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Register a new userOn the mechanism of high-temperature superconductivity in the iron based pnictides
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K.P.Sinha
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The recent discovery of superconductivity at moderately high temperature (26 K to 55 K) in doped iron-based pnictides (LnO_{1-x}F_xFeAs, where Ln = La, Ce, Sm, Pr, Nd, etc.), having layered-structure-like cuprates, has triggered renewed challenge towards understanding the pairing mechanism. After reviewing the current findings on these systems, a theoretical model of a combined mechanism is suggested in which the phonon-mediated and distortion-field-mediated pairing processes give the right order of superconducting critical temperature T_c. The distortion-field modes arise from Jahn-Teller or pseudo Jahn-Teller effects due to degenerate or near-degenerate iron 3d_{xz} and 3d_{yz} orbitals.
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Non-trivial topology and unconventional pairing are two central guiding principles in the contemporary search for and analysis of superconducting materials and heterostructure compounds. Previously, a topological superconductor has been predominantly conceived to result from a topologically non-trivial band subject to intrinsic or external superconducting proximity effect. Here, we propose a new class of topological superconductors which are uniquely induced by unconventional pairing. They exhibit a boundary-obstructed higher-order topological character and, depending on their dimensionality, feature unprecedently robust Majorana bound states or hinge modes protected by chiral symmetry. We predict the 112-family of iron pnictides, such as Ca$_{1-x}$La$_x$FeAs$_2$, to be a highly suited material candidate for our proposal, which can be tested by edge spectroscopy. Because of the boundary-obstruction, the topologically non-trivial feature of the 112 pnictides does not reveal itself for a bulk-only torus band analysis without boundaries, and as such had evaded previous investigations. Our proposal not only opens a new arena for highly stable Majorana modes in high-temperature superconductors, but also provides the smoking gun evidence for extended s-wave order in the iron pnictides.
The elementary CuO2 plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO5 pyramids (Fig 1a). Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate $t/{hbar}$ and across the charge-transfer energy gap E, generate superexchange spin-spin interactions of energy $Japprox4t^4/E^3$ in an antiferromagnetic correlated-insulator state1. Hole doping the CuO2 plane disrupts this magnetic order while perhaps retaining superexchange interactions, thus motivating a hypothesis of spin-singlet electron-pair formation at energy scale J as the mechanism of high-temperature superconductivity. Although the response of the superconductors electron-pair wavefunction $Psiequiv<c_uparrow c_downarrow>$ to alterations in E should provide a direct test of such hypotheses, measurements have proven impracticable. Focus has turned instead to the distance ${delta}$ between each Cu atom and the O atom at the apex of its CuO5 pyramid. Varying ${delta}$ should alter the Coulomb potential at the planar Cu and O atoms, modifying E and thus J, and thereby controlling ${Psi}$ in a predictable manner. Here we implement atomic-scale imaging of E and ${Psi}$, both as a function of the periodic modulation in ${delta}$ that occurs naturally in $Bi_2Sr_2CaCu_2O_{8+x}$. We demonstrate that the responses of E and ${Psi}$ to varying ${delta}$, and crucially those of ${Psi}$ to the varying E, conform to theoretical predictions. These data provide direct atomic-scale verification that charge-transfer superexchange is key to the electron-pairing mechanism in the hole-doped cuprate superconductor ${Bi_2Sr_2CaCu_2O_{8+x}}$.
We present an ab-initio study of Ru substitution in two different compounds, BaFe2As2 and LaFeAsO, pure and F-doped. Despite the many similarities among them, Ru substitution has very different effects on these compounds. By means of an unfolding technique, which allows us to trace back the electronic states into the primitive cell of the pure compounds, we are able to disentangle the effects brought by the local structural deformations and by the impurity potential to the states at the Fermi level. Our results are compared with available experiments and show: i) satisfying agreement of the calculated electronic properties with experiments, confirming the presence of a magnetic order on a short range scale; ii) Fermi surfaces strongly dependent on the internal structural parameters, more than on the impurity potential. These results enter a widely discussed field in the literature and provide a better understanding of the role of Ru in iron pnictides: although isovalent to Fe, the Ru-Fe substitution leads to changes in the band structure at the Fermi level mainly related to local structural modifications.
Dome-shape superconductivity phase diagram can commonly be observed in cuprate and iron-based systems via tuning parameters such as charge carrier doping, pressure, bond angle, and etc. We report doping electrons from transition-metal elements (TM = Co, Ni) substitution can induce high-Tc superconductivity around 35 K in Ca0.94La0.06Fe2As2, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Unexpectedly, the onset critical temperature for the high-Tc superconductivity stays constant for a wide range of TM doping. Possible extrinsic factors like phase separation, chemical inhomogeneity, and charge carrier cancelation effect are all excluded. This anomalous charge carrier density independent SC is very similar to the interface superconductivity in La2-xSrxCuO4-La2CuO4 bilayer system. The further verified two-dimensional (2D) nature of superconductivity by the Tinkhams angular-dependent critical field model as well as by the angle-resolved magneto-resistance measurements jointly supports the idea of interfacial effect induced high-Tc superconductivity.
Developing a theory of high-temperature superconductivity in copper oxides is one of the outstanding problems in physics. It is a challenge that has defeated theoretical physicists for more than twenty years. Attempts to understand this problem are hindered by the subtle interplay among a few mechanisms and the presence of several nearly degenerate and competing phases in these systems. Here we present some crucial experiments that place essential constraints on the pairing mechanism of high-temperature superconductivity. The observed unconventional oxygenisotope effects in cuprates have clearly shown strong electron-phonon interactions and the existence of polarons and/or bipolarons. Angle-resolved photoemission and tunneling spectra have provided direct evidence for strong coupling to multiple-phonon modes. In contrast, these spectra do not show strong coupling features expected for magnetic resonance modes. Angle-resolved photoemission spectra and the oxygen-isotope effect on the antiferromagnetic exchange energy J in undoped parent compounds consistently show that the polaron binding energy is about 2 eV, which is over one order of magnitude larger than J = 0.14 eV. The normal-state spin-susceptibility data of holedoped cuprates indicate that intersite bipolarons are the dominant charge carriers in the underdoped region while the component of Fermi-liquid-like polarons is dominant in the overdoped region. All the experiments to test the gap or order-parameter symmetry consistently demonstrate that the intrinsic gap (pairing) symmetry for the Fermi-liquid-like component is anisotropic s-wave and the order-parameter symmetry of the Bose-Einstein condensation of bipolarons is d-wave.
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