Iron Oxypnictide Research Articles

Heavily Hydride-ion-doped 1111-type Iron-based Superconductors: Synthesis, Physical Properties and Electronic Structure

Iron-based superconductors have grown to be a new continent of high Tc superconductors comparable to cuprates. The optimal critical temperature (Tc) of 56 K in electron-doped 1111-type iron oxypnictides attracts considerable attention of physicists and chemists. Carrier doping is not only essential to induce superconductivity but also is a critical parameter that governs the electronic, magnetic, and crystallographic properties of ground states in high-Tc superconductors. Hydride ion (H-) which is an anionic state of hydrogen acts as an efficient electron donor in the 1111-type, leading to several important discoveries such as two-dome structure of superconducting phase and bipartite parent phase. This article summarizes the synthesis, physical properties, and electronic structure of the H- bearing 1111-type iron-based superconductors along with the relevant phenomena of other superconductors. We show several common characteristics of iron-based superconductors with high-Tc over 50 K and suggest a way to achieve higher Tc.

Magnetism driven by strong electronic correlations in the heavily carrier-doped iron oxypnictide LaFeAsO0.49H0.51

The magnetism of the second antiferromagnetic phase (AF2) arising in the iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was investigated by muon spin rotation measurements under hydrostatic pressure up to 2.6 GPa. The N\'eel temperature ($T_{\rm N}$) obtained for a sample with $x=0.51$ exhibits considerably greater sensitivity to pressure than that in the pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the AF1 phase is always accompanied by the structural transition (from tetragonal to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly higher than $T_{\rm N}$, the AF2 phase prevails at higher pressures above $\sim$1.5 GPa where the structural transition is suppressed ($T_{\rm s}=0$). These features indicate that the microscopic origin of the AF2 phase is distinct from that of AF1, suggesting that electronic correlation plays important role in the former phase. We argue that the orbital-selective Mott transition is a plausible scenario to account for the observed pressure dependence of $T_{\rm N}$ and $T_{\rm s}$ in the AF2 phase.

Characteristics of superconducting subsystems in magnesium diborides and iron oxypnictides from data on spectroscopy of multiple Andreev reflections

Physical parameters of condensates in multiple-gap superconductors are determined by coupling both within and between bands. The simplest description of the physics of the system in case of two superconducting order parameters is offered by the Moskalenko-Suhl two-band model, in which strength of coupling is determined by four constants of electron–boson interaction. The characteristic ratio 2Δ0/kBTc usually exceeds the limit of the BCS theory equal to 3.53, thus requiring renormalization to be introduced for Δ0 or Tc in both BCS integrals. This implies that at least six parameters are to be handled in the Moskalenko–Suhl model to describe a two-gap superconducting system. The quantities observed using various techniques are superposition of contributions from each band and interband interaction, and thus usually cannot be separated in the experiment. Moreover, it is not possible to explore in the experiment individual properties of each of the superconducting subsystems, i.e. to study them in the absence of crossband interaction. In contrast to the Eliashberg model extended for the two-band case, the Moskalenko-Suhl model provides the simplest technique to describe the superconducting state using a minimal set of quantities, a feature that is undoubtedly attractive for experimentalists. The factor that is required for such an estimate to be reliable is direct, simultaneous, and accurate measurement of both order parameters as a function of temperature, a task that is very challenging for the experiment. Multiple Andreev reflection effect (MARE) spectroscopy may be used to determine dependences of order parameters Δ1,2(T) directly without involving additional approximation of experimental spectra of the dynamic conductance of Andreev contacts. We have fitted the experimental dependences Δ1,2(T) obtained within the extended Moskalenko–Suhl model to estimate parameters of superconducting systems such as MgB2 + MgO, Mg1-xAlxB2, and iron-containing oxypnictides ReO1-xFyFeAs (Re = Gd, Sm:Th, La). The intraband coupling was shown to be stronger than the crossband coupling by a factor of 15 for magnesium diborides and 10 for ferrous arsenides with maximal Tc, this ratio decreases together with Tc. The estimated eigen characteristic ratios for “strong” bands are ≈5.5 and 4.6, respectively, are almost independent on chemical composition in the explored range Tc > 20 K. This ratio for “weak” bands is close to the weak-coupling BCS limit 3.5.

Intrinsic Charge Dynamics in High-T_{c} AFeAs(O,F) Superconductors.

We report the first determination of the in-plane complex optical conductivity of 1111 high-T_{c} superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at T_{c} and experiences an anomalous T-linear softening and narrowing below T^{*}≈110 K≫T_{c}. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2Δ/k_{B}T_{c}≈7 in both materials, indicating strong pairing.

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

The fluorine-doped rare-earth iron oxypnictide series SmFeAsO1−xFx (0 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies (Margadonna et al 2009 Phys. Rev. B. 79 014503), the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at = 130 K which is rapidly suppressed by 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120 K. The temperature dependence of the thermal expansion coefficient reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy (Mertelj et al 2010 Phys. Rev. B 81 224504) whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering (Nandi et al 2011 Phys. Rev. B 84 055419). At much lower temperatures than , SmFeAsO exhibits a significant negative thermal expansion on the order of −40 ppm · K−1 in contrast to the behaviour of other rare-earth oxypnictides such as PrFeAsO (Kimber et al 2008 Phys. Rev. B 78 140503) and the actinide oxypnictide NpFeAsO (Klimczuk et al 2012 Phys. Rev. B 85 174506) where the onset of 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements (Tropeano et al 2009 Supercond. Sci. Technol. 22 034004).

Proximity and Josephson effects in microstructures based on multiband superconductors (Review Article)

Emerging in the 1950s, the multiband superconductivity has been considered for a long time as an approximate model in the form of a generalization of the BCS theory to the case of two bands for a more accurate quantitative description of the properties and characteristics of such superconductors as cuprates, heavy fermions compounds, metal boron carbides, fullerides, strontium ruthenate etc. due to their complex piecewise-continuous Fermi surfaces. However the discovery of the multiband structure of the superconducting state in magnesium diboride in 2001 and iron oxypnictides and halides in 2008 led to the appearance of many papers in which effects and different dependences well known for usual one-band s-wave superconductors were re-examined. The main purpose of these studies was to reveal the symmetry type of the order parameter, which provides an important information about the mechanism of Cooper pairing in these superconductors. One of the most effective methods of obtaining information on the symmetry properties of the order parameter in the multiband superconductors is phase-sensitive techniques. This review summarizes the results of theoretical and experimental studies of the proximity and Josephson effects in systems based on multiband superconductors in contact with normal metals, insulators and other superconductors.

Pulsed laser deposition of SmFeAsO1−δ on MgO(100) substrates

Layered iron oxyarsenides are novel interesting semimetallic compounds that are itinerant antiferromagnets in their ground state with a transition to high-temperature superconductivity upon charge carrier doping. The rare earth containing mother compounds offer rich physics due to different antiferromagnetic orderings: the alignment of Fe magnetic moments within the FeAs sublattice, which is believed to play a role for the superconducting pairing mechanism, and the ordering of the rare-earth magnetic moments at low temperatures. Here, we present thin film preparation and a film growth study of SmFeAsO on MgO(100) substrates using pulsed laser deposition (PLD). In general, the PLD method is capable to produce iron oxyarsenide thin films, however, competition with impurity phase formation narrows the parameter window. We assume that the film growth in an ultra-high vacuum (UHV) environment results in an oxygen-deficient phase, SmFeAsO1−δ. Despite the large lattice misfit, we find epitaxial oxyarsenide thin film growth on MgO(100) with evolving film thickness. Bragg reflections are absent in very thin films although they locally show indications for pseudomorphic growth of the first unit cells. We propose the possibility for a Stranski-Krastanov growth mode as a result of the large in-plane lattice misfit between the iron oxypnictide and the MgO unit cells. A columnar 3-dimensional film growth mode dominates and the surface roughness is determined by growth mounds, a non-negligible parameter for device fabrication as well as in the application of surface sensitive probes. Furthermore, we found evidence for a stratified growth in steps of half a unit cell, i.e. alternating growth of (FeAs)− and (SmO1−δ)+ layers, the basic structural components of the unit cell. We propose a simple model for the growth kinetics of this compound.

Experimental Evidence for Static Charge Density Waves in Iron Oxypnictides.

In this Letter we report high-resolution synchrotron x-ray powder diffraction and transmission electron microscope analysis of Mn-substituted LaFeAsO samples, demonstrating that a static incommensurate modulated structure develops across the low-temperature orthorhombic phase, whose modulation wave vector depends on the Mn content. The incommensurate structural distortion is likely originating from a charge-density-wave instability, a periodic modulation of the density of conduction electrons associated with a modulation of the atomic positions. Our results add a new component in the physics of Fe-based superconductors, indicating that the density wave ordering is charge driven.

Pressure effect on iron-based superconductor LaFeAsO1-xHx: Peculiar response of 1111-type structure.

A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1−xHx as a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As–Fe–As angle of the FeAs4 tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs4 coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (Tc) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-Tc under pressure.

In-situ growth of superconducting SmO1-xFxFeAs thin films by pulsed laser deposition.

Oxypnictide thin film growth by pulsed laser deposition (PLD) is one of many insufficiently resolved issues in the research of iron-based superconductors. Here we report on the successful realization of superconducting SmO1−xFxFeAs oxypnictide thin film growth by in-situ PLD on CaF2 (fluorite) substrates. CaF2 acts as fluorine supplier by diffusion and thus enables superconducting oxypnictide thin film growth by PLD. Films are grown heteroepitaxially and characteristically have a broad resistive normal-to-superconducting transition. Best films have onset transition temperatures around 40 K. The proposed in-situ PLD film growth offers an alternative and cheap route for the fabrication of iron oxypnictides. PLD becomes now an additional option for iron oxypnictide synthesis.

Iron pnictide thin films: Synthesis and physics

AbstractSuperconducting thin films play a prominent role in applications of superconductivity and provide an essential source for studying physical phenomena. Here, we summarize the activities for iron pnictide thin films with a focus on the German Special Priority Programme from 2009 until today. The quick availability of such films after the discovery of superconductivity in the iron‐based superconductors enabled a number of experiments. After a general introduction and a brief historical overview we focus on film synthesis of iron pnictides by a two‐step method and by pulsed laser deposition, the assessment of their application potential, the upper critical fields in iron oxypnictides of F‐doped LaOFeAs and F‐doped SmLaOFeAs and on superconductivity in Fe/BaFeAs heterostructures.

Recent progress in pulsed laser deposition of iron based superconductors

Pulsed laser deposition (PLD) is the most commonly used deposition technique for Fe-based superconductor thin films today. The number of grown compounds using PLD is still quite limited to so-called 11 compounds (FeTexSy, FeSe1−xTex) and 122 compounds (primarily Co- and P-substituted BaFe2As2). Especially in the growth of Fe-chalcogenides, PLD is challenged by the strong volatility of the elements and their non-negligible vapour pressure. In addition, in situ PLD of the high-temperature superconducting F-doped iron oxypnictides seemed to be feasible only under reactive deposition and stayed disregarded for some time. Here, we summarise the progress that was recently made in the growth of Fe-based superconducting thin films towards an improved control of thin film stoichiometry and the in situ growth of F-doped iron oxypnictides. The presented new ideas deviate from the standard approach of an adjustment of target composition. We first focus on the growth of FeSe1−xTex films, where the introduction of a buffer layer of same composition decreased surface roughness and allowed epitaxial film growth at reduced deposition temperatures with enhanced reproducibility. Second, we illustrate how F-doping in iron oxypnictide thin films can be obtained during in situ PLD using a diffusive reaction between substrate and the growing film.

High-temperature superconductivity from fine-tuning of Fermi-surface singularities in iron oxypnictides.

In the family of the iron-based superconductors, the REFeAsO-type compounds (with RE being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures (Tc) up to 55 K and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe0.92Co0.08AsO (Tc = 18 K) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the REFeAsO-type materials and if so, whether their exact topology is responsible for the aforementioned record Tc. In this work, we use angle-resolved photoemission spectroscopy (ARPES) to investigate the inherent electronic structure of the NdFeAsO0.6F0.4 compound with a twice higher Tc = 38 K. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine-tuning of one of the band-edge singularities to within a fraction of the superconducting energy gap Δ below the Fermi level. Our results provide compelling evidence that the band-structure singularities near the Fermi level in the iron-based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these materials.

A ferromagnetic quantum critical point in heavy-fermion iron oxypnictide CeFe1−xCrxPO

We report crystallographic and magnetic properties of layered iron oxypnictide CeFe1−xCrxPO (x = 0.000–0.692). Interlayer distances between Ce2O2 and (Fe1−xCrx)2P2 layers increase as a function of x, suggesting suppression of Kondo coupling among hybridized conducting orbitals and localized Ce 4f orbitals. CeFe1−xCrxPO (x = 0.100–0.384) exhibits finite ferromagnetic transition temperatures (Tcurie) obtained by Arrott plots, although 57Fe Mössbauer spectra reveal paramagnetic Fe sublattice at T ≥ 4.2 K. These results indicate that the ferromagnetic phase transitions of samples are mainly due to Ce sublattice. For the samples with x ≥ 0.500, no ferromagnetic order is observed down to 2 K. These results verify that ferromagnetic quantum critical points of CeFe1−xCrxPO appear at 0.045 ≤ x ≤ 0.100 and 0.384 ≤ x ≤ 0.500.

The Role of Yttrium Doping on the Structural and Superconducting Properties of Sm1111 and Its Comparison with Other (RE, Y)1111 (RE $$\varvec{=}$$ = La, Ce, and Nd) Iron Oxypnictides

The role of yttrium doping on the structural and superconducting properties of SmFeAsO $$_{1-x}$$ F $$_x$$ (Sm1111) is studied in detail and is compared with other yttrium-doped RE1111 (RE—rare earth) iron oxypnictide superconductors on the basis of the existing reports. It is found that the partial substitution of lanthanides such as La, Ce, and Nd with yttrium systematically enhances $$T_\mathrm{C}$$ of the system to different levels depending on their crystal ionic radii difference with yttrium. The isovalent substitution of these lanthanides at relatively smaller yttrium ion site causes inner chemical pressure and enhances the $$T_\mathrm{C}$$ . However, yttrium doping does not seem to be beneficial for Sm1111 because of its larger ionic size.

Controlling factors of <i>T<span class="aps-inline-formula"><math><msub><mrow></mrow><mi>c</mi></msub></math></span></i> dome structure in 1111-type iron arsenide superconductors

We investigated the effects of phosphorus substitution on the shape of the Tc(x) dome in 1111-type SmFeAs1-yPyO1-xHx (0 < x < 0.5). Hydride ion substitution of oxide sites (O2- -> H-) exerts a chemical pressure effect, i.e., a structural reduction of the Pn-Fe-Pn angle {\alpha} (Pn = P, As) and also dopes electrons into the FePn layer to induce superconductivity. Isovalent phosphorus substitution (P3- -> As3-) can induce only a chemical pressure effect, i.e., an increase of {\alpha} for La-substitution of Sm-sites. As y increases from 0.0 to 0.5, the single Tc dome gradually splits into two domes, similar to those of LaFeAsO1-xHx with a Tc valley at x ~ 0.16. We found that the Tc valley is located around (x, {\alpha}) ~ (0.16, 113{\deg}) for both SmFeAs1-yPyO1-xHx and LaFeAsO1-xHx series, irrespective of changes in the Pn anion and Ln cation species. This result suggests that suppression of Tc leads to the emergence of a Tc valley when both the shape of FePn4 tetrahedra represented by {\alpha} and electron doping level of x meet the above criterion in 1111 type iron oxypnictide superconductors.

Charge Density and Bonding Properties of LaFeAsO under Pressure

Some interesting aspects of the electronic structure of iron oxypnictides depend sensitively on pressure. We investigate this sensitive pressure dependence in the parent compound LaFeAsO by full-potential electronic structure calculations based on density functional theory. We evaluate the sensitivity of the electronic structure to the Fe–As bond length as being due to the rather high degree of covalency associated with the bond, by visualizing the charge density distributions in detail. A strong pressure dependence in the charge density distributions is observed in the two bands, that form cylindrical electron sheets of the Fermi surface at ambient pressure. The bonding states associated with the Fe and As atoms rapidly grow up to 4 GPa and gradually start to decrease above 4 GPa. These pressure dependences of the covalency for the Fe–As bond are similar to the observed pressure dependences in superconductive transition temperatures. It is suggested that this result play an important role in exploring the origin of the superconductivity of LaFeAsO.

Role of Dirac cones in magnetotransport properties of REFeAsO (RE = rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE = rare earth). On one hand we carry out density functional theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-Γ and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.

Synthesis of New Layered Oxypnictides Sr2CrO2(FeAs)2

We successfully synthesized new layered iron oxypnictide Sr2CrO2(FeAs)2 by solid-state-reaction techniques. This is the first report on the Fe-substituted Sr2Mn3As2O2-type oxypnictides to our knowledge. A pellet of stoichiometric mixtures of SrO, Cr and FeAs wrapped with tantalum foil was sintered at 900{\deg}C for 96 h. Results of x-ray diffraction measurement and Rietveld refinements show that the structure of the sample belongs to the space group of I4/mmm with the lattice constants of a = 3.9948(1) {\AA} and c = 18.447(1) {\AA}. Temperature dependence of the resistivity for Sr2CrO2(FeAs)2 shows semiconductor-like behavior. Additionally, the sample does not exhibit superconducting transition in measurements of resistivity and magnetization even in the low temperature down to 3 K.

Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca(4) tetrahedron, leading to notable anisotropic expansion of the unit cell along the c axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca-H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi(1-x)Mn(x)Ge reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state.