Andreev bound states in iron pnictide superconductors (original) (raw)
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Small-q phonon-mediated unconventional superconductivity in the iron pnictides
Physical Review B, 2011
We report self-consistent calculations of the gap symmetry for the iron-based high-temperature superconductors using realistic small-q phonon mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless s± state that was first encountered in a spin-fluctuations mechanism picture. Nodal gap structures such as d x 2 −y 2 and s± +d x 2 −y 2 and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase sensitive experiments reporting a gap changing sign attributed previously only to a non-phononic mechanism and isotope effect measurements proving the involvement of phonons in the pairing. PACS numbers: 71.27.+a, One of the foremost issues in contemporary condensedmatter physics is the nature of the medium-hightemperature superconductivity (SC) in the recently discovered iron pnictides . The focal point of the current challenge is to understand whether the bosonic pairing that drives the SC is due to spin-fluctuations or phonons, and how the pairing mechanism concurs with the emerging symmetry of the superconducting gap [2, 3]. Experimental techniques have recently given controversial results . Angular-resolved photoemission spectroscopy and Andreev spectroscopy indicate one nearly isotropic gap or two isotropic gaps , whereas penetration depth [9, 10] and nuclear magnetic resonance (NMR) measurements support both nodeless and nodal gap structures. Notably, a nodeless gap is not necessarily a conventional isotropic s-wave gap here. An important ingredient for discussing the gap symmetry is the Fermi surface (FS) of both 1111 (LaOFeAs) and 122 (BaFe 2 As 2 ) parent compounds, which is remarkably simple. The strongly two-dimensional FS mainly consists of two electron and two hole pockets, with a nesting wavevector Q = (π, π) connecting them in the folded Brillouin zone (BZ) . Fermi surface nesting stabilizes a spin density wave which yields to superconductivity upon doping . If spin-fluctuations mediate the pairing then a nodeless unconventional superconducting gap reversing its sign between the electron and hole FS sheets is expected, known as the s ± state .
Journal of the Physical Society of Japan, 2009
We study whether Fermi-surface (FS) nesting can give rise to high-temperature superconductivity in iron pnictides. Starting with ab initio construction of an effective four-orbital model, we employ the fluctuation-exchange approximation to show that FS does not necessarily favor the stripe antiferromagnetic order observed in experiments, especially for realistic electronic correlations. If superconductivity in iron pnictides is magnetically mediated and has fully-gapped sign-reversing s-wave symmetry, our results suggest that the pairing interaction does not arise only from FS nesting and exchange interactions between local moments in the Fe 3d orbitals may play a crucial role.
Fermi-surface Shrinking, Interband Coupling and Multiple Gaps in Iron-based Pnictides
Journal of Superconductivity and Novel Magnetism, 2010
In this contribution we present a comprehensive explanation for the origin of the band shifts observed in dHvA and ARPES experiments. Using a four-band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V ≈ 0.46 eV, which corresponds to a mass enhancement Z ≈ 1.4. We also employ such fourband model to investigate the magnitude of the superconducting gap on different Fermi sheets of Ba 0.6 K 0.4 Fe 2 As 2 , and we show that the same four-band model provides a simple explanation of the different gap values on different Fermi sheets and of the thermodynamics properties (specific heat, superfluid density,. . .).
Looking at the superconducting gap of iron pnictides
Journal of Physics and Chemistry of Solids, 2011
THz and infrared spectroscopy is widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.
Three-band s± Eliashberg theory and the superconducting gaps of iron pnictides
Physical Review B, 2009
The experimental critical temperatures and gap values of the superconducting pnictides of both the 1111 and 122 families can be simultaneously reproduced within the Eliashberg theory by using a three-band model where the dominant role is played by interband interactions and the order parameter undergoes a sign reversal between hole and electron bands (s±-wave symmetry). High values of the electron-boson coupling constants and small typical boson energies (in agreement with experiments) are necessary to obtain the values of all the gaps and to correctly reproduce their temperature dependence. PACS numbers: 74.70.Dd, 74.20.Fg, 74.20.Mn The recently discovered Fe-based pnictide superconductors [1, 2, 3] have aroused great interest in the scientific community. They have indeed shown that high T c superconductivity does not uniquely belong to cuprates but can take place in Cu-free systems as well. Nevertheless, as in cuprates, superconductivity occurs upon charge doping of a magnetic parent compound above a certain critical value. However, important differences exist: the parent compound in cuprates is a Mott insulator with localized charge carriers and a strong Coulomb repulsion between electrons; in the pnictides, on the other hand, it is a bad metal and shows a tetragonal to orthorhombic structural transition below ≈ 140 K, followed by an antiferromagnetic (AF) spin-density-wave (SDW) order . Charge doping gives rise to superconductivity and, at the same time, inhibits the occurrence of both the static magnetic order and the structural transition. The Fermi surface consists of two or three hole-like sheets around Γ and two electron-like sheets around M . Up to now, the most intensively studied systems are the 1111 compounds, ReFeAsO 1−x F x (Re = La, Sm, Nd, Pr, etc.) and especially the 122 ones, hole-or electron-doped AFe 2 As 2 (A = Ba, Sr, Ca). The huge amount of experimental work already done in 122 compounds is due to the availability of rather big high-quality single crystals.
The European Physical Journal B, 2014
We uncover the low-energy spectrum of a t-J model for electrons on a square lattice of spin-1 iron atoms with 3d xz and 3d yz orbital character by applying Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of one hole roaming over a 4 × 4 × 2 lattice. Hopping matrix elements are set to produce hole bands centered at zero two-dimensional (2D) momentum in the free-electron limit. Holes can propagate coherently in the t-J model below a threshold Hund coupling when long-range antiferromagnetic order across the d+ = 3d (x+iy)z and d− = 3d (x−iy)z orbitals is established by magnetic frustration that is off-diagonal in the orbital indices. This leads to two hole-pocket Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate spin-density wave (cSDW) that exists above the threshold Hund coupling results in emergent Fermi surface pockets about cSDW momenta at a quantum critical point (QCP). This motivates the introduction of a new Gutzwiller wavefunction for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta indicates that the dispersion of the nested band of oneparticle states that emerges is electron-type. Increasing Hund coupling past the QCP can push the hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy level, in agreement with recent determinations of the electronic structure of mono-layer iron-selenide superconductors.
Tunneling spectroscopy of spis pispi pairing state as a model for FeAs superconductors
Eprint Arxiv 0807 4604, 2008
We present the self-consistent Bogoliubov-de Gennes calculations of an spis\pispi pairing state of two band superconductivity as a model for the FeAs superconductors. The spis\pispi state is an s-wave pairing state with an internal pi\pipi phase, that is, nodeless gaps on each band but with the opposite sign. The novel features of this state are investigated by calculating the local density of states of the pi\pipi phase superconductor/normal metal bilayers. Because of the sign reversal between the two condensates, the zero bias conductance peak appears as observed in tunneling spectroscopy experiments on FeAs superconductors. This eliminates the major obstacle to establish the spis\pispi state as the pairing symmetry of the FeAs superconductors.
Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor
New Journal of Physics, 2009
For the iron-based high T c superconductor LaFeAsO 1−x F x , we construct a minimal model, where all of the five Fe d bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.
Tunneling spectroscopy of spis pispi pairing state as a model for the FeAs superconductors
2008
We present the self-consistent Bogoliubov-de Gennes calculations of an sπ pairing state of two band superconductivity as a model for the FeAs superconductors. The sπ state is an s-wave pairing state with an internal π phase, that is, nodeless gaps on each band but with the opposite sign. The novel features of this state are investigated by calculating the local density of states of the π phase superconductor/normal metal bilayers. Because of the sign reversal between the two condensates, the zero bias conductance peak appears as observed in tunneling spectroscopy experiments on FeAs superconductors. This eliminates the major obstacle to establish the sπ state as the pairing symmetry of the FeAs superconductors.