Thermal propagation and stability in superconducting films

Abstract

Abstract. Thermal propagation and stable hot spots (normal domains) are studied in various high T, superconducting films (NblSn, Nb, NbN and NbjGe). The prediction of the thermal propagation velocity of the long-standing model of Broom and Rhoderick is verified quantitatively in the regime of its validity. A new energy balance model is shown to give reasonable quantitative agreement of the dependence of the propagation velocity on the length of short normal domains. The steady state (zero velocity) measurements indicate the existence of two distinct situations for films on high thermal conductivity (sapphire) substrates. For low power per unit area the film and substrate have the same temperature, and the thermal properties of the substrate dominate. However, for higher power densities in short hot spots, the coupling is relatively weak and the thermal properties of the film alone are important. Here a connection is made between the critical current stability of superconducting films and a critical hot spot size for thermal propagation. As a result efficient heat removal is shown to dominate the stabilisation of superconducting films. The strong and weak coupling situations also lead to modifications of the models for propagation velocities on sapphire substrates. These models are used to explain quantitatively self-healing of hot spots and a curious switching phenomena in a superconducting film in which a hot spot grows and collapses repeatedly over a very narrow range of current. Finally there is a discussion of the potential use of the thermal propagation model in applications of superconductors, especially switches.

Modulating Sub-THz Radiation with Current in Superconducting Metamaterial

“…This agrees with the directly measured critical current density of 3 × 10 6 A/cm 2 reported by Zhuravel et al for niobium metamaterials [31]. The transition to the normal state typically starts at a small defect in the wire and rapidly spreads until it reaches the final size, which is a function of the applied bias voltage [32,33]. The DC-resistance of niobium in the normal state changes very little with temperature below 20 K. Since the DC-resistance of the superconducting niobium is zero, we can estimate the proportion of the metamaterial in the normal state at any time by comparing its resistance with the normal-state value measured at temperature 10 K. Even for the longest ramp pulses (with peak amplitude of 38 V) no more than 20% percent of metamaterial went into normal state.…”

Integrated Joule switches for the control of current dynamics in parallel superconducting strips

“…This means that the resistive region is expanding beyond the constriction edge in the thermal buffer, toward the strips, increasing the local temperature and degrading its I C,S (see inset figure 1(b)), according to the model of the selfheating hotspot propagating in a superconducting short bridge described in [21] for l/η≈1 and w/η<0.1-0.2. In the formula, the thermal healing length is equal to η»1 μm and it was obtained by estimating the thermal conductivity of the Nb thin film K∼0.0056 W cm −1 K −1 from the Weidmann-Franz law and using a value of the coefficient heat transfer per unit are to the substrate α∼2 W cm −2 K −1 [21,22]. The resistive region would extend outside the constriction over a length of several thermal healing length η [21].…”

Current-biased superconducting strips as position-sensitive particle detectors

“…Stuivinga et al [21,22] showed wd tm pt. =·&C®(T·7L<I.,t..))· I t T Sl and describing the power-dissipation density by ®() x 2 (12 ) O for xS0 1 for x > 0 region of the strip. This can be modelled by introducing a Heaviside step-function:…”

“…Because this [9], and W [10,11] strips, for monitoring 6keV X·rays. Gray et al [12] suggested that [2,3,5,6], and Al [7] strips. Recently Gabutti et al demonstrated the feasibility of granular Nb [8], Al suitable detection of 5.5MeV-0r-particles has previously been demonstrated with In [2,3,4], Sn realisation of radiation-hard vertex detectors for applications in future high-luminosity colliders.…”