朗道量子化

朗道量子化

选定对称规范:

A

^

=

1

2

(

B

y

B

x

0

)

{\displaystyle {\hat {\mathbf {A} }}={\frac {1}{2}}{\begin{pmatrix}-By\\Bx\\0\end{pmatrix}}}

对于哈密顿算符进行去量纲化:

H

^

=

1

2

[

(

i

x

y

2

)

2

+

(

i

y

+

x

2

)

2

]

{\displaystyle {\hat {H}}={\frac {1}{2}}\left[\left(-i{\frac {\partial }{\partial x}}-{\frac {y}{2}}\right)^{2}+\left(-i{\frac {\partial }{\partial y}}+{\frac {x}{2}}\right)^{2}\right]}

实际值可以通过引入

q

{\displaystyle q}

c

{\displaystyle c}

{\displaystyle \hbar }

B

{\displaystyle \mathbf {B} }

m

{\displaystyle m}

等常数得出。

引入算符

a

^

=

1

2

[

(

x

2

+

x

)

i

(

y

2

+

y

)

]

{\displaystyle {\hat {a}}={\frac {1}{\sqrt {2}}}\left[\left({\frac {x}{2}}+{\frac {\partial }{\partial x}}\right)-i\left({\frac {y}{2}}+{\frac {\partial }{\partial y}}\right)\right]}

a

^

=

1

2

[

(

x

2

x

)

+

i

(

y

2

y

)

]

{\displaystyle {\hat {a}}^{\dagger }={\frac {1}{\sqrt {2}}}\left[\left({\frac {x}{2}}-{\frac {\partial }{\partial x}}\right)+i\left({\frac {y}{2}}-{\frac {\partial }{\partial y}}\right)\right]}

b

^

=

1

2

[

(

x

2

+

x

)

+

i

(

y

2

+

y

)

]

{\displaystyle {\hat {b}}={\frac {1}{\sqrt {2}}}\left[\left({\frac {x}{2}}+{\frac {\partial }{\partial x}}\right)+i\left({\frac {y}{2}}+{\frac {\partial }{\partial y}}\right)\right]}

b

^

=

1

2

[

(

x

2

x

)

i

(

y

2

y

)

]

{\displaystyle {\hat {b}}^{\dagger }={\frac {1}{\sqrt {2}}}\left[\left({\frac {x}{2}}-{\frac {\partial }{\partial x}}\right)-i\left({\frac {y}{2}}-{\frac {\partial }{\partial y}}\right)\right]}

这些算符的对易关系为:

[

a

^

,

a

^

]

=

[

b

^

,

b

^

]

=

1

{\displaystyle [{\hat {a}},{\hat {a}}^{\dagger }]=[{\hat {b}},{\hat {b}}^{\dagger }]=1}

.

哈密顿算符可记为:

H

^

=

a

^

a

^

+

1

2

{\displaystyle {\hat {H}}={\hat {a}}^{\dagger }{\hat {a}}+{\frac {1}{2}}}

朗道能级序数

n

{\displaystyle n}

a

^

a

^

{\displaystyle {\hat {a}}^{\dagger }{\hat {a}}}

的本征值。

角动量z方向上的分量为:

L

^

z

=

i

θ

=

(

b

^

b

^

a

^

a

^

)

{\displaystyle {\hat {L}}_{z}=-i\hbar {\frac {\partial }{\partial \theta }}=-\hbar ({\hat {b}}^{\dagger }{\hat {b}}-{\hat {a}}^{\dagger }{\hat {a}})}

利用其与哈密顿算符可对易,即

[

H

^

,

L

^

z

]

=

0

{\displaystyle [{\hat {H}},{\hat {L}}_{z}]=0}

,我们选定

L

^

z

{\displaystyle {\hat {L}}_{z}}

的本征值

m

{\displaystyle -m\hbar }

为使

H

^

{\displaystyle {\hat {H}}}

L

^

z

{\displaystyle {\hat {L}}_{z}}

对角化的本征函数。易见,在第

n

{\displaystyle n}

个朗道能级上存在

m

n

{\displaystyle m\geq -n}

。然而

m

{\displaystyle m}

的值可能非常大。在下面将推导系统表现出的有限简并度。

使用

b

^

{\displaystyle {\hat {b}}^{\dagger }}

可以使

m

{\displaystyle m}

减小一个单位同时使

n

{\displaystyle n}

保持不变,而

a

^

{\displaystyle {\hat {a}}^{\dagger }}

则可以使

n

{\displaystyle n}

增大一个单位,同时令

m

{\displaystyle m}

减小一个单位。类比量子谐振子,可以得到:

H

^

|

n

,

m

=

E

n

|

n

,

m

{\displaystyle {\hat {H}}|n,m\rangle =E_{n}|n,m\rangle }

E

n

=

(

n

+

1

2

)

{\displaystyle E_{n}=\left(n+{\frac {1}{2}}\right)}

|

n

,

m

=

(

b

^

)

m

+

n

(

m

+

n

)

!

(

a

^

)

n

n

!

|

0

,

0

{\displaystyle |n,m\rangle ={\frac {({\hat {b}}^{\dagger })^{m+n}}{\sqrt {(m+n)!}}}{\frac {({\hat {a}}^{\dagger })^{n}}{\sqrt {n!}}}|0,0\rangle }

在朗道规范与对称规范下,每个朗道能级上的简并轨道分别以量子数ky及

m

{\displaystyle m}

表征,每个朗道能级上单位面积的简并度是相同的。

可以证明选定下面这个波函数时,也可以得到上面得到的结果:

ψ

n

,

m

(

x

,

y

)

=

(

w

w

¯

4

)

n

w

n

+

m

e

|

w

|

2

/

4

{\displaystyle \psi _{n,m}(x,y)=\left({\frac {\partial }{\partial w}}-{\frac {\bar {w}}{4}}\right)^{n}w^{n+m}e^{-|w|^{2}/4}}

式中

w

=

x

+

i

y

{\displaystyle w=x+iy}

特别地,对于最低的朗道能级,即

n

=

0

{\displaystyle n=0}

时,波函数为任意一个解析函数与高斯函数的乘积:

ψ

(

x

,

y

)

=

f

(

w

)

e

|

w

|

2

/

4

{\displaystyle \psi (x,y)=f(w)e^{-|w|^{2}/4}}

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