# Search results

1. ### Diamond/Silicon's band structure

Homework Statement I am asked to discuss the band structure of diamond. I saw the band structure of diamond has 4 filled valence bands and then 4 conduction bands. Silicon, the same. Homework Equations --- The Attempt at a Solution I'm feeling really silly because I don't understand why it is...
2. ### Partial derivatives and thermodynamics

Hi all. Suppose I have the ideal gas law $$P=\frac{RT}{v}$$If I'm asked about the partial derivative of P with respect to molar energy ##u##, I may think "derivative of P keeping other quantities (whatever those are) constant", so from the formula above I get \frac{\partial P}{\partial...
3. ### I Values of Lagrange multipliers when adding new constraints

Say we have a Lagrange function with one multiplier a times a constrain. I minimize and solve the system to find a. I now add another constrain to the same system multiplied by the constant b. Is the value of a the same or can it change?
4. ### I Why does low surface energy imply hydrophobicity?

I don't understand why materials with low surface energy are hydrophobic and viceversa. All I can find are quick phenomenological explanations that don't quite deal with the physical (microscopic) process going on. Could anyone provide a good microscopic picture of why it is that way? What's...
5. ### I Is the Fermion number operator squared equal to itself?

What the title says. Acting on a fermionic state with the number operator to a power is like acting with the fermionic operator itself. Does this allow us to define ## \hat{n}^k=\hat{n} ##? Or is there any picky mathematical reason not to do so?
6. ### A Surfaces that present expansion instead of relaxation?

Since there are no bonds at the other side of the surface, external layers of solids are usually closer to the next layer. This process is called relaxation. (Example in picture a here). However, at a lecture I attended the other day it was mentioned that some surfaces present expansion...
7. ### I Density matrix on a diagonal by blocks Hamiltonian.

If I have a Hamiltonian diagonal by blocks (H1 0; 0 H2), where H1 and H2 are square matrices, is the density matrix also diagonal by blocks in the same way?
8. ### A Question about Berry phase in 1D polyacetylene

Hi. I'm taking a look at some lectures by Charles Kane, and he uses this simple model of polyacetylene (1D chain of atoms with alternating bonds which give alternating hopping amplitudes) [view attached image]. There are two types of polyacetylene topologically inequivalent. They both give the...
9. ### I Probabilities in many body systems...

Hi there. Excuse that I write it all in standard text but I don't know how to write it otherwise. I'll get right to the question and try to see if I understand the theory from the answer. Suppose we have a system of two electrons, one spin up and one spin down in a system with 3 possible...
10. ### Solid State Books for second quantization and condensed matter

Hi. I'll be doing a master's degree in nanophysics and working on electron transport in arrays of qubits. I don't know anything (or barely) about the second quantization and would like a book which covers it, and on condensed matter overall. So far I've been told about Bruus&Flensberg's...
11. ### I Why are scalar fields Lorentz invariant?

Hi. This question most probably shows my lack of understanding on the topic: why are scalar fields Lorentz invariant? Imagine a field T(x) [x is a vector; I just don't know how to write it, sorry] that tells us the temperature in each point of a room. We make a rotation in the room and now...
12. ### I Active vs Passive Lorentz transformation

Hi. First, excuse my English. In my lecture notes on classical electrodynamics, we are introduced to the Lorentz transformations: a system S' moves relative to a system S with positive veloticy v in the x axis (meassured in S), spacial axis are parallel, origin of times t and t' coincide...
13. ### I (2,0) tensor is not a tensor product of two vectors?

Hi. I'm trying to understand tensors and I've come across this problem: "Show that, in general, a (2, 0) tensor can't be written as a tensor product of two vectors". Well, prior to that sentence, I would have thought it could... Why not?
14. ### I Understanding Etherington's reciprocity theorem

Hi, Etherington't reciprocity theorem states that distances measured by angular separation and by luminosity differ. My question is which one (if any of them) is the actual distance. I can understand they might differ in an expanding universe, but there's still a physical distance in such one...
15. ### A Measurement without energy cost?

Hi, On the context of Maxwell's Demon, it's accepted that Landauer's principle (the erasure of a bit of information requires kTln2 of work) gives a solution to the paradox. The erasure of a bit of information can be seen as the modulation of a double well potential, as in the right side of the...
16. ### Why does Landauer define erasure of information as resetting

Hi. I hope this is the right forum for the question. Landauer's principle says that the erasure of one bit of information (1 or 0) requires kBTln2 of heat production. When he talks about erasure, it means "resetting back to 0 (or 1)" after measuring. He says something along "this is needed as...
17. ### Schools Finishing undergrad school -- What are my options?

Hi I'm finishing undergrad school at a university in Spain. It's taken me 5 years (was supposed to be 4) and my grades are mediocre (trying to reach a 7 out of 10). My university itself is a trainwreck, a disaster. Most of what I've actually, properly learnt was during my year at Nottingham...
18. ### Help understanding Shannon entropy

Hi I'm having some trouble understanding Shannon entropy and its relation to "computer" bits (zeros and ones). Shannon entropy of a random variable is (assume b=2 so that we work in bits) and everywhere I've read says it is "the number of bits on the average required to describe the random...
19. ### Shouldn't a metal at 0K emit electromagnetic radiation?

Electrons pile up inside a metal up to a maximum value in k-space (meaning the have velocity) and occupying all levels at 0K, solving the Schroedinger equation as running waves. So electrons moving => radiation being emitted. But electrons can't move to a lower energy state, so that would mean...