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Nonfrustrated Antiferromagnet in a Frustrated Lattice Due to Charge Ordering: A Scenario for Pairing in Layered Cobaltates

100%
Wróbel P., Suleja W.
Acta Physica Polonica A
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2007
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vol. 111
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issue 4
563-572
EN
The mechanism of pairing-symmetry selection in the weakly electron doped t-J model on the honeycomb lattice has been analyzed. The discussion of that problem has been motivated by some recent suggestions that due to charge ordering which may take place in the unconventional superconductor Na_xCoO_2· yH_2O at doping levels near x=1/3 the physics of CoO_2 planes may be effectively described in terms of a model for a weakly electron doped antiferromagnet on the honeycomb lattice. In the current publication the main emphasis has been put on reviewing experimental and theoretical work, the results of which support the scenario of charge order. In the calculation, the so-called string picture has been used. It has been demonstrated that spin fluctuations may induce in the honeycomb lattice the formation of an unconventional two-particle bound state. Upon the condensation of bound particles this mechanism may give rise to unconventional pairing. The critical value of the ratio J/t which is sufficient to induce binding has not been evaluated. It has been assumed instead that in the case of cobaltates some additional isotropic attractive interaction, for example phonon mediated, is active. Despite that the exchange of spin fluctuations is not a dominating interaction, it selects the symmetry of the paired state because it is anisotropic. C_{3v} is the relevant point group for the t-J model on the honeycomb lattice. It has been shown that the bound state of two additional electrons doped to the half-filled antiferromagnetically ordered system has zero total momentum and p-wave symmetry of the irreducible representation E. The expected paired state is a mixture of a singlet and a triplet because the honeycomb lattice does not possess the inversion symmetry.
2
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Pairing-Symmetry Selection in a Weakly Doped Canted Antiferromagnet on the Triangular Lattice

100%
Wróbel P., Suleja W.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
225-228
EN
The mechanism of superconductivity generation by spin fluctuations in the electron doped canted antiferromagnet on the triangular lattice was analyzed. The underlying assumption is that the formation of the bound state is the prerequisite of pairing. The outcome of this analysis is also valid if an additional isotropic attraction is active but the anisotropic spin-fluctuation mediated force decides on the symmetry of the two-particle bound state. When the canted antiferromagnetic state is generated, the symmetry of the point group C_{6v} for the triangular lattice is lowered to the symmetry of C_{3v}. It is demonstrated that spin fluctuations definitely favor the p-wave bound state, which transforms according to the E representation of C_{3v}. Since the inversion is not an element of C_{3v}, the parity is not a good quantum number and thus the predicted paired state will be a mixture of singlet and triplet. Such a scenario may be relevant to physics of superconducting triangular cobaltates or organics.
3
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Spin Polarons in Weakly Doped Antiferromagnets: Experimental Evidence Obtained for Cuprates

81%
Wróbel P., Suleja W., Eder R.
Acta Physica Polonica A
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2008
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vol. 114
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issue 1
51-58
EN
The mechanism of spin polaron formation in moderately doped cuprates is discussed. These objects represent holes embedded into heavy clouds formed by spin fluctuations. Wave functions of spin polarons are spatially confined due to the increase in the exchange energy which is induced by hole motion giving rise to the creation of spin fluctuations. These wave functions are eigenstates of an "unperturbed" Hamiltonian which is defined by processes responsible for the tendency toward confinement. The eigenstates transform according to different irreducible representations of the point group reflecting the symmetry of the problem. Thus, the spin polarons being local wave functions resemble orbital states. The spectrum of optical conductivity in the mid-infrared range is determined by transitions between s-wave and p-wave spin polarons. The hybridization between different spin polarons which is induced by some high order processes gives rise to the formation of energy bands. The pronounced transfer of the spectral weight between different bands is induced by the coupling between spin fluctuations created by the hopping hole and local quantum fluctuations in the empty antiferromagnetic background from which an electron has been rejected, for example during the photoemission process. The form of the energy dispersion for spin polarons gives rise to the formation of a small Fermi surface at the low hole-doping range. These three above mentioned phenomena were observed in cuprates which seems to confirm the spin polaron scenario. The discussion of related experiments is the additional objective of this paper.
4
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Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

71%
Wróbel P., Maciąg A., Suleja W., Eder R., Micnas R.
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issue 2
292-298
EN
We argue that three gaps observed in underdoped cuprates can be attributed to the formation of antiferromagnetic spin polarons and bipolarons. Within the spin polaron scenario the antinodal pseudogap at he high energy scale originates from the change of the Fermi surface topology, induced by antiferromagnetic correlations. That change gives rise to the diminishing of the spectral weight at the antinodal region near the Brillouin zone boundary. We demonstrate that effect by analyzing effective models of doped antiferromagnets. The second type of pseudogap appearing at the intermediate energy scale originates from the phenomena which are precursory to superconductivity and predominantly concern the portion of the Fermi surface near the nodal region. In order to analyze the latter phenomenon we use the negative U Hubbard model, in which many details typical to spin polaron physics are neglected, but which contains the essential ingredient of it, that is the strong short range attraction. The lowest energy scale is related to the true superconducting gap which develops with doping, although both types of pseudogap diminish with doping. This behavior can be explained by the fact that the spin polaron band is empty in the undoped system and therefore the formation of the superconducting state in the system is forbidden. Due to a pedagogical character of this report, we present in the introduction a short overview of mostly recent experimental results which are related to the gap-pseudogap physics.
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