Electronic, magnetic and optical properties of two Fe-based superconductors and related parent compounds (original) (raw)
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Gap features of layered iron-selenium-tellurium compound below and above the superconducting transition temperature by break-junction spectroscopy combined with STS Abstract. We studied correlations between the superconducting gap features of Te-substituted FeSe observed by scanning tunnelling spectroscopy (STS) and break-junction tunnelling spectroscopy (BJTS). At bias voltages outside the superconducting gap-energy range, the broad gap structure exists, which becomes the normal-state gap above the critical temperature, T c. Such behaviour is consistent with the model of the partially gapped density-wave superconductor involving both superconducting gaps and pseudogaps, which has been applied by us earlier to high-T c cuprates. The similarity suggests that the parent electronic spectrum features should have much in common for these classes of materials. 1. Introduction From the early stages of our break-junction tunnel spectroscopy (BJTS) measurements on the superconducting FeSe x Te 1-x single crystals, the normal-state gaps were manifested in addition to the Bardeen-Cooper-Schrieffer (BCS)-type superconducting gap features [1,2]. The observed unusual electronic properties of this superconductor including the gap distributions extending to several tenth of meV are in fact intriguing in view of the possible manifestation of the local anomalously high (about 100 K [3]) superconducting critical temperature (T c). On the other hand, it is reasonable to consider that the normal-state gap is a consequence of the charge-or spin-density-wave formation below the structural/magnetic phase transition, as stems from the phase diagram [4]. In particular, there has been suggested that the lower symmetry C 2 nematic/smectic charge ordering emerges in the iron-based superconductors [5,6]. In such states, the normal-state gap structures can be readily observed by tunnelling spectroscopy, which was proposed theoretically [7]. However, there were not enough direct tunnel measurements of such a normal-state gap. In this paper, we report the observations of both superconducting and the normal-state gaps for the simple iron-based superconductor, FeSe x Te 1-x , using the BJTS techniques as well as the scanning tunnel spectroscopy (STS). We emphasize that the gap of ± 10-20 mV observed by STS in the superconducting state below T c = 15 K is larger than that revealed in the BJTS measurements [1]. The attention should be paid to the fact that these measurements constitute the most direct and precise tool among the gap-probing experimental techniques. This probe dealing with the conduction electrons themselves immediately leads to the well energy-resolved data needing no further assumptions and relatively easy in interpretation [8].
Theoretical investigation of the superconducting gaps in the Fe-based superconductors
Physica C: Superconductivity, 2010
Based on an antiferromagnetic (AFM) spin fluctuation approximation, we study the superconducting gaps in Fe-based compound using two-band model. We find that our results are consistent with the previous work that concludes sign-reversal extended s-wave pairings between different Fermi surface sheets. The different superconducting gap magnitude around different Fermi surface sheets is probably due to the different density of states on them. This calculation can give insight to the recent angle-resolved photoemission (ARPES) experiments on these materials. To detect the phase variation of the superconducting gap over the Fermi surfaces, we propose a new method for measuring the particular wave vector phonon linewidth. In the case of the sign-reversal superconducting pairing, the linewidth shows continuities compared to the case of no phase variation.
Physical Review B, 2010
We report magnetic penetration depth and thermal conductivity data for high-quality single crystals of BaFe2(As1−xPx)2 (Tc = 30 K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba,K)Fe2As2 which has similar Tc and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-Tc superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable Tc.
Superconducting properties of thes±-wave state: Fe-based superconductors
Journal of Physics: Condensed Matter, 2017
Although the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been settled with consensus with regard to the pairing symmetry and the superconducting (SC) gap function, the vast majority of experiments support the existence of spin-singlet signchanging s-wave SC gaps on multi-bands (± s-wave state). This multi-band ± s-wave state is a very unique gap state per se and displays numerous unexpected novel SC properties, such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with a SH jump and condensation energy versus T c , etc. In particular, many of these non-trivial SC properties can easily be mistaken as evidence for a nodal-gap state such as a d-wave gap. In this review, we provide detailed explanations of the theoretical principles for the various non-trivial SC properties of the ± s-wave pairing state, and then critically compare the theoretical predictions with experiments on FeSCs. This will provide a pedagogical overview of to what extent we can coherently understand the wide range of different experiments on FeSCs within the ± s-wave gap model.
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.
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.
Probing the Energy Gaps of a Multi-Gap Superconductor: Ba(1-x)KxFe2As2
Journal of Undergraduate Reports in Physics
In order to spectroscopically probe the superconducting energy gap of potassium-doped Ba122 iron pnictides, in particular Ba (1-x) K x Fe 2 As 2 where x = 0.33 (under-doped regime), we have performed four-wire conductance measurements from T = 2K to 52K. We report evidence for multi-gap features with gaps corresponding to directional tunneling through the ab-axes of this iron pnictide. The multi-gap features a predominant result of tunneling across the ab-plane with gaps of Δ 1 = 2-4 meV and Δ 2 = 9-11 meV. These gap values are temperature dependent.
TOPICAL REVIEW: Magnetism in Fe-based superconductors
J Phys Condens Matter, 2010
In this review, we present a summary of experimental studies of magnetism in Fe-based superconductors. The doping dependent phase diagram shows strong similarities to the generic phase diagram of the cuprates. Parent compounds exhibit magnetic order together with a structural phase transition both of which are progressively suppressed with doping allowing superconductivity to emerge. The stripe-like spin arrangement of Fe moments in the magnetically ordered state shows the identical in-plane structure for the RFeAsO (R=rare earth) and AFe 2 As 2 (A=Sr, Ca, Ba, Eu and K) parent compounds, notably different than the spin configuration of the cuprates. Interestingly, Fe 1+y Te orders with a different spin order despite very similar Fermi surface topology. Studies of the spin dynamics in the parent compounds shows that the interactions are best characterized as anisotropic three-dimensional (3D) interactions. Despite the room temperature tetragonal structure, analysis of the low temperature spin waves under the assumption of a Heisenberg Hamiltonian indicates strong in-plane anisotropy with a significant next-near neighbor interaction. In the superconducting state, a resonance, localized in both wavevector and energy is observed in the spin excitation spectrum as in the cuprates. This resonance is observed at a wavevector compatible with a Fermi surface nesting instability independent of the magnetic ordering of the relevant parent compound. The resonance energy (E r) scales with superconducting transition temperature (T C) as E r ∼4.9 k B T C consistent with the canonical value of ∼5 k B T C observed in the cuprates. Moreover, the relationship between the resonance energy and the superconducting gap, ∆, is similar to that observed in many unconventional superconductors (E r /2∆ ∼ 0.64). Material Max. T C (K) LaFeAsO 1−x F x [1] 26 NdFeAsO 1−x F x [2] 52 PrFeAsO 1−x F x [3] 52 SmFeAsO 1−x F x [4] 55 CeFeAsO 1−x F x [5] 41 GdFeAsO 1−x F x [7] 50 TbFeAsO 1−x F x [9] 46 DyFeAsO 1−x F x [9] 45 Gd 1−x Th x FeAsO [10] 56 LaFeAsO 1−y [11, 12] 28 NdFeAsO 1−y [11, 12, 13] 53 PrFeAsO 1−y [11, 12] 48 SmFeAsO 1−y [11] 55 GdFeAsO 1−y [14, 12] 53 TbFeAsO 1−y [12] 52 DyFeAsO 1−y [12] 52 LaFe 1−x Co x AsO [17] 14 SmFe 1−x Ni x AsO [26] 10 SmFe 1−x Co x AsO [27] 15 LaFe 1−x Ir x AsO [28] 12 Material Max. T C (K) Ba 1−x K x Fe 2 As 2 [16] 38 Ba 1−x Rb x Fe 2 As 2 [29] 23 K 1−x Sr x Fe 2 As 2 [30] 36 Cs 1−x Sr x Fe 2 As 2 [30] 37 Ca 1−x Na x Fe 2 As 2 [31] 20 Eu 1−x K x Fe 2 As 2 [32] 32 Eu 1−x Na x Fe 2 As 2 [33] 35 Ba(Fe 1−x Co x) 2 As 2 [18, 34] 22-24 Ba(Fe 1−x Ni x) 2 As 2 [19] 20 Sr(Fe 1−x Ni x) 2 As 2 [35] 10 Ca(Fe 1−x Co x) 2 As 2 [36] 17 Ba(Fe 1−x Rh x) 2 As 2 [20] 24 Ba(Fe 1−x Pd x) 2 As 2 [20] 19 Sr(Fe 1−x Rh x) 2 As 2 [21] 22 Sr(Fe 1−x Ir x) 2 As 2 [21] 22 Sr(Fe 1−x Pd x) 2 As 2 [21] 9 Ba(Fe 1−x Ru x) 2 As 2 [22] 21 Sr(Fe 1−x Ru x) 2 As 2 [23] 13.5 LiFeAs [37, 38, 39] 18 Na 1−x FeAs [40] 25 Fe 1+y Se x Te 1−x [41] 15
Physical Review B, 2019
Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba(Fe1-xCox)2As2 family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and Ba(Fe1-xCox)2As2 is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap ∆e(x,φ) has an intermediate value between the dominant inner Δ2h(x) and outer Δ1h(x) hole gaps. Close to the AFM transition the electron gap ∆e(x, φ) increases sharply and becomes closer in magnitude to the dominant inner hole gap Δ2h(x). The same two-gap state with close electron and inner hole gaps Δ2h(x) ≈ ∆e(x, φ) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap ∆e(x, φ) is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap ∆e(x, φ), the doping dependence of the hole gaps Δ1,2h(x) and the critical temperature Tc(x), both before and after the AFM transition, are associated with a change of the density of states γnh(x) and the intraband electron-phonon interaction in the hole bands. The non-phonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the Ba(Fe1-xCox)2As2 family. The high-Tc iron-based superconductors (FeSCs) are multiband quasi-two-dimensional compounds with strongly anisotropic Fermi surface and low carrier density in the hole-like and electron-like bands [1]. The Fermi surface (FS) of these compounds consists of hole-like (h) pockets at the Γ point and electron-like (e) pockets centered at the X = (π, 0) and Y = (0, π) points of the Brillouin zone. Compared to strongly correlated high-Tc cuprates, which are similar in their basic characteristics, electron-electron correlations in FeSCs are not large (see, for example, reviews [2, 3]).The parent orthorhombic (Ort) Fe-based compounds are antiferromagnetic (AFM) metals of spin-density wave (SDW) type with the magnetic ordering vectors Q = (π, 0), (0, π). Unlike dielectric parent high-Tc cuprates, they have free electronic states at the FS that are not associated with magnetism but can, in principle, be involved in superconducting (SC) pairing. The electronic structure of these compounds is very sensitive to small changes in doping, pressure, and degree of disorder. When in parent compounds the magnetic atoms Fe (3d 6) in the a-b plane are replaced by atoms with larger number of d electrons (electron doping) or the non-magnetic atoms out of this plane are replaced by atoms with smaller valence (hole doping), antiferromagnetism is gradually suppressed which leads to the onset of superconductivity. In this regime, the AFM and SC gaps coexist at the Fermi surface