The nuclear Overhauser effect (nOe) is a well-known phenomenon in NMR circles. Essentially, it is the transfer of spin polarisation from a species with a bigger gyromagnetic ratio (e.g. electrons) to a species with a smaller gyromagnetic ratio (e.g. protons, carbon-13 atoms). NOe occurs through space, and so this is quite useful in NMR because it allows you to work out which atoms are closest to which other atoms (so you can find out how many hydrogens a given carbon has, for example). The bit that's relevant to condensed matter is that it was first theorised by Overhauser (the guy who basically derived NMR physics) as a way to enhance the spin-resonance signal of metal atoms via electron spin-resonance. The NMR spectra of metal atoms is today used (among other things) to check the Korringa ratio—metals that do weird stuff don't follow the Korringa ratio! And we all know that weird stuff is an excellent starting point for condensed matter study!
See you at the tutorial,
Josh
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So I thought that this was a pretty interesting paper on the Korringa ratio.
ReplyDeletehttp://prl.aps.org/pdf/PRL/v72/i12/p1933_1
They go through and derive some interesting relations. The Korringa ratio is
KR = 1/ T_(relaxation) T K^2
where K is the chemical shift of the metal, and T, I presume, is the temperature. T_(relaxation) is the relaxation time. I'll have to have more than a skim read of this paper to see the physical significance of this ratio however...
Beat me to it!
DeleteSorry, I forgot the squared.
So, you're saying that nOe is a process that gives spin from something with many electrons to something with fewer electrons.
ReplyDeleteSo this nOe is measurable such that you can tell the difference in spin between the two entities (without the need for a medium).
So reading up on the Korringa ratio, it is defined as κ s.t.
κ = 1/ (T1 T K)
for T1 the spin relaxation rate of a given nucleus in the material
and K the magnetic resonance field shift
and T I assume is temperature.
There's a particular material whose Korringa ratio is derived from a plot of 1/ (T1 T) against temperature, as in Figure 3.
Looking at this figure, the ratio isn't constant => must be an interesting material!
Also if you look at the plot of the ratios in Figure 6, the ratio does indeed change over varying temperatures!
That material is quite strange, especially considering its disorder plays such a significant part on its properties. Relates somewhat back to those materials that Ross was talking about that showed the quantum Hall affect, and how it was disorder that helped produce such striking features.
DeleteWith this one, if the disorder (or dirtiness of the metal I think they called it) is set correctly, you could have the material start out with an antiferromagnetic enhancement, and then turn ferromagnetic just by increasing the coulomb interaction. Pretty cool!
Ann,
ReplyDeleteI have done more reading and your idea of nOe isn't quite right. It's very important for my project so I will have to post on it again soon!
Essentially, a system with an electron and a nucleus has 4 energy levels based on spin (as expected). Often, these are non-degenerate (like in my ones), and sometimes the nuclear spin-flip energy difference is much less than the electron spin-flip difference. So, if you can change the electron's spin, you can end up changing the nuclear spin spontaneously as the electron relaxes back to its original spin (I'll have to draw a picture sometime...). The energy has to go somewhere though, so if the molecule is rotating in solution at about the same energy as the spin-flip energy, you can 'dump' the energy into rotation. This is why the Overhauser effect is movement dependent. NOe is the case where it happens for nuclei rather than a nucleus and an electron. So, you could have a similar but more complex case for multiple electrons (many more energy levels).
Dale,
That would be cool! Although changing the Coulomb interaction may not be as easy as it sounds.... I have heard of this in other things, like in big organic complexes that change their ferromagneticity based on (I think) concentration—it's an example in the first year Chem text book, so I shall have to look again sometime. Of course, with organic compounds, you could change the pH and the resulting conformational change of a metal complex could change its magneticity.
Dave,
Perhaps I'll get a chance to look at the paper too and think about it a bit.