Anisotropy in the electrical resistivity and susceptibility of superconducting BaFe2As2 single crystals

Intrinsic Spin Susceptibility and Pseudogaplike Behavior in Infinite-Layer LaNiO_{2}

The recent discovery of superconductivity in doped infinite-layer nickelates has stimulated intensive interest, especially for similarities and differences compared to that in cuprate superconductors. In contrast to cuprates, although earlier magnetization measurement reveals a Curie-Weiss-like behavior in undoped infinite-layer nickelates, there is no magnetic ordering observed by elastic neutron scattering down to liquid helium temperature. Until now, the nature of the magnetic ground state in undoped infinite-layer nickelates was still elusive. Here, we perform a nuclear magnetic resonance (NMR) experiment through ^{139}La nuclei to study the intrinsic spin susceptibility of infinite-layer LaNiO_{2}. First, the signature for magnetic ordering or freezing is absent in the ^{139}La NMR spectrum down to 0.24 K, which unambiguously confirms a paramagnetic ground state in LaNiO_{2}. Second, a pseudogaplike behavior instead of Curie-Weiss-like behavior is observed in both the temperature-dependent Knight shift and nuclear spin-lattice relaxation rate (1/T_{1}), which is widely observed in both underdoped cuprates and iron-based superconductors. Furthermore, the scaling behavior between the Knight shift and 1/T_{1}T has also been discussed. Finally, the present results imply a considerable exchange interaction in infinite-layer nickelates, which sets a strong constraint for the proposed theoretical models.

Interfacial Superconductivity Achieved in Parent AEFe2As2 (AE = Ca, Sr, Ba) by a Simple and Realistic Annealing Route

Materials with interfaces often exhibit extraordinary phenomena exemplified by rich physics, such as high-temperature superconductivity and enhanced electronic correlations. However, demonstrations of confined interfaces to date have involved intensive effort and fortuity, and no simple path is consistently available. Here, we report the achievement of interfacial superconductivity in the nonsuperconducting parent compounds AEFe₂As₂, where AE = Ca, Sr, or Ba, by simple subsequent annealing of the as-grown samples in an atmosphere of As, P, or Sb. Our results indicate that the superconductivity originates from electron transfer at the interface of the hybrid van der Waals heterostructures, consistent with the two-dimensional superconducting transition observed. The observations suggest a common origin of interfaces for the nonbulk superconductivity previously reported in the AEFe₂As₂ compound family and provide insight for the further exploration of interfacial superconductivity.

Evolution of Structure and Electronic Correlations in a Series of BaT2As2 (T = Cr-Cu) Single Crystals

We present a systematic study of the evolution of structural parameters and electronic correlations as a function of 3d band filling in a single crystal series of BaT₂As₂ (T = Cr-Cu). The structure trends are discussed in relation to the orbital occupation of the corresponding d elements supported by calculations of the charge density and electron localization function. Analysis of our specific heat data yields the mass enhancement (m*/m_b) throughout the series. By combining the structural data with the mass enhancement values, we find that the decrease in m*/m_b for n > 5 follows an increase of the crystal field splitting, determined by the progressive distortion of the As-T-As angle from the ideal tetrahedral environment. This study finds a strong interplay between crystal structure, bonding behavior, band filling, and electronic properties.

Identifying materials with charge-spin physics using charge-spin susceptibility computed from first principles

The authors present a quantity termed charge-spin susceptibility, which measures the charge response to spin degrees of freedom in strongly correlated materials. This quantity is simple to evaluate using both standard density functional theory and many-body electronic structure techniques, enabling comparison between different levels of theory. A benchmark on 28 layered magnetic materials shows that large values of charge-spin susceptibility correlate with unconventional ground states such as disordered magnets and unconventional superconductivity.

Decoupling of itinerant and localized d-orbital electrons in the compound Sc0.5Zr0.5Co

By using the arc-melting method, we successfully synthesize the compound Sc0.5Zr0.5Co with the space group ofPm-3m. Both the resistivity and magnetic susceptibility measurements reveal a phase transition at about 86 K. This transition might be attributed to the establishment of an antiferromagnetic order. The magnetization hysteresis loop measurements in wide temperature region show a weak ferromagnetic feature, which suggests a possible canted arrangement of the magnetic moments. Bounded by the phase transition temperature, the resistivity at ambient pressure shows a change from Fermi liquid behavior to a super-linear behavior as temperature increases. By applying pressure up to 32.1 GPa, the transition temperature does not show a clear change and no superconductivity is observed above 2 K. The density functional theory calculations simulate the antiferromagnetic order and reveal a gap between the spin-up and spin-down d-orbital electrons. This kind of behavior may suggest that the antiferromagnetic order in this compound originates from the localized d-electrons which do not contribute to the electric conduction. Thus the itinerant and localized d-orbital electrons in the compound are decoupled.

Strain-Induced Spin-Nematic State and Nematic Susceptibility Arising from 2×2 Fe Clusters in KFe_{0.8}Ag_{1.2}Te_{2}

Spin nematics break spin-rotational symmetry while maintaining time-reversal symmetry, analogous to liquid crystal nematics that break spatial rotational symmetry while maintaining translational symmetry. Although several candidate spin nematics have been proposed, the identification and characterization of such a state remain challenging because the spin-nematic order parameter does not couple directly to experimental probes. KFe_{0.8}Ag_{1.2}Te_{2} (K_{5}Fe_{4}Ag_{6}Te_{10}, KFAT) is a local-moment magnet consisting of well-separated 2×2 Fe clusters, and in its ground state the clusters order magnetically, breaking both spin-rotational and time-reversal symmetries. Using uniform magnetic susceptibility and neutron scattering measurements, we find a small strain induces sizable spin anisotropy in the paramagnetic state of KFAT, manifestly breaking spin-rotational symmetry while retaining time-reversal symmetry, resulting in a strain-induced spin-nematic state in which the 2×2 clusters act as the spin analog of molecules in a liquid crystal nematic. The strain-induced spin anisotropy in KFAT allows us to probe its nematic susceptibility, revealing a divergentlike increase upon cooling, indicating the ordered ground state is driven by a spin-orbital entangled nematic order parameter.

Momentum Dependence of the Nematic Order Parameter in Iron-Based Superconductors

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe_{2}As_{2} without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

In-plane and out-of-plane properties of a BaFe2As2 single crystal

Anisotropy of transport and magnetic properties of parent compounds of iron based superconductors is a key ingredient of superconductivity. In this work, we investigate in-plane and out-of-plane properties, namely thermal, electric, thermoelectric transport and magnetic susceptibility in a high quality BaFe₂As₂ single crystal of the 122 parent compound, using a combined experimental and theoretical approach. Combining the ab initio calculation of the band structure and the measured in-plane and out-of-plane resistivity, we evaluate the scattering rates which turn out to be strongly anisotropic and determined by spin excitations in the antiferromagnetic state. The observed anisotropy of thermal conductivity is discussed in terms of anisotropy of sound velocities which we estimate to be [Formula: see text]. Remarkably, we find that thermal conductivity is characterized by a sizeable electronic contribution at low temperature, which is ascribed to the high purity of our crystal.

Low-temperature microstructural studies on superconducting CaFe2As2

Undoped CaFe₂As₂ (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T_c up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals.

Angular-dependent magnetoresistance study in Ca0.73La0.27FeAs2: a 'parent' compound of 112-type iron pnictide superconductors

We report a study of angular-dependent magnetoresistance (AMR) with the magnetic field rotated in the plane perpendicular to the current on a Ca_0.73La_0.27FeAs₂ single crystal, which is regarded as a 'parent' compound of 112-type iron pnictide superconductors. A pronounced AMR with twofold symmetry is observed, signifying the highly anisotropic Fermi surface. By further analyzing the AMR data, we find that the Fermi surface above the structural/antiferromagnetic (AFM) transition (T _s/T _N) is quasi-two-dimensional (quasi-2D), as revealed by the 2D scaling behavior of the AMR, Δρ/ρ(0) (H, θ)  =  Δρ/ρ(0) (µ ₀ Hcosθ), θ being the magnetic field angle with respect to the c axis. While such 2D scaling becomes invalid at temperatures below T _s/T _N, the three-dimensional (3D) scaling approach by inclusion of the anisotropy of the Fermi surface is efficient, indicating that the appearance of the 3D Fermi surface contributes to anisotropic electronic transport. Compared with other experimental observations, we suspect that the additional 3D hole pocket (generated by the Ca d orbital and As1 p _z orbital) around the Γ point in CaFeAs₂ will disappear in the heavily electron doped regime, and moreover, the Fermi surface should be reconstructed across the structural/AFM transition. Besides, a quasi-linear in-plane magnetoresistance with H//ab is observed at low temperatures and its possible origins are also discussed. Our results provide more information to further understand the electronic structure of 112-type IBSs.

Dichotomy between in-plane magnetic susceptibility and resistivity anisotropies in extremely strained BaFe2As2

High-temperature superconductivity in the Fe-based materials emerges when the antiferromagnetism of the parent compounds is suppressed by either doping or pressure. Closely connected to the antiferromagnetic state are entangled orbital, lattice, and nematic degrees of freedom, and one of the major goals in this field has been to determine the hierarchy of these interactions. Here we present the direct measurements and the calculations of the in-plane uniform magnetic susceptibility anisotropy of BaFe₂As₂, which help in determining the above hierarchy. The magnetization measurements are made possible by utilizing a simple method for applying a large symmetry-breaking strain, based on differential thermal expansion. In strong contrast to the large resistivity anisotropy above the antiferromagnetic transition at T _N, the anisotropy of the in-plane magnetic susceptibility develops largely below T _N. Our results imply that lattice and orbital degrees of freedom play a subdominant role in these materials.Interplay between lattice, orbital, magnetic and nematic degrees of freedom is crucial for the superconductivity in Fe-based materials. Here, the authors demonstrate the subdominant roles of pure lattice distortions and/or orbital ordering in BaFe₂As₂ by characterizing the in-plane magnetic susceptibility anisotropy.

Three-dimensional magnetic interactions in quasi-two-dimensional PdAs2O6

Millimeter-sized PdAs₂O₆ single crystals are grown using the vapor transport technique. The magnetic order at [Formula: see text] K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd²⁺ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) [Formula: see text]/Pd²⁺ using the fitted magnetic form factor of Pd²⁺ . A weak λ-type anomaly around T _N was observed in specific heat and the magnetic entropy change across T _N is 1.72 J mol^-1 K.This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range [Formula: see text] 250 K. The comparison with SrRu₂O₆ suggests that the magnetic interactions in PdAs₂O₆ are dominated by Pd-(O-[Formula: see text]-O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. The comparison with NiAs₂O₆ suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO3-δ: X-ray absorption and photoemission studies

The local electronic and atomic structures of the high-quality single crystal of SrFeO_3-δ (δ~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, T_m~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L_3,2-edge X-ray linear dichroism results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d e_g states remain in persistently out-of-plane 3d_3z²_-r² orbitals. In contrast, in the warming process below the transition temperature, they change from 3d_3z²_-r² to in-plane 3d_x²_-y² orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below T_m. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3d_x²_-y² and out-of-plane 3d_3z²_-r² orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO_3-δ (δ~0.19) single crystal.

The effects of post-growth annealing on the structural and magnetic properties of BaFe2As2

We investigate the effects of post-growth annealing on the structural and magnetic properties of BaFe2As2. Magnetic susceptibility measurements, which exhibit a signal corresponding to the magnetic phase transition, and high-resolution x-ray diffraction measurements, which directly probe the structural order parameter, show that annealing causes the ordering temperatures of both the phase transitions to increase, sharpen and converge. In the as grown sample, our measurements show two distinct transitions corresponding to structural and magnetic ordering, which are separated in temperature by approximately 1 K. After 46 days (d) of annealing at 700 °C, the two become concurrent in temperature. These measurements demonstrate that the structural phase transition is second-order like when the magnetic and structural phase transitions are separated in temperature, and first-order like when the two phase transition temperatures coincide. This observation indicates that annealing causes the system to cross a hitherto undiscovered tricritical point. In addition, x-ray diffraction measurements show that the c-axis lattice parameter increases with annealing up to 30 d, but remains constant for longer annealing times. Comparisons of BaFe2As2 to SrFe2As2 are made when possible.

Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe2As2

Within the BaFe2As2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba(1-x)Tl(x)Fe2As2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Néel temperature rises with small x, and then falls with additional x. Specifically, we find that Néel and structural transitions in BaFe2As2 (T(N) = T(s) = 133 K) increase for x = 0.05 (T(N) = 138 K, T(s) = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x = 0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (T(N) = T(s) = 131 K), and this is due to charge doping. We illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.

Superconductivity and disorder effect in TlNi2Se(2-x)S(x) compounds

After our first discovery of multi-band superconductivity (SC) in the TlNi2Se2 crystal, we successfully grew a series of TlNi2Se(2-x)S(x) (0.0 ≤ x ≤ 2.0) single crystals. Measurements of resistivity, specific heat, and susceptibility were carried out on these crystals. Superconductivity with T(C) = 2.3 K was first observed in the TlNi2S2 crystal, which also appears to involve heavy electrons with an effective mass m* = 13-25 m(b), as inferred from the normal state electronic specific heat and the upper critical field, H(C2)(T). It was found that bulk SC and heavy-electron behavior is preserved in all the studied TlNi2Se(2-x)S(x) samples. In the mixed state, a novel change of the field dependence of the residual specific heat coefficient, γ(N)(H), occurs in TlNi2Se(2-x)S(x) with increasing S content. We also found that the T(C) value changes with the disorder degree induced by the partial substitution of S for Se, characterized by the residual resistivity ratio (RRR). Thus, the TlNi2Se(2-x)S(x) system provides a platform to study the effect of disorder on the multi-band SC.

Hidden non-Fermi liquid behavior caused by magnetic phase transition in Ni-doped Ba-122 pnictides

We studied two BaFe(2-x)N(I)xAs2 (Ni-doped Ba-122) single crystals at two different doping levels (underdoped and optimally doped) using an optical spectroscopic technique. The underdoped sample shows a magnetic phase transition around 80 K. We analyze the data with a Drude-Lorentz model with two Drude components (D1 and D2). It is known that the narrow D1 component originates from electron carriers in the electron-pockets and the broad D2 mode is from hole carriers in the hole-pockets. While the plasma frequencies of both Drude components and the static scattering rate of the broad D2 component show negligible temperature dependencies, the static scattering rate of the D1 mode shows strong temperature dependence for the both samples. We observed a hidden quasi-linear temperature dependence in the scattering rate of the D1 mode above and below the magnetic transition temperature while in the optimally doped sample the scattering rate shows a more quadratic temperature dependence. The hidden non-Fermi liquid behavior in the underdoped sample seems to be related to the magnetic phase of the material.

An itinerant antiferromagnetic metal without magnetic constituents

The origin of magnetism in metals has been traditionally discussed in two diametrically opposite limits: itinerant and local moments. Surprisingly, there are very few known examples of materials that are close to the itinerant limit, and their properties are not universally understood. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn₂ and Sc₃In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet with a spin density wave ground state, its 3d electron character has been deemed crucial to it being magnetic. Here, we report evidence for an itinerant antiferromagnetic metal with no magnetic constituents: TiAu. Antiferromagnetic order occurs below a Néel temperature of 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This itinerant antiferromagnet challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing insights into the effects of spin fluctuations in itinerant–electron systems.

Sc₃In and ZrZn₂ are the only two known itinerant ferromagnets that form from non-magnetic constituents. Now, Svanidze et al., evidence itinerant antiferromagnetism in TiAu below 36 K using thermodynamic, transport, muon-based and neutron-based measurements, and density functional analysis.

Optical conductivity of iron-based superconductors

The new family of unconventional iron-based superconductors discovered in 2006 immediately relieved their copper-based high-temperature predecessors as the most actively studied superconducting compounds in the world. The experimental and theoretical effort made in order to unravel the mechanism of superconductivity in these materials has been overwhelming. Although our understanding of their microscopic properties has been improving steadily, the pairing mechanism giving rise to superconducting transition temperatures up to 55 K remains elusive. And yet the hope is strong that these materials, which possess a drastically different electronic structure but similarly high transition temperatures compared to the copper-based compounds, will shed essential new light onto the several-decade-old problem of unconventional superconductivity. In this work we review the current understanding of the itinerant-charge-carrier dynamics in the iron-based superconductors and parent compounds largely based on the optical conductivity data the community has gleaned over the past seven years using such experimental techniques as reflectivity, ellipsometry, and terahertz transmission measurements and analyze the implications of these studies for the microscopic properties of the iron-based materials as well as the mechanism of superconductivity therein.

Magnetic and structural transitions of SrFe2As2 at high pressure and low temperature

One of key issues in studying iron based superconductors is to understand how the magnetic phase of the parent compounds evolves. Here we report the systematic investigation of paramagnetic to antiferromagnetic and tetragonal to orthorhombic structural transitions of “122” SrFe₂As₂ parent compound using combined high resolution synchrotron Mössbauer spectroscopy and x-ray diffraction techniques in a cryogenically cooled high pressure diamond anvil cell. It is found that although the two transitions are coupled at 205 K at ambient pressure, they are concurrently suppressed to much lower temperatures near a quantum critical pressure of approximately 4.8 GPa where the antiferromagnetic state transforms into bulk superconducting state. Our results indicate that the lattice distortions and magnetism jointly play a critical role in inducing superconductivity in iron based compounds.

Annealing effects on the properties of BFe2As2 (B = Ca, Sr, Ba) superconducting parents

Thermal annealing results in ∼6 K shifts in the structural and magnetic transition temperatures of BaFe₂As₂ and SrFe₂As₂.

Growth and characterizations of Ba2Ti2Fe2As4O single crystals

Single crystals of a new iron-based superconductor Ba2Ti2Fe2As4O have been grown successfully via a Ba2As3-flux method in a sealed evacuated quartz tube. Bulk superconductivity with Tc ∼ 21.5 K was demonstrated in resistivity and magnetic susceptibility measurements after the as-grown crystals were annealed at 500 °C in vacuum for a week. X-ray diffraction patterns confirm that the annealed and the as-grown crystals possess the identical crystallographic structure of Ba2Ti2Fe2As4O. Energy-dispersive x-ray spectra indicate that partial Ti/Fe substitution exists in the [Fe2As2] layers and the annealing process redistributes the Ti within the Fe-plane. The ordered Fe-plane stabilized by annealing exhibits superconductivity with magnetic vortex pinned by Ti.

A temperature dependent tunneling study of the spin density wave gap in EuFe2As2 single crystals

We report temperature dependent scanning tunneling microscopy and spectroscopy measurements on single crystals of EuFe2As2 in the 15-292 K temperature range. The in situ cleaved crystals show atomic terraces with homogeneous tunnel spectra that correlate well with the spin density wave (SDW) transition at a temperature, TSDW ≈ 186 K. Above TSDW the local tunnel spectra show a small depression in the density of states (DOS) near the Fermi energy (EF). The gap becomes more pronounced upon entering the SDW state with a gap value ∼90 meV at 15 K. However, the zero bias conductance remains finite down to 15 K indicating a finite DOS at the EF in the SDW phase. Furthermore, no noticeable change is observed in the DOS at the antiferromagnetic ordering transition of Eu(2+) moments at 19 K.

Co-substitution effects on the Fe valence in the BaFe2As2 superconducting compound: a study of hard x-ray absorption spectroscopy

The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

High T(c) electron doped Ca10(Pt3As8)(Fe2As2)5 and Ca10(Pt4As8)(Fe2As2)5 superconductors with skutterudite intermediary layers

It has been argued that the very high transition temperatures of the highest T(c) cuprate superconductors are facilitated by enhanced CuO(2) plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T(c) in the new high T(c) iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca(10)(Pt(3)As(8))(Fe(2)As(2))(5) (the "10-3-8 phase") and Ca(10)(Pt(4)As(8))(Fe(2)As(2))(5) (the "10-4-8 phase"). Based on -Ca-(Pt(n)As(8))-Ca-Fe(2)As(2)- layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T(c) of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T(c) to As-Fe-As bond angles does not explain the observed differences in T(c) of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt(4)As(8) layer. The interlayer coupling is thus revealed as important in enhancing T(c) in the iron pnictide superconductors.

Measurements of the anisotropic in-plane resistivity of underdoped FeAs-based pnictide superconductors

We systematically investigated the in-plane resistivity anisotropy of electron-underdoped EuFe(2-x)Co(x)As(2) and BaFe(2-x)Co(x)As(2) and hole-underdoped Ba(1-x)K(x)Fe(2)As(2). Large in-plane resistivity anisotropy was found in the former samples, while tiny in-plane resistivity anisotropy was detected in the latter ones. When it is detected, the anisotropy starts above the structural transition temperature and increases smoothly through it. As the temperature is lowered further, the anisotropy takes a dramatic enhancement through the magnetic transition temperature. We found that the anisotropy is universally tied to the presence of T-linear behavior of resistivity. Our results demonstrate that the nematic state is caused by electronic degrees of freedom, and the microscopic orbital involvement in the magnetically ordered state must be fundamentally different between the hole- and electron-doped materials.

Linear-temperature dependence of static magnetic susceptibility in LaFeAsO from dynamical mean-field theory

In this Letter we report the local density approximation with dynamical mean field theory results for magnetic properties of the parent superconductor LaFeAsO in the paramagnetic phase. Calculated uniform magnetic susceptibility shows linear dependence at intermediate temperatures in agreement with experimental data. Contributions to the temperature dependence of the uniform susceptibility are strongly orbitally dependent. For high temperatures (>1000  K) susceptibility first saturates and then decreases with temperature. Our results demonstrate that linear-temperature dependence of static magnetic susceptibility in pnictide superconductors can be reproduced without invoking antiferromagnetic fluctuations.

Out-of-plane momentum and symmetry-dependent energy gap of the pnictide Ba0.6K0.4Fe2As2 superconductor revealed by angle-resolved photoemission spectroscopy

The three-dimensional band structure and superconducting gap of Ba0.6K0.4Fe2As2 are studied with angle-resolved photoemission spectroscopy. In contrast with previous results, we have identified three holelike Fermi surface sheets near the zone center with sizable out-of-plane or kz dispersion. The superconducting gap on certain Fermi surface sheets shows significant kz dependence. Moreover, the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries (one odd, one even). Our results further reveal the three-dimensional and orbital-dependent structure of the superconducting gap in iron pnictides, which facilitates the understanding of momentum-integrated measurements and provides a distinct test for theories.

Evidence for competing magnetic and superconducting phases in superconducting Eu 1-x Sr x Fe 2-y Co y As 2 single crystals

In single crystals of Eu(1-x)Sr(x)Fe(2-y)Co(y)As(2), Co doping suppresses spin-density wave (SDW) ordering and induces a superconducting transition. A resistivity reentrance due to the antiferromagnetic ordering of Eu(2+) spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity (SC). It is striking that the resistivity reentrance can be completely suppressed by a small magnetic field due to a field-induced metamagnetic transition from AFM to ferromagnetism (FM). The resistivity reentrance can also be suppressed by the substitution of Eu(2+) ions with nonmagnetic Ba(2+)/Sr(2+) to completely destroy the AFM ordering. These results indicate that the AFM order appears destructive to SC, while FM can coexist with the superconductivity. Further we find that magnon excitation exists in AFM ordering and can be suppressed by an applied field. Coexistence of SC from the FeAs layer and the inner field produced by the ferromagnetic Eu(2+) layer suggest a possible p-wave component in the superconducting order parameter.

A perspective on the Fe-based superconductors

FeSe is employed as reference material to elucidate the observed high T(c) superconducting behaviour of the related layered iron pnictides. The structural and ensuing semimetallic band structural forms are here rather unusual, with the resulting ground state details extremely sensitive to the precise shape of the Fe-X coordination unit. The superconductivity is presented as coming from a combination of resonant valence bond and excitonic insulator physics, and incorporating boson-fermion degeneracy. Although sourced in a very different fashion, the latter leads to some similarities with the high temperature superconducting (HTSC) cuprates. The excitonic insulator behaviour sees spin density wave, charge density wave/periodic structural distortion, and superconductive instabilities all vie for ground state status. The conflict leads to a very sensitive and complex set of properties, frequently mirroring HTSC cuprate behaviour. The delicate balance between ground states is made particularly difficult to unravel by the micro-inhomogeneity of structural form which it can engender. It is pointed out that several other notable superconductors, layered in form, semimetallic with indirect overlap and possessing homopolar bonding, would look to fall into the same general category, β-ZrNCl and MgB(2) and the high pressure forms of several elements, like sulfur, phosphorus, lithium and calcium, being cases in point.

Magnetism in Fe-based superconductors

In this review, we present a summary of experimental studies of magnetism in Fe-based superconductors. The doping dependent phase diagram shows strong similarities to the generic phase diagram of the cuprates. Parent compounds exhibit magnetic order together with a structural phase transition, both of which are progressively suppressed with doping, allowing superconductivity to emerge. The stripe-like spin arrangement of Fe moments in the magnetically ordered state shows identical in-plane structure for the RFeAsO (R = rare earth) and AFe(2)As(2) (A = Sr, Ca, Ba, Eu and K) parent compounds, notably different than the spin configuration of the cuprates. Interestingly, Fe(1 + y)Te orders with a different spin order despite having very similar Fermi surface topology. Studies of the spin dynamics of the parent compounds show that the interactions are best characterized as anisotropic three-dimensional interactions. Despite the room temperature tetragonal structure, analysis of the low temperature spin waves under the assumption of a Heisenberg Hamiltonian indicates strong in-plane anisotropy with a significant next-nearest-neighbor interaction. For the superconducting state, a resonance, localized in both wavevector and energy, is observed in the spin excitation spectrum as for the cuprates. This resonance is observed at a wavevector compatible with a Fermi surface nesting instability independent of the magnetic ordering of the relevant parent compound. The resonance energy (E(r)) scales with the superconducting transition temperature (T(C)) as E(r) ∼ 4.9k(B)T(C), which is consistent with the canonical value of ∼ 5k(B)T(C) observed for the cuprates. Moreover, the relationship between the resonance energy and the superconducting gap, Δ, is similar to that observed for many unconventional superconductors (E(r)/2Δ ∼ 0.64).

Template engineering of Co-doped BaFe2As2 single-crystal thin films

Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.5 K, where rho=resistivity), a small transition width (DeltaT(c)=1.3 K), a superior critical current density J(c) of 4.5 MA cm(-2) (4.2 K) and strong c-axis flux pinning. Implementing SrTiO(3) or BaTiO(3) templates to match the alkaline-earth layer in the Ba-122 with the alkaline-earth/oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multifunctional single-crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin-film overlayers.

Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2

The in-plane resistivity rho and thermal conductivity kappa of the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior rho(T) approximately T{1.5} at H{c{2}}=5 T, and the development of a Fermi liquid state with rho(T) approximately T{2} when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field H{c{2}}. In zero field, there is a large residual linear term kappa{0}/T, and the field dependence of kappa_{0}/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

Low ordered magnetic moment by off-diagonal frustration in undoped parent compounds to iron-based high-T(c) superconductors

A Heisenberg model over the square lattice recently introduced by Si and Abrahams to describe local-moment magnetism in the new class of Fe-As high-T(c) superconductors is analyzed in the classical limit and on a small cluster by exact diagonalization. In the case of spin-1 iron atoms, large enough Heisenberg exchange interactions between neighboring spin-1/2 moments on different iron 3d orbitals that frustrate true magnetic order lead to hidden magnetic order that violates Hund's rule. It accounts for the low ordered magnetic moment observed by elastic neutron diffraction in an undoped parent compound to Fe-As superconductors. We predict that low-energy spin-wave excitations exist at wave numbers corresponding to either hidden Néel or hidden ferromagnetic order.

Hall effect and resistivity study of the magnetic transition, carrier content, and Fermi-Liquid Behavior in Ba(Fe(1-x) Co(x))(2)As(2)

The negative Hall constant R(H) measured all over the phase diagram of Ba(Fe(1-x) Co(x))(2)As(2) allows us to show that electron carriers always dominate the transport properties. The evolution of R(H) with x at low doping (x<2%) indicates that important band structure changes happen for x<2% prior to the emergence of superconductivity. For higher x, a change with T of the electron concentration is required to explain the low T variations of R(H), while the electron scattering rate displays the T(2) law expected for a Fermi liquid. The T=0 residual scattering is affected by Co disorder in the magnetic phase, but is rather dominated by incipient disorder in the paramagnetic state.

Superconducting and ferromagnetic phases induced by lattice distortions in stoichiometric SrFe2As2 single crystals

Single crystals of SrFe2As2 grown using a self-flux solution method were characterized via x-ray, transport, and magnetization studies, revealing a superconducting phase below Tc=21 K characterized by a full electrical resistivity transition and partial diamagnetic screening. The reversible destruction and reinstatement of this phase by heat treatment and mechanical deformation studies, along with single-crystal x-ray diffraction measurements, indicate that internal crystallographic strain originating from c-axis-oriented planar defects plays a central role in promoting the appearance of superconductivity under ambient-pressure conditions in approximately 90% of as-grown crystals. The appearance of a ferromagnetic moment with magnitude proportional to the tunable superconducting volume fraction suggests that these phenomena are both stabilized by lattice distortion.