- #1
Xyius
- 508
- 4
Homework Statement
Consider the Landau free energy ##\mathcal{L}=-hm+r_1 t m^2+Cm^3+s_0 m^4## with an additional ##m^3## term. We consider zero magnetic field case ##h=0## and ##s_0>0##.
a.) Please show that there are two critical temperatures ##t*## and ##t_1##. When ##t<t*##, a second minimal of free energy (not the one at m=0) appears, and when ##t<t_1##, the second minimal has smaller free energy than the minimal at m=0.
b.) Comparing with the second order phase transition, please show that there is no symmetry breaking or ergodicity breaking for the first order phase transition. Why is symmetry breaking related to the second order phase transition, but not the first order phase transition?
Homework Equations
Nothing
The Attempt at a Solution
So the first thing I did was to find all the minimals.
##\frac{\partial \mathcal{L}}{\partial m}=0 \rightarrow m=-\frac{3C}{8s_0} \pm \frac{1}{2}\sqrt{(\frac{3C}{4s_0})^2-\frac{2r_1 t}{s_0}} =m_{\pm}##
In order for these minimals to exist, we require the expression under the square root to be positive, thus we get.
##t<\frac{s_0}{2 r_1}\left(\frac{3C}{4s_0}\right)^2=t*##
Thus as long as ##t<t*## there will be two additional minimals, besides the minimal at m=0. However, these minimals will be less than the minimal at m=0 until a certain value of ##t##. In order to find the value of ##t## that equates all the minimals together, we do the following.
##\mathcal{L}(m=m_1)<\mathcal{L}(m=0) \rightarrow r_1t m^2+Cm^3+s_0m^4<0 \rightarrow m=\frac{-C}{2s_0} \pm \frac{1}{2}\sqrt{(\frac{C}{s_0})^2-\frac{4r_1 t}{s_0}}##
We again require the expression under the square root to be positive, this gives us,
##t<\frac{s_0}{4r_1}\left(\frac{C}{s_0}\right)^2=t_1##
Thus for ##t<t_1##, the minima will be less than the minima at m=0.
So now I believe I found the expressions for ##t_1## and ##t*##, my issue comes with dealing with the phase transitions.
My understanding is, when the absolute minimum changes from one value at ##m=0##, to two values at ##m=m_{\pm}## suddenly, then this indicates a phase transition.
My confusion comes into determining what exactly is the first order and second order phase transitions. I believe this is a first order phase transition because ##m_{min}## changes abruptly, and the first derivative of the free energy corresponds to ##m_{min}##, so therefore the discontinuity occurs at the first derivative of the free energy and this is a first order phase transition.
I do not know how to find the second order phase transition, and how to determine if symmetry is broken. The only thing I can think of is to plug the values of ##m_{min}## into ##\mathcal{L}## and see if both minimums are at the same value. Any help would be appreciated!