Fractional Schrödinger equation

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The fractional Schrödinger equation is a fundamental equation of fractional quantum mechanics. It was discovered by Nick Laskin (1999) as a result of extending the Feynman path integral, from the Brownian-like to Lévy-like quantum mechanical paths. The term fractional Schrödinger equation was coined by Nick Laskin.^[1]

Fundamentals

The fractional Schrödinger equation in the form originally obtained by Nick Laskin is:^[2]

$i\hbar {\frac {\partial \psi ({\mathbf {r}},t)}{\partial t}}=D_{\alpha }(-\hbar ^{2}\Delta )^{{\alpha /2}}\psi ({\mathbf {r}},t)+V({\mathbf {r}},t)\psi ({\mathbf {r}},t)$

r is the 3-dimensional position vector,
ħ is the reduced Planck constant,
ψ(r, t) is the wavefunction, which is the quantum mechanical probability amplitude for the particle to have a given position r at any given time t,
V(r, t) is a potential energy,
Δ = ∂²/∂r² is the Laplace operator.

Further,

D_α is a scale constant with physical dimension [D_α] = [energy]^{1 − α}·[length]^α[time]^−α, at α = 2, D₂ =1/2m, where m is a particle mass,
the operator (−ħ²Δ)^α/2 is the 3-dimensional fractional quantum Riesz derivative defined by (see, Ref.[2]);

$(-\hbar ^{2}\Delta )^{{\alpha /2}}\psi ({\mathbf {r}},t)={\frac 1{(2\pi \hbar )^{3}}}\int d^{3}pe^{{i{\frac {{\mathbf {pr}}}\hbar }}}|{\mathbf {p}}|^{\alpha }\varphi ({\mathbf {p}},t),$

Here, the wave functions in the position and momentum spaces; $\psi ({\mathbf {r}},t)$ and $\varphi ({\mathbf {p}},t)$ are related each other by the 3-dimensional Fourier transforms:

$\psi ({\mathbf {r}},t)={\frac 1{(2\pi \hbar )^{3}}}\int d^{3}pe^{{i{\mathbf {p}}\cdot {\mathbf {r}}/\hbar }}\varphi ({\mathbf {p}},t),\qquad \varphi ({\mathbf {p}},t)=\int d^{3}re^{{-i{\mathbf {p}}\cdot {\mathbf {r}}/\hbar }}\psi ({\mathbf {r}},t).$

The index α in the fractional Schrödinger equation is the Lévy index, 1 < α ≤ 2. Thus, the fractional Schrödinger equation includes a space derivative of fractional order α instead of the second order (α = 2) space derivative in the standard Schrödinger equation. Thus, the fractional Schrödinger equation is a fractional differential equation in accordance with modern terminology.^[3] This is the main point of the term fractional Schrödinger equation or a more general term fractional quantum mechanics.^[4] At α = 2 fractional Schrödinger equation becomes the well-known Schrödinger equation.

The fractional Schrödinger equation has the following operator form

$i\hbar {\frac {\partial \psi ({\mathbf {r}},t)}{\partial t}}=\widehat {H}_{\alpha }\psi ({\mathbf {r}},t)$

where the fractional Hamilton operator $\widehat {H}_{\alpha }$ is given by

$\widehat {H}_{\alpha }=D_{\alpha }(-\hbar ^{2}\Delta )^{{\alpha /2}}+V({\mathbf {r}},t).$

The Hamilton operator, $\widehat {H}_{\alpha }$ corresponds to the classical mechanics Hamiltonian function

$H_{\alpha }({\mathbf {p}},{\mathbf {r}})=D_{\alpha }|{\mathbf {p}}|^{\alpha }+V({\mathbf {r}},t),$

where p and r are the momentum and the position vectors respectively.

Time-independent fractional Schrödinger equation

The special case when the Hamiltonian $H_{\alpha }$ is independent of time

$H_{\alpha }=D_{\alpha }(-\hbar ^{2}\Delta )^{{\alpha /2}}+V({\mathbf {r}}),$

is of great importance for physical applications. It is easy to see that in this case there exist the special solution of the fractional Schrödinger equation

$\psi ({\mathbf {r}},t)=e^{{-(i/\hbar )Et}}\phi ({\mathbf {r}}),$

where $\phi ({\mathbf {r}})$ satisfies

$H_{\alpha }\phi ({\mathbf {r}})=E\phi ({\mathbf {r}}),$

$D_{\alpha }(-\hbar ^{2}\Delta )^{{\alpha /2}}\phi ({\mathbf {r}})+V({\mathbf {r}})\phi ({\mathbf {r}})=E\phi ({\mathbf {r}}).$

This is the time-independent fractional Schrödinger equation.

Thus, we see that the wave function $\psi ({\mathbf {r}},t)$ oscillates with a definite frequency. In classical physics the frequency corresponds to the energy. Therefore, the quantum mechanical state has a definite energy E. The probability to find a particle at ${\mathbf {r}}$ is the absolute square of the wave function $|\psi ({\mathbf {r}},t)|^{2}.$ Because of time-independent fractional Schrödinger equation this is equal to $|\phi ({\mathbf {r}})|^{2}$ and does not depend upon the time. That is, the probability of finding the particle at ${\mathbf {r}}$ is independent of the time. One can say that the system is in a stationary state. In other words, there is no variation in the probabilities as a function of time.

Probability current density

The continuity equation for probability current and density follows from the fractional Schrödinger equation:

${\frac {\partial \rho ({\mathbf {r}},t)}{\partial t}}+\nabla \cdot {\mathbf {j}}({\mathbf {r}},t)=0,$

where $\rho ({\mathbf {r}},t)=\psi ^{{\ast }}({\mathbf {r}},t)\psi ({\mathbf {r}},t)$ is the quantum mechanical probability density and the vector ${\mathbf {j}}({\mathbf {r}},t)$ can be called by the fractional probability current density vector

${\mathbf {j}}({\mathbf {r}},t)={\frac {D_{\alpha }\hbar }i}\left(\psi ^{{*}}({\mathbf {r}},t)(-\hbar ^{2}\Delta )^{{\alpha /2-1}}{\mathbf {\nabla }}\psi ({\mathbf {r}},t)-\psi ({\mathbf {r}},t)(-\hbar ^{2}\Delta )^{{\alpha /2-1}}{\mathbf {\nabla }}\psi ^{{*}}({\mathbf {r}},t)\right),$

where we use the notation (see also matrix calculus): ${\mathbf {\nabla =\partial /\partial r}}$ .

Introducing the momentum operator $\widehat {{\mathbf {p}}}={\frac {\hbar }{i}}{\frac {\partial }{\partial {\mathbf {r}}}}$ we can write the vector ${\mathbf {j}}$ in the form (see, Ref.[2])

${\mathbf {j=}}D_{{\alpha }}\left(\psi (\widehat {{\mathbf {p}}}^{{2}})^{{\alpha /2-1}}\widehat {{\mathbf {p}}}\psi ^{{\ast }}+\psi ^{{\ast }}(\widehat {{\mathbf {p}}}^{{\ast 2}})^{{\alpha /2-1}}\widehat {{\mathbf {p}}}^{{\ast }}\psi \right).$

This is fractional generalization of the well-known equation for probability current density vector of standard quantum mechanics (see, Ref.[7]).

Velocity operator

The quantum mechanical velocity operator $\widehat {{\mathbf {v}}}$ is defined as follows:

$\widehat {{\mathbf {v}}}={\frac {i}{\hbar }}(H_{{\alpha }}\widehat {{\mathbf {r}}}{\mathbf {-}}\widehat {{\mathbf {r}}}H_{{\alpha }}),$

Straightforward calculation results in (see, Ref.[2])

$\widehat {{\mathbf {v}}}=\alpha D_{{\alpha }}|\widehat {{\mathbf {p}}}^{{2}}|^{{\alpha /2-1}}\widehat {{\mathbf {p}}}\,.$

Hence,

${\mathbf {j=}}{\frac {1}{\alpha }}\left(\psi \widehat {{\mathbf {v}}}\psi ^{{\ast }}+\psi ^{{\ast }}\widehat {{\mathbf {v}}}\psi \right),\qquad 1<\alpha \leq 2.$

To get the probability current density equal to 1 (the current when one particle passes through unit area per unit time) the wave function of a free particle has to be normalized as

$\psi ({\mathbf {r}},t)={\sqrt {{\frac {\alpha }{2{\mathrm {v}}}}}}\exp \left[{\frac {i}{\hbar }}({\mathbf {p}}\cdot {\mathbf {r}}-Et)\right],\qquad E=D_{{\alpha }}|{\mathbf {p}}|^{{\alpha }},\qquad 1<\alpha \leq 2,$

where ${\mathrm {v}}$ is the particle velocity, ${\mathrm {v}}=\alpha D_{{\alpha }}p^{{\alpha -1}}$ .

Then we have

${\mathbf {j=}}{\frac {{\mathbf {v}}}{{\mathrm {v}}}},\qquad {\mathbf {v}}=\alpha D_{{\alpha }}|{\mathbf {p}}^{{2}}|^{{{\frac {\alpha }{2}}-1}}{\mathbf {p,}}$

that is, the vector ${\mathbf {j}}$ is indeed the unit vector.

Physical applications

Fractional Bohr atom

When $V({\mathbf {r}})$ is the potential energy of hydrogenlike atom,

$V({\mathbf {r}})=-{\frac {Ze^{{2}}}{|{\mathbf {r}}|}},$

where e is the electron charge and Z is the atomic number of the hydrogenlike atom, (so Ze is the nuclear charge of the atom), we come to following fractional eigenvalue problem,

$D_{{\alpha }}(-\hbar ^{{2}}\Delta )^{{\alpha /2}}\phi ({\mathbf {r}})-{\frac {Ze^{{2}}}{|{\mathbf {r|}}}}\phi ({\mathbf {r}})=E\phi ({\mathbf {r}}).$

This eigenvalue problem has first been solved in.^[5]

Using the first Niels Bohr postulate yields

$\alpha D_{{\alpha }}\left({\frac {n\hbar }{a_{{n}}}}\right)^{{\alpha }}={\frac {Ze^{{2}}}{a_{{n}}}},$

and it gives us the equation for the Bohr radius of the fractional hydrogenlike atom

$a_{{n}}=a_{{0}}n^{{\alpha /(\alpha -1)}}.$

Here a₀ is the fractional Bohr radius (the radius of the lowest, n = 1, Bohr orbit) defined as,

$a_{{0}}=\left({\frac {\alpha D_{{\alpha }}\hbar ^{{\alpha }}}{Ze^{{2}}}}\right)^{{1/(\alpha -1)}}.$

The energy levels of the fractional hydrogenlike atom are given by

$E_{{n}}=(1-\alpha )E_{{0}}n^{{-\alpha /(\alpha -1)}},\qquad 1<\alpha \leq 2,$

where E₀ is the binding energy of the electron in the lowest Bohr orbit that is, the energy required to put it in a state with E = 0 corresponding to n = ∞,

$E_{{0}}=\left({\frac {Ze^{{2}}}{\alpha D_{{\alpha }}^{{1/\alpha }}\hbar }}\right)^{{\alpha /(\alpha -1)}}.$

The energy (α − 1)E₀ divided by ħc, (α − 1)E₀/ħc, can be considered as fractional generalization of the Rydberg constant of standard quantum mechanics. For α = 2 and Z = 1 the formula $(\alpha -1)E_{{0}}/\hbar c$ is transformed into

${\mathrm {Ry}}=me^{{4}}/2\hbar ^{{3}}c$ ,

which is the well-known expression for the Rydberg formula.

According to the second Niels Bohr postulate, the frequency of radiation $\omega$ associated with the transition, say, for example from the orbit m to the orbit n, is,

$\omega ={\frac {(1-\alpha )E_{{0}}}{\hbar }}\left[{\frac {1}{n^{{{\frac {\alpha }{\alpha -1}}}}}}-{\frac {1}{m^{{{\frac {\alpha }{\alpha -1}}}}}}\right]$ .

The above equations are fractional generalization of the Bohr model. In the special Gaussian case, when (α = 2) those equations give us the well-known results of the Bohr model.^[6]

The infinite potential well

A particle in a one-dimensional well moves in a potential field $V({x})$ , which is zero for $-a\leq x\leq a$ and which is infinite elsewhere,

$V(x)=\infty ,\qquad x<-a\qquad \qquad ({\mathrm {i}})$

$V(x)=0,\quad -a\leq x\leq a\quad \quad \quad \ ({\mathrm {ii}})$

$V(x)=\infty ,\qquad \ x>a\qquad \qquad \ ({\mathrm {iii}})$

It is evident a priori that the energy spectrum will be discrete. The solution of the fractional Schrödinger equation for the stationary state with well-defined energy E is described by a wave function $\psi (x)$ , which can be written as

$\psi (x,t)=\left(-i{\frac {Et}{\hbar }}\right)\phi (x)$ ,

where $\phi (x)$ , is now time independent. In regions (i) and (iii), the fractional Schrödinger equation can be satisfied only if we take $\phi (x)=0$ . In the middle region (ii), the time-independent fractional Schrödinger equation is

$D_{\alpha }(\hbar \nabla )^{\alpha }\phi (x)=E\phi (x).$

This equation defines the wave functions and the energy spectrum within region (ii), while outside of the region (ii), x<-a and x>a, the wave functions are zero. The wave function $\phi (x)$ has to be continuous everywhere, thus we impose the boundary conditions $\phi (-a)=\phi (a)=0$ for the solutions of the time-independent fractional Schrödinger equation (see, Ref.[5]). Then the solution in region (ii) can be written as

$\phi ^{{{\mathrm {even}}}}(x)=A\cos kx,\qquad {\text{or}}\qquad \phi ^{{{\mathrm {odd}}}}(x)=A\sin kx,$

where k is given by

$k={\frac 1\hbar }({\frac E{D_{\alpha }}})^{{1/\alpha }},\qquad 1<\alpha \leq 2.$

The even (under reflection $x\rightarrow -x$ ) solution $\phi ^{{{\mathrm {even}}}}(x)$ satisfies the boundary conditions if

$k=(2m-1){\frac \pi {2a}},\quad \quad m=1,2,3,...$

The odd (under reflection $x\rightarrow -x$ ) solution $\phi ^{{{\mathrm {odd}}}}(x)$ satisfies the boundary conditions if

$k={\frac {m\pi }a},\quad \quad m=1,2,3,...$

It is easy to check that the normalized solutions are

$\phi _{m}^{{{\mathrm {even}}}}(x)={\frac 1{{\sqrt {a}}}}\cos \left[(m-{\frac 12}){\frac {\pi x}a}\right],$

and

$\phi _{m}^{{{\mathrm {odd}}}}(x)={\frac 1{{\sqrt {a}}}}\sin {\frac {m\pi x}a}.$

Solutions $\phi ^{{{\mathrm {even}}}}(x)$ and $\phi ^{{{\mathrm {odd}}}}(x)$ have the property that

$\int \limits _{{-a}}^{{a}}dx\phi _{{m}}^{{{\mathrm {even}}}}(x)\phi _{{n}}^{{{\mathrm {even}}}}(x)=\int \limits _{{-a}}^{{a}}dx\phi _{{m}}^{{{\mathrm {odd}}}}(x)\phi _{{n}}^{{{\mathrm {odd}}}}(x)=\delta _{{mn}},$

where $\delta _{{mn}}$ is the Kronecker symbol and

$\int \limits _{{-a}}^{{a}}dx\phi _{{m}}^{{{\mathrm {even}}}}(x)\phi _{{n}}^{{{\mathrm {odd}}}}(x)=0.$

The eigenvalues of the particle in an infinite potential well are (see, Ref.[5])

$E_{n}=D_{\alpha }\left({\frac {\pi \hbar }a}\right)^{\alpha }n^{\alpha },\qquad \qquad n=1,2,3....,\qquad 1<\alpha \leq 2.$

It is obvious that in the Gaussian case (α = 2) above equations are transformed into the standard quantum mechanical equations for a particle in a box (for example, see Eq.(20.7) in ^[7])

The state of the lowest energy, the ground state, in the infinite potential well is represented by the $\phi _{n}^{{{\mathrm {even}}}}(x)$ at n=1,

$\phi _{{{\mathrm {ground}}}}(x)\equiv \phi _{1}^{{{\mathrm {even}}}}(x)={\frac 1{{\sqrt {a}}}}\cos \left({\frac {\pi x}{2a}}\right),$

and its energy is

$E_{{{\mathrm {ground}}}}=D_{{\alpha }}\left({\frac {\pi \hbar }{2a}}\right)^{{\alpha }}.$

Fractional quantum oscillator

Fractional quantum oscillator introduced by Nick Laskin (see, Ref.[2]) is the fractional quantum mechanical model with the Hamiltonian operator $H_{{\alpha ,\beta }}$ defined as

$H_{{\alpha ,\beta }}=D_{{\alpha }}(-\hbar ^{{2}}\Delta )^{{\alpha /2}}+q^{{2}}|{\mathbf {r}}|^{{\beta }},\quad 1<\alpha \leq 2,\quad 1<\beta \leq 2,$ ,

where q is interaction constant.

The fractional Schrödinger equation for the wave function $\psi ({\mathbf {r}},t)$ of the fractional quantum oscillator is,

$i\hbar {\frac {\partial \psi ({\mathbf {r}},t)}{\partial t}}=D_{{\alpha }}(-\hbar ^{{2}}\Delta )^{{\alpha /2}}\psi ({\mathbf {r}},t)+q^{{2}}|{\mathbf {r}}|^{{\beta }}\psi ({\mathbf {r}},t)$

Aiming to search for solution in form

$\psi ({\mathbf {r}},t)=e^{{-iEt/\hbar }}\phi ({\mathbf {r}}),$

we come to the time-independent fractional Schrödinger equation,

$D_{{\alpha }}(-\hbar ^{{2}}\Delta )^{{\alpha /2}}\phi ({\mathbf {r}},t)+q^{{2}}|{\mathbf {r}}|^{{\beta }}\phi ({\mathbf {r}},t)=E\phi ({\mathbf {r}},t).$

The Hamiltonian $H_{{\alpha ,\beta }}$ is the fractional generalization of the 3D quantum harmonic oscillator Hamiltonian of standard quantum mechanics.

Energy levels of the 1D fractional quantum oscillator in semiclassical approximation

The energy levels of 1D fractional quantum oscillator with the Hamiltonian function $H_{{\alpha }}=D_{{\alpha }}|p|^{{\alpha }}+q^{{2}}|x|^{{\beta }}$ can be found in semiclassical approximation.

We set the total energy equal to E, so that

$E=D_{{\alpha }}|p|^{{\alpha }}+q^{{2}}|x|^{{\beta }},$

whence

$|p|=\left({\frac {1}{D_{{\alpha }}}}(E-q^{{2}}|x|^{{\beta }})\right)^{{1/\alpha }}$ .

At the turning points $p=0$ . Hence, the classical motion is possible in the range $|x|\leq (E/q^{{2}})^{{1/\beta }}$ .

A routine use of the Bohr-Sommerfeld quantization rule yields

$2\pi \hbar (n+{\frac {1}{2}})=\oint pdx=4\int \limits _{{0}}^{{x_{{m}}}}pdx=4\int \limits _{{0}}^{{x_{{m}}}}D_{{\alpha }}^{{-1/\alpha }}(E-q^{{2}}|x|^{{\beta }})^{{1/\alpha }}dx,$

where the notation $\oint$ means the integral over one complete period of the classical motion and $x_{{m}}=(E/q^{{2}})^{{1/\beta }}$ is the turning point of classical motion.

To evaluate the integral in the right hand we introduce a new variable $y=x(E/q^{{2}})^{{-1/\beta }}$ . Then we have

$\int \limits _{0}^{{x_{m}}}D_{\alpha }^{{-1/\alpha }}(E-q^{2}|x|^{\beta })^{{1/\alpha }}dx={\frac 1{D_{\alpha }^{{1/\alpha }}q^{{2/\beta }}}}E^{{{\frac 1\alpha }+{\frac 1\beta }}}\int \limits _{0}^{1}dy(1-y^{\beta })^{{1/\alpha }}.$

The integral over dy can be expressed in terms of the Beta-function,

$\int \limits _{{0}}^{{1}}dy(1-y^{{\beta }})^{{1/\alpha }}={\frac {1}{\beta }}\int \limits _{{0}}^{{1}}dzz^{{{\frac {1}{\beta }}-1}}(1-z)^{{{\frac {1}{\alpha }}}}={\frac {1}{\beta }}\mathrm{B} \left({\frac {1}{\beta }},{\frac {1}{\alpha }}+1\right).$

Therefore

$2\pi \hbar (n+{\frac 12})={\frac 4{D_{\alpha }^{{1/\alpha }}q^{{2/\beta }}}}E^{{{\frac 1\alpha }+{\frac 1\beta }}}{\frac 1\beta }\mathrm{B} \left({\frac 1\beta },{\frac 1\alpha }+1\right).$

The above equation gives the energy levels of stationary states for the 1D fractional quantum oscillator (see, Ref.[2]),

$E_{{n}}=\left({\frac {\pi \hbar \beta D_{{\alpha }}^{{1/\alpha }}q^{{2/\beta }}}{2\mathrm{B} ({\frac {1}{\beta }},{\frac {1}{\alpha }}+1)}}\right)^{{{\frac {\alpha \beta }{\alpha +\beta }}}}\left(n+{\frac {1}{2}}\right)^{{{\frac {\alpha \beta }{\alpha +\beta }}}}.$

This equation is generalization of the well-known energy levels equation of the standard quantum harmonic oscillator (see, Ref.[7]) and is transformed into it at α = 2 and β = 2. It follows from this equation that at ${\frac {1}{\alpha }}+{\frac {1}{\beta }}=1$ the energy levels are equidistant. When $1<\alpha \leq 2$ and $1<\beta \leq 2$ the equidistant energy levels can be for α = 2 and β = 2 only. It means that the only standard quantum harmonic oscillator has an equidistant energy spectrum.

References

↑ N. Laskin, (2000), Fractional Quantum Mechanics and Lévy Path Integrals. Physics Letters 268A, 298-304.
↑ N. Laskin, (2002), Fractional Schrödinger equation, Physical Review E66, 056108 7 pages. (also available online: http://arxiv.org/abs/quant-ph/0206098)
↑ S. G. Samko, A. A. Kilbas, and O. I. Marichev, Fractional Integrals and Derivatives, Theory and Applications ~Gordon and Breach, Amsterdam, 1993
↑ N. Laskin, (2000), Fractional Quantum Mechanics, Physical Review E62, 3135-3145. (also available online: http://arxiv.org/abs/0811.1769)
↑ N. Laskin, (2000), Fractals and quantum mechanics. Chaos 10, 780-790
↑ N. Bohr, (1913), Phil. Mag. 26, 1, 476, 857
↑ L.D. Landau and E.M. Lifshitz, Quantum mechanics (Non-relativistic Theory), Vol.3, Third Edition, Course of Theoretical Physics, Butterworth-Heinemann, Oxford, 2003

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