The Division of Physics and Applied Physics has notched up an impressive number of world-first research achievements, ranging from optical cooling of semiconductors to the creation of an invisibility cloak. Here are a few of our recent research highlights.
Magnetic Skyrmions at Room Temperature
The team of Prof. Christos Panagopoulos develops novel materials with “strong spin-orbit coupling”, meaning that the motion of the electrons inside the materials is strongly influenced by their intrinsic magnetic moment and angular momentum. These materials have unusual physical properties that are robust to impurities and temperature fluctuations, which makes them extremely promising for practical next-generation electronic and spintronic technologies.
Recently, the team developed an innovative technique for making materials containing tunable room-temperature “skyrmions” — tiny particle-like magnetic entities that only appear in specially-designed materials. Due to their stability, skyrmions have the potential to be used as magnetic bits in future magnetic storage and computing applications. By engineering a multi-layered Ir/Fe(x)/Co(y)/Pt nano-structure, the team was able to generate room temperature skyrmions with just the right size (less than 50 nm) to be suitable for technological applications. Furthermore, the team showed that the skyrmions could be detected and manipulated using processes commonly used in the electronics industry. This work was published in the journal Nature Materials in 2017. Their next step is to demonstrate digital operations using these skyrmions, in nano-scale devices that can be integrated into microchips.
- A. Soumyanarayanan, N. Reyren, A. Fert and C. Panagopoulos, Spin-orbit coupling induced emergent phenomena at surfaces and interfaces, Nature 539, 507 (2016).
- A. Soumyanarayanan, M. Raju, A. Oyarce, A. Tran, M. Im, A. Petrovic, P. Ho, K. Khoo, M. Tran, C. Gan, F. Ernult and C. Panagopoulos, Tunable room temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers, Nature Materials 16, 898 (2017).
Discovery of Left-Handed DNA G-Quadruplex
DNA, the molecule in which genes are contained, is well-known for having a double-helix structure. Less well-known is the fact that DNA can take an alternative four-stranded form known as G-quadruplex (or G4). The G4 structure is known to take part in cellular processes, and understanding it may lead to breakthroughs in therapeutics and nanotechnology. G4 DNA is highly polymorphic, meaning that it can exist in many different forms. Before 2015, however, only right-handed helical forms had ever been observed.
Using nuclear magnetic resonance and X-ray crystallography, Prof. Phan Anh Tuân
and his team were the first to observe left-handed G4 DNA. Their work was published in the journal Proceedings of the National Academy of Sciences of the United States of America
Perovskite Materials for Solar Cells And Lasers
When a molecular-scale composite is created from a hybrid of organic and inorganic constituents, it possesses valuable features of both organic and inorganic elements. One such organic-inorganic hybrid composite is the semiconductor CH3NH3PbI3. Since its first discovery in 2009, solar cells made of these “perovskite” materials have improved rapidly in performance, and have attained efficiencies exceeding 20%.
In 2013, an interdisciplinary team led by Prof. Sum Tze Chien succeeded in uncovering the mechanism behind the efficiency of perovskite solar cells. Their work was published in the journal Science.
This discovery is currently being utilized by Prof. Sum's collaborators at the Energy Research Institute @ NTU (ERI@N) to develop a commercial perovskite solar cell prototype, in collaboration with Australian clean-tech firm Dyesol Limited. The team's findings have also led to spin-off discoveries of novel phenomena, such as lasing in CH3NH3PbI3 halide perovskites, which has been patented and published in the journal Nature Materials.
- G.C. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, and T. C. Sum, Long-range balanced electron and hole-transport lengths in organic-inorganic CH3NH3PbI3, Science 342, 344 (2013).
- G. C. Xing, N. Mathews, S. S. Lim, N. Yantara, X. Liu, S. Dharani, M. Grätzel, S. Mhaisalkar, and T.C. Sum, Low-temperature solution-processed wavelength tunable perovskites for lasing, Nature Materials 13, 476 (2014).
Invisibility Cloaks for Light And Heat
A team led by Prof. Zhang Baile have developed a device that sounds like something out a Harry Potter movie! They created a “cloak” consisting of two pieces of calcite (a commonly-available carbonate mineral), which renders objects placed inside invisible to the human eye. This cloak has even been successfully demonstrated for cloaking living creatures, including a cat and a fish. This work was published in the journal Nature Communications.
Prof. Zhang and his colleagues have applied similar principles to create a “thermal cloak”, which makes a body inside the cloak immune to heat conduction. They have demonstrated the thermal cloaking of a three-dimensional air bubble inside a block of metal. This discovery has potential applications for protecting electronic components from heat, and for enhancing heat conduction in electronic devices. This work was published in the journal Physical Review Letters.
- H. Chen, B. Zheng, L. Shen, H. Wang, X. Zhang, N. I. Zheludev, and B. Zhang, Ray-optics cloaking devices for large objects in incoherent natural light, Nature Communications 4, 2652 (2013).
- H. Xu, F. Gao, X. Shi, H. Sun, and B. Zhang, Ultrathin three-dimensional thermal cloak, Physical Review Letters 112, 054301 (2014).
First Laser Cooling of a Semiconductor
Lasers can be used to cool specially-designed materials in a phenomenon called optical refrigeration. This is also known as laser cooling of solids. Optical refrigeration offers several important advantages over conventional cooling methods. Since no coolant or moving part is involved, it is a vibration-free technique that requires little space, while delivering highly stable and reliable results. Optical refrigeration may potentially be applied to systems such as aerospace detectors and remote sensors.
A research group led by Prof. Xiong Qihua scored a breakthrough by achieving net laser cooling of 40 Kelvin on cadmium sulphide nanobelts. This was the first time that optical refrigeration had been successfully employed to cool semiconductors.
This discovery points the way toward the development of all-solid-state semiconductor cryocoolers, which have considerable promise for the integration of solid-state semiconductor cryocoolers into electronic devices in the future. This research also opened the door to the creation of materials with strong electron-phonon couplings specifically to cater for laser-cooling.
This discovery was featured on the cover of the journal Nature in 2013.