Metastability and Phase Separation in a Simple Model of a Superconductor with Extremely Short Coherence Length

Abstract

We present studies of the atomic limit of the extended Hubbard model with pair hopping for arbitrary electron density and arbitrary chemical potential. The Hamiltonian consists of (i) the effective on-site interaction U and (ii) the intersite charge exchange term I , determining the hopping of electron pairs between nearest-neighbour sites. In the analysis of the phase diagrams and thermodynamic properties of this model we treat the intersite interactions within the mean-field approximation. In this report we focus on metastable phases and determine their ranges of occurrence. Our investigations in the absence of the external magnetic field show that the system analysed exhibits tricritical behaviour. Two metastable phases (superconducting and nonordered) can exist inside the regions of the phase separated state stability and a first-order transition occurs between these metastable phases.

Superconductivity, Charge Orderings, Magnetism, and Their Phase Separations in the Ground State of Lattice Models of Superconductor with Very Short Coherence Length

“…Names of transition orders used are consistent with our earlier works [7][8][9][10][11][12][13][14][33][34][35][36]. In particular, a transition between a homogeneous phase and the PS state is symbolically named as a "third-order" transition.…”

“…The resulting diagram as a function of n is shown in For J = 0 the M phase reduces into the NO phase with ω NO = −μ − U/2 (n = 1) and E NO (n) = 0 (for n < 1) [8][9][10][11], and the PS states on Fig. 2b change into PS :SC/NO and PS :NO/NO states, whose energies can be derived from (5) analogously.…”

“…refs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and references therein). Moreover, the phase separation (PS) has been evidenced (by various microscopy techniques such as STM, MFM, etc.)…”

“…The effective model considered has a form of single-band extended Hubbard model with pair hopping at the atomic limit (t = 0) [4][5][6][7][8][9][10][11][12][13][14][15][16][17]: …”

“…The analysis of the ground state of model (1) has been performed by means of a variational approach (VA) in the grand canonical ensemble [6][7][8][9][10][11][12][13][14], which treats on-site U term exactly and the intersite interactions within the mean-field approximation (MFA). The MFA is a rigorous treatment of the intersite terms in the limit of infinite dimensions d → +∞ (or large coordination number z); thus, the result obtained in this paper for model (1) are also exact in the limit d → +∞.…”