Chongfan Technology
News
31
2025
-
03
ELight | Discovery of a new quasiparticle, the tetrameron, in two-dimensional semiconductors
Author:
Introduction
QuasiparticleIt is a collective quantum excitation in condensed matter, which is fundamentally important for understanding materials physics and semiconductor technology applications.
The findings were published inThe quadruplon in a monolayer semiconductorunder the titleeLight。
One of the important tasks of physics is to understand why various materials have corresponding electrical, optical, thermal, or magnetic properties. These properties are the basis for various material applications. Modern quantum theory uses the concept of "elementary excitation" or quasiparticles to explain these properties. Typical quasiparticles include phonons in crystals, electrons and holes in semiconductors modulated by the lattice, or polarons further "decorated" by phonons, and hydrogen-like structures composed of electron-hole pairs, called excitons. Two excitons can combine to form an exciton molecule, called a biexciton, similar to a hydrogen molecule. Although a biexciton contains four particles (2 electrons + 2 holes), it can still be reduced to a combination of 2-body structures. Although there have been theoretical predictions that more general true four-body quantum entities exist in different physical systems, and they cannot be reduced to a combination of two bodies, experimental evidence is scarce, especially in semiconductors, where they have never been discovered.
A team from Shenzhen Technology University and Tsinghua University led by Ning Cunzheng reported experimental evidence for the existence of a new four-body quasiparticle (quadruplon) in two-dimensional molybdenum ditelluride semiconductor. The team used monolayer molybdenum ditelluride (MoTe2) as the core material, encapsulated it between two layers of hexagonal boron nitride (BN) insulators, and formed electrical contact with molybdenum ditelluride through a metal electrode on the top; a gate electrode was placed below the bottom BN layer (see Figure 1a). Using ultrafast pump-probe technology, the material was excited with a strong pump pulse, and then its light absorption characteristics were analyzed with a weak probe pulse. By adjusting parameters such as gate voltage, pump intensity, delay time, probe photon energy and polarization, and temperature, they systematically studied the evolution law of the absorption spectral response.
Figure 1: Key results and theoretical interpretation
a. Device structure:Monolayer molybdenum ditelluride (MoTe2) sandwiched between two layers of hexagonal boron nitride (BN), with electrical contact formed between the top layer and molybdenum ditelluride through a graphene layer;
b. Energy level diagram:The pump light excites electrons to form a two-body (2B) state (such as an exciton), and the absorption of the probe light causes the system to transition from the two-body (2B) state to the four-body (4B) state. The four-body state may contain different quantum states such as biexcitons and quadruplons;
c. Spectral comparison:The red-yellow contour map (upper part) is the continuous-wave absorption spectrum, and the white dashed line marks the spectral positions of the exciton (X) and triexciton (T). Under electrically neutral gate voltage, the exciton peak intensity is the highest, and the triexciton peak is the weakest, and there is no significant characteristic peak below the exciton peak. The ultrafast pump-probe differential absorption spectrum (lower part) shows six new characteristic peaks (marked by solid lines), corresponding to the 2B→4B transition related to the fourth-order group in the theoretical model (see Figure b)
The physical principle of this experiment is as follows:A strong pump pulse excites electron transitions in the crystal, forming quasiparticles and many-body composite states such as excitons and their hydrogen-like series, triexcitons (2 electrons-1 hole or its inverse), biexcitons, and plasmas. Subsequently, the probe pulse photons are absorbed by these newly generated quasiparticles, transitioning to various four-particle final states (i.e., four-body states). By measuring the reflected or transmitted light signal of the probe pulse, key information about the four-body state can be obtained. Because the quasiparticle lifetime is extremely short (picosecond level), the probe pulse needs to interact with the material and be absorbed by the material before the quasiparticles generated by the pump light annihilate, thus obtaining useful spectral information.
Traditional experiments of this kind can only observe characteristic peaks such as excitons, triexcitons, and biexcitons. In contrast, the team discovered six new absorption peaks on the low-energy side of the exciton peak. Because the team has prepared hundreds of such samples over the past few years, they can easily rule out spectral peaks caused by external factors such as defects. The experimental results showing that these characteristic peaks systematically change with temperature and pump power also confirm that these spectral features originate from the intrinsic properties of the material, not external factors such as defects or test noise.
"To confirm that these new characteristic peaks originate from the intrinsic properties of the material, we systematically studied their variation laws with temperature and pump power. We tried many possible experimental methods, and all evidence shows that these spectral features are caused by the intrinsic properties of the material, and these spectral features are stable and reliable." explained Tang Jiacheng, the first author of the paper and a doctoral student in Ning Cunzheng's research group.
After confirming the experimental phenomena, the team tried to explain these spectral features using traditional theoretical models such as excitons, triexcitons, and biexcitons, but found that existing theories could not obtain results consistent with the experiment. Therefore, they broke through traditional approximation methods and completely considered all possible Coulomb interaction terms between four bodies (2 electrons-2 holes). Surprisingly, the new theoretical model based on all four-body quantum states perfectly reproduced all key experimental features.
"Normally, such a high degree of agreement between theory and experiment is sufficient to prove the correctness of the conclusion." Ning Cunzheng said, "But the four-body interaction involves many terms, and we need to know exactly which terms caused the generation of the new characteristic peaks. In addition, the above theoretical explanation lacks intuitive physical image support. Therefore, I further challenged my students to try to build a more intuitive theoretical framework."
Another equivalent theoretical framework that Ning Cunzheng considered is the "group expansion" method, which is another equivalent method for handling multi-body problems. This method approximates the treatment of complete Coulomb interactions that cannot be solved exactly by gradually increasing the size of the "particle group". Its core advantage lies in its ability to establish a clear physical image.Figure 1b）。
"It is gratifying that we were finally able to establish a correspondence between the two theoretical methods, and both can well explain the experimental results." Ning Cunzheng further elaborated, "The unique advantage of group expansion lies in the ability to selectively consider or ignore specific group terms, thereby accurately determining which group is the physical source of the new characteristic peak."
The research team finally found that only when irreducible fourth-order groups (2 electrons-2 holes) are included can all experimental phenomena be accurately reproduced. This fourth-order group corresponds to a new type of composite quantum entity—a four-body quasiparticle formed by the direct association of 2 electrons and 2 holes. Unlike traditional biexcitons, this four-body entity does not require the presence of excitons. Following the naming conventions in previous theoretical literature, they named it a "quadruplon".
The team believes that there is still much worth further investigating. They plan to explore similar phenomena in other material systems. Another area worth studying is the luminescence characteristics of this new quadruplon, to reveal the quantum nature of this strongly correlated many-body system, particularly the characteristics of the new quantum entangled states. Due to the experimental verification of the quadruplon being rare in various fields of physics, the recent discovery of a similar quadruplon in quark systems at CERN provides an easier opportunity to achieve experimental research on the physical nature of quadruplons within a typical energy range. It is believed that this discovery is of significant importance for understanding the essence of quadruplons. The discovery of quadruplons in semiconductors has also opened up new research directions for semiconductor physics. The application of devices based on this quadruplon, especially in quantum device development, is another important area worth studying.
This research was supported by the following projects:
National Natural Science Foundation of China (91750206, 61861136006); Shenzhen High-tech Zone Development Special Plan Innovation Platform Construction Project (29853M-KCJ-2023-002-01); Guangdong Provincial University Engineering Technology Center (2023GCZX005); Guangdong Provincial Key Construction Discipline Scientific Research Capacity Improvement Project (2022ZDJS111); Shenzhen Technology University High-end Talent Research Fund (GDRC202301); Guangdong Provincial Engineering Research Center for Compound Semiconductor Devices and Chips; Tsinghua University Independent Research Project
Publication Information
Tang, J., Ning, C., Sun, H. et al. Quadruplons in monolayer semiconductors.eLight5, 3 (2025).
https://doi.org/10.1186/s43593-025-00081-1
LATEST NEWS
Thank you for visiting the official website of Chongfan Technology. If you have cooperation intentions or suggestions, please contact us through the following methods, and we will reply as soon as possible, thank you!
Address: Room 403, Building 6, Phase III of R&D, No. 36 Xiyong Avenue, High tech Zone, Chongqing, China.
Telephone: +86-13658337211
E-mail: Sales@cfkeji.net
Website: www.cfkeji.net
Mobile Version
