- #1
FranzDiCoccio
- 342
- 41
Hi all,
I am looking into the discussions of Pauli paramagnetism (arising from free neutral fermions with spin 1/2) in chapter 11.6 of K. Huang's Stat. Mech. (II ed) and in chapter 31 of Ashcroft and Mermin's Solid State Physics.
It seems to me that these books do not agree on signs.
So in this discussion electrons are neutral, i.e. they interact with a (constant) magnetic field only through their dipole moment. The interaction energy should be [tex]-\vec{\mu}\cdot \vec{H}[/tex], i.e. the dipole wants to be aligned with the field.
But
[tex]
\vec{\mu} = g_s \frac{q}{2 m c} \vec{s} = -g_s \frac{e}{2 m c} \vec{s}
[/tex]
so that the interaction energy becomes
[tex]
g_s \frac{e}{2 m c} \vec{s} \cdot \vec{H} = \pm \mu_B H
[/tex]
for an electron of spin [tex]\pm[/tex] (recalling that the g factor of the electron is basically 2). That is the energy is raised for spin up and decreased for spin down.
Now this is what A&M say.
On the other hand, it seems to me that K. Huang says the opposite when writing the Hamiltonian for a single dipole ("neutral electron") as
[tex]
{\cal H} = \frac{p^2}{2 m}-\mu_B \vec{\sigma} \cdot \vec{H}
[/tex]
Could it be that Huang is using the intrinsic dipole moment for a particle of charge e instead of -e? Then he gets the correct result because his definition (11.112) of magnetization agrees with this assumption:
[tex]
{\cal M} = \frac{\mu_B}{V}(N_{+}-N_{-})
[/tex]
which differs from A&M's (31.55) by a sign.
Does this make sense to you?
Sorry about all this fuss, but all the story here is about signs, you know, paramagnetism vs diamagnetism, and the sign discrepancy bothers me a little.
Anyway, the bottom line seems to be that one gets paramagnetism independent of the sign in the definition of the dipole moment. That sort of makes sense, since magnetization here is the average dipole moment.
Thanks
F
I am looking into the discussions of Pauli paramagnetism (arising from free neutral fermions with spin 1/2) in chapter 11.6 of K. Huang's Stat. Mech. (II ed) and in chapter 31 of Ashcroft and Mermin's Solid State Physics.
It seems to me that these books do not agree on signs.
So in this discussion electrons are neutral, i.e. they interact with a (constant) magnetic field only through their dipole moment. The interaction energy should be [tex]-\vec{\mu}\cdot \vec{H}[/tex], i.e. the dipole wants to be aligned with the field.
But
[tex]
\vec{\mu} = g_s \frac{q}{2 m c} \vec{s} = -g_s \frac{e}{2 m c} \vec{s}
[/tex]
so that the interaction energy becomes
[tex]
g_s \frac{e}{2 m c} \vec{s} \cdot \vec{H} = \pm \mu_B H
[/tex]
for an electron of spin [tex]\pm[/tex] (recalling that the g factor of the electron is basically 2). That is the energy is raised for spin up and decreased for spin down.
Now this is what A&M say.
On the other hand, it seems to me that K. Huang says the opposite when writing the Hamiltonian for a single dipole ("neutral electron") as
[tex]
{\cal H} = \frac{p^2}{2 m}-\mu_B \vec{\sigma} \cdot \vec{H}
[/tex]
Could it be that Huang is using the intrinsic dipole moment for a particle of charge e instead of -e? Then he gets the correct result because his definition (11.112) of magnetization agrees with this assumption:
[tex]
{\cal M} = \frac{\mu_B}{V}(N_{+}-N_{-})
[/tex]
which differs from A&M's (31.55) by a sign.
Does this make sense to you?
Sorry about all this fuss, but all the story here is about signs, you know, paramagnetism vs diamagnetism, and the sign discrepancy bothers me a little.
Anyway, the bottom line seems to be that one gets paramagnetism independent of the sign in the definition of the dipole moment. That sort of makes sense, since magnetization here is the average dipole moment.
Thanks
F