Andreev–Saint-James reflections: A probe of cuprate superconductors

Abstract

Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint James and de Gennes and by Andreev, and their spectroscopic features discussed in detail by Saint James in the early sixties. They are generally called Andreev reflections but, for that reason, we call them Andreev -Saint James (ASJ) reflections. We present a historical review of ASJ reflections and spectroscopy in conventional superconductors, and review their application to the High Tc cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy range. Low energy surface bound states have been observed in all studied hole doped cuprates, in agreement with a dominant d-wave symmetry order parameter. Results are mixed for electron doped cuprates. In overdoped Y Ba 2 Cu 3 O 7−δ (δ < 0.08) and La 2−x SrxCuO 4 , ASJ spectroscopy is consistent with the presence of an additional imaginary component of the order parameter. Results of ASJ spectroscopy under applied magnetic fields are also reviewed. A short section at the end is devoted to some recent results on spin effects. * Electronic address: guyde@tau.ac.il 2

s-Wave Symmetry Along the c-Axis and s+d In-plane Superconductivity in Bulk YBa2Cu4O8

“…[15], an inflection point in λ −2 (T ) suggests the presence of at least two superconducting gaps in YBa 2 Cu 4 O 8 with very different gap values, i.e., a large gap and a small one. The same behavior was observed recently in La 1.83 Sr 0.17 CuO 4 by μSR [14], as well as in other HTS using various techniques [9][10][11][12][13], supporting a two-gap behavior with the larger gap being of d-wave and the smaller one of s-wave symmetry. The above data were analyzed by assuming that an s-wave and a d-wave gap contribute to σ according to: where both components are expressed like [14]:…”

“…In all of the above mentioned superconductors, the combined order parameters exhibit always the same symmetry, namely s + s in Nb doped SrTiO 3 [4] and MgB 2 [5] and d + d in heavy fermion compounds [6]. A totally novel situation is met in cuprate hightemperature superconductors (HTS), since the order parameters are of different symmetries, i.e., s + d [7][8][9][10][11][12][13]. However, theoretical modeling suggested a single d-wave order parameter in the CuO 2 planes, and unfortunately biased further research and partly inhibited the experimental efforts to characterize TGS in more detail in HTS.…”

Asymmetric tunneling, Andreev reflection and dynamic conductance spectra in strongly correlated metals

“…2 as well as that below the critical temperature T c of superconducting phase transition arises from Andreev reflection, see e.g. [14]. It is seen from Fig.…”

“…Now we turn to a consideration of the tunneling current at low temperatures which in the case of ordinary metals is given by [14] …”

Superconductivity from Repulsion: Ginzburg–Landau Phenomenology of Cuprates

“…Nontrivial momentum dependence of the coherence factors resulting from the η K -pairing scheme allows to explain observed tunnel conductance asymmetry, peak and dip-hump structure of both tunnel and photoemission spectra [29], and typical of cuprates features [30] of Andreev reflection [31].…”