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Register a new userThe strange metal Hall effect connects quantum criticality and superconductivity in an iron-based superconductor
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Many unconventional superconductors exhibit a common set of anomalous charge transport properties that characterize them as `strange metals, which provides hope that there is single theory that describes them. However, model-independent connections between the strange metal and superconductivity have remained elusive. In this letter, we show that the Hall effect of the unconventional superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ contains an anomalous contribution arising from the correlations within the strange metal. This term has a distinctive dependence on magnetic field, which allows us to track its behavior across the doping-temperature phase diagram, even under the superconducting dome. These measurements demonstrate that the strange metal Hall component emanates from a quantum critical point and, in the zero temperature limit, decays in proportion to the superconducting critical temperature. This creates a clear and novel connection between quantum criticality and superconductivity, and suggests that similar connections exist in other strange metal superconductors.
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Motivated by the close correlation between transition temperature ($T_c$) and the tetrahedral bond angle of the As-Fe-As layer observed in the iron-based superconductors, we study the interplay between spin and orbital physics of an isolated iron-arsenide tetrahedron embedded in a metallic environment. Whereas the spin Kondo effect is suppressed to low temperatures by Hunds coupling, the orbital degrees of freedom are expected to quantum mechanically quench at high temperatures, giving rise to an overscreened, non-Fermi liquid ground-state. Translated into a dense environment, this critical state may play an important role in the superconductivity of these materials.
We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, together with the ferromagnetic quantum criticality and paramagnetic singularity on the Ising 5f-itinerant system UCoAl. Thanks to the variety of ordered moment in ferromagnetic superconductors from 1.5 muB to 0.05 muB, interesting systematic changes or similarities are clarified. All ferromagnetic superconductors show large upper critical field Hc2, and the field-reentrant (-reinforced) phenomena are observed in the field-temperature phase diagram, when the pressure or field direction is tuned for particular conditions. These phenomena are well explained by the ferromagnetic longitudinal fluctuations, which are induced by the magnetic field in transverse configurations. The large Hc2 might be also associated with possible additional effects of Fermi surface instabilities, such as Lifshitz-type singularities.
We investigate a two-orbital model for iron-based superconductors to elucidate the effect of interplay between electron correlation and Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory combined with the exact diagonalization method. When the intra- and inter-orbital Coulomb interactions, $U$ and $U$, increase with $U=U$, both the local spin and orbital susceptibilities, $chi_{s}$ and $chi_{o}$, increase with $chi_{s}=chi_{o}$ in the absence of the Hunds rule coupling $J$ and the electron-phonon coupling $g$. In the presence of $J$ and $g$, there are distinct two regimes: for $J stackrel{>}{_sim} 2g^2/omega_0$ with the phonon frequency $omega_0$, $chi_{s}$ is enhanced relative to $chi_{o}$ and shows a divergence at $J=J_c$ above which the system becomes Mott insulator, while for $J stackrel{<}{_sim} 2g^2/omega_0$, $chi_{o}$ is enhanced relative to $chi_{s}$ and shows a divergence at $g=g_c$ above which the system becomes bipolaronic insulator. In the former regime, the superconductivity is mediated by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is found to be largely dependent on carrier doping. On the other hand, in the latter regime, the superconductivity is mediated by ferro-orbital fluctuations and is observed for wide doping region including heavily doped case without the Fermi-surface nesting.
Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H_{c2}$) of the iron-based superconductor FeSe ($T_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of $H_{c2}$ for magnetic field applied both parallel (textbf{H} $|$ $ab$) and perpendicular (textbf{H} $|$ $c$) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for textbf{H} $|$ $c$, a Pauli paramagnetic contribution is evidenced for textbf{H} $|$ $ab$ with Maki parameter $alpha$ = 2.1, corresponding to Pauli field $H_{P}$ = 36.5 T
We investigated the elastic properties of the iron-based superconductor Ba(Fe1-xCox)2As2 with eight Co concentrations. The elastic constant C66 shows large elastic softening associated with the structural phase transition. The C66 was analyzed base on localized and itinerant pictures of Fe-3d electrons, which shows the strong electron-lattice coupling and a possible mass enhancement in this system. The results resemble those of unconventional superconductors, where the properties of the system are governed by the quantum fluctuations associated with the zero-temperature critical point of the long-range order; namely, the quantum critical point (QCP). In this system, the inverse of C66 behaves just like the magnetic susceptibility in the magnetic QCP systems. While the QCPs of these existing superconductors are all ascribed to antiferromagnetism, our systematic studies on the canonical iron-based superconductor Ba(Fe1-xCox)2As2 have revealed that there is a signature of structural quantum criticality in this material, which is so far without precedent. The elastic constant anomaly is suggested to concern with the emergence of superconductivity. These results highlight the strong electron-lattice coupling and effect of the band in this system, thus challenging the prevailing scenarios that focus on the role of the iron 3d-orbitals.
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