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.

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 3 He 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.

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.

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 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.

Hafnium is a metal with rare properties, such as a low superconducting transition temperature slightly above 100 mK, which makes it attractive for a lot of applications, especially for design and fabrication of microcalorimeters with high energy resolution. In this paper we describe the results of fabrication and experimental investigation of superconducting properties of hafnium thin films in a dilution refrigerator. The considered Hf films, whose thickness is in the range from 60 to 115 nm, are deposited on substrates by electron beam evaporation. The combinations with thin layers of normal metals to decrease the critical temperature of superconducting hafnium films by proximity effect, are also considered. The critical temperature of the studied films varies from 56 to 302 mK. Also, significant increase of the critical temperature of some films over time was observed and investigated.

We have developed the fabrication technology of transition-edge sensors made of e-beam-deposited hafnium films coated with palladium to prevent oxidation and suppress their critical temperature. The measured sensors have critical temperatures of 43 –57 mK and temperature sensitivity coefficients of up to 500. The resistive state of TES near the critical temperature is fully reversible, as shown in experiments, unlike the current-initiated switching. To clarify the state of the film during the temperature change, we use the generalized TDGL model. The transition width in terms of temperature has a finite value close to experimental, even in the absence of heating terms in the model. Fabricated sensors can be used in applications that require an energy resolution below 1 eV.