0.7 anomaly and the Fermi liquid

Nanophysics is one of the research acitvities  full of promises for the improvement of our lives through the realization of new devices. This application of solid state physics becomes relevant when quantum mechanics comes into play in conduction phenomena. The aspect people may not be aware is that these researches produced several unexpected results. One of these is the so called 0.7 anomaly. This effect appears in the QPC or quantum point contacts. This can be seen as a waveguide for the wavefunction of the electrons. As such, the main effect is that conductance is quantized in integer multiples of an universal constant 2e^2/\hbar. qpc
Measurements on these devices are realized at very low temperature so to have quantum effects at work. The result of such measurements come out somewhat unexpected. Indeed, the quantization of conductance appeared as due but a further step occurred at 0.7\times 2e^2/\hbar and was called the 0.7 anomaly.

Theoretical physicists proposed two alternatives to explain this effect. The first one claimed that the Fermi liquid of conduction electrons was spin polarized while the second claimed that the Kondo effect was at work. Kondo effect appears in presence of magnetic impurities modifying the resistance curve of the material. In any case, both proposals have effects on the electron conductance and are able to explain the observed anomaly. The only way to achieve an understanding is then through further experimental work.

I have found a recent paper by Leonid Rokhinson at Purdue University, and Loren Pfeiffer and Kenw West both at Bell Lab producing a consistent result that proves that the conducting electrons are spin polarized (see here). I cannot expect a different result also in view of my paper about another problem in nanophysics and this is the appearence of a finite coherence time in nanowires, a rather shocking result for the community as the standard result should be an infinite coherence time (see here). Indeed, I have accomoned both effects as due to the same reason and this is the polariztion of the Fermi liquid (see here). This matter is still open and under hot debating in the nanophysics community. What I see here are the premises of a relevant new insight into condensed matter physics.

5 Responses to 0.7 anomaly and the Fermi liquid

  1. hermann says:

    Dear Marco,
    I read about four years ago your cited paper: http://arxiv.org/abs/cond-mat/0403678. I read this statement: “Let us see this in a simple model. We consider as a possible Hamiltonian…

    where we consider negligible the dynamical part of the N spins that
    interact with a single one through the coupling constant J.”

    What are the arguments you can produce in order to prove the negligibility of the J interaction?

  2. mfrasca says:

    Dear hermann,

    It is a property of exchange models. This approximation favors the tendency of the spins to align. For the simple model that sentence refers to, there is a more mundane reason and it is that the effect can be seen when you do this approximation. Indeed, there can be some experiments with polarized states that do not see the decoherence effect and the reason is that a small perturbation approximation applies. The other way round grants the effects I discuss.



  3. hermann says:

    Dear Marco,
    In effect, in May 2000 a paper has been submitted to Phys Rev B: http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Acond-mat%2F0005082. The autors, in the conclusions, had already the intuition that the 0.7 anomaly could be a spin polarization effect.


  4. mfrasca says:

    Dear hermann,

    I was aware of it. The novelty in my works is to apply this same idea to the saturation of dephasing time in nanowires and quantum dots. This latter problem has got a resurgence after the work in PRL by Mohanty, Jariwala and Webb


    where it was proved that this is an intrinsic effect contrarily to common wisdom.
    I worked this out after my works about decoherence in thermodynamic limit.



  5. […] neutralizing background of positive ions. A 2DEG is an essential part of any nanoscale device (see my preceding post) and we know that a lot of unexpected effects are seen when the temperature is lowered to few nK°, […]

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