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Saturday, 8 February 2020

A new quasiparticle is discovered: Pi-ton

Two electrons and two gaps, conjugated by the injection of a photon, remain together, forming the quasiparticle Π-ton.


There are very different types of particles: Elementary particles are the fundamental blocks of matter. Atoms, for example, are "linked" - or associated - states that consist of several minor constituents, such as quarks.

And there are so-called "quasiparticles" - excitations in a system formed by many particles, but which behave together exactly as if they were a single particle.

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It is one of those complex particles - dubbed Π-ton - that was discovered by Anna Kauch and her colleagues at the Vienna University of Technology, Austria.

In addition to describing the behavior of Pi-ton in simulations, the team also indicated the way for experimentalists to detect it in the laboratory.


Quasiparticles

The simplest and most well-known quasiparticle is the Hole, the carrier of positive charge. When an electron, which carries the negative charge, moves, it leaves a Hole in its place. There doesn't seem to be anything concrete there - hence the name Hole - but that "absence of electron" behaves in many ways as if it were a particle.

However, unlike an electron, which can also be seen outside a crystal, the Hole exists only in conjunction with the other particles. It is for these and others thinhs that it is interpreted as a quasiparticle.

But there are more complex quasiparticles, such as excitons , that play a central role in semiconductor physics , at the basis of the functioning of various hardware components. Exciton is a bonded state of an electron and a Hole, which is created when light hits a material. Instead of the electron and the Hole annihilating, they form a bond, and that bonded state is a quasiparticle.


Sketch of the physical processes (top) and Feynman diagrams (bottom) behind an exciton (left) and a  π -ton (right). The yellow wiggled line symbolizes the incoming (and outgoing) photon, which creates an electron-hole pair denoted by open and filled circles, respectively. The Coulomb interaction between the particles is symbolized by a red wiggled line; dashed line indicates the recombination of the particle and hole; dotted line denotes the creation of a second particle-hole pair (right); black lines the underlying band structure (top panels).


Pi-ton

Anna and her colleagues Petra Pudleiner and Katharina Astleithner were just studying the excitons when they realized that their calculations were showing something much broader than expected: electrons and Holes don't have to bond just in pairs.

In fact, the calculations showed the possibility that two electrons could bind to two Holes, forming an unprecedented quasiparticle: they called it Pi-ton, or Π-ton.

"The name pi-ton comes from the fact that the two electrons and the two Holes are held together by charge density fluctuations or spin fluctuations that always invert their character 180 degrees from one point in the crystalline network to the next - or that is, by an angle of pi, measured in radians, "said Anna.

"This constant shift from more to less can be imagined as a shift from black to white on a chessboard," illustrated Petra.

Like exonium, pi-ton is created spontaneously when the material absorbs a photon. When the quasiparticle falls apart, a photon is emitted again.



"Although we are constantly surrounded by countless quasiparticles, the discovery of a new species of quasiparticle is something very special. In addition to exxciton, now there is also pi-ton. Anyway, this contributes to a better understanding of the coupling between light and solids. , a topic that plays an important role not only in basic research, but also in many technical applications - from semiconductor technology to photovoltaic energy," said Professor Karsten Held.


Bibliography:

Article: Generic Optical Excitations of Correlated Systems: π-tons

Authors: Anna Kauch, Petra Pudleiner, Katharina Astleithner, P. Thunström, T. Ribic, Karsten Held

Magazine: Physical Review Letters

Vol .: 124, 047401

DOI: 10.1103 / PhysRevLett.124.047401

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