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23PLASMA PHASE TRANSITION
Norman G.E., Saitov I.M., Stegailov V.V.
Joint Institute for High Temperatures RAS, Izhorskaya street, 13, bldg 2, Moscow 125412, Russia
A plasma-plasma first-order phase transition is predicted theoretically in 1968 by Norman and Starostin [1, 2]. The origin of the phase transition is derived from the competition of the short-range quantum repulsion, long-range effective Coulomb attraction and temperature, similar to the van der Waals equation. The hypothesis initiated a great number of theoretical works, see e.g. [3, 4]. The theory is complicated by the strong coupling of Coulomb interaction. Finally some experimental evidences are obtained. Both [5] and [6] focus on the first-order phase transition that separates two distinct coexisting phases of warm dense hydrogen, which differ in density from each other. Metallization of fluid molecular hydrogen is observed at shock compression; resistivity decreases 4-5 orders of magnitude just in the density range where the 20% increase of density is demonstrated [5]. Pulsed-laser heating above the melting line of hydrogen at static pressures in a diamond anvil cell is used in the megabar pressure region [6]; the anomaly observed in the heating curve correlates with theoretical predictions for the plasma phase transition. The authors [7] demonstrates a laser-driven shock wave in a hydrogen sample, pre-compressed in a diamond anvil cell; optical reflectance probing at two wavelengths reveals the onset of the conducting fluid state; the boundary line to conducting fluid hydrogen is suggested; reflectivity jump points to a phase transition. The liquid-liquid-solid (in fact, fluid-fluid-solid) triple point is considered for hydrogen (deuterium) melt [6, 7].
Contrary to [1, 2] the liquid-liquid phase transition in selenium is observed experimentally from the very beginning in 1989 by Brazhkin et al [8, 9]. The phase difference is related to jumps of conductivity and viscosity. The transition in the melt is attached to the liquid-solid coexistence curve with a triple point not far from the extremum point of the curve. The discovery [8] initiated studies for different substances.
It looks as if Brazhkin et al transition and Norman-Starostin transition belong to one and the same coexistence curve. The first manifests itself in the vicinity of the liquid-liquid-solid triple point. Another one can be attributed to the partially ionized area near the critical point of the transition. Such a coexistence curve is predicted in [2]. Possibility of the plasma phase transition limited from lower temperatures by the second critical point is also pointed to in [2]. The physical nature of the Brazhkin et al transition is announced as semiconductor-to-metal transition [8]. The plasma phase transition is related to different degrees of ionization in [1, 2]. Ab initio quantum modeling is applied to check the ideas that motivated both [1, 2] and [8] studies and to analyze both similarity and difference between Brazhkin et al and Norman-Starostin phase transitions as well as with the Wigner metallization. Electron density of states and the characteristic gap in it are investigated to verify the semiconductor-to-metal nature of the transition [8] at least near the triple point. Variation of the density of states with increase of temperature is calculated for selenium.
The concepts of the "degree of ionization" and of the "free electrons" do not work anymore in the strongly coupled plasmas. Uncertainty of the both concepts in dense plasmas is emphasized. Electron states cannot be classified as only free and bound states even conditionally in dense plasmas, contrary to the ideal plasmas. The wide range of the intermediate multi-particle non-stationary states of the fluctuation nature spans between free and bound states, the borders of latter being rather diffuse [10]. The range width expands with the increase of plasma density. The excited bound states disappear. Moreover free electrons number density is not an observable value at all. Plasma frequency is suggested to study difference between two fluid phases along the coexistence curve investigated in warm dense hydrogen, since the plasma frequency is an observable value in any plasma. Method to calculate plasma frequency, as well as conductivity and reflectivity in dense plasmas and warm dense matter is developed. The approach suggested presents a method of the self-consistent calculation of both electron density of states and dielectric properties. Density of electron states is discussed for both classical and quantum VASP cases. Plasma frequency jump is calculated to verify the nature of the phase transition in warm dense hydrogen. Conductivity jump as well as electron density of states and pair distribution functions are also calculated. The change of the pair distribution function points to the consistent change of the ionic structure as a result of the electronic structure change. The relation between jumps in plasma frequency, conductivity, reflectivity and a firstorder phase transition is discussed.
The work is supported in part by the RAS Program 43 Fundamental problems of mathematical modeling.
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