Josephson vortex lattice melting in Bi-2212
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Josephson Vortex Lattice Melting in Bi-2212 Yu. I. Latyshev, V. N. Pavlenko*, and A. P. Orlov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia * e-mail: [email protected] Abstract—The B−T diagram of Josephson vortex lattice melting in Bi-2212 is analyzed (B is magnetic induction parallel to the layers, T is temperature). It is shown that the Josephson vortex lattice melting at B > B* = 0.6–0.7 T is associated with Berezinsky–Kosterlitz–Thouless transition in individual Bi-2212 superconducting layers and is a second-order phase transition. PACS numbers: 74.25.Qt, 74.25.Dw, 74.50.+r, 74.72.Hs DOI: 10.1134/S1063776107070515
In a recent study [1], a method was found for identifying a triangular lattice of Josephson vortices from observation of Josephson vortex flow resistance oscillations due to commensurability between the period of a moving vortex lattice and the size of the underlying layered structure. We used this phenomenon to examine Josephson vortex lattice melting [2], assuming that the vortex flow resistance oscillations vanish with increasing temperature because of breakdown of long-range order in a Josephson vortex system. The experiment was conducted on overdoped Bi2212 layered structures (mesas) (see Fig. 1). The structures were stacks of size La × Lb = (15–30) × (3–5) µm2 consisting of approximately 100 junctions (see Fig. 1). The structures were fabricated from Bi-2212 whiskers by double-sided focused-ion-beam etching [3]. The samples were aligned so that the applied magnetic field was parallel to the ab planes and perpendicular to the a axis (the long side of the sample). Parallel alignment to an accuracy of approximately 0.01° was achieved by precise adjustment of the angle of sample rotation about the a axis to a sharp peak of the Josephson vortex flow resistance with the use of an additional coil generating a perpendicular field. Data acquisition was performed by means of a current source and a nanovoltmeter, which were controlled by a computer. We measured the oscillating Josephson vortex flow resistance of the mesa as a function of the parallel magnetic field at several constant temperatures, with a small temperature step (see Fig. 2). The oscillation period was exactly corresponding to half a flux quantum per junction, ∆B = 0.5Φ0/Ls, where L is the structure size in the direction perpendicular to the applied magnetic field and the separation between neighboring layers, s = 1.5 nm, corresponds to a triangular lattice of Josephson vortices. The oscillation amplitude decreased with increasing temperature and vanished at a temperature T0 that was 3.5 K below Tc. It is clear from Fig. 2 that oscillations are observed at constant temperature over a
certain interval of magnetic induction. The endpoints of the intervals, marked in the figure, determine the upper and lower boundaries of the triangular-lattice state in the B−T diagram of Fig. 3. The point in the diagram where the upper and lower boundaries meet at T = T0 corresponds to B = 0.6–0.7 T.
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