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A new method to control superconductivity with spin current

As we saw previously, superconductivity is the property of some material to conduct electricity without any resistance. Different materials can be used with the main difference based on the transition temperature which can be in some case a high temperature (actually not so high for now, only 63 K, the temperature of liquid nitrogen). Beyond these special metals, iron-based superconductors have shown intriguing phenomena related to the coexistence of magnetism and superconductivity below the superconducting transition temperature. The origin of such phenomena is still under debate, and recent studies have shown new antiferromagnetic phases coexisting with superconductivity and have reported that the superconducting can be suppressed by the magnetic field. Being able to manipulate this effect could lead to novel antiferromagnetic memory devices and transistors, opening a new way to more efficient electronic systems.

To better understand what’s happening scientists had the idea to do a direct atomic-scale control of the magnetic field. And recently, a team from South Korea has tried this with success by controling with local probes the atomic magnetism demonstrating a correlation with superconductivity.

General phase diagram and electronic structure of iron-based superconductors. (a) General phase diagram of iron-based superconductors. (b) Lattice and magnetic structure in the symmetry breaking states of iron-based superconductors. (c) Calculation of the band dispersion. (d) Illustration of the general Fermi surface topology of an iron-based superconductor. Source: Nature

M. Lee and his team explored a new mechanism for switching magnetism and superconductivity by manipulating the magnetic field at an atomic level using spin-polarized scanning tunneling microscopy. Their study on single-crystal Sr2VO3FeAs showed that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial magnetic order, which cannot be achieved by thermal excitation with an unpolarized current.

To our knowledge, our study is the first report of a direct real-space observation of this type of control by a local probe, as well as the first atomic-scale demonstration of the correlation between magnetism and superconductivity.

Jhinhwan Lee, a physicist at the Korea Advanced Institute of Science and Technology.

The tunneling spectroscopy study showed that the induced magnetic field has characteristics of antiferromagnetic and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases this effect. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.

Schematic illustrations of FeAs-layer configuration potential landscapes. (a) The imaginary case layers being separated sufficiently apart. (b) The natural separation found results in interlayer coupling and near degeneracy among the magnetic states with different symmetries, with the magnetism with strong superconductivity still being the ground state. (c) If a sufficiently strong spin-polarized current is injected, the balances among these states may change, possibly resulting in magnetic states with weak superconductivity in the FeAs layer. (d) When the sample is thermally annealed globally or locally with a high bias tunneling current injection, it may return to the ground states with magnetism and strong superconductivity.

These results are a unique and clear demonstration of switching the Fe-layer magnetism and superconductivity by spin-polarized current injection and thermal agitation. These findings may be extended toward future studies for heterostructure superconductor devices manipulating magnetism and superconductivity using spin-polarized and unpolarized currents.

Our findings may be extended to future studies where magnetism and superconductivity are manipulated using spin-polarized and unpolarized currents, leading to novel antiferromagnetic memory devices and transistors controlling superconductivity.

Continue reading at: https://phys.org/news/2017-12-scientists-superconductivity-currents.html

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