Continuing on a series of interesting arxiv articles,
Clarification as to why alcoholic beverages have the ability to induce superconductivity in Fe_{1+d}Te_{1-x}S_x claims to have investigated the key components in alcoholic beverages and clarified the mechanism to induce superconductivity in Fe1+dTe1-xSx.
Actually reading this, I agree that they have investigated the key components in alcoholic beverages.
They have mentioned previous work with that material, specifically regarding the presence (necessitating absence) of iron in that material to achieve superconductivity.
Also, they have not experimentally induced superconductivity in that material.
Procedure:
They considered the concentrations of three acids: malic acid, citric acid and β-alanine and water (which are major components of their drinks).
Temperature susceptibility measured to 2K.
Shielding volumes were calculated for the drinks showing increase proportional to concentrations of of the three acids.
X-ray diffraction shows that the drink solutions does not decompose the crystal structure.
So reasonable question (which they mention) would be how does/why would malic acid, citric acid and β-alanine make things superconduct?
They mention Fe content is interesting, bu these three acids don't contain Fe.
Their procedure raises a question which they have not addressed/supported: what's shielding volume and how is it relevant?
I suppose superconductivity involves conduction without loss, so there's some term that must shield a current from the outside?
Anyway, if alcoholic beverages had the ability to induce superconductivity, that'd be a pretty good party trick...
Sidenote: There are some great articles on arXiv...
"How many zombies do you know?" Using indirect survey methods to measure alien attacks and outbreaks of the undead
(footnotes...)
Thursday, May 31, 2012
Wednesday, May 30, 2012
Further remarks on the thermoelectric paper
Here is my final blogpost for Condensed Matter (there might a few more after this).
In my talk, I mentioned that to have a good thermoelectric material, you need to have high electrical conductivity and low thermal conductivity. The simplest reason I could think of is that we want to maintain the temperature difference so we do not want the temperature at both ends to equalize quickly-hence the need for low thermal conductivity.
While I was thinking about thermal conductivity, I was wondering how do they measure the thermal conductivity. It turns out that measuring thermal conductivity is not an easy as measuring electrical conductivity. Several methods are listed on wiki and one of the most widely used one was laser flash analysis whose basic working principle is basically heating up one side of the material and measure the temperature change at the other side.
This makes you think about the uncertainties of the measurements that they made of the thermoelectric efficiency which depends on how accurately you can measure electrical conductivity, thermal conductivity and thermopower (hopefully everyone memorized the formula). It was mentioned in the paper that uncertainty in each quantity may vary between 5% to 20% which means the uncertainty of zT (the maximum efficiency) might be as high as 50%! Hence, the guys measuring those stuff need to make careful with measurements so that results of high zT would be actually meaningful.
In my talk, I mentioned that to have a good thermoelectric material, you need to have high electrical conductivity and low thermal conductivity. The simplest reason I could think of is that we want to maintain the temperature difference so we do not want the temperature at both ends to equalize quickly-hence the need for low thermal conductivity.
While I was thinking about thermal conductivity, I was wondering how do they measure the thermal conductivity. It turns out that measuring thermal conductivity is not an easy as measuring electrical conductivity. Several methods are listed on wiki and one of the most widely used one was laser flash analysis whose basic working principle is basically heating up one side of the material and measure the temperature change at the other side.
This makes you think about the uncertainties of the measurements that they made of the thermoelectric efficiency which depends on how accurately you can measure electrical conductivity, thermal conductivity and thermopower (hopefully everyone memorized the formula). It was mentioned in the paper that uncertainty in each quantity may vary between 5% to 20% which means the uncertainty of zT (the maximum efficiency) might be as high as 50%! Hence, the guys measuring those stuff need to make careful with measurements so that results of high zT would be actually meaningful.
Course summary
Write 10-15 pages summarising the main points learnt in the semester. Each point must contain one figure and one equation.
The summary must be emailed as a .pdf file to me.
It may be run through Turnitin in order to test for plagiarism.
This assessment piece counts for 10% of the summative assessment.
This is officially due at 5pm this friday June 1.
I may not start marking until tuesday morning.
The summary must be emailed as a .pdf file to me.
It may be run through Turnitin in order to test for plagiarism.
This assessment piece counts for 10% of the summative assessment.
This is officially due at 5pm this friday June 1.
I may not start marking until tuesday morning.
Tuesday, May 29, 2012
Here a link to my slides.
https://docs.google.com/presentation/d/1OcztPPBoxEhgPVTbcdryazAjZtcNiodDdbLW-Ka0k_4/edit
Inform me if it does not work. Cheers.
Inform me if it does not work. Cheers.
Monday, May 28, 2012
Umklapp scattering
So this wasn't really mentioned in the notes, but it popped up a few times in my paper, and is in chapter 25 'anharmonic effects in crystals' in the book. It's a bit of a step back to Brillouin Zones and interactions, and whilst a simple concept it comes across as quite counter intuitive!
So say we have two phonons that lie within the first brillouin zone, and they scatter off each other to produce a third phonon with a different wavevector k3. If this third wavevector lies within the first brillouin zone, then this is just a simple scattering situation. Umklapp scattering occurs when the wavevector lies outside of this zone. However, as any wavevector lying outside the FBZ can be equally well described by a wavevector lying inside of it, it is possible to have two phonons interact, and have a third phonon that appears almost reflected from the scattering. This is a bit clearer with this diagram here
http://en.wikipedia.org/wiki/File:Phonon_nu_process.png
Obviously this has implications about the phonons momenta, although at first glance I can't find anything related to conservation of it. Anyway, a little aside concerning phonons and scattering!
So say we have two phonons that lie within the first brillouin zone, and they scatter off each other to produce a third phonon with a different wavevector k3. If this third wavevector lies within the first brillouin zone, then this is just a simple scattering situation. Umklapp scattering occurs when the wavevector lies outside of this zone. However, as any wavevector lying outside the FBZ can be equally well described by a wavevector lying inside of it, it is possible to have two phonons interact, and have a third phonon that appears almost reflected from the scattering. This is a bit clearer with this diagram here
http://en.wikipedia.org/wiki/File:Phonon_nu_process.png
Obviously this has implications about the phonons momenta, although at first glance I can't find anything related to conservation of it. Anyway, a little aside concerning phonons and scattering!
Sunday, May 27, 2012
Energy bands and semiconductors
Just thinking about the band structures we learnt with the Bloch model.
Bands determine the state, so insulator, metal, semimetal, semiconductor by the filling (or not filling) of electrons or holes.
We saw the band structures again in Paul's last set of lectures on semiconductors.
We were introduced to the donor and acceptor bands, also the valance and conduction bands.
How are these bands related to the energy bands from the Bloch picture?
Are they the same thing?
Also, I still don't quite understand the distinction between the valance/conduction bands and the donor/acceptor bands?
I take it that the donaor accept are within the conduction/valance pair, but why are there two pairs of them?
Final set of questions, when we consider these two sets of bands, their expressions (p592-3 A&M) no longer contain the Fermi energy. Are they related to the Fermi energy, else where has the Fermi gone?
Apologies for all the questions.
Thanks.
Bands determine the state, so insulator, metal, semimetal, semiconductor by the filling (or not filling) of electrons or holes.
We saw the band structures again in Paul's last set of lectures on semiconductors.
We were introduced to the donor and acceptor bands, also the valance and conduction bands.
How are these bands related to the energy bands from the Bloch picture?
Are they the same thing?
Also, I still don't quite understand the distinction between the valance/conduction bands and the donor/acceptor bands?
I take it that the donaor accept are within the conduction/valance pair, but why are there two pairs of them?
Final set of questions, when we consider these two sets of bands, their expressions (p592-3 A&M) no longer contain the Fermi energy. Are they related to the Fermi energy, else where has the Fermi gone?
Apologies for all the questions.
Thanks.
Friday, May 25, 2012
Conduction mechanisms
Differences in conduction mechanisms between different classes of materials can lead to some interesting differences. For instance, the conductivity of silicon decreases with temperature. This is because as you heat silicon, the crystalline of the structure is degraded. As we know, it is the crystalline structure that allows such a well defined band structure, leading to conduction electrons.
Conversely, organic semiconductors rely on a different type of conduction. Electrons are conducted via "hopping" between molecules. This can only happen when molecules are very close to each other. An increase in temperature increases the rate of these phononic interactions, thus increasing the conductivity.
I think that this is a good example of how a relatively simple experiment, like temperature dependence of conductivity, can lead to some insight into much more fundamental electronic processes. Can you guys think of some other examples? Or perhaps some other fundamental differences between seemingly similar materials?
Conversely, organic semiconductors rely on a different type of conduction. Electrons are conducted via "hopping" between molecules. This can only happen when molecules are very close to each other. An increase in temperature increases the rate of these phononic interactions, thus increasing the conductivity.
I think that this is a good example of how a relatively simple experiment, like temperature dependence of conductivity, can lead to some insight into much more fundamental electronic processes. Can you guys think of some other examples? Or perhaps some other fundamental differences between seemingly similar materials?
Thursday, May 24, 2012
Presentation logistics
Each presentation will be 15 minutes plus 5 minutes for questions.
Time limits will be rigidly enforced.
I expect you all to attend all the presentations.
I also expect you to fill out the feedback and assessment forms I will provide.
i.e. you will be grading each others presentations. Paul and I will take this into account when assigning your final mark.
It is your responsibility to bring a laptop and to check beforehand that you can get it to work with the data projector.
On the same day as your talk please post on the blog your Powerpoint slides.
The material below is from John Wilkins one page guides and should be read and applied before giving your talk.
Time limits will be rigidly enforced.
I expect you all to attend all the presentations.
I also expect you to fill out the feedback and assessment forms I will provide.
i.e. you will be grading each others presentations. Paul and I will take this into account when assigning your final mark.
It is your responsibility to bring a laptop and to check beforehand that you can get it to work with the data projector.
On the same day as your talk please post on the blog your Powerpoint slides.
The material below is from John Wilkins one page guides and should be read and applied before giving your talk.
- Preparing a talk. [html ], [postscript ]
- Talk Grit [html], [postscript] (Jim Garland)
- How to give a Terrible Talk
- Viewgraphs Preparation: NOT
Schedule is
Monday 11am Kiran, Josh, Dale
Tuesday 11am Andy, Joseph
Wednesday 11am Ann, Dave + course evaluation
Tuesday 11am Andy, Joseph
Wednesday 11am Ann, Dave + course evaluation
Monday, May 21, 2012
Isotope effect
One of the key ideas that pointed out that phonons are the pairing mechanism for BCS superconductors is the isotope effect -why the critical temperature was lower for heavier metal isotopes (elements with the same number of protons but different number of neutrons).
I have read somewhere before a simple explanation on how BCS theory accounts for this. A simple mental picture of the BCS theory is as follows. When electron moves through the lattice made up of positively charge metal ions, it distorts the ion lattice, since it attracts the ions creating a relatively positive region. This region then attracts another electron with the opposite spin and both electrons form what is known as a Cooper pair! Since heavier ions are harder to move, they are less attracted by the electrons. This results smaller binding energy between the Cooper pairs which means lower critical temperature for heavier isotopes.
Overhauser and DNP
An interesting fact I just discovered whilst reading about dynamic nuclear polarisation on Wikipedia: apparently, when Albert Overhauser first described the process in his paper (which just talked about using DNP on copper centres in organo-metallic complexes, all the rage in the 50s and 60s), famous physicists like Norman Ramsey and Felix Bloch (and 'other renowned physicists of the time', Wikipedia) initially criticised Overhauser because they thought the process was 'thermodynamically improbable' (Wikipedia). Obviously their Force senses were clouded by the Dark Side since Overhauser essentially invented NMR and worked out its equations... :) and it works. Then when Carver and Slichter gave experimental confirmation, Overhauser apparently received an apology from Ramsey (via mail) in the same year! Fortunately, DNP most definitely does exist, I've witnessed it!
More facts: originally, the term 'dynamic' was used to represent the random and time-dependent nature of Overhauser cross-relaxation. It has been kept in the name so that we now have a cool acronym for my project! :)
What is this 'cross-relaxation' you may ask? (Ask, or else!) :).
Solution state DNP (i.e. liquid, so that molecules can move around a lot) works by coupling molecular motion to the relaxation energies of the electron spin. With a nucleus and an electron, your system has 4 levels, 2 corresponding to the nuclear spin transition and 2 corresponding to the electron spin transition. The nuclear spin transition is of much lower energy than the electron spin transition due to the smaller gyromagnetic ratio of nuclei compared with electrons. The process the works like this:
1. Nucleus and electron are in low energy state
2. Electron is excited via microwave radiation (the wavelength appropriate for the temperatures and fields of normal electron paramagnetic resonance).
3. Electron has two choices: it can decay back to its original state (with nuclear spin unchanged) or it can decay to a state where the nuclear spin changes from low to high energy ('cross-relaxation'); because the nuclear transition energy is smaller than the electron transition energy, these two choices are not much different in energy. This cross-relaxation can occur if the energy of the electron decay with the nuclear spin flip is equal to the energy of molecular motions (hence the need for solution state), and if the cross-relaxation process is faster than the other relaxation (no nuclear spin flip) the nuclei are polarised in the high-spin state.
Now you have 'hyperpolarised' nuclei! Hooray! After doing all of this work, we now have a greater nucleus spin population difference between low and high spin states, and so our NMR signal is enhanced (the Force is strong with this one). :)
More facts: originally, the term 'dynamic' was used to represent the random and time-dependent nature of Overhauser cross-relaxation. It has been kept in the name so that we now have a cool acronym for my project! :)
What is this 'cross-relaxation' you may ask? (Ask, or else!) :).
Solution state DNP (i.e. liquid, so that molecules can move around a lot) works by coupling molecular motion to the relaxation energies of the electron spin. With a nucleus and an electron, your system has 4 levels, 2 corresponding to the nuclear spin transition and 2 corresponding to the electron spin transition. The nuclear spin transition is of much lower energy than the electron spin transition due to the smaller gyromagnetic ratio of nuclei compared with electrons. The process the works like this:
1. Nucleus and electron are in low energy state
2. Electron is excited via microwave radiation (the wavelength appropriate for the temperatures and fields of normal electron paramagnetic resonance).
3. Electron has two choices: it can decay back to its original state (with nuclear spin unchanged) or it can decay to a state where the nuclear spin changes from low to high energy ('cross-relaxation'); because the nuclear transition energy is smaller than the electron transition energy, these two choices are not much different in energy. This cross-relaxation can occur if the energy of the electron decay with the nuclear spin flip is equal to the energy of molecular motions (hence the need for solution state), and if the cross-relaxation process is faster than the other relaxation (no nuclear spin flip) the nuclei are polarised in the high-spin state.
Now you have 'hyperpolarised' nuclei! Hooray! After doing all of this work, we now have a greater nucleus spin population difference between low and high spin states, and so our NMR signal is enhanced (the Force is strong with this one). :)
Hey all,
Just a question in class today about how the interaction between electrons can be strong even though they are moving in opposite directions. Remember that in classical superconductors the electron-electron interaction is mediated by phonons. I.e. the actual paring interaction is electron-phonon interaction. An electron can interact with the phonon field at any point in space. This interaction can propagate in the phonon field until it reaches the other electron (i.e. the only electron with the exact opposite wave vector sign and spin) when it interacts with it. Giving an effective long distance paring between the two spatially separated electrons.
This is kinda similar to what we have been doing in quantum with the field interactions. I.e. we have an electron field interacting with a phonon field. Giving effective electron, electron interactions mediated by a virtual phonons.
Just a question in class today about how the interaction between electrons can be strong even though they are moving in opposite directions. Remember that in classical superconductors the electron-electron interaction is mediated by phonons. I.e. the actual paring interaction is electron-phonon interaction. An electron can interact with the phonon field at any point in space. This interaction can propagate in the phonon field until it reaches the other electron (i.e. the only electron with the exact opposite wave vector sign and spin) when it interacts with it. Giving an effective long distance paring between the two spatially separated electrons.
This is kinda similar to what we have been doing in quantum with the field interactions. I.e. we have an electron field interacting with a phonon field. Giving effective electron, electron interactions mediated by a virtual phonons.
Semiconductors lecture slides
You can download a copy of my lecture slides on semiconductors here. I recommend reading chapters 28 and 29 from A&M.
Sunday, May 20, 2012
Why no Copper superconductors?
This intrigued me in the last (or second last?) lecture. The question is; why do Copper, Silver, and Gold, three very good conductors not superconduct?
Well, it seems that it is due to them not being able to form Cooper pairs, required by BCS theory. These elements all have a free conduction electron which they can easily use to conduct well. However, they all have a FCC lattice structure which prevents eletron-phonon coupling required to form a cooper pair.
For a basic overview, I thought this site was pretty cool: http://www.superconductors.org/
It's aimed at beginners, but is surprisingly concise, and has some nice, comprehensive tables of superconducting materials.
On another note, A&M says in one of the footnotes that amorphous Bismuth superconducts at higher temperatures than crystalline Bismuth. This is all kinds of crazy! I decided to have a quick look around, and I found a couple of papers:
http://prl.aps.org/pdf/PRL/v22/i11/p526_1
These guys talk about the phonon spectrum of Bismuth and Gallium. To be perfectly honest, I don't really understand what the phonon spectrum is (perhaps some series of resonances within the crystal?). If anyone can shed some light on this is would be great. Speaking of shedding light, this paper:
http://iopscience.iop.org/0305-4608/5/11/034/pdf/0305-4608_5_11_034.pdf
looks at the optical properties of both amorphous and crystalline Bismuth films. I haven't had time to properly read this one yet, but it seems pretty interesting. Also, I'm sure it'll have a reference to a paper talking about electronic properties of the two types as well. In saying that, this paper is not particularly thoroughly referenced, which is slightly irritating. These guys;
http://iopscience.iop.org/0305-4608/11/3/013/pdf/0305-4608_11_3_013.pdf
talk about Bismuth films, both amorphous and crystalline, which are pretty interesting. Also, this paper is from 1980, as opposed to 1968 and 1975 for the last two, so maybe they have a better idea about the theory. However, the first ceramic superconductors weren't around until 1986, and the first with Tc>77K were discovered a year later. I guess what I'm trying to say is that these guys were in for a surprise...
Well, it seems that it is due to them not being able to form Cooper pairs, required by BCS theory. These elements all have a free conduction electron which they can easily use to conduct well. However, they all have a FCC lattice structure which prevents eletron-phonon coupling required to form a cooper pair.
For a basic overview, I thought this site was pretty cool: http://www.superconductors.org/
It's aimed at beginners, but is surprisingly concise, and has some nice, comprehensive tables of superconducting materials.
On another note, A&M says in one of the footnotes that amorphous Bismuth superconducts at higher temperatures than crystalline Bismuth. This is all kinds of crazy! I decided to have a quick look around, and I found a couple of papers:
http://prl.aps.org/pdf/PRL/v22/i11/p526_1
These guys talk about the phonon spectrum of Bismuth and Gallium. To be perfectly honest, I don't really understand what the phonon spectrum is (perhaps some series of resonances within the crystal?). If anyone can shed some light on this is would be great. Speaking of shedding light, this paper:
http://iopscience.iop.org/0305-4608/5/11/034/pdf/0305-4608_5_11_034.pdf
looks at the optical properties of both amorphous and crystalline Bismuth films. I haven't had time to properly read this one yet, but it seems pretty interesting. Also, I'm sure it'll have a reference to a paper talking about electronic properties of the two types as well. In saying that, this paper is not particularly thoroughly referenced, which is slightly irritating. These guys;
http://iopscience.iop.org/0305-4608/11/3/013/pdf/0305-4608_11_3_013.pdf
talk about Bismuth films, both amorphous and crystalline, which are pretty interesting. Also, this paper is from 1980, as opposed to 1968 and 1975 for the last two, so maybe they have a better idea about the theory. However, the first ceramic superconductors weren't around until 1986, and the first with Tc>77K were discovered a year later. I guess what I'm trying to say is that these guys were in for a surprise...
Friday, May 18, 2012
Essay
I was just wondering if anyone knows the exact date that this is due? It says in the course notes end of semester, but isn't any more specific than that. Is that end of week 13, or swot-vac, or possibly up until our exam on the 18th? I know I have been a bit bad and haven't been to a few lectures lately, was just wondering if it's come up in any of them.
Thanks!!!
Thanks!!!
Crystal Defects
Throughout this course we have come across a couple of examples where a metal possessing defects actually leads to some pretty interesting features, most notably the quantum Hall Effect. Chapter 30 goes into a bit more detail about these, but as it's not required reading I shall summarise a few key points here.
Firstly, whilst any disruption from the perfect crystal state is a defect, only two are considered in this chapter. They are:
1) Vacancies and Interstitials
2) Dislocations
The first are known as point defects, as the imperfection bounds this one point in three dimensions (as opposed to a line or surface). Examples of these are the addition of removal of additional ions from the crystal structure, the former of these known as a Schottky Defect. This particular defect can arise naturally without disrupting any thermodynamic laws as long as the number of vacancies n is given by
n = N*exp(-(E+Pv)/kT)
Here N is the total number of ions, E is the derivative of (U-TS) with respect to n, where U is the potential energy, T is Temperature and S Entropy, P is pressure, v velocity and k is the boltzmann constant!
Consequences of this type of defect include changes in the electrical conductivity and optical properties. Similar consequences arise if an additional ion was at the lattice point, if particular ions were polarons or excitons (polarised or excited state ions).
Dislocations, on the other hand, refer to line defects, where the imperfection is bounded in two dimensions. This defect is important as it helps explain why a much smaller force is necessary to deform a crystal than predicted by all the previous theories. There are two kinds, screw and edge dislocations, and the diagram for these is on page 632. The basic requirements for this type of dislocation are (from page 634)
1) Everywhere else in the crystal except at the point of dislocation the crystal can be described as perfect
2) In the region around the dislocation the atomic positions differ substantially to that predicted by a pure crystal
3) There exists a nonvanishing Burgess Vector, defined as a loop of vectors that when mapped out in the perfect lattice return one to their starting point, but when the same vectors are traversed in the dislocation region one ends up at a different lattice.
As well as describing the weakness of crystals, these dislocations also affect the rate at which a crystal can grow.
Firstly, whilst any disruption from the perfect crystal state is a defect, only two are considered in this chapter. They are:
1) Vacancies and Interstitials
2) Dislocations
The first are known as point defects, as the imperfection bounds this one point in three dimensions (as opposed to a line or surface). Examples of these are the addition of removal of additional ions from the crystal structure, the former of these known as a Schottky Defect. This particular defect can arise naturally without disrupting any thermodynamic laws as long as the number of vacancies n is given by
n = N*exp(-(E+Pv)/kT)
Here N is the total number of ions, E is the derivative of (U-TS) with respect to n, where U is the potential energy, T is Temperature and S Entropy, P is pressure, v velocity and k is the boltzmann constant!
Consequences of this type of defect include changes in the electrical conductivity and optical properties. Similar consequences arise if an additional ion was at the lattice point, if particular ions were polarons or excitons (polarised or excited state ions).
Dislocations, on the other hand, refer to line defects, where the imperfection is bounded in two dimensions. This defect is important as it helps explain why a much smaller force is necessary to deform a crystal than predicted by all the previous theories. There are two kinds, screw and edge dislocations, and the diagram for these is on page 632. The basic requirements for this type of dislocation are (from page 634)
1) Everywhere else in the crystal except at the point of dislocation the crystal can be described as perfect
2) In the region around the dislocation the atomic positions differ substantially to that predicted by a pure crystal
3) There exists a nonvanishing Burgess Vector, defined as a loop of vectors that when mapped out in the perfect lattice return one to their starting point, but when the same vectors are traversed in the dislocation region one ends up at a different lattice.
As well as describing the weakness of crystals, these dislocations also affect the rate at which a crystal can grow.
Magnetic photons
There is an interesting article about magnetic photons:
Their claim is that magnetic photons are responsible for the potency of homeopathic solutions.
From what we have learnt in our undergrad years, a photon is a packet of electromagnetic wave.
We understand electromagnetic waves to be self-sustaining disturbance in the electric and magnetic field.
Here's our magnetic field.
I'm finding it a bit difficult to read, but it is bizarre. From what I understand:
Premise: information is carried an oscillatory wave
Premise: Tesla coil produces longitudinal wave
Premise: solution in Tesla coil gives nonzero response
Conclusion: this response is the information
?
There is something relating the frequency spectrum (power spectral density?) to the homeopathic information.
The figures from 3 and onwards could be clearer. Anyone?
Relation back to our work on the magnetism:could what they're measuring be the diagmanetic response?
Homeopathic solutions contain water...and you can levitate a frog with diamagnetism...levitate the solution?
Big sidenote:
Sort by magic!!
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