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We demonstrate a direct writing method for the fabrication of planar Josephson junctions from high quality superconducting niobium titanium nitride (NbTiN) thin films, by creating local disorder using focused He-ion beam irradiation in a helium ion microscope. We show that we can control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, enabling us to successfully fabricate Josephson junctions with highly tunable weak links ranging from metallic to insulating phase, due to the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both SNS and SIS type of devices, and show that we can achieve exceptionally wide range ($\sim$5 orders of magnitude) of critical current densities and junction resistances. The SNS type junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages up to $\sim$ 1.5\,mV and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that junctions fabricated with this method from NbTiN are suitable for a wide range of applications in superconducting electronics because of the excellent mechanical, electrical and microwave properties of NbTiN. In particular, as NbTiN has previously been used to fabricate high quality factor microwave resonators, we see the method as a promising and simple way to realize superconducting qubits and other quantum devices using only a single superconducting film., Comment: Supplementary infmormation included

In contradictin to the common concept that acoustic phonons can only travel inside a material medium, they can in fact "tunnel" across a vacuum gap with the help of piezoelectricity, transmitting a significantly stronger heat flux than that of blackbody radiation. Here, we present a theoretical formulation for the heat flux of such piezoelectrically mediated heat transfer, applicable to any anisotropic piezoelectric crystals with an arbitrary orientation. A few numerical results are demonstrated and compared to heat transfer driven by other close-range mechanisms, including near-field radiative heat transfer and other acoustic phonon tunneling mechanisms. We find that piezoelectrically mediated heat transfer has a significant effect when the vacuum gap size is smaller than the phonon characteristic thermal wavelength, and its heat flux can dominate heat transfer between piezoelectric solids over all other known heat transfer mechanisms at temperatures below 50 K.

We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on the fabrication and characterization of Al/EuS/Cu and EuS/Al/Co-based detectors, along with their noise analysis. Additionally, we suggest some approaches for multiplexing such self-biased detectors., Comment: 30 pages, 21 figures

When two piezoelectric solids are placed in close proximity, acoustic waves (phonons) can "tunnel" across a vacuum gap transmitting energy between the two solids. Here, we demonstrate analytically that not only is such a phenomenon possible, but that a simple resonance condition exists for which complete transmission of the incoming wave is possible, physically corresponding to the excitation of leaky surface waves. This result is derived for an arbitrary anisotropic crystal symmetry and orientation. We also show that the complete transmission condition can be related to the surface electric impedance and the effective surface permittivity of the piezoelectric material, making it possible to be determined experimentally. In addition, we present numerical results for the maximum power transmittance of a slow transverse wave, tunneling between identical ZnO crystals, as function of all possible crystal orientations. The results show a large range of orientations for which complete tunneling can be achieved., Comment: 6 pages, 3 figures

Specific heat of phonon cavities is investigated in order to analyse the effect of phonon confinement on thermodynamic properties. The specific heat of free standing very thin SiN membranes in the low dimensional limit is measured down to very low temperatures (from 6~K to 50~mK). In the whole temperature range, we measured an excess of specific heat orders of magnitude bigger than the typical value observed in amorphous solids. Below 1~K, a cross-over in $c_p$ to a lower power law is seen, and the value of specific heat of thinner membranes becomes larger than that of thicker ones demonstrating a significant contribution coming from the surface. We show that this high value of the specific heat cannot be explained by the sole contribution of 2D phonon modes (Lamb waves). The excess specific heat, being thickness dependent, could come from tunneling two level systems (TLS) that form in low density regions of amorphous solids located on the surfaces. We also show that the specific heat is strongly tuned by the internal stress of the membrane by orders of magnitude, giving unprecedentedly high values, making low stress SiN very efficient for energy storage at very low temperature.

The superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductorferromagnet tunnel junctions. We demonstrate analytical models and solutions in the time domain for a SFTED operated as a microcalorimeter (pulse excitation), in the linear smallsignal limit. Based on these solutions, the signal current and temperature pulse response were studied for two different electrical circuit models, providing design conditions for stable and non-oscillatory response.

It is not widely appreciated that an acoustic wave can "jump" or "tunnel" across a vacuum gap between two piezoelectric solids, nor has the general case been formulated or studied in detail. Here, we remedy that situation, by presenting a general formalism and approach to study such an acoustic tunneling effect between two arbitrarily oriented anisotropic piezoelectric semi-infinite crystals. The approach allows one to solve for the reflection and transmission coefficients of all the partial wave modes, and is amenable to practical numerical or even analytical implementation, as we demonstrate by a few chosen examples. The formalism can be used in the future for quantitative studies of the tunneling effect in connection not only with the manipulation of acoustic waves, but with many other areas of physics of vibrations such as heat transport, for example.

Controlling thermal transport at the nanoscale is vital for many applications. Previously, it has been shown that this control can be achieved with periodically nanostructured two-dimensional phononic crystals, for the case of suspended devices. Here we show that thermal conductance can also be controlled with three-dimensional phononic crystals, allowing the engineering of the thermal contact of more varied devices without the need of suspension in the future. We show experimental results measured at sub-Kelvin temperatures for two different period three-dimensional crystals, as well as for a bulk control structure. The results show that the conductance can be enhanced with the phononic crystal structures in our geometry. This result cannot be fully explained by the simplest theory taking into account the coherent modification of the phonon band structure, calculated with finite element method simulations.

Superconducting junctions are widely used in multitude of applications ranging from quantum information science and sensing to solid-state cooling. Traditionally, such devices must be fabricated on flat substrates using standard lithographic techniques. In this study, we demonstrate a highly versatile method that allows for superconducting junctions to be fabricated on a more complex topography. It is based on maskless direct laser writing (DLW) two-photon lithography, which allows writing in 3D space. We show that high-quality normal metal-insulator-superconductor (NIS) tunnel junctions can be fabricated on top of a 20 $\mu$m tall three-dimensional topography. Combined with more advanced resist coating methods, this technique could allow sub-micron device fabrication on almost any type of topography in the future.

A nanoscale phononic crystal filter (reflector) is designed for a kinetic inductance detector where the reflection band is matched to the quasiparticle recombination phonons with the aim to increase quasiparticle lifetime in the superconducting resonator. The inductor is enclosed by a 1 um wide phononic crystal membrane section with two simple hole patterns that each contain a partial spectral gap for various high frequency phonon modes. The phononic crystal is narrow enough for low frequency thermal phonons to propagate unimpeded. With 3D phonon scattering simulations over a 40 dB attenuation in transmitted power is found for the crystal, which was previously estimated to give a lifetime enhancement of nearly two orders of magnitude., Comment: 7 pages, 3 figures, 18th International Workshop on Low Temperature Detectors (Milan, 2019)

We discuss a robust method to simultaneously fit a complex model both to the complex impedance and the noise data for transition-edge sensors (TES). It is based on a differential evolution (DE) algorithm, providing accurate and repeatable results with only a small increase in computational cost compared to the standard least squares (LS) fitting method. Test fits are made using both DE and LS methods, and the results compared with previously determined best fits, with varying initial value deviations and limit ranges for the parameters. The robustness of DE is demonstrated with successful fits even when parameter limits up to a factor of 5 from the known values were used. It is shown that the least squares fitting becomes unreliable beyond a 10% deviation from the known values.

Superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductor-ferromagnet tunnel junctions. This type of detector can be operated without the need of additional bias lines, and is predicted to provide a performance rivaling transition-edge sensors and kinetic inductance detectors. Here we report our numerical studies on the SFTED noise equivalent power, energy resolution and time constant, and the feasibility of a SQUID readout in both bolometric and calorimetric regimes, with the goal to provide practical design parameters for the detector fabrication and the readout circuitry implementation., Comment: 6 pages, 4 figures, conference proceeding

Periodic hole array phononic crystals (PnC) can strongly modify the phonon dispersion relations, and have been shown to influence thermal conductance coherently, especially at low temperatures where scattering is suppressed. One very important parameter influencing this effect is the period of the structure. Here, we measured the sub-Kelvin thermal conductance of nanofabricated PnCs with identical hole filling factors, but three different periodicities, 4, 8, and 16 $\mu$m, using superconducting tunnel junction thermometry. We found that all the measured samples can suppress thermal conductance by an order of magnitude, and have a lower thermal conductance than the previously measured smaller period, 1 $\mu$m and 2.4 $\mu$m structures. The 8 $\mu$m period PnC gives the lowest thermal conductance of all the samples above, and has the lowest specific conductance/unit heater length observed to date in PnCs. In contrast, coherent transport theory predicts that the longest period should have the lowest thermal conductance. Comparison to incoherent simulations suggests that diffusive boundary scattering is likely the mechanism behind the partial breakdown of the coherent theory., Comment: Supplement included

We present a systematic study of the magnetic domain wall induced modulation of superconducting transition temperature (Tc) in Nb/Ni bilayer stripes. By varying the thickness of the Ni layer from 20 nm to 100 nm we have been able to measure the low field Tc-H phase diagram spanning the Neel domain wall and Bloch domain wall range of thicknesses. Micromagnetic simulations and magnetic force microscopy measurements confirmed a stronger out-of-plane stray field in the Bloch domain walls compared to the Neel walls. A suppression in Tc was observed in the magnetization reversal region of the Ni film, the magnitude of which followed linearly to the strength of the out-of-plane stray field due to the domain walls. The magnitude of the stray field was quantified by comparing the Tc of the suppressed region of H-Tc phase diagrams with the unaffected part of the H-Tc curve. With Bloch domain walls a change in Tc of more than 60 mK was observed which is much more compared to the earlier reports. We believe that the narrow stripe geometry of the bilayers and the transverse external field maximized the effect of the domain walls in the Ni layer on the overlying superconducting film, leading to a larger change in Tc. This observation may be useful for domain wall controlled switching devices in superconducting spintronics., Comment: 13 pages, 4 figures

We demonstrate an original method -- based on controlled oxidation -- to create high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires. We show clean tunnel characteristics with a current suppression by over $4$ orders of magnitude for a junction bias well below the Al gap $\Delta_0 \approx 200\,\mu {\rm eV}$. The experimental data are in close agreement with the BCS theoretical expectations of a superconducting tunnel junction. The studied devices combine small-scale tunnel contacts working as thermometers as well as larger electrodes that provide a proof-of-principle active {\em cooling} of the electron distribution in the nanowire. A peak refrigeration of about $\delta T = 10\,{\rm mK}$ is achieved at a bath temperature $T_{bath}\approx250-350\,{\rm mK}$ in our prototype devices. This method opens important perspectives for the investigation of thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration., Comment: 6 pages, 4 color figures

We have developed a technique to fabricate sub-micron, 0.6 um x 0.6 um Al-AlOx-Nb tunnel junctions using a standard e-beam resist, angle evaporation and double oxidation of the tunneling barrier, resulting in high quality niobium, as determined by the the high measured values of the critical temperature TC 7.5 K and the gap 1.3 meV. The devices show great promise for local nanoscale thermometry in the temperature range 1 - 7.5 K. Electrical characterization of the junctions was performed at sub-Kelvin temperatures both with and without an external magnetic field, which was used to suppress superconductivity in Al and thus bring the junction into a normal-metal-insulator-superconductor (NIS) configuration. We observed excess sub-gap current, which could not be explained by the standard tunneling theory. Evidence points towards materials science issues of the barrier or Nb/AlOx interface as the culprit.

In a previous publication, we discussed the formalism and some computational results for phononic thermal conduction in the suspended membrane geometry for radial heat flow from a central source, which is a common geometry for some low-temperature detectors, for example. We studied the case where only diffusive surface scattering is present, the so called Casimir limit, which can be experimentally relevant at temperatures below $\sim$ 10 K in typical materials, and even higher for ultrathin samples. Here, we extend our studies to much thinner membranes, obtaining numerical results for geometries which are more typical in experiments. In addition, we interpret the results in terms of a small signal and differential thermal conductance, so that guidelines for designing devices, such as low-temperature bolometric detectors, are more easily obtained. Scaling with membrane dimensions is shown to differ significantly from the bulk scattering, and, in particular, thinning the membrane is shown to lead to a much stronger reduction in thermal conductance than what one would envision from the simplest bulk formulas., Comment: submitted to AIP Advances

We have statistically characterized the self-assembly of multi-layer polystyrene colloidal crystals, using the technique of vertical deposition, with parameters chosen to produce thick layers of self-assembled crystals in one deposition step. In addition, using a lithographically directed self-assembly method, we have shown that the size of multi-layer, continuous crack-free domains in lithographically defined areas can be many times larger than in the surrounding areas. In a single deposition step, we have produced continuous colloidal crystal films of 260 nm diameter polystyrene spheres approximately 30 -- 40 layers thick, with a controllable lateral size of 80 -- 100 $\upmu$m without lithography, and as high as 250 $\upmu$m with the lithographic template.

We report the successful fabrication of superconducting tantalum nitride (TaN) thin films using a pulsed laser deposition technique with 1064 nm radiation. Films with thickness $ \sim $ 100 nm deposited on MgO (100) single crystals and on oxidized silicon (SiO$_{2} $) substrates exhibited a superconducting transition temperature of $\sim $ 8 K and 6 K, respectively. The topography of these films were investigated using atomic force and scanning electron microscopy, revealing fairly large area particulate free and smooth surfaces, while the structure of the films were investigated using standard $ \theta -2 \theta $ and glancing angle X-ray diffraction techniques. For films grown on MgO a face-centered cubic phase of TaN was observed, while films grown on SiO$_{2} $ exhibited the face-centered cubic and as well as a mononitride hexagonal phase. The transition temperature of the TaN deposited on SiO$_{2} $ was found to be more sensitive to the nitrogen pressure during deposition as compared to the TaN deposited on MgO.

The responsivity and noise of a voltage-biased superconducting transition-edge sensor depends strongly on the details of its thermal model, and the simplest theory for TES response assumes a single heat capacity connected to the heat bath. Here, analytical results are derived and discussed for the complex impedance, the responsivity and the noise of a transition-edge sensor, when the thermal model is not simple but consists of either two or three connected heat capacities. The implications of the differences of the models are discussed, as well., Comment: 33 p. 12 fig version 2, final, AIP Advances, in press (2012)

We have studied the physics of transition-edge sensor (TES) devices with an insulating AlOx layer on top of the device to allow implementation of more complex detector geometries. By comparing devices with and without the insulating film, we have observed significant additional noise apparently caused by the insulator layer. In addition, AlOx was found to be a relatively good thermal conductor. This adds an unforeseen internal thermal feature to the system., Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conference

We discuss computational analysis of phononic thermal conduction in the suspended membrane geometry, in the experimentally commonly appearing case where heat can flow out radially in two dimensions from a central source. As we are mostly interested in the low-temperature behavior where bulk scattering of phonons becomes irrelevant, we study the limit where all phonon scattering takes place at the membrane surfaces. Moreover, we limit the discussion here to the case where this surface scattering is fully diffusive, the so called Casimir limit. Our analysis shows that in the two-dimensional case, no analytic results are available, in contrast to the well known 1D Casimir limit. Numerical solutions are presented for the temperature profiles in the membrane radial direction, for several different membrane thicknesses. Our results can be applied, for example, in the design of membrane-supported bolometric radiation detectors.

We report that annealing Al-AlOx-Al tunnel junctions in a vacuum chamber at temperature of 400C reduces the characteristic 1/f noise in the junctions, in some cases by an order of magnitude. Both ultra high vacuum and high vacuum fabricated samples demonstrated a significant reduction in the 1/f noise level. Temperature dependence of the noise was studied between 4.2 and 340 Kelvin, with a linear dependence below 100 K, but a faster increase above. The results are consistent with a model where the density of charge trapping two level-systems within the tunneling barrier is reduced by the annealing process.

Kinetic inductance detectors (KID) have received increased interest due to their relative ease of fabrication, low noise, and scalability to large format arrays required by next generation of astronomical telescopes. The development of KIDs has progressed rapidly, with very low noise equivalent power demonstrated by several groups and KIDs arrays implemented in several ground-based and air-borne instruments. In this paper, we describe a new fabrication process which consists of a membrane isolated KID incorporating a phononic bandgap structure tuned to block recombination phonons from escaping to the thermal bath. This architecture is designed to increase the quasi-particle lifetime and results in increased responsivity to signal photons and lower noise. These devices have been fabricated as lumped-element resonators with hafnium inductors and Nb capacitors on low stress silicon nitride and silicon-on-insulator membranes. We discuss the fabrication process, which uses a combination of sub-micron laser based direct write lithography and nanoscale electron beam lithography.