- #1

LCSphysicist

- 646

- 162

- Homework Statement
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- Relevant Equations
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$$H = - J ( \sum_{i = odd}) \sigma_i \sigma_{i+1} - \mu H ( \sum_{i} \sigma_i ) $$

So basically, my idea was to separate the particles in this way::

##N_{\uparrow}## is the number of up spin particles

##N_{\downarrow}## "" down spin particles

##N_1## is the number of pairs of particles close to each other with spin up

##N_2## "" with spin down

##N_3## "" with spin antiparallel

Therefore

$$\hat{H} = - J (N_1 + N_2 - N_3 ) - \mu H (N_{\uparrow} - N_{\downarrow})$$

Subject to the following constraints:

$$\frac{N}{2} = N_1 + N_2 + N_3$$

$$N_{\uparrow} = 2N_1 + N_3$$

$$N_{\downarrow} = 2N_2 + N_3$$

$$N = N_{\uparrow} + N_{\downarrow}$$

But, even make this clarifications, i can't see how to find the partition! If we substitute the above expressions on ##\sum e^{-\hat{H}/kT}##, we will find Z as a function of, for example, ##Z=Z(N_{\uparrow},N_3)##. But i don't know how to evaluate such a sum! I mean, it can't factoriz in the sum of two expoents, because both arguments are constrained! (We can't have ##N_3 = N/2, N_{\uparrow} = 0##, for example)

So basically, my idea was to separate the particles in this way::

##N_{\uparrow}## is the number of up spin particles

##N_{\downarrow}## "" down spin particles

##N_1## is the number of pairs of particles close to each other with spin up

##N_2## "" with spin down

##N_3## "" with spin antiparallel

Therefore

$$\hat{H} = - J (N_1 + N_2 - N_3 ) - \mu H (N_{\uparrow} - N_{\downarrow})$$

Subject to the following constraints:

$$\frac{N}{2} = N_1 + N_2 + N_3$$

$$N_{\uparrow} = 2N_1 + N_3$$

$$N_{\downarrow} = 2N_2 + N_3$$

$$N = N_{\uparrow} + N_{\downarrow}$$

But, even make this clarifications, i can't see how to find the partition! If we substitute the above expressions on ##\sum e^{-\hat{H}/kT}##, we will find Z as a function of, for example, ##Z=Z(N_{\uparrow},N_3)##. But i don't know how to evaluate such a sum! I mean, it can't factoriz in the sum of two expoents, because both arguments are constrained! (We can't have ##N_3 = N/2, N_{\uparrow} = 0##, for example)

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