25 research outputs found
Phase diagram of microcavity exciton-polariton condensates
In this work, we study the exciton-polariton condensate phase transition in a
microcavity matter-light system in which electron-hole Coulomb interaction and
matter-light coupling effects are treated on an equal footing. In the framework
of the unrestricted Hartree-Fock approximation applying the two-dimensional
exciton-polariton model, we derive the self-consistent equations determining
simultaneously the excitonic and the photonic condenstate order parameters. In
the thermal equilibrium limit, we find a condensed state of the
exciton-polariton systems and phase diagrams are then constructed. At a given
low temperature, the condensate by its nature shows a crossover from an
excitonic to a polaritonic and finally photonic condensed state as the
excitation density increases at large detuning. Without the detuning, the
excitonic condensed state disappears whereas the polaritonic or photonic phases
dominate. The crossover is also found by lowering the Coulomb interaction at a
finite matter-light coupling. Lowering the Coulomb interaction or increasing
the temperature, the excitonic Mott transition occurs, at which the
exciton-polariton condensates dissociate to free electron-hole/photon.
Depending on temperature and excitation density, the phase transition of the
exciton-polariton condensates is also addressed in signatures of
photoluminescence mapping to the photonic momentum distribution.Comment: 7 pages, 6 figure
Valence transition and superconductivity in the extended periodic Anderson model
In this thesis, an extended periodic Anderson model with an additional local Coulomb repulsion U f c between localized f electrons and conduction electrons is investigated by use of the projector-based renormalization method (PRM). First, it is shown that the model in one dimension shows a valence transition, which becomes sharper, when the energy of the f level approaches the Fermi level. The transition becomes also enhanced, when the hybridization V between the localized and conduction electrons decreases, for the case that the total number of electrons is fixed. In the two-dimensional case, one finds a similar valence transition behavior. However, in the valence transition regime also a superconducting phase may occur. To investigate this phase, we start from an Hamiltonian which includes small gauge symmetry breaking fields. We derive renormalization equations, from which the superconducting pairing functions are self-consistently determined. Our analytical and numerical results show that d- wave superconductivity becomes dominant in the valence transition regime. This confirms the suggestion by Miyake that valence fluctuations may lead to superconductivity in the Ce based heavy-fermion systems under high pressure.In dieser Arbeit wird mit Hilfe der projektiven Renormierungsmethode (PRM) ein erweitertes periodische Anderson Modell untersucht, das zusätzlich eine Coulomb-Abstoßung zwischen den lokalisierten f-Elektronen und den Leitungselektronen enthält. In einer Dimension zeigt das Modell einen Valenzübergang, wenn sich die Energie des f-Niveaus der Fermienergie nähert. Der Übergang wird ebenfalls schärfer, wenn bei festgehaltener Gesamtelektronenzahl die Hybridisierung V zwischen den lokalisierten und den Leitungselekronen abnimmt. In zwei Dimensionen findet man ein ähnliches Valenzübergangsverhalten. Allerdings kann zusätzlich eine supraleitende Phase im Valenzübergangsgebiet auftreten. Um die supraleitende Phase zu untersuchen, betrachten wir einen Hamiltonoperator mit kleinen zusätzlichen Feldern, die die Eichsymmetrie brechen. Wir leiten Renormierungsgleichungen her, aus denen sich die supraleitenden Paarfunktionen selbstkonsistent bestimmen lassen. Unsere analytischen und numerischen Resultate zeigen, dass im Valenzübergangsgebiet d-Wellen-Supraleitung dominiert. Dies bestätigt eine Vermutung von Miyake, dass Valenzfluktuationen in Ce-basierten Schwerfermionensystemen bei hohen Drücken zur Supraleitung führen können
Doping change and distortion effect on double-exchange ferromagnetism
Doping change and distortion effect on the double-exchange ferromagnetism are
studied within a simplified double-exchange model. The presence of distortion
is modelled by introducing the Falicov-Kimball interaction between itinerant
electrons and classical variables. By employing the dynamical mean-field theory
the charge and spin susceptibility are exactly calculated. It is found that
there is a competition between the double-exchange induced ferromagnetism and
disorder-order transition. At low temperature various long-range order phases
such as charge ordered and segregated phases coexist with ferromagnetism
depending on doping and distortion. A rich phase diagram is obtained.Comment: 8 pages, 8 figure
Excitonic resonances in the 2D extended Falicov-Kimball model
Using the projector-based renormalization method we investigate the formation
of the excitonic insulator phase in the two-dimensional (2D) spinless
Falicov-Kimball model with dispersive electrons and address the existence
of excitonic bound states at high temperatures on the semiconductor side of the
semimetal-semiconductor transition. To this end we calculate the imaginary part
of the dynamical electron-hole pair susceptibility and analyze the wave-vector
and energy dependence of excitonic resonances emerging in the band gap. We
thereby confirm the existence of the exciton insulator and its exciton
environment within a generic two-band lattice model with local Coulomb
attraction.Comment: 6 pages, 5 figures, final versio
Transport properties in Simplified Double Exchange model
Transport properties of the manganites by the double-exchange mechanism are
considered. The system is modeled by a simplified double-exchange model, i.e.
the Hund coupling of the itinerant electron spins and local spins is simplified
to the Ising-type one. The transport properties such as the electronic
resistivity, the thermal conductivity, and the thermal power are calculated by
using Dynamical mean-field theory. The transport quantities obtained
qualitatively reproduce the ones observed in the manganites. The results
suggest that the Simplified double exchange model underlies the key properties
of the manganites.Comment: 5 pages, 5 eps figure