In condensed matter physics, biexcitons are created from two free excitons, analogous to di-positronium in vacuum.

Formation of biexcitons

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In quantum information and computation, it is essential to construct coherent combinations of quantum states. The basic quantum operations can be performed on a sequence of pairs of physically distinguishable quantum bits and, therefore, can be illustrated by a simple four-level system.

In an optically driven system where the   and   states can be directly excited, direct excitation of the upper   level from the ground state   is usually forbidden and the most efficient alternative is coherent nondegenerate two-photon excitation, using   or   as an intermediate state.[1][2]

 
Model for a single quantum dots.   is the biexciton binding energy

Observation of biexcitons

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Three possibilities of observing biexcitons exist:[3]

(a) excitation from the one-exciton band to the biexciton band (pump-probe experiments);

(b) two-photon absorption of light from the ground state to the biexciton state;

(c) luminescence from a biexciton state made up from two free excitons in a dense exciton system.

Binding energy of biexcitons

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The biexciton is a quasi-particle formed from two excitons, and its energy is expressed as

 

where   is the biexciton energy,   is the exciton energy, and   is the biexciton binding energy.

When a biexciton is annihilated, it disintegrates into a free exciton and a photon. The energy of the photon is smaller than that of the exciton by the biexciton binding energy, so the biexciton luminescence peak appears on the low-energy side of the exciton peak.

The biexciton binding energy in semiconductor quantum dots has been the subject of extensive theoretical study. Because a biexciton is a composite of two electrons and two holes, we must solve a four-body problem under spatially restricted conditions. The biexciton binding energies for CuCl quantum dots, as measured by the site selective luminescence method, increased with decreasing quantum dot size. The data were well fitted by the function

 

where   is biexciton binding energy,   is the radius of the quantum dots,   is the binding energy of bulk crystal, and   and   are fitting parameters.[4]

A simple model for describing binding energy of biexcitons

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In the effective-mass approximation, the Hamiltonian of the system consisting of two electrons (1, 2) and two holes (a, b) is given by

 

where   and   are the effective masses of electrons and holes, respectively, and

 

where   denotes the Coulomb interaction between the charged particles   and   (  denote the two electrons and two holes in the biexciton) given by

 

where   is the dielectric constant of the material.

Denoting   and   are the c.m. coordinate and the relative coordinate of the biexciton, respectively, and   is the effective mass of the exciton, the Hamiltonian becomes

 

where  ;   and   are the Laplacians with respect to relative coordinates between electron and hole, respectively. And   is that with respect to relative coordinate between the c. m. of excitons, and   is that with respect to the c. m. coordinate   of the system.

In the units of the exciton Rydberg and Bohr radius, the Hamiltonian can be written in dimensionless form

 

where   with neglecting kinetic energy operator of c. m. motion. And   can be written as

 

To solve the problem of the bound states of the biexciton complex, it is required to find the wave functions   satisfying the wave equation

 

If the eigenvalue   can be obtained, the binding energy of the biexciton can be also acquired

 

where   is the binding energy of the biexciton and   is the energy of exciton.[5]

Numerical calculations of the binding energies of biexcitons

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The diffusion Monte Carlo (DMC) method provides a straightforward means of calculating the binding energies of biexcitons within the effective mass approximation. For a biexciton composed of four distinguishable particles (e.g., a spin-up electron, a spin-down electron, a spin-up hole and a spin-down hole), the ground-state wave function is nodeless and hence the DMC method is exact. DMC calculations have been used to calculate the binding energies of biexcitons in which the charge carriers interact via the Coulomb interaction in two and three dimensions,[6] indirect biexcitons in coupled quantum wells,[7][8] and biexcitons in monolayer transition metal dichalcogenide semiconductors.[9][10][11]

Binding energy in nanotubes

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Biexcitons with bound complexes formed by two excitons are predicted to be surprisingly stable for carbon nanotube in a wide diameter range. Thus, a biexciton binding energy exceeding the inhomogeneous exciton line width is predicted for a wide range of nanotubes.

The biexciton binding energy in carbon nanotube is quite accurately approximated by an inverse dependence on  , except perhaps for the smallest values of  .

 

The actual biexciton binding energy is inversely proportional to the physical nanotube radius.[12] Experimental evidence of biexcitons in carbon nanotubes was found in 2012. [13]

Binding energy in quantum dots

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The binding energy of biexcitons in a quantum dot decreases with size. In CuCl, the biexciton's size dependence and bulk value are well represented by the expression

  (meV)

where   is the effective radius of microcrystallites in a unit of nm. The enhanced Coulomb interaction in microcrystallites still increase the biexciton binding energy in the large-size regime, where the quantum confinement energy of excitons is not considerable.[14]

References

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  1. ^ Chen, Gang; Stievater, T. H.; Batteh, E. T.; Li, Xiaoqin; Steel, D. G.; Gammon, D.; Katzer, D. S.; Park, D.; Sham, L. J. (2002). "Biexciton Quantum Coherence in a Single Quantum Dot". Physical Review Letters. 88 (11): 117901. Bibcode:2002PhRvL..88k7901C. doi:10.1103/PhysRevLett.88.117901. ISSN 0031-9007. PMID 11909428.
  2. ^ Li, X. (2003). "An All-Optical Quantum Gate in a Semiconductor Quantum Dot". Science. 301 (5634): 809–811. Bibcode:2003Sci...301..809L. doi:10.1126/science.1083800. ISSN 0036-8075. PMID 12907794. S2CID 22671977.
  3. ^ Vektaris, G. (1994). "A new approach to the molecular biexciton theory". The Journal of Chemical Physics. 101 (4): 3031–3040. Bibcode:1994JChPh.101.3031V. doi:10.1063/1.467616. ISSN 0021-9606.
  4. ^ Park, S.; et al. (2000). "Fabrication of CuCl Quantum Dots and the Size Dependence of the Biexciton Binding Energy". Journal of the Korean Physical Society. 37 (3): 309–312.
  5. ^ Liu, Jian-jun; Kong, Xiao-jun; Wei, Cheng-wen; Li, Shu-shen (1998). "Binding Energy of Biexcitons in Two-Dimensional Semiconductors". Chinese Physics Letters. 15 (8): 588–590. Bibcode:1998ChPhL..15..588L. doi:10.1088/0256-307X/15/8/016. ISSN 0256-307X. S2CID 250889566.
  6. ^ D. Bressanini; M. Mella & G. Morosi (1998). "Stability of four-body systems in three and two dimensions: A theoretical and quantum Monte Carlo study of biexciton molecules". Physical Review A. 57 (6): 4956–4959. Bibcode:1998PhRvA..57.4956B. doi:10.1103/PhysRevA.57.4956.
  7. ^ M.Y.J. Tan; N.D. Drummond & R.J. Needs (2005). "Exciton and biexciton energies in bilayer systems". Physical Review B. 71 (3): 033303. arXiv:0801.0375. Bibcode:2005PhRvB..71c3303T. doi:10.1103/PhysRevB.71.033303. S2CID 119225682.
  8. ^ R.M. Lee; N.D. Drummond & R.J. Needs (2009). "Exciton-exciton interaction and biexciton formation in bilayer systems". Physical Review B. 79 (12): 125308. arXiv:0811.3318. Bibcode:2009PhRvB..79l5308L. doi:10.1103/PhysRevB.79.125308. S2CID 19161923.
  9. ^ M.Z. Mayers; T.C. Berkelbach; M.S. Hybertson & D.R. Reichman (2015). "Binding energies and spatial structures of small carrier complexes in monolayer transition-metal dichalcogenides via diffusion Monte Carlo". Physical Review B. 92 (16): 161404. arXiv:1508.01224. Bibcode:2015PhRvB..92p1404M. doi:10.1103/PhysRevB.92.161404. S2CID 118607038.
  10. ^ Szyniszewski, M.; et al. (2017). "Binding energies of trions and biexcitons in two-dimensional semiconductors from diffusion quantum Monte Carlo calculations". Physical Review B. 95 (8): 081301(R). arXiv:1701.07407. Bibcode:2017PhRvB..95h1301S. doi:10.1103/PhysRevB.95.081301. S2CID 17859387.
  11. ^ Mostaani, E.; et al. (2017). "Diffusion quantum Monte Carlo study of excitonic complexes in two-dimensional transition-metal dichalcogenides". Physical Review B. 96 (7): 075431. arXiv:1706.04688. Bibcode:2017PhRvB..96g5431M. doi:10.1103/PhysRevB.96.075431. S2CID 46144082.
  12. ^ Pedersen, Thomas G.; Pedersen, Kjeld; Cornean, Horia D.; Duclos, Pierre (2005). "Stability and Signatures of Biexcitons in Carbon Nanotubes". Nano Letters. 5 (2): 291–294. Bibcode:2005NanoL...5..291P. doi:10.1021/nl048108q. ISSN 1530-6984. PMID 15794613.
  13. ^ Colombier, L.; Selles, J.; Rousseau, E.; Lauret, J. S.; Vialla, F.; Voisin, C.; Cassabois, G. (2012). "Detection of a Biexciton in Semiconducting Carbon Nanotubes Using Nonlinear Optical Spectroscopy". Physical Review Letters. 109 (19): 197402. Bibcode:2012PhRvL.109s7402C. doi:10.1103/PhysRevLett.109.197402. ISSN 0031-9007. PMID 23215424. S2CID 25249444.
  14. ^ Masumoto, Yasuaki; Okamoto, Shinji; Katayanagi, Satoshi (1994). "Biexciton binding energy in CuCl quantum dots". Physical Review B. 50 (24): 18658–18661. Bibcode:1994PhRvB..5018658M. doi:10.1103/PhysRevB.50.18658. hdl:2241/98241. ISSN 0163-1829. PMID 9976308.