Observation of Two Andreev-Like Energy Scales inLa2−xSrxCuO4Superconductor–Normal-Metal–Superconductor Junctions

Abstract

Conductance spectra measurements of highly transparent ramp-type junctions made of superconducting La 2−x Sr x CuO 4 electrodes and non superconducting La 1.65 Sr 0.35 CuO 4 barrier are reported. At low temperatures below T c , these junctions have two prominent Andreev-like conductance peaks with clear steps at energies ∆ 1 and ∆ 2 with ∆ 2 > 2∆ 1 . No such peaks appear above T c . The doping dependence at 2 K shows that both ∆ 1 and ∆ 2 scale roughly as T c . ∆ 1 is identified as the superconducting energy gap, while a few scenarios are proposed as for the origin of ∆ 2 . PACS numbers: 74.45.+c, 74.25.Dw, The issue of two distinct energy gaps in the cuprates has been discussed by many authors, and the question whether both are related to superconductivity is still controversial [1][2][3][4]. In one scenario, one energy gap is the coherence gap which opens at T c with the onset of phase coherent superconductivity, while the other gap opens at T * which marks the cross over to the pseudogap regime and possibly the creation of uncorrelated pairs [5]. In contrast to this scenario, some ARPES measurements show only a single energy gap, which indicate that the superconducting gap and the pseudogap might be the same entity [6,7]. In another scenario, the regime above T c in the underdoped cuprates which exhibits a signature of the condensate, can be attributed to strong superconducting fluctuations. This behavior was found in measurements of the Nernst effect [8], whose T c (onset) values scale with doping roughly as the superconducting dome. This effect therefore, is related to T c and apparently depends on more than one energy scale of the condensate. Previous point contact measurements of tunneling and Andreev conductance have shown that the tunneling gap which scales as T * is larger than the Andreev gap which follows T c [2,[9][10][11]. In the present study we report on similar conductance measurements in ramp-type junctions of the La 2−x Sr x CuO 4 (LSCOx or LSCO) system, but due to their high transparency we observe mostly Andreev gaps. Surprisingly, we find two different such gaps in this system below T c both of which scale versus doping roughly as the superconducting dome. Only single gaps were observed in previous conductance measurements in . The results though show that in Refs. [10][11][12] the gaps follow T c while in Ref.[13] the gaps scale as T * . The present low energy Andreev peak in the conductance spectra is attributed to the superconducting gap, while a few scenarios are discussed in relation to the origin of the second high energy feature in the spectra. Fig. 1. This spectrum has three pronounced features. The first is a narrow zero bias conductance peak (ZBCP), the second is a dome like peak of intermediate width which is superimposed on a third feature which is even broader. The conductance data is therefore the result of a sum of three components which can be written as The inset shows the derivative of the conductance data of the main panel.A typical conductance spectrum of a LS...

Pairing and the phase diagram of the normal coherence length ξN(T, x) above Tc of La2−xSrxCuO4 thin films probed by the Josephson effect

“…Although reminiscent of the pseudogap T * behavior as depicted from ARPES measurements by the dotted line31, the slopes of the two lines are very different, possibly indicating the presence of additional effects such as phase fluctuations or that the two phenomena are unrelated2021. Similar phase diagram trends were observed before in the cuprates in Nernst effect measurements32, in high magnetic field results33, in infrared and terahertz spectroscopy1734, and in higher energy gap results obtained in Andreev conductance spectroscopy measurements35. These previous results, as well as the new one presented here, provide additional support for strong superconducting fluctuation effects and the preformed pairs scenario in the underdoped regime of the cuprates above T c , but not necessarily up to the T * transition-line of the pseudogap.…”

Gating of Au-$Bi_2Se_3$-NbN ramp-type junctions with superconducting NbN gate-electrode and $SrTiO_3$ film as the gate-insulator

“…The resistance spread above the transition can be due to thickness variations over the 10 mm width of the wafer. Similar but smaller spreads however, were also observed in our previous studies of all-epitaxial cuprate ramp junctions [12,13]. Thus the obviously non-epitaxial nature of the present junctions (hexagonal Bi 2 Se 3 on cubic NbN or STO can never be epitaxial) can also contribute to the large spread of the junctions properties.…”

Transport and spectroscopic properties of superconductor/ferromagnet/superconductor junctions of La1.9Sr0.1CuOKoren1, Kirzhner2 2011Phys. Rev. BSelf Cite110170View full textAdd to dashboardCite

“…We note however, that PITS could be at the origin of previous results where a small supercurrents of 5 kA/cm 2 at 4 K had been observed in similar SFS junctions but with Y Ba 2 Cu 3 O 7−δ electrodes and a 20 nm thick La 0.67 Sr 0.33 MnO 3 barrier [10]. Following our recent study of similar SNS junctions but with a normal La 1.65 Sr 0.35 CuO 4 barrier where two Andreev-like energy scales were observed [11], in the present study we concentrated on low biases and observed only the lower superconducting energy scale. The negative magnetoresistance of the LCMO barrier at 2 K had also been detected in the conductance spectra of the present work, either under a high magnetic field in the superparamagnetic state of the LCMO barrier or after field cycling.…”

“…This means that our preparation process is quite clean and introduces only a negligible amount of contamination at the interfaces of our junctions. In passing we note that in the supplementary material of a previous publication by our group concerning these short junctions [11], the given J c (2K) values were in error and the correct values are given here.…”