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Spintronics are promising devices that work utilizing not only the charge of electrons, like conventional electronics, but also their spin (i.e., their intrinsic angular momentum). The development of fast and energy-efficient spintronic devices greatly depends on the identification of materials with a tunable spin-selective conductivity, which essentially means that engineers can control how electrons with different spin orientations move through these materials, ideally using external magnetic or electric fields.

Researchers at Columbia University and the National High Magnetic Field Laboratory recently unveiled a new mechanism that enables the spin-selective transport of charge carriers in an atomically thin transition metal dichalcogenide, namely tungsten diselenide (WSe₂). Their paper, in Nature Â鶹ÒùÔºics, could open new possibilities for the development of compact and energy-efficient components for spintronic devices.

"Spin is a fundamental quantum property of electrons, which—in a simplified picture—can be thought of like a tiny internal compass needle pointing either 'up' or 'down,'" said En-Min Shih, first author of the paper. "Spin lies at the heart of magnetism and plays a crucial role in many technologies. For example, in a hard disk drive, information is stored based on whether the magnetization of nanoscale regions points up or down. In this way, you can 'write' information by forcing the spins in a certain region to order along a particular direction, but how do you 'read' the information?"

One common approach to "reading" the information stored in spintronic devices entails measuring how easily electrical current passes through magnetized regions in a material. This approach relies on the fact that electrical current is carried by moving electrons, which also have spin.

"The essential idea is that when the spin of the moving electrons matches the spins in the nearby magnet, electrical conductivity is high (the electrons can easily move), and when the spins don't match, the conductivity is low (electrons are impeded)," explained Cory Dean, lead PI of the project.

"While existing technology works well, it is challenging and expensive to make, primarily because you have to integrate different materials into complicated structures. In our work, we asked a simple question: Can we achieve spin-selective transport—where only electrons with a certain spin state can move—using just a single, non-magnetic material?"

As part of their recent study, the team specifically tried to realize spin-selective transport in WSe₂, a material that can be synthesized to be atomically thin, similarly to graphene. Unlike graphene, however, WSe₂ exhibits a large energy splitting between spin "up" and "down" energy levels when an external magnetic field is applied to it and stronger Coulomb interactions between charge carriers, while also having a larger effective mass.

"We performed transport measurements in high-quality WSe₂ devices under high magnetic fields," said co-first author of the paper, Qianhui Shi. "The magnetic field induces discrete energy levels known as Landau levels. The appearance of Landau levels is a general feature of any two-dimensional material in the presence of large magnetic fields.

"In most systems, the Landau level sequence consists of alternating spin-up and spin-down levels. So, if you add electrons to these levels to fill them up, the first level has all spins pointing up, then in the second level, all spins point down, and so on. In this regard, WSe₂ is exceptionally unique where several levels in a row can all have the same spin before you encounter the opposite spin state."

In their earlier studies, Shi and her colleagues observed unique and interesting spin patterns in WSe_2. In one study, for instance, they found that when the material was under a large magnetic field, as many as six spin-up levels would be filled before a spin-down level appeared.

"What we realized in this new study is that when we looked at current flowing in the highest landau level it behaves similar to the sandwich structures used for magnetic memory," explained Shi. "For example, if the spin in the highest level is the same as all the lower levels, the conductivity is high. However, when the spin of the highest level is opposite to the lower levels, the conductivity is low.

"This is a kind of spin-selective transport effect, but with the current carrying and magnetic layers separated between energy levels in a single material, rather than physically confined to two different materials."

Interestingly, the researchers observed a dramatic contrast in the electrical conductivity of WSe₂, depending on the spin of the mobile carriers at the Fermi level. This is essentially the energy level that represents the highest occupied state of charge carriers in materials.

"When these carriers belong to the majority spin group (same spin as the lower energy 'filled' levels), they can move freely and contribute to significant conductance; when they are in the minority spin group (opposite spin to the filled levels), their motion is effectively suppressed—they become localized, and conductivity drops sharply," said Shih.

"Most surprising to us was that this is not a small effect and indeed, under the right conditions, can be quite dramatic with the low conductance state showing effectively no conductivity at all—i.e., a true 'off' state."

In their paper, Shih and his colleagues tried to offer a possible explanation for the interesting effect they observed in WSe₂. Ultimately, they proposed that the effect could arise from the strength with which the mobile carriers interact with the background of already-filled, inert electronic states.

"When the mobile carriers have a different spin from the background, they experience strong Coulomb interactions with the immobile charges—they are essentially 'dragged' by the background immobile charges and are forced to move slowly or even not at all," explained Kun Yang, theoretical collaborator of the paper.

"In contrast, when the spins match, the quantum principle of Pauli exclusion prevents them from interacting very strongly and in this regime the mobile electrons pass by without much regard for the immobile charges."

The researchers demonstrated that the effect observed in their experiments can be leveraged to realize spin-selective transport. Moreover, the mechanism through which spin-selective transport is achieved (i.e., the interplay between strong Coulomb interactions and the separation of spin in distinct energy bands) was found to have notable advantages over other previously outlined mechanisms prompting spin-selective transport.

"In contrast to magnetoresistive heterostructures with spatially separated magnetic domains, this mechanism achieves spin filtering within a single material, driven by the interaction between free and localized spins residing in energy-separated bands," said Dean. "Furthermore, since we can tune the Fermi level to reside in either the spin majority or minority levels by either a magnetic or electric field, the spin selectivity is highly tunable."

This recent work by Shih, Dean, Shi and their colleagues introduces a new promising pathway for the fabrication of compact, efficient and tunable spintronic devices. In their next studies, the researchers plan to explore the mechanism they identified in more depth, to understand whether it can also be exploited without the use of an external magnetic field.

"Moiré structures, such as those formed by twisting two layers of WSe₂ by a small angle, host flat bands that resemble the Landau levels produced but without requiring the magnetic field," added Shi. "In addition, we showed in our study that this effect does not only rely on what is called 'real particle spin' but also other quantum numbers associated with these levels such as valley pseudospin—a quantum property related to how the electron moves the atomic structure of the material.

"This could be significant for future technology opportunities since valley pseudospin may be manipulated with optical rather than magnetic fields."

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