Electronic correlation is of fundamental importance to high temperature superconductivity. Iron-based superconductors are believed to possess moderate correlation strength, which combined with their multi-orbital nature makes them a fascinating platform for the emergence of exotic phenomena. A particularly striking form is the emergence of an orbital selective Mott phase, where the localization of a subset of orbitals leads to a drastically reconstructed Fermi surface. Here, we report spectroscopic evidence of the reorganization of the Fermi surface from FeSe to FeTe as Se is substituted by Te. We uncover a particularly transparent way to visualize the localization of the $d_{xy}$ electron orbital through the suppression of its hybridization with the more coherent $d$ electron orbitals, which leads to a redistribution of the orbital-dependent spectral weight near the Fermi level. These noteworthy features of the Fermi surface are accompanied by a divergent behavior of a band renormalization in the $d_{xy}$ orbital. All of our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase towards an orbital-selective Mott phase in FeTe$_{1-x}$Se$_x$ as Se concentration is reduced. 

/pdf/correlation-driven-electronic-reconstruction-in-fete1-xsex-7cgq64bl.pdf

Correlation-driven electronic reconstruction in FeTe1−xSex

RbEuFe4As4 exhibits both superconducting order in the FeAs layers and ferromagnetic order in the Eu layers, providing a good platform to study the interaction and microscopic coexistence of these two traditionally incompatible orders. Growing high-quality RbEuFe4As4 single crystals is essential to investigate these phenomena at a deeper level. Here we report the successful growth of the RbEuFe4As4 single crystals with millimeter-size dimensions using a RbAs flux. The high-quality crystals were characterized via resistivity, magnetization, and heat capacity measurements. Single crystal X-ray diffraction data refinements reveal almost regular FeAs4 tetrahedra (As1–Fe–As1 = 108.60(7)°, As2–Fe–As2 = 109.01(8)°, and As1–Fe–As2 = 109.81(2)°) in RbEuFe4As4, providing structural support for the highest superconducting transition temperature (Tc = 36.8 K) among all known AAeFe4As4 (A = K, Rb, Cs; Ae = Ca, Sr, Eu) compounds. Our flux method using RbAs can also be employed to grow other desired transition metal comp...

Single Crystal Growth and Study of the Ferromagnetic Superconductor RbEuFe4As4

We report an inelastic neutron scattering study on the spin resonance in the bilayer iron-based superconductor ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ In contrast to its quasi-two-dimensional electron structure, three strongly $L$-dependent modes of spin resonance are found below ${T}_{c}=35\text{ }\text{ }\mathrm{K}$ The mode energies are below and linearly scale with the total superconducting gaps summed on the nesting hole and electron pockets, essentially in agreement with the results in cuprate and heavy fermion superconductors This observation supports the sign-reversed Cooper pairing mechanism under multiple pairing channels and resolves the long-standing puzzles concerning the broadening and dispersive spin resonance peak in iron pnictides More importantly, the triple resonant modes can be classified into odd and even symmetries with respect to the distance of Fe-Fe planes within the Fe-As bilayer unit Thus, our results closely resemble those in the bilayer cuprates with nondegenerate spin excitations, suggesting that these two high-${T}_{c}$ superconducting families share a common nature

/pdf/odd-and-even-modes-of-neutron-spin-resonance-in-the-bilayer-qizc6uckk2.pdf

Odd and even modes of neutron spin resonance in the bilayer iron-based superconductor CaKFe 4 As 4

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs) In contrast to those SCs, the newly discovered SC $\mathrm{CaK}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{)}_{4}{\mathrm{As}}_{4}$ exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal (SVC) structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations Our $^{75}\mathrm{As}$ nuclear magnetic resonance studies on $\mathrm{CaK}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{)}_{4}{\mathrm{As}}_{4}$ ($0\ensuremath{\le}x\ensuremath{\le}0049$) revealed that ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP) The magnetic QCP without nematicity in $\mathrm{CaK}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{)}_{4}{\mathrm{As}}_{4}$ highlights the close connection of spin fluctuations and superconductivity in iron-based SCs The advantage of stoichiometric composition also makes ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects

https://link.aps.org/accepted/10.1103/PhysRevLett.121.137204

Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality in CaK(Fe1-xNix)4As4 Revealed by NMR

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe4As4 was investigated by using a combination of inelastic neutron scattering (INS) measurements and random...

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

The $\mathit{ab}$-plane optical properties of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ single crystals have been measured over a wide temperature and frequency range. Below ${T}^{*}\ensuremath{\simeq}155$ K, where this material undergoes an incoherent-coherent crossover, a new coherent response emerges in the optical conductivity. A spectral weight analysis suggests that this feature originates from high-energy bound states. Below $\ensuremath{\simeq}75$ K the scattering rate for this feature is quadratic in temperature, indicating a Fermi-liquid response. Theoretical calculations suggest this crossover is dominated by the ${d}_{xy}$ orbital. Our results indicate Kondo-type screening is the likely mechanism for the incoherent-coherent crossover in hole-overdoped ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$.

Observation of an emergent coherent state in the iron-based superconductor KFe 2 As 2

The temperature-dependence of the in-plane optical properties of $({\mathrm{CaFe}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}{\mathrm{As})}_{10}{\mathrm{Pt}}_{3}{\mathrm{As}}_{8}$ have been investigated for the undoped ($x=0$) parent compound, and the optimally doped ($x=0.1$) superconducting material (${T}_{c}\ensuremath{\simeq}12$ K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and magnetic transitions at $\ensuremath{\simeq}96$ and 83 K, respectively, strength is transferred from the free-carrier components into a bound excitation at $\ensuremath{\simeq}1000\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and the material exhibits semiconducting-like behavior. In the optimally doped sample, at room temperature the optical properties are again described by narrow and broad Drude responses comparable to the parent compound; however, below ${T}^{*}\ensuremath{\simeq}100$ K, strength from the narrow Drude is transferred into a newly emergent low-energy peak at $\ensuremath{\simeq}120\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which arises from a localization process, resulting in semiconducting-like behavior. Interestingly, below ${T}_{c}$, this peak also contributes to the superfluid weight, indicating that some localized electrons condense into Cooper pairs; this observation may provide insight into the pairing mechanism in iron-based superconductors.

Unravelling the mechanism of the semiconducting-like behavior and its relation to superconductivity in ( CaFe 1 − x Pt x As ) 10 Pt 3 As 8

The temperature dependence of $ab$-plane optical conductivity of ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ has been measured below and above its superconducting transition temperature ${T}_{c}\ensuremath{\simeq}35.5$ K. In the normal state, analysis with the two-Drude model reveals a $T$-linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 ${\mathrm{cm}}^{\ensuremath{-}1}$ vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\simeq}9$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\simeq}14$ meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 ${\mathrm{cm}}^{\ensuremath{-}1}$, we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.

/pdf/anomalous-phonon-behavior-in-superconducting-cakfe4as4-an-2npqfulabx.pdf

Anomalous phonon behavior in superconducting CaKFe4As4: An optical study

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Observation of an emergent coherent state in the iron-based superconductor KFe 2 As 2

Unravelling the mechanism of the semiconducting-like behavior and its relation to superconductivity in ( CaFe 1 − x Pt x As ) 10 Pt 3 As 8

Anomalous phonon behavior in superconducting CaKFe4As4: An optical study