We study NbTiN resonators by measurements of the temperature dependent resonance frequency and frequency noise. Additionally, resonators are studied covered with SiO x dielectric layers of various thicknesses. The resonance frequency develops a nonmonotonic temperature dependence with increasing SiO x layer thickness. The increase in the noise is independent of the SiO x thickness, demonstrating that the noise is not dominantly related to the low temperature resonance frequency deviations.

We demonstrate experimentally that the critical current in superconducting NbTiN wires is dependent on their geometrical shape, due to current-crowding effects. Geometric patterns such as 90• corners and sudden expansions of wire width are shown to result in the reduction of critical currents. The results are relevant for single-photon detectors as well as parametric amplifiers.PACS numbers: 74.25.SvSuperconducting wires made of strongly disordered superconducting materials such as NbN and NbTiN are used for single-photon detection [1][2][3], single-electron detection [4] and parametric amplification [5]. In all cases, for optimal performance, the devices are biased at as high currents as possible, without exceeding the critical current. In principle, for wires smaller than both the Pearl length Λ = 2λ 2 /d (λ is the dirty London penetration depth and d is the film thickness)[6] and the dirty-limit coherence length ξ, the critical current is determined by the critical pair-breaking current, which has been theoretically calculated over the full temperature range by Kupriyanov and Lukichev[7]. These predictions have been tested experimentally in aluminium by Romijn et al. [8] and Anthore et al. [9]. In many practical cases, it is found that the critical current varies from device to device and is significantly lower than this intrinsic maximum value. This reduction is usually attributed to defects in the films and slight variations in width. In addition, for strongly disordered superconductors electronic inhomogeneity may develop, even for homogeneously disordered materials [10]. However, Clem and Berggren [11], responding to the observed dependence of the critical currents of superconducting single-photon detectors on the fill factor of the pattern [12], explained that the critical current may depend on geometric factors in the wires, such as bends. In their model analysis, the superconducting wires were narrower than the Pearl length, but wider than the coherence length. Consequently, the current is not necessarily uniform and the critical current is reached when the current density locally exceeds the critical pair-breaking current. At this current a vortex enters the superconducting wire, causing the transition to a resistive state [13]. In this letter, we present an explicit comparison of critical currents of superconducting NbTiN nanowires with different geometrical shapes and confirm that the observed critical current depends on the geometry.

Abstract-We present measurements of the frequency noise and resonance frequency temperature dependence in planar superconducting resonators on both silicon and sapphire substrates. We show, by covering the resonators with sputtered SiO layers of different thicknesses, that the temperature dependence of the resonance frequency scales linearly with thickness, whereas the observed increase in noise is independent of thickness. The frequency noise decreases when increasing the width of the coplanar waveguide in NbTiN on hydrogen passivated silicon devices, most effectively by widening the gap. We find up to an order of magnitude more noise when using sapphire instead of silicon as substrate. The complete set of data points towards the noise being strongly affected by superconductor-dielectric interfaces.

The voltage-carrying state of superconducting NbTiN nanowires, used for single-photon detectors, is analyzed. Upon lowering the current, the wire returns to the superconducting state in a steplike pattern, which differs from sample to sample. Elimination of geometrical inhomogeneities, such as sharp corners, does not remove these steplike features. They appear to be intrinsic to the material. Since the material is strongly disordered, electronic inhomogeneities are considered as a possible cause. A thermal model, taking into account random variations of the electronic properties along the wire, is used as an interpretative framework.

Antenna-coupled suspended single carbon nanotubes exposed to 108 GHz microwave radiation are shown to be selectively heated with respect to their metal contacts. This leads to an increase in the conductance as well as to the development of a power-dependent DC voltage. The increased conductance stems from the temperature dependence of tunneling into a one-dimensional electron system. The DC voltage is interpreted as a thermovoltage, due to the increased temperature of the electron liquid compared to the equilibrium temperature in the leads.The temperature response of carbon nanotubes exposed to microwave and terahertz radiation is of great interest both from a practical and a fundamental point of view. One of the many potential applications of carbon nanotubes lies in their response to far-infrared radiation. The bolometric response of either bundles or films of carbon nanotubes has been shown in the far-to mid-infrared range. 1,2 Recently the bolometric and rectifying response of a single carbon nanotube was studied at 77 K and at 4.2 K. 3 Also, detectors based on thermoelectric properties of suspended films of carbon nanotubes have been proposed for the terahertz range and have been demonstrated to work at optical frequencies. 4 The unique one-dimensional electronic states of carbon nanotubes play an important role in the description of their DC electrical response. 5 Due to the strong interactions between the electrons, the response is a collective effect of the entire electron system, as expressed in the Tomonaga-Luttinger-liquid theory. 6 In this theory, these interactions induce a reduced tunneling density of states around the Fermi energy, which leads to a power-law scaling of the conductance with bias voltage and temperature. [7][8][9][10] In studying transport through a carbon nanotube, an important consideration is the energy relaxation. In recent experimental work, indications of weak energy relaxation were reported. 11 In subsequent theoretical work, the intrinsic relation between weak energy relaxation and one-dimensional physics was emphasized. 12,13 It is to be expected that the conductivity of a carbon nanotube will also depend on this weak energy relaxation, when exposed to radiation.In previous work, carbon nanotubes in direct contact with a substrate were studied. This contact leads to interaction with charges in surface dielectrics, an increase in the scattering probability, and phonon-exchange with the substrate. 14-16 In order to avoid these potential complications and increase the thermal response, we have developed suspended carbon nanotubes in broadband antennas. We describe the realization of these suspended carbon nanotubes and their response to 108 GHz radiation. Two main effects are observed and analyzed: the