Pnictide Superconductors Research Articles

Solid state single crystal growth of three-dimensional faceted LaFeAsO crystals

Solid state single crystal growth (SSCG) is a crystal growth technique where crystals are grown from a polycrystalline matrix. Here, we present single crystals of the iron pnictide LaFeAsO grown via SSCG using NaAs as a liquid phase to aid crystallization. The size of the as-grown crystals are up to 2 × 3 × 0.4 mm3. Typical for this method, but very uncommon for crystals of the pnictide superconductors and especially for the oxypnictides, the crystals show pronounced facets caused by considerable growth in c direction. The crystals were characterized regarding their composition, structure, magnetic, and thermodynamic properties. This sets the stage for further measurements for which single crystals are crucial such as any c axis and reciprocal space dependent measurements.

Study of the second magnetization peak and the pinning behaviour in Ba(Fe0.935Co0.065)2As2 pnictide superconductor

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the axis on a single crystal of Ba(Fe0.935 Co0.065)2As2 pnictide superconductor having Tc = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from Tc to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (Hspt) between Hon (onset of SMP in M(H)) and Hp (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near Hspt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley’s criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual and nature of pinning. The critical current density (Jc), estimated using the Bean critical state model, was found to be 5 × 105 A cm−2 at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.

La(Zn1−2xMnxCux)SbO: A new diluted magnetic semiconductor isostructural to 1111-type iron pnictide superconductors

A new diluted magnetic semiconductor La(Zn1−2xMnxCux)SbO in bulk form has been successfully synthesized. Spins and carriers are introduced by co-doping Mn and Cu into Zn sites of the parent semiconductor LaZnSbO. Doping Mn or Cu alone does not induce magnetic ordering. Only when Mn and Cu are co-doped, can the system undergo a ferromagnetic transition below the Curie temperature . The ferromagnetic La(Zn1−2xMnxCux)SbO shares the same two-dimensional crystal structure as that of the 1111-type iron pnictide superconductor . A relatively large coercive field of is observed from the iso-thermal magnetization measured at 2 K.

Transport current density at temperatures up to 25 K of Cu/Ag composite sheathed 122-type tapes and wires

The fabrication of iron-based superconductors with high transport critical current density (Jc) and low cost is a crucial determinant of whether they can be used for practical applications. In this paper, Cu/Ag composite sheathed Sr0.6K0.4Fe2As2 (Sr122) tapes and Ba0.6K0.4Fe2As2 (Ba122) round wires were fabricated via an ex situ powder-in-tube method and heat-treated by the hot pressing and hot isostatic pressing process respectively. In order to thoroughly reveal the application potential of Cu/Ag composite sheathed ‘122’ iron pnictide superconductors, transport Jc of tapes and wires in high fields at temperatures up to 25 K was measured. High transport Jc of 4.4 × 104 A cm−2 at 4.2 K and 3.6 × 103 A cm−2 at 20 K in 10 T was achieved in Cu/Ag composite sheathed Sr122 tapes. Transport Jc of Ba122 wires is 9.4 × 103 A cm−2 at 4.2 K and 1.9 × 103 A cm−2 at 20 K in 10 T. These results demonstrate the great potential of Cu/Ag composite sheathed ‘122’ iron pnictide superconducting tapes and wires for high-field applications at intermediate temperatures around 20 K, which can be easily obtained by using cryocoolers.

Upper critical and irreversibility fields in Ni- and Co-doped pnictide bulk superconductors

A comprehensive study of upper critical and irreversibility magnetic fields in Ba(Fe0.95Ni0.05)2As2 (large grain and small grain samples), Ba(Fe0.94Ni0.06)2As2, Ba(Fe0.92Co0.08)2As2, and Ba(Fe0.92Co0.09)2As2 polycrystalline bulk pnictide superconductors was made in pulsed fields of up to 65T. The full magnetic field-temperature (H-T) phase diagrams, starting at 1.5K, were measured. The higher temperature, upper critical field Hc2 data are well described by the one-band Werthamer, Helfand, and Hohenberg (WHH) model. At low temperatures, the experimental data depart from the fitted WHH curves, suggesting an emergence of a new phase that could be attributed to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The large values of the Maki fitting parameter α indicate that the Zeeman pair breaking dominates over the orbital pair breaking and spin-paramagnetic pair-breaking effect is significant in these materials. Possible multi-band structure of these materials is lumped into effective parameters of the single-band model. Table of measured physical parameters allows us to compare these pnictide superconductors for different Co- and Ni- doping levels and granularity.

Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO

We investigate the temperature dependence of the lower critical field $H_{c1}(T)$, the field at which vortices penetrate into the sample, of a high-quality fluorine-doped NdFeAsO single crystal under static magnetic fields $H$ parallel to the $c$-axis. The temperature dependence of the first vortex penetration field has been experimentally obtained and pronounced changes of the $H_{c1}$(T) curvature are observed, which is attributed to the multiband superconductivity. Using a two-band model with $s$-wave-like gaps, the temperature-dependence of the lower critical field $H_{c1}(T)$ can be well described. These observations clearly show that the superconducting energy gap in fluorine-doped NdFeAsO is nodeless. The values of the penetration depth at $T$ = 0\,K have been determined and confirm that the pnictide superconductors obey an Uemura-style relationship between $T_{c}$ and $\lambda_{ab}(0)^{-2}$

Universal linear-temperature resistivity: possible quantum diffusion transport in strongly correlated superconductors

The strongly correlated electron fluids in high temperature cuprate superconductors demonstrate an anomalous linear temperature (T) dependent resistivity behavior, which persists to a wide temperature range without exhibiting saturation. As cooling down, those electron fluids lose the resistivity and condense into the superfluid. However, the origin of the linear-T resistivity behavior and its relationship to the strongly correlated superconductivity remain a mystery. Here we report a universal relation d{boldsymbol{rho }}/{boldsymbol{dT}}=({{boldsymbol{mu }}}_{{bf{0}}}{{boldsymbol{k}}}_{{boldsymbol{B}}}/{boldsymbol{hslash }}),{{boldsymbol{lambda }}}_{{boldsymbol{L}}}^{{bf{2}}}, which bridges the slope of the linear-T-dependent resistivity (dρ/dT) to the London penetration depth λL at zero temperature among cuprate superconductor Bi2Sr2CaCu2O8+δ and heavy fermion superconductors CeCoIn5, where μ0 is vacuum permeability, kB is the Boltzmann constant and ħ is the reduced Planck constant. We extend this scaling relation to different systems and found that it holds for other cuprate, pnictide and heavy fermion superconductors as well, regardless of the significant differences in the strength of electronic correlations, transport directions, and doping levels. Our analysis suggests that the scaling relation in strongly correlated superconductors could be described as a hydrodynamic diffusive transport, with the diffusion coefficient (D) approaching the quantum limit D ~ ħ/m*, where m* is the quasi-particle effective mass.

Tunneling conductance and twofold spin-singlet Andreev reflections in ferromagnet/ferromagnet/iron pnictide superconductor hybrid structures with collinear magnetizations

Iron-based superconductors, as some other high-temperature superconducting materials such as the cuprates, are confronted with uncovering the unconventional pairing symmetry, although most researchers favor the so-called s±-wave pairing state. Herein, we theoretically investigate the tunneling conductance of clean ferromagnet (FM)/FM/iron pnictide superconductor (SC) hybrid structures with the SC having s±(two energy gaps have phase difference) pairing symmetry. Novel twofold spin-singlet pairing states near the FM/SC interface in collinear magnetizations emerge due to the presence of two bands in the SC. Conversions of the differential conductance in the ferromagnetic alignment of the two FMs between the peak and valley are shown to be much different from those in the antiferromagnetic alignment. More importantly, two rather different properties in contrast with tunneling into a conventional s-wave SC and an s++-wave SC (two energy gaps have the same sign) are also exhibited, which can be experimentally used to probe and identify the s±pairing symmetry in the iron pnictide SC.

Memory matrix theory of the dc resistivity of a disordered antiferromagnetic metal with an effective composite operator

We perform the calculation of the dc resistivity as a function of temperature of the “strange-metal” state that emerges in the vicinity of a spin-density-wave phase transition in the presence of weak disorder. This scenario is relevant to the phenomenology of many important correlated materials, such as, e.g., the pnictides, the heavy-fermion compounds and the cuprates. To accomplish this task, we implement the memory-matrix approach that allows the calculation of the transport coefficients of the model beyond the quasiparticle paradigm. Our computation is also inspired by the ϵ=3−d expansion in a hot-spot model embedded in d-space dimensions recently put forth by Sur and Lee (2015), in which they find a new low-energy non-Fermi liquid fixed point that is perturbatively accessible near three dimensions. As a consequence, we are able to establish here the temperature and doping dependence of the electrical resistivity at intermediate temperatures of a two-dimensional disordered antiferromagnetic metallic model with a composite operator that couples the order-parameter fluctuations to the entire Fermi surface. We argue that our present theory provides a good basis in order to unify the experimental transport data, e.g., in the cuprates and the pnictide superconductors, within a wide range of doping regimes.

Superconducting Properties of 100-m Class Sr0.6K0.4Fe2As2 Tape and Pancake Coils

Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a long wire form which can be used for different devices. In this work, 100 m class 7-core Sr0.6K0.4Fe2As2 (Sr122) tapes have been made using the powder-in-tube technique, which is reported for the first time. Clearly, an average Jc of 1.3x10^4 A/cm^2 at 10 T was reached over the 115 meter length, showing a high property and good uniformity of 100 meter level Sr122 tapes. Using the 10 m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the mono-filamentary tape. However, transport Ic was obtained in the coil made from the 7-filamentary tape. The factors which affect the superconducting property of the coil were discussed in this work.

Critical Slowing Down of Quadrupole and Hexadecapole Orderings in Iron Pnictide Superconductor

Ultrasonic measurements have been carried out to investigate the critical dynamics of structural and superconducting transitions due to degenerate orbital bands in iron pnictide compounds with the formula Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. The attenuation coefficient $\alpha_{\mathrm{L}[110]}$ of the longitudinal ultrasonic wave for $(C_{11}+C_{12}+2C_{66})/2$ for $x = 0.036$ reveals the critical slowing down of the relaxation time around the structural transition at $T_\mathrm{s} = 65$ K, which is caused by ferro-type ordering of the quadrupole $O_{x'^2-y'^2}$ coupled to the strain $\varepsilon_{xy}$. The attenuation coefficient $\alpha_{66}$ of the transverse ultrasonic wave for $C_{66}$ for $x = 0.071$ also exhibits the critical slowing down around the superconducting transition at $T_\mathrm{SC} = 23$ K, which is caused by ferro-type ordering of the hexadecapole $H_z^\alpha \bigl( \boldsymbol{r}_i, \boldsymbol{r}_j \bigr) = O_{x'y'}\bigl( \boldsymbol{r}_i \bigr) O_{x'^2 - y'^2}\bigl( \boldsymbol{r}_j \bigr) + O_{x'^2 - y'^2}\bigl( \boldsymbol{r}_i \bigr) O_{x'y'}\bigl( \boldsymbol{r}_j \bigr)$ of the bound two-electron state coupled to the rotation $\omega_{xy}$. It is proposed that the hexadecapole ordering associated with the superconductivity brings about spontaneous rotation of the macroscopic superconducting state with respect to the host tetragonal lattice.

A density functional theory study of the influence of exchange-correlation functionals on the properties of FeAs

We use density functional theory within the local density approximation (LDA), LDA + U, generalised gradient approximation (GGA), GGA + U, and hybrid-functional methods to calculate the properties of iron monoarsenide. FeAs, which forms in the MnP structure, is of current interest for potential spintronic applications as well as being the parent compound for the pnictide superconductors. We compare the calculated structural, magnetic and electronic properties obtained using the different functionals to each other and to experiment, and investigate the origin of a recently reported magnetic spiral. Our results indicate the appropriateness or otherwise of the various functionals for describing FeAs and the related Fe-pnictide superconductors.

Two-band and pauli-limiting effects on the upper critical field of 112-type iron pnictide superconductors

The temperature dependence of upper critical field μ0Hc2 of Ca0.83La0.17FeAs2 and Ca0.8La0.2Fe0.98Co0.02As2 single crystals are investigated by measuring the resistivity for the inter-plane (H//c) and in-plane (H//ab) directions in magnetic fields up to 60 T. It is found that μ0Hc2(T) of both crystals for H//c presents a sublinear temperature dependence with decreasing temperature, whereas the curve of μ0Hc2(T) for H//ab has a convex curvature and gradually tends to saturate at low temperatures. μ0Hc2(T) in both crystals deviates from the conventional Werthamer-Helfand-Hohenberg (WHH) theoretical model without considering spin paramagnetic effect for H//c and H//ab directions. Detailed analyses show that the behavior of μ0Hc2(T) in 112-type Iron-based superconductors (IBSs) is similar to that of most IBSs. Two-band model is required to fully reproduce the behavior of μ0Hc2(T) for H//c, while the effect of spin paramagnetic effect is responsible for the behavior of μ0Hc2(T) for H//ab.

Neutron powder diffraction study on the iron-based nitride superconductor ThFeAsN

We report neutron diffraction and transport results on the newly discovered superconducting nitride ThFeAsN with . No magnetic transition, but a weak structural distortion around 160 K, is observed by cooling from 300 K to 6 K. Analysis on the resistivity, Hall transport and crystal structure suggests that this material behaves as an electron optimally doped pnictide superconductor due to extra electrons from nitrogen deficiency or oxygen occupancy at the nitrogen site, which, together with the low arsenic height, may enhance the electron itinerancy and reduce the electron correlations, thus suppressing the static magnetic order.

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

The recently discovered high-T$_c$ superconductor Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a unique compound not only because of its low symmetry crystal structure, but also because of its electronic structure which hosts Dirac-like metallic bands resulting from (spacer) zig-zag As chains. We present a comprehensive first principles theoretical study of the electronic and crystal structures of Ca$_{1-x}$La$_{x}$FeAs$_{2}$. After discussing the connection between the crystal structure of the 112 family, which Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs$_{2}$, and similar hyphothetical compounds SrFeAs$_{2}$ and BaFeAs$_{2}$ which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca$_{1-x}$La$_{x}$FeAs$_{2}$ using the DFT + DMFT method, and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the $yy$ component of the optical conductivity of the 30\% La doped compound, which correponds to coherent interband transitions within a fast-dispersing band arising from the zig-zag As-chains which are unique to this compound. We also study the Landau free energy for Ca$_{1-x}$La$_{x}$FeAs$_{2}$ including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca$_{1-x}$La$_{x}$FeAs$_{2}$.

Superconductivities of pressurized iron pnictide superconductors

As one of the independent control parameters, pressure plays an important role in finding new phenomena, testing related theories and guiding the explorations for new superconductors. In this review article, we will briefly review the progress achieved from high pressure studies on some main types of the iron pnictide superconductors, including 1111-type, 122-type, 111-type, 10-3-8 type and 112-type. A few typical results from high pressure studies are introduced in more detail, including the positive pressure effect on the superconducting transition temperature TC of 1111-type iron pnictide superconductors, which indicates a way to enhance the TC by using a smaller cation to replay La ion; the maximum TC of iron pnictide superconductors estimated by high-pressure studies on a series of 1111-type iron-based superconductors etc. More importantly, high pressure studies on the parent compounds of iron pnictide superconductors clearly demonstrate that pressure can suppress the transition temperatures of magnetic order and crystal structure, and then drive a superconducting transition. Furthermore, many examples are given in this review to reveal how the magnetic order competes with superconductivity under pressure, which provides new constrains for the establishment of the theory on superconductivity. These high pressure results are expected to be helpful for the studies of high-TC superconductors and for the exploring of new superconductors.

A Mott insulator continuously connected to iron pnictide superconductors.

Iron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.

Competing antiferromagnetism in a quasi-2D itinerant ferromagnet: Fe3GeTe2

Fe3GeTe2 is known as an air-stable layered metal with itinerant ferromagnetism with a transition temperature of about 220 K. From our extensive dc and ac magnetic measurements, we have determined that the ferromagnetic layers of Fe3GeTe2 actually order antiferromagnetically along the c-axis below 152 K. The antiferromagnetic state was further substantiated by theoretical calculation to be the ground state. A magnetic structure model was proposed to describe the antiferromagnetic ground state as well as competition between antiferromagnetic and ferromagnetic states. Fe3GeTe2 shares many common features with pnictide superconductors and may be a promising system in which to search for unconventional superconductivity.

Josephson effects at iron pnictide superconductors: Approaching phase‐sensitive experiments

The electrical transport in the pnictides especially the quasiparticle tunnelling and Josephson effects in inhomogeneous superconducting systems can provide information on the symmetry of the order parameter. Different kinds of Josephson junctions with one or two pnictide electrodes were realised and their properties were experimentally studied in detail (e.g., current–voltage characteristics, microwave response, current‐phase relations, dependencies on temperature and external magnetic field). These results were compared to well‐known Josephson junctions and recent theories taking into account the special properties (multiband superconductivity, exotic pairing symmetry) of the pnictides. By combination of two junctions in different crystal directions with different barrier transparencies one forms corner junctions or SQUIDs as phase‐sensitive devices. If the technological problems can be solved the investigations on such devices will finally clear up the symmetry of the pnictides, directly. These investigations will lead to a better understanding of fundamental properties of the pnictides as well as to a realistic judgement on the potential for applications of pnictides in superconducting electronics and sensor applications, both established ones and novel ones utilising the special pnictide properties.

Discovery of FeBi2.

Recent advances in high-pressure techniques offer chemists access to vast regions of uncharted synthetic phase space, expanding our experimental reach to pressures comparable to the core of the Earth. These newfound capabilities enable us to revisit simple binary systems in search of compounds that for decades have remained elusive. The most tantalizing of these targets are systems in which the two elements in question do not interact even as molten liquids—so-called immiscible systems. As a prominent example, immiscibility between iron and bismuth is so severe that no material containing Fe–Bi bonds is known to exist. The elusiveness of Fe–Bi bonds has a myriad of consequences; crucially, it precludes completing the iron pnictide superconductor series. Herein we report the first iron–bismuth binary compound, FeBi2, featuring the first Fe–Bi bond in the solid state. We employed geologically relevant pressures, similar to the core of Mars, to access FeBi2, which we synthesized at 30 GPa and 1500 K. The compound crystallizes in the Al2Cu structure type (space group I4/mcm) with a = 6.3121(3) Å and c = 5.4211(4) Å. The new binary intermetallic phase persists from its formation pressure of 30 GPa down to 3 GPa. The existence of this phase at low pressures suggests that it might be quenchable to ambient pressure at low temperatures. These results offer a pathway toward the realization of new exotic materials.