Acoustic matching of superconducting films to substrates

Kaplan, Steven B.

doi:10.1007/bf00119193

Cited by 210 publications

(123 citation statements)

References 27 publications

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“…The phonon escape time τ e = 8.96 ns = 34τ φ was set to maintain power balance for the absorbed optical power for ≈ 15%. This value is about an order of magnitude larger than the equivalent value used by Goldie and Withington 31 and de Visser et al 13 , however it is within the range of reported values for τ e for Al on sapphire 37 . We found that relatively large changes in τ e resulted in relatively small changes in σ 1 and σ 2 .…”

Section: Quality Factor and Frequencysupporting

confidence: 54%

“…Here τ e is the escape time for phonons to leave the superconductor and go into the substrate 37 . We use a version of the Parker heating model 35 and set (4) Although n opt takes the form of a Bose-Einstein thermal distribution with effective temperature T eff for Ω > 2∆, it is not a thermal distribution since n opt = 0 for Ω < 2∆.…”

Section: Loss From Nonequilibrium Distribution Of Quasiparticlesmentioning

confidence: 99%

“…We assume the ballistic phonon limit where τ e is proportional to thickness 37 and assume the volume of Al forming the cavity is proportional to the illuminated area. With these assumptions, Eq.…”

Section: Quality Factor and Frequency Shift At Higher Temperaturesmentioning

confidence: 99%

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Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

et al. 2016

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We have illuminated a thin-film superconducting Al lumped-element microwave resonator with 780 nm light and observed the resonator quality factor and resonance frequency as a function of illumination and microwave power in the 20 to 300 mK temperature range. The optically-induced microwave loss increases with increasing illumination but decreases with increasing microwave power. Although this behavior may suggest the presence of optically activated two-level systems, we find that the loss is better explained by the presence of nonequilibrium quasiparticles generated by the illumination and excited by the microwave drive. We model the system by assuming that the illumination creates an effective source of phonons with energy higher than double the superconducting gap and solve the coupled quasiparticle-phonon rate equations. We fit the simulation results to our measurements and find good agreement with the observed dependence of the resonator quality factor and frequency shift on temperature, microwave power, and optical illumination. Examination of the model reveals approaches to reducing optically-induced loss and improving the relaxation time of superconducting quantum devices.

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Section: Quality Factor and Frequencysupporting

confidence: 54%

Section: Loss From Nonequilibrium Distribution Of Quasiparticlesmentioning

confidence: 99%

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Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

et al. 2016

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“…For our 80-nm bi-layer Al film on sapphire, the best estimate of F is in the range of 5-10 considering the strong acoustic mismatch at the interface 40 . The reported values of r for Al in the literature range from 1/(120 ns) to 1/(8 ns) 14,18,19,28,41 .…”

Section: Qp Recombination Constant Across Seven Devices (Supplementarymentioning

confidence: 99%

Measurement and control of quasiparticle dynamics in a superconducting qubit

et al. 2014

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Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been taken to completely shield these circuits from external magnetic fields to protect the integrity of the superconductivity. Here we show vortices can improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we quantitatively distinguish between recombination and trapping mechanisms in controlling the dynamics of residual quasiparticle, and show quantized changes in quasiparticle trapping rate because of individual vortices. These results highlight the prominent role of quasiparticle trapping in future development of superconducting qubits, and provide a powerful characterization tool along the way.

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“…The phonon distribution in the substrate n sub ðΩÞ is assumed to have a Bose-Einstein distribution at the bath temperature T bath . τ esc ¼ 0.17 ns is the phonon escape time, calculated for Al on sapphire using the acoustic mismatch model [26]. N ion is the number of ions per unit volume and DðΩÞ ¼ 3Ω 2 =Ω 3 D is the phonon density of states.…”

mentioning

confidence: 99%

Evidence of a Nonequilibrium Distribution of Quasiparticles in the Microwave Response of a Superconducting Aluminum Resonator

et al. 2014

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In a superconductor, absorption of photons with an energy below the superconducting gap leads to redistribution of quasiparticles over energy and thus induces a strong nonequilibrium quasiparticle energy distribution. We have measured the electrodynamic response, quality factor, and resonant frequency of a superconducting aluminium microwave resonator as a function of microwave power and temperature. Below 200 mK, both the quality factor and resonant frequency decrease with increasing microwave power, consistent with the creation of excess quasiparticles due to microwave absorption. Counterintuitively, above 200 mK, the quality factor and resonant frequency increase with increasing power. We demonstrate that the effect can only be understood by a nonthermal quasiparticle distribution. DOI: 10.1103/PhysRevLett.112.047004 PACS numbers: 74.25.nn, 07.57.Kp, 74.40.Gh, 74.78.-w A superconductor can be characterized by the density of states, which exhibits an energy gap due to Cooper pair formation, and the distribution function of the electrons, which in thermal equilibrium is the Fermi-Dirac distribution. When a superconductor is driven by an electromagnetic field, nonlinear effects in the electrodynamic response can occur, which are usually assumed to be due to a change in the density of states, the so-called pair-breaking mechanism [1]. These nonlinear effects can be described along the lines of a current dependent superfluid density n s ðT; jÞ ∝ n s ðTÞ½1 − ðj=j c Þ 2 , where j is the actual current density, j c the critical current density, and T the temperature. Observations such as the nonlinear Meissner effect [2] and nonlinear microwave conductivity [3,4] can be explained by a broadening of the density of states and a decreased n s . The quasiparticles are assumed to be in thermal equilibrium and a Fermi-Dirac distribution fðEÞ ¼ 1=½expðE=k B TÞ þ 1 is assumed, with E the quasiparticle energy and k B Boltzmann's constant.Here we demonstrate that a microwave field also has a strong effect on fðEÞ in the superconductor, and induces a nonlinear response. We present measurements of the electrodynamic response, quality factor, and resonant frequency of an Al superconducting resonator (at 5.3 GHz) as a function of temperature and microwave power at low temperatures T c =18 < T < T c =3. The response measurements, complemented with quasiparticle recombination time measurements, are explained consistently by a model based on a microwave-induced nonequilibrium fðEÞ. Redistribution of quasiparticles [5,6] due to microwave absorption [7] has been shown earlier to cause enhancement of the critical current [8], the critical temperature (T c ), and the energy gap [9]. These enhancement effects are most pronounced close to T c and were observed for temperatures T > 0.8T c . A representation of gap suppression and gap enhancement is shown in the inset to Fig. 1(b) [8]. The consequences of the redistribution of quasiparticles for the electrodynamic response were only studied theoretically for T > 0.5T c [10]. Redistributi...

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Acoustic matching of superconducting films to substrates

Cited by 210 publications

References 27 publications

Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

Measurement and control of quasiparticle dynamics in a superconducting qubit

Evidence of a Nonequilibrium Distribution of Quasiparticles in the Microwave Response of a Superconducting Aluminum Resonator

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