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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.


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A color-map of the phase diagram of ξN(T, x) representing the normal coherence length of LSCO-x in nm as function of temperature T and doping x.The dotted line represents the pseudogap T* temperature of Ref. 32, while the dashed line describes the trend of the present data of ξN for 0.18 ≥ x ≥ 0.1 and T > 55 K. For comparison we plot also Tc(x) measured on LSCO single crystals by Matsuzaki et al.36.
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f5: A color-map of the phase diagram of ξN(T, x) representing the normal coherence length of LSCO-x in nm as function of temperature T and doping x.The dotted line represents the pseudogap T* temperature of Ref. 32, while the dashed line describes the trend of the present data of ξN for 0.18 ≥ x ≥ 0.1 and T > 55 K. For comparison we plot also Tc(x) measured on LSCO single crystals by Matsuzaki et al.36.

Mentions: To further elucidate and explain this interpretation of our results, we plot in Fig. 5 a color-map of the full phase diagram of ξN(T, x). All the measured ξN(T) values of the x = 0.07, 0.1, 0.18 and 0.24 doping levels were used (12 × 4 measured values at 12 temperatures per each doping level), and the color-map extrapolates and draws the contours in between these doping levels. The contours in between the measured data points should thus be considered only as guides to the eye. We have data also below 40 K, but this is less reliable due to flux flow effects and we have chosen not to show it here. A clear feature in Fig. 5 is that the contours of constant ξN follow roughly the superconducting dome, but this occurs much above the Tc values of the LSCO-x barrier. Moreover, above 55 K, the maximum ξN values for each contour occurs at moderate under-doping (x = 0.1). One can see this behavior also by looking at the dashed line which shows the general trend of the contours in the 0.1 < x < 0.18 doping range at high temperatures. 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 Tc, but not necessarily up to the T* transition-line of the pseudogap.


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)

A color-map of the phase diagram of ξN(T, x) representing the normal coherence length of LSCO-x in nm as function of temperature T and doping x.The dotted line represents the pseudogap T* temperature of Ref. 32, while the dashed line describes the trend of the present data of ξN for 0.18 ≥ x ≥ 0.1 and T > 55 K. For comparison we plot also Tc(x) measured on LSCO single crystals by Matsuzaki et al.36.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: A color-map of the phase diagram of ξN(T, x) representing the normal coherence length of LSCO-x in nm as function of temperature T and doping x.The dotted line represents the pseudogap T* temperature of Ref. 32, while the dashed line describes the trend of the present data of ξN for 0.18 ≥ x ≥ 0.1 and T > 55 K. For comparison we plot also Tc(x) measured on LSCO single crystals by Matsuzaki et al.36.
Mentions: To further elucidate and explain this interpretation of our results, we plot in Fig. 5 a color-map of the full phase diagram of ξN(T, x). All the measured ξN(T) values of the x = 0.07, 0.1, 0.18 and 0.24 doping levels were used (12 × 4 measured values at 12 temperatures per each doping level), and the color-map extrapolates and draws the contours in between these doping levels. The contours in between the measured data points should thus be considered only as guides to the eye. We have data also below 40 K, but this is less reliable due to flux flow effects and we have chosen not to show it here. A clear feature in Fig. 5 is that the contours of constant ξN follow roughly the superconducting dome, but this occurs much above the Tc values of the LSCO-x barrier. Moreover, above 55 K, the maximum ξN values for each contour occurs at moderate under-doping (x = 0.1). One can see this behavior also by looking at the dashed line which shows the general trend of the contours in the 0.1 < x < 0.18 doping range at high temperatures. 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 Tc, but not necessarily up to the T* transition-line of the pseudogap.

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