Nanotechnology + Superconductivity = Spintronics 88
karvind writes "Spintronics is a nanoscale technology in which information is carried not by the electron's charge, as it is in conventional microchips, but by the electron's intrinsic spin and if a reliable way can be found to control and manipulate the spins spintronic devices could offer higher data processing speeds, lower electric consumption, and many other advantages over conventional chips--including, perhaps, the ability to carry out radically new quantum computations. PhysOrg is reporting that University of Notre Dame physicist Boldizsar Janko and his colleagues have found a way to achieve this control using a magnetic semiconductor, insulator and superconducting material stack of thicknesses of order of few dozen nanometers. IBM and Stanford are also looking into spintronics."
Lots of research (Score:5, Informative)
Spintronics also represents one of the quickest transitions from lab to market, next to the transistor via GMR sensors. The hard disk read heads on the hard drives in your computer, if you bought a new disk in the past few years, already incorporates spintronic effects through GMR (Giant MagnetoResistance). Most major media storage and also electronics companies have been heavily investigating spintronics for years too, not to mention a good percentage of condensed-matter physicsists, electrical and materials-science engineers.
Spintronics is also being investigated for quantum computation because the two electron eigenstates in any direction (up / down) can make a good basis for the Zero and One states of a qubit.
But to repeat the hype, spintronics does have potential to revolutionize the electronics industry by offering a whole new degree of freedom to manipulate of the electrons. 'Classical' transistors move/detect/switch charge, adding spin to the picture allows much more flexibility, and probably higher device speeds or data densities. Eg, perhaps microprocessors can go from binary as presence/lack of charge to spintronic up/down charge. Or perhaps even base-4 using presence/absence of both spin up and spin down flavors of electrons.
Re:Need Wikipedia Update? (Score:4, Informative)
Re-read TFA where it says "Boldizsar Janko and his colleagues believe they have found such a control technique" and "Although Janko and his colleagues have tested their approach so far only through computer simulations".
Not exactly a practical, demonstrated technology yet. Wikipedia is therefore current.
Re:DIY? (Score:5, Informative)
Here's a semi-serious reply to your obviously tongue-in-cheek question. I'll assume by 'certain value' you mean direction, since the total spin of an electron is fixed to hbar/2.
It depends how many spins you want to align, what percentage of the total number of spins you want to align, and how accurately you want to control the direction the spins are aligned to. In a nutshell a magnet will align the spins, cooling will also align the spins (for ferromagnets and antiferromagnets). doing both will do it faster and give more control. But that adds to the cost.
At absolute zero the slightest applied magnetic field to a paramagnetic system will line the spins entirely along the direction of the applied field.
If you get a ferromagnet, you only need to cool below the curie point and then apply a field to get the spins aligned. You'll need to go to a stronger field than above to overcome the hysteresis, though.
As someone said above, a simple refrigerator magnetic will put out weak-enough fields that will allow you to align several spins, and it will have an effect on coulombs per second if you move it fast enough. Not to high degree of polarization, but enough to attract the magnet to the refrigerator, so that should answer your question.
Re:Mildly disappointing (Score:5, Informative)
Particles with integer spin, such as phonons (spin 0), photons (spin 1), gravitons (spin 2) are called Bosons and obey Bose-Einstein statistics. Any number of bosons can be found in any quantum state, and at low temperatures they can condense into the ground state via Bose-Einstein Condensation.
Particles with half-integer spin, such as electrons, protons, neutrons (all spin 1/2) are called Fermions, and obey Fermi-Dirac statistics. This means interchanging two fermions in a system will cause the wavefunction of the system to acquire a factor of negative one. So if two fermions are in the same quantum state, that component of the wavefunction must be equal to it's negative - meaning zero. This is the Pauli Exclusion Principle, meaning no two fermions can ever exist in the same quantum state of a system. This effect has profound impact on physics, accounting for orbital nature of atoms, band structure of semiconductors, etc.
Anyway, back to your question about spin, another aspect of spin is that the allowable spin values must differ by integer units of hbar. So electrons, with total spin of hbar/2 are allowed two states that differ by hbar - +hbar/2 and -hbar/2. Usually the direction is chosen by an applied field, or whatever direction is chosen to measure the electron spin.
Spin is tricky because it isn't simply additive, but follows appropriate group theory. Electrons are part of SU(2) algebra, and spin interactions are weird. For example, you can simultaneously know the total spin (electrons are always hbar/2) and the spin component along one direction (for electrons this could be +hbar/2 and -hbar/2). But you cannot know the x, y, and z components simultaneously, basically because the Pauli matrices don't commute (Heisenberg uncertainty principle). So in actuality a spin-up electron really points somewhere along a cone that mostly points up, but you don't know more than that.
With two electrons, you can simultaneously know EITHER the total spin of the pair AND the total spin projected along one axis, OR you can know the projections of the two independent spins along one axis. If one electron is up and another is down, the system is in a state of 1/sqrt(2) (spin-Zero + spin-One). Also - this means that the two-electron system can exist in a Spin-1 state with the spin in one direction zero, or a Spin-0 also with the spin in one direction zero. Since the two electrons would have an integral number of spin, the system acts like a Boson. This is what allows superconductors, which are mentioned in TFA, to pair up and effectively condense.
Additionally, the spin-zero state of two electronss is very important in quantum communication, quantum teleportation, and quantum computation. This is the state with total spin zero, so no matter what direction you measure one spin, the other spin is aligned opposite.
Re:DIY? (Score:3, Informative)
Not normally, no, in general.
You can find some situations where there is a difference in the energy between the two states. The most common one is the application of a magnetic field - then, the spin state that is most aligned to the field is favoured over the other one.
The other major case is in chemcial species that have partially filled orbitals. Most of these tend to have a net magnetic moment anyway, but are much more complex, and less generally applicable.
Given the presence of a magnetic field merely changes the potential landscape, no work is going to be done.
By analogy - if you want all the cars on a road to be on the left hand side - block the right hand side of the road off. Once the block is in place, no other work need be done to ensure that all the cars are on the left. (That's a sucky analogy, but it's the best I can think of).
Re:Mildly disappointing (Score:3, Informative)
Oddly enough, free electrons do not have well-defined spin directions (interference phenomena destroy any possibility of measuring it, so it does not exist). Because of this it is not the case that electron-spin correlation is important to quantum communciation. Photon linear polarization alignment in J=0 states, which is the spin-1 analogue of the spin-zero state of two free electrons, is important, though. And bound electrons do have well-defined spin directions, which is what creates interesting effects in superconductors etc.
--Tom
Re:So, what, Base 4 Computing? (Score:2, Informative)
Re:DIY? (Score:3, Informative)
If you want to send a 100% polarized current of spin-up electrons into your batteries, your batteris will have a horrible coherence time and you'll eventually lose the coherence. Ie, after probably a few seconds any free electrons chosen at random from your battery will have 50% chance of being polarized up or down. Now put that battery in a magnetic field, you'll probably have more electrons of one polarity than another. but if you're not doing anything with spintronic materials or devices, this is entirely useless to you.
If you just want aligned spins, if you took a chunk of iron and cooled it sufficiently and put it in a sufficiently-high field, you can fully polarize that chunk so all the 'free' electrons point in the field direction. Of course most of the inner electrons in the iron atoms will be 'locked' into place, and unchangeable.
Re:DIY? (Score:3, Informative)
For free electrons there is only an energy difference in the presence of a magnetic field.
For atoms, an energy difference comes about from Zeeman splitting, which can be seen by standard textbook perturbation theory of the hydrogen atom in a magnetic field, where the otherwise degenerate levels split. This Zeeman splitting is how astronomers are able to detect the magnetic fields of astronomic objects.
Can you explain exactly what you are trying to accomplish, and why you think you need a fully polarized set of spins?
Re:So, what, Base 4 Computing? (Score:1, Informative)
Re:DIY? (Score:3, Informative)
Re:Is it too late? (Score:5, Informative)
Of course the buzzword 'spintronics' is is just 'electronics' with the word spin substituted in. The actual less-trendy synonym for spintronics is Magnetoelectronics, which is what it's usually referred to in the "real" science journals, not popular outlets like PhysOrg. magnetoelectronics.
BTW - since you mention Greek I thought a better example would be using the suffix Thon, as from Marathon, to refer to any excessivly long activity. Eg Bowl-a-thon, Dance-a-thon, Phone-a-thon, etc.
Re:DIY? (Score:4, Informative)
Quantum mechanics forces a measure of the electron's spin (and hence the direction of the dipole moment) into one of the allowable eigenstates. For a spin-1/2 fermion, such as an electron, there are only two states.
now - if you apply a field in the z direction and measure the spin in the z direction, there is a definite preference for the spin to align with the field.
if you apply the field in the y direction and measure in the z direction, then both states are of equal energies and there is no preference.
If you turn on interactions between electrons, like ferromagnetic or anti-ferromagnetic coupling, you get interesting effects, esecially at points where there the electron-electron interaction is countered by the field, and you have phase transitions at that point. if you allow for different couplings, different field directions, you can build up very rich phase diagrams of such systems, which are actually being studied by top physicists today.
Eg - anti-ferromagnetic interactions (neighbors want to be anti-aligned) on a triangle lattice is a frustrated magnet. A spin will be up, another neighbor will be down, the third is equally frustrated and doesn't know where to go. This makes very degenerate ground states, which have interesting properties.
Re:DIY? (Score:2, Informative)
Electrons placed in a magnetic field will have a potential difference between the populations aligned with and against the field, but the difference is so small that the applications at this point are severely limited.
The population difference in a 5 tesla field (the kind you need a liquid helium superconducting floor mounted magent to create and sustain) is still only a tiny fraction of the total. This population difference is utilised in ESR (Electron Spin Resonance) spectroscopy, and the spin differnce of protons is utilised in NMR (Nuclear Magnetic Resonance) spectroscopy, which is called MRI when used in medicine (because people get nervous about anything that mentions 'nuclear').
These scientists are excited because if the spin state of an electron can be controled, manipulated and stored, it presents a new pathway into the quantum computing field. This is because, as a quantum property, electron spin is affected by the same uncertainties and dualities as other quantum properties.
As to your final points, yes conducting individual electons through a permanent magnet will create the aforementioned population difference, but this will revert to a totally random population within milliseconds of being removed from the magnetic field (unless at absolute zero, but then you couldn't have current flowing anyway). I also think it unlikely that this could be used to store energy, as the energy needs to hold a sizable population of electrons in one spin orientation would rise exponentially with the population. I am also unaware of any means influence the spin orientation of photons, possibly something akin to polarising film could be employed (polarisation is a property of electromagnetic fields, and is IIRC seperate from quantum spin).
Pheew....