Magnetic and superconducting structures near twin boundaries in low doped Fe-pnictides (original) (raw)
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Nano letters, 2017
Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe1-xCox)2As2 crystals with x=0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.
Unconventional superconductivity in iron pnictides: Magnon mediated pairing
Physica C: Superconductivity and its Applications
We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe 2 As 2 and BaFe 2 As 2 in pure form for umklapp scattering with scattering wave vector Q = (1 , 1) , in the unit of π / a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe 2 As 2 and BaFe 2 As 2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.
Physical Review Letters, 2009
The puzzling nature of magnetic and lattice phase transitions of iron pnictides is investigated via a first-principles Wannier function analysis of representative parent compound LaOFeAs. A rare ferroorbital ordering is found to give rise to the recently observed highly anisotropic magnetic coupling, and drive the phase transitions-without resorting to widely employed frustration or nesting picture. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the magnons advocates active roles of light orbitons in spin dynamics and electron pairing in iron pnictides. PACS numbers: 74.25.Jb, 74.25.Ha, 74.20.Mn, 71.15.Mb With a striking similarity to the cuprate families [1], recently discovered high-temperature superconductivity in iron pnictides emerges upon suppression of magnetic order via additional charge doping . This new example of close proximity of superconductivity to the magnetic phase has raised again the long standing questions concerning the intimate relationship between these two seemingly different (or even exclusive) phases of solids . Following the cuprate research, a big thrust of current efforts has been to establish superconductivity in these compounds via magnetic correlations, despite the apparently diverse perspectives of the magnetism itself [4, 5, 6]. Clearly, a solid understanding of the parent compounds and their magnetism is an essential first step toward a convincing resolution of superconductivity in the doped systems, especially within the heavily discussed spin fluctuation scenario of pairing .
Applied Physics Letters, 2009
We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe 1-x Co x ) 2 As 2 . Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe 1-x Co x ) 2 As 2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field H c2 both parallel and perpendicular to the c-axis, the irreversibility field H irr c (T) and a rather unusual symmetric volume pinning force curve F p (H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ = H c2 ab /H c2 c deduced from the slopes of dH c2 ab /dT = 4.9T/K and dH c2 c /dT = 2.5T/K decreases from ~2 near T c , to ~1.5 at lower temperatures, much smaller than g for 1111 pnictides and high-T c cuprates.
Physical Review Letters, 2010
Directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe1−xCox)2As2 single crystals (Tc=24.5 K) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The PCAR spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy Ω b (T ) is obtained, very similar to the spinresonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band s± Eliashberg model by using an electron-boson spectral function peaked at Ω0 = 12 meV ≃ Ω b (0). PACS numbers: 74.50.+r , 74.70.Dd, 74.45.+c The discovery of the first class of non-cuprate, Febased high-temperature superconductors in 2008 brought great excitement in the scientific community [1]. The phase diagram of these compounds (although still imperfectly known) looks similar to that of copper-oxide superconductors [2] and, as in cuprates, superconductivity emerges "in the vicinity" of a magnetic parent compound. The electron-phonon interaction seems not to be sufficient [3] to explain their high T c (up to 55 K [4]) even by considering a magnetic ground state . A spinfluctuation-mediated pairing mechanism has been early proposed instead, which predicts the occurrence of a sign change of the order parameter on different sheets of the Fermi surface (s±-symmetry) . This picture is naturally based on the proximity of the superconducting phase to a magnetic one, on the existence of disconnected Fermi surface (FS) sheets, and on the multiband character of superconductivity in these compounds, which are nowadays almost universally accepted . The s± model itself is strongly supported by various experimental results [8] which indicate the existence of multiple nodeless gaps on different sheets of the FS, although the possible emergence of gap nodes in some systems, along certain directions or in particular conditions [9, 10] is still debated. The role of spin fluctuations (SF) in the pairing has also found support in neutron scattering experiments that have revealed a spin resonance energy which scales linearly with T c [2]. Finally, it has been recently shown that a multiband s± Eliashberg model can reproduce several experimental quantities (such as gaps, T c , kinks in the band dispersion and effective masses ) by assuming that the mediating boson has a characteristic energy similar to the spin-resonance one. In this paper we report on directional PCAR measurements on high-quality single crystals of the e-doped 122 compound BaFe 1.8 Co 0.2 As 2 . The results prove the existence of two superconducting gaps with no line nodes on the FS, and whose amplitude is almost the same in the ab plane or along the c axis. The PCAR spectra also present structures that can be related to a strong electron-boson interaction (EBI). The characteristic energy Ω b of the mediating boson extracted from the PCAR curves decreases with temperature and is very similar to the resonance energy of the spin excitation spectrum . Moreover, both the gaps and the additional EBI structures in the PCAR spectra can be reproduced within an effective three-band s± wave Eliashberg model using a boson energy Ω 0 = 12 meV ≃ Ω b (0). All these results strongly support a spin-fluctuation-mediated mechanism for superconductivity in this compound. The BaFe 1.8 Co 0.2 As 2 (10% Co) single crystals were prepared by the self-flux method under a pressure of 280 MPa at the National High Magnetic Field Laboratory in Tallahassee. The typical crystal sizes are ≈ 1 × 1 × 0.1 mm 3 . The onset of the resistive transition is T on c = 24.5 K with ∆T c (10%-90%) = 1 K (see inset to ). Instead of using the standard technique where a sharp metallic tip is pressed against the material under study, the point contacts were made by putting a small drop of Ag paste on a fresh surface exposed by breaking the crystal. Contacts made in this way are very stable and the differential conductance curves, obtained by numerical differentiation of the I-V characteristics, can be recorded up to ≈ 200 K . As an example, shows the raw conductance curves, recorded up to 180 K, of a Ag/BaFe 1.8 Co 0.2 As 2 point contact (R N = 25 Ω) with current injection along the c axis ("c-axis contact"). The clear signatures of AR in the low-T curve and the absence of heating effects or dips indicate ballistic conduction through the point contact, so that energy-resolved