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Continuum of many-particle states near the metal-insulator transition in the Hubbard model
The strong coupling diagram technique is used for investigating states near
the metal-insulator transition in the half-filled two-dimensional repulsive
Hubbard model. The nonlocal third-order term is included in the irreducible
part along with local terms of lower orders. Derived equations for the electron
Green's function are solved by iteration for moderate Hubbard repulsions and
temperatures. Starting iteration from Green's functions of the Hubbard-I
approximation with various distances of poles from the real frequency axis
continua of different metallic and insulating solutions are obtained. The
insulating solutions vary in the width of the Mott gap, while the metallic
solutions differ in the shape of the spectral function in the vicinity of the
Fermi level. Besides, different scenarios of the metal-insulator transition --
with a sudden onset of a band of mobile states near the Fermi level and with
gradual closure of the Mott gap -- are observed with a change in temperature.
In spite of these dissimilarities, all solutions have a common curve separating
metallic and insulating states in the phase diagram. Near this curve metallic
and insulating solutions coexist. For moderate Hubbard repulsions metallic
solutions are not Fermi liquids.Comment: 10 pages, 9 figure
Cooling by conversion of para to ortho-hydrogen
The cooling capacity of a solid hydrogen cooling system is significantly increased by exposing vapor created during evaporation of a solid hydrogen mass to a catalyst and thereby accelerating the endothermic para-to-ortho transition of the vapor to equilibrium hydrogen. Catalyst such as nickel, copper, iron or metal hydride gels of films in a low pressure drop catalytic reactor are suitable for accelerating the endothermic para-to-ortho conversion
Magnetic properties and temperature variation of spectra in the Hubbard model
Using the strong coupling diagram technique, magnetic and spectral properties
of the two-dimensional repulsive Hubbard model are investigated in the ranges
of repulsions , temperatures and
electron concentrations with the hopping
constant. The approach takes into account interactions of electrons with spin
and charge fluctuations of all ranges and fulfils the Mermin-Wagner theorem.
Temperature and concentration dependencies of the uniform magnetic
susceptibility, the variation of the double occupancy with the repulsion and
the temperature dependence of the square of the site spin are in satisfactory
agreement with Monte Carlo results. Three types of the temperature variation of
the electron energy spectrum can be distinguished at half-filling. For
, at low temperatures, there are two nonintersecting bands, which
approach each other on the boundary of the magnetic Brillouin zone. With
increasing these bands merge into one band crossing the Fermi level. For
, the low-temperature picture described above is
supplemented with a low-intensity spin-polaron band located near the Fermi
level. As its counterpart in the strong-correlation case, the band is formed by
bound states of electrons and spin excitations. However, in contrast to the
former case, the band exists even at half-filling and occupies the entire
Brillouin zone. As for lower , with increasing temperature, all bands
coalesce into a single band. For and low temperatures the
spectrum has a pronounced four-band structure, which with increasing
transforms into two Hubbard subbands.Comment: 12 pages, 14 figure
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