Phase Transitions in Electron-Hole Systems

Ph. Brune, A. P. Kampf

Igor N. Karnaukhov, Ilya Eremin

We propose a strongly correlated (1+1)D electron model with particle–hole asymmetry which is exactly solvable by means of the nested Bethe ansatz. The particle–hole asymmetry in this many-body system causes a metal–insulator transition with a phase separation. Here, we give a detailed analysis of the zero-temperature phase diagram of the model and demonstrate how this unusual behavior of the metal–insulator transition with phase separation arises. In particular, we show that a homogeneous phase of itinerant electrons and a mixed phase of itinerant electrons and localized electron pairs (for which two different species reside in spatially separated regions) define the ground state of a fermion chain. We also find that the metal–insulator phase transition induces phase transition on the surface with simultaneous generation of the interphase boundary.

T. O. Voronkova, A. M. Sarry, M. F. Sarry +1 more

It has been experimentally found that, under the static compression of a calcium crystal at room temperature, it undergoes a series of structural phase transitions: face-centered cubic lattice → body-centered cubic lattice → simple cubic lattice. It has been decided to investigate precisely the simple cubic lattice (because it is an alternative lattice) with the aim of elucidating the possibility of the existence of other (nonstructural) phase transitions in it by using for this purpose the Hubbard model for electrons with half-filled ns-bands and preliminarily transforming the initial electronic system into an electron–hole system by means of the known Shiba operators (applicable only to alternative lattices). This transformation leads to the fact that, in the new system of fermions, instead of the former repulsion, there is an attraction between electrons and holes. Elementary excitations of this new system are bound boson pairs—excitons. This system of fermions has been quantitatively analyzed by jointly using the equation-of-motion method and the direct algebraic method. The numerical integration of the analytically exact transcendental equations derived from the first principles for alternative (one-, two-, and three-dimensional) lattices has demonstrated that, in systems of two-species (electrons + hole) fermions, temperature-induced metal–insulator phase transitions of the Mott type are actually possible. Moreover, all these crystals are in fact excitonic insulators. This conclusion is in complete agreement with the analytically exact calculations of the ground state of a one-dimensional crystal (with half-filled bands), which were performed by Lieb and Wu with the aim to find out the Mott insulator–metal transition of another type.

Xin Wan

S. I. Shevchenko

The unusual mechanism of superconductivity, which was predicted earlier and is due to the pairing of spatially separated electrons and holes, is dramatically dependent on interband transitions capable of fixing the order parameter phase and thus eliminating superfluidity. In this paper the conditions are found under which the interband transitions do not prohibit superfluidity of pairs. A phase diagram is plotted for the corresponding systems.