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Abstract:
Researchers at the National University of Singapore (NUS) and Yale-NUS College have established the mechanisms for spin motion in molybdenum disulfide, an emerging two-dimensional (2D) material. Their discovery resolves a research question on the properties of electron spin in single layers of 2D materials, and paves the way for the next generation of spintronics and low-power devices. The work was published online in the journal Physical Review Letters on 29 January 2016.

Spin dynamics in an atomically thin semi-conductor

Singapore | Posted on February 1st, 2016

Molybdenum disulfide (MoS2), a class of transition metal dichalcogenide compounds, has attracted great attention due to wide recognition of its potential for manipulating novel quantum degrees of freedom such as spin and valley. Due to its unique material properties, a single layer of MoS2 has the potential to be used for spin transistors, where both electric current and spin current can be switched on and off independently. Despite this potential for application, there have not been any experimental studies on the mechanism for spin dynamics in MoS2.

To address this gap, scientists from the Centre for Advanced 2D Materials at NUS used highly precise measurements of the classical and quantum motion of electrons to extract information on how long spins live in this new material.

The team of scientists led by Assistant Professor Goki Eda, co-leader of this study who is from the NUS Department of Physics and Department of Chemistry, thinned down a crystal of molybdenite, a mineral of MoS2, to less than one nanometer. Here, the electrons live in a purely 2D plane that is just one atom thick. The researchers then successfully injected a high density of electrons in this ultra-thin material to enable measurements in the quantum mechanical regime. Quantum transport measurements at low temperatures of 2 Kelvin (‑271 degrees Celsius) revealed a surprising transition, where quantum mechanical wave interference switched from constructive to destructive with increasing magnetic field.

Mr Indra Yudhistira, a Research Associate with the NUS Department of Physics who is under the supervision of Assistant Professor Shaffique Adam, co-leader of the NUS study who is from Yale-NUS College and NUS Department of Physics, demonstrated that this crossover was caused by spin dynamics.

By comparing the theoretical and experimental results, the two research groups were able to extract spin lifetimes and also determine that the relaxation was driven by the Dyakonov-Perel type where electron spins live longer in dirtier samples.

“Aside from investigating the fundamental properties of low field magnetotransport in molybdenum disulfide, our team was able to establish the mechanism for spin scattering to reveal the properties of the electron spin,” said Dr Hennrik Schmidt, who was a Research Fellow working under the supervision of Asst Prof Eda when the study was conducted.

Commenting on the significance of the discovery, Asst Prof Adam noted that spin-based devices would generally lead to lower energy consumption as compared to conventional electronics. He explained, “The combination of MoS2 being a semiconductor and the long spin lifetimes open up opportunities in spintronics, where the electron spin and not the electron charge is used to transport information. Such unconventional devices could allow for next generation low-power devices.”

Professor Yoshihiro Iwasa, Director of the Center for Quantum-Phase Electronics at the University of Tokyo, and a world expert on quantum devices who first reported superconductivity in this class of materials remarked, “2D materials have been anticipated as a promising platform for spintronics. I feel that this very comprehensive study of the analysis of the electron spin life time will provide crucial information for further pushing the research toward the realisation of a new generation of spintronic devices.”

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About National University of Singapore
A leading global university centred in Asia, the National University of Singapore (NUS) is Singapore’s flagship university, which offers a global approach to education and research, with a focus on Asian perspectives and expertise.

NUS has 16 faculties and schools across three campuses. Its transformative education includes a broad-based curriculum underscored by multi-disciplinary courses and cross-faculty enrichment. Over 38,000 students from 100 countries enrich the community with their diverse social and cultural perspectives.

NUS has three Research Centres of Excellence (RCE) and 26 university-level research institutes and centres. It is also a partner in Singapore’s fifth RCE. NUS shares a close affiliation with 16 national-level research institutes and centres. Research activities are strategic and robust, and NUS is well-known for its research strengths in engineering, life sciences and biomedicine, social sciences and natural sciences. It also strives to create a supportive and innovative environment to promote creative enterprise within its community.

About Yale-NUS College

Established in 2011, Yale-NUS College is Singapore's first liberal arts college with a full residential programme that integrates living and learning. Drawing on the resources and traditions of Yale University and the National University of Singapore, Yale-NUS aims to redefine liberal arts and science education for a complex, interconnected world.

A Yale-NUS College education emphasises broad-based multi-disciplinary learning across the natural and social sciences, humanities and the arts. Our curriculum and pedagogy, built from scratch by the inaugural faculty, seeks to draw on the strengths of established liberal arts traditions, while introducing our students to the diverse intellectual traditions and cultures of Asia and the world.

We are nurturing young minds and equipping the next generation with the means to appreciate and understand the breadth and complexity of issues, the capacity to think critically and solve problems, and the skills to effectively communicate and lead. Our inaugural class will graduate in 2017.

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