Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device.

We have successfully fabricated Cu/AlOx-Al/Nb normal-metal/insulator/superconductor tunnel junction devices with a high value of the superconducting gap (up to ∼ 1 mV), using electron-beam lithography and angle evaporation techniques in the sub-micron scale. The subgap conductance of these junctions shows the expected strong temperature dependence, rendering them suitable for thermometry all the way from 100 mK to 6 K. In addition, some direct electronic cooling of the normal metal was also seen at bias values near the gap edge. The device performance was strongly influenced by the details of the Al layer geometry, with lateral spilling of the aluminium giving rise to strong extra subgap features, and the thickness of Al layer affecting the proximised superconducting gap value of the superconducting Al/Nb bilayer.Normal-metal-insulator-superconductor (NIS) tunnel junction devices have already proven as promising solid state coolers 1-4 , accurate low temperature thermometers and sensitive bolometers 2,5,6 , and near-ideal single electron transistors 7,8 . Some active research on such devices is aimed at enhancing the cooling power at sub-300 mK temperatures 9 , but increasing the operational range in temperature by changing the superconducting material (gap ∆) from the traditional aluminum has not been very successful yet, although a promising new alternative is to use SIS' structures, instead 10 .To extend the range of NIS devices beyond the operational range of Al, which has an upper limit for thermometry at T C ∼1.5 K and the maximum cooling power at T ∼ 300 mK, one should consider the elemental metal with highest gap, niobium (T C ∼ 9 K). It is already routinely used for SIS tunnel junction applications in SQUIDs, radiation detectors and digital electronics 11 . In those applications, fairly large micron-scale junctions can typically be tolerated, and fabrication usually proceeds by the robust Nb/Al/AlOx/Nb trilayer deposition and etching techniques 12,13 , where Nb and Al are typically sputter deposited, and the high quality AlOx barrier is thermally grown on a thin < 10 nm Al layer. This process typically yields a high gap ∆ ∼ 1.3 mV and a near bulk T C ∼ 9 K 14,15 .In contrast, for single-charge devices and for small thermometers and bolometers, sub-micron scale junctions are desired. They are easier to fabricate using angle-evaporation and lift-off 16 , although a successful but more complex sub-micron Nb trilayer Josephson junction process has also been demonstrated15 . The quality of evaporated Nb, unfortunately, has turned out to be quite sensitive to the exact chamber and substrate conditions, such as vacuum level, evaporation speed, substrate to Nb crucible distance and especially to the type of resist used [17][18][19][20][21][22] . Problems with standard polymer resists are typically attributed to decomposition and outgassing during Nb evaporation, leading to the suppression of T C and ∆.Here, we demonstrate that a fairly simple angleevaporation process can, nevertheless, be used to fabricate good...

Combined voltage and magneto-optical study of magnetic flux flow in superconducting NbN films is reported. The nanosecond-scale voltage pulses appearing during thermomagnetic avalanches have been recorded in films partially coated by a metal layer. Simultaneous magneto-optical imaging and voltage measurements allowed the pulses to be associated with individual flux branches penetrating the superconductor below the metal coating. From detailed characteristics of pulse and flux branches, the electrical field in the superconductor is found to be in the range of 5-50 kV/m, while the propagation speed of the avalanche during its final stage is found to be close to 5 km/s.

Experimental evidence of wave properties of dendritic flux avalanches in superconducting films is reported. Using magneto-optical imaging the propagation of dendrites across boundaries between a bare NbN film and areas coated by a Cu-layer was visualized, and it was found that the propagation is refracted in full quantitative agreement with Snell's law. For the studied film of 170 nm thickness and a 0.9 µm thick metal layer, the refractive index was close to n = 1.4. The origin of the refraction is believed to be caused by the dendrites propagating as an electromagnetic shock wave, similar to damped modes considered previously for normal metals. The analogy is justified by the large dissipation during the avalanches raising the local temperature significantly. Additional time-resolved measurements of voltage pulses generated by segments of the dendrites traversing an electrode confirm the consistency of the adapted physical picture.

We report the development of superconducting tantalum nitride (TaNx) normal metal-insulator-superconductor (NIS) tunnel junctions. For the insulating barrier, we used both AlOx and TaOx (Cu-AlOx-Al-TaNx and Cu-TaOx-TaNx), with both devices exhibiting temperature dependent current-voltage characteristics which follow the simple one-particle tunneling model. The superconducting gap follows a BCS type temperature dependence, rendering these devices suitable for sensitive thermometry and bolometry from the superconducting transition temperature TC of the TaNx film at ∼5 K down to ∼0.5 K. Numerical simulations were also performed to predict how junction parameters should be tuned to achieve electronic cooling at temperatures above 1 K.

Superconducting titanium nitride (TiN) thin films were deposited on magnesium oxide, sapphire and silicon nitride substrates at 700 • C, using a pulsed laser deposition (PLD) technique, where infrared (1064 nm) pulses from a solid-state laser were used for the ablation from titanium target in a nitrogen atmosphere. Structural studies performed with X-ray diffraction showed the best epitaxial crystallinity for films deposited on MgO. In the best films, superconducting transition temperatures, T C , as high as 4.8 K were observed, higher than in most previous superconducting TiN thin films deposited with reactive sputtering. A room temperature resistivity down to ∼ 17µΩcm and residual resistivity ratio (RRR) up to 3 were observed in the best films, approaching reported single crystal film values, demonstrating that PLD is a good alternative to reactive sputtering for superconducting TiN film deposition. For less than ideal samples, the suppression of the film properties were correlated mostly with the unintended incorporation of oxygen (5-10 at. %) in the film, and for high oxygen content films, vacuum annealing was also shown to increase the T C. On the other hand, superconducting properties were surprisingly insensitive to the nitrogen content, with high quality films achieved even in the highly nitrogen-rich, Ti:N = 40/60 limit. Measures to limit oxygen exposure during deposition must be taken to guarantee the best superconducting film properties, a fact that needs to be taken into account with other deposition methods, as well.

We have successfully fabricated Cu-AlOx-Al-NbN normal metal-insulator-superconductor (NIS) tunnel junction devices, using pulsed laser deposition (PLD) for NbN film growth, and electronbeam lithography and shadow evaporation for the final device fabrication. The subgap conductance of these devices exhibit a strong temperature dependence, rendering them suitable for thermometry from ∼ 0.1 K all the way up to the superconducting transition temperature of the NbN layer, which was here ∼ 11 K, but could be extended up to ∼ 16 K in our PLD chamber. Our data fits well to the single particle NIS tunnel junction theory, with an observed proximised superconducting gap value ∼ 1 meV for a 40 nm thick Al overlayer. Although this high value of the superconducting energy gap is promising for potential electronic NIS cooling applications as well, the high value of the tunneling resistance inhibits electronic cooling in the present devices. Such opaque barriers are, however, ideal for thermometry purposes as self-induced thermal effects are thus minimized.

Modulation of charge carrier dynamics and hence electrical conductivity of solids by photoexcitation has been a rich field of research with numerous applications. Similarly, electric and magnetic field assisted enhancement of conductivity are of fundamental importance and technological use.