Superconducting type transition by proximity effect between a frustrated antiferromagnetic insulator and a normal metal

doi:10.21203/rs.3.rs-3117195/v1

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Superconducting type transition by proximity effect between a frustrated antiferromagnetic insulator and a normal metal

https://doi.org/10.21203/rs.3.rs-3117195/v1

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A superconductor-like transition in a copper channel is observed when an antiferromagnetic layer is in contact with this channel. This reminds the superconducting cuprates that from the phase diagram one observes an antiferromagnetic phase close to the superconducting phase. In my proposal, there is a subtle difference, the magnetic phase is not killed as in cuprates. [Cu2(OH)3Cl] was used as the antiferromagnetic insulator and Copper (Cu) as the normal metal. The copper conductive channel is formed by applying a voltage of 70 V.

Soft Condensed-matter Physics

Magnetism

Magnetics Materials and Devices

Superconductivity

Proximity effect

Dicopper chloride trihydroxide [Cu2(OH)3Cl]

Copper (Cu)

It seems that in common sense, superconductivity and magnetism are two incompatible phenomena in the same material, it is such that one of the fundamental properties of superconductors is the expulsion of the magnetic field from its interior [1, 2, 3]. In addition, the exchange interaction forces the alignment of the Cooper pairs' spins in the same direction, preventing pairing. On the other hand, in some classes of materials such as cuprates [4], heavy fermions [5], iron pnictides [6] and organic superconductors [7], the superconductivity is close to the antiferromagnetic state and in some cases, there is a coexistence of these two phenomena as observed by the phase diagram of these materials, a satisfactory explanation for the emergence of superconductivity in these materials is still controversial. In practice, superconductivity and antiferromagnetism can harmoniously coexist because, on average, the exchange interaction is zero, such that the magnetic moment has no effect on Cooper pairs. Although the antiferromagnetic exchange interaction does not prevent the superconductor pairing, some anomalies arise in these antiferromagnetic superconductors such as: anomalous dependence of the critical field as a function of temperature [8], gapless superconductivity [9], appearance of new phases in antiferromagnetic materials that exhibit weak ferromagnetism [10]. Pure cuprates, a class of undoped copper-based oxides, are antiferromagnetic insulators that arise from the electron spins of neighbouring coppers in the CuO₂ planes. Due to strong indirect exchange interactions between copper spins, the long-range antiferromagnetic ordering is established with T_N ~ 300–500 K, above a certain doping value, the antiferromagnetic phase disappears giving way to a superconducting phase in these compounds. A feature of all cuprates is the presence of CuO₂ planes and is where superconductivity occurs, whose planes are separated by other layers called charge reservoirs.

The phase diagram of the cuprates is quite interesting, but quite complex due to the numerous phases when doping these materials. On the other hand, this can be circumvented by mimicking those, building an antiferromagnetic insulating material in contact with a normal metal. On the other hand, this can be circumvented by mimicking those, building an antiferromagnetic insulating material in contact with a normal metal. And by proximity effect, verify how much antiferromagnetism can influence the properties of the metallic layer from which the new properties will be observed. In the literature there are already few theoretical works dealing with such a study [11, 12, 13], but it seems that the choice of materials to observe superconductivity is not the best. I recently published a preprint [14] that is under peer review showing that there are changes in resistance as a function of temperature when a highly frustrated antiferromagnetic insulator is in contact with normal metal. My results clearly show an increase in resistance as a function of temperature in the copper path exactly where the clinoatacamite magnetic transitions occur. Such behaviour is related to the change of spin states of copper where such state would be the creation of singlet pairs in the copper metallic path.

In this work, I show superconducting type transitions by proximity effect that occur in the metallic copper path when a highly frustrated antiferromagnetic layer is in contact with it.

The samples of Dicopper chloride trihydroxide Cu₂(OH)₃Cl as antiferromagnetic insulators and Copper as powder metal used in this work were from Sigma Aldrich company with 99.9% purity. The samples were prepared in pellet form by mixing the powders forming a composite in a steel matrix of diameter, d = 5 mm. The pressure used was 15 kbar for about 10 minutes to compact the powder into a 0.5 mm thick pellet. With the prepared wafer-shaped composites, we made electrical contacts using silver paint and gold wire. The cryostat used for the electrical transport measurements was the Janis Research, model Supervaritemp.

Before transport measurement, it is fundamental induce a cooper channel, where the current will flow, by applying a voltage of 70 V at 300 K to the volume of the composite. The figure (1a) shows the behavior of resistance as a function of temperature for the Cu₂(OH)₃Cl (60%)-Cu (40%) after applying 70 V to create the copper channel in the composite. As we can see in this figure, there is a drop in resistance around 5 K that resembles a superconducting transition and that is close to the T_N2 magnetic transition observed in clinoatacamite. The figure (1b) shows the behavior of resistance as a function of temperature for the Cu₂(OH)₃Cl (75%)-Cu (25%) after applying 70 V to create the copper channel in the composite. Calls our attention, an increase in resistance as a function of temperature at 6.4 K which coincides with the T_N2 transition of clinoatacamite, this increase could be indicative of a gap opening or a change in the state of the copper channel spins induced by the strong spin fluctuations of clinoatacamite that are well known. The same behavior was observed for a second sample around 18 K. The two transitions were never observed to occur at the same time in a single sample. On the other hand, the reproducibility of the results is good. It is not possible to apply a magnetic field to know the behavior of the superconducting transition, because from the second measurement onwards the superconducting transition disappears. In this scenario we can propose some hypotheses, if a copper channel is formed by application of external voltage in the Cu₂Cl(OH)₃ matrix: (1) the copper channel can function as a charge reservoir, transferring electrons to clinoatacamite, on the other hand, there is no impediment to the opposite. O role of chlorine and oxygen is very difficult to inform, but they could be the dopants as they occur in cuprates superconductors: (2) from the proximity effect, the spins at the interface of the antiferromagnetic insulating layer of clinoatacamite can polarize the channel spins in the conductor of copper creating superconducting pairing: (3) interfacial superconductivity due to non-trivial electronic reconstruction when the metal - (Cu) is in contact with the antiferromagnetic insulator – Cu₂Cl(OH)₃.

It is concluded that it is possible to induce superconductivity in the copper channel by approaching a highly frustrated antiferromagnetic layer. This is very similar to superconducting cuprates where the superconductivity occurs close to the superconducting phase, but there is a certain difference because in cuprates the dope kills the magnetic phase, and the superconducting phase appears. In this proposal to approximate a highly frustrated antiferromagnetic material, the magnetic phase is never killed because it is essential to induce superconductivity in the copper channel since the transition occurs practically in the magnetic transitions of Clinoatacamite. I believe that from numerical calculations one can quickly arrive at the explanation of such behavior and shed light on the phase diagram of various magnetic superconductors.

Data Availability

The data used to support the findings of this study are available upon reasonable request from the corresponding authors.

Conflicts of Interest

The author declares that there are no conflicts of interest.

Acknowledgements

This work was carried out with the support of the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES) - Financing Code 001 for 1 year. I thank the “Ministry of Science, Technology and Innovations” and “National Council of Scientific and Technological Development” – CNPq” for the financial support granted to me for 3 years.

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Superconducting type transition by proximity effect between a frustrated antiferromagnetic insulator and a normal metal

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