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NUS ECE Yang's Group

 

About Our Research

Fig 1. Magnetic random access memory.

Source: Chappert et al., Nature Materials 6, 813 - 823 (2007)

Spin transfer torque (STT) MRAM

Magnetoresistive random-access memory (MRAM), with its non-volatile characteristic, fast read/write speed, longer retention time and small cell size, is expected as an alternative to traditional transistor based memories such as dynamic RAM (DRAM) and static RAM (SRAM).

Spin transfer torque (STT) provides new approuch to flip the magnetization, showing that a magnetic material can be reversed by an electric currents that transport spin angular momentum. Thus the error rate problem (as the cell size scaling down) of traditional MRAM could be overcome.

Selected publications
Loong et al., Adv. Mat. 28, 4983 (2016)
Loong et al., Sci. Rep. 4, 6505 (2014)
Spin orbit torque, Dzyaloshinskii-Moriya interaction and magnetic chiral structures

Spin orbit torque (SOT), originating from spin orbit coupling, has gained considerable research interests in recent years due to its promising role in switching magnetic cells. Heterostructures with breaking invention symmetry, typically a heavy metal/ferromagnet/oxide tri layer, is expected to provide large spin orbit coupling.  
Recently there have been extensive research works on Dzyaloshinskii-Moriya interaction (DMI) and accompanying chiral spin textures due to their potential application in fast domain wall motion and less-power-hungry skyrmions bubbles, all of which are predicted as essential building blocks for future “Racetrack memory”. 

Selected publications
Yoon et al., Science Advances 3, e1603099 (2017)
Qiu et al., Nat. Nanotechnol. 10, 333 (2015)
Pollard et al., Nat. Commun. 8, 14761 (2017)

Fig 2. Experimental L-TEM images of a Ne´el skyrmion at varying tilt angles.

Source: Pollard et al., Nat. Commun. 8, 14761 (2017)

Fig. 3. A image plot of a topological insulator, showing Dirac dispersing surface states lying in the bulk band gap.

Source: Chen et al., Science 325, 178 (2009)

Fig. 4. Fermi arcs (left) and Weyl nodes (right).

Source: Xu et al., Science 349, 613-617 (2015)

Topological Insulator and Weyl semimetal

Topological insulators (TIs) are an emerging state of quantum matter which have insulating bulk and conducting surfaces. TIs have strong spin-orbit coupling which results in the band inversion and forms the surface state bands with linear dispersion inside the bulk band gap. The unique properties of TIs are this conducting topological surface states (TSS) protected by the time reversal symmetry, on which the electron spin and orbital momentum are locked.

Our research mainly focuses on the characterizing the electrical transport and optical properties of these materials, and tries to manipulate magnetization by utilizing the TSS. We are interested in the practical TI based applications at room temperature and with low power consumptions.

Weyl semimetals are topologically nontrivial phase of matter which resides the massless Weyl fermions in the Weyl nodes of low energy excitation. The Weyl fermions associated with the Weyl nodes process the distinct chirality, either left handed or right handed (see the Figure below).  And the projection of Weyls nodes on the surface gives the non-closed topological Fermi arcs. Each Weyl nodes behave as topological charges, i.e. monopoles and anti-monopoles of Berry curvature. The unique physical properties, together with distinct and non-degenerate spin texture, make the Weyl semimetals promising candidate for the next-generation (opto) electronics using the spin degree of freedom

Selected publications
Wang et al., Phys. Rev. Lett. 114, 257202 (2015)
Deorani et al., Phys. Rev. B 90, 094403(2014)
Terahertz Spintronics

Terahertz (0.1–10 THz) technologies have been studied intensively for a wide range of promising applications such as the chemical composition analysis, integrated circuits failure analysis, and spintronics characterization. Recently THz emission spectroscopy is playing an important role in unveiling the spin dynamics at a terahertz (THz) frequency range.
The development in the field of spintronics reveals a new possibility for the generation and manipulation of spin and charge currents. Spin-to-charge conversion has been discovered as a new mechanism for ultrafast photocurrent generation. The metallic spintronic THz emitter based on the inverse spin Hall effect is fast becoming a superior candidate among conventional THz sources.

Selected publications
Wu et al., Adv. Mat. 29, 1603031 (2017)
Chen et al., Adv. Opt. Mat. 6, 1800450 (2018)

Fig 5. A typical NM/FM sample structure (top) and a typical experimental THz signal emitted along the y-axis induced by transient charge currents due to ISHE (bottom)

Source: Wu et al., Adv. Mat. 29, 1603031 (2017)