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visits member for 2 years, 1 month
seen Apr 12 at 12:15

I'm an experimental condensed matter physicist, but I'm very curious about high energy physics as well.


Apr
10
accepted Where does the phase difference come from in a Josephson Junction?
Apr
9
comment Where does the phase difference come from in a Josephson Junction?
I am beginning to understand now. So, if we tie both superconductors to a common voltage $V$, there will be no current? And from $\dot{\phi}=2eV/\hbar$, $\phi =2eVt/\hbar+C$. What determines the integration constant?
Apr
7
comment Where does the phase difference come from in a Josephson Junction?
I would have assumed that if we prepared two superconductors in the exact same way at the exact same time (of course, experimentally impossible), they would have the same phase. Or is that wrong? Is there something else that determines the initial phase?
Apr
7
comment Where does the phase difference come from in a Josephson Junction?
Thanks. What I meant to ask was: what determines the phase that the superconductor starts with? Is it completely random? If I take a superconductor, break it into two, will the Josephson current between them be zero at $V=0$ because they have the same phase?
Apr
7
comment Where does the phase difference come from in a Josephson Junction?
Thanks. This was well explained. Why must there be a current at $V=0$? Why not $I=0$?
Apr
6
asked Where does the phase difference come from in a Josephson Junction?
Mar
30
awarded  Tumbleweed
Mar
30
comment Why is the Gibbs Free Energy $F-HM$?
What do you mean the Legendre Transform leave the value of the function unaltered? It changes the value of the function by $-HM$.
Mar
30
comment Why is the Gibbs Free Energy $F-HM$?
The first half of my response is a proof that the Helmholtz Free Energy is minimized at equilibrium, and $E_2=T S_2$ comes from integrating the definition of temperature: $1/T=\partial S/\partial E$, modulo a constant. Note that a crucial part of this proof is $E=E_1+E_2$, or $V=V_1+V_2$ or $N=N_1+N_2$, something that we can't write for the magnetic case. It doesn't matter that $V=\infty$ because $dV_1=-dV_2$ is what is really important, and also $\mu=-T\partial S/\partial N$ covers non-conservation of particles.
Mar
27
comment Why is the Gibbs Free Energy $F-HM$?
Perhaps I need to be a little more explicit. If we couple a system (labeled 1) to a heat bath (labeled 2) we maximize the total entropy $S=S_1+S_2$. The total energy is conserved $E=E_1+E_2$. If we take the heat bath energy to be $E_2=T S_2$, then $S=S_1+E_2/T=S_1+(E-E_1)/T$. Maximizing $S$ is the same as minimizing $E_1-T S_1$, which is the Helmholtz Free Energy. You can do the exact same analysis for $PV$ and $\mu N$, and it doesn't really matter if particles aren't conserved and that $V=\infty$. However, how do you prove that $F-HM$ maximizes global entropy?
Mar
26
asked Why is the Gibbs Free Energy $F-HM$?
Mar
23
asked Phase factor for nearest neighbor hopping in the Haldane Model
Mar
23
asked Average value of consecutive measurements of two observables
Mar
15
awarded  Yearling
Feb
16
comment Is energy lost in an elastic tunneling process?
@annav Yes, I am talking about quantum mechanical tunneling. And yes, if the tunneling is elastic, electrons will tunnel from one state to another of the same energy. However, $P=IV$ indicates that energy will be lost at some point.
Feb
15
asked Is energy lost in an elastic tunneling process?
Feb
15
comment Is the resistance 0 in the ballistic regime?
@Slaviks What if the contacts were superconducting?
Feb
1
accepted Do holes have wavefunctions?
Jan
30
comment Do holes have wavefunctions?
Thanks for the reply. I guess what I meant was that, is it valid to work with a 2x2 Slater determinant with the electron and hole wavefunction rather than the giant NxN many body Slater determinant? Of course, we could bypass this whole discussion by just working with creation and annihilation operators, but lets not talk about them.
Jan
30
comment Do holes have wavefunctions?
Ok, makes sense. So if you have an electron hole excitation (e.g. we shine light to promote an electron to the conduction band), do we need to write a Slater determinant with the electron and hole wavefunctions in it?