Gauss's inequality: Difference between revisions
en>Qwfp Undid revision 484688847 by DrMicro (talk) per WP:SEEALSO: The "See also" section should not link to pages that do not exist (red links) |
en>Addbot m Bot: Migrating 1 interwiki links, now provided by Wikidata on d:q5527816 |
||
Line 1: | Line 1: | ||
The | In [[condensed matter physics]], '''biexcitons''' are created from two free [[excitons]]. | ||
== Formation of biexcitons == | |||
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 <math>| 0 1 \rangle</math> and <math>| 1 0 \rangle</math> states can be directly excited, direct excitation of the upper <math>| 1 1 \rangle</math> level from the ground state <math>| 0 0 \rangle</math> is usually forbidden and the most efficient alternative is coherent nondegenerate two-photon excitation, using <math>| 0 1 \rangle</math> or <math>| 1 0 \rangle</math> as an intermediate state. | |||
<ref>G. Chen et al, "Biexciton Quantum Coherence in a Single Quantum Dot", ''Phys. Rev. Lett.'', '''88''' (11), 117901 (2002)</ref> | |||
<ref>Xiaoqin Li et al, "An All-Optical Quantum Gate in a Semiconductor Quantum Dot", ''Science'', '''301''', 809 (2003)</ref> | |||
[[Image:Biexciton levels.gif|thumb|Model for a single [[quantum dots]]. <math>E_b</math> is the biexciton binding energy]] | |||
== Observation of biexcitons == | |||
Three possibilities of observing biexcitons exist: | |||
<ref>G. Vektrais, "A new approach to the molecular biexciton theory", ''J. Chem. Phys.'', '''101''' (4), 3031 (1994)</ref> | |||
(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 == | |||
The biexciton is a [[quasi-particle]] formed from two [[excitons]], and its energy is expressed as | |||
:<math>E_{b} = 2 E_{X} - E_{XX}</math> | |||
where <math>E_{XX}</math> is the biexciton energy, <math>E_{X}</math> is the [[exciton]] energy, and | |||
:<math>E_{b}</math> 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 biexciton 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 | |||
:<math>B_{XX} = \frac{c_1}{a^2} + \frac{c_2}{a} + B_{bulk}</math> | |||
where <math>B_{XX}</math> is biexciton binding energy, <math>a</math> is the radius of the [[quantum dots]], <math>B_{bulk}</math> is the binding energy of bulk crystal, and <math>c_1</math> and <math>c_2</math> are fitting parameters. | |||
<ref>S. Park et al, "Fabrication of CuCl Quantum Dots and the Size Dependence of the Biexciton Binding Energy", ''Journal of the Korean Physical Society'', '''37''' (3), 309-312 (2000)</ref> | |||
=== A simple model for describing binding energy of biexcitons === | |||
In the effective-mass approximation, the [[Hamiltonian (quantum mechanics)|Hamiltonian]] of the system consisting of two electrons (1, 2) and two holes (a, b) is given by | |||
:<math>H_{XX} = - \frac{\hbar^2}{2 m_e^*} ({\nabla_1}^2 + {\nabla_2}^2) - \frac{\hbar^2}{2 m_h^*} ({\nabla_a}^2 + {\nabla_b}^2) + V</math> | |||
where <math>m_e^*</math> and <math>m_h^*</math> are the effective masses of electrons and holes, respectively, and | |||
:<math>V = V_{12} - V_{1a} - V_{1b} - V_{2a} - V_{2b} + V_{ab}</math> | |||
where <math>V_{ij}</math> denotes the [[Coulomb interaction]] between the charged particles <math>i</math> and <math>j</math> (<math>i, j = 1, 2, a, b</math> denote the two electrons and two holes in the biexciton) given by | |||
:<math>V_{ij} = \frac{e^2}{\epsilon |\mathbf{r}_i - \mathbf{r}_j|}</math> | |||
where <math>\epsilon</math> is the dielectric constant of the material. | |||
Denoting <math>\mathbf{R}</math> and <math>\mathbf{r}</math> are the c.m. coordinate and the relative coordinate of the biexciton, respectively, and <math>M = m_e^* + m_h^*</math> is the [[Effective mass (solid-state physics)|effective mass]] of the [[exciton]], the Hamiltonian becomes | |||
:<math>H_{XX} = - \frac{\hbar^2}{4 M} {\nabla_R}^2 - \frac{\hbar^2}{M} {\nabla_r}^2 - \frac{\hbar^2}{2 \mu} ({\nabla_{1a}}^2 + {\nabla_{2b}}^2) + V</math> | |||
where <math>1/\mu = 1/{m_e^*} + 1/{m_h^*}</math>; <math>{\nabla_{1a}}^2</math> and <math>{\nabla_{2b}}^2</math> are the Laplacians with respect to relative coordinates between electron and hole, respectively. | |||
And <math>{\nabla_r}^2</math> is that with respect to relative coordinate between the c. m. of [[excitons]], and <math>{\nabla_R}^2</math> is that with respect to the c. m. coordinate <math>\mathbf{R}</math> of the system. | |||
In the units of the [[exciton]] Rydberg and [[Bohr radius]], the Hamiltonian can be written in dimensionless form | |||
:<math>H_{XX} = - ({\nabla_{1a}}^2 + {\nabla_{2b}}^2) - {2 \sigma}{(1 + \sigma)^2} {\nabla_r}^2 + V </math> | |||
where <math>\sigma = {m_e^*}/{m_h^*}</math> with neglecting kinetic energy operator of c. m. motion. And <math>V</math> can be written as | |||
:<math>V = 2 (\frac{1}{r_{12}} - \frac{1}{r_{1a}} - \frac{1}{r_{1b}} - \frac{1}{r_{2a}} - \frac{1}{r_{2b}} + \frac{1}{r_{ab}})</math> | |||
To solve the problem of the bound states of the biexciton complex, it is required to find the wave functions <math>\psi</math> satisfying the wave equation | |||
:<math>H_{XX} \psi = E_{XX} \psi</math> | |||
If the eigenvalue <math>E_{XX}</math> can be obtained, the binding energy of the biexciton can be also acquired | |||
:<math>E_{b} = 2 E_{X} - E_{XX}</math> | |||
where <math>E_{b}</math> is the binding energy of the biexciton and <math>E_{X}</math> is the energy of [[exciton]]. | |||
<ref>J. Liu et al, "Binding Energy of biexcitons in Two-Dimensional Semiconductors", ''Chin. Phys. Lett.'', '''15''' (8), 588 (1998)</ref> | |||
=== Binding energy in nanotubes === | |||
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 <math>r</math>, except perhaps for the smallest values of <math>r</math>. | |||
:<math>E_{XX} \approx \frac{0.195 eV}{r}</math> | |||
The actual biexciton binding energy is inversely proportional to the physical nanotube radius. | |||
<ref>T. G. Perdersen et al, "Stability and Signatures of biexcitons in Carbon nanotubes", ''Nanolett.'', '''5''' (2), 291 (2005)</ref> | |||
Experimental evidence of biexcitons has yet to be found. | |||
=== Binding energy in CuCl QDs === | |||
The binding energy of biexcitons increase with the decrease in their size and its size dependence and bulk value are well represented by the expression | |||
:<math>\frac{78}{{a^*}^2} + \frac{52}{{a^*}} + 33</math> (meV) | |||
where <math>a^*</math> 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. | |||
<ref>Y. Masumoto et al, "Biexciton binding energy in CuCl quantum dots", ''Phys. Rev. B'', '''50''' (24), 18658 (1994)</ref> | |||
== References == | |||
{{reflist|2}} | |||
[[Category:Condensed matter physics]] | |||
[[Category:Spintronics]] | |||
[[Category:Quasiparticles]] |
Revision as of 10:55, 20 March 2013
In condensed matter physics, biexcitons are created from two free excitons.
Formation of biexcitons
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]
Observation of biexcitons
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
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
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 biexciton 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
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]
Binding energy in nanotubes
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. [6] Experimental evidence of biexcitons has yet to be found.
Binding energy in CuCl QDs
The binding energy of biexcitons increase with the decrease in their size and its size dependence and bulk value are well represented by the expression
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. [7]
References
43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.
- ↑ G. Chen et al, "Biexciton Quantum Coherence in a Single Quantum Dot", Phys. Rev. Lett., 88 (11), 117901 (2002)
- ↑ Xiaoqin Li et al, "An All-Optical Quantum Gate in a Semiconductor Quantum Dot", Science, 301, 809 (2003)
- ↑ G. Vektrais, "A new approach to the molecular biexciton theory", J. Chem. Phys., 101 (4), 3031 (1994)
- ↑ S. Park et al, "Fabrication of CuCl Quantum Dots and the Size Dependence of the Biexciton Binding Energy", Journal of the Korean Physical Society, 37 (3), 309-312 (2000)
- ↑ J. Liu et al, "Binding Energy of biexcitons in Two-Dimensional Semiconductors", Chin. Phys. Lett., 15 (8), 588 (1998)
- ↑ T. G. Perdersen et al, "Stability and Signatures of biexcitons in Carbon nanotubes", Nanolett., 5 (2), 291 (2005)
- ↑ Y. Masumoto et al, "Biexciton binding energy in CuCl quantum dots", Phys. Rev. B, 50 (24), 18658 (1994)