Continuous canonical transformation for the double exchange model

2010

Phys. Rev. B

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The intrinsic structural metastability in cuprate high-T c materials, evidenced in a checkerboard domain structure of the CuO 2 planes, locally breaks translational and rotational symmetry. Dynamical chargedeformation fluctuations of such nanosize unidirectional domains, involving Cu-O-Cu molecular bonds, result in resonantly fluctuating diamagnetic pairs embedded in a correlated Fermi liquid. As a consequence, the single-particle spectral properties acquire simultaneously ͑i͒ fermionic low-energy Bogoliubov branches for propagating Cooper pairs and ͑ii͒ bosonic localized glassy structures for tightly bound states of them at high energies. The partial localization of the single-particle excitations leads to a fractionation of the Fermi surface as the strength of the exchange coupling between itinerant fermions and partially localized fermion pairs increases upon moving from the nodal to the antinodal point. This is also the reason why bound fermion pairs accumulate near the antinodal points and thereby control the doping dependence of the cuprates upon approaching the singular universal optimal doping rate.

Section: Discussionmentioning

confidence: 99%

Section: The Boson-fermion Dualitymentioning

confidence: 99%

Boson-fermion duality and metastability in cuprate superconductors

Ranninger

2010

Phys. Rev. B

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“…Out of 8 possible configurations being a product of the fermionic states |0 >, | ↑>, | ↓>, | ↑↓> and the hard-core bosonic ones |0), |1) two of them ↑↓> ⊗|0) and |0 > ⊗|1) get mixed due to the Andreev-like interaction. A complete set of the eigenstates can be obtained using the transformation [13] …”

mentioning

confidence: 99%

Electron pair current through the correlated quantum dot

Physica Status Solidi (b)

Donabidowicz

2009

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We study the charge current transmitted through the correlated quantum dot characterized by a finite magnitude of the Coulomb interaction |U|. At low temperatures the correlations can lead to appearance of the spin (for U>0) or charge (for U<0) Kondo states qualitatively affecting the transport properties. We explore an influence of the charge Kondo effect on the electron pair tunneling introducing the auxiliary two-component model which describes the fluctuations between empty and doubly occupied states of the quantum dot.Comment: 4 pages, 2 figure

Renormalization Group Approach for the Double Exchange Ferromagnets

Zapalska

2012

Acta Phys. Pol. A

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The discovery of the colossal magnetoresistance (CMR) in the manganese oxides with perovskite structures T1−xDMnO3 (T = La, Pr, Nd; D=Sr, Ca, Ba, Pb) and its potential technological application motivated theoretical and experimental researchers to study the itinerant ferromagnetism. A rst theoretical description of this phenomenon in terms of the double-exchange mechanism was given a long time ago by Zener. In this model, the spin orientation of adjacent Mn-moments is associated with kinetic exchange of conduction eg electrons.Consequently, alignment of the core Mn-spins by an external magnetic eld causes higher conductivity. The Mn ions are considered as localized forming a spin of S = 3 2 and they are coupled to the itinerant electrons by a strong ferromagnetic Hund coupling, JH > 0. We apply the ow equation technique (nonperturbative method, based on continuous canonical transformation) to the double-exchange model for ferromagnetism described by the Kondo type Hamiltonian. We want to eliminate the interaction term responsible for non-conservation of magnon number and to take into account fermion and magnon degrees of freedom. We express the spin operators of Mn ions via the magnon operators (the HolsteinPrimako transformation) and investigate the magnon excitation spectrum determined by Green's function.