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I Electrons with no mass?

  1. Aug 30, 2016 #1
    I was reading this article: http://www.eurekalert.org/pub_releases/2016-08/ac-irj082916.php

    The following paragraph has me scratching my head, wondering what they are talking about:

    "The team studied what happened to the current passing through the exotic material zirconium pentatelluride (ZrTe5) when exposed to a very high magnetic field. Measuring a current in a high magnetic field is a standard way of characterising the material's electronic structure. In the absence of a magnetic field the current flows easily through ZrTe5. This is because in ZrTe5 the electrons responsible for the current have no mass. However, when a magnetic field of 60 Tesla is applied (a million times more intense than the earth's magnetic field) the current is drastically reduced and the electrons acquire a mass. An intense magnetic field in ZrTe5 transforms slim and fast electrons into fat and slow ones. "

    What do they mean the electrons responsible for the current have no mass? I thought electrons always have mass (i.e. ~1/1800th mass of a proton)?
     
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  3. Aug 30, 2016 #2

    mfb

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    Electrons always have mass. For conduction in solids, the regular electrons are not a good model any more. They get replaced by quasiparticles. Those can have a mass different from the regular electron mass - but as they look similar to electrons they can be called electrons as well. There are also holes as quasiparticles, "places" where you normally expect an electron but do not have one.
     
  4. Aug 30, 2016 #3
    If these quasiparticles are not electrons, but look similar to electrons, then what are they actually made of? Are "holes" the same as positrons?
     
  5. Aug 30, 2016 #4

    mfb

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    They are quantum mechanical states in the crystal. They are not made out of some set of particles, you have to consider the whole crystal structure to understand them, and there is no good classical analog to it - you need quantum mechanics.
    No.
     
  6. Aug 30, 2016 #5
    Quasiparticles are the collective response of all electrons in the system.
    The idea is a bit similar to water waves. It is useless to think about water waves as the movement of single water molecules. Instead you look at the emerging collective behavior of the system as a whole.
    In the solid state and semiconductors in particular it turns out that in most cases the quasiparticles look very much like electrons, but often with a modified "effective" mass. Holes are another common phenomenon. They behave like electrons with a positive charge and modified effective mass.
     
  7. Aug 30, 2016 #6
    The "effective mass" of electrons in solids is a convenient model.
    In a solid, if you need to describe the dynamics of an electron, you need to take into account the interaction of the electron with all the other particles in the lattice. besides the interaction with any external fields applied to the crystal. Fortunately, the average effect of the internal interactions can be taken into account by assuming a "free| electron but with a modified mass. This is called "effective mass" and you may read that in graphene too there are electrons with zero effective mass.
    I did not know about the ZrTe5 so far.

    Edit. After reading the article, I am not sure what they are talking about. I thought they mean effective mass but the talk about implication in particle physics and first time when a massless particle acquires mass does sound odd.
     
  8. Aug 31, 2016 #7
    Then perhaps, the article is wrong? Most of you guys seem to think it refers to quasi-particles, so maybe it does refer to those, and the article mistook quasi-electrons with real electrons? Science journalism has been known to do this before, afterall.
     
  9. Aug 31, 2016 #8

    ZapperZ

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    It is a "press-release" meant for journalists and the general public. So that's the level that it is appealing to.

    Please note that many "artifacts" of elementary particle, QFT, and QED can already be found in condensed matter systems. We've already seen analogous version of magnetic monopole, Majorana fermions, etc.. in such systems. And let's not forget that the Higgs mechanism came directly out of such a system as well. That possibly is why these types of discovery are often looked at for possible relationship to elementary particle physics.

    Zz.
     
  10. Aug 31, 2016 #9
    So if this is actually referring to a quasi-electron then how do quasi-electrons acquire mass? Does it just mean that they've slowed down from their expected velocity?
     
  11. Aug 31, 2016 #10

    ZapperZ

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    Did you pay attention to the post you quoted from M Quack? Read it again, because I will give you the same answer as that.

    Zz.
     
  12. Aug 31, 2016 #11
    Yeah I did, but it wasn't very specific. He talked about "effective masses", but how do you determine effectively massless vs. effectively massive. Is it determined in the same way as masses in regular particles? A massless particle traverses at the speed of light, while a massive particle doesn't? So does an effectively massless quasi-particle also traverse at the speed of light?
     
  13. Aug 31, 2016 #12

    ZapperZ

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    This will get complicated. The effective mass is determined by the "curvature" of the electronic band structure. The band structure will also tell you the group velocity of these quasiparticles, which is the"speed" that is being discussed here.

    http://www2.physics.ox.ac.uk/sites/default/files/BandMT_05.pdf

    Zz.
     
  14. Sep 1, 2016 #13
    Okay, so I didn't quite understand all of the equations in that PDF, but if I summarize, does it mean that various quasi-particles have different normal group velocities, and if a force is applied to them, they slow down to a slower group velocity, which gives them an effective mass due to the slow down? So their normal group velocities would be their effective zero-mass level, while slower group velocities would be the effective massive level?
     
  15. Sep 1, 2016 #14

    ZapperZ

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    There is no "force" being applied. The band structure describes the dispersion, the E vs. k of the electrons in the material. So they already have momentum and energy. These electrons already have these group velocities.

    Zz.
     
  16. Sep 2, 2016 #15
    Just to clarify the vocabulary:

    "effective mass" means the apparent mass of the quasi-particle. It is determined by the band structure.

    When condensed matter physicists talk amongst themselves it is clear to everybody that they are dealing with quasi-particles. For the sake of brevity these are referred to "electrons" and "holes" if they have negative or positive charge.

    As you cannot include a full textbook on condensed matter physics in a 1 page press release, such details get brushed under the table.

    To simplify the relation between band structure and effective mass:
    In classical physics, E=1/2 m v^2 = 1/(2m) p^2, where v is velocity, p is momentum, m mass and E energy.
    In quantum mechanics, p = hbar k, where k is the wave vector.
    The band structure describes the relation between E and k. At the minimum of the conduction band, the band can locally be approximated as E=E0 + 1/(2m*) k^2, where m* is the famous effective mass and E0 is the energy at the band minimum.

    Effectively massless means that locally the band structure is best approximated by m* very very close to zero.
     
  17. Sep 11, 2016 #16
    Okay thanks, that's put me on the path to some understanding of this.
     
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