Anomalous Structural Change Induced byStrong Electron Exchange-Correlationin an Expanded Liquid Alkali Metal
Takada GroupOne of the central issues in condensed matterphysics is to understand the strong exchangecorrelation (XC) effect between electrons which may
cause some kinds of attraction related to a variety ofphenomena such as superconductivity, ferromagnetism, and Wigner crystallization. Besides those kinds
of attraction, the XC effect always acts as an attractionin the symmetric Landau’s Fermi-liquid interaction for
the entire region of the electron density n; thisattraction increases monotonically with decreasing n,leading to a strong tendency toward electronic phaseseparation, which is, however, prohibited fromdeveloping macroscopically by the charge neutrality.
An expanded liquid alkali metal is an interesting
system in which the latter attraction plays an essential
role not only in transport properties but also structural
ones. This attraction is, in fact, a driving force for theliquid-gas phase transition necessarily accompaniedby the metal-insulator transition [1].
Here we report on our recent study of the local ionic
structure of an expanded liquid alkali metal on thebasis of a model ionic system derived from thefirst-principles Hamiltonian by treating the electronion pseudopotential as perturbation [2]. Near theliquid-gas phase transition, we have found that RNN
the distance between adjacent ions decreases despite
the increase of the mean interionic distance (or rs theWigner-Seitz radius) as shown in Fig. 1. Thisanomalous structural change is in sharp contrast with
the normal one in compressed liquid alkali metals near
the liquid-solid phase transition, for which RNN
increases with increasing rs.
In order to elucidate the physical origin of the local
contraction of RNN, we consider the interionic equilibration distance Req, which is determined by the minimum position in the effective interionic interaction
potential, because RNN coincides with Req for low n.
From Fig. 2, it is found that Req decreases against theincrease of rs provided that the condition of 2rc< rs<4rc is satisfied with rc being the radius of the ion core.
By the careful analysis on the spatial dependence ofthe effective interionic interaction, we have revealed
that this decrease of Req is attributed to an enhanced
attraction working between valence electrons (or thestrong tendency toward electronic phase separation
caused by the XC effect), combined with the effect of
excluding those electrons from the ion cores.
In Fig. 2, we have also indicated Req at nsolid (thesolid densities at ambient pressure and temperature)
and at ncritical (the critical densities) for Na, K, Rb, andCs. Since the density in the expanded liquid state issmaller than nsolid but larger than ncritical, we canconclude that the local contraction of Req and therefore
RNN is a universal phenomenon of an expanded liquidalkali metal.

Fig. 1. Comparison between our theoretical results of the radial
ion-ion distribution function g(R) obtained by Monte-Carlosimulations on the model ionic system (the red open circle) andthe experimental results by Matsuda et al. [Phys. Rev. Lett. 98,096401 (2007)] (the blue solid curves) for Rb. The distance
between adjacent ions RNN, which is determined by thefirst-peak position in g(R), decreases with increasing rs.

Fig. 2. Interionic equilibration distance Req with changing rc
from 1.5 to 3.0 by 0.1 versus the ratio of rs to 2rc. For eachspecies, Req at nsolid is marked by a blue circle, while Req at
ncritical by a red triangle. Req decreases against the increase of rs
in the region of 1< rs/2rc <2, roughly corresponding to theexpanded-liquid regime.

References[1] H. Maebashi and Y. Takada, J. Phys.: Condens.
Matter 21, 064205 (2009).
[2] H. Maebashi and Y. Takada, J. Phys. Soc. Jpn. 78,053706 (2009).
AuthorsH. Maebashi and Y. Takada