Succeeding Polystyrene

Do we get marks for participating after the exam? :)

I read about this in the National Geographic magazine for June: a group in the USA created 'ultralight metallic microlattices. 'Ultralight' refers to < 10 mg per cubic centimetre. Of course, styrofoam already exists, but it isn't particularly strong because of the rather disordered structure of the ultralight-version of the material.

These microlattices apparently overcome this weakness because they can be created using much more regular internal structures. The way they did this: a self-propagating photopolymer was grown as a template, the template was coated with nickel via 'electroless' plating (not sure what this is yet...) and then etching away the template.

Ultralight materials are useful because of their thermal and kinetic insulating abilities, and now because they can be used as light structural supports.

Here is the doi and other stuff:
Science 18 November 2011: 
Vol. 334 no. 6058 pp. 962-965 
DOI: 10.1126/science.1211649


Josh Harbort

PS: Has anyone though that doi would be pronounced "DOO-ee"? :)

Today's exam (Monday 18th June)

This is just a reminder that the final exam will be in room 407 physics annex (conference room) at 2:30 pm today.

Please note that you are allowed a formula sheet that must be no longer than two sides of a single A4 sheet of paper and contain no more than 40 equations.

Semiconductors and synthetic biology

 I know points have been added and everything, and not that I've been distracted or anything, but there is an interesting (news) article: Artificial cells evolve proteins to structure semiconductors(snazzy title!)
This is based on PNAS article pubished in April (there's a very exciting intro in this).
(The news article seems much more optimistic/daring than the article)

The claim is that they have biofabricated silicon dioxide and titanium doxide (which they point out is in solar cells!).

From what I understand, they're taking advantage of biomineralisation -> sort of like skeleton of sea sponge (and humans and bones).
They place a seed inside a cell (fungus). DNA will then wrapped around the bead, which grows the material (the function of this DNA string). The proteins from these genes have been produced.

I suppose it's not too far-fetched of an application, but now the genes that could involved with making SiO2 and TiO2, have been identified and proteins synthetised.
It's not a big step to suggest we could use these proteins on a small labscale - on a larger scale is a different question entirely. Perhaps with regards to manufacturing very small structures, biofabrication would be most useful.

I suppose one must also question the efficiency of engineering bacteria to make such materials compared to convential methods.


Sidenote: the comments with the news article are interesting, and quite entertaining.

Classical Hall Resistance

Hello everyone, I was working through the mid semester exam again, and in Question 3 (d) we are required to find the classical Hall resistance in 2D by analogy with the 3D case. I'm not sure which quantity this Hall resistance is exactly, though, because of the 'hints' given in the question. The question itself seems to suggest use of the Drude relation, is this right?

Good work too, everyone, on passing so far! :)

Formative assessment marks

Marks are out of 100.
First three numbers are the last 3 numbers of student number

712 - 63.5
653 - 62
375 - 90
235 - 91.5
762 - 69.5
613 -59.5
398 - 70


Good news. Everyone scored more than 50 and so no one has an automatic fail.
Maxwell Smart says Extra good news for agents  no. 375 and 235. 
Because of they scored above 85 this enhanced the effect of their summative marks. e.g. they only need 45% to pass and 75% to get a 7. (see course profile for more details).
Consistent engagement pays off!

Marks for student presentations

These were well done.


Marks are out of 10.
First three numbers are the last 3 numbers of your student number

712 - 8.3
653 - 7.9
375 - 8.8
235 - 7.5
762 - 7.3
613 -7
398 - 7.8

Final exam location

It will be in room 407 physics annex [conference room]
at 2:30pm monday June 18

Josh's Slides from Talk, at last!

Hello everyone, sorry for not getting these up earlier! I had terrible troubles with Google Docs and infinite redirect loops, but I think that I've found a temporary solution... don't use Google Chrome for Google docs....

Here is the link to the document: https://docs.google.com/presentation/d/1Pr2rWBp_ujcko5YNP-wQKz-wRiZrbQGgx8KfUczHr_k/edit
Enjoy! :)

Three most important equations

Related to the essay, but I figured I'd go through what I considered to be the three most important equations of the course.  Reckon it would be interesting to see if they differ from others opinions.  I apologise for the notation in advance, it'll probably be set out how I would write it in matlab

Fermi wave vector: n = (kf^3)/(3*pi^2)

Might seem like a bit of an odd one, but the construction of the Fermi surface and the subsequent Fermi wave vector has so many implications in all of the solid state that we dealt with.  This definition first appeared in chapter 2 using the Sommerfield model, but was still used once the crystal lattice was introduced.

Bloch's Theorem:  psi_nk(r+N*a) = exp(i*N*k.*a)psi_nk(r)

Bloch rocks, need I say more?

Semiclassical equations of motion:  dr/dt = 1/hbar * de_n(k)/dk
                                                          hbar* dk/dt = -e(E + 1/c * dr/dt * B)

Grouped these two together, but with them we can determine the dynamics of electrons in magnetic and electric fields with certain energy levels defined by e_n(k).  Requires just a few assumptions about position and momentum.

Anyway they're my three equations, and with them i feel you can describe most of the dynamics of electrons and ions in a crystal lattice.  Any major disagreements with my choices?

Optional revision session

Thursday June 7
1-2pm
Interaction room.

n.b. I will be overseas from June 9 onwards.

Final exam date?

Please talk amongst yourselves and propose a possible date.

Dirac points

During my presentation, the Dirac point was mentioned.
This also popped up in Andy's paper.


From an article:  Pairs of bands are degenerate at a point of high symmetry of the Brillouin zone.
Electromagnetic one-way edge modes were first predicted in 2008, 2006, to occur in systems possessing Dirac points.
Here, they take a Dirac point to be points s.t. modes near each degeneracy point can be described by Dirac Hamiltonian. (We may recall Dirac Hamiltonian from Quantum, similar to Schrodinger, but with spin with

We may interpret the Dirac points to be points of high symmetry of the Brillouin zone, which is where electrons can be described by the Dirac equation. This is consistent with description given in another article of where two energy bands intersect linearly and the electrons behave as relativistic Dirac fermions.
The points of high symmetry would be the centre and corners of the Brilouin zone (wikipedia), verified in graphene since this is where the kx and ky must be equal (in magnitude), with the periodic nature of Bloch model.


This was related to my presentation with the plot Dave pointed out on Page 9, this figure, with the band structure of of the Bi2Se3.

 

For Figure a, we see our Dirac point is at the centre of the Brillouin zone. We see our surface band meets our bulk band at the one point at the Dirac point, which is a condition for being a topological insulator.

We have the yellow for a high electron density of states. Then there is a higher density of states at the bulk band, which we can infer as most electrons are in the bulk state, i.e. insulating. Thus there are fewer states in the surface state, i.e. conducting

This the conduction is small compared to the insulation, as we would expect since conduction is only at the edges.

Sidenote: Figure b would be the dispersion relation for the material. Resembles the cone of graphene.

So our Fermi surface would be circles, of diameter proportional to the Fermi energy/wavenumber.