|
|
|
 |
Clustered fluid: a novel state of charged quantum fluid mixtures
(
Department of Physical Sciences, University of Oulu, Finland
)
Wednesday, 25 Oct 2006, 14.00, W0.02
Quantum mixtures of charged particles appear in a large variety of different systems in condensed matter physics. Examples range from electrons in metals and electron-hole fluids in semiconductors to electron-proton liquids and solids in cold stars and in high pressure and shock wave experiments. Recent advances in experimental techniques have brought new interest into electron-hole liquids in narrow channels [1] and new large scale simulations [2, 3] have been performed in order to understand phases of such a basic system as the mixture of electrons and protons. At the room temperature and low pressures protons and electrons mixed together form a molecular gas. At zero temperature molecules solidify into a quantum crystal. Molecular phases are mainly insulators, but molecular solids may become metallic at high pressures due to overlap of the bands. Thus by increasing the pressure (or density) at zero temperature molecular crystals undergo series of phase transitions before protons form a metallic, atomic crystal. Due to the softness of the Coulomb repulsion the atomic crystal melts into liquid metallic hydrogen, which is the high pressure phase of the mixture. In the laboratory shock wave experiments metallization of the liquid molecular hydrogen has been found at temperatures of about 3000 K and at the pressure of 140 GPa [4]. High pressure experiments up to 320 GPa at low temperatures have not yet reached the metallic state. Recent simulations [2] find that the melt line of the molecular H2 crystal has a negative slope in the pressure-temperature plane. Above 300GPa the molecular solid melts into a weakly metallic fluid. It is still an open question how low in temperature this fluid phase extends [3]. In this work we discuss homogeneous charged mixtures ranging from the electron-hole liquid to the liquid metallic hydrogen and let the density decrease until homogeneous phases become unstable. At high densities the Coulomb interaction is strongly screened and that is why the homogeneous metallic liquid is stable, whereas at low densities bound entities like excitons or hydrogen molecules form the ground state. At intermediate densities, as we will show, the bare Coulomb potential is over-screened, which changes the character of interactions. This may lead to formation of bound clusters of different numbers of particles and charges [5]. In semiconductors this phase has been identified in narrow channel experiments in Si [1] and we suggest that the weakly metallic fluid found in the electron-proton mixture simulations has the same origin. [1] N. Pauc, V. Calvo, J. Eymery, F. Fournel and N. Magnea, Phys. Rev. Lett. 92, 236802-1 (2004). [2] S. A. Bonev, E. Schwegler, T. Ogitsu and G. Galli, Nature 431, 669 (2004). [3] C. Pierleoni, D. M. Ceperley and M. Holzmann, Phys. Rev. Lett. 93, 146402-1 (2004). [4] S. T. Weir, A. C. Mitchel and W. J. Nellis, Phys. Rev. Lett. 76, 1860 (1996). [5] M. Saarela, T. Taipaleenmäki and F. V. Kusmartsev, J. Phys. A 36, 9223 (2003). |
||
|
|