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Digix
2008-Sep-21, 12:58 AM
I would like to know if electron tunneling is somehow affected by insulators or magnetic field.

for example if we have gap between conductors, where electrons can tunnel at some rate what if we fill that gap with insulator lets say some diamond?
will the current change or there will be absolutely nos difference if gap or vacuum or some insulator.

also if we apply strong magnetic field in such way which normally redirects electrons back like in magnetron how it will affect tunneling current?

cjameshuff
2008-Sep-21, 03:42 AM
I would like to know if electron tunneling is somehow affected by insulators or magnetic field.

for example if we have gap between conductors, where electrons can tunnel at some rate what if we fill that gap with insulator lets say some diamond?
will the current change or there will be absolutely nos difference if gap or vacuum or some insulator.

It will change the local electrostatic field, in a way that depends on the dielectric constant of the material. If it contains dipoles that may orient to the field, or electron shells that can be induced to form dipoles in an external field, it will tend to concentrate and store electrostatic fields. If the charge difference remains the same, this can change the potential energy difference across the dielectric, and increase (or decrease) the likelyhood of tunneling.



also if we apply strong magnetic field in such way which normally redirects electrons back like in magnetron how it will affect tunneling current?

Remember that a magnetic field exerts a force on moving charges perpendicular to their direction of movement. In a magnetron, it doesn't redirect them back, it redirects them to one side, causing them to spiral outward in a plane perpendicular to the magnetic field.

If there is a current flowing parallel to the insulator, a magnetic field could reduce the potential across it and make it less likely for electrons to tunnel across, or do the opposite. SQUIDs are extremely sensitive magnetometer devices based on tunneling between superconductors separated by insulating barriers...however, I have no idea if they use that effect, or some more subtle quantum mechanical effect.

Digix
2008-Sep-21, 05:26 PM
It will change the local electrostatic field, in a way that depends on the dielectric constant of the material. If it contains dipoles that may orient to the field, or electron shells that can be induced to form dipoles in an external field, it will tend to concentrate and store electrostatic fields. If the charge difference remains the same, this can change the potential energy difference across the dielectric, and increase (or decrease) the likelyhood of tunneling.

ok. but in short, can we somehow use very thin insulators or after you make any insulator few atoms thin, it does nothing.


Remember that a magnetic field exerts a force on moving charges perpendicular to their direction of movement. In a magnetron, it doesn't redirect them back, it redirects them to one side, causing them to spiral outward in a plane perpendicular to the magnetic field
I was talking about same thing, if we have something like magnetron with very small gap. then electrons will tunnel from cathode to anode but if we apply strong magnetic field they cant pass the gap in the usual magnetron because electrons circle or return back.

from what you say I suppose that everything will be about same in tunneling mode like in thermionic emission mode .

cjameshuff
2008-Sep-21, 07:07 PM
ok. but in short, can we somehow use very thin insulators or after you make any insulator few atoms thin, it does nothing.

It still imposes a barrier, a minimum difference in potential energy is required in order for electrons to tunnel across it. If there were no insulator, there would be no such minimum energy required. Flash memory uses this behavior...electrons can be tunneled into or out of a floating gate, modifying its charge, by application of a voltage sufficient to tunnel electrons through a thin barrier but not through a thick one, either injecting or removing electrons from the gate. When the voltage is removed, the electrons lack the energy they need to tunnel through the thin barrier, and the gate retains its charge.



I was talking about same thing, if we have something like magnetron with very small gap. then electrons will tunnel from cathode to anode but if we apply strong magnetic field they cant pass the gap in the usual magnetron because electrons circle or return back.

They don't return to the cathode, they spiral outward from it, at least when in the stronger part of the cathode's electrostatic field. Electrostatic force accelerates them outward from the cathode, and the Lorentz force bends their trajectories to the side with a strength proportional to their velocity.

If you're asking if Lorentz forces affect particles tunneling across a barrier...I really have no idea. They don't tunnel at any definable velocity, by my understanding. If there's an analogous effect, it likely requires quantum mechanics to accurately describe, and probably can't be visualized very well by comparison with a magnetron.



from what you say I suppose that everything will be about same in tunneling mode like in thermionic emission mode .

Not at all sure what you mean.

Digix
2008-Sep-21, 09:14 PM
It still imposes a barrier, a minimum difference in potential energy is required in order for electrons to tunnel across it. If there were no insulator, there would be no such minimum energy required. Flash memory uses this behavior...electrons can be tunneled into or out of a floating gate, modifying its charge, by application of a voltage sufficient to tunnel electrons through a thin barrier but not through a thick one, either injecting or removing electrons from the gate. When the voltage is removed, the electrons lack the energy they need to tunnel through the thin barrier, and the gate retains its charge.
ok, that part is clear



If you're asking if Lorentz forces affect particles tunneling across a barrier...I really have no idea. They don't tunnel at any definable velocity, by my understanding. If there's an analogous effect, it likely requires quantum mechanics to accurately describe, and probably can't be visualized very well by comparison with a magnetron.
yes, that was what I was asking.
if there is electric current the there is electron velocity.


Not at all sure what you mean.

I was asking for difference if the vacuum gap in the setup similar to magnetron is big, or if it is very small. If gap is big, only high energy thermal electrons can cross it. if gap is very little then lots of relatively cold electrons can tunnel.

but never mind that, since it was same question as before

I just presented illustration how magnetic field prevents electrons from reaching anode because if anode voltage is insufficient magnetron will not pass any current at all. it can be even used as magnetic switch since if you remove magnetic field magnetron becomes simple vacuum diode.