Editorial Feature

Tunability of Spin-Orbit Electron Coupling in Indium Selenide Films

New research looks at the superconducting properties of indium selenide films to assess their viability in spintronic devices.

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Indium selenide (InSe) is an inorganic compound of indium and selenium with a wide range of applications in technology as diverse as optoelectronics, field-effect transistors, and methanol gas sensors.

The solid has a structure consisting of two-dimensional layers bonded together with van der Waals forces — interactions between molecules or atoms that do not arise from a chemical or electronic bond that vanishes at increasing distances between molecules.

InSe is a superconductor, meaning that at certain temperatures it allows the free movement of electrons with no resistance and it can hold a magnetic field externally without it penetrating through the material.

InSe films that are atomically thin can be produced from bulk crystals or can be produced by chemical vapor deposition (CVD), a method used in the semiconductor industry to create high quality, and high-performance, solid materials in thin films.

InSe is already used to create field-effect transistors (FET devices) — a type of transistor that uses an electric field to control the flow of current in a semiconductor. But, one of the most promising potential applications of InSe films arising thanks to the persistent high-mobility of electrons throughout it, is in so-called spintronic devices.

A new research paper published in the journal Physical Review B demonstrates the spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer γ-InSe films.

The authors reveal that because of competition between film-thickness and the electric-field-induced SOC, this material is tuneable. This tunability makes the creation of electrically switchable spintronic devices possible.

What is Spintronics?

Spintronics — otherwise known as spin transport electronics or just spin electronics — is the study of a quantum quality of electrons called “spin.” 

Spin is an intrinsic form of angular momentum that is carried by elementary particles like electrons and quarks that carries up to the composite particles they form like protons, neutrons, and even atoms. It is separate from another form of quantum angular momentum called orbital angular momentum.

Spin is a vector meaning it has two values, charge and direction. While all particles of the same type have spin with the same magnitude of spin angular momentum, the directions of spins can vary between particles of the same type. The traditional values for a spin in electrons are spin up or spin down.

The spin of an electron has an associated magnetic moment, meaning that passing electrons through a magnetic field should induce some electrons to move upwards and others to move downwards depending on their spin. 

More on Thin Films: Can Phone Screens Be Improved With Anisotropic Light-Diffusing Films?

In a solid, the spins of various electrons can act in unison to change the magnetic and electronic properties of a material.

The field of spintronics concerns spin-charge coupling in metallic systems and differs from traditional electronics because, in addition to charge, electron spins are used as an additional degree of freedom.

This has implications for spintronic devices as this extra degree of freedom can be exploited in both the storage and transportation of data. Spintronic devices manipulate the spins of electrons polarizing them in such a way that there are an abundance of spin-up or spin-down particles.

Superconductors enhance the qualities of spintronics like magnetoresistance effects — the tendency of a material to change the value of its electrical resistance in an externally-applied magnetic field — and the lifetimes of spin values. 

A Spintronic Synergy

A paper published in Nature Physics in 2015 explains that traditional studies that combine spintronics and superconductivity have mainly focused on the injection of spin-polarized quasiparticles into superconducting materials. 

It adds that the creation of spin-triplet Cooper pairs, which are generated at carefully engineered superconductor interfaces with ferromagnetic materials, could lead to a complete synergy between superconducting and magnetic orders.

The authors add: “currently, there is intense activity focused on identifying materials combinations that merge superconductivity and spintronics to enhance device functionality and performance.”

The authors of this new study published in Physical Review B could have taken a step towards this synergy by revealing the superconducting properties of InSe films in greater detail.

They conclude that the description of SOC strength as a function of the number of layers and the applied electric field piercing the multilayer film they obtained in experiments match well with theoretically calculated values.

The size of the SOC constant that they computed from InSE films with between 2 and 10 layers is similar to the strength of quantum wells — the discrete energy levels that confine electrons or quasi-particles to a dimension perpendicular to the surface of the layer — in conventional semiconductors.

The quality the authors found distinguished 2D InSe films from other spintronic systems is that its SOC strength can be tuned over a wide range. They also found that the spin-orbit coupling strength for electrons near the conduction band edge in few-layer γ-InSe films can also be tuned over a wide range.

This tunability results from a competition between film-thickness-dependent intrinsic and electric-field-induced SOC, potentially, allowing for electrically switchable spintronic devices.

References and Further Reading 

Ceferio. A., Magorrian. S. J., Zólyomi. V. et al, [2021], ‘Tunable spin-orbit coupling in two-dimensional InSe,’ Physical Review B, [DOI: 10.1103/PhysRevB.104.125432] https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.125432

Linder. J., Robinson. J. W. A., [2015], ‘Superconducting spintronics,’ Nature Physics, https://www.nature.com/articles/nphys3242

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Robert Lea

Written by

Robert Lea

Robert is a Freelance Science Journalist with a STEM BSc. He specializes in Physics, Space, Astronomy, Astrophysics, Quantum Physics, and SciComm. Robert is an ABSW member, and aWCSJ 2019 and IOP Fellow.

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