How Many Excitons Can Combine?
© The Physical Society of Japan
This article is on
Polyexcitons in Two Dimensions
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 073702 (2023).
Quantum diffusion Monte Carlo simulation demonstrated the formation of polyexcitons in twodimensional multivalley semiconductor systems, where all exciton pairs were energetically bound by equalstrength “chemical bonds.”
Polyexcitons are composites of multiple electronhole pairs. In strongly photoexcited intrinsic semiconductors or type II semiconductor heterostructures, an electron in conduction bands and a hole in valence bands naturally form an exciton, and further, a pair of excitons coalesces into a biexciton. These composites have been studied in an analogy to hydrogen atoms or molecules. A common expectation is that excitons cannot form triexcitons, similar to hydrogen atoms that cannot form trimers. However, in multivalley semiconductors, more than two excitons can be bound, because electrons and holes acquire the additional valley degrees of freedom that, combined with the spins, allow more than two electrons and holes to occupy the same position. This possibility was first identified by Wang and Kittel in 1972 and has been experimentally observed in pure bulk samples of silicon and diamond as the photoluminescence (PL) peaks almost equally spaced at energy intervals.
We consider the twodimensional simplified model of the type II doublebilayer graphene heterostructures. The electrons and holes stay on different layers separated by a distance and interact with each other via Coulomb potentials. They also acquire four internal degrees of freedom because they have two spin and two valley degrees of freedom. This implies that our system can afford triexcitons and tetraexcitons, and that the quantum diffusion Monte Carlo simulation is free of the negative sign problem and can precisely evaluate the energies.
Notably, the separation energy required to pull out one exciton from the polyexciton grows almost exactly linearly with the exciton number. This behavior resembles the aforementioned PL peaks, indicating that the underlying physics is common between two and three dimensions. This further implies that all pairs of excitons inside the polyexciton are energetically bound by “chemical bonds” of equal strength, irrespective of the bound exciton numbers or the details of the system, for example, the interlayer distances.
(Written by K. Asano on behalf of all the authors)
Polyexcitons in Two Dimensions
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 073702 (2023).
Share this topic
Fields
Related Articles

Current Melt Frozen Electrons
Dielectric, optical, and other properties in condensed matter
Magnetic properties in condensed matter
2024115
The origin of the currentinduced insulatortometal transition of samarium monosulfide was explained by the 4f−5d hybridization observed using optical reflectivity and photoelectron spectroscopies.

Towards Uncovering the Hidden Order of URu_{2}Si_{2} Phase Transition
Magnetic properties in condensed matter
Electronic structure and electrical properties of surfaces and nanostructures
2024111
We propose a chiral charge as the hidden order parameter in URu_{2}Si_{2} and present experiments to detect it by focusing on breakings of mirror and inversion symmetries at the local uranium ion.

Possible Origin of High Thermoelectric Power Factor in Ultrathin FeSe: A Twoband Model
Electronic structure and electrical properties of surfaces and nanostructures
Structure and mechanical and thermal properties in condensed matter
Crossdisciplinary physics and related areas of science and technology
20231221
The high thermoelectric power factor observed in ultrathin FeSe can be theoretically explained by a twoband model with chemical potential between upper and lower band bottoms.

OneWay Optical Waveguide Realized by Edge Modes of Topological Photonic Crystals
Electromagnetism, optics, acoustics, heat transfer, and classical and fluid mechanics
Dielectric, optical, and other properties in condensed matter
20231212
A oneway optical waveguide was realized on the boundary between two types of photonic crystals with different topological properties, which were demonstrated by highresolution infrared reflection measurements.

Dzyaloshinskii–Moriya Interactions in Magnetism, Electricity, and Electronics
Magnetic properties in condensed matter
Electron states in condensed matter
Dielectric, optical, and other properties in condensed matter
20231012
A collection of papers in the Journal of the Physical Society of Japan advances our understanding of Dzyaloshinskii–Moriya interaction and paves the way for developing next generation computing and electronic systems.