Electrical injection of THz spin excitations in topological solid state devices

Summary

To support the future growth of the digital economy, we must explore computing paradigms that overcome the inherent limitations of semiconductors. A proposed way to do this is to use the spin of electrons to carry information: a field known as spintronics. This technology promises to provide a route to low power, high speed computers of the future. An emerging area within spintronics is Terahertz spintronics, where interactions between electrons at trillionths of a second are used. This is 1000 times faster than the current state of the art in silicon. To make this technology work, we need to find ways to inject and control THz spin currents into materials. This project will try to accomplish this using low-temperature deposited Gallium Arsenide (LT GaAs) and topological insulators, materials with a graphene like surface state that can efficiently transport and control spin. The successful student will learn state-of-the-art cleanroom techniques that are currently used in the semiconductor industry, low-temperature measurement techniques widely used in the development of quantum computers, and materials growth relevant to next generation computing technology; all while performing cutting edge research in a world leading THz group.

To support the future growth of the digital economy, we must explore computing paradigms that overcome the inherent limitations of semiconductors. Spintronics seeks to build novel devices for computing and communications that utilise electron spin. Applications include low power electronics, high speed logic, neuromorphic computing and novel memory devices. [1-3] Cutting edge research in this area focuses on the high-speed interactions between spins, which allow spintronic devices to operate at THz frequencies, more than 1000 times faster than the current state of the art in silicon. [4] However, integration of spintronics with conventional electronics is extremely challenging, due to the inherent difficulty of converting magnetic and spin excitations into electrical currents. A promising approach is to utilise spin orbit effects at interfaces. [5]

Topological Insulators (TIs) are materials with strong spin-orbit interactions that are semiconducting in the bulk but with conducting surface states. The spin-momentum relationship of the high mobility surface states also provides a route to high-speed spin-charge conversion. [6] To utilise these interfaces, they must be integrated with a THz source in a solid state device. Low Temperature GaAs (LT-GaAs) is a commonly used source of THz radiation. [7] While some initial research has suggested LT GaAs can be integrated with topological materials, heterostructures and devices utilising these materials have yet to be comprehensively understood. [8] This project will develop LT GaAs injectors for the study of the THz spin response of TI devices, using a range of facilities including ultra-low temperature dilution refrigerator facilities.

University of Leeds is a world leader in the growth of both high-quality Bi2Se3/BiInSe topological insulator multilayers utilising the Royce Deposition System and the growth of LT-GaAs, grown on dedicated III-V MBE growth chambers. As a post graduate researcher on this project you will be involved in the cleanroom fabrication, electrical and optical characterisation, and simulation of these novel materials and devices. These techniques will be combined to develop LT-GaAs/TI heterostructures with electrically injected, amplified, tuneable THz spin dynamics for future computing and communications technology.

[1] – S D Bader and S S P Parkin, Annu. Rev. Condens. Matter Phys. 1, 1, 2010.

[2] – C H Marrows et al, NPJ Spintronics, 2, 12, 2024.

[3] – S S P Parkin et al, Science, 320, 5873, 2008.

[4] – J Walowski and M Munzenberg, Jour of Appl Phys. 120, 14, 2016.

[5] – M Tong et al, Nano Letters, 21, 1, 2020.

[6] – P Di Pietro et al, Nat Nano. 8, 556 – 560, 2013.

[7] – M C Beard et al, Jour Appl. Phys. 90, 12, 2001.

[8] – M Eddrief et al, Nanotechnology, 25, 245701, 2014

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