15,937 research outputs found
Quantum properties of two-dimensional electron gas in the inversion layer of Hg1âxCdxTe bicyrstals
The electronic and magnetotransport properties of conduction electrons in the grain boundary interface of p-type Hg1âxCdxTe bicrystals are investigated. The results clearly demonstrate the existence of a two-dimensional degenerate n-type inversion layer in the vicinity of the grain boundary. Hydrostatic pressure up to 103 MPa is used to characterize the properties of the two-dimensional electron gas in the inversion layer. At atmospheric pressure three series of quantum oscillations are revealled, indicating that tthree electric subbands are occupied. From quantum oscilations of the magnetoresistivity the characteristics parameters of the electric subbands (subband populations nsi, subband energies EFâEi, effective electron masses m*ci) and their pressure dependences are established. A strong decrease of the carrier concentration in the inversion layer and of the corresponding subband population is observed when pressure is applied A simple theoretical model based on the triangular-well approximation and taking into account the pressure dependence of the energy band structure of Hg1âxCdxTe is use to calculate the energy band diagram of the quantum well and the pressure dependence of the subband parameters
Optimization of alloy-analogy-based approaches to the infinite-dimensional Hubbard model
An analytical expression for the self-energy of the infinite-dimensional
Hubbard model is proposed that interpolates between different exactly solvable
limits. We profit by the combination of two recent approaches that are based on
the alloy-analogy (Hubbard-III) solution: The modified alloy-analogy (MAA)
which focuses on the strong-coupling regime, and the Edwards-Hertz approach
(EHA) which correctly recovers the weak-coupling regime. Investigating the
high-energy expansion of the EHA self-energy, it turns out that the EHA
reproduces the first three exactly known moments of the spectral density only.
This may be insufficient for the investigation of spontaneous magnetism. The
analysis of the high-energy behavior of the CPA self-consistency equation
allows for a new interpretation of the MAA: The MAA is the only (two-component)
alloy-analogy that correctly takes into account the first four moments of the
spectral density. For small U, however, the MAA does not reproduce Fermi-liquid
properties. The defects of the MAA as well as of the EHA are avoided in the new
approach. We discuss the prospects of the theory and present numerical results
in comparison with essentially exact quantum Monte Carlo data. The correct
high-energy behavior of the self-energy is proved to be a decisive ingredient
for a reliable description of spontaneous magnetism.Comment: LaTeX, 18 pages, 12 eps figures include
On the magnetic stability at the surface in strongly correlated electron systems
The stability of ferromagnetism at the surface at finite temperatures is
investigated within the strongly correlated Hubbard model on a semi-infinite
lattice. Due to the reduced surface coordination number the effective Coulomb
correlation is enhanced at the surface compared to the bulk. Therefore, within
the well-known Stoner-picture of band ferromagnetism one would expect the
magnetic stability at the surface to be enhanced as well. However, by taking
electron correlations into account well beyond the Hartree-Fock (Stoner) level
we find the opposite behavior: As a function of temperature the magnetization
of the surface layer decreases faster than in the bulk. By varying the hopping
integral within the surface layer this behavior becomes even more pronounced. A
reduced hopping integral at the surface tends to destabilize surface
ferromagnetism whereas the magnetic stability gets enhanced by an increased
hopping integral. This behavior represents a pure correlation effect and can be
understood in terms of general arguments which are based on exact results in
the limit of strong Coulomb interaction.Comment: 6 pages, RevTeX, 4 eps figures, accepted (Phys. Rev. B), for related
work and info see http://orion.physik.hu-berlin.d
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