The study of physical properties of condensed phases of matter, including solids and liquids.
Questions tagged [condensed-matter]
5091 questions
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What is spontaneous symmetry breaking in quantum systems?
Most descriptions of spontaneous symmetry breaking, even for spontaneous symmetry breaking in quantum systems, actually only give a classical picture.
According to the classical picture, spontaneous symmetry breaking can only happen for non-linear…
Xiao-Gang Wen
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73
votes
5 answers
What does it mean for a Hamiltonian or system to be gapped or gapless?
I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean?
Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
Jordan
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68
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5 answers
What challenges needed to be overcome to create (blue) LEDs?
In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask:
What is the physical significance of blue LEDs, which challenges had to be…
Martin
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Why do we expect our theories to be independent of cutoffs?
Final edit: I think I pretty much understand now (touch wood)! But there's one thing I don't get. What's the physical reason for expecting the correlation functions to be independent of the cutoff? I.e. why couldn't we just plump for one "master…
Edward Hughes
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63
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What is a $p_x + i p_y$ superconductor? Relation to topological superconductors
I often read about s-wave and p-wave superconductors. In particular a $p_x + i p_y$
superconductor - often mentioned in combination with topological superconductors.
I understand that the overall Cooper pair wavefunction may have orbital angular…
Mike
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58
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3 answers
Why are relativistic quantum field theories so much more restrictive than non-relativistic ones?
Part of the reason that relativistic QFT is so hard to learn is that there are piles of 'no-go theorems' that rule out simple physical examples and physical intuition. A very common answer to the question "why can't we do X simpler, or think about…
knzhou
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votes
4 answers
How are magnets held together, and why do they not explode?
Imagine we have a magnet (red side is the north pole, blue side is the south pole), and imagine two ways to split it.
The first way:
When we split it by separating the north pole from the south pole, we see that the two pieces are themselves dipole…
MaximusIdeal
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54
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5 answers
A pedestrian explanation of conformal blocks
I would be very happy if someone could take a stab at conveying what conformal blocks are and how they are used in conformal field theory (CFT). I'm finally getting the glimmerings of understanding from reading Moore and Read's wonderful paper. But…
user346
53
votes
8 answers
Books for Condensed Matter after Ashcroft/Mermin
What are some good condensed matter physics books that can fill the gap between Ashcroft & Mermin and research papers? Suggestions for any specialized topics (such as superconductivity, CFT, topological insulators) are welcomed.
leongz
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53
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5 answers
Good reading on the Keldysh formalism
I'd like some suggestions for good reading materials on the Keldysh formalism in the condensed matter physics community. I'm familiar with the imaginary time, coherent state, and path integral formalisms, but lately, I've been seeing Keldysh more…
user129
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51
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4 answers
What's the rigorous definition of phase and phase transition?
I always feel unsure about the definitions of phase and phase transition. First, let's discuss in Laudau's paradigm. For example, some people say that phase is classified by symmetry. Some people say that phase is classified by order parameter and…
346699
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47
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Do photons gain mass when they travel through glass?
Please correct me if I'm wrong, but I believe that photons slow down when travelling through glass. Does this mean they gain mass? Otherwise, what happens to extra kinetic energy?
I understand now that the apparent slowing down is due to electron…
dan_waterworth
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45
votes
2 answers
Periodic vs Open boundary conditions
In condensed matter, people often use periodic boundary conditions to perform calculations about the bulk properties of a material. It's generally argued that in the $N\rightarrow\infty$ limit the boundary conditions don't affect the bulk…
Jahan Claes
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38
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3 answers
Reconciling topological insulators and topological order
We make an important distinction between the topological insulators (which are essentially uncorrelated band insulators, "with a twist") and topological order (which covers a variety of exotic properties in certain quantum many-body ground states).…
wsc
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38
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Emergent symmetries
As we know, spontaneous symmetry breaking(SSB) is a very important concept in physics. Loosely speaking, zero temprature SSB says that the Hamiltonian of a quantum system has some symmetry, but the ground state breaks the symmetry.
But what about…
Kai Li
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