Condensed Matter: More Is Different

To the Editor,

Half a century ago, one of the most important theoretical physicists who shaped the field of condensed matter physics, Professor Philip Anderson, expressed in his landmark article “More is Different” [1] that “…the behavior of large and complex aggregates of elementary particles, it turns out, is not to be understood in terms of a simple extrapolation of the properties of a few particles. Instead, at each level of complexity entirely new properties appear…”. The concept “More is Different” emphasized the significance of emergence in complex systems, where emergent behaviors and properties that cannot be found from the individual parts. Professor Anderson's own research in condensed matter physics also reflects the importance of emergence and complexity science. He was most concerned about how complex phenomena emerge from simple systems [2, 3].

In recent years, Professor Ben Zhong Tang and his team at the Chinese University of Hong Kong (Shenzhen) have proposed the concept of aggregate science, building on their pioneering research on aggregation-induced emission (AIE). AIE is a phenomenon in which certain organic luminophores exhibit stronger emission of light in their aggregated or solid state compared to when they are in solution [4]. While molecular science usually focuses on the study of free isolated particles that are not affected by interactions between molecules, the object of study of aggregate science is a complex system where various interactions influence each other. Therefore, aggregate matter could be viewed as a special type of complex system, in which increased and enhanced interactions give rise to distinct physical and chemical properties—an idea that aligns with Professor Anderson's concept of “More is Different”.

To showcase the recent developments in the study of aggregate matter in condensed matter physics, Aggregate has organized a Special Collection:Condensed Matter featuring topics such as proliferation and aggregation of topological spin textures, designs of multiresonant thermally activated delayed fluorescence emitters that suppress severe aggregation-caused quenching, multiple-phase aggregation process of Si and C atoms, and voltage-induced hysteresis resistance in nematic order.

Zhang et al. [5] theoretically show that the proliferation and aggregation effects of topologically nontrivial bimerons in chiral magnets can be induced by magnetic fields and electric currents. They find that small damping and a relatively large nonadiabatic spin-transfer torque could lead to more pronounced bimeron proliferation and aggregation. They analyze the micromagnetic energy terms during the proliferation and aggregation of bimerons. These results may help to understand the mechanism for the proliferation and aggregation of bimerons and motivate the development of new aggregate science.

Ye et al. [6] develop a new characterization method to identify the microscopic defects in 4H-SiC material and trace back the evolution of the SiC aggregation process. Usually, the aggregation process is too complex and dynamic to study and trace back in a non-destructive and comprehensive way. First, they implement the observations by using micro-computed tomography (CT) scanning in a common transmission mode with a tungsten-target reflection for the high yield of X-rays to obtain high-resolution images. Then, deep learning technologies have been used to process the visual data from micro-CT to achieve an effective feature extraction and identification of different SiC defects. At last, the evolution process of SiC growth is reconstructed based on the deep leaning-enhanced SiC characterization. This new method developed in this work is a nondestructive, rapid, but precise way. It can provide more comprehensive information on the SiC aggregation process and work for not only the SiC but also other high-cost third-generation semiconductor single crystals.

Wang et al. [7] design two new multiresonant thermally activated delayed fluorescence emitters, triphenylphosphine oxide (TPPO)-tBu-DiKTa and triphenylamine (TPA)-tBu-DiKTa, to address the severe aggregation-caused quenching (ACQ) and slow reverse intersystem crossing (RISC) for display applications. Ortho-substituted TPPO and TPA groups help increase the intermolecular distance and largely suppress the ACQ. The contributions from intermolecular charge transfer states for the TPA groups help to accelerate the RISC process. These results may help to improve the performance of LEC devices.

Li et al. [8] experimentally study the conductance and resistance by using point contact spectroscopy. They demonstrate that a bias voltage applied by a point contact could induce a resistance hysteresis in the nematic order. They also study temperature-dependent resistance of the sample, but no such behavior has been observed. The authors further show that the unique voltage-driven conductance hysteresis is not affected by a magnetic field. These results may help to understand the electronic origin of the nematic order in SmFeAsO.

We hope this Special Collection will inspire further exploration and discussion in the field of aggregate science, and encourage the discovery of new emergent phenomena in complex systems.

The authors declare no conflicts of interest.