Metastability

Metastability is a weak shape stability. A metastable state is stable against small changes, but unstable to larger changes.

An example of this is the system " wood and air oxygen " at room temperature: From a thermodynamic perspective, the spontaneous combustion of chemically bound in carbon would lead with the oxygen to form carbon dioxide to a more stable state. Without activation, ie a large enough supply of energy such as the lighting of the wood, this will not happen.

Clearly this is shown in the image on the right: A ball lying in a small hollow on the mountainside. As long as the ball is only slightly deflected in the dish, it rolls back to its lowest point. This is called a " local minimum ". But if it is deflected more, he can roll down the mountainside. First, then, a certain minimum energy must be applied before the state changes of the system.

The weakest form of metastability is instability. An unstable system loses its initial state after an arbitrarily small ( infinitesimal ) perturbation.

Thermodynamics

Have a higher energy metastable phases (correct: Gibbs - under defined conditions, such as constant pressure and constant temperature ) than the stable phase. Due to a high activation energy they do not or only slowly transform into the stable phase.

An example of a metastable phase is the diamond, which was to transform spontaneously into graphite at atmospheric pressure; the speed of this process is, however, negligibly small at room temperature. Another example is the tin pest: the metallic phase of the tin is metastable below 13 ° C and slowly converts to the more stable at these temperatures nonmetallic phase. Other examples are subcooled water, glass ( the stable state would be the crystalline silicates) and supersaturated solutions, see for example in hand warmers use.

Atomic physics, solid state physics

In atomic physics, is a metastable state, an excited state of the atomic shell, the back does not fall within the usual, very short period of less than one microsecond by spontaneous emission of photons ( dipole ) to the ground state. This is explained by conflicting selection rules. The decay of a metastable state is sometimes referred to as forbidden transition. Examples:

• The metastable state of helium atom He * ( 3S1 ) lives in the middle 8000 s
• Metastable states of certain crystals in ceramic can be maintained several thousand years, obviously, as the thermoluminescence shows.
• The Wigner energy is stored in lattice defects of a graphite moderator energy.

Nuclear physics

Although the selection rules for atomic nuclei much more transitions and decays allow as in the atomic shell, there are at atomic nuclei as metastable -defined conditions, the Kernisomere. Examples (in brackets the half-life for the transition to the ground state by gamma emission ):

• Tin - 119m (293 days)
• Employed in nuclear medicine, technetium-99m ( 6 hours).

Digital circuits

In digital technology, there are circuits which are to store at any given time a binary information (0 or 1). The simplest form of such a flip-flop consists of a ring consists of two inverters. This feedback circuit has two stable states, 0 and 1, ie near the bottom and near the upper operating voltage. With a slight interference with a cable, the circuit will always return in the stable state, which makes the storing effect of the circuit of. Additionally, there is a metastable point, approximately in the center of the operating voltage. Idealized could remain in this point the circuit. However, effects such as noise, interference, and they will eventually leave (usually within a few nanoseconds) in one of the stable states tilt. It can be predicted only statistically, the time after which this will be done. The typical problem case is the sampling of a changing signal. If the signal is sufficiently close to 0 or 1, the flip-flop will store this state as expected without any problems. However, the input voltage approaches a particular area, it is observed that

This may cause serious malfunction of circuits result. Critical is less that the flip- flop moves only when re- sampling in the current state, but that it remains for an indefinite time in the metastable state. Thus, it can display a information at the output, which lies between the two logic levels, which in turn can lead to metastable states and errors in nachgeschaltenen flip -flops. Thus, the computation time required for the clock conditions, one also speaks of the setup and hold times for the next flip- flop stages violated under certain circumstances.

It is essential that the metastable situation can be avoided by any measure whatsoever. Every " Solution " in this direction is always based on a fallacy that ignores the occurrence of metastability at any point. By daisy-chaining of multiple sampling stages ( flip- flops), the probability of error can only be arbitrarily greatly reduced. In modern logic circuits, the residence time is in the metastable state with a probability of 99.9% of the switching operations at less than 1 ns. Thus, it is possible for the overall probability of failure by means of one scan, for example, To reduce 10ns to irrelevant values ​​.