Homoepitaxial diamond films heavily doped with boron, grown by CVD on single-crystal type IIa diamond substrates are investigated. Quantitative analysis of the nominal, structural, and active concentrations of boron atoms in the grown layers was made using secondary ion mass spectrometry, X-ray diffractometry, and van der Pauw method, respectively. Nominal concentrations ranged from 8.2∙10 20 cm -3 to 2∙10 22 cm -3 . Electrophysical measurements revealed metallic conductivity in the epitaxial layers, and a superconducting transition was observed. The dependence of the superconducting critical temperature on the boron doping level was studied. At nominal concentrations of 10 22 cm -3 and higher, diamond lost its perfection, transitioning to a graphite-containing phase that lacks the superconducting transition. Thus, this study examined the crossover from the superconducting phase to the boron-carbon phase with a gradual increase in the nominal boron concentration. The critical boron doping level was determined for diamond to exhibit superconductivity while maintaining its crystalline perfection, which is an important parameter for the formation of heavily doped diamond layers with a superconducting transition.

Cold-electron bolometers have shown suitability for use in modern fundamental physical experiments. In this study, the fabrication and measurements of the samples with cold-electron bolometers integrated into coplanar antennas are performed. The absorber layer was made using combined aluminum-hafnium technology to improve quality of aluminum oxide layer and decrease electron-phonon coupling. The samples of two types were measured in a dilution cryostat at various temperatures from 20 to 300 mK. The first sample with Ti/Au/Pd antenna shows response in the two frequency bands, at 7-9 GHz with bandwidth of about 20%, and also at 14 GHz with 10% bandwidth. The NEP below 10 aW Hz-1 is reached at 300 mK for 7.7 GHz signal. The second sample with aluminum made antenna shows response in the frequency range 0.5-3 GHz due to the effect of kinetic inductance of superconducting aluminum.

We predict that the threshold detectors based on Al Josephson junctions, with critical currents below 100 nA, exhibiting a phase diffusion regime, can be exploited for the microwave photon detection both at 17 mK and 700 mK. We demonstrate a detection of two- and one-photon energies at 5 GHz with 90 15 observed weak temperature dependence of the detector's performance in the sub-kelvin range fully confirms its phase-diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of the detector noise. Consequently, there is still a room to optimize the detector's performance. These results are important for axion search experiments in the range of 5-25 GHz (20-100 μeV).

We present an electromagnetic study of a metamaterial receiver based on split-ring resonators with integrated cold-electron bolometers. We suggest a modified antenna design that allows one to significantly increase the absorbed power and the bandwidth. The trade-off between the bandwidth expansion due to miniaturization and the reduction in absorption efficiency determined by the Airy spot size of the coupling lens is investigated. To solve this issue, a simultaneous miniaturization of the size of the entire structure with an increase in the number of array elements is proposed. The design with a 37-element array demonstrates an increase in power absorption by a factor of 1.4 compared to the original 19-element single-ring array, as well as an increase in operating bandwidth from 160 to 820 GHz.

Properties of long Josephson junctions, fabricated from YBCO high-temperature superconducting thin films, are investigated experimentally and by numerical modeling. Since the junctions are placed into broadband log-periodic antennas, it poses limitations on junction's bias feed properties, affecting their dynamics. It is shown that biasing junction at the output end, connected to the antenna, makes current-voltage characteristics more steep in comparison with biasing along the whole length and at the input end. The radiation properties of Josephson junctions are studied by receiving signals using dipole antenna array with cold electron bolometers. Broadband frequency tuning of the oscillator with radiation up to 800 GHz, being restricted by the receiver antenna range, is demonstrated. It is shown that for the considered junction lengths and bias feed types, the maximal radiation power varies weakly, leading to a high bolometric response.

We predict that threshold detectors based on Al Josephson junctions with critical currents below 100 nA exhibiting a phase diffusion regime can be exploited for microwave photon detection at both 17 mK and 700 mK. We demonstrate the detection of two- and one-photon energies at 5 GHz with 90% and 15% efficiency and dark count times of about 0.1 s and 0.01 s, respectively. The weak temperature dependence of the detector’s performance observed in the sub-kelvin range fully confirms its phase diffusion mode of operation. On the other hand, these results show that inevitable thermal fluctuations are not the main source of detector noise. Consequently, there is still room to optimize the detector’s performance. These results are important for axion search experiments in the range of 5–25 GHz (20–100 μeV).

When measuring electromagnetic radiation of frequency $f$, the most sensitive detector is the one that counts the single quanta of energy $h f$. Single photon detectors (SPDs) were demonstrated from $\gamma$-rays to infrared wavelengths, and extending this range down to the microwaves is the focus of intense research. The energy of $10\,\mathrm{GHz}$ microwave photon, about $40\,\mathrm{\mu eV}$ or $7\, \mathrm{yJ},$ is enough to force a superconducting Josephson junction into its resistive state, making it suitable to be used as a sensor. In this work, we use an underdamped Josephson junction to detect single thermal photons stochastically emitted by a microwave copper cavity at millikelvin temperatures. After characterizing the source and detector, we vary the temperature of the resonant cavity and measure the increased photon rate. The device shows an efficiency up to 40% and a dark count rate of $0.1\,\mathrm{Hz}$ in a bandwidth of several gigahertz. To confirm the thermal nature of the emitted photons we verify their super-Poissonian statistics, which is also a signature of quantum chaos. We discuss detector application in the scope of Dark Matter Axion searches, and note its importance for quantum information, metrology and fundamental physics.

A metamaterial receiver, featuring a 19-ring facet structure and intended for usage with a lens, has been successfully designed, fabricated, and tested. The presented concept uses cold-electron bolometers (CEBs) as sensing elements, which enables the design of multiabsorber receivers maintaining a balance between power load and noise. The micrometric size of the CEBs eliminates the need for microwave feed lines, facilitating integration within antenna slots. Using a YBa2Cu3O7-x (YBCO) Josephson junction oscillator, it is demonstrated that the CEB metamaterial receiver has broadband response, approaching 400 GHz, in two frequency bands: 0.15-0.55 and 0.9-1.3 THz. Additionally, the photon-noise-limited operation of the CEB metamaterial, operating at 0.3 K, is shown.

We present the analysis of soliton dynamics, current–voltage characteristics and AC power for a two-dimensional lattice of Josephson junctions using an effective alternating direction implicit (ADI) numerical scheme. The lattice is supplemented by a resistive-capacitive load (RC-load) and the bias feed is supplied from boundaries of the structure only and is redistributed between junctions via internal dynamics. At fixed length of such a lattice, its critical current have a maximum as a function of a lattice width, since not all junctions inside the lattice can be biased equally. The use of RC-load allows transferring generated AC power to a load device out of the array, but it changes the dynamics significantly. We show that regimes of solitons with plasma tails, generated due to array discreteness and leading to strong superradiant power, become unstable with matched RC-load and the corresponding zero field steps at the current–voltage characteristics disappear completely. Instead, at matched RC-load, standing wave regimes demonstrate high generation efficiency, reaching 21% of supplied dc power.

We consider properties of dichroic antenna arrays on a silicon substrate with integrated cold-electron bolometers to detect radiation at frequencies of 210 and 240 GHz. This frequency range is widely used in cosmic microwave background experiments in space, balloon, and ground-based missions such as BICEP Array, LSPE, LiteBIRD, QUBIC, Simons Observatory, and AliCPT. As a direct radiation detector, we use cold-electron bolometers, which have high sensitivity and a wide operating frequency range, as well as immunity to spurious cosmic rays. Their other advantages are the compact size of the order of a few micrometers and the effect of direct electron cooling, which can improve sensitivity in typical closed-loop cycle 3He cryostats for space applications. We study a novel concept of cold-electron bolometers with two SIN tunnel junctions and one SN contact. The amplitude–frequency characteristics measured with YBCO Josephson Junction oscillators show narrow peaks at 205 GHz for the 210 GHz array and at 225 GHz for the 240 GHz array; the separation of these two frequency bands is clearly visible. The noise equivalent power level at an operating point in the current bias mode is 5 × 10−16 W/√Hz.

Switching current distributions (SCDs), describing switching statistics from a superconducting to a resistive state, are measured for a set of aluminum-made Josephson junctions (JJ) in a temperature range from 15 mK to 1 K. The measured data are compared with existing theories and computer simulation in the frame of the second order pendulum model, with account of noise and temporal driving. Generalizing the obtained data, it is shown that the crossover temperature between the running state and the phase diffusion regime scales exponentially versus k B T / h& for JJ critical currents from 70 to 1000 nA. Besides, the quantum crossover temperature decreases with decreasing critical current. Also, the quantum floor (the SCD width) below the quantum crossover temperature is not constant, but has a finite tilt, proportional to temperature as h& + k B T , due to residual thermal activation switches.

The superconducting properties of 85 nm thick hafnium thin films with a 5 nm thick titanium layer on top have been investigated for three different geometries, that is, a film covering the entire 7 × 7 mm2 chip surface, bridges with a width of 200 μm and length up to 1800 μm, and bridges in the form of squares with sides from 100 to 1000 μm. The bridges were formed by a photolithographic lift-off process and are intended to be used as the main sensing element of a microcalorimeter based on a transition-edge sensor (TES) in experiments to determine the magnetic moment of neutrinos. Based on the measurements of the critical current, the critical temperature, and the width of the superconducting transition, we estimate the energy resolution δE of the TES prototypes, showing that it is possible to fabricate microcalorimeters with δE less than 1 eV using these films.

We consider mutual effects of soliton dynamics and noise in a chain of Josephson junctions in the framework of the Frenkel-Kontorova model. The investigations are performed by the analysis of experimentally relevant quantities such as oscillation power and spectral linewidth, which both can be derived from the power spectral density of the system as, respectively, its integral and its width. Taking into account both ohmic and surface losses together with bias inhomogeneity, and considering dense soliton chain under the effect of external magnetic field, it is demonstrated that increase of the system discreteness (decrease of coupling between neighboring elements) leads to the oscillation power reduction and spectral linewidth increase, which signals about increase of the total noise in the system. Drastic increase of the linewidth is observed when the distance between elements becomes comparable with the soliton size, which is explained by an increase of stochasticity due to reflections of solitons from the system edges and noise.

We present the results of a comparative analysis of the available atmospheric absorption data for millimeter waves on the Suffa plateau (southern Uzbekistan). It is shown that the atmospheric absorption values at wavelengths of 3 and 2 mm published in G. Bubnov, et al., EPJ Web of Conferences 195, 09002 (2018) are much smaller than the data obtained for the Suffa site according to other sources. The optical depth we calculated on the basis of the global model of the atmosphere from the NASA website also shows that the data of the above work are underestimated by a factor of two. As a result, the actual astroclimate conditions of the Suffa plateau are, in particular, much worse than the Terskol peak in Russia and Koluch-Kul site in Tajikistan. Also, in the same work, we have found an overestimation of atmospheric absorption values by about 1.5 times at a wavelength of 2 mm for another site (Karadag landfill) compared with the values we obtained during independent processing of the same data. Therefore, the data given in the above article for the Suffa plateau cannot be considered reliable, and they should not be used when choosing sites for millimeter-wave antennas.

A theoretical analysis of the efficiency of detecting photon-like pulses at frequencies of the order of 10 GHz by the Josephson junction is performed with parameters available for aluminum technology. Numerical simulation of the junction switching dynamics under the influence of a switching pulse is carried out within the framework of a linear resistive model of the Josephson junction. For comparison with simulations, the experimental data of a sample made using aluminum technology by the shadow evaporation technique are used. The times between dark counts for which the junction is sensitive to single photons are determined. Ways to increase sensitivity are considered.

The Josephson junction, as a threshold detector, is studied in the presence of noise and a weak external signal in the framework of the pendulum model. A range of parameters, efficient for weak signal detection has been found. While in the low noise limit the signal can be enhanced via dynamic resonant activation mechanism, even weaker signals can be efficiently detected with the help of noise via stochastic resonant activation mechanism. The parameter optimizations of a microwave single photon counter prototype based on aluminum Josephson junction are discussed.

Another major side of Dr. Kuzmin's life was being a permanent magnet to attract many undergraduate and graduate students to his research field.His passionate mentoring and personal touch have contributed much to their current scientific careers.Very typical of Leonid was his enthusiasm and cheerfulness in whatever he was doing.His laughter was never far away, and he was always bubbling with new ideas-not only scientific ones.He was happy to talk about his participation in the famous long-distance skiing event-the Vasaloppet.He arranged a series of symposia in the mountains of Lapland and was proud to beat again and again the world record in constructing the strongest igloo, helped by the many international participants.

Hafnium is a superconductor with a transition temperature slightly above 100 mK. This makes it attractive for such applications as microcalorimeters with high energy resolution. We report the superconducting properties of Hf films of thicknesses ranging from 60 to 115 nm, deposited on Si and Al2O3 substrates by electron beam evaporation. Besides that, we fabricated and measured combinations of hafnium with thin layers of normal metals, decreasing the critical temperature by the proximity effect. The critical temperature of the studied films varied from 56 to 302 mK. We have observed a significant change in the critical temperature of some films over time, which we propose to prevent by covering hafnium films with a thin layer of titanium.

Here, we experimentally test the applicability of an aluminium Josephson junction of a few micrometers size as a single photon counter in the microwave frequency range. We have measured the switching from the superconducting to the resistive state through the absorption of 10 GHz photons. The dependence of the switching probability on the signal power suggests that the switching is initiated by the simultaneous absorption of three and more photons, with a dark count time above 0.01 s.