Influence of the spatially inhomogeneous gap distribution on the quasiparticle current inc-axis junctions involvingd-wave superconductors with charge density waves

Abstract

The quasiparticle tunnel current J(V) between the superconducting ab-planes along the c-axis and the corresponding conductance [Formula: see text] were calculated for symmetric junctions composed of disordered d-wave layered superconductors partially gapped by charge density waves (CDWs). Here, V is the voltage. Both the checkerboard and unidirectional CDWs were considered. It was shown that the spatial spread of the CDW-pairing strength substantially smears the peculiarities of G(V) appropriate to uniform superconductors. The resulting curves G(V) become very similar to those observed for a number of cuprates in intrinsic junctions, e.g. mesas. In particular, the influence of CDWs may explain the peak-dip-hump structures frequently found for high-T c oxides.

Quasiparticle conductance-voltage characteristics for break junctions involvingd-wave superconductors: charge-density-wave effects

“…Experimental CVCs so measured [136] differ substantially from their counter parts obtained in the 'c-axis' experimental configuration. As is demonstrated below, the theoretical CVCs obtained in the framework of our approach also differ rather strongly from the dependences calculated for the case of 'c-axis' tunneling [83][84][85]155]. It should be noted that, although BJs are most often prepared from single crystals in such a manner that the observed Josephson or quasiparticle tunneling should occur along the CuO 2 planes [137,186,187], the crystals can be easily (unintentionally) cleaved along the ab planes during the junction processing, so that there is a large probability for a c-axis-directed tunneling junction to be formed [139].…”

“…We also consider the tunneling process to be coherent and take into account tunnel directionality [143][144][145][146][147]. Here, the coherent character of tunneling is supported by experiments for Josephson tunneling [140][141][142], and both coherence and directionality turn out crucial to make the actual CVC substantially different from its counterpart in the incoherent case appropriate to the c-axis-directed current [85,155]. On the basis of the adopted assumptions, the formulas for the quasiparticle tunnel current can be obtained in the conventional way [189,191] and similarly to our earlier calculations made for other cases dealing with both s-wave and d-wave SCDWs [6,82,[194][195][196][197].…”

“…the quasiparticle momentum k, where k is the quasiparticle wave vector and Planck's constant, is not conserved. The obtained CVCs revealed both superconductivity-and CDW-related features [83][84][85]. However, the BJ tunneling in high-T c oxides (perpendicularly to the c-axis) is quite different [114,[136][137][138], because it is concentrated in the ab-plane (at least predominately [139]) and should be coherent similarly to the Josephson current in the same configuration [115,117,[140][141][142].…”

How does the break-junction quasiparticle tunnel conductance look like for d-wave superconductors?

“…Such a situation is realized, e.g., in mesas [52][53][54][55][56][57]. In this case, tunneling is predominantly incoherent [9], and it was considered by us earlier [58][59][60][61] with the special emphasis on the possible influence of the charge density wave (CDW) manifestations. It might also happen, that in the specific case of Bi 2 Sr 2 CaCu 2 O 8+δ , the incoherent tunneling along the c axis, which is inherent to underdoped oxide compositions, becomes coherent for overdoped ones due to the "metallization" of the Bi-O layers [56].…”

“…At the same time, the two broken pieces (the junction electrodes) of the original d -wave superconducting sample can be arbitrarily rotated (in this plane) with respect to the junction face. The theoretical picture is intentionally somewhat idealized to trace only the gross features of the superconductivity itself; in particular, without the inevitable (for highc T oxides) CDWs [26,60,[76][77][78], although the additional energy gaps induced by the latter are well known to severely distort the actual conductance ( ) G V dependences of highc T oxides [58][59][60][61]. The quasiparticle tunneling is assumed coherent (see below), because the coherent Josephson currents are usually observed in such geometrical configurations [79][80][81].…”