Anion Height Research Articles

Effect of aliovalent substitution on the local structure of CaKFe4As4 superconductor

We have investigated the local structure of the iron-based CaKFe4As4superconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFe4As4, CaK0.9Sr0.1Fe4As4, CaK0.9Ba0.1Fe4As4and Ca0.9Na0.1K0.9Ba0.1Fe4As4. Temperature-dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements are used to determine the near-neighbors bondlengths and their stiffness. The EXAFS analysis reveals that the Fe-As bondlength undergoes negligible changes by substitution, however, the Fe-Fe bondlength and the As height are affected by the Sr substitution. The superconducting transition temperatures of CaK0.9Sr0.1Fe4As4and CaK0.9Ba0.1Fe4As4are very similar even if the mean As heights are significantly different suggesting that the anion height may not be a unique parameter to describe the superconductivity in CaKFe4As4. The mean As heights show a peculiar temperature dependence characteristic of CaKFe4As4system. Furthermore, the temperature-dependent mean square relative displacements reveal similar Fe-Fe bond stiffness in all samples, instead the Fe-As bond is substantially stiffer in case of CaK0.9Sr0.1Fe4As4. The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.

Hydrothermal synthesis of tetragonal FeS: Dome-shaped superconductivity vs Fe:S actual molar ratios

We successfully synthesized tetragonal FeS with various Fe:S molar ratios using a hydrothermal method, and the EDS and XRF results confirmed that FeS exhibits superconductivity within the Fe and S element ratio range of 1.155–1.274, rather than a strict stoichiometric ratio. The reaction temperature can not only play a crucial role in determining the purity of the obtained FeS phase but also significantly alter its micromorphology. Significantly, by analyzing the relationship between the Fe:S actual molar ratio and Tc as well as the Fe:S actual molar ratio and unit cell volume, FeS exhibits dome-shaped superconductivity and shows a negative correlation with Tc and chemical pressure. Furthermore, considering the correlation between anion height (hanion) and Tc, Tc reaches the maximum value of 4.53 K for hanion ≈ 1.28 Å, a behavior distinct from that of observed in Fe-pnictide superconductors. In short, our experimental results provide a unique perspective to deepen our understanding of FeS superconductors.

Pressure-induced antiferromagnetic-tetragonal to nonmagnetic-collapse-tetragonal insulator-metal transition in ThMnAsN

We report first-principles numerical discovery of hydrostatic pressure-driven tetragonal to collapsed tetragonal transition in 1111-type material ThMnAsN accompanied by simultaneous magneto-structural, insulator to metal transition together with complete collapse of Mn moments. We present detailed evolution of various structural parameters, magnetism and electronic structures of ThMnAsN with increasing hydrostatic pressure. All the structural parameters show anomalies at a critical pressure $$P_\text {c} \sim$$ 8.1 GPa; c-lattice parameter, out-of-plane As–As bond length, anion height ( $$h_\text {As}$$ ) undergo drastic modifications compared to the in-plane parameters which is manifested in an iso-structural phase transition from tetragonal to a collapsed tetragonal (cT) phase. These modifications in “local structural correlations” due to pressure destroy the usually localized nature of Mn moments which gets completely quenched. Apart from that the elastic constant, the electronic structures also bear the finger prints of insulator-metal and magneto-structural transition at higher pressures accompanying a total collapse of magnetic moment at the vicinity of $$P_\text {c}$$ . The critical value of the pressure $$P_\text {c}$$ at which tetragonal to collapse tetragonal phase transition occurs, remains robust with respect to the on-site Hubbard correlation (U). The dynamical stability of the compound at higher pressures (above and below the magneto-structural transition) is affirmed through detailed computations of phonon dispersion curves endowed with positive phonon frequency throughout the Brillouin zone. The effect of magnetic spin structure on the electronic band structures is obtained through band unfolding. The electronic structure of ThMnAsN at higher pressures “orbital selectively” influences bands, band gap and closely resembles with the electronic structure of Fe-based superconductors (quasi-two-dimensional Fermi surfaces) with the occurrences of orbital-selective Lifshitz transition.

Transport properties and phase diagrams of FeSe1−S (0 ≤ x ≤ 1) single crystals

Based on the high quality FeSe1−xSx (0 ≤ x ≤ 1) single crystals synthesized via a hydrothermal method, we carried out systematic measurement of the normal state magnetoresistance and Hall effect. Through the maximum entropy mobility spectrum analysis (MEMSA), we investigate the evolution of mobility spectrum, which reflects the multi-band structures for different doping content x and temperature T. The relationships between Tc and carrier properties calculated by MEMSA, as well as bond angles and anion height calculated by first-principle method are investigated. Nonlinear to linear transition of Hall resistivity(ρxy(B)) is mainly attributed to the decreasing mobilities and merging of mobility peaks with the reductive x or increasing T. The relationship between Tc and carrier density n show weak interaction for the orthogonal phases when x ≤ 0.2, but nearly positive correlation for the tetragonal phases when x > 0.2. Bond angle as well as the Se/S anion height change almost linearly with x within the tetragonal phases, and the V-shaped relationships with Tc indicate that structural factors play a key role in the regulation of superconductivity. Our researches are helpful for understanding the multi-band and complex phase diagram of iron-based superconductors.

Stronger quantum fluctuation with larger spins: Emergent magnetism in the pressurized high-temperature superconductor FeSe

A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" quantum spin liquid state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. The ordering further benefits from the ferro-orbital order and shows the observed enhancement around 1GPa. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. In addition, the versatile itinerant carriers produce interesting correlated metal behavior in a large region of phase space. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.

Lix(C5H5N)yFe2-zSe2: A Defect-Resilient Expanded-Lattice High-Temperature Superconductor.

Two-dimensional iron chalcogenide intercalates display a remarkable correlation of the interlayer spacing with enhancement of the superconducting critical temperature (Tc). In this work, synchrotron X-ray absorption (XAS; at the Fe and Se K-edges) and emission (XES; at the Fe Κβ) spectroscopies allow one to discuss how the important rise of Tc (∼44 K) in the molecule-intercalated Lix(C5H5N)yFe2-zSe2 relates to the electronic and local structural changes felt by the inorganic host upon doping (x). XES shows that widely separated layers of edge-sharing FeSe4 tetrahedra carry low-spin moieties, with a local Fe magnetic moment slightly reduced compared to the parent β-Fe2-zSe2. Pre-edge XAS expresses the progressively reduced mixing of metal 3d-4p states upon lithiation. Doping-mediated local lattice modifications, probed by conventional Tc optimization measures (cf. the anion height and FeSe4 tetrahedra regularity), become less relevant when layers are spaced far away. On the basis of extended X-ray absorption fine structure, such distortions are compensated by a softer Fe network that relates to Fe-site vacancies, alleviating electron-lattice correlations and superconductivity. Density functional theory (DFT) guided modification of the isolated Fe2-zSe2 (z, vacant sites) planes, resembling the host layers, identify that Fe-site deficiency occurs at low energy cost, giving rise to stretched Fe sheets, in accordance with experiments. The robust high-Tc in Lix(C5H5N)yFe2-zSe2, arises from the interplay of electron-donating spacers and the iron selenide layer's tolerance to defect chemistry, a tool to favorably tune its Fermi surface properties.

Suppression of Superconductivity and Nematic Order in Fe1-ySe1-xSx (0 ≤ x ≤ 1; y ≤ 0.1) Crystals by Anion Height Disorder.

Connections between crystal chemistry and critical temperature Tc have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry, and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that Tc correlates with the average anion height above the Fe plane, i.e., with the geometry of the FeAs4 or FeCh4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe1-ySe1-xSx (0 ≤ x ≤ 1; y ≤ 0.1), we find that in alloyed crystals Tc is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in Tc(x) and tetragonal-to-orthorhombic (nematic) transition Ts(x) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S)4 tetrahedra with different Fe-Se and Fe-S bond distances. Our results provide evidence of Tc and Ts suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.

In Situ Visualization of Local Distortions in the High-Tc Molecule-Intercalated Lix(C5H5N)yFe2-zSe2 Superconductor.

A time-resolved synchrotron X-ray total scattering study sheds light on the evolution of the different structural length scales involved during the intercalation of the layered iron-selenide host by organic molecular donors, aiming at the formation of the expanded-lattice Lix(C5H5N)yFe2-zSe2 hybrid superconductor. The intercalates are found to crystallize in the tetragonal ThCr2Si2-type structure at the average level, however, with an enhanced interlayer iron-selenide spacing (d = 16.2 Å) that accommodates the heterocyclic molecular spacers. Quantitative atomic pair distribution function (PDF) analysis at variable times suggests distorted FeSe4 tetrahedral local environments that appear swollen with respect to those in the parent β-FeSe. Simultaneously acquired in situ synchrotron X-ray powder diffraction data disclose that secondary phases (α-Fe and Li2Se) grow significantly when a higher lithium concentration is used in the solvothermal reaction or when the solution is aged. These observations are in line with the strongly reducing character of the intercalation medium's solvated electrons that mediate the defect chemistry of the expanded-lattice superconductor. In the latter, intralayer correlated local distortions indicate electron-donating aspects that reflect in somewhat enlarged Fe-Se bonds. They also reveal a degree of relief of chemical pressure associated with a large distance between Fe and Se sheets ("taller" anion height) and a stretched Fe-Fe square planar topology. The elongation of the latter, derived from the in situ PDF study, speaks for a plausible increase in the Fe-site vacancy concentration. The evolution of the local structural parameters suggests an optimum reaction window where kinetically stabilized phases resemble the distortions of the edge-sharing Fe-Se tetrahedra, required for a high-Tc in expanded-lattice iron-chalcogenides.

Relationship between anion height and superconducting properties for Fe1+x Te0.6 Se0.4 bulk single crystals

We have synthesized Fe1+xTe0.6Se0.4 large bulk single crystals by the melting method with a homogenization heat treatment. The relationship between the crystal structures and the superconducting properties was investigated. Critical temperature Tc onset reached the highest value of 14 K for the crystal with x = - 0.005. The maximum critical current densities Jc in the a-b plane at 4.2 K achieved 1.53 and 0.83×105 A/cm2 at 0 T and 5 T for the crystal with x = 0, respectively. The crystal lattice and structural parameters were refined by Rietveld analysis. The anion height was estimated by using the obtained parameters. The anion height dependence of Tc and Jc were investigated. The Tc decreased as the anion height increased, and it showed the same tendency as other Fe-based superconductors. On the other hand Jc increased initially and then decreased with the increase in the anion height. We believe that it comes from the trade-off between the increase in flux pinning centres of excess Fe sites and the deterioration of superconductivity.

Anion height dependence of critical current densities for Fe Te1-x Sex bulk single crystals

We have fabricated FeTe1-xSex high quality bulk single crystals. The crystals with different composition of x = 0.375, 0.4, 0.45, 0.46 and 0.475 were prepared by the melting method with two stage heat treatment. The crystal structure and superconducting properties were evaluated. Temperature dependence of magnetization showed that low-T c region exists inside the crystals. Critical temperature T c was the highest value of 14.3 K for the crystal with x = 0.45. Critical current densities J c under the magnetic fields parallel to the c-axis at 4.2 K achieved 1.52 and 0.72 × 105 A / cm2 at 0 T and 5 T respectively for the crystal with x = 0.4. The crystal lattice parameters were examined by Rietveld refinement. The anion height from Fe layer was calculated using the obtained parameters. We investigated the anion height dependence of T c and J c and mentioned an important role of excess Fe about the magnetic flux pinning.

Superconductivity of KFe2As2 Under Pressure: Ab Initio Study of Tetragonal and Collapsed Tetragonal Phases

KFe2As2 is one of the representatives of iron-based superconductors. Many interesting features distinguish this compound from other iron-based superconductors, e.g., a realization of the Pauli limit or an occurrence of the superconducting gap with nodal lines. Moreover, with increasing pressure, the isostructural phase transition from the tetragonal to collapsed tetragonal phase is experimentally observed. We discuss the structural, electronic, and superconducting properties of the KFe2As2 under pressure using the ab initio density functional theory (DFT) methods. We analyze the untypical properties of this superconductor considering, among others, the Fermi surfaces or the dependence of the anion height from the iron layer on the superconducting critical temperature.

Asymmetrically optimized structure in a high-Tc single unit-cell FeSe superconductor

We report asymmetric Se heights in a single unit-cell (UC) FeSe on SrTiO3(0 0 1) substrate with the highest superconducting transition temperature (Tc) among the Fe-based superconductors revealed by total-reflection high-energy positron diffraction measurements. Among various iron-based superconductors, this single UC FeSe on the SrTiO3(0 0 1) has been the best material to achieve the highest-Tc above 50 K. We found the asymmetric Se heights of 1.44 ± 0.03 and 1.33 ± 0.03 Å from the single Fe layer by the intensity analysis based on dynamical diffraction theory. The average Se height results in 1.39 ± 0.04 Å, corresponding to the optimum value for Fe-based superconductors. In addition, the average of bond angles of Se–Fe–Se, 107.2 ± 1.1 and 111.5 ± 1.2° becomes 109.3 ± 1.6°, which is close to the optimum value of 109.5° for a regular tetrahedron. Thus, this single UC FeSe is found to have asymmetrically optimized structure. Based on our first-principles calculations, the asymmetry does not change the bandwidth whereas it splits the electron bands at the M point only at the bottom. These calculations suggest that at low electron doping, the structural asymmetry is expected to lead to exotic properties of non-centrosymmetric superconductivity, whereas after a certain amount of electron doping, average anion height plays an important role for high-Tc.

Realization of continuous electron doping in bulk iron selenides and identification of a new superconducting zone.

It is known that iron selenide superconductors exhibit unique characteristics distinct from iron pnictides, especially in the electron-doped region. However, a comprehensive study of continuous carrier doping and the corresponding crystal structures of FeSe is still lacking, mainly due to the difficulties in controlling the carrier density in bulk materials. Here we report the successful synthesis of a new family of bulk Lix(C3N2H10)0.37FeSe, which features a continuous superconducting dome harboring Lifshitz transition within the wide range of . We demonstrate that with electron doping, the anion height of FeSe layers deviates linearly away from the optimized values of pnictides and pressurized FeSe. This feature leads to a new superconducting zone with unique doping dependence of the electronic structures and strong orbital-selective electronic correlation. Optimal superconductivity is achieved when the orbitals have almost the same intermediate electronic correlation strength, with moderate mass enhancement between 3 ~ 4 in the two separate superconducting zones. Our results shed new light on achieving unified mechanism of superconductivity in iron-based superconductors.

Pressure effects on superconductivity and structural parameters of ThFeAsN

Here we report effects of pressure on the recently discovered iron-based superconductor ThFeAsN, the first FeAs-based superconductor containing an actinide element, by in situ high-pressure resistance and synchrotron X-ray diffraction measurements. Our results show that the superconducting transition temperature (TC) decreases monotonously with increasing pressure and then vanishes at 25.4 GPa. The analysis of the As anion height above the Fe layer (), as well as the bond angle of As-Fe-As (α), for the pressurized ThFeAsN sample found that its TC vs. or α follows the same universal curve describing most of the iron-based superconductors. The results indicate that the lattice parameters also control the superconductivity of the ThFeAsN superconductor, similar to other FeAs-based superconductors, as long as the “superconducting gene” is a FeAs layer. Furthermore, we demonstrate that the ambient-pressure ThFeAsN is an “intrinsic” superconductor, while its lattice structure is not in the optimal state of the FeAs-based superconductors.

Coexistence of superconductivity and short-range double-stripe spin correlations in Te-vapor annealed FeTe1−xSex ( x≤0.2 )

In as-grown bulk crystals of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ with $x\lesssim0.3$, excess Fe ($y>0$) is inevitable and correlates with a suppression of superconductivity. At the same time, there remains the question as to whether the character of the antiferromagnetic correlations associated with the enhanced anion height above the Fe planes in Te-rich samples is compatible with superconductivity. To test this, we have annealed as-grown crystals with $x=0.1$ and 0.2 in Te vapor, effectively reducing the excess Fe and inducing bulk superconductivity. Inelastic neutron scattering measurements reveal low-energy magnetic excitations consistent with short-range correlations of the double-stripe type; nevertheless, cooling into the superconducting state results in a spin gap and a spin resonance, with the extra signal in the resonance being short-range with a mixed single-stripe/double-stripe character, which is different than other iron-based superconductors. The mixed magnetic character of these superconducting samples does not appear to be trivially explainable by inhomogeneity.

Anisotropic Pressure Effects on Superconductivity in Fe1+yTe1−xSx

We have investigated uniaxial and hydrostatic pressure effects on superconductivity in Fe1.07Te0.88S0.12 through magnetic-susceptibility measurements down to 1.8 K. The superconducting transition temperature Tc is enhanced by out-of-plane pressure (uniaxial pressure along the c-axis); the onset temperature of the superconductivity reaches 11.8 K at 0.4 GPa. In contrast, Tc is reduced by in-plane pressure (uniaxial pressure along the ab-plane) and hydrostatic pressure. Taking into account these results, it is inferred that the superconductivity of Fe1+yTe1-xSx is enhanced when the lattice constant c considerably shrinks. This implies that the relationship between Tc and the anion height for Fe1+yTe1-xSx is similar to that applicable to most iron-based superconductors. We consider the reduction of Tc by hydrostatic pressure due to suppression of spin fluctuations because the system moves away from antiferromagnetic ordering, and the enhancement of Tc by out-of-plane pressure due to the anion height effect on Tc.

Observation of direct evolution from antiferromagnetism to superconductivity in Cu1−xLixFeAs ( 0≤x≤1.0 )

We report the structure, antiferromagnetism, and superconductivity in $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{i}}_{x}\mathrm{FeAs}$ ($0\ensuremath{\le}x\ensuremath{\le}1.0$) samples. A direct evolution from antiferromagnetism to superconductivity is observed as increasing doping level of Li. A phase diagram is constructed to show this evolution, which features no coexistence region between superconductivity and antiferromagnetism. This behavior shows that antiferromagnetic CuFeAs can be regarded as a parent compound to the observed superconductivity by equivalent doping, which is different from the cases with other FeAs-based superconductors. Structural analyses and first-principles calculations indicate that the anion height of $\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ tetrahedral layer plays a crucial role on the physical properties. Moreover, the simple Fermi surface nesting picture adopted to explain the evolution from spin-density wave to superconductor in other FeAs-based superconductors might be not applicable to $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{i}}_{x}\mathrm{FeAs}$.

Electronic Properties and Fermi Surface for New Layered High-Temperature Superconductors CaAFe4As4 (A = K, Rb, and Cs): FLAPW-GGA Calculations

Recently, new FeAs-based high-temperature superconductors CaAFe4As4 (A = K, Rb, and Cs) with a layered tetragonal crystal structure were synthesized (T C ∼ 30 K). In this letter, we report for the first time the band structures, Fermi surface topology, total and partial densities of electronic states, and interatomic interactions for CaAFe4As4 as estimated by means of the first-principle FLAPW-GGA calculations. The obtained data for CaAFe4As4 phases are analyzed in comparison with each other and with other related FeAs systems. The interatomic bonding picture can be represented as a highly anisotropic mixture of metallic, covalent, and ionic contributions, which are realized inside Fe4As4-layered blocks and between these blocks and Ca, A atomic sheets. The Fermi surfaces of these systems have a multisheet character and are compiled of a large number of cylinders at the edges and in the central part of the Brillouin zone. It is established that the high-temperature superconductivity in CaAFe4As4 compounds as in other related systems correlates well with such structure parameters as bond angles and anion height.

Direct response of the spin-density wave transition and superconductivity to anion height inSrFe2As2

We explored the effects of isovalent doping on the evolution of structure, spin-density wave (SDW), and superconductivity (SC) in three series of Ba-doped $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2\ensuremath{-}y}{\mathrm{P}}_{y}$ ($y=0.5,\phantom{\rule{0.16em}{0ex}}0.7,\phantom{\rule{0.16em}{0ex}}1.0$). For the $y=0.5$ series, the SDW and SC can coexist in the all-Ba-doped $\mathrm{SrF}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{1.5}{\mathrm{P}}_{0.5}$, while the SC first decreases and then saturates in the case of $y=0.7$. Besides, it is strikingly found that the SC of Ba-doped ${\mathrm{SrFe}}_{2}\mathrm{AsP}$ exhibits a dome-shaped ${T}_{\mathrm{c}}$, in which the anion height, ${h}_{\mathrm{pn}}$, approaching the optimal value of 1.308(2) \AA{} and the ${T}_{\mathrm{c}}$ simultaneously increase to the maxima. The phase diagrams of SDW and SC as a function of ${h}_{\mathrm{pn}}$ and Pn-Fe-Pn angle are first constructed here. It is argued that the isovalent substitutions in FeAs-based superconductors could significantly alter SC and ground states equally as well as carrier doping. The theoretical calculations show that electronic band structure is strongly dependent on the parameter of ${h}_{\mathrm{pn}}$ without introducing carriers by doping. The transition from three-dimensional-like to two-dimensional electronic states and the nesting of the Fermi surface can be realized by changing the ${h}_{\mathrm{pn}}$.

Magnetism and site exchange in CuFeAs and CuFeSb: A microscopic and theoretical investigation

We have investigated the magnetic ground state of CuFeAs and CuFeSb by means of $^{57}\mathrm{Fe}$-M\ossbauer spectroscopy, muon spin rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$, neutron diffraction, and electronic structure calculations. Both materials share the 111-LiFeAs crystal structure and are closely related to the class of iron-based superconductors. In both materials there is a considerable occupancy of the Cu site by Fe, which leads to ferromagnetic moments, which are magnetically strongly coupled to the regular Fe site magnetism. Our study shows that CuFeAs is close to an antiferromagnetic instability, whereas a ferromagnetic ground state is observed in CuFeSb, supporting theoretical models of anion height driven magnetism.