Interlayer transport in the misfit-layer superconductor (LaSe)_1.14(NbSe₂)

    The interlayer transport in the superconducting state of the misfit-layer crystal (LaSe)_1.14(NbSe₂), consisting of conducting NbSe sheets separated by insulating LaSe sheets, reveals the presence of two competing channels of conduction between the layers which involve the tunnelling of quasiparticles and Cooper pairs.

Figure 4:Temperature dependence of the interlayer resistivity r_c of (LaSe)_1.14(NbSe₂).

Figure 5: a) Intralayer and b) interlayer magnetoresistive superconducting transitions at different temperatures.

    The temperature dependence of both r_c and r_ab is metallic-like between 1.5 and 300 K with a saturation below 30 K. At lower temperatures, figure 1 displays the transition to the superconducting state at zero magnetic field. The interlayer resistivity r_ab(T) has a conventional narrow transition with a midpoint at T_c = 1.23 K. The interlayer resistivity r_c(T) shows a very peculiar increase below 1.4 K by about three times and then decreases below 1.15 K to zero. Figure 5 displays the full set of our interlayer as well as interlayer magnetotransport data measured at different temperatures from 100 mK up to 1.3 K. The interlayer magnetoresistivities (Fig. 5a) show conventional transitions to the superconducting state which are shifted to higher fields and broadened as the temperature is decreased. The interlayer magnetotransport data plotted in Fig. 5b have a peak structure in the transition to the normal state. The peak position in r_c(B) is always found in the range of magnetic fields where the superconducting transition of the interlayer resistivity r_ab(B) takes place at the respective temperature. Therefore, the peak effect in the temperature as well as in magnetic field dependencies of the interlayer resistivity r_c can be related to the superconducting transition. For analogous phenomena observed in the layered high-T_c superconductors, Gray and Kim [1] proposed a model where the peak is due to an interplay of two different conductance channels in the superconducting state of the sample. The model assumes a highly anisotropic superconductor as a stack of weakly coupled internal Josephson junctions with interlayer transport by tunnelling of quasiparticles and Cooper pairs. Below the upper critical magnetic field due to the opening of the superconducting gap in the quasiparticle spectrum, r_cincreases but at a sufficiently small field the Cooper pair tunnelling channel is opened and r_c decreases to zero. Using a high-field extrapolation of r_c(B) to the normal state [2], the upper critical field B_c2(T) of the superconducting state can be deduced showing the standard behavior for a type-II superconductor.

[1] K.E. Gray and D.H. Kim, Phys. Rev. Lett. 70, 1693 (1993).
[2] P. Szabó, P. Samuely, J. Kačmarčík etal., Phys. Rev. Lett. 86, 5990 (2001).

P. Szabó, P. Samuely, J. Kačmarčík
A. G. M. Jansen (GHMFL MPI & CNRS Grenoble), A. Lafond, A.Meerschaut (Institut des Mat'eriaux Jean Rouxel, Nantes)