Transient Increase of the Energy Gap of Superconducting NbN Thin Films Excited by Resonant Narrow-Band Terahertz Pulses

Beck, Matthias; Rousseau, Ian; Klammer, Maximilian; Leiderer, Paul; Mittendorff, Martin; Winnerl, Stephan; Helm, M.; Gol’tsman, G.; Demšar, Jure

doi:10.1103/physrevlett.110.267003

Cited by 76 publications

(53 citation statements)

References 42 publications

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“…Apart from an overall time-dependent phase (which is important for certain observables), these equations are equivalent to the classical spin equations of motion (1.8) and spins are related to the amplitudes as 32) where s p is the length of the spin. For quench initial conditions s p = 1/2, as explained below Eq.…”

Section: Quench Setup and Initial Conditionsmentioning

confidence: 99%

“…Later studies [17,18] mapped out the full quantum quench "phase diagram" for weakly coupled s-wave BCS superconductors finding that three distinct regimes occur depending on the strength of the quench: Volkov-and-Kogan-like behavior, persistent oscillations, and exponential vanishing of the order parameter. Most recent research [27][28][29][30] fueled by experimental breakthroughs [25,31,32] investigates nonadiabatic dynamics of s-wave BCS superconductors in response to fast electromagnetic perturbations. Closely related subjects developing in parallel are exciton dynamics [33], collective neutrino oscillations [34,35], quenched p-wave superfluids [36,37], etc.…”

Section: Introductionmentioning

confidence: 99%

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Quantum quench phase diagrams of ans-wave BCS-BEC condensate

et al. 2015

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We study the dynamic response of an s-wave BCS-BEC (atomic-molecular) condensate to detuning quenches within the two-channel model beyond the weak-coupling BCS limit. At long times after the quench, the condensate ends up in one of three main asymptotic states (nonequilibrium phases), which are qualitatively similar to those in other fermionic condensates defined by a global complex order parameter. In phase I the amplitude of the order parameter vanishes as a power law, in phase II it goes to a nonzero constant, and in phase III it oscillates persistently. We construct exact quench phase diagrams that predict the asymptotic state (including the many-body wave function) depending on the initial and final detunings and on the Feshbach resonance width. Outside of the weak-coupling regime, both the mechanism and the time dependence of the relaxation of the amplitude of the order parameter in phases I and II are modified. Also, quenches from arbitrarily weak initial to sufficiently strong final coupling do not produce persistent oscillations in contrast to the behavior in the BCS regime. The most remarkable feature of coherent condensate dynamics in various fermion superfluids is an effective reduction in the number of dynamic degrees of freedom as the evolution time goes to infinity. As a result, the long-time dynamics can be fully described in terms of just a few new collective dynamical variables governed by the same Hamiltonian only with "renormalized" parameters. Combining this feature with the integrability of the underlying (e.g., the two-channel) model, we develop and consistently present a general method that explicitly obtains the exact asymptotic state of the system.

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Section: Quench Setup and Initial Conditionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Quantum quench phase diagrams of ans-wave BCS-BEC condensate

et al. 2015

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“…However, we estimate that pair breaking due to Cooper pair acceleration by the THz electric field is not large enough to completely destroy the condensate. For instance, using a rough approximation [29], the maximum supercurrent density can be calculated as j max = (ne 2 E max )/mω. We estimate this supercurrent density to be ∼10 11 A/m 2 , which is smaller compared to the depairing [30] current density of 3 × 10 12 A/m 2 in YBCO.…”

Section: B(tt ) = R(t T )/R(t )mentioning

confidence: 99%

Enhanced coherent oscillations in the superconducting state of underdopedYBa2Cu3O6+x

et al. 2015

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We utilize intense, single-cycle terahertz pulses to induce collective excitations in the charge-density-waveordered underdoped cuprate YBa 2 Cu 3 O 6+x . These excitations manifest themselves as pronounced coherent oscillations of the optical reflectivity in the transient state, accompanied by minimal incoherent quasiparticle relaxation dynamics. The oscillations occur at frequencies consistent with soft phonon energies associated with the charge-density-wave, but vanish above the superconducting transition temperature rather than that at the charge-density-wave transition. These results indicate an intimate relationship of the terahertz excitation with the underlying charge-density-wave and the superconducting condensate itself.

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“…These quasiparticles impose a limit for detectors for astrophysics based on microwave resonators [13,14]. Related phenomena have been reported in superconductingnormal metal devices [15], terahertz pulse experiments [16], and holographic superconductivity [17].To measure the microwave response, microwave resonators were patterned into a 60 nm thick Al film, which was sputter deposited on a sapphire substrate. T c was measured to be 1.17 K, from which the energy gap at zero temperature is taken to be Δ ¼ 1.76k B T c ¼ 177 μeV.…”

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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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Transient Increase of the Energy Gap of Superconducting NbN Thin Films Excited by Resonant Narrow-Band Terahertz Pulses

Cited by 76 publications

References 42 publications

Quantum quench phase diagrams of ans-wave BCS-BEC condensate

Quantum quench phase diagrams of ans-wave BCS-BEC condensate

Enhanced coherent oscillations in the superconducting state of underdopedYBa2Cu3O6+x

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

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