Tunnel Effect

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Tunnel Effect

De Broglie's idea of "matter as wave" suggests a startling consequence, and that's what is called thetunnel effect. More precisely, Schrodingers wave equation can show this rigorously. Unlike the classical

mechanics of particles, quantum mechanics allows light as well as particles (such as electrons and

protons) to appear even where the "wall" of potential should prevent them from appearing. In the

following figure, you may imagine the wall as the "wall of potential", in that any particle must have an

energy, greater than a certain amount, for going through it and appearing on the other side. But even

when the particle has a lower energy than that, it can go through the wall, just as a wave can appear on

the other side (since its oscillation can go through the wall). Since particles as well as light have particle-

wave duality, matter (with an appropriate energy) can go through the wall according to quantum

mechanics. This can explain the spontaneous disintegration of radioactive substances (such as radium);

even though the strong interaction within the nucleus forms a high wall of potential, alpha-

disintegration can occur because of the tunnel effect.

Bohr briefly mentions this (58); but for a more rigorous treatment, the reader is referred to Dr.

Tomonaga's book (1997, 101-106).


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APPLICATIONS OF TUNNEL EFFECTS

The scanning tunnelling microscope (STM)

The device, with which one could see for the first time to down there to the atoms, is

Called scanning tunnelling microscope. It has since its invention, for which there was1986 a Nobelpreis, which revolutionizes figure of surfaces.

A fine metal needle is the transmitter of the messages from the quantum world. Their

Point consists in the ideal case of only one atom.

As is the case for the record player it ertastet the atomic landscape line for line.Owing to quantum physics that occurs everything without contact. Electrons branch

From the point on the copper surface. It flows a tunneling current. This current is kept

Constant. But the distance of the needle to the surface must be kept always alike.The height adjustment functions with the help of a Piezoroehrchens (Piezoro pipe?),This is controlled with the tunneling current. Thus the mountains and valleys of the

Atomic landscape can be illustrated exactly. The distance of the needle to the surfaceAmounts to only few atomic diameters. So that it does not come to disturbances,

Therefore a thought out oscillation damping is necessary.

The principle

The principle is simple: With a fine point the surface is scanned. If an individual atomSits at the end of the point, surfaces with atomic accuracy can be illustrated. Despite

the extremely high enlargement a scanning tunneling microscope is often onlyMatchbox large

.


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The point of the scanning tunneling microscope is so closely over the surface of the

Sample that it comes to quantum -mechanical tunnels of electrons by the vacuumBarrier between point and sample. If a voltage between point and surface is applied,

Then flows a tunneling current, which depends exponentially on the distance between

Point and sample. This exponential spacer dependency is the reason for the

Enormous sensitivity of the STM. During a change in distance of only one atomic

Diameter the tunneling current changes around more than tenfold.A certain current value corresponds thus to a certain distance of the point to the

Surface. During the horizontal movement of the point over the sample the current isregulated by vertical descendant of the point on a fixed desired value. The correcting

Variable, the distance between point and sample, is recorded thereby always andCorresponds to the surface morphology. If the point drives o ver. a obstacle ", e.g. an

Atom, then the tunneling current became larger. The regulation causes a withdrawingof the point, to again the desired distance (= desired current) is achieved. Thus the

Point of the surface morphology always follows in constant distance.

Scanning the surface the point in all three directions in space with an accuracy

Must be moved by fractions of an atomic diameter. In addition the point is fastened toThe end of a ceramic(s) tube, whose length changes when creating an electrical

Voltage (piezoelectric effect). The point can be laterally moved by creation of differentVoltages to the different pages of the Piezoroehrchens; by creation the same voltage

to all four electrodes the point is vertically moved. The compensated height of thePoint is now recorded, while a part of the sample is scanned. Thus results an elevator

Profile of the sample which can be examined, which is represented as picture. If thePoint is sharp enough, and then individual atoms can be illustrated.

Moving atoms


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The needle for scanning the atoms can fulfill even still another additional function. Todo this, it must be more deeply lowered: so far, until attractive or repulsive binding

forces become effective. Now an atom can be shifted by the surface either pulled toanother place (attractive strength) or (repulsive strength). Physicists of the free

University of Berlin in this way built the smallest Q of the world from copper atoms. Ithas a diameter of approximately eight nanometers, that is 0.000000008 meters.

Atomic structures are however not the only application of this new technique: The currentbetween point and surface can be used also to loosen chemical compounds in a molecule directly

on the surface and make new connections. So a chemical reaction can be executed gradually atonly one molecule and examined during its.

Applications

One of the first applications of tunneling was an atomic clock based on the tunneling

frequency of the nitrogen atom in an ammonia (NH3) molecule. In this molecule, the

nitrogen atom tunnels back and forth across the energy barrier presented by the

hydrogen atoms in a pattern that is reliable and easily measured. This characteristic

made it ideal for use as one of the earliest atomic clocks.

A current and rapidly growing application of tunneling is Scanning Tunneling

Microscopy (abbreviated STM). This technique can produce high-resolution images that

accurately map the surface of a material. Some of the best STM pictures may actually

show us what individual atoms look like. As with many high-tech tools, the operation of

a scanning tunneling microscope is fairly simple in principle, while its actual

construction is quite challenging.


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The working part of a tunneling microscope is an incredibly sharp metal tip. This tip is

electrically charged and held near the surface of an object that is to be imaged. The

energy barrier in this case is the gap between the tip and the sample. When the tip gets

sufficiently close to the sample surface, the energy barrier becomes thin enough that a

noticeable number of electrons begin to tunnel from the tip to the object. Classically, the

technique could never work because the electrons would not pass from the tip to the

sample until the two actually touched. The number of tunneling electrons, measured by

highly sensitive equipment, can eventually yield enough information to create a picture

of the sample surface.

Another application of tunneling has resulted in the tunnel diode. The tunnel diode is a

small electronic switch that can process electronic signals much faster than any ordinary

physical switch. At peak performance, it can switch on and then off again ten billion

times in a single second.

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Tunnel Effect