Named π-ton (pi-ton), the newly-discovered quasiparticle consists of two electrons and two holes.
Two electrons and two holes, created by light quanta, held together by a chessboard-like background. Image credit: TU Wien
Quasiparticle is a disturbance in a medium that behaves as a particle and that may conveniently be regarded as one.
They are studied in connection with solid-state physics and nuclear physics because they play an important role in determining the properties of matter.
“The simplest quasiparticle is a hole,” said Professor Karsten Held, a researcher in the Institute for Solid State Physics at TU Wien.
“Let us imagine, for example, that many atoms are arranged in a regular pattern in a crystal and that there is a moving electron at each atom.”
“Only at one particular atom the electron is missing — this is called a hole.”
“Now an electron can move up from the neighboring atom. The original hole is closed, a new hole opens.”
But there are several more complex quasiparticles: excitons, phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from plasma oscillations), magnons (collective excitations of the electrons’ spin structure in a crystal lattice), and polarons (electrons dressed by a phonon cloud).
“The exciton, which plays an important role in semiconductor physics, is a bound state consisting of an electron and a hole, which is created by light,” the physicists said.
“The electron is negatively charged, the hole is the absence of a negative charge — and thus positively charged. Both attract each other and can form a bond.”
In the new study, they developed computer simulations to calculate quantum physical effects in solids.
But soon they realized that they had come across something totally different in their calculations: a completely new type of quasiparticle.
“The name pi-ton comes from the fact that the two electrons and two holes are held together by charge density fluctuations or spin fluctuations that always reverse their character by 180 degrees from one lattice point of the crystal to the next — i.e. by an angle of pi, measured in radians,” said Dr. Anna Kauch, also from the Institute for Solid State Physics at TU Wien.
“This constant change from plus to minus can perhaps be imagined like a change from black to white on a chessboard,” added Dr. Petra Pudleiner, from the Institute for Solid State Physics at TU Wien and the Institute of Theoretical and Computational Physics at the Graz University of Technology.
“The pi-ton is created spontaneously by absorbing a photon. When it disappears, a photon is emitted again.”
The team’s work appears in the journal Physical Review Letters.
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A. Kauch et al. 2020. Generic Optical Excitations of Correlated Systems: π-tons. Phys. Rev. Lett 124 (4): 047401; doi: 10.1103/PhysRevLett.124.047401
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