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Pairing and the phase diagram of the normal coherence length ξN(T, x) above Tc of La(2-x)Sr(x)CuO4 thin films probed by the Josephson effect.

Kirzhner T, Koren G - Sci Rep (2014)

Bottom Line: The long range proximity effect in high-Tc c-axis Josephson junctions with a high-Tc barrier of lower Tc is still a puzzling phenomenon.It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect.This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier.

View Article: PubMed Central - PubMed

Affiliation: Physics Department, Technion-Israel Institute of Technology, Haifa 32000, Israel.

ABSTRACT
The long range proximity effect in high-Tc c-axis Josephson junctions with a high-Tc barrier of lower Tc is still a puzzling phenomenon. It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect. Here we measured the T - x (Temperature-doping level) phase diagram of the barrier coherence length ξN(T, x), and found an enhancement of ξN at moderate under-doping and high temperatures. This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier. In more details, we measured the supercurrents Ic of Superconducting - Normal - Superconducting SNS c-axis junctions, where S was optimally doped Y Ba2Cu3O(7-δ) below Tc (90 K) and N was La(2-x)Sr(x)CuO4 above its Tc (<25 K) but in the pseudogap regime. From the exponential decay of Ic(T) ∝ exp[-d/ξN(T)], where d is the barrier thickness, the ξN(T) values were extracted. By repeating these measurements for different barrier doping levels x, the whole phase diagram of ξN(T, x) was obtained.

No MeSH data available.


Related in: MedlinePlus

Normal coherence lengths ξN of LSCO-0.1 and LSCO-0.18 as a function of temperature.The inset shows the normal state resistivity of the corresponding LSCO-x barriers as a function of temperature.
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f4: Normal coherence lengths ξN of LSCO-0.1 and LSCO-0.18 as a function of temperature.The inset shows the normal state resistivity of the corresponding LSCO-x barriers as a function of temperature.

Mentions: We now turn to the main result of this study which shows the normal coherence lengths of the LSCO-x barriers at different doping levels. For any given temperature T and doping level x, the normal coherence length of the barrier can be extracted from the ratio of the critical currents in junctions with two different barrier thicknesses di using the exponential part of the De-Gennes formula (Ici ∝ exp[−di/ξN]28). To further clarify the procedure of extracting ξN(T) from the data, a detailed description is given in the supplementary material for the case of LSCO-0.24 film and junctions. Fig. 4 shows the normal coherence lengths ξN(T, x) for x = 0.1 and x = 0.18 LSCO-x barriers as a function of temperature. The temperature range of the coherence lengths plots is limited here to 40–60 K. The lower bound of the temperature range is set by the flux flow phenomenon in the junctions with the thinner barrier due to the high Ic values and rounding of the I-V curves which make the determination of Ic difficult. The upper bound is set by the low critical currents in the junctions with the thicker barrier which are noisy and therefore hard to measure. Fig. 4 shows that the measured normal coherence length values range between 4–6 nm. These values are much higher than expected from the conventional proximity effect theory28, where the coherence length should be limited by the short c-axis superconductor coherence length ξS and the corresponding mean free path lN, both of which are shorter than 1 nm. Previous experiments on SNS cuprate junctions of the type LSCO-LCO-LSCO had also shown very long coherence lengths22. This “giant proximity effect” was explained by a number of theories which took into account superconducting phase fluctuations above Tc in the barrier2425.


Pairing and the phase diagram of the normal coherence length ξN(T, x) above Tc of La(2-x)Sr(x)CuO4 thin films probed by the Josephson effect.

Kirzhner T, Koren G - Sci Rep (2014)

Normal coherence lengths ξN of LSCO-0.1 and LSCO-0.18 as a function of temperature.The inset shows the normal state resistivity of the corresponding LSCO-x barriers as a function of temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150101&req=5

f4: Normal coherence lengths ξN of LSCO-0.1 and LSCO-0.18 as a function of temperature.The inset shows the normal state resistivity of the corresponding LSCO-x barriers as a function of temperature.
Mentions: We now turn to the main result of this study which shows the normal coherence lengths of the LSCO-x barriers at different doping levels. For any given temperature T and doping level x, the normal coherence length of the barrier can be extracted from the ratio of the critical currents in junctions with two different barrier thicknesses di using the exponential part of the De-Gennes formula (Ici ∝ exp[−di/ξN]28). To further clarify the procedure of extracting ξN(T) from the data, a detailed description is given in the supplementary material for the case of LSCO-0.24 film and junctions. Fig. 4 shows the normal coherence lengths ξN(T, x) for x = 0.1 and x = 0.18 LSCO-x barriers as a function of temperature. The temperature range of the coherence lengths plots is limited here to 40–60 K. The lower bound of the temperature range is set by the flux flow phenomenon in the junctions with the thinner barrier due to the high Ic values and rounding of the I-V curves which make the determination of Ic difficult. The upper bound is set by the low critical currents in the junctions with the thicker barrier which are noisy and therefore hard to measure. Fig. 4 shows that the measured normal coherence length values range between 4–6 nm. These values are much higher than expected from the conventional proximity effect theory28, where the coherence length should be limited by the short c-axis superconductor coherence length ξS and the corresponding mean free path lN, both of which are shorter than 1 nm. Previous experiments on SNS cuprate junctions of the type LSCO-LCO-LSCO had also shown very long coherence lengths22. This “giant proximity effect” was explained by a number of theories which took into account superconducting phase fluctuations above Tc in the barrier2425.

Bottom Line: The long range proximity effect in high-Tc c-axis Josephson junctions with a high-Tc barrier of lower Tc is still a puzzling phenomenon.It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect.This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier.

View Article: PubMed Central - PubMed

Affiliation: Physics Department, Technion-Israel Institute of Technology, Haifa 32000, Israel.

ABSTRACT
The long range proximity effect in high-Tc c-axis Josephson junctions with a high-Tc barrier of lower Tc is still a puzzling phenomenon. It leads to supercurrents in junctions with much thicker barriers than would be allowed by the conventional proximity effect. Here we measured the T - x (Temperature-doping level) phase diagram of the barrier coherence length ξN(T, x), and found an enhancement of ξN at moderate under-doping and high temperatures. This indicates that a possible origin of the long range proximity effect in the cuprate barrier is the conjectured pre-formed pairs in the pseudogap regime, which increase the length scale over which superconducting correlations survive in the seemingly normal barrier. In more details, we measured the supercurrents Ic of Superconducting - Normal - Superconducting SNS c-axis junctions, where S was optimally doped Y Ba2Cu3O(7-δ) below Tc (90 K) and N was La(2-x)Sr(x)CuO4 above its Tc (<25 K) but in the pseudogap regime. From the exponential decay of Ic(T) ∝ exp[-d/ξN(T)], where d is the barrier thickness, the ξN(T) values were extracted. By repeating these measurements for different barrier doping levels x, the whole phase diagram of ξN(T, x) was obtained.

No MeSH data available.


Related in: MedlinePlus