### electronic band theory - What is the exact relation between honeycomb atomic arrangement and Dirac cones?

The question is in the title. I couldn't find any resembling question from the past.Any link to thorough study (with the answer to my question) would be incredibly helpful....Read more

### electronic band theory - Effective mass for density of states calculation and for conductivity calculation

In silicon, for the effective mass for density of states calculation, electron mass (1.08) is more than hole mass (0.81). Whereas, the effective mass for conductivity calculation, hole mass (0.386) is more than electron mass (0.26). Why is it so?...Read more

### absorption - What does "Metal based electronic transition" means?

Recently I've been reading on the chiroptical activity of metal nanoparticles protected by biomolecular ligands, which have optical activity signals that are manifested in the metal-based electronic transition in the visible region, implying an electronic coupling effect.Any idea what does these two concepts mean?For what I have read it just that... electronic transitions in the metal nuclei, but I can't fully understand it....Read more

### Energy Diagram of Band Theory of Solids for Semiconductors

I was trying to understand the band theory of solids and came across this graph for a semi-conductor. In between the areas between b and c, why does the graph display two curves? From the description that I've read it says that when the atoms of the semi-conductor lattice approach, the 3s and 3p energy levels show no difference. How is that possible and if it is what is the reason for the other curve in that region? Also, it is said that between B and C, the number of energy gaps are no longer what it would normally dictate as I've learn in C...Read more

### Electronic band structure

I want to know why the bands like valence or conduction splits up into two parts as shown in this diagram. why the energy gaps exist with in these bands? As you can see in this diagram that their is an energy gap with in conduction as well as valence band. what are their physical significance?...Read more

### fermi liquids - What is the shape of a band electron in momentum space?

Band electrons occupy adjacent sharply defined momentum states that in xyz space take the form of a spectrum of wave functions. These wave functions span the entire xyz volume of any compact unit of metal, e.g. a 1 cm diameter perfect crystal of silver.For any selected xyz space axis crossing such a unit of metal, the wave function of a particular band electron will have the form of two sharply defined momentum states, one for "leftward electron travel," and one for "rightward electron travel." There will also be lower momentum components creat...Read more

### Band formation in compounds

How are energy bands formed in compounds(solids)? Take for example Al2O3. Which orbitals of Al atom or O atom will form conduction band or valence band. Are the conduction or valence bands have s, p, d.... etc character? Is it that conduction band is formed out of atomic orbital of one type of atom and valence band is formed out of the orbitals of the other type of atom? In which band do the shared electrons reside in case of a covalently bonded compound? They are highly localized and can not conduct (i suppose). But then in silicon crystal or ...Read more

### electronic band theory - Is Fermi Level a property of the material?

I have gotten myself confused about the band structure explanation of semi-conductors vs insulators. The Wikipedia picture and the explanation surrounding it seem to suggest that Fermi Level is a property of the material. I understand how Fermi-Energy is a property of the material: it is the highest energy occupied by an electron at zero temperature. But Fermi level as shown in the picture doesn't make sense to me as there is nothing special about that level intrinsically for semi-conductors and insulators. Furthermore, there is no state there ...Read more

### electronic band theory - What holds metal atoms together? And what accounts for the strength of metallic bonds?

From the wikipedia page for metallic bonding, I've noticed that there seem to be a few things at play:(1) the delocalization of electrons, and(2) the fact that there are a far larger number of delocalized energy states available to the electrons than there are electrons themselves (dubbed "electron deficiency").However, I'm still struggling to understand how this constitutes a bond between the atoms (i.e. how these two factors holds metal atoms, and specifically their nuclei, together) and what makes this bond so strong.One quote that really in...Read more

### electronic band theory - Dispersion relation near a Dirac point

I have very simple energy spectrum,$$E_{\pm}(k_x,k_y)=\sqrt{2+2\cos(k_x)\cos(k_y)}.$$The band gap closes at (for example) $k_x=0, k_y=\pi$. Both $E_+$ and $E_-$ are zero there. This is seen in the plot:Apparently the function is supposed to be linear in momenta, so I expect the second term in the Taylor expansion to be important. However, I can't seem to compute the Taylor expansion because the terms either vanish or become undefined. A Taylor expansion about the point $(a,b)$ of the function $f(k_x,k_y)$ is,f(k_x,k_y)\approx f(a,b)+\partial_...Read more

### Mobile Charge effects on Band Diagrams

Hi I have a question which I think is related more to definitions than anything else. Consider a band diagram. PN junction, heterostructure, whatever it might be. It's a very useful tool to determine where there is field, where there is space charge by slopes, band bending etc. However, as I recall, the strictest definition is that this is the potential an electron sees due to the hybridization of the atomic lattice states. Any charges, i.e. defect/traps, surfaces, polarization charges, etc. are all fixed. Does this mean that mobile charge ...Read more

### electronic band theory - Do III-V based photovoltaics "glow" (photo-luminesce) when illuminated but not loaded?

In this nice answer I started to learn about how the silicon photovoltaics cells on the International Space Station are managed. In that case, the cells are generally either delivering useful power to the station, or shunted to a dummy load. However, some satellites may leave some fraction of their photovoltaics open circuit if there is low power demand. In that case, the electron-hole pairs produced by sunlight are left to recombine within the junction itself.Does this mean that photovoltaic cells made from III-V compounds with direct band gap...Read more