Electromagnetic properties of impure superconductors with pair-breaking processes

Herman, František; Hlubina, R.

doi:10.1103/physrevb.96.014509

Cited by 23 publications

(28 citation statements)

References 34 publications

(82 reference statements)

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“…2 the superfluid fraction n s (0)/n for a series of Dynes superconductors with varying coupling strength g, but fixed scattering rates Γ and Γ tot , is plotted as a function of their transition temperature T c , making use of the explicit formula for n s (0)/n given in Ref. 6. In order to emphasize the power-law behavior, the log-log plot is used; the curve for Γ = 0 coincides with the result of Ref.…”

Section: Modified Homes' Lawmentioning

confidence: 99%

Thermodynamic properties of Dynes superconductors

Herman¹,

Hlubina²

2018

Phys. Rev. B

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The tunneling density of states in dirty superconductors is often well described by the phenomenological Dynes formula. Recently we have shown that this formula can be derived, within the coherent potential approximation, for superconductors with simultaneously present pair-conserving and pair-breaking impurity scattering. Here we demonstrate that the theory of such so-called Dynes superconductors is thermodynamically consistent. We calculate the specific heat and critical field of the Dynes superconductors, and we show that their gap parameter, specific heat, critical field, and penetration depth exhibit power-law scaling with temperature in the low-temperature limit. We also show that, in the vicinity of a coupling constant-controlled superconductor to normal metal transition, the Homes law is replaced by a different, pair-breaking dominated scaling law.

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Section: Modified Homes' Lawmentioning

confidence: 99%

Thermodynamic properties of Dynes superconductors

Herman¹,

Hlubina²

2018

Phys. Rev. B

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“…29,30 The Mattis-Bardeen equations are extended by adding a Lorentzian broadening parameter, Γ, to the superconducting density of states. 31 Physically, this parameter represents a Cooper-pair breaking scattering process caused by impurities in the film, which have the effect of modifying both the temperature dependence of the superconducting gap energy and the complex conductivity. 32 The extended Mattis-Bardeen equations fit well to the hafnium data when we also allow the zero temperature gap energy, ∆ 0 , to vary.…”

mentioning

confidence: 99%

Design and performance of hafnium optical and near-IR kinetic inductance detectors

Zobrist

Coiffard

Bumble

et al. 2019

Applied Physics Letters

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We report on the design and performance of Microwave Kinetic Inductance Detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 µΩ cm with an RRR = 1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process which resulted in pixels with decay times of about 40 µs and resolving powers of ∼9 at 800 nm.Optical and near-IR (OIR) MKIDs are superconducting sensors capable of measuring the arrival time and energy of optical to near-infrared photons. 1 They are less sensitive to false counts and radiation damage 2 than semiconductor devices operating in the same wavelength range and can achieve higher readout speeds. Moreover, each MKID is a high quality factor resonator which allows for natural frequency domain multiplexing and distinguishes the technology from other superconducting detectors. These advantages make arrays of OIR MKIDs useful as astrophysics cameras focusing on timedomain astronomy 3-5 and high contrast imaging. 6-8 To date, commissioned instruments have used either nonstoichiometric titanium nitride or platinum silicide alloys as the photon-sensitive material in the resonators and have achieved resolving powers, R = E/∆E, of up to 8 at 800 nm. 9 This resolving power has been shown to be limited equally by stationary noise, generated by two-level systems (TLS) in the device and amplifiers in the readout chain, as well as an intrinsic variance in the photon signal pulse height, likely caused by phonon escape from the superconductor to the substrate during the initial photon energy down-conversion. 10

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“…This Γ-induced Q-rise comes from the same mechanism as the Q rise due to current-induced DOS broadening [46,47]. On the other hand, finite subgap states in the vicinity of the Fermi level induce a residual dissipative conductivity lim T →0 σ 1 [2,3,21,38,48].…”

Section: Introductionmentioning

confidence: 95%

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

Kubo

2021

Preprint

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We investigate how a combination of a nonmagnetic-impurity scattering rate γ and finite subgap states parametrized by Dynes Γ affects various physical quantities relevant to to superconducting devices: kinetic inductance L k , complex conductivity σ, surface resistance Rs, quality factor Q, and depairing current density J d . All the calculations are based on the Eilenberger formalism of the BCS theory. We assume the device materials are extreme type-II s-wave superconductors. It is well known that the optimum impurity concentration (γ/∆0 ∼ 1) minimizes Rs. Here, ∆0 is the pair potential for the idealized (Γ → 0) superconductor for the temperature T → 0. We find the optimum Γ can also reduce Rs by one order of magnitude for a clean superconductor (γ/∆0 < 1) and a few tens % for a dirty superconductor (γ/∆0 > 1). Also, we find a nearly-ideal (Γ/∆0 ≪ 1) clean-limit superconductor exhibits a frequency-independent Rs for a broad range of frequency ω, which can significantly improve Q of a very compact cavity with a few tens of GHz frequency. As Γ or γ increases, the plateau disappears, and Rs obeys the ω 2 dependence. The subgap-state-induced residual surface resistance Rres is also studied, which can be detected by an SRF-grade high-Q 3D resonator. We calculate L k (γ, Γ, T ) and J d (γ, Γ, T ), which are monotonic increasing and decreasing functions of (γ, Γ, T ), respectively. Measurements of (γ, Γ) of device materials can give helpful information on engineering (γ, Γ) via materials processing, by which it would be possible to improve Q, engineer L k , and ameliorate J d .

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Electromagnetic properties of impure superconductors with pair-breaking processes

Cited by 23 publications

References 34 publications

Thermodynamic properties of Dynes superconductors

Thermodynamic properties of Dynes superconductors

Design and performance of hafnium optical and near-IR kinetic inductance detectors

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

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