorporation of a silicon atom under a Cl monolayer proved to be the most energetically favorable process. Our results show that chlorine segregates on the surface during Si deposition and does not incorporate into homoepitaxial layers. In addition, we found that SiCl2*, SiCl3*, and SiCl4* clusters can be formed above a Si(100)-2 × 1-Cl surface while Si is adsorbed. SiCl2* clusters are bound weakly to the substrate, and their desorption leaves the silicon surface free of chlorine. To check whether the Si epitaxy is possible on the chlorine resist, we compare our results with the well-studied case of a hydrogen resist. We find the two processes to be similar; moreover, epitaxy on chlorine resist appears to have an advantage.
We apply first principles calculations to compare the carbon and boron nitride nanotube unzipping under atomic oxygen impact. We show that the attack of several oxygen atoms can cause bond breaking in nanotubes, but the structure of boron nitride nanotubes is less damaged than the structure of carbon ones. With increasing diameter, the structural damage of nanotubes reduces
We have investigated the dependence of hole mobility on thickness in free-standing films of bisphenol-Apolycarbonate (PC) doped with 30 wt% p-diethylaminobenzaldehyde diphenylhydrazone (DEH). Carrier generation in a time-of-flight (TOF) experiment was achieved through direct ionization of dopant molecules by electron impact using an electron gun supplying pulses of monoenergetic electrons in the range of 2–50 keV. The position of dopant ionization depends upon the electron energy and three TOF variants have been recently developed and used in this study. We have found that the hole mobility generally decreased with increasing film thickness with concomitant acceleration of the post-flight current decay indicating that the transport process approaches the steady-state regime, this process happening slightly faster than our model predicts. Numerical calculations have been compared with experimental data. The results are discussed in detail. The way to reconcile ostensibly contradictory interpretations of our results and those commonly reported in literature relying on photo injection technique has been proposed.
Aim. The main stages of scientific activity of V.A. Solntsev from the student's bench to the leader of the scientific school on radiophysics and microwave electronics are investigated. Method. Based on the primary publications, the main directions of the creative path of the scientist, whom determined the electronic age of the time, are analyzed. Results. It is shown how timely V.A. Solntsev found a substitute for research and calculation methods that ceased to satisfy the demands of science and production, and resolutely went on to develop new principles of amplification and generation, accurately determining the expiration time of previous methods and principles and giving way only to the routine apparatus. Discussion. Great educational work and scientific and organizational role of the outstanding scientist were noted. © 2018 Saratov State University. All rights reserved.
We investigate the absorption properties of U-shaped niobium nitride (NbN) nanowires atop nanophotonic circuits. Nanowires as narrow as 20nm are realized in direct contact with Si3N4 waveguides and their absorption properties are extracted through balanced measurements. We perform a full characterization of the absorption coefficient in dependence of length, width and separation of the fabricated nanowires, as well as for waveguides with different cross-section and etch depth. Our results show excellent agreement with finite-element analysis simulations for all considered parameters. The experimental data thus allows for optimizing absorption properties of emerging single-photon detectors co-integrated with telecom wavelength optical circuits.
Plasmon spectroscopy methods are highly sensitive to the small volumes of material due to subwavelength localization of light increasing light-matter interaction. Recent research has shown a high potential of plasmon quantum generator (spaser) or amplifier (sped) for sensing in the infrared optical region. Trinitrotoluene (TNT) molecules fingerprints are considered as an example. Basing on Lindblad equations, we implement full quantum mechanical theory of graphene plasmon generator to investigate how a small amount of absorbing atoms influences the spectrum of a graphene spaser. We analyze the optimal type of an active medium, the number of active molecules, and the pump level to achieve the highest sensitivity and show that optimized structure is sensitive to dozens of atoms. Our research is useful for the development of near- and mid-IR spectroscopy based on plasmon quantum amplifier.
A consideration of the acceleration mechanism which supplies the fast electrons to the source of Saturnian kilometric radiation (SKR) and an interpretation of the recently reported observational indications of the influence of Titan on the SKR are presented. The proposed mechanism operates by the effect of the different magnetization of the electrons and ions in Titan’s ionosphere which in the course of Titan’s motion through the Saturnian magnetic field causes the creation of a charge‐separation electric field. This field has a component parallel to the magnetic field and accelerates part of the ionospheric electrons (called “runaway electrons”). The performed estimates show that the mechanism accelerates the runaway electrons up to an energy of ∼5 keV. The power of the acceleration mechanism is sufficient for SKR generation and also for the ultraviolet luminescence of Titan’s atmosphere. The weakening of the SKR when Titan passes on the dayside of Saturn is due to a decrease of the magnetic field strength near the dayside magnetopause, when the Moon escapes the Saturnian magnetosphere, as well as due to the break in the magnetic connection between the electron acceleration region on Titan and the SKR sources. The latter prevents the penetration of the accelerated electrons into the radiation generation region. When Titan is on the nightside of Saturn, it enters into shell L∼14, which is stretched owing to the ring current. In this case, the electrons that accelerated in the ionosphere of Titan can reach the nightside SKR sources and activate them and therefore being the reason for the Titan influence on the SKR.
Fluxes of energetic protons in the range from 30 keV up to several MeV measured at the Voyager 1/2spacecraft downstream of the heliospheric termination shock can be explained by shock-drift acceleration theory, which includes variations of the magnetic field direction in a vicinity of the shock. The variations can be connected with the sector structure of the interplanetary magnetic field near the solar equatorial plane. Theoretical fluxes of accelerated protons are calculated numerically in the framework of a 3D kinetic-magnetohydrodynamic model of the interaction of the solar wind and local interstellar medium.
This work is devoted to the investigation of particle acceleration during magnetospheric dipolarizations. A numerical model is presented taking into account the four scenarios of plasma acceleration that can be realized: (A) total dipolarization with characteristic time scales of 3 min; (B) single peak value of the normal magnetic component Bz occurring on the time scale of less than 1 min; (C) a sequence of rapid jumps of Bz interpreted as the passage of a chain of multiple dipolarization fronts (DFs); and (D) the simultaneous action of mechanism (C) followed by the consequent enhancement of electric and magnetic fluctuations with the small characteristic time scale 1 s. In a frame of the model, we have obtained and analyzed the energy spectra of four plasma populations: electrons e, protons Hþ, helium Heþ, and oxygen Oþ ions, accelerated by the above-mentioned processes (A)–(D). It is shown that Oþ ions can be accelerated mainly due to the mechanism (A); Hþ and Heþ ions (and to some extent electrons) can be more effectively accelerated due to the mechanism (C) than the single dipolarization (B). It is found that high-frequency electric and magnetic fluctuations accompanying multiple DFs (D) can strongly accelerate electrons e and really weakly influence other populations of plasma. The results of modeling demonstrated clearly the distinguishable spatial and temporal resonance character of particle acceleration processes. The maximum particle energies depending on the scale of the magnetic acceleration region and the value of the magnetic field are estimated. The shapes of energy spectra are discussed.
We study properties of Markov chain Monte Carlo simulations of classical spin models with local updates. We derive analytic expressions for the mean value of the acceptance rate of single-spin-flip algorithms for the one-dimensional Ising model. We find that for the Metropolis algorithm the average acceptance rate is a linear function of energy. We further provide numerical results for the energy dependence of the average acceptance rate for the three- and four-state Potts model, and the XY model in one and two spatial dimensions. In all cases, the acceptance rate is an almost linear function of the energy in the critical region. The variance of the acceptance rate is studied as a function of the specific heat. While the specific heat develops a singularity in the vicinity of a phase transition, the variance of the acceptance rate stays finite.