Metastable state

Metastability on the example of a ball in a pit

A metastable state (from the Greek. Μετα “through” and Latin stabilization “stable”) is a state of quasi-stable equilibrium of a physical system in which the system can stay for a long time.

What is a metastable state can be understood from the figure on the right:

state 1: metastable - a state whose stability is maintained under not very large disturbances;
state 2: unstable - a state whose stability is violated by arbitrarily small disturbances;
state 3: stable - a state whose stability is maintained under large disturbances.

Metastable states are widely found in nature and are used in science and technology. The existence of metastable states is associated, for example, with the phenomena of magnetic, electrical, and elastic hysteresis , the formation of supersaturated solutions, hardening of steel, glass production, etc.

Content

In thermodynamics

Van der Waals gas isotherms:
P is the pressure;
V is the volume;
K is the critical point;
abKcd - binodal (the boundary of the region of two-phase equilibrium; the region under the binodal bell is the region of two-phase equilibrium liquid - vapor);
eKf is the spinodal (the boundary between the regions of metastable and thermodynamically unstable states; the region under the spinodal bell is unrealizable state);
bc is the condensation line ;
abKe - region of superheated liquid;
dcKf — region of supercooled steam;
the areas of the filled figures under the isobar bc and above it are equal ( Maxwell's rule , 1875)

Metastable states correspond to one of the minima of the thermodynamic potential of the system under given external conditions. A steady (stable) state corresponds to the deepest minimum. A homogeneous system in a metastable state satisfies the conditions of stability of thermodynamic equilibrium $C_{P}>C_{V}>0$ ${\ displaystyle C_ {P}> C_ {V}> 0}$ , $(dP/dV)_{T}<0$ ${\ displaystyle (dP / dV) _ {T} <0}$ , relatively small perturbations of physical parameters ( entropy , density, etc.). With sufficiently large perturbations, the system goes into an absolutely stable state. A large class of metastable states is associated with first-order phase transitions (crystal $\leftrightarrow$ ${\ displaystyle \ leftrightarrow}$ liquid $\leftrightarrow$ ${\ displaystyle \ leftrightarrow}$ gas).

In quantum systems

Metastable states in quantum systems are states with lifetimes ( $\tau$ ${\ displaystyle \ tau}$ ), large enough so that the uncertainty of the width of the energy levels, according to the uncertainty principle for energy and time $\Delta E\sim {\frac {\hbar }{\tau }}$ ${\ displaystyle \ Delta E \ sim {\ frac {\ hbar} {\ tau}}}$ , there was much less difference between them. ^[1] Usually, excited states are considered metastable, radiative (radiation) transitions from which to other states are prohibited by strict selection rules . Metastable states differ in the type of transitions that are possible for them: magnetic dipole, electric quadrupole, two-photon, and other transitions.

Notes

↑ Landau L.D., Lifshits E.M. Quantum mechanics. - M., Science, 1972. - p. 83

Literature

Landau L.D. , Lifshits E.M. Statistical Physics. Part 1. - 2nd edition, revised. - M .: Nauka , 1964 .-- 568 p. - (“ Theoretical Physics ”, Volume V).

Links

Metastable state - an article from the Physical Encyclopedia .

[1] Landau L.D., Lifshits E.M. Quantum mechanics. - M., Science, 1972. - p. 83