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ABSTRACT: Recent experiments on the alkali-intercalated iron selenides have raised
questions about the symmetry of the superconducting phase. Random phase
approximation calculations of the leading pairing eigenstate for a tight-
binding 5-orbital Hubbard-Hund model of AFe2Se2 find that a d-wave (B1g) state
evolves into an extended s{\pm} (A1g) state as the system is hole-doped.
However, over a range of doping these two states are nearly degenerate. Here,
we calculate the imaginary part of the magnetic spin susceptibility
\chi"(q,{\omega}) for these gaps and discuss how the evolution of neutron
scattering resonances can distinguish between them.
06/2012;
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ABSTRACT: We introduce an effective low-energy pairing model for Fe-based superconductors with s- and d-wave interaction components and a small number of input parameters and use it to study the doping evolution of the symmetry and the structure of the superconducting gap. We argue that the model describes the entire variety of pairing states found so far in the Fe-based superconductors and allows one to understand the mechanism of the attraction in s(±) and d(x(2)-y(2)) channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.
Physical Review Letters 09/2011; 107(14):147002. · 7.37 Impact Factor
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ABSTRACT: We present a detailed study of the symmetry and structure of the pairing gap
in Fe-based superconductors (FeSC). We treat FeSC as quasi-2D, decompose the
pairing interaction in the XY plane in s-wave and d-wave channels into
contributions from scattering between different Fermi surfaces and analyze how
each scattering evolves with doping and input parameters. We verify that each
interaction is well approximated by the lowest angular harmonics. We use this
simplification to analyze the interplay between the interaction with and
without spin-fluctuation components, the origin of the attraction in the
s+/-and d_{x2-y2} channels, the competition between them, the angular
dependence of the s+/- gaps along the electron Fermi surface, the conditions
under which s+/- gap develops nodes, and the origin of superconductivity in
heavily electron- or hole-doped systems, when only Fermi surfaces of one type
are present. We also discuss the relation between RPA and RG approaches for
FeSC.
09/2011;
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ABSTRACT: The bilayer Hubbard model with an intra-layer hopping $t$ and an inter-layer
hopping $t_\perp$ provides an interesting testing ground for several aspects of
what has been called unconventional superconductivity. One can study the type
of pair structures which arise when there are multiple Fermi surfaces. One can
also examine the pairing for a system in which the structure of the
spin-fluctuation spectral weight can be changed. Using a dynamic cluster
quantum Monte Carlo approximation, we find that near half-filling, if the
splitting between the bonding and anti-bonding bands $t_\perp/t$ is small, the
gap has $B_{1g}$ ($d_{x^2-y^2}$-wave) symmetry but when the splitting becomes
larger, $A_{1g}$ ($s^\pm$-wave) pairing is favored. We also find that in the
$s^\pm$ pairing region, the pairing is driven by inter-layer spin fluctuations
and that $T_c$ is enhanced.
07/2011;
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ABSTRACT: We study the symmetry and the structure of the gap in Fe-based
superconductors by decomposing the pairing interaction obtained in the RPA into
s- and d-wave components and into contributions from scattering between
different Fermi surfaces. We show that each interaction is well approximated by
the lowest angular harmonics and use this simplification to analyze the origin
of the attraction in the two channels, the competition between s- and d-wave
solutions, and the origin of superconductivity in heavily doped systems, when
only electron or only hole pockets are present.
04/2011;
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ABSTRACT: Neutron spin-flip scattering observations of a resonance in the
superconducting state is often taken as evidence of an unconventional
superconducting state in which the gap changes sign $\Delta(k+Q)=-\Delta(k)$
for momentum transfers $Q$ which play an important role in the pairing.
Recently questions regarding this identification for the Fe-pnictide
superconductors have been raised and it has been suggested that
$\Delta(k+Q)=\Delta(k)$. Here we propose that inelastic neutron or x-ray
scattering measurements of the spectral weight of a phonon of momentum $Q$ can
distinguish between these two pairing scenarios.
03/2011;
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ABSTRACT: Angle-resolved photoemission spectroscopy measurements on the recently
discovered superconduc- tors in the KFe2Se2 family with critical temperatures
up to - 33K suggest that no Fermi pockets of hole character centered on the
{\Gamma} point of the Brillouin zone are present, in contrast to all other
known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation
exchange approximation and a 5-orbital tight-binding description of the band
structure, we calculate the effective pairing interaction. We find that the
pairing state in this system is most likely to have d-wave symmetry due to pair
scattering between the remaining electron Fermi pockets at wave vector q -
({\pi}, {\pi}), but without any symmetry-imposed nodes for the given Fermi
surface. We propose experimental tests of this result, including the form of
the resonance spectrum probed by inelastic neutron scattering.
01/2011;
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ABSTRACT: Using dynamic cluster quantum Monte Carlo simulations, we study the superconducting behavior of a 1/8 doped two-dimensional Hubbard model with imposed unidirectional stripelike charge-density-wave modulation. We find a significant increase of the pairing correlations and critical temperature relative to the homogeneous system when the modulation length scale is sufficiently large. With a separable form of the irreducible particle-particle vertex, we show that optimized superconductivity is obtained for a moderate modulation strength due to a delicate balance between the modulation enhanced pairing interaction, and a concomitant suppression of the bare particle-particle excitations by a modulation reduction of the quasiparticle weight.
Physical Review Letters 06/2010; 104(24):247001. · 7.37 Impact Factor
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ABSTRACT: Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^\pm$). In particular we focus on the role of the hole pocket at the $(\pi,\pi)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {\it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^\pm$ state. The pocket's contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap.
03/2010;
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ABSTRACT: Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective 5-orbital tight-binding fit of the full DFT band structure for BaFeAs including the kz dispersions. We compare the 5-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the fluctuation exchange approximation to determine the leading pairing instability, we then examine the differences between a strictly two dimensional model calculation over a single kz cut of the BZ and a completely three dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheets at kz = 0 which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ. Comment: 12 pages, 15 figures
02/2010;
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ABSTRACT: Using dynamic cluster quantum Monte Carlo simulations, we study the superconducting behavior of a 1/8 doped two-dimensional Hubbard model with imposed uni-directional stripe-like charge density wave modulation. We find a significant increase of the pairing correlations and critical temperature relative to the homogeneous system when the modulation length-scale is sufficiently large. With a separable form of the irreducible particle-particle vertex, we show that optimized superconductivity is obtained for moderate modulation strength due to a delicate balance between the modulation enhanced pairing interaction, and a concomitant suppression of the bare particle-particle excitations by a modulation reduction of the quasi-particle weight. Comment: published version
12/2009;
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ABSTRACT: We present a numerical solution of the parquet approximation, a conserving diagrammatic approach which is self-consistent at both the single-particle and the two-particle levels. The fully irreducible vertex is approximated by the bare interaction thus producing the simplest approximation that one can perform with the set of equations involved in the formalism. The method is applied to the Hubbard model on a half-filled 4x4 cluster. Results are compared to those obtained from determinant quantum Monte Carlo (DQMC), FLuctuation EXchange (FLEX), and self-consistent second-order approximation methods. This comparison shows a satisfactory agreement with DQMC and a significant improvement over the FLEX or the self-consistent second-order approximation.
Physical Review E 10/2009; 80(4 Pt 2):046706. · 2.26 Impact Factor
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ABSTRACT: We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA spin-fluctuation and functional renormalization group calculations and show how the simple arguments leading to isotropic sign-switched s-wave states in these systems need to be supplemented by a consideration of pair scattering within Fermi surface sheets and between the individual electron sheets as well. In addition, accounting for the orbital makeup of the states on the Fermi surface is found to be crucial. Comment: 6 pages, 7 figures
03/2009;
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ABSTRACT: Using a dynamical cluster quantum Monte Carlo approximation, we investigate the effect of local disorder on the stability of d-wave superconductivity including the effect of electronic correlations in both particle-particle and particle-hole channels. With increasing impurity potential, we find an initial rise of the critical temperature due to an enhancement of antiferromagnetic spin correlations, followed by a decrease of Tc due to scattering from impurity-induced moments and ordinary pair breaking. We discuss the weak initial dependence of Tc on impurity concentration found in comparison to experiments on cuprates.
Phys. Rev. B. 03/2009; 79(10).
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ABSTRACT: The existence of a neutron scattering resonance at a wavevector q* implies a sign change of the gap between two Fermi surface regions separated by wavevector q* . For the Fe pnictides, a resonance has been observed for a wavevector q* which connects a hole Fermi surface around the $\Gamma$ point with an electron Fermi surface around the X or Y points of the 1 Fe/unit cell Brillouin zone. Here we study the neutron scattering resonance for a five orbital model within an RPA-BCS approximation. Our results show that both sign-switched and extended s-wave gaps are consistent with the present data for q* near ($\pi$, 0) and that scattering at other momentum transfers can be useful in distinguishing between gap structures. Comment: 5 pages, 4 figures
02/2009;
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ABSTRACT: Weak-coupling approaches to the pairing problem in the iron pnictide superconductors have predicted a wide variety of superconducting ground states. We argue here that this is due both to the inadequacy of certain approximations to the effective low-energy band structure, and to the natural near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we review attempts to construct two-orbital effective band models, the argument for their fundamental inconsistency with the symmetry of these materials, and the comparison of the dynamical susceptibilities in two- and five-band models. We then present results for the magnetic properties, pairing interactions, and pairing instabilities within a five-band Random Phase Approximation model. We discuss the robustness of these results for different dopings, interaction strengths, and variations in band structure. Within the parameter space explored, an anisotropic, sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to the near nesting of Fermi surface sheets.
01/2009;
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ABSTRACT: Using a combined local density functional theory (LDA-DFT) and quantum Monte Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of the superconducting transition temperature Tc of several single-layer hole-doped cuprate superconductors with experimentally very different Tcmax is investigated. The parameters of two different three-band Hubbard models are obtained using the LDA and the downfolding Nth-order muffin-tin orbital technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show that the d-wave transition temperature Tc depends sensitively on the parameters. While the N=1 MTO basis set which reproduces all three $pd\sigma$ bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA Fermi surface and velocities does not.
07/2008;
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ABSTRACT: Using a dynamical cluster quantum Monte Carlo approximation, we investigate the effect of local disorder on the stability of d-wave superconductivity including the effect of electronic correlations in both particle-particle and particle-hole channels. With increasing impurity potential, we find an initial rise of the critical temperature due to an enhancement of anti-ferromagnetic spin correlations, followed by a decrease of Tc due to scattering from impurity-induced moments and ordinary pairbreaking. We discuss the weak initial dependence of Tc on impurity concentration found in comparison to experiments on cuprates.
07/2008;
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ABSTRACT: The question of whether one should speak of a "pairing glue" in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an "instantaneous glue," this interaction differs from the traditional picture of a retarded pairing interaction. However, as we will show, the dominant contribution to the pairing interaction for both of these models arises from energies reflecting the spectrum seen in the dynamic spin susceptibility. In this case, the basic interaction is retarded, and one speaks of a spin-fluctuation glue which mediates the d-wave pairing.
Physical Review Letters 06/2008; 100(23):237001. · 7.37 Impact Factor
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ABSTRACT: Using a dynamical cluster quantum Monte Carlo approximation we investigate the d-wave superconducting transition temperature $T_c$ in the doped 2D repulsive Hubbard model with a weak inhomogeneity. The inhomogeneity is introduced in the hoppings $\tp$ and $t$ in the form of a checkerboard pattern where $t$ is the hopping within a $2\times2$ plaquette and $\tp$ is the hopping between the plaquettes. We find inhomogeneity suppresses $T_c$. The characteristic spin excitation energy and the strength of d-wave pairing interaction decrease with decreasing $T_c$ suggesting a strong correlation between these quantities. Comment: Five pages, four figures. Accepted for Phys. Rev. B (Rapid Com.)
06/2008;
