Researchers at the Division of Physics and Applied Physics have made many interesting discoveries and inventions, ranging from the optical cooling of semiconductors to the development of invisibility cloaks. Here is a selection of our notable accomplishments:
Theory of internal structures in plasmons
Electrons in a metal can oscillate collectively to produce a wave called a “plasmon”, which can be used to compress and manipulate light at nanometer length scales. Although the properties of plasmons have found applications in many areas of science and technology, ranging from bioimaging to photodetection, the plasmons themselves have long been regarded as relatively simple wave-like objects.
Theoretical work by the group of Prof. Justin Song has challenged this assumption. In a paper published in Physical Review X, the team reports that plasmons in ordinary metals contain hidden internal structures that can affect their motion. Much as a duck’s frantic paddling is hidden beneath the water surface as it glides across a pond, a plasmon that appears as a simple wave actually consists of swirling microscopic currents forming various intricate patterns. The researchers showed that these patterns can be exploited to alter the trajectory of the plasmons; for instance, plasmons reflecting from a surface undergo parallel shifts that can be controlled by a magnetic field.
Pattern of microscopic currents within a plasmonic wave. Credit: Justin Song.
This fundamental theoretical finding may lead, in the future, to novel techniques for controlling plasmons in optical devices. This work has been featured in Physics.
Reference: L.-K. Shi and J.C.W. Song, Plasmon geometric phase and plasmon Hall shift, Physical Review X 8, 021020 (2018).
Understanding How Cockroaches Sense Magnetic Fields
Certain animals can sense magnetic fields, and even use magnetic fields for navigation. However, the underlying mechanism for this ability is still a puzzle. One of the leading scientific hypotheses is that these animals make use of special cells containing rotatable magnetic nanoparticles, similar to tiny compasses.
The research groups of Prof. Rainer Dumke and Prof. Tomasz Paterek launched a collaboration to investigate this phenomenon. By creating a customized, highly sensitive atomic magnetometer, they were able to peform the first study of the dynamics of magnetic particles in a living insect: the American Cockroach (Periplaneta Americana). They discovered that the nanoparticles behave very differently in living and dead animals. Their results have narrowed down the range of possibilities of what the magnetic nanoparticles in cockroach bodies consist of, but also imply that these nanoparticles are not responsible for cockroaches’ magnetic field-sensing abilities.
Prof. Rainer Dumke (left), Prof. Tomasz Paterek (right), and their test subject (front). Photo credit: Janis Zhang.
These findings, published in the journal Scientific Reports in March 2018, are an important step in the long-standing puzzle of how animals sense magnetic fields—including the intriguing question of whether humans are able to do so. Progress in this research topic may have applications in future magnetic sensors based on biological principles.
This work has been featured in Physics World and the MIT Technology Review.
Reference: L.-J. Kong, H. Crepaz, A. Górecka, A. Urbanek, R. Dumke, and T. Paterek, Scientific Reports 8, 5140 (2018).
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 in 2015.
Reference: W. J. Chung, B. Heddi, E. Schmitt, K. W. Lim, Y. Mechulam, and A. T. Phan, Structure of a left-handed DNA G-quadruplex, Proc. Natl. Acad. Sci. U.S.A. 112, 2729 (2015).
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 also used 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.
Reference: J. Zhang, D. H. Li, R. J. Chen, and Q. H. Xiong, Laser cooling of a semiconductor by 40 Kelvin, Nature 493, 504 (2013).