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Two coexisting vortex phases in the peak effect regime in a superconductor

Abstract

The critical current in the vortex phase of a type-II superconductor such as NbSe2 displays a striking anomaly in the vicinity of the superconductor-to-normal-metal transition. Instead of going to zero smoothly, it rebounds to a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect1,2,3, has remained an unsolved problem for 40 years. Here we use a scanning a.c. Hall microscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter phases with intrinsically different pinning strengths coexist on a macroscopic scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects4,5,6 and anomalous voltage response4,5 commonly seen when external parameters such as temperature, magnetic field or transport current are varied. We argue that the observed phase coexistence is, in fact, the hallmark of a disorder-driven non-thermal phase transition.

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Figure 1: Imaging the peak effect.
Figure 2: Plot of ‘local’ Bac averaged over the areas I–III shown in frame 1 of Fig. 1b.
Figure 3: Plot of frame-averaged Bac.
Figure 4: The ‘phase diagram’ for the peak effect.

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Acknowledgements

We thank E. Zeldov and E. Andrei for helpful discussions.

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Correspondence to M. Marchevsky.

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Marchevsky, M., Higgins, M. & Bhattacharya, S. Two coexisting vortex phases in the peak effect regime in a superconductor. Nature 409, 591–594 (2001). https://doi.org/10.1038/35054512

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