Recent content by AJS2011

If your question is in the context of superconductivity then: The coherence length is the "natural scale" set by parameters of GL equation. Likewise the penetration depth sets a "natural scale" for phenomena related to EM field. For example if the size of your system is much less than...

Thanks RedX! In superconductivity such mixing terms appear, and the way around it is through using Bogolubov-Valatin transformation. Linear response, aka Kubo formalism, studies near equilibrium properties of a system when a small external field is perturbing the system. For example, to...

Thanks for the explanation! You are right. Well, I meant an external electromagnetic field for example. For magnetic fied in z direction for example in symmetric gauge we can have A_{\mu}= \frac{B_0}{2}(0,y,-x,0)) \rightarrow \vec{B}= B_0 \hat{a}_z. Then external A can cause the break...

Thanks, dextercioby! Based on what you say, the presence of of non-constant gauge fields will ruin the the translational invariance. However in calculations of the linear response theory to electromagnetic field I see that people have \left\langle J_{\mu}(x) J_{\nu}(y) \right\rangle where...

Hi, Assume the following action: \int d^4 x L[\phi,A]+ \int d^4 x A_{\mu} (x) J^{\mu}(x) What are the conditions on the form of action to have space/time translational invariance for a two point function: \left\langle J_{\mu}(x) J_{\nu}(y) \right\rangle = G_{\mu \nu}(x-y)...

Thanks, SW VandeCarr! Intuitively, I understand that because of huge number of scatterings from impurities in all directions, preferred directions in space disappear; however I do not see how it can happen in mathematics through a concrete example.

Hey, In doing calculations on superconductors, I often hear that people say "averaging over the random potential of impurities make the theory translationally invariant both in time and space". I do not exactly understand it? Could you please explain it through a simple example or by citing a...

Thanks to all of you Avodyne, tom.stoer, RedX, A. Neumaier!

Thanks, Avodyne! My action looks similar to what you suggested. The action I have in (1+1)dimension is of the form: \begin{equation} \begin{split} S_E = \int d\omega dq & \Bigl\{ f(q,\omega)(q^2 VV^* +q\omega VA^*+q\omega V^*A+\omega^2AA^*)\\&+g(q,\omega)(VV^*-i\omega V\varphi^*+i\omega...

Quantum Field Theory Purly in Momentum Space? Hello, I have a complicated nonlinear-nonlocal-nonrelativistic-effective action in momentum space and would like to do perturbation theory with that. I need to find propagator and Feynman rules. I can not go to x-space and follow the standard...