Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2 (original) (raw)
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Physical Review B, 2011
We investigate the electronic properties and the superconducting gap characteristics of a single crystal of hole-doped 122 Fe-pnictide Ba0.65Na0.35Fe2As2 by means of specific heat measurements. The specific heat exhibits a pronounced anomaly around the superconducting transition temperature Tc = 29.4 K, and a small residual part at low temperature. In a magnetic field of 90 kOe, the transition is broadened and Tc is lowered insignificantly by an amount ∼ 1.5 K. We estimate a high electronic coefficient in the normal state with a value 57.5 mJ mol −1 K 2 , being consistent with holedoped 122 compounds. The temperature-dependent superconducting electronic specific heat cannot be described with single-gap BCS theory under weak coupling approach. Instead, our analysis implies a presence of two s-wave like gaps with magnitudes ∆1(0)/kBTc = 1.06 and ∆2(0)/kBTc = 2.08 with their respective weights of 48% and 52%. While our results have qualitative similarities with other hole-doped 122 materials, the gap's magnitude and their ratio are quite different.
Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe 1− x Co x ) 2 As 2
Journal of the Physical Society of Japan, 2014
We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe 1−x Co x) 2 As 2 (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 superconductors 1 have the second-highest superconducting transition temperature (T c) and a wide variety of superconducting (SC) states, particularly as SC-gap structure owing to multiple Fermi surfaces (FSs) and orbital characters. 2, 3 Focusing on the SC state of the 122-type iron pnictides, the Ba 1−x K x Fe 2 As 2 system shows a fully opened SC gap and a nodal SC gap around optimally doped 4 and a hole-end KFe 2 As 2 , 5 respectively, observed by angle-resolved photoemission spectroscopy (ARPES). 6-9 On the other hand, the BaFe 2 (As 1−x P x) 2 system has nodes in the whole SC region. 10, 11 There are different ARPES
Phase diagram of Ba_{1−x}K_{x}Fe_{2}As_{2}
Physical Review B, 2012
We report the results of a systematic investigation of the phase diagram of the iron-based superconductor, Ba1−xKxFe2As2, from x = 0 to x = 1.0 using high resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of ∆x 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1−xKxFe2As2 crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with space group F mmm and a striped antiferromagnet (AF) with space group Fcmm m . The transition temperature falls from a maximum of 139 K in the undoped compound to 0 K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3) and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 falling to 3 K at x = 1.0. We discuss reasons for the suppression of the spin-density-wave order and the electron-hole asymmetry in the phase diagram.
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
Microscopic Coexistence of Superconductivity and Magnetism in Ba_{1-x}K_{x}Fe_{2}As_{2}
Physical Review Letters, 2011
We use 75 As nuclear magnetic resonance (NMR) to investigate the local electronic properties of Ba(Fe1−xRux)2As2 (x = 0.23). We find two phase transitions, to antiferromagnetism at TN ≈ 60 K and to superconductivity at TC ≈ 15 K. Below TN , our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4 µB/Fe. The spin-lattice relaxation rate 1/ 75 T1 is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below TC, 1/ 75 T1 on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity. In the iron-based superconductors, superconductivity (SC) is achieved on suppressing a long-ranged antiferromagnetic order [1] by doping or pressure. At this phase boundary, much attention has been drawn to the question of whether SC may coexist with antiferromagnetism (AFM). Proposals for possible coexisting phases have included commensurate [2] and incommensurate [3-5] magnetic structures, competition between AFM and SC , and variations in the size of the ordered moment or the pairing symmetry . No consensus has yet been reached on the pairing mechanism or the possible phenomena arising from the interplay of AFM and SC. For most materials, local-probe studies on high quality samples are required as a matter of urgency to distinguish the key properties of microscopic coexistence from any form of phase separation.