Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2 (original) (raw)
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.
New Fe-based superconductors: properties relevant for applications
Superconductor Science and Technology, 2010
Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T c , huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families. 2 , to the ab-plane. 12 The temperature dependence is very different in the two directions, strongly departing from the WHH behaviour 16 mainly in the direction parallel to c. The anisotropy evaluated as γ = ab c ab c H H H ⊥ = 2 // 2 / γ
JETP Letters, 2008
Based on first-principles FLAPW-GGA calculation, we have investigated electronic structure of newly discovered oxygen-free 38K superconductor Ba 1-x K x Fe 2 As 2 in comparison with parent phase-tetragonal ternary iron arsenide BaFe 2 As 2. The density of states, magnetic properties, near-Fermi bands compositions, together with Sommerfeld coefficients γ and molar Pauli paramagnetic susceptibility χ are evaluated. The results allow us to classify these systems as quasi-two-dimensional ionic metals, where the conduction is strongly anisotropic, only happening on the (Fe-As) layers. According to our calculations, at the hole doping of BaFe 2 As 2 the density of states at the Fermi level grows, and this can be a possible factor of occurrence of superconductivity for Ba 1-x K x Fe 2 As 2. On the other hand, Ba 1-x K x Fe 2 As 2 lays at the border of magnetic instability and the pairing interactions might involve magnetic or orbital fluctuations.
Upper critical field, anisotropy, and superconducting properties of Ba1−xKxFe2As2 single crystals
Physical Review B, 2008
The temperature dependent resistivity of Ba 1−x K x Fe 2 As 2 (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba 0.6 K 0.4 Fe 2 As 2 single crystals were measured in magnetic fields up to 9 T. The data measured on samples with different doping levels revealed very high upper critical fields which increase with the transition temperature, and a very low superconducting anisotropy ratio Γ = H ab c2 /H c c2 ≈ 2. By scaling the resistivity within the framework of the anisotropic Ginzburg-Landau theory, the angle dependent resistivity of the Ba 0.6 K 0.4 Fe 2 As 2 single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy Γ was alternatively determined for each temperature and was found to be between 2 and 3.