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Comparative analysis of tunneling magnetoresistance in low-Tc Nb/Al-AlOx/Nb and high-Tc Bi2−yPbySr2CaCu2O8+δ intrinsic Josephson junctions

V. M. Krasnov, H. Motzkau, T. Golod, A. Rydh, S. O. Katterwe, and A. B. Kulakov
Phys. Rev. B 84, 054516 – Published 9 August 2011
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  • INTRODUCTION
  • THEORY
  • EXPERIMENT
  • RESULTS AND DISCUSSION
  • SCALING LAWS OF TUNNELING…
  • ANALYSIS OF INTRINSIC MAGNETOTUNNELING…
  • CONCLUSIONS
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    We perform a detailed comparison of magnetotunneling in conventional low-Tc Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2−yPbySr2CaCu2O8+δ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field Hc2. For Nb an extended region of surface superconductivity at Hc2<H<Hc3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that Hc2(0) for Bi(Pb)-2212 is ≃T→Tc T and decreases significantly at T→Tc. The amplitude of subgap magnetoresistance is suppressed exponentially at T>Tc/2, but remains negative, although very small, above Tc. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at Tc.

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    • Received 5 May 2011

    DOI:https://doi.org/10.1103/PhysRevB.84.054516

    ©2011 American Physical Society

    Authors & Affiliations

    V. M. Krasnov*, H. Motzkau, T. Golod, A. Rydh, and S. O. Katterwe

    • Department of Physics, Stockholm University, AlbaNova University Center, SE-10691 Stockholm, Sweden

    A. B. Kulakov

    • Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia

    • *Vladimir.Krasnov@fysik.su.se

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    Vol. 84, Iss. 5 — 1 August 2011

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    Images

    • Figure 1
      Figure 1
      (a) A sketch of the hexagonal Abrikosov vortex lattice and the circular approximation of a unit cell. (b) Calculated spatial distribution of the order parameter between two vortices. (c) Local density of states at three points A, B, C, indicated in (a) and (b). Calculations are made for H=0.5Hc2, T=4.7 K, and Tc=8.8 K.Reuse & Permissions
    • Figure 2
      Figure 2
      Temperature dependence of tunneling I−V characteristics at zero magnetic field for (a) calculations for a BCS superconductor (Nb, Tc=8.8 K), (b) for the Nb/Al-AlOx/Nb junction, (c) for slightly overdoped Bi(Pb)-2212 mesa, and (d) for slightly underdoped Bi(Y)-2212 mesa (data from Ref. 13). (e),(f) Measured T dependence of the sum-gap voltage. Solid lines represent BCS T dependence. (g),(h) Comparison of the correlation between the subgap resistance (solid symbols) and the sum-gap conductance (open symbols) for the same Nb/Al-AlOx/Nb junction and Bi-2212 mesas, respectively.Reuse & Permissions
    • Figure 3
      Figure 3
      Magnetic-field evolution of I−V characteristics at low T for (a) BCS calculations at T≃2 K and Tc=8.8 K, (b) the Nb/Al-AlOx/Nb junction at T≃2 K. Semilogarithmic plots of (c) theoretical I−Vs at T≃0.5 K, and (d) I−Vs of a Bi(Pb)-2212 mesa in the subgap region at T≃2 K for different c-axis field components. Note a remarkable almost parallel translation of I−Vs in the semilogarithmic scale in (c) and (d).Reuse & Permissions
    • Figure 4
      Figure 4
      Magnetic-field evolution of dI/dV(V) characteristics at low T: (a) BCS calculations at T=2 K, (b) the Nb/Al-AlOx/Nb junction at T=2 K, and (c) a small Bi(Pb)-2212 mesa [normalized by dI/dV(T≳Tc)] at T=1.7 K. All plots are shown in the semilogarithmic scale. Low bias structure in (c) is caused by excitation of phonon-polariton resonances by the ac-Josephson effect (Ref. 69). Due to state conservation, the magnetoresistance is negative in the subgap region, but positive at and above the sum-gap peak. Note also an almost parallel translation of lndI/dV(V) curves with H.Reuse & Permissions
    • Figure 5
      Figure 5
      Analysis of scaling of magnetotunneling characteristics at different T as a function of H/Hc2 in theory (thick lines) and for the Nb junction (symbols). (a) Zero-bias conductance. Note that there is no scaling of dI/dV(0) for different T due to progressive development of the zero-bias logarithmic singularity with increasing T. Thin solid lines and the inset show evolution of the conductance of Nb electrodes at H>Hc2. It is seen that the surface superconductivity in electrodes survives up to Hc3≃1.69Hc2. (b) Scaling of the subgap conductance at finite bias V=Δ/e. (c) Scaling of the sum-gap resistance. (d) Scaling of the sum-gap peak voltage. (e) The upper critical field for Nb, extracted from the scaling.Reuse & Permissions
    • Figure 6
      Figure 6
      (a) Field dependence of the sum-gap resistance in Bi(Pb)-2212 at different T (normalized to the normal-state value). Dotted lines indicate extrapolations to H=Hc2. (b) Magnetic-field evolution of dI/dV(V) curves at T=75 K [normalized by dI/dV(V) at T≃Tc]. (c) Relative change of dI/dV upon rotation of the crystal in field H=10 T from the in-plane to the c-axis direction. Inset shows enhanced view of the curve at T=95 K.Reuse & Permissions
    • Figure 7
      Figure 7
      Magnetic-field dependence of the voltage at a given current for (a) theoretical calculation, current levels correspond to horizontal lines in Fig. 3c, and for Bi(Pb)-2212 mesa at (b) T=3 K and (c) T=70 K at different bias currents. Inset in (b) shows normal-state I−Vs obtained by adjustment of V−Vn so that they vanish at the same H=Hc2, as indicated by dotted lines in (b) and (c).Reuse & Permissions
    • Figure 8
      Figure 8
      Temperature dependence of the maximum magnetoresistance at a constant current, ΔV(0−H)=V(I,H=0)−V(I,H), (a) for Bi(Pb)-2212 mesas at H=17 T, (b) for the Nb/Al-AlOx/Nb junction at H≃0.44 T with the same H/Hc2(0) ratio. The inset in (a) shows the same data in the semilogarithmic scale. It is seen that the negative subgap magnetoresistance decays almost exponentially with increasing T, but retains a finite value at T>Tc. (c) Extracted upper critical field in the c-axis direction for Bi(Pb)-2212.Reuse & Permissions
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