180 reputation
9
bio website brendan.sdf-eu.org
location Bristol, United Kingdom
age 32
visits member for 3 years, 9 months
seen Sep 21 at 9:32
Amatuer programmer. Briefly a software tester, then a server admin. Now a researcher working on low temperature physics experiments.

Mar
15
awarded  Popular Question
Dec
9
comment What really allows airplanes to fly?
@David, although the low pressure according to Bernoulli is probably localised to above the wing (assuming the same air meets at the trailing edge) so there might be no upwards movement associated with Bernoulli. More importantly though, how is the 'downwash' explanation reconciled with the stall? (where the laminar flow over the top of the wing causes pretty much all lift to be lost when it becomes turbulent). If the downwash explanation were true then provided the air over the top does not move upwards then there should still be lift if the bottom continues to push air down ...
Dec
9
comment The requirements for superconductivity
Mazin does however outline some new 'rules' though in that paper, paraphrased: Layered structures are good, Carrier density should not be too high (c.f. conventional metals). Period 4 Transition metals (V, Cr, Mn, Fe, Co, Ni and Cu) are good. Magnetism is essential. Fermi surface geometry is essential. Enlist theorists, at least to compute the Fermi surfaces. Also, materials of interest are likely to be complex chemical compounds — work closely with solid-state chemists. See also "Manifesto for a higher Tc" Basov, Nature 7, 272, (2011)
Dec
9
comment What really allows airplanes to fly?
The 'push downward' explanation is also supported by the ground effect principle where the upforce of the aerofoil increases as you approach a surface (e.g. the ground) - there is no reason why this would cause pressure to be lower on top of the wing in the pure Bernoulli case
Dec
9
comment What really allows airplanes to fly?
Shouldn't this be easy to test? If the air behind the wing moves down after the aerofoil passes then it is due to the downwash from the wing, if it moves up then it is due to reduced pressure above the wing - or is it not that simple?
Dec
9
comment Superconductor: What form of paramters (like London penetration depth) to use?
I did answer his question (as I understood it). He asks what his temperature parameter should be for calculating the penetration depth, 0K or 77K. Since he is at LN2 temperatures, he should use 77K ...
Dec
2
comment Fermi surface nesting and CDW/SDW/SC orders
As wsc said, you don't need nesting to get SC. However certain types of unconventional SC may result from partial Fermi surface nesting (although it is still unresolved in the literature). Broadly speaking in these cases (such as the pnictides) strong nesting leads to the SDW state and weak nesting results in a paramagnetic metal, but the region inbetween where there are fluctuations between the two states, the fluctuations are thought to pair electrons into a SC state - see for example papers by I. Mazin who is big into this idea
Dec
2
comment Impurity scattering temperature dependence
I don't know the answer :-) Pretty much all I know about the impurity term is that it has a zero temperature contribution to resistivity. Actual impurities in a material should not increase with T but the effects could potentially increase with T. As far as I am aware, most 'textbook' models leave the impurity contribution as a constant but these are simplified and only reflect current established theory. The likely answer is that temperature dependence of the impurity contribution in most metals etc. is slight but in some cases it may be important ... Sorry I can't give a definitive answer!
Dec
2
answered Mean of a measurement on periodic data: what is the use of the inverse of correlation length?
Dec
2
comment Impurity scattering temperature dependence
Might want to emphasise 'impurity scattering' in the title, since this is the key part to this question ...
Jul
25
answered Superconductor: What form of paramters (like London penetration depth) to use?
Jul
25
revised trying to understand Bose-Einstein Condensate (BEC)
added 32 characters in body
Jan
18
comment What is “Quantum Levitation”?
Some note on terminology: Both Type I and Type II superconductors exhibit the Meissner effect (i.e. perfect diamagnetism), the difference is how they behave when a strong magnetic field is gradually applied (which eventually destroys superconductivity). Type I superconductors go from a superconducting state to non-superconducting abruptly. Type II begin to let some magnetism through in quantised 'vortices' (the 'mixed state') before becoming saturated. I've never heard of the 'anti-Meissner effect' nor the 'Ginzburg-Landau effect'.
Jan
18
comment Yet another question on the Lindhard function
The relation $z = \epsilon_{k+q} - \epsilon_{k} - \hbar\omega$ does not have to occur between an occupied and an unoccupied state, it can occur between two occupied states (or two unoccupied states). It is the density terms in the numerator that make occupied-occupied or unoccupied-unoccupied contributions zero - not sure if that helps...
Dec
11
awarded  Scholar
Dec
11
accepted What does an electron's wavevector mean inside of a crystal?
Dec
11
comment What does an electron's wavevector mean inside of a crystal?
This makes a lot of sense, however it isn't necessarily a quasiparticle concept is it? I've written what I think it is at this address (basically Bloch wavefunctions cause the modulation) - would you say that is right? brendan.sdf-eu.org/phy/…
Nov
30
awarded  Commentator
Nov
30
comment What does an electron's wavevector mean inside of a crystal?
Some more reading into Fermi-liquid theory tells me that the quasiparticles have their momentum enhanced, so it appears that maybe the Fermi velocity is the velocity of the electron, whereas the $k$ vector corresponds to the momentum of the quasiparticle. However if this is the case, it still leaves the question as to what the physical meaning of the quasiparticle $k$ values are and why we are so interested in them (instead of the electrons).
Nov
29
awarded  Student