Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe 1− x Co x ) 2 As 2 (original) (raw)
Related papers
2016
We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe1−xCox)2As2 (x = 0.1), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors. Iron pnictide superconductors1 have the second-highest superconducting transition tem-perature (Tc) and a wide variety of superconducting (SC) states, particularly as SC-gap struc-ture owing to multiple Fermi surfa...
Anisotropy of the superconducting gap in the iron-based superconductor BaFe2(As(1-x)P(x))2
Scientific reports, 2014
We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As(1-x)P(x))2 (x = 0.30, Tc = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s ± gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.
In the iron pnictide superconductors, two distinct unconventional mechanisms of superconductivity have been put forth: One is mediated by spin fluctuations leading to the s ± state with sign change of superconducting gap between the hole and electron bands, and the other is orbital fluctuations which favor the s ++ state without sign reversal. Here we report direct observation of peculiar momentum-dependent anisotropy in the superconducting gap from angle-resolved photoemission spectroscopy (ARPES) in BaFe 2 (As 1-x P x) 2 (T c =30 K). The large anisotropy found only in the electron Fermi surface (FS) and the nearly isotropic gap on the entire hole FSs are together consistent with modified s ± gap with nodal loops, which can be theoretically reproduced by considering both spin and orbital fluctuations whose competition generates the gap modulation. This indicates that these two fluctuations are nearly equally important to the high-T c superconductivity in this system.
Physical Review B, 2010
We use angle-resolved photoemission spectroscopy to study the band structure of BaFe 2 As 2 and CaFe 2 As 2 , two of the parent compounds of the iron arsenic high-temperature superconductors. Our high quality data reveals that although the Fermi surface is strongly three-dimensional, it does indeed have long parallel segments along the k z direction that can lead to the emergence of magnetic order. More interestingly, we find very unusual incommensurate nesting of the Fermi surface in the a-b plane that is present only at low temperatures. We speculate that this is a signature of a failed charge density wave state that was predicted by renormalization-group studies.
Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe 2 As 2 , and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3d xz,yz orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.
Nature Physics, 2011
The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional [1] (3D) electronic structure of these hightemperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k z -capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba 0.6 K 0.4 Fe 2 As 2 samples. We found a marked k z dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer ∆ 1 and interlayer ∆ 2 pairing. The anisotropy ratio ∆ 2 /∆ 1 , determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J c /J ab in the parent compound [2]. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations. Angle-resolved photoemision spectroscopy (ARPES) has played an important role in revealing the electronic structure of the pnictides. These measurements have typically been carried out at a fixed incident photon energy (hν) and varying incident angles that map out the planar band dispersion as a function of k x and k y . Thus far, four Fermi-surface (FS) sheets have been observed with two hole pockets centred around the Γ (0, 0) point and two electron pockets around the M (π, 0) point in the unfolded two-dimensional (2D) Brillouin zone. Below the superconducting transition temperature T c , nodeless superconducting (SC) gaps open everywhere on the FS sheets [3-6], pointing to a pairing order parameter with an s-wave symmetry in the a-b plane, in agreement with a number of theoretical results [7-11]. However there are other experiments which have indicated possible nodes in the superconducting gap of some pnictides, either line nodes in the a-b plane or nodes along the c-axis [ . It is well known that upon tuning the incident photon energy hν, the allowed direct transitions will shift in energy and consequently in the momentum perpendicular to the a-b plane (k z ) which enables the determination of the electronic dispersion along the c-axis. In the free electron final state approximation, the conversion is given by k z = 2m[(hν − φ − E B ) cos 2 θ + V 0 ]/ , where V 0 is an experimentally determined inner