Bound Electron Pairs Research Articles

Superfluid Precursor Effects in a Model of Hybridized Bosons and Fermions.

We examine how a superfluid state is approached in a system of localizedbosons (tightly bound electron pairs) in contact with a reservoir of itinerantfermions (electrons). Assuming spontaneous decay and recombination betweenthese two species, the initially localized states of the bosons change overinto free-particle--like propagating statesas the temperature is lowered and the superfluid transition at ${T}_{c}$ is approached. Concomitantly a pseudogap opens up in the fermionic density of states which deepens with decreasing temperature.

Bound electron pairs in two dimensions at quantizing magnetic fields

We use the exact diagonalization of the Hamiltonian to study the ground state of a double-layer system in which a higher Landau level in one layer is degenerate with the ground Landau level in the other. It is shown that electrons form bound pairs over a wide range of layer separations and filling factors. The binding is due to the attraction between an electron in the low-density layer and the interior of the larger cyclotron orbit in the high-density layer.

Evolution from BCS superconductivity to Bose condensation: Role of the parameter kF xi.

We argue that the natural variable to establish the crossover from Cooper-pair-based superconductivity to Bose-Einstein condensation of bound-electron pairs is the product ${\mathit{k}}_{\mathit{F}}$\ensuremath{\xi} of Fermi electron wave vector times the coherence length for two-electron correlation, and that Cooper-pair-based superconductivity is stable against bosonization down to ${\mathit{k}}_{\mathit{F}}$\ensuremath{\xi}\ensuremath{\simeq}2\ensuremath{\pi}. We also propose that the experimental plot by Uemura et al., relating the superconducting temperature ${\mathit{T}}_{\mathit{c}}$ to the Fermi temperature ${\mathit{T}}_{\mathit{F}}$ for a variety of ``exotic'' as well as conventional superconductors, can be rationalized by correlating the ratio ${\mathit{T}}_{\mathit{c}}$/${\mathit{T}}_{\mathit{F}}$ to ${\mathit{k}}_{\mathit{F}}$\ensuremath{\xi}. It appears in this way that the high-${\mathit{T}}_{\mathit{c}}$ superconductors lie in the plot by Uemura et al. near the ``instability'' line ${\mathit{k}}_{\mathit{F}}$\ensuremath{\xi}=2\ensuremath{\pi}.

Pairing ground states in fermion systems: A static method.

We present a static recursive method that can straightforwardly be applied to variational treatments of canonical many-body systems whose ground states generally exhibit pairing or higher clustering correlations. We illustrate the utility of the method by applying it to an electron system with pairing interactions where we obtain, as expected, a smooth transition between a Bose condensate of tightly bound electron pairs and a superconducting ground state. The method also reproduces the mean-field results for the electron-hole liquid problem as well as the antiferromagnetic insulator ground state of the Hubbard model but in a simpler way. We give a nontrivial application to a model of dense molecular hydrogen consisting of condensed generalized molecules. We show that the effective interaction between identical particles is of such a form that the state undergoes a dielectric catastrophe at a certain finite density. The competition between this abrupt transition and a smooth transition of atomic pairs in the spinless and translationally invariant version of the problem is also discussed.

Superconductivity in narrow-band systems with local nonretarded attractive interactions

In narrow-band systems electrons can interact with each other via a short-range nonretarded attractive potential. The origin of such an effective local attraction can be polaronic or it can be due to a coupling between electrons and excitons or plasmons. It can also result from purely chemical (electronic) mechanisms, especially in compounds with elements favoring disproportionation of valent states. These mechanisms are discussed and an exhaustive list of materials in which such local electron pairing occurs is given. The authors review the thermodynamic and electromagnetic properties of such systems in several limiting scenarios: (i) Systems with on-site pairing which can be described by the extended negative-$U$ Hubbard model. The strong-attraction limit of this model, at which it reduces to a system of tightly bound electron pairs (bipolarons) on a lattice, is extensively discussed. These electron pairs behaving as hard-core charged bosons can exhibit a superconducting state analogous to that of superfluid $^{4}\mathrm{He}$ II. The change-over from weak-attraction BCS-like superconductivity to the superfluidity of charged hard-core bosons is examined. (ii) Systems with intersite pairing described by an extended Hubbard model with $Ug0$ and nearest-neighbor attraction and/or nearest-neighbor spin exchange as well as correlated hopping. (iii) A mixture of local pairs and itinerant electrons interacting via a charge-exchange mechanism giving rise to a mutually induced superconductivity in both subsystems. The authors discuss to what extent the picture of local pairing, and in particular superfluidity of hard-core charged bosons on a lattice, can be an explanation for the superconducting and normal-state properties of the high-${T}_{c}$ oxides: doped BaBi${\mathrm{O}}_{3}$ and the cuprates.

Structure of K+(cryptand[2.2.2J) electride and evidence for trapped electron pairs

Electrides are crystalline salts in which stoichiometric amounts of trapped or itinerant electrons serve as the anions1–8. The cations are alkali metal cations complexed by cyclic or bicyclic polyethers of the crown ether9 or cryptand10 classes. Optical spectra, powder conductivities and magnetic susceptibilities show that, in most electrides, individual electron localization occurs, presumably centred at the anionic sites, with trap depths of 0.5–1.0 eV. An exception is K+(cryptand[2.2.2]) ˙e-, which has a plasma-like optical absorption spectrum2, high microwave conductivity, a low activation energy for direct current powder conductivity (∼0.02 eV), and a weak, temperature-dependent electronic paramagnetic susceptibility. The crystal structure of this electride shows the presence of large (4×6×12A) vacancies of complex shape, interconnected in two directions by zigzag channels, but blocked in the third direction. The structure and properties suggest the presence of weakly bound electron pairs.

Bound Electron Pairs in Metal–Ammonia Solutions: Self-Consistent Local Field and Correlation Energy

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Inelastic electron scattering from organic compounds

AbstractIn calculations of possible superconductivity in a system, knowledge of certain matrix elements determines whether or not bound electron pairs can exist and thus lead to superconductivity. These matrix elements involve ground and excited state wave functions. We are presently attempting to experimentally determine such matrix elements in certain organic compounds by measuring differential cross sections for inelastic electron scattering. Such cross sections involve the square of the matrix elements of interest. To do this we are using a crossed beam technique with an electrostatic electron spectrometer for the production of the incident electron beam and analysis of the scattered beam. The apparatus is described, as well as the present status of the experiment.