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Effect of a magnetic field on the quasiparticle recombination in superconductors

Xiaoxiang Xi, J. Hwang, C. Martin, D. H. Reitze, C. J. Stanton, D. B. Tanner, and G. L. Carr
Phys. Rev. B 87, 140502(R) – Published 5 April 2013
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Abstract

Quasiparticle recombination in a superconductor with an s-wave gap is typically dominated by a phonon bottleneck effect. We have studied how a magnetic field changes this recombination process in metallic thin-film superconductors, finding that the quasiparticle recombination process is significantly slowed as the field increases. The magnetic field disrupts the time-reversal symmetry of the pairs, giving them a finite lifetime and decreasing the energy gap. The field could also polarize the quasiparticle spins, producing different populations of spin-up and spin-down quasiparticles. Both processes favor slower recombination; in our materials we conclude that strong spin-orbit scattering reduces the spin polarization, leaving the field-induced gap reduction as the dominant effect and accounting quantitatively for the observed recombination rate reduction.

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  • Received 21 November 2012

DOI:https://doi.org/10.1103/PhysRevB.87.140502

©2013 American Physical Society

Authors & Affiliations

Xiaoxiang Xi1,2, J. Hwang1,3, C. Martin1, D. H. Reitze1, C. J. Stanton1, D. B. Tanner1, and G. L. Carr2

  • 1Department of Physics, University of Florida, Gainesville, Florida 32611, USA
  • 2Photon Sciences, Brookhaven National Laboratory, Upton, New York 11973, USA
  • 3Department of Physics, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea

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Vol. 87, Iss. 14 — 1 April 2013

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Images

  • Figure 1
    Figure 1
    Experimental setup. Electrons circulate in bunches in the synchrotron storage ring, generating pulses of far-infrared radiation with a repetition frequency of 52.9 MHz. The Ti:sapphire laser produces pulses with a repetition frequency of 105.8 MHz and a pulse picker selects every other pulse to match the synchrotron pulse pattern. The selected laser pulses are delivered over a fiber optic cable to the sample and the synchrotron pulse probes the photoinduced transmission at a fixed time delay afterward. To synchronize the synchrotron and laser pulses, the 52.9 MHz bunch timing signal from a pair of electrodes inside the synchrotron ring chamber is used by the Synchro-Lock laser control system as a reference for the laser pulse emission. The pulse generator introduces an adjustable delay between the laser and synchrotron pulses. The transmitted far-infrared light is detected by a bolometer detector and recorded on a computer.Reuse & Permissions
  • Figure 2
    Figure 2
    Photoinduced transmission S(t) vs time t for Nb0.5Ti0.5N [(a) and (b)] and for NbN [(c) and (d)], all measured in parallel fields at T≤2 K. Low-fluence and high-fluence data are compared. Note the semilog scale; simple exponential decay produces a straight line.Reuse & Permissions
  • Figure 3
    Figure 3
    Effective instantaneous recombination rate vs photoinduced transmission. (a) For Nb0.5Ti0.5N, data at each field include fluences ranging from 0.4 to 10.7 nJ/cm2. (b) For NbN, data at each field include fluences ranging from 2.4 to 18.1 nJ/cm2 except for 8 T and 10 T, where data were collected at 18.1 nJ/cm2. A 4-point moving average was performed on the data to reduce noise. The lines are linear fits to the data.Reuse & Permissions
  • Figure 4
    Figure 4
    Panels (a) and (b) show the excitation gap ΩG (squares) and the pair-correlation gap Δ (triangles) for Nb0.5Ti0.5N and NbN, obtained from the optical conductivity (left scale). The solid lines are theoretical calculations of Δ and ΩG. The square root of the condensate density Nsc (proportional to the order parameter) is shown as circles (right scale). Panels (c) and (d) show the slope extracted from Fig. 3 vs ΩG from (a) and (b). The error bars in both plots are calculated deviations of the slope from the linear fit in Fig. 3. The lines are linear fits to the circles.Reuse & Permissions
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