The tag has no wiki summary.

learn more… | top users | synonyms

8
votes
2answers
750 views

Modern and complete references for the $k\cdot p$ method?

I've recently started studying the $k\cdot p$ method for describing electronic bandstructures near the centre of the Brillouin zone and I've been finding it hard to find any pedagogical references on ...
8
votes
1answer
2k views

Why do electrons in graphene behave as Dirac fermions near the Dirac points?

I've been learning about graphene, and I recently calculated the band structure for it using a nearest-neighbor tight-binding model for the $\pi$ electrons: $$\varepsilon(\vec k)=\pm t\sqrt{3+2 \cos ...
8
votes
1answer
678 views

Counterexamples to the bulk-boundary correspondence (topological insulators)

In the literature on topological insulators and superconductors the 'bulk-boundary correspondence' features quite heavily. One version of this conjecture says roughly: "At an interface between two ...
8
votes
1answer
357 views

Intuition on topologically nontrivial 2D-band structures?

I want to get more intuition on topologically nontrivial band structures. There's this popular 2D two-band model for a topological insulator where $H=\sum_{k}h(\boldsymbol{k})$ (see Qi, Hughes, and ...
8
votes
0answers
826 views

Where does the Berry phase of $\pi$ come from in a topological insulator?

The Berry connection and the Berry phase should be related. Now for a topological insulator (TI) (or to be more precise, for a quantum spin hall state, but I think the Chern parities are calculated in ...
7
votes
5answers
19k views

What's the difference between Fermi Energy and Fermi Level?

I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the ...
7
votes
1answer
270 views

What is the simplest possible topological Bloch function?

Kohmoto (1985) pointed out in Topological Invariant and the Quantization of the Hall Conductance how TKNN's calcuation of Hall conducance is related to topology, in which topologically nontriviality ...
6
votes
2answers
163 views

Floquet and Bloch's theorems : connection?

It is often stated that Bloch's theorem and Floquet's theorem are equivalent, even the Bloch's theorem is often referred as Floquet-Bloch theorem. However, it seems quite confusing to me since the ...
6
votes
3answers
1k views

A question on the existence of Dirac points in graphene?

As we know, there are two distinct Dirac points for the free electrons in graphene. Which means that the energy spectrum of the 2$\times$2 Hermitian matrix $H(k_x,k_y)$ has two degenerate points $K$ ...
6
votes
3answers
326 views

Are electronic wavefunctions in band gap insulators localized? is a single-particle picture sufficient in this case?

I am having trouble understanding the physics of band gap insulators. Usually in undergrad solid state physics one looks at non-interacting electrons in a periodic potential, with no disorder. Then, ...
5
votes
1answer
2k views

Transparency of solids using bandgaps and relation to conduction and valence bands

I think I understand how a solid can appear transparent as long as the energy of the photons travelling through it are not absorbed in the material's bandgap. But how does this band gap relate to ...
5
votes
3answers
633 views

How do charge carriers move thermal energy? (Peltier effect)

I am having hard time understanding how the charge carriers (electrons and holes) are able to move thermal energy. I am on a high school physics level, so I will probably have a hard time ...
5
votes
1answer
330 views

A (smart) way to map out the Brillouin zone of a 2-D material

I am currently writing a tight-binding calculation model for various 2-D cells as part of a homework assignment. Whilst solving the problem set was quite easy I struggle at a smart way to plot the ...
4
votes
2answers
629 views

How does Bloch's theorem generalize to a finite sized crystal?

I would be fine with a one dimensional lattice for the purpose of answering this question. I am trying to figure out what more general theorem (if any) gives Bloch's theorem as the number of unit ...
4
votes
1answer
866 views

Can surface dipoles/charges change the work function of a metal?

As typically drawn in simplified band diagrams (see picture below), the metal Fermi Level is shown as the top of the conduction band, with the entire band filled. In many situations, including ...
4
votes
1answer
580 views

What is an electron/hole pocket and what is the significance?

What is an electron/hole pocket and what is the significance? I'm trying to get my head around this. I've read what Ashcroft and Mermin have to say on the subject, but it's a little convoluted. They ...
4
votes
3answers
155 views

How does temperature affect photovoltaics (PV) efficiency?

I know that photovoltaic panels are more efficient at lower temperatures: As the temperature increases, the output voltage decreases. I am looking for an explanation of the mechanism behind this ...
4
votes
1answer
106 views

Electron degeneracy pressure

Why is it that in stars undergoing gravitational collapse electron degeneracy kicks in? Why couldn't the electrons form energy bands like in semiconductors?
4
votes
2answers
830 views

Need help understanding Semiconductor physics

I am trying to read Kittel for a project, and he mentions the properties on silicon and germanium so briefly, that I don't understand it at all. He talks about p states, and I don't really know what ...
4
votes
1answer
179 views

Is it possible to excite a semiconductor @ k=0?

Let us assume a direct semiconductor with parabolic band structure around the $\Gamma$ point (which is not bad for most semiconductors) in momentum ($k$) space. Now we excite it with light: I ...
4
votes
2answers
300 views

Why are there direct bandgaps?

This Question has been bugging me for sometime. Some semiconductors have direct bandgaps and indirect bandgaps. So what causes a direct bandgap to occur? The physics behind ,why there are direct ...
4
votes
1answer
176 views

In a positively biased PN junction, where do the injection carriers come from?

I am not quite understand i-v character of PN-junction diode. Here is the model in textbook. The PN junction diode can be divided into three regions. They are One depletion region near the PN ...
3
votes
1answer
74 views

Semiconductors and energy bands

The valence and conduction band of a semi-conductor are often drawn as here click. This plot has essentially two features and I would like to understand them. The peak and the valley of the two ...
3
votes
1answer
102 views

Why according to Hund's first rule all electron with same spin should occupy orbitals when partially filling?

I get that because of coulomb repulsion initially all the electrons will not occupy the same site but will single occupy the orbitals.But while doing so how do they know to keep their spins aligned ...
3
votes
1answer
44 views

Periodic momentum space in band structure

I often see pictures like this in physics, this one for Silicon band structure. (source, NB: it's the German page for Silicon). There you see the plot of the energy in terms of the momentum $k$. ...
3
votes
3answers
65 views

Do electrons in the conduction band have a quantum number?

Sorry if this is a silly question, but I haven't seen the concept anywhere and was wondering why. If valence electrons can have a quantum number, why can't conduction electrons? Is there something ...
3
votes
1answer
568 views

Dopant concentration and changes in band gap energy

Thanks to this lovely website, I was able to pop out reasonable values for my band gap energies from a translucent material. As expected, I found a decrease in band gap energy due to my treatments. ...
3
votes
1answer
46 views

Can a strained diamond actually become conductive?

According to about.com, "The electrical resistivity of most diamonds is on the order of 1011 to 1018 Ω·m" (source) However, according to the diamond band diagram, it seems that under a certain ...
3
votes
1answer
124 views

Hartree-Fock correction to $e$-$e$ interaction

The corrections to the energy per electron in a jellium model (uniform distribution of positive ion charge approximation to the regulated long range order ionic array) is given by (in units of Ry) ...
3
votes
1answer
205 views

Confused about charge seperation in solar cells

I'm a bit confused about how solar cells work. My understanding is that there is a p-n junction. A photon is absorbed which creates an electron-hole pair, and the idea is to separate the electron ...
3
votes
2answers
215 views

Bloch oscillations - Scattering to other bands

In the free electron approximation, a Bloch state $|k\rangle$ is the linear superposition of free plane wave states $\sum_G C_G(k) |k+G\rangle$, where $G$ are the conjugate lattice. Since the ...
2
votes
3answers
998 views

Reverse bias P-N junction [duplicate]

I am not able to understand why the PN junction does not conduct when reverse biased. Can't electrons travel from the N side through the power supply to the P side where they can jump from one hole to ...
2
votes
1answer
48 views

Where does Fermi energy place?

As you can see, this diagram determines the place of Fermi energy in different types of matter. I don't understand the later part which is related to semiconductors and insulators? Why do we set ...
2
votes
1answer
66 views

How to calculate the speed of electrons in a metal

According to the Sommerfeld model, the electrons on the Fermi level has the relation $$ \epsilon_F=\frac{\hbar^2k_F^2}{2m_e}=\frac{1}{2}m_ev_F^2 $$ i.e. $\hbar k_F=m_ev_F$ with $k_F=(3\pi^2n)^{1/3}$ ...
2
votes
1answer
582 views

Fermi level and conductivity

Can someone in a simple way explain me what the Fermi level is and what does it have to do with conductivity. My teacher said that Cu conducts electric current better than Al because of something in ...
2
votes
1answer
2k views

Valence band and conduction band, trying to get a clear picture!

I am trying to get a clear picture of the valence band, conduction band, and the band gap. Now I've been researching it for a little while now and understand most of what's going on. I'm still a ...
2
votes
1answer
237 views

Connecting Fermi levels and band diagrams to potential diagrams?

I'm trying to make sense of how you can find the potential diagram given the band diagrams of a few adjacent materials. As a simple example, in semiconducting heterostructures, if you have sandwich ...
2
votes
1answer
209 views

Does anyone know the difference and relation between $k\cdot p$ method and tight binding (TB) method?

Among the methods of calculating energy bands for crystals, first-principles method is the most accurate. Besides first principles, two commonly used modeling methods are the $k\cdot p$ method and ...
2
votes
1answer
1k views

What does the Fermi Energy really signify in a Semiconductor?

In understanding the behavior of semiconductors, I'm coming across a description of the Fermi Energy here and at Wikipedia's page (Fermi Energy, Fermi Level). If I understand correctly, the Fermi ...
2
votes
1answer
535 views

Energy band diagram of a system of Silicon Quantum dots

Suppose that we have a system of Silicon nanoparticles embedded in ZnO dielectric matrix. i'm thinking about how to construct the energy band structure of this system , suppose that we already have ...
2
votes
1answer
87 views

Electron density in metals at non zero temperature

When computing the electron density in metals, the usual crude result is computed for zero temperature. That is, we integrate \begin{equation} n=\frac{8\sqrt{2}\pi m^{3/2}}{h^{3}} ...
2
votes
1answer
164 views

Effective mass of a particle

I was reading about concept of effective mass on Wikipedia and came across the line that effective mass of a particle can be negative and even zero. How is this possible?
2
votes
2answers
936 views

How to interpret band structures

I'm currently taking a Solid State Physics class, and is currently reading about the quantum mechanical description of solids. I then came across the following figure: It's supposed to be the band ...
2
votes
0answers
217 views

Derivation of existence of energy band gap in semiconductor (solid State)

I am looking for both a mathematical and a physical reason for energy band gap in metals. For Physical reason, I was told that at each reciprocal lattice, you could have Bragg scattering, that would ...
2
votes
0answers
232 views

Topological band theory [closed]

Why topological insulators were discovered so late? While the band theory was known long time ago! I mean why the topological properties of electronic bands were not noticed in the past?
2
votes
0answers
128 views

Why does silicon have an indirect gap?

Is there an intuitive explanation as to why silicon has an indirect gap? I have heard that this can explained using pseudopotentials.
2
votes
2answers
57 views

How to check the ohmic contact to the film?

I have a thin semiconductor film deposited on an isolating substrate. I would like to check different metals to find out do they form the ohmic contact or Schottky barrier. What is the best way to do ...
1
vote
1answer
241 views

Semiconductors and localization of the electrons

When looking at the band diagram of a semi-conductor, direct conclusion of the invariance under discrete translations, for a filled state with an electron, one does know precisely it's momentum, so my ...
1
vote
4answers
116 views

The conduction electrons in metals is a thermal phenomenon?

When applying an external electric field in a metal at absolute zero, there is electrical current? There must be thermal fluctuations in the electron's band to be occurs current?
1
vote
1answer
386 views

Band gaps: are they at the centre or at the edge of the Brillouin zone?

Reading about electronic band structures, I came across the following: Band gaps open at the edges of the Brillouin zone (BZ), since that is where the Bragg scattering occurs. I am slightly ...