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Register a new userNematicity at the Hunds metal crossover in iron superconductors
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The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments calls for a prominent role of local correlations and place iron superconductors at the entrance of a Hund metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hunds coupling. We show that Hunds physics strongly affects the nematic properties of the system. It severely constraints the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx=yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes, so far overlooked in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Gamma and M points are modified, with important consequences for ARPES measurements.
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Superconductivity in cuprates peaks in the doping regime between a metal at high p and an insulator at low p. Understanding how the material evolves from metal to insulator is a fundamental and open question. Early studies in high magnetic fields revealed that below some critical doping an insulator-like upturn appears in the resistivity of cuprates at low temperature, but its origin has remained a puzzle. Here we propose that this metal-to-insulator crossover is due to a drop in carrier density n associated with the onset of the pseudogap phase at a critical doping p*. We use high-field resistivity measurements on LSCO to show that the upturns are quantitatively consistent with a drop from n=1+p above p* to n=p below p*, in agreement with high-field Hall data in YBCO. We demonstrate how previously reported upturns in the resistivity of LSCO, YBCO and Nd-LSCO are explained by the same universal mechanism: a drop in carrier density by 1.0 hole per Cu atom.
A new class of high temperature superconductors based on iron and arsenic was recently discovered, with superconducting transition temperature as high as 55 K. Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a Curie Weiss susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence-incoherence crossover temperature is a very strong function of the strength of the Hunds rule coupling J_Hund. On the basis of the normal state properties, we estimate J_Hund to be 0.35-0.4 eV. In the atomic limit, this value of J_Hund leads to the critical ratio of the exchange constants J_1/J_2~2. While normal state incoherence is in common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron-oxypnictides, is unique due to its multiorbital electronic structure.
The experimental transport scattering rate was determined for a wide range of optimally doped transition metal-substituted FeAs-based compounds with the ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of suppression with increasing scattering rate indicative of a common pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a limit of sim26 K on the maximum Tc for all transition metal-substituted 122 systems, in agreement with experimental observations, and sets a critical scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting phase. The observed critical scattering rate is much weaker than that expected for a sign-changing order parameter, providing important constraints on the nature of the superconducting gap in the 122 family of iron-based superconductors.
We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.
Insight into the electronic structure of the pnictide family of superconductors is obtained from quantum oscillation measurements. Here we review experimental quantum oscillation data that reveal a transformation from large quasi-two dimensional electron and hole cylinders in the paramagnetic overdoped members of the pnictide family to significantly smaller three-dimensional Fermi surface sections in the antiferromagnetic parent members, via a potential quantum critical point at which an effective mass enhancement is observed. Similarities with the Fermi surface evolution from the overdoped to the underdoped normal state of the cuprate superconducting family are discussed, along with the enhancement in antiferromagnetic correlations in both these classes of materials, and the potential implications for superconductivity.
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