Superconductor Science and Technology PAPER • OPEN ACCESS Unconventional long range triplet proximity effect in planar YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 Josephson junctions To cite this article: David Sanchez-Manzano et al 2023 Supercond. Sci. Technol. 36 074002   View the article online for updates and enhancements. You may also like Strong-coupling topological Josephson effect in quantum wires Flavio S Nogueira and Ilya Eremin - Magnetoelectric effects in Josephson junctions I V Bobkova, A M Bobkov and M A Silaev - Anomalous Josephson effect Yu M Shukrinov - This content was downloaded from IP address 147.96.28.131 on 31/08/2023 at 15:17 https://doi.org/10.1088/1361-6668/accb11 /article/10.1088/0953-8984/24/32/325701 /article/10.1088/0953-8984/24/32/325701 /article/10.1088/1361-648X/ac7994 /article/10.1088/1361-648X/ac7994 /article/10.3367/UFNe.2020.11.038894 Superconductor Science and Technology Supercond. Sci. Technol. 36 (2023) 074002 (8pp) https://doi.org/10.1088/1361-6668/accb11 Unconventional long range triplet proximity effect in planar YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 Josephson junctions David Sanchez-Manzano1, S Mesoraca1, F Cuellar2, M Cabero3,4, S Rodriguez-Corvillo2, V Rouco2, F Mompean5,6, M Garcia-Hernandez5,6, J M Gonzalez-Calbet4,7, C Feuillet-Palma8, N Bergeal8, J Lesueur8, C Leon2, Javier E Villegas1 and J Santamaria2,6,∗ 1 Unité Mixte de Physique, CNRS, Thales, Université Paris-Saclay, 91767 Palaiseau, France 2 GFMC. Department Fisica de Materiales. Facultad de Fisica. Universidad Complutense, Madrid 28040, Spain 3 IMDEA Nanoscience Campus Universidad Autonoma, 28049 Cantoblanco, Spain 4 Centro Nacional de Microscopia Electronica. Universidad Complutense, Madrid 28040, Spain 5 Instituto de Ciencia de Materiales de Madrid ICMM-CSIC, 28049 Cantoblanco, Spain 6 Unidad Asociada UCM/CSIC, ‘Laboratorio de Heteroestructuras con aplicación en spintrónica’, Cantoblanco 28049, Spain 7 Department Quimica Inorganica. Facultad de Quimica. Universidad Complutense, Madrid 28040, Spain 8 Laboratoire de Physique et d’Etude des Matériaux, CNRS, ESPCI Paris, PSL Research University, UPMC, 75005 Paris, France E-mail: jacsan@ucm.es Received 1 January 2023, revised 13 March 2023 Accepted for publication 6 April 2023 Published 5 June 2023 Abstract The proximity effect between superconductors and ferromagnets may become long range due to the generation of triplet pairs. The recent finding of a long, one micron-range unconventional Josephson effect between YBa2Cu3O7 high Tc cuprates separated by a half metallic La0.7Sr0.3MnO3 manganite ferromagnet has uncovered a novel unconventional triplet proximity effect. In this paper, we examine the temperature dependence of the critical current in planar Josephson junctions. We find that the critical current—normal resistance product follows the predictions of traditional superconductor-normal metal-superconductor junctions, which implies that triplet pairs in a ferromagnet are transported in the diffusive limit similarly to singlet pairs in a normal metal. This result calls for theoretical studies of the new triplet Josephson effect and underlines its potential in future superconducting spintronics. Keywords: proximity effect, unconventional superconductivity, Josephson effect (Some figures may appear in colour only in the online journal) ∗ Author to whom any correspondence should be addressed. Original content from this workmay be used under the terms of the Creative Commons Attribution 4.0 licence. Any fur- ther distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1361-6668/23/074002+8$33.00 Printed in the UK 1 © 2023 The Author(s). Published by IOP Publishing Ltd https://doi.org/10.1088/1361-6668/accb11 https://orcid.org/0000-0003-1229-6868 https://orcid.org/0000-0002-5987-0647 https://orcid.org/0000-0002-8389-5756 https://orcid.org/0000-0002-5843-187X https://orcid.org/0000-0002-3262-1843 https://orcid.org/0000-0003-4594-2686 mailto:jacsan@ucm.es http://crossmark.crossref.org/dialog/?doi=10.1088/1361-6668/accb11&domain=pdf&date_stamp=2023-6-5 https://creativecommons.org/licenses/by/4.0/ Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al 1. Introduction The traditional antagonism between ferromagnetism and superconductivity had to be reconsidered in the early 2000s when seminal theoretical papers showed the possibility of triplet Cooper pairs being generated at ferromagnet/supercon- ductor (F/S) interfaces [1–9]. Soon after, extensive experi- mental evidence appeared of long range supercurrents in F’s in contact with S’s, supporting the existence of a long range triplet F/S proximity effect [10–13]. At the microscopic level, triplet generation results from spin mixing and spin rotation processes driven by magnetic inhomogeneities and spin flip scattering [5, 7, 8, 10–15]. Long range supercurrents have been observed in vertical and lateral F/S structures in several mater- ial scenarios fueling the dream of superconducting spintronics [16, 17]. Despite many hints of triplet superconductivity producing long range supercurrents, very long range Josephson effects have remained elusive, especially in the case of unconven- tional high-temperature S’s combined with half-metallic F’s [18–23]. A major breakthrough is the recent finding of a long micron-range unconventional Josephson effect across a half metal [24]. Planar Josephson junctions where supercon- ducting cuprate YBa2Cu3O7 (YBCO) electrodes are coupled across half-metallic manganite La0.7Sr0.3MnO3 (LSMO) wire show in addition to large critical currents, the hallmarks of Josephson effect: (i) critical current oscillations under applied magnetic fields driven by magnetic flux quantization (Fraunhofer pattern); and (ii) quantum phase locking effects under microwave excitation (Shapiro steps). Being YBCO an unconventional d-wave S, this study has raised important ques- tions regarding the origin of the triplet proximity effect, and what is the symmetry of the pairing amplitude driving the long range Josephson coupling. In this paper, we aim at exploring the proximity effect in the YBCO/LSMO/YBCO system from an analysis of the temperature dependence of the critical cur- rent in planar Josephson junctions with different lengths of the LSMO weak link. The presence of supercurrent and Josephson effects in S- normalmetal (N)- Superconductor structures (SNS) has its ori- gin in the proximity effect [25], bywhich superconducting cor- relations in the N are mediated by Andreev pairs generated by phase coherent Andreev reflections at the S-N interfaces. The coherence length in the N ξ N takes the form ξ N = ( ℏD 2π kBT )1/2 (where D is the diffusion coefficient) which can take long (micron-size) values at low temperatures. If the N is an F the superconducting correlations decay fast as phase coherence between Andreev pairs is broken by the exchange field. i.e. the correlation length in the F ξ FM takes the form ξ FM = (ℏDhex ) 1/2 (where hex is the exchange field, namely the exchange split- ting between both spin bands due to the exchange interaction) [3, 4]. For homogeneous exchange fields ξ FM is typically very short, a few nanometers in the case of weak F’s with small val- ues of the exchange field, and much shorter values are expec- ted in the case of strong F with ∼1 eV range exchange split- ting. The situation, however, changes under inhomogeneous exchange fields, if spin triplet pairs are generated [3, 4], since they would not be affected by the exchange field. Moreover, in the case (like ours) of half metals where the conduction band is fully spin polarized, the (femto-second) spin flip route charac- teristic of metals, the Elliott Yafet mechanism, caused by spin– orbit driven spin mixing at high symmetry points, is blocked resulting from the vanishing minority spin density of states at the Fermi level. As a result, there is no spin channel for spin– flip scattering and the energy has to be transferred through the less efficient spin–lattice relaxation with a longer time scale (τSF ∼ 200–400 ps) as discussed in [26]. In the half metal/S proximity scenario, as a result of the blocked spin–flip scatter- ing channel, only thermal de-phasing of the Andreev pairs can occur. In this sense, a half metallic weak link where the pairing amplitude is carried by triplet correlations should behave akin to a N one and follow the predictions of SNS theories for sing- let pairs de-phasing only thermally. This is precisely the aim of this paper, we want to examine the temperature dependence of the critical current in planar junction devices to assess if triplets in a ferromagnetic weak link follow the predictions of the theory for conventional SNS junctions, which we briefly describe immediately below. The critical current of an SNS junction is limited exponen- tially by the length L of the N weak link: Icαe − L ξN . The the- ory of the critical current of SNS junctions in the diffusive limit was initially developed by Likharev [27, 28] usingUsadel [29] equations. This is a high temperature approach valid close to the critical temperature where the thermal energy is larger than the superconducting gap. The extension to low temperat- ures, where experimental data have shown significant depar- tures from early theories by de Gennes [25], was completed by Dubos and collaborators [30] using quasi-classical Green’s functions. Diffusive transport in junctions with an N weak link longer than the electron mean free path but shorter than the de- phasing length limits electron–hole coherence to an energy window given by the Thouless energy [31] ETh = ℏD L2 (which should be smaller than the width of the thermal distribution). It is important to note that the Thouless energy is a single elec- tron energy scale set by the diffusion rate of normal electrons across the sample. The low temperature calculation kBT< ETh by Dubos et al [30] involving the solution of Usadel equations at all energies provides, in the long junction limit ( ∆ ETh ≫ 1, with∆ the superconducting gap), a simple analytic expression for the normal resistance-critical current eIcRn product, which reads: eIcRn = 32 3+ 2 √ 2 ETh [ L ξ N ]3 e− L ξN . (1) With a zero temperature limit, which cuts-off the diver- gence of the critical current at low temperature of the early theories. It is worth to remark that at high temperatures where the thermal energy scale is larger than the gap ( kBT∆ ≫ 1) the full calculation recovers the result predicted by Likharev [28]. In discussing the superconducting properties of all oxide planar junction devices, an interesting connection is the so called ‘long range proximity effect’ observed in planar HTSC junctions with oxide semiconductor barriers showing long 2 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al range supercurrents for spacer layer thicknesses in excess of 100 nm [32–34]. Theoretical analysis [35–37] has shown that the long range transport of the Josephson current is governed by resonant tunneling via the localized states in the barrier. 2. Results and discussion The samples used in this study were planar Josephson junctions [24] based on LSMO and YBCO microwires 20– 25 µm wide grown epitaxially on (001) SrTiO3. LSMO was grown on (001)-oriented SrTiO3 single crystals in a pure oxy- gen d.c. sputtering system at high pressure (3.2 mbar) and elevated temperature 900 ◦C. In situ annealing was done at 800 ◦C with 900 mbar O2 pressure for 1 h. Electron beam lithography was performed in a Raith50 module mounted on a SEM Zeiss EVO50 to obtain LSMO microwires and to define amorphous alumina patterns. This is an import- ant step, since the usual process resists do not withstand the high temperatures used to grow these oxides. Amorph- ous alumina was grown in a d.c. sputtering at 7.3 · 10−3 mbar atmosphere (Argon/Oxygen 2:1) at room temperature. YBCO was grown on top of LSMO and a-ALO template in a high O2 pressure (3.4 mbar) d.c. sputtering system at 900 ◦C. In situ annealing was done at 800 ◦C with 900 mbar O2 pressure for 1 h. The spacing between YBCO contacts was ∼1 µm. Figure 1 shows an optical microscope image of a typical device. The ex situ fabrication strategy followed where LSMOwires were first patterned by optical lithography, and then amorphous alumina templates (mechanical masks) were engineered by electron beam lithography to define the YBCO electrodes that did not degrade the structure nor the chemistry of the interfaces as compared to vertical struc- tures grown in situ. Figure 1(c) shows a scanning transmis- sion electron microscopy (STEM) high-resolution image of the YBCO/LSMO interface. It appears to be atomically sharp and highly ordered as in in-situ samples, which indicates that the growth at 900 ◦C in a pure oxygen plasma has the effect of (re)conditioning the surface after exposure to atmosphere or processing. Figure 1(c) displays an atomic resolution electron energy loss spectroscopy (EELS) false color elemental map of the interface showing Ba M4,5 (red), La M4,5 (yellow) and Mn L2,3 (blue). As in the in-situ samples, interface termina- tion corresponds to BaO facing MnO2 planes with missing CuO chains at the interface [38, 39] thus providing a direct Cu- O-Mn superexchange path across the interface which induces a magnetic state in the Visani et al [40]. Moreover, the dif- ference in chemical potentials gives rise to charge (electrons) transfer from the YBCO to the LSMO which depresses the magnetization [38] as assessed by polarized neutron reflecto- metry. Thus, charge transfer (depressed Mn magnetism) and bond reconstruction (induced magnetism in the Cu at the inter- face) may be, together with the magnetic domain structure of the LSMO below the YBCO electrodes, sources of mag- netic inhomogeneity which may play a role in triplet genera- tion. The planar junctions were wired in a four-point contacts configuration (see sketch in figure 1(b)) for transport meas- urements. Resistance was measured by injecting current along the YBCO electrodes and measuring voltage between the ends of the LSMO microwire. Figure 1(c) shows the temperature dependence of the resistance for three different devices. An S- like resistive transition can be seen with steps corresponding to the resistive transition of the different LSMO regions with different strength of the proximity interaction. The LSMO directly underneath the YBCO electrodes proximitizes first and then, gradually, the bare LSMO between the YBCO electrodes. To obtain the critical current, I(V) curves were measured as a function of temperature (figure 2(a)) using the same four point contacts configuration. R(T), R(H) and I(V) were per- formed in a helium close cycle cryostat down to 15 K apply- ing a maximum current of 800 uA and magnetic field up to 4000 Oe while measuring the voltage. Current was injected along the YBCO leads and voltage wasmeasured at the ends of the LSMOwire. Although Josephson junctions are in principle two terminal devices, this configuration was chosen mainly because Al wire bonding yields less noisy, more stable elec- trical contacts for LSMO than for YBCO, yet it provides a measurement without the influence of the interface resistance between LSMO and YBCO. Control experiments in the usual Josephson junction geometry with all four contacts on the YBCOwires (with interface resistance now included in series) showed very similar results pointing to a highly transparent interface, with indeed negligible contribution of interface res- istance compared to the normal state resistance of the LSMO wire. In our junctions, we obtain normal resistances Rn in the range of 1–5 ohms, consistent with the usual low temper- ature resistivity values of single LSMO wires in the range 80µΩ cm—200µΩ cm. I–V curves are non-hysteretic indicat- ing a negligible internal capacitance as expected from strongly overdamped intrinsically shunted devices. I–V plots show considerable rounding and low current and voltage levels, as recently observed in other Josephson junctions with fer- romagnetic barriers [41]. In the following, we describe the temperature dependence of the critical current in our planar Josephson devices. In fact, see figure 2(b), the actual val- ues of the critical current are much lower than anticipated from the linear scale plots, and a rather restrictive voltage criterion (∼hundred of nanovolts) had to be used to reach the non-linear critical current regime. The analysis of the low voltage regime (below a few µV) allows us to estim- ate the critical current, which takes values in the range of 102−103 A cm−2, much lower than the typical ones (a few 107 A cm−2) found in YBCO wires in a similar temperature range. Fits of the low-voltage portion of I(V) curves to the Ivachenko—Zil’berman model [42], see figure 2(c)), showed that the critical current is limited by thermal fluctuations. Figure 2(c) shows the IV curves measured at 18, 30, 35 and 40 K (the same as in figure 2(b)) together with fits to the (ana- lytic) expressions of the Ivachenko—Zil’berman model [42] (black lines). It becomes clear from these fits that, although the low-current rounding can be explained by the effect of thermal fluctuations, the whole voltage range cannot be fit- ted by that model. In order to fit the complete IV curves (yel- low lines in figure 2(c)), a non-linear excess voltage has to be added in series that accounts for the contribution, unavoidable 3 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al Figure 1. Structure and transport of planar YBCO/LSMO/YBCO devices. (a) Optical image of the device showing the amorphous Al2O3 template to deposit YBCO. (b) Sketch showing the wiring of the devices to measure transport. (c) High resolution STEM-HAADF image of the same interface. Left: Atomic resolution HAADF image where atomic positions of each element are indicated by arrows in the different atomic columns. Right: Ba M4,5 (red), Mn L2,3 (blue) and La M4,5 (yellow) elemental EELS map in a color mix of a spectrum image of a region of interest from the same HAADF image. (d) Resistance vs. temperature of three different devices. Separation between YBCO contacts is 1 µm. The width of the LSMO wire electrodes are 20 µm (red) 25 µm (blue and green). Reproduced from [24], with permission from Springer Nature. Figure 2. Temperature dependence of the critical current. (a) IV curves as a function of temperature (left panel) (b) I(V) curves at different temperatures between 18 K and 40 K. Panel adapted from [24] (c) IV curves measured at 18, 30, 35 and 40 K from top to bottom in double logarithmic scale (symbols). The black lines are fits within the low current range, made according to the thermal rounding scenario described in [29]. These fits yield values of Rn = 0.5 Ω at all temperatures and Ic = 60, 30, 15 and 5 µA (increasing temperature). Yellow lines result from adding to the black curves a nonlinear voltage term that accounts for contributions of the superconducting parts with higher critical current that are in series with the LSMO Josephson barrier. Reproduced from [24], with permission from Springer Nature. is this type of planar Josephson devices [41], of other super- conducting parts of the planar devices (YBCO electrodes and proximitized LSMO regions directly underneath the YBCO electrodes) which, despite their higher critical current, yield a significant contribution to the device’s resistance when a suffi- ciently high current is injected. The yellow lines in figure 2(c)) have been obtained by adding a V ∝ In term which is charac- teristic of YBCO in the critical current regime [43], with the 4 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al exponent n changing between 3 and 6 when the temperature is varied. The analysis, based on the Ivachenko—Zil’berman model [42], yields values of Rn = 0.5Ω at all temperatures and Ic = 60, 30, 15 and 5 µA (increasing temperature), which may be somewhat overestimated compared to the actual values. On the one hand, it validates independent estimates of the critical current and the junction’s normal-state resistance Rn ∼ 1 ohm as obtained from this device (i) from the low-temperature res- istivity of LSMO and (ii) from the resistance at the onset of resistive transition’s last step. Notice that the normal state res- istance cannot be obtained from a direct reading of the resist- ance at the critical temperature of the device due to the (dom- inant) series contribution of the resistance of other parts of the device. On the other hand, it provides an explanation for the substantial rounding of the IV curves and small critical cur- rent of the junctions in terms of thermal fluctuations, as one would indeed expect from the wide ferromagnetic barrier and the high critical temperature of the studied Josephson devices, showing that, in fact, thermal energy becomes comparable to Josephson energy (kBT∼ ℏIc 2e ) at temperatures higher 40 K. Notice, finally that we cannot rule out that magnetic scatter- ing of quasi particles may also contribute to the rounding of the I(V), as suggested earlier [41], but in the present junctions this effect is probably weaker than that of thermal fluctuations given the high critical temperatures and the fact that the ana- lysis based on thermal fluctuations provides by itself a close quantitative description of the data. The finding of a long range supercurrent across a one micron long ferromagnetic barrier is a strong indication of triplet proximity, which was further confirmed [24] by flux matching effects of low temperature resistance and phase lock- ing under microwave irradiation. We found a modulation of the resistance at the resistive transition by the (in plane) mag- netic field at different angles with the [100] direction of the LSMO wire. Figure 3 shows the dependence of the resistance with the angle of the in plane magnetic field with the [100] (LSMO wire) direction measured T = 40 K. Injected current was 20 µA. This temperature is just below the onset of the res- istive transition of the superconducting weak link. Notice that while at small magnetic fields (0.01 T) resistance is close to zero in the whole angular range, at larger fields (0.55 T) resist- ance is strongly modulated displaying sharp resistance peaks when the magnetic field is aligned with the [110] easy axes of the LSMO. Resistance is modulated by the variation of the (very small) critical current with magnetic field, and evidences its modulation with magnetization direction. This result, apart from proving the magnetic state of the weak link, strongly sup- ports the triplet state of the pairing amplitude. Finally, we also found resistancemodulation by out of planemagnetic field due to the Fraunhofer pattern modulation of the critical current by magnetic field (not shown) which is being analyzed and will be reported in the near future. We have analyzed the scaling of the critical current with weak link length and temperature to verify that it follows the predictions of the theory of SNS junctions described above. We have prepared samples with different lengths of the LSMO weak link and looked at the temperature dependence of the critical current. Three different samples will be described here, Figure 3. Magnetization dependence of the resistance. Angle dependence of the resistance at 0.01 T (blue symbols) and 0.55 T (black symbols) measured at 25 µA and 40 K. Angle θ refers to different orientations of the magnetic field with the [100] LSMO wire. with weak link lengths 0.75, 1 and 2 µm. The temperature dependence of the critical current of the sample with 0.75 µm is displayed in figure 4(a). Notice the larger values of the critical current as compared to the 1 µm sample shown in figure 4(b). This scaling is in accordance with the 1 L2 size dependence of the Thouless energy. The 2 µm showed no measurable critical current within our temperature window, which is also expected from the exponential suppression of the critical current (see below). Notice that in the low temperature long junction approxim- ation, the critical current features a temperature dependence of the form IcαT3/2e − L ξN . This dependence can be used to obtain directly the Thouless energy noting that the product IcRn in expression [1] can be written explicitly in terms of the Thou- less energy as eIcRn = 32 3+ 2 √ 2 ETh [ 2π kBT ETh ]3/2 e−[2π kBT/ETh] 1/2 . (2) It follows, according to the analysis by Anwar et al [36, 37] that the critical current is proportional to T3/2 exp (√ 2π kBT/ETh ) . The linearity of ln(Ic)− 3 2 ln(T) vs √ T is shown in figure 3(b) for the two superconducting samples. The slope provides a direct measure of the Thouless energy yielding 95 µeV for the sample with 1 µmLSMOweak link and 150 µeV for the 0.75 µm sample. These values, as we demonstrate below, are physically reasonable and compat- ible with previous analyses of the temperature dependence of SNS Josephson junctions, and in particular, with the analysis [12, 44] of the long range proximity effect in CrO2 (also a half metal) with low Tc S’s. The normal state resistance of the devices Rn was obtained from the lowest temperature step of the resistance curves and 5 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al Figure 4. Analysis of the temperature dependence of the critical current. (a) Current–voltage (I–V) curves of a device with a smaller length of LSMO spacing of L = 0.75 µm. Lines connecting points are guides to the eye. (b) Analysis of the temperature dependence of the critical current to extract the Thouless energy (see main text). Notice the larger values of the critical current and the Thouless energy (smaller slope) of the 0.75 µm long weak link (black symbols) compared with the L = 1 µm long weak link (blue symbols). (c) IcRn product for L=2, 1 and 0.75µm. Reproduced from [24], with permission from Springer Nature. was found to be between 1 and 2 Ω for the 1 µm sample, in agreement with the low temperature LSMO resistivity (80 µΩ cm) and bridge dimensions (width 20 µm× thickness 30 nm for this device). This yields Rn values of the order of 1.3 Ω, which are consistent with the value estimated from the resistive transition. FollowingDubos et al [30], further (rough) estimates can be gained from the current at a voltage value corresponding to the Thouless energy (95 µeV) of the low temperature curves. The IV curve at 18 K yields a value of 1.3 Ω in good agreement with our previous estimate. For the sample with 0.75 µm LSMO spacer Rn was estimated to be 5 Ohm, a value which, considering the device dimensions, is also consistent within the values of the resistivity and its range of variation for single LSMOwires. To assess whether samples with different spacer lengths scale according to the predic- tions of the SNS theory by Dubos et al [30], we constructed IcRn products with measured values of the critical current and the estimated Rn which follow the asymptotic expression in terms of the Thouless energy, equation (2). Figure 4(c) shows that experimental points closely follow the theoretical IcRn curve (equation (2)), for both samples. The open blue sym- bols correspond to the sample with LSMO spacing L = 1 µm and a Thouless energy of 95 µeV. Red lines reflect the uncer- tainty in the determination of the Thouless energy according to error bars in the IcRn product: ETh = 85µeV (lower curve) and ETh = 105µeV (upper curve). The open black squares corres- pond to the sample with L = 0.75 µm and higher Thouless energy of 150µeV, as expected for the 1/L2 scaling. Notice that for the sample with L= 2 µm no measurable critical current is expected in the accessible temperature range, as experiment- ally observed. This confirms that the critical current scales with device length as expected from the theory of SNS weak links, confirming that the measured critical current is phys- ically reasonable in this theoretical framework. Furthermore, the fair agreement of the experimental long range triplet super- currents to the predictions of the theory of (singlet pairs) SNS junctions indicates that triplet Andreev pairs diffuse in a half metallic F similar to how singlets do in an N. A further important question concerns the symmetry of the triplet pairs in the orbital sector. The long range triplet supercurrent has interesting implications on the symmetry of the pairing amplitude in the F [45]. Diffusive transport over micron long distances in the diffusive limit requires isotropic pairing to avoid de-phasing by random scatter- ing events. This implies that the anisotropic d-wave pair- ing symmetry of the YBCO, established from Josephson interference experiments [46], would drastically suppress the critical current due to scattering induced de-phasing. Iso- tropic (s-wave) pairing is thus expected on general theoretical grounds [47]. In fact, it has been proposed that in junctions involving high Tc S’s and F’s with domain walls perpendicu- lar to the interface [40], the generation of an odd-frequency triplet s-wave component of the condensate may give rise to long range penetration of the superconductivity in the F along the domain wall. On the other hand, strong on-site Coulomb repulsion present in all cuprates and manganites would, in principle, not favor simple s-wave pairing to avoid the strong repulsion at short range. An intriguing possibil- ity is whether pairing may happen in some higher angular momentum channel in order to avoid the short-range repul- sion. Pairing states with broken time-reversal symmetry, such as mixed (dx2 − y2 ± is)- or (dx2 − y2 ± idxy)-wave pairing states [48–50] are possible in the presence of broken spatial and temporal symmetries in our planar devices with ferromag- netic weak links. Moreover, for a cylindrical Fermi surface, this state is nodeless which optimizes the condensation energy and it avoids the [110] zero energy Andreev bound states (which suppress the d-wave order parameter upon scattering). Future experiments will be directed to address these exciting possibilities. In summary, we have found that the critical current of planar Josephson junctions combining high Tc S’s and half metallic manganites follows the predictions of the SNS theory for conventional proximity coupled Josephson junctions. This result suggests that fully spin polarized triplet Andreev pairs diffuse in the half metal similarly to singlets in an N. The very long (micrometric) distance over which phase coherent trans- port has been found underlines the potential of these devices in future superconducting spintronics. Furthermore, the pos- sibility of controlling the spin polarized supercurrents with the 6 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al domain state of the F opens interesting opportunities for novel concepts of quantum switches and quantum memories. Data availability statement Data will be available from the corresponding author upon reasonable request. Acknowledgments Work supported by Spanish AEI through Grant PID2020- 118078RB-I00 and by Regional Government of Madrid CAM through SINERGICO Project Y2020/NMT-6661 CAIRO- CM. Work at CNRS/Thales lab supported by ERC Grant No. 647100 ‘SUSPINTRONICS’, French ANR Grant ANR-15- CE24-0008-01 ‘SUPERTRONICS’ and COST action ‘Nano- cohybri’.We acknowledge funding from Flag ERAERA-NET To2Dox Project. J E V thanks C Ulysse and L Vila for collab- oration in related projects. Conflict of interest Authors declare that they have no conflict of interest. ORCID iDs David Sanchez-Manzano https://orcid.org/0000-0003- 1229-6868 M Garcia-Hernandez https://orcid.org/0000-0002-5987- 0647 C Feuillet-Palma https://orcid.org/0000-0002-8389-5756 J Lesueur https://orcid.org/0000-0002-5843-187X C Leon https://orcid.org/0000-0002-3262-1843 J Santamaria https://orcid.org/0000-0003-4594-2686 References [1] Bergeret F S, Volkov A F and Efetov K B 2001 Long-range proximity effects in superconductor-ferromagnet structures Phys. Rev. Lett. 86 4096–9 [2] Eschrig M, Kopu J, Cuevas J C and Schön G 2003 Theory of half-metal/superconductor heterostructures Phys. Rev. Lett. 90 4 [3] Buzdin A I 2005 Proximity effects in superconductor-ferromagnet heterostructures Rev. Mod. Phys. 77 935–76 [4] Bergeret F S, Volkov A F and Efetov K B 2005 Odd triplet superconductivity and related phenomena in superconductor-ferromagnet structures Rev. Mod. Phys. 77 1321 [5] Houzet M and Buzdin A 2007 Long range triplet Josephson effect through a ferromagnetic trilayer Phys. Rev. B 76 060504 [6] Asano Y, Tanaka Y and Golubov A A 2007 Josephson effect due to odd-frequency Pairs in diffusive half metals Phys. Rev. Lett. 98 107002 [7] Sperstad I B, Linder J and Sudbø A 2008 Josephson current in diffusive multilayer superconductor/ferromagnet/ superconductor junctions Phys. Rev. B 78 104509 [8] Halterman K, Valls O T and Barsic P H 2008 Induced triplet pairing in clean s -wave superconductor/ferromagnet layered structures Phys. Rev. B 77 174511 [9] Eschrig M and Löfwander T 2008 Triplet supercurrents in clean and disordered half-metallic ferromagnets Nat. Phys. 4 138–43 [10] Keizer R S, Goennenwein S T B, Klapwijk T M, Miao G, Xiao G and Gupta A 2006 A spin triplet supercurrent through the half-metallic ferromagnet CrO2 Nature 439 825–7 [11] Khaire T S, Khasawneh M A, Pratt W P and Birge N O 2010 Observation of spin-triplet superconductivity in co-based Josephson junctions Phys. Rev. Lett. 104 137002 [12] Anwar M S, Czeschka F, Hesselberth M, Porcu M and Aarts J 2010 Long-range supercurrents through half-metallic ferromagnetic CrO2 Phys. Rev. B 82 100501 [13] Robinson A W J Witt S D J and Blamire M G 2010 Controlled injection of spin-triplet supercurrents into a strong ferromagnet Science 329 59–61 [14] Klose C et al 2012 Optimization of spin-triplet supercurrent in ferromagnetic Josephson junctions Phys. Rev. Lett. 108 127002 [15] Glick J A, Aguilar V, Gougam A B, Niedzielski B M, Gingrich E C, Loloee R, Pratt W P and Birge N O 2018 Phase control in a spin-triplet SQUID Sci. Adv. 4 eaat9457 [16] Linder J and Robinson J W A 2015 Superconducting spintronics Nat. Phys. 11 307–15 [17] Eschrig M 2015 Spin-polarized supercurrents for spintronics: a review of current progress Rep. Prog. Phys. 78 104501 [18] Sefrioui Z, Arias D, Peña V, Villegas J E, Varela M, Prieto P, León C, Martinez J L and Santamaria J 2003 Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices Phys. Rev. B 67 214511 [19] Dybko K, Werner-Malento K, Aleshkevych P, Wojcik M, Sawicki M and Przyslupski P 2009 Possible spin-triplet superconducting phase in the La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 trilayer Phys. Rev. B 80 144504 [20] Visani C, Sefrioui Z, Tornos J, Leon C, Briatico J, Bibes M, Barthélémy A, Santamaría J and Villegas J E 2012 Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions Nat. Phys. 8 539–43 [21] Kalcheim Y, Millo O, Egilmez M, Robinson J W A and Blamire M G 2012 Evidence for anisotropic triplet superconductor order parameter in half-metallic ferromagnetic La0.7Ca0.3Mn3O proximity coupled to superconducting Pr1.85Ce0.15CuO4 Phys. Rev. B 85 104504 [22] Egilmez M, Robinson J W A, MacManus-Driscoll J L, Chen L, Wang H and Blamire M G 2014 Supercurrents in half-metallic ferromagnetic La0.7Ca0.3MnO3 Europhys. Lett. 106 37003 [23] Visani C, Cuellar F, Pérez-Muñoz A, Sefrioui Z, León C, Santamaría J and Villegas J E 2015 Magnetic field influence on the proximity effect at YBa2Cu3O7/La2/3 Ca1/3MnO3 superconductor/half-metal interfaces Phys. Rev. B 92 014519 [24] Sanchez Manzano D et al 2022 Extremely long-range, high-temperature Josephson coupling across a half-metallic ferromagnet Nat. Mater. 21 188 [25] de Gennes P G 1964 Rev. Mod. Phys. 36 225 [26] Müller G M et al 2009 Nat. Mater. 2008 56 [27] Likharev K K 1976 Sov. Tech. Phys. Lett. 2 12 [28] Likharev K K 1979 Superconducting weak links Rev. Mod. Phys. 51 101 [29] Usadel K D 1970 Phys. Rev. Lett. 25 507 7 https://orcid.org/0000-0003-1229-6868 https://orcid.org/0000-0003-1229-6868 https://orcid.org/0000-0003-1229-6868 https://orcid.org/0000-0002-5987-0647 https://orcid.org/0000-0002-5987-0647 https://orcid.org/0000-0002-5987-0647 https://orcid.org/0000-0002-8389-5756 https://orcid.org/0000-0002-8389-5756 https://orcid.org/0000-0002-5843-187X https://orcid.org/0000-0002-5843-187X https://orcid.org/0000-0002-3262-1843 https://orcid.org/0000-0002-3262-1843 https://orcid.org/0000-0003-4594-2686 https://orcid.org/0000-0003-4594-2686 https://doi.org/10.1103/PhysRevLett.86.4096 https://doi.org/10.1103/PhysRevLett.86.4096 https://doi.org/10.1103/PhysRevLett.90.137003 https://doi.org/10.1103/PhysRevLett.90.137003 https://doi.org/10.1103/RevModPhys.77.935 https://doi.org/10.1103/RevModPhys.77.935 https://doi.org/10.1103/RevModPhys.77.1321 https://doi.org/10.1103/RevModPhys.77.1321 https://doi.org/10.1103/PhysRevB.76.060504 https://doi.org/10.1103/PhysRevB.76.060504 https://doi.org/10.1103/PhysRevLett.98.107002 https://doi.org/10.1103/PhysRevLett.98.107002 https://doi.org/10.1103/PhysRevB.78.104509 https://doi.org/10.1103/PhysRevB.78.104509 https://doi.org/10.1103/PhysRevB.77.174511 https://doi.org/10.1103/PhysRevB.77.174511 https://doi.org/10.1038/nphys831 https://doi.org/10.1038/nphys831 https://doi.org/10.1038/nature04499 https://doi.org/10.1038/nature04499 https://doi.org/10.1103/PhysRevLett.104.137002 https://doi.org/10.1103/PhysRevLett.104.137002 https://doi.org/10.1103/PhysRevB.82.100501 https://doi.org/10.1103/PhysRevB.82.100501 https://doi.org/10.1126/science.1189246 https://doi.org/10.1126/science.1189246 https://doi.org/10.1103/PhysRevLett.108.127002 https://doi.org/10.1103/PhysRevLett.108.127002 https://doi.org/10.1126/sciadv.aat9457 https://doi.org/10.1126/sciadv.aat9457 https://doi.org/10.1038/nphys3242 https://doi.org/10.1038/nphys3242 https://doi.org/10.1088/0034-4885/78/10/104501 https://doi.org/10.1088/0034-4885/78/10/104501 https://doi.org/10.1103/PhysRevB.67.214511 https://doi.org/10.1103/PhysRevB.67.214511 https://doi.org/10.1103/PhysRevB.80.144504 https://doi.org/10.1103/PhysRevB.80.144504 https://doi.org/10.1038/nphys2318 https://doi.org/10.1038/nphys2318 https://doi.org/10.1103/PhysRevB.85.104504 https://doi.org/10.1103/PhysRevB.85.104504 https://doi.org/10.1209/0295-5075/106/37003 https://doi.org/10.1209/0295-5075/106/37003 https://doi.org/10.1103/PhysRevB.92.014519 https://doi.org/10.1103/PhysRevB.92.014519 https://doi.org/10.1038/s41563-021-01162-5 https://doi.org/10.1038/s41563-021-01162-5 https://doi.org/10.1103/RevModPhys.36.225 https://doi.org/10.1103/RevModPhys.36.225 https://doi.org/10.1038/nmat2341 https://doi.org/10.1038/nmat2341 https://doi.org/10.1103/RevModPhys.51.101 https://doi.org/10.1103/RevModPhys.51.101 https://doi.org/10.1103/PhysRevLett.25.507 https://doi.org/10.1103/PhysRevLett.25.507 Supercond. Sci. Technol. 36 (2023) 074002 D Sanchez-Manzano et al [30] Dubos P, Courtois H, Pannetier B, Wilhelm F K, Zaikin A D and Schön G 2001 Josephson critical current in a long mesoscopic S-N-S junction Phys. Rev. B 63 064502 [31] Courtois H, Ph. Gandit D M and Pannetier B 1996 Phys. Rev. Lett. 76 130 [32] Kabasawa U, Tanitani Y, Fukazawa T, Tsukamoto A, Masahiko Hiratani M H and Kazumasa Takagi K T 1991 Jpn. J. Appl. Phys. 1 1670 [33] Kozono Y, Kasai M and Kanke Y Ohno T, Hanazono M, and Sugita Y 1991 Physica C 185–189 1919 [34] Boguslavskii Y M and Gao J 1992 Physica C 194 268 [35] Satoh T, Hidaka M, Yu K M, Hidaka M and Tsuge H 1995 IEEE Trans. Appl. Supercond. 5 2612 [36] Kupriyanov M Y and Tsai J S 1995 IEEE Trans. Appl. Supercond. 5 2531 [37] Devyatov I A and Kupriyanov M Y 1997 JETP 85 189 [38] Hoffmann A, te Velthuis S G E, Sefrioui Z, Santamaría J, Fitzsimmons M R, Park S and Varela M 2005 Suppressed magnetization in La0.7Ca0.3MnO3 YBa2Cu3O7-δ superlattices Phys. Rev. B 72 140407 [39] Salafranca J and Okamoto S 2010 Unconventional proximity effect and inverse spin-switch behavior in a model manganite-cuprate-manganite trilayer system Phys. Rev. Lett. 105 256804 [40] Visani C et al 2011 Phys. Rev. B 84 060405(R) [41] Bhatia E, Srivastava A, Devine-Stoneman J, Stelmashenko N A, Barber Z H, Robinson J W A and Senapati K 2021 Nanoscale domain wall engineered spin-triplet Josephson junctions and SQUID Nano Lett. 21 3092–7 [42] Ivachenko Y M and Zil’berman L A 1969 The Josephson effect in small tunnel contacts Sov. J. Exp. Theor. Phys. 28 1272 [43] Koch R H, Foglietti V, Gallagher W J, Koren G, Gupta A and Fisher M P A 1989 Experimental evidence for vortex-glass superconductivity in Y-Ba-Cu-O Phys. Rev. Lett. 63 1511 [44] Anwar M S, Veldhorst M, Brinkman A and Aarts J 2012 Long range supercurrents in ferromagnetic CrO2 using a multilayer contact structure Appl. Phys. Lett. 100 052602 [45] Tanaka Y and Golubov A A 2007 Theory of the proximity effect in junctions with unconventional superconductors Phys. Rev. Lett. 98 037003 [46] Tusei C C and Kirtley J R 2000 Rev. Mod. Phys. 72 969 [47] Volkov A F and Efetov B K 2009 Proximity effect and its enhancement by ferromagnetism in high-temperature superconductor-ferromagnet structures Phys. Rev. Lett. 102 077002 [48] Laughlin R B 1998 Magnetic induction of dx2−y2+idxy order in high- Tc superconductors Phys. Rev. Lett. 80 5188 [49] Franz M and Tesannovic Z 1998 Self-consistent electronic structure of a dx2−y2 and a dx2−y2+idxy Vortex Phys. Rev. Lett. 80 4763 [50] Balatsky A V 2000 Field-induced dx2−y 2+idxy state and marginal stability of high-Tc superconductors Phys. Rev. Lett. 61 6940 8 https://doi.org/10.1103/PhysRevB.63.064502 https://doi.org/10.1103/PhysRevB.63.064502 https://doi.org/10.1103/PhysRevLett.76.130 https://doi.org/10.1103/PhysRevLett.76.130 https://doi.org/10.1143/JJAP.30.1670 https://doi.org/10.1143/JJAP.30.1670 https://doi.org/10.1016/0921-4534(91)91084-H https://doi.org/10.1016/0921-4534(91)91084-H https://doi.org/10.1016/S0921-4534(05)80004-8 https://doi.org/10.1016/S0921-4534(05)80004-8 https://doi.org/10.1109/77.403125 https://doi.org/10.1109/77.403125 https://doi.org/10.1109/77.403105 https://doi.org/10.1109/77.403105 https://doi.org/10.1134/1.558305 https://doi.org/10.1134/1.558305 https://doi.org/10.1103/PhysRevB.72.140407 https://doi.org/10.1103/PhysRevB.72.140407 https://doi.org/10.1103/PhysRevLett.105.256804 https://doi.org/10.1103/PhysRevLett.105.256804 https://doi.org/10.1103/PhysRevB.84.060405 https://doi.org/10.1103/PhysRevB.84.060405 https://doi.org/10.1021/acs.nanolett.1c00273 https://doi.org/10.1021/acs.nanolett.1c00273 https://doi.org/10.1103/PhysRevLett.63.1511 https://doi.org/10.1103/PhysRevLett.63.1511 https://doi.org/10.1063/1.3681138 https://doi.org/10.1063/1.3681138 https://doi.org/10.1103/PhysRevLett.98.037003 https://doi.org/10.1103/PhysRevLett.98.037003 https://doi.org/10.1103/RevModPhys.72.969 https://doi.org/10.1103/RevModPhys.72.969 https://doi.org/10.1103/PhysRevLett.102.077002 https://doi.org/10.1103/PhysRevLett.102.077002 https://doi.org/10.1103/PhysRevLett.80.5188 https://doi.org/10.1103/PhysRevLett.80.5188 https://doi.org/10.1103/PhysRevLett.80.4763 https://doi.org/10.1103/PhysRevLett.80.4763 https://doi.org/10.1103/PhysRevB.61.6940 https://doi.org/10.1103/PhysRevB.61.6940 Unconventional long range triplet proximity effect in planar YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 Josephson junctions 1. Introduction 2. Results and discussion References