Graphene, a two-dimensional surface of carbon-atoms is only one atom thick true interface. This property of graphene can be exploited by bringing it in contact with other two-dimensional or bulk materials and engineering its pristine properties in a desired way. This unique ability of graphene makes it a rich playground to explore exotic quantum-degrees of freedom which are not possible to explore in materials in their pristine forms but can be accessed on the surface of graphene when it is contacted with other materials.

In this project, in a collaboration with the research teams from MIT and Harvard, we explore the effect of a ferromagnetic exchange field by a ferromagnetic material europium sulfide (EuS) and the spin-orbit coupling (SOC) by tungsten disulfide (WS2), a transition-metal dichalcogenide. Graphene is sandwiched between these two materials. Both effects and their interplay are expected to have a measurable effect which can be seen in the graphene Landau level splitting in Quantum Hall measurements.

A fabricated device which has graphene on WSe2 is shown at the left figure.

The project is in collaboration with the group of Prof. Jagadish Moodera at MIT, USA and Prof. Amir Yakobi at Harvard,USA. Theoretical support is provided by Prof. Patrick Lee at the MIT, USA

In 1956 Brown and Twiss (Nature 177, 27 (1956)) performed an experiment where they show that the photons emitted from a coherent light source are fully correlated. The measured feature is a consequence of bosonic statistics. Therefore, the effect represents the qunatum-statistical nature of any bosonic system.

Recently, Bender et al. (Phys.Rev. Lett. 122, 187701 (2019)) proposed that the same effect could be measured even in solid state devices. In ferromagnetic insulators such as Yittium Iron Garnate (YIG), the information communication occurs via quantum-mechanical excitation 'magnon' quasi-particle which are bosonic in nature, and the similar correlation between magnonic signals should appear as was earlier observed in the light experiment. The measurement scheme is shown in the image at the left

The project is an ongoing collaboration between the theory group of Prof. Rambert Duine at Utrecht, the Netherlands and the group of Dr. Marcos Guimaraes at Groningen, the Netherlands.

In our recent seminal work, we showed that up to 100% spin-injection and detection efficiency can be achieved in graphene if hBN is used as a tunnel barrier. We demonstrated the effect by using a finite dc current bias on top of the ac signal across the injector/detector contact. This observation clearly suggests that the spin-injection/detection efficiency is bias dependent (is not constant), and introduces nonlinearity in the spin-injection process.

In the present work, we explore this nonlinearity in the spin-signal in graphene and measure this effect in form of higher harmonic spin-signals. We also show the cool application of nonlinearity in form of spin-current amplification, amplitude modulation and heterodyne detection of the spin-signal for the first time which paves way for analog spintronics!

More details will follow soon :)