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physica status solidi (b)

Theoretical Study of Hole-Pair System in a Periodical Lattice

Oracio Navarro, Ilya G. Kaplan

Published Jul 1, 2000• 1 citation
Publisher

Abstract

We investigate the statistics of the coupled hole pairs in a crystal lattice in high Tc superconductors, following the previously developed approach for the collective charge state (holon). The exact commutation relations for the hole pair operators correspond to a modified parafermi statistics of rank M′ (M′ is the number of sites in a “superlattice” formed by centers of mass for each hole pair), i.e. one state can be occupied by up to M′ pairs. Even in the absence of dynamic interaction, the system of hole pairs is characterized by some immanent interaction (kinematic interaction), which depends on the hole-pairs concentration. In spite of the kinematic interaction, there is no statistical prohibition on the Bose-Einstein condensation of coupled hole pairs.

DOI
10.1002/1521-3951(200007)220:1<493::aid-pssb493>3.0.co;2-d

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•Jan 1, 2008•13 citations
We investigate the statistics of the coupled hole pairs in a crystal lattice in high Tc superconductors, following the previously developed approach for the collective charge state (holon). The exact commutation relations for the hole pair operators correspond to a modified parafermi statistics of rank M′ (M′ is the number of sites in a “superlattice” formed by centers of mass for each hole pair), i.e. one state can be occupied by up to M′ pairs. Even in the absence of dynamic interaction, the system of hole pairs is characterized by some immanent interaction (kinematic interaction), which depends on the hole-pairs concentration. In spite of the kinematic interaction, there is no statistical prohibition on the Bose-Einstein condensation of coupled hole pairs.
The collective charge state (holon) in a periodical lattice is defined. The exact commutation relations for the holon operators correspond to the modified para-Fermi statistics of rank M ( M is the number of lattice sites at which the hole can be created), i.e. one state can be occupied by up to M holons. At small concentration, the holon gas can be treated as a bosonic system. In the general case, the holon gas is characterized by the immanent interaction and coupling among the different holon band states, even in the absence of dynamic interactions (the so-called kinematic interaction). In spite of the kinematic interaction, there is no statistical prohibition of Bose-Einstein condensation in the holon system. The possible extension of the approach developed to the coupled hole pairs in high- T c superconductors is discussed.
Abstract The Cooper pair has the total spin S = 0. So, in accordance with the Pauli principle, the wave functions describing the Cooper pair system should have the boson permutation symmetry, but the pairon operators (Cooper's pair operators) do not obey the boson commutation relations. The pairon operators may not be considered neither as the Bose operators, nor as the Fermi operators. In this work, we analyze the statistical properties and the commutation relations for the pairon operator and reveal that they correspond to the modified parafermi statistics of rank p = 1. Two different expressions for the Cooper pair number operator are presented. We demonstrate that the calculations with a Hamiltonian expressed via pairon operators is more convenient using the commutation properties of these operators without presenting them as a product of fermion operators. This allows to study problems in which the interactions between Cooper's pair are also included. The problem of two interacting Cooper's pairs is discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Quantum field-theory is developed for treating electrons and holes in planar systems. Non-unitary representations of Dirac equation in the plane are developed. These equations can be used for treating holes-electron pairing in high TC materials where in a non-relativistic approximation we have a relatively small energy band gap. It is shown that while holes or electrons satisfy separately Pauli-exclusion principle, with anti-commutation relations, the holes-electron pairs follow boson commutation relations. The distribution for electrons-holes pairs in planar systems is calculated for the case in which scattering effects between different pairs can be neglected. It is shown that for high surface density in which many pairs are included in an area with wave length dimension, Bose-Einstein condensation can occur. The ground state of the Bose condensation is calculated and the calculations are compared with those of the BCS theory. The physical implications to high TC currents in planar systems are discussed. The present model is related to the chemistry of high-TC materials. PACS: 03.65.
Unification of the BCS and the Bose-Einstein condensation (BEC) theories is surveyed in detail via a generalized BEC (GBEC) finite-temperature statistical formalism. Its major difference with BCS theory is that it can be diagonalized exactly. Under specified conditions it yields the precise BCS gap equation for all temperatures as well as the precise BCS zero-temperature condensation energy for all couplings, thereby suggesting that a BCS condensate is a BE condensate in a ternary mixture of kinematically independent unpaired electrons coexisting with equally proportioned weakly-bound two-electron and two-hole Cooper pairs. Without abandoning the electron-phonon mechanism in moderately weak coupling it suffices, in principle, to reproduce the unusually high values of Tc (in units of the Fermi temperature TF) of 0.01-0.05 empirically reported in the so-called "exotic" superconductors of the Uemura plot, including cuprates, in contrast to the low values of Tc/TF ≤ 10-3 roughly reproduced by BCS theory for conventional (mostly elemental) superconductors. Replacing the characteristic phonon-exchange Debye temperature by a characteristic magnon-exchange one more than twice in size can lead to a simple interaction model associated with spin-fluctuation-mediated pairing.
Although BCS pairs of fermions are known not to obey Bose-Einstein (BE) commutation relations nor BE statistics, we show how Cooper pairs (CPs), whether the simple original ones or the CPs recently generalized in a many-body Bethe-Salpeter approach, being clearly distinct from BCS pairs at least obey BE statistics. Hence, contrary to widespread popular belief, CPs can undergo BE condensation to account for superconductivity if charged, as well as for neutral-atom fermion superfluidity where CPs, but uncharged, are also expected to form. Keywords: Cooper pairs; Bose-Einstein condensation; superconductors; superfluids 1