Measurements ofΔandvFfrom Andreev reflections and McMillan-Rowell oscillations in edge junctions ofYBa2Cu3O6.6/YBa2Cu2.55Fe0.45Oy/YBa2Cu3O6.6

O. Nesher, G. Koren
1999 Physical Review B (Condensed Matter)  
We report measurements of dynamic resistance of high quality oxygen deficient SNS edge junctions with YBa 2 Cu 3 O 6.6 (T c ϭ60 K) as the superconductor ͑S͒ and YBa 2 Cu 2.55 Fe 0.45 O y as the barrier ͑N͒. Below the gap a series of peaks in the dynamic resistance is identified as due to multiple Andreev reflections in the barrier (V n ϭ2⌬/en), while above the gap, the peaks are attributed to McMillan-Rowell oscillations (⌬V ϭhv F Ј /4ed N ). Analysis of the subgap series in the 60 K YBa 2 Cu 3
more » ... the 60 K YBa 2 Cu 3 O 6.6 yields a gap value ⌬ aϪb of 16 Ϯ1.5 meV, and a coupling constant 2⌬/kT c of 6.2Ϯ0.5. The McMillan-Rowell series yield the renormalized Fermi velocity of quasiparticles in the barrier v F Ј ϭ(1.5Ϯ0.1)ϫ10 7 cm/sec. ͓S0163-1829͑99͒15233-0͔ Tunneling spectroscopy in NIS or SIS junctions is an important tool in the study of the high-T c superconductors. It can yield the density of states of the superconductor, the symmetry of the order parameter, and other physical constants of the superconductor such as the gap energy and the Fermi velocity. A model of quasiparticles tunneling based on the Andreev reflection process was introduced by Blonder, Tinkham, and Klapwijk ͑BTK͒. 1 This model has been very successful in explaining tunneling results in junctions with low-T c superconductors which have isotropic s-wave symmetry. 2 More recently, this model was extended by Tanaka and Kashiwaya to include anisotropic d-wave superconductors. 3,4 Experimental results of scanning tunneling microscopy ͑STM͒ and point contact spectroscopy were found to be consistent with the predictions of this extended model. 5,6 However, additional important physical properties can be obtained from the investigation of SNS junction. Recently, we observed geometrical resonances in the dynamic conductance of this kind of junction, which were attributed to Tomasch oscillations in one of the superconducting electrodes ͑the S layer͒. 7 This indicates that the interfaces in our edge junctions are sufficiently smooth to allow for the observation of this kind of interference effect. In the present study we continued to investigate the dynamic resistance of our SNS edge junctions with barriers of different thickness and properties. This time, we focus on geometrical resonances in the barrier of the junctions ͑the N layer͒. In these junctions the incoming and reflected quasiparticles at each of the NS interfaces, or the bare S surface, are interfering and producing multiple Andreev reflections below the gap, or McMillan-Rowell and Tomasch oscillations above the gap. 8-10 It should be noted here that generally a junction shows either Tomasch oscillations, multiple Andreev reflections, or McMillan-Rowell oscillations, depending on its specific parameters such as the barrier strength and thickness. In the multiple Andreev scattering process, an incoming electronlike quasiparticle is reflected from one of the NS interfaces as a holelike quasiparticle with an opposite momentum. The holelike quasiparticle can then undergo Andreev reflection from the other SN interface as an electronlike quasiparticle, and so on. In every reflection pro-cess the quasiparticle gains an energy eV, where V is the voltage across the junction. Whenever the total energy gained by the quasiparticle reaches twice the gap energy 2⌬, it makes an interference process which leads to a peak in the dynamic resistance at V n ϭ2⌬/en, where n is the number of reflections that the quasiparticle has gone through. 8, 11 The multiple Andreev phenomena, also called subharmonic gap structures in the literature, was observed in the past in the dynamic resistance of low-T c junctions with smooth interfaces. [12] [13] [14] [15] Recently, similar series of peaks ͑with nϭ1 and 2͒ have also been observed in high-T c edge junctions and attributed to a peculiar SNcNS structure. 16 Quasiparticles in the barrier with energies above the gap can also be reflected from the interfaces and produce interference effects. This yields series of peaks ͑or resonances͒ in the dynamic resistance, which are known as McMillan-Rowell oscillations. 10,17 The period of these oscillations is equal to hv F Ј /4ed N , where v F Ј is the renormalized Fermi velocity in the barrier, and d N is the thickness of the barrier. In the present study we investigate geometrical resonances in the dynamic resistance as described above, in oxygen-deficient edge junctions with YBa 2 Cu 3 O 6.6 (T c ϭ60 K) electrodes. We observed subharmonic gap structures due to Andreev scattering up to nϭ5, and found the energy gap ⌬ and the coupling constant 2⌬/kT c . Above the gap, we observed linear series of peaks in the dynamic resistance up to nϭ18 which were identified as McMillan-Rowell oscillations that also yield v F Ј in the barrier. We prepared oxygen-deficient edge junctions with YBa 2 Cu 3 O 6.6 (T c ϭ60 K) as the superconductor and YBa 2 Cu 2.55 Fe 0.45 O y as the barrier. 7 This barrier in the welloxygenated phase has a T c onset Ϸ10 K, while in its oxygendeficient phase it shows localization with resistivity (4 K) of about 10 m⍀ cm. A schematic diagram of the junction is shown in Fig. 1 . The fabrication process is described in detail elsewhere. 7 Briefly, the base electrode and insulator layers were deposited by laser ablation deposition on a 10 ϫ10 mm 2 wafer of ͑100͒ SrTiO 3 . Then using a waterless deep UV photholigraphy process, the edge was made by Ar ion milling at an angle of ␣ϭ36°. After this, a second deposition run was carried out, in which a barrier, a cover elec-PHYSICAL REVIEW B 1 OCTOBER 1999-I VOLUME 60, NUMBER 13 PRB 60 0163-1829/99/60͑13͒/9287͑4͒/$15.00 9287
doi:10.1103/physrevb.60.9287 fatcat:atoioy7pjvcnphmk2qecqy7e6q