A new quasiparticle’s discovery is equivalent to the discovery of a new molecule. However, molecules include various elements, whereas quasiparticles are formed of basic particles and interactions.
Similar to each molecule that has its own exclusive properties, each quasiparticle also has its own properties. Therefore, the discovery of a new quasiparticle paves the way for a wide array of potential technological applications.
A research team from NUS has discovered a new quasiparticle called “polaronic trion” in molybdenum disulfide (MoS2), which could be exploited to develop an optical modulator for visible light that is regulated by both temperature and electric fields.
The research effort for this advancement was headed by Professor T Venky Venkatesan, Director of NUS Nanoscience and Nanotechnology Institute (NUSNNI), and was reported in Advanced Materials on August 26th, 2019.
The Formation of the New Quasiparticle
Typically, a quasiparticle is a composite that forms as a result of the interaction between elementary particles. For example, a quasiparticle called the “exciton” is formed due to the Coulomb interaction between oppositely charged particles, both holes and electrons, in a semiconductor. “Recently it was reported that in electron-rich semiconductors, an extra electron can bind to an exciton to form a new quasiparticle called the ‘trion’,” explained Prof. Venkatesan.
As part of the study, the team found that the growth of an atomically thin MoS2 layer on a single crystal of strontium titanate (SrTiO3) enables the charged trion in MoS2 to further interact with the atomic vibrations of the SrTiO3 lattice to give rise to a new quasiparticle.
This interaction can be characterized as analogous to the one that occurs between electrons and lattice vibrations (or phonons) in solids, forming another quasiparticle called a “polaron.” Therefore, they termed the new quasiparticle a “polaronic trion.”
The polaronic trion can be visualised like a Russian Tea doll, or Matryoshka. Inside the polaronic trion is a bare trion, inside which is an exciton that itself is made from electrons and holes.
Shaffique Adam, Associate Professor, NUS Department of Physics, Yale-NUS College, Centre for Advanced 2D Materials (CA2DM)
Adam is one of the lead authors of the study.
The Significance of the Polaronic Trion
“Trions and excitons in 2D materials like MoS2 are interesting because these can absorb and emit light,” stated Dr Soumya Sarkar, the first author of the study who is from NUSNNI and the NUS Graduate School for Integrative Sciences and Engineering. He chanced upon this phenomenon during his doctoral research under Prof. Venkatesan.
He continued, “Usually, phonons have energies that are too large to couple with a trion. This is where the SrTiO3 crystal is special because it undergoes a structural phase transition at temperatures below −120 °C and gives rise to a particular atomic vibration, the soft mode.”
The energy of this soft mode is of the same order of that of the bare trion, and allows a strong coupling between the SrTiO3 phonons and the trion of MoS2 to create the new entity, the “polaronic trion.” There is a decrease in normal lattice vibrations when the crystal freezes at low temperatures; by contrast, the soft mode vibration is highly optimized, consistent with the observations.
One more significant characteristic of this quasiparticle is its sensitivity to electric fields. According to Dr Sreetosh Goswami from NUSNNI, who is one of the lead authors of this study, “What we are observing here is a many-body interaction and tuning that interaction with an external electric field. This is the Holy Grail in condensed matter physics, and such examples are quite rare.”
For me, the most exciting part of this entire study is the electric field tunability of polaronic trions by manipulating the soft phonons in SrTiO3. The ability to tune its binding energy by almost 40 meV using a voltage bias is much more than any other that was previously reported, and requires only a meagre amount of external energy.
Dr Sreetosh Goswami, Study Lead Author, NUS Nanoscience and Nanotechnology Institute
In theory, the coupling is strange as this is the first time such a strong interfacial phonon coupling that involves rotational phonons has been observed.
“We have extended an old result by Feynman and Fröhlich to explain this interaction. In fact, 2D materials interact strongly with their environment and this was crucial to this coupling,” explained Dr Maxim Trushin, a theoretical physicist at CA2DM who conducted all the calculations involved in the study and put forward the quasiparticle picture to elucidate the observed phenomenon.
A wider picture of this discovery was offered by Dr Sinu Mathew, who started the 2D materials effort at NUSNNI under Prof. Venkatesan and is a main player in this study.
He stated that “90 per cent of the research on 2D materials uses SiO2 or hexagonal boron nitride as substrates. Those might be great to explore quantum properties of 2D materials, but if you want to explore interface interactions, oxide substrates can be far more interesting as they have rich quantum functionalities. In this paper we report the interaction between MoS2 and SrTiO3, but there is a lot more room to explore.”
In the recent past, exciton-based interconnects have gained a lot of attention.
The polaronic trion is charged and hence it would be easier to guide with applied voltages, thereby making it a key player in this area. In fact we have already started observing polaronic trions in other 2D semiconductors and are working to demonstrate a functional device based on this new quasiparticle.
T Venky Venkatesan, Professor, Director, NUS Nanoscience and Nanotechnology Institute