Negative electron compressibility in the Hubbard model

Abstract

Using the strong coupling diagram technique, we study the one-band repulsive Hubbard model on a twodimensional square lattice in a wide range of chemical potentials µ. Infinite sequences of diagrams describing interactions of electrons with spin and charge fluctuations are taken into account. At low temperatures, regions of the negative electron compressibility are found for µ < 0 and µ > U , where U is the Hubbard repulsion. For even lower temperatures, in the neighborhood of these regions, states with the phase separation are revealed. arXiv:1905.02929v1 [cond-mat.str-el]

Phase Separation and Pairing Fluctuations in Oxide Materials

“…In systems with strong electron correlations, phase separation takes place with the inclusion of charge and spin fluctuations. This result was recently demonstrated in the one-band repulsive Hubbard model on a two-dimensional square lattice [72]. It was shown that at low temperatures, regions of negative electron compressibility (NEC), κ = x −2 (dx/dµ) < 0, arise near certain values of the chemical potential µ.…”

“…Figure 3 exhibits a cartoon image of one of the NEC regions. In the one-band Hubbard model, the topmost point of this dependence may be close to x = 1 at low temperatures [72]. Let us suppose that the crystal is divided into two parts with the electron concentration x 1 and chemical potential µ 1 in one of them, and x 2 < x 1 and µ 2 > µ 1 in another.…”

Magnetic properties and superconductivity in the two-dimensional repulsive Hubbard model

“…6) Outside of this range of chemical potentials, for |µ| T and |U − µ| T , regions of the negative charge compressibility are located. 15) This peculiarity of the model can lead to the formation of stripes. By choosing the mentioned range of µ, we exclude them from consideration.…”

“…On the other hand, such a value of µ allows us to avoid the regions of the negative electron compressibility located near µ = 0 and µ = U. 15) In calculations including phonons or with an unfixed chemical potential, these regions lead to charge instability and phase separation.…”

“…10 have distinctive features of the d x 2 −y 2 symmetry, they are drastically different. It is clear that the cause of these peculiarities is in the two multipliers θ(p) and θ(−p), which together with V + p ′ p form the matrix in the Eliashberg equation (15).…”