What
is Spin?
Spin manifests itself as a magnetic field, which causes electrons to act like tiny bar magnets (magnetic dipole). The arrow which represents the direction of the spin points from the south pole to the north pole, just like the compass needle. Despite the exotic and elusive nature of spin, we are all familiar with its large scale effects. Permanent magnets exist when the electrons in a material are arranged so the arrows representing their spins point in the same general direction. And yet this still gives us no clue as to what is actually happening to produce this quantity
spin. To simplify we can think of electrons in atoms like the planets in our solar system, orbiting around the sun (nucleus) with a given angular velocity. The arrow (vector) which represents the magnitude and direction of the angular momentum for this rotational motion, is perpendicular to the orbital plane. The values of electron’s angular momentum in this orbit are a great example of what it means to be quantized; the momentum can only take certain special values. Only certain quantities of angular momentum are allowed, mathematically they correspond to the orbital quantum number L (which denotes different orbit shapes around the nucleus and can only be a whole number!-- 1,2,3,4). If the angular momentum associated with the revolution of the electron around the nucleus corresponds to the orbital quantum number,L, the angular momentum associated with the earth rotating on its axis would represent the spin quantum number S. Just like the orbital angular momentum S is also quantized and can only have the values 1/2 and -1/2 corresponding to spin up and spin down. No matter how neat and pleasing this analogy may seem, there is a paradox lurking in this planetary model of the electron. I assure you spin cannot be fully understood (some would say cannot be understood at all) using this analogy. What was said before still stands: spin doesn’t correlate exactly with any physical process in space as we perceive it. A well known equation relates the spin angular momentum to the magnetic field produced by the spin. When we try to calculate the speed of an electron which would, produce the appropriate angular momentum, we find the electrons would have to spin about 10,000 times faster than the speed of light (the fastest thing in our universe!). This is just one of the many paradoxes of spintronics and quantum physics, which can make them seem nonsensical by everyday standards, but make them such rich and interesting topics for research.
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Electrons and Spin! What is Spin? Continue on for Spin and Light! Spin Precession Spin Transport! Spin Populations! Spin Transistor! |