Reentrant quantum melting of exciton crystals

Nature knows two opposite types of solids: one that emerges upon compression from a liquid and a second that appears if the pressure on a liquid is reduced. While the former is typical for substances in our everyday life the latter occurs for example in a dense quantum liquid of electrons (such as in metals) or ions (in exotic white dwarf or neutron stars). Now it has been shown that there exists yet a third form of matter that inherits both of these properties. This unusual beharior has been predicted to exist in crystals of excitons - hydrogen atom-like bound states of electrons and holes - in a semiconductor quantum well placed in a strong electric field.
In his phd thesis Dr. Jens Boening, together with Privat-dozent Alexei Filinov and Prof. Michael Bonitz, has performed extensive accurate path integral Monte Carlo simulations that shed light on the mysterious properties of this material. They found a simple explanation for the coexistence of the two seemingly contradicting melting behaviors. The secret lies in the character of the forces acting between two excitons: at low pressure excitons repel each other via a dipole force and form a quantum liquid. Upon compression this fluid freezes into an exciton crystal. Further compression brings two excitons so close together that the quantum wave nature of their constituents (electrons and holes) starts to weaken the forces. As a consequence, further compression leads to an increasing overlap of the exciton quantum waves that is no longer balanced by the inter-exciton repulsion, and the crystal melts again.
The researchers have made precise predictions where to search for this exotic crystal of excitons (particularly suited are gallium arsenide or zinc selenide quantum wells)- it is now up to the experimentalists to find this new state of matter.

Density distribution (pair distribution function) of the quantum particles (excitons) in the plane of the quantum well. Top left (b): dipolar fluid, (c) dipolar crystal, (e) Coulomb-like crystal, (f) superfluid exciton liquid. Yellow color corresponds to high density, red to lower, green to zero.
From top left to bottom right the density is increased at constant temperature.
The two quantum phase tansitions persist down to zero temperature.

Phase diagram of 2D indirect excitons for a fixed quantum well width d=13.3,a_B*$. Circles and squares mark our PIMC results. Vertical dashed lines (D=17 and r_s=9.4) indicate the two density induced quantum freezing (melting) transitions. Filled symbols mark the two triple points. The normal fluid--superfluid phase boundary is marked by the red line and is below the ideal estimate T_KT, cf. thick solid line labeled chi=4. The line T_dip marks the freezing transition of a classical 2D dipole system.

Our original paper is: Crystallization of an Exciton Superfluid by J. Boening, A.V. Filinov, and M. Bonitz,
Phys. Rev. B 84 , 075130 (2011) | local pdf-file

Press release of Kiel university | Deutsche Pressemitteilung

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