Family Of Iron Pnictides Research Articles

Современное состояние исследований сверхпроводящей и электронной подсистемы стехиометрических ферропниктидов семейства 1144

Due to multiple-band structure, an interplay between superconducting and magnetic subsystems, the members of the 1144 iron-pnictide family show a number of unique properties not typical to other ferropnictide families. Nonetheless, a quick degradation in open air causes a lack of experimental probes of such extraordinary compounds. In the review, we briefly consider the crystal structure, doping and pressure phase diagram, band-structure features, as well as summarize the available experimental data on the gap structure of the 1144 family pnictides. We compare the properties of the 1144 materials with those of a sister 122 family compounds.

Characteristic Timescales of the Local Moment Dynamics in Hund's Metals.

We study the characteristic timescales of the fluctuating local moments in Hund's metal systems for different degrees of correlation. By analyzing the dynamical spin susceptibility in the real-time domain, we determine the timescales controlling oscillation and damping of on-site fluctuations-a crucial factor for the detection of local moments with different experimental probes. We apply this procedure to different families of iron pnictides or chalcogenides, explaining the material trend in the discrepancies reported between experimental and theoretical estimates of their magnetic moments.

Single-crystal growth of the iron-based superconductor La0.34Na0.66Fe2As2

We report single-crystal growth of a La0.34Na0.66Fe2As2 iron-based superconductor several millimeters in size. The samples were accidentally obtained in trying to grow LaFeAsO1−yFy (y ≥ 0.8 ) single crystals with NaAs and NaF flux. The sample shows both antiferromagnetic and structural transitions at 106 K. The superconducting transition temperature is about 27 K with a superconducting anisotropy of about 1.9. These values and the temperature dependence of the Hall coefficient suggest that La0.34Na0.66Fe2As2 belongs to the hole-doped ‘122’ families of iron pnictides with the doping level slightly lower than optimal doping. Compared with previous reports on the polycrystalline samples, our results suggest that either the Tc of the this system can be further increased, or the superconducting dome may not be well formed in this system. In either case, the La0.5−xNa0.5+xFe2As2 system provides a new platform to study the antiferromagnetic and superconducting properties of iron-based superconductors.

Electronic and spin dynamics in the insulating iron pnictideNaFe0.5Cu0.5As

NaFe$_{0.5}$Cu$_{0.5}$As represents a rare exception in the metallic iron pnictide family, in which a small insulating gap is opened. Based on first-principles study, we provide a comprehensive theoretical characterization of this insulating compound. The Fe$^{3+} $spin degree of freedom is quantified as a quasi-1D $S=\frac{5}{2}$ Heisenberg model. The itinerant As hole state is downfolded to a $p_{xy}$-orbital hopping model on a square lattice. A unique orbital-dependent Hund's coupling between the spin and the hole is revealed. Several important material properties are analyzed, including (a) factors affecting the small $p-d$ charge-transfer gap; (b) role of the extra interchain Fe; and (c) the quasi-1D spin excitation in the Fe chains. The experimental manifestations of these properties are discussed.

Quantum oscillations in the SmFeAsO parent compound and superconducting SmFeAs(O,F)

By carrying out magnetotransport measurements up to 30 T, we succeeded in detecting and analyzing quantitatively Shubnikov--de Haas oscillations in two crystals of the $RE\mathrm{FeAsO}$ ($RE=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth}$) family of iron pnictides, namely, the undoped SmFeAsO and the superconducting fluorine underdoped SmFeAs(O,F). We combine experimental outcomes and ab initio calculations of electronic structure to get the following direct evidences: (i) the Shubnikov--de Haas oscillations are determined by a two-dimensional hole band, whose effective mass is ${m}^{*}=(0.50\ifmmode\pm\else\textpm\fi{}0.1)\phantom{\rule{0.16em}{0ex}}{m}_{e}$ in the undoped sample and ${m}^{*}=(0.89\ifmmode\pm\else\textpm\fi{}0.1)\phantom{\rule{0.16em}{0ex}}{m}_{e}$ in the F-doped sample, while classical magnetotransport is dominated by an electron band; (ii) in the superconducting compound we give a quantitative account of the low effectiveness of electron doping associated with fluorine substitution.

Collective magnetic excitations of C4 -symmetric magnetic states in iron-based superconductors

We study the collective magnetic excitations of the recently discovered $C_{4}$ symmetric spin-density wave states of iron-based superconductors with particular emphasis on their orbital character based on an itinerant multiorbital approach. This is important since the $C_{4}$ symmetric spin-density wave states exist only at moderate interaction strengths where damping effects from a coupling to the continuum of particle-hole excitations strongly modifies the shape of the excitation spectra compared to predictions based on a local moment picture. We uncover a distinct orbital polarization inherent to magnetic excitations in $C_{4}$ symmetric states, which provide a route to identify the different commensurate magnetic states appearing in the continuously updated phase diagram of the iron-pnictide family.

Phase transitions in a frustrated biquadratic Heisenberg model with coupled orbital degrees of freedom for iron-based superconductors

In this work, we study a biquadratic Heisenberg model with coupled orbital degree of freedom using Monte Carlo simulation in order to investigate the phase transitions in iron-based superconductors. The antiferro-quadrupolar state, which may be related to the magnetism of FeSe [Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most recent experiments [Nat. Mater. 14, 210 (2015)]. In addition, it is suggested that the orbital interaction may lead to the separation of the structural and magnetic phase transitions as observed in many families of iron pnictides.

Weak-coupling superconductivity in a strongly correlated iron pnictide.

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.

Electron–phonon coupling in BaFe2As2: A possible origin of nematic phase

Iron-based superconductors have the second-highest superconducting transition temperature next to cuprate superconductors. There are many electronic phases such as superconducting, antiferromagnetic and nematic phases, coexisting in the family of iron pnictides. The charge/spin orders are the key factors for understanding the mechanism of high-temperature superconductivity. In this paper we have performed a Raman scattering study of parent compound BaFe2As2. A relatively strong electron–phonon coupling is observed for B1g phonon mode and is weakened after the structural transition at ∼140 K. Through a careful symmetry analysis, we revealed the possibility that B1g mode can be effectively coupled with dxz and dyz orbits. The coupling can remove the degeneracy of the two orbits and further cause the fluctuations of electronic nematic phases. The study suggests that electron–phonon coupling can be a possible origin of nematic phase.

Structure and composition of the superconducting phase in alkali iron selenideKyFe1.6+xSe2

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide K$_y$Fe$_{1.6+x}$Se$_2$. In the high temperature paramagnetic state, both materials have a single phase with crystal structure similar to that of the BaFe$_2$As$_2$ family of iron pnictides. While the insulating K$_y$Fe$_{1.6+x}$Se$_2$ forms a $\sqrt{5}\times\sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5}\times\sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K$_z$Fe$_{2}$Se$_2$ and BaFe$_2$As$_2$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K$_z$Fe$_{2}$Se$_2$ in the matrix of the insulating block AF phase.

Electronic nematicity revealed by torque magnetometry inEuFe2(As1−xPx)2

Electronic nematics, an electron orientational order which breaks the underlying rotational symmetry, were observed in iron pnictide superconductors several years after their discovery. However, the universality of the doping dependence of this phase and its relation to other symmetry-breaking orders (such as superconductivity) in distinct families of iron pnictides remain outstanding questions. Here we use torque magnetometry as a probe to study the rotational symmetry breaking in $\mathrm{Eu}{\mathrm{Fe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ without introducing external pressure. The nematic phase is found to proliferate well above the structural transition and to persist into the superconducting regime at optimal doping, after which it becomes absent or very weak, in sharp contrast to the behavior observed in $\mathrm{Ba}{\mathrm{Fe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$. These measurements suggest a putative quantum nematic transition near optimal doping under the superconducting dome.

Importance of itinerancy and quantum fluctuations for the magnetism in ironpnictides

By applying density functional theory, we find strong evidence for an itinerant nature of magnetism in two families of iron pnictides. Furthermore, by employing dynamical mean field theory with continuous time quantum Monte Carlo as an impurity solver, we observe that the antiferromagnetic metal with small magnetic moment naturally arises out of coupling between unfrustrated and frustrated bands. Our results point to a possible scenario for magnetism in iron pnictides where magnetism originates from a strong instability at the momentum vector (π,π,π) while it is reduced by quantum fluctuations due to the coupling between weakly and strongly frustrated bands.

Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron–phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.

Electrodynamics of electron-doped iron pnictide superconductors: Normal-state properties

The electrodynamic properties of $\text{Ba}{({\text{Fe}}_{0.92}{\text{Co}}_{0.08})}_{2}{\text{As}}_{2}$ and $\text{Ba}{({\text{Fe}}_{0.95}{\text{Ni}}_{0.05})}_{2}{\text{As}}_{2}$ single crystals have been investigated by reflectivity measurements in a wide frequency range. In the metallic state, the optical conductivity consists of a broad incoherent background and a narrow Drude-type component which determines the transport properties; only the latter contribution strongly depends on the composition and temperature. This subsystem reveals a ${T}^{2}$ behavior in the dc resistivity and scattering rate disclosing a hidden Fermi-liquid behavior in the 122 iron pnictide family. An extended Drude analysis yields the frequency dependence of the effective mass (with ${m}^{\ensuremath{\ast}}/{m}_{b}\ensuremath{\approx}5$ in the static limit) and scattering rate that does not disclose a simple power law. The spectral weight shifts to lower energies upon cooling; a significant fraction is not recovered within the infrared range of frequencies.

The Phenomenology of Iron Pnictides Superconductors Explained in the Framework of -Wave Three-Band Eliashberg Theory

Thes-wave three-band Eliashberg theory can simultaneously reproduce the experimental critical temperatures and the gap values of the superconducting materials , and as exponent of the more important families of iron pnictides. In this model the dominant role is played by interband interactions and the order parameter undergoes a sign reversal between hole and electron bands (-wave symmetry). The values of all the gaps (with the exact experimental critical temperature) can be obtained by using high values of the electron-boson coupling constants and small typical boson energies (in agreement with experiments).

Five-fold way to new high T c superconductors

Discovery of high T c superconductivity in La2−x Ba x CuO4 by Bednorz and Muller in 1986 was a breakthrough in the 75-year long search for new superconductors. Since then new high T c superconductors, not involving copper, have also been discovered. Superconductivity in cuprates also inspired resonating valence bond (RVB) mechanism of superconductivity. In turn, RVB theory provided a new hope for finding new superconductors through a novel electronic mechanism. This article first reviews an electron correlation-based RVB mechanism and our own application of these ideas to some new noncuprate superconducting families. In the process we abstract, using available phenomenology and RVB theory, that there are five directions to search for new high T c superconductors. We call them five-fold way. As the paths are reasonably exclusive and well-defined, they provide more guided opportunities, than before, for discovering new superconductors. The five-fold ways are (i) copper route, (ii) pressure route, (iii) diamond route, (iv) graphene route and (v) double RVB route. Copper route is the doped spin-½ Mott insulator route. In this route one synthesizes new spin-½ Mott insulators and dopes them chemically. In pressure route, doping is not external, but internal, a (chemical or external) pressure-induced self-doping suggested by organic ET-salts. In the diamond route we are inspired by superconductivity in boron-doped diamond and our theory. Here one creates impurity band Mott insulators in a band insulator template that enables superconductivity. Graphene route follows from our recent suggestion of superconductivity in doped graphene, a two-dimensional broadband metal with moderate electron correlations, compared to cuprates. Double RVB route follows from our recent theory of doped spin-1 Mott insulator for superconductivity in iron pnictide family.

Point contact Andreev reflection spectroscopy of superconducting energy gaps in 122-type family of iron pnictides

A brief overview of the superconducting energy gap studies on 122-type family of iron pnictides is given. It seems that the situation in the hole doped Ba 1− x K x Fe 2As 2 is well resolved. Most of the measurements including the presented here point contact Andreev reflection spectra agree on existence of multiple nodeless gaps in the excitation spectrum of this multiband system. The gaps have basically two sizes – the small one with a strength up to the BCS weak coupling limit and the large one with a very strong coupling with 2 Δ L / kT c > 6–8. In the electron doped Ba(Fe 1− x Co x ) 2As 2 the most of the experiments including our point contact measurements reveal in quite broadened spectra only a single gap with a strong coupling strength. The high precision ARPES measurements on this system identified two gaps but very close to each other, both showing a strong coupling with 2 Δ/ kT c ∼ 5 and 6, respectively.

The good samarium

Replacing lanthanum with samarium boosts the critical temperature of the iron pnictide family of superconductors to 43 K