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arXiv:1602.07484

New many-particle system consisting of electron pairs and holes in a two-dimensional crystal

G.‐Q. Hai, Ladir Cândido

Published Feb 24, 2016• 0 citations
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Abstract

When atoms are organized into a crystal, the single-electron energy levels of individual atoms form energy bands. However, there also exist electron-pair states in atoms. We found previously that the counterpart of these electron-pair states of individual atoms may survive in a two-dimensional square-lattice crystal. When the amplitude of the crystal potential is larger than a certain value, a metastable electron-pair band arises between the two lowest single-electron energy bands. In this paper, we demonstrate that the many-particle exchange-correlation corrections renormalize the energy bands and may stabilize the electron pairs for certain hole and electron-pair densities in the two-dimensional crystal. Therefore, depending upon the crystal potential and free carrier densities, there may exist a new many-particle system consisting of electron pairs and holes. Furthermore, we calculate the charge-density excitation spectrum in this new many-particle system.

DOI
10.48550/arxiv.1602.07484

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When atoms are organized into a crystal, the single-electron energy levels of individual atoms form energy bands. However, there also exist electron-pair states in atoms. We found previously that the counterpart of these electron-pair states of individual atoms may survive in a two-dimensional square-lattice crystal. When the amplitude of the crystal potential is larger than a certain value, a metastable electron-pair band arises between the two lowest single-electron energy bands. In this paper, we demonstrate that the many-particle exchange-correlation corrections renormalize the energy bands and may stabilize the electron pairs for certain hole and electron-pair densities in the two-dimensional crystal. Therefore, depending upon the crystal potential and free carrier densities, there may exist a new many-particle system consisting of electron pairs and holes. Furthermore, we calculate the charge-density excitation spectrum in this new many-particle system.
We study possible quantum states of two correlated electrons in a two-dimensional periodic potential and find a metastable energy band of electron pairs between the two lowest single-electron bands. These metastable states result from interplay of the electron-electron Coulomb interaction and the strength of the crystal potential. The paired electrons are bound in the same unit cell in relative coordinates with an average distance between them of approximately one third of the crystal period. Furthermore, we discuss how such electron pairs can possibly be stabilized in a many-electron system.
Starting from the shell structure in atoms and the significant correlation within electron pairs, we distinguish the exchange-correlation effects between two electrons of opposite spins occupying the same orbital from the average correlation among many electrons in a crystal. In the periodic potential of the crystal with lattice constant larger than the effective Bohr radius of the valence electrons, these correlated electron pairs can form a metastable energy band above the corresponding single-electron band separated by an energy gap. In order to determine if these metastable electron pairs can be stabilized, we calculate the many-electron exchange-correlation renormalization and the polaron correction to the two-band system with single electrons and electron pairs. We find that the electron-phonon interaction is essential to counterbalance the Coulomb repulsion and to stabilize the electron pairs. The interplay of the electron-electron and electron-phonon interactions, manifested in the exchange-correlation energies, polaron effects, and screening, is responsible for the formation of electron pairs (bipolarons) that are located on the Fermi surface of the single-electron band.
If the bare interaction between two electrons is dressed in the two-dimensional electron gas by the response of the many-body environment, pairing may occur. Here, we study numerically the existence and character of bound states in the case where the dressing in the pair-interaction energy is described by the Overhauser geminal model [A. W. Overhauser, Can. J. Phys. 75, 683 (1995)]. The starting point of the model is the exchange correlation hole around a single electron. We constrain the hole by arguments based on the results for the exchange-correlation energy and the contact density of the pair-correlation function. We find a bound state with the energy minimized at a certain electron gas density. On the basis of the behavior of the binding energy we discuss recent experimental findings for the electron density dependence of the critical temperature in cuprate, Chevrel-phase, and Li-intercalated LixZrNCl{\mathrm{Li}}_{x}\mathrm{Zr}\mathrm{N}\mathrm{Cl}Lix​ZrNCl superconductors.
We show that two tight-binding electrons that repel may form a bounded pair in two dimensions. The paired states form a band with energies that scale like the strength of the interaction potential. By applying an electric field we show that the dynamics of such states is that of a composite particle of charge 2e2e2e. The system still sustains Bloch-type states so that if the two bands overlap single and paired states might coexist allowing for a bosonic fluid component that, if condensed, would decrease the resistance at low temperatures. The presence of two bands allows for new oscillations whose experimental detection would permit a direct measurement of the interaction potential strength.
We study the ``t-J'' model with strongly correlated electrons (infinite-U Hubbard-model kinetic energy plus superexchange J) by exact diagonalization on small two-dimensional clusters of six, eight, and twelve sites on a square lattice with periodic boundary conditions. We find an attractive pairing potential for two holes when J/t\ensuremath{\ge}(J/t)c{)}_{\mathit{c}})c​\ensuremath{\simeq}0.2, with t the hopping matrix element. Ground-state calculations show that extended s-wave and d-wave pairing susceptibilities and equal-time pairing correlations grow weakly with increasing J and system size at fixed filling n. Pairing correlations may be enhanced for (J/t)>(J/t)c{)}_{\mathit{c}})c​\ensuremath{\approxeq}0.2. We compare equal-time pairing correlations to a mean-field theory for J=0 and note potential difficulties with interpreting numerical results for such small systems. The widths of one- and two-hole energy bands in low-spin states are of order J; the one-hole effective mass is larger than the two-hole effective mass in such states. Spin-charge correlation functions for a single hole in the eight- and twelve-site lattices are consistent with the spin-charge decoupling scenario for resonating-valence-bond states.
On the basis of quantum Monte Carlo simulations we study the influence of "breathing" lattice distortions on the two-dimensional Hubbard model. In the half-filled band case the system remains stable against the formation of a charge-density wave already for intermediate values (U=4tU=4tU=4t) of the electronic interaction. However, the addition of holes allows for a charge-density-wave state. Thus, the structural instability is not driven by nesting of the Fermi surface, but by the combined effect of doping and Coulomb correlation. The influence of the distortion on the various pairing correlation functions is calculated.