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The Meissner effect in a strongly underdoped cuprate above its critical temperature.

Morenzoni E, Wojek BM, Suter A, Prokscha T, Logvenov G, Božović I - Nat Commun (2011)

Bottom Line: The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state.In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed.The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.

View Article: PubMed Central - PubMed

Affiliation: Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. elvezio.morenzoni@psi.ch

ABSTRACT
The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed. Here we probe the local diamagnetic response in the normal state of an underdoped La(1.94)Sr(0.06)CuO(4) layer (T(c)'≤5 K), which is brought into close contact with two nearly optimally doped La(1.84)Sr(0.16)CuO(4) layers (T(c)≈32 K). We show that the entire 'barrier' layer of thickness, much larger than the typical c axis coherence lengths of cuprates, exhibits a Meissner effect at temperatures above T(c)' but below T(c). The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.

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Depth profile of the local field at different temperatures.The grey shaded areas indicate the top and bottom layers of the La1.84Sr0.16CuO4 (46 nm)/La1.94Sr0.06CuO4 (46 nm)/La1.84Sr0.16CuO4 (46 nm) heterostructure. The horizontal dashed line shows the applied field of 9.5 mT. Points: measured average fields. The entire heterostructure excludes the magnetic flux like a superconductor: it shows the Meissner effect with the UD layer active in the screening. Note that in this geometry all the supercurrent must pass through the region of the heterostructure, which, taken as an isolated layer, would be in the normal state at T〉Tc′. This functional form can only be observed if shielding supercurrents flow across (that is, along the c axis) as well as in the ab planes of the UD barrier. The lines are the fits using the London model described in the text. The fit takes into account the energy-dependent muon stopping profiles, which are also used to calculate the average stop depth 〈zμ〉 (upper scale). The grey dash-dotted line shows the field profile that would be expected (at T=4.35 K) if the shielding current flow were restricted to the upper and lower superconducting electrodes. The dashed lines are obtained if one assumes that supercurrents in the barrier flow only in c direction.
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f6: Depth profile of the local field at different temperatures.The grey shaded areas indicate the top and bottom layers of the La1.84Sr0.16CuO4 (46 nm)/La1.94Sr0.06CuO4 (46 nm)/La1.84Sr0.16CuO4 (46 nm) heterostructure. The horizontal dashed line shows the applied field of 9.5 mT. Points: measured average fields. The entire heterostructure excludes the magnetic flux like a superconductor: it shows the Meissner effect with the UD layer active in the screening. Note that in this geometry all the supercurrent must pass through the region of the heterostructure, which, taken as an isolated layer, would be in the normal state at T〉Tc′. This functional form can only be observed if shielding supercurrents flow across (that is, along the c axis) as well as in the ab planes of the UD barrier. The lines are the fits using the London model described in the text. The fit takes into account the energy-dependent muon stopping profiles, which are also used to calculate the average stop depth 〈zμ〉 (upper scale). The grey dash-dotted line shows the field profile that would be expected (at T=4.35 K) if the shielding current flow were restricted to the upper and lower superconducting electrodes. The dashed lines are obtained if one assumes that supercurrents in the barrier flow only in c direction.

Mentions: The depth profile of the mean field 〈Bx〉 at different temperatures is shown in Figure 6. At 10, 15 and 17 K—that is, well above Tc′—the local field is lower than the applied field at all depths, meaning that the entire heterostructure excludes the magnetic flux like a conventional superconductor. This is unexpected when one recalls that in this geometry the supercurrent must pass through the 'barrier' La1.94Sr0.06CuO4 region that is 46-nm thick. This is over two orders of magnitude larger than the c axis coherence length ξc in the electrodes. Note that at T〉Tc′ a single-phase La1.94Sr0.06CuO4 is not superconducting, and not even metallic along the c axis. In Figure 7a, we compare the temperature dependence of the average field in the centre of a single-phase film of UD La1.94Sr0.06CuO4 with that in the barrier of the same composition inside a trilayer heterostructure. In the former case no shift is observed, whereas in the latter case the shift is observable up to Teff≈22 K. The observed field profile reflects the shielding supercurrent that runs along the c axis as well as in the ab planes of the barrier; note that 〈jab〉=〈(1/μ0) dBx/dz〉/≠0. The profile has the form of an exponential field decay in the Meissner state with the flux penetrating from both sides and looks like that for two superconductors with different magnetic penetration depths.


The Meissner effect in a strongly underdoped cuprate above its critical temperature.

Morenzoni E, Wojek BM, Suter A, Prokscha T, Logvenov G, Božović I - Nat Commun (2011)

Depth profile of the local field at different temperatures.The grey shaded areas indicate the top and bottom layers of the La1.84Sr0.16CuO4 (46 nm)/La1.94Sr0.06CuO4 (46 nm)/La1.84Sr0.16CuO4 (46 nm) heterostructure. The horizontal dashed line shows the applied field of 9.5 mT. Points: measured average fields. The entire heterostructure excludes the magnetic flux like a superconductor: it shows the Meissner effect with the UD layer active in the screening. Note that in this geometry all the supercurrent must pass through the region of the heterostructure, which, taken as an isolated layer, would be in the normal state at T〉Tc′. This functional form can only be observed if shielding supercurrents flow across (that is, along the c axis) as well as in the ab planes of the UD barrier. The lines are the fits using the London model described in the text. The fit takes into account the energy-dependent muon stopping profiles, which are also used to calculate the average stop depth 〈zμ〉 (upper scale). The grey dash-dotted line shows the field profile that would be expected (at T=4.35 K) if the shielding current flow were restricted to the upper and lower superconducting electrodes. The dashed lines are obtained if one assumes that supercurrents in the barrier flow only in c direction.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3104550&req=5

f6: Depth profile of the local field at different temperatures.The grey shaded areas indicate the top and bottom layers of the La1.84Sr0.16CuO4 (46 nm)/La1.94Sr0.06CuO4 (46 nm)/La1.84Sr0.16CuO4 (46 nm) heterostructure. The horizontal dashed line shows the applied field of 9.5 mT. Points: measured average fields. The entire heterostructure excludes the magnetic flux like a superconductor: it shows the Meissner effect with the UD layer active in the screening. Note that in this geometry all the supercurrent must pass through the region of the heterostructure, which, taken as an isolated layer, would be in the normal state at T〉Tc′. This functional form can only be observed if shielding supercurrents flow across (that is, along the c axis) as well as in the ab planes of the UD barrier. The lines are the fits using the London model described in the text. The fit takes into account the energy-dependent muon stopping profiles, which are also used to calculate the average stop depth 〈zμ〉 (upper scale). The grey dash-dotted line shows the field profile that would be expected (at T=4.35 K) if the shielding current flow were restricted to the upper and lower superconducting electrodes. The dashed lines are obtained if one assumes that supercurrents in the barrier flow only in c direction.
Mentions: The depth profile of the mean field 〈Bx〉 at different temperatures is shown in Figure 6. At 10, 15 and 17 K—that is, well above Tc′—the local field is lower than the applied field at all depths, meaning that the entire heterostructure excludes the magnetic flux like a conventional superconductor. This is unexpected when one recalls that in this geometry the supercurrent must pass through the 'barrier' La1.94Sr0.06CuO4 region that is 46-nm thick. This is over two orders of magnitude larger than the c axis coherence length ξc in the electrodes. Note that at T〉Tc′ a single-phase La1.94Sr0.06CuO4 is not superconducting, and not even metallic along the c axis. In Figure 7a, we compare the temperature dependence of the average field in the centre of a single-phase film of UD La1.94Sr0.06CuO4 with that in the barrier of the same composition inside a trilayer heterostructure. In the former case no shift is observed, whereas in the latter case the shift is observable up to Teff≈22 K. The observed field profile reflects the shielding supercurrent that runs along the c axis as well as in the ab planes of the barrier; note that 〈jab〉=〈(1/μ0) dBx/dz〉/≠0. The profile has the form of an exponential field decay in the Meissner state with the flux penetrating from both sides and looks like that for two superconductors with different magnetic penetration depths.

Bottom Line: The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state.In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed.The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.

View Article: PubMed Central - PubMed

Affiliation: Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. elvezio.morenzoni@psi.ch

ABSTRACT
The Meissner effect and associated perfect 'bulk' diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates, unusual diamagnetic signals and anomalous proximity effects have been detected, but a Meissner effect has never been observed. Here we probe the local diamagnetic response in the normal state of an underdoped La(1.94)Sr(0.06)CuO(4) layer (T(c)'≤5 K), which is brought into close contact with two nearly optimally doped La(1.84)Sr(0.16)CuO(4) layers (T(c)≈32 K). We show that the entire 'barrier' layer of thickness, much larger than the typical c axis coherence lengths of cuprates, exhibits a Meissner effect at temperatures above T(c)' but below T(c). The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers, but this induced order is sensitive to thermal excitation.

Show MeSH
Related in: MedlinePlus