Fermi-surface reconstruction and complex phase equilibria in CaFe2As2

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.

Lattice disorder effect on magnetic ordering of iron arsenides

This study investigates magnetic ordering temperature in nano- and mesoscale structural features in an iron arsenide. Although magnetic ground states in quantum materials can be theoretically predicted from known crystal structures and chemical compositions, the ordering temperature is harder to pinpoint due to potential local lattice variations that calculations may not account for. In this work we find surprisingly that a locally disordered material can exhibit a significantly larger Néel temperature (T_N) than an ordered material of precisely the same chemical stoichiometry. Here, a EuFe₂As₂ crystal, which is a ‘122’ parent of iron arsenide superconductors, is found through synthesis to have ordering below T_N = 195 K (for the locally disordered crystal) or T_N = 175 K (for the ordered crystal). In the higher T_N crystals, there are shorter planar Fe-Fe bonds [2.7692(2) Å vs. 2.7745(3) Å], a randomized in-plane defect structure, and diffuse scattering along the [00 L] crystallographic direction that manifests as a rather broad specific heat peak. For the lower T_N crystals, the a-lattice parameter is larger and the in-plane microscopic structure shows defect ordering along the antiphase boundaries, giving a larger T_N and a higher superconducting temperature (T_c) upon the application of pressure. First-principles calculations find a strong interaction between c-axis strain and interlayer magnetic coupling, but little impact of planar strain on the magnetic order. Neutron single-crystal diffraction shows that the low-temperature magnetic phase transition due to localized Eu moments is not lattice or disorder sensitive, unlike the higher-temperature Fe sublattice ordering. This study demonstrates a higher magnetic ordering point arising from local disorder in 122.

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.

Superconductivity in Uncollapsed Tetragonal LaFe2As2

We report synthesis, crystal structure, and superconductivity in ThCr₂Si₂-type LaFe₂As₂ (La122). La122 was synthesized at 960 °C for 1.5 h under a pressure of 3.4 GPa. An as-synthesized La122 (nonsuperconductor) had a collapsed tetragonal structure with a short c-axis length of 11.0144(4) Å as observed in CaFe₂As₂ under pressure. The collapsed tetragonal structure transformed into an uncollapsed tetragonal structure by annealing the as-synthesized La122 at 500 °C. The c-axis length remarkably extended to 11.7317(4) Å, and superconductivity emerged at 12.1 K in the uncollapsed tetragonal La122. A cylindrical hole-like Fermi surface around the Γ point that plays an important role for an s± wave pairing in iron-based superconductors was missing in the uncollapsed tetragonal La122 because of heavy electron doping. Superconductivity in La122 may be closely related to that induced in CaFe₂As₂ under pressure.

Evolution of the magnetic order of Fe and Eu sublattices in Eu1-x Ca x Fe2As2 (0 ⩽ x ⩽ 1) single crystals

Single crystals of Eu_1-x Ca _x Fe₂As₂ ([Formula: see text]) are grown using the high-temperature solution-growth method employing FeAs self-flux. Structural and chemical analysis indicates that these crystals are homogeneous and their lattice parameters exhibit a gradual monotonic decrease with increasing Ca concentration. Detailed magnetic, specific heat and resistivity data were used to construct a phase diagram which depicts the evolution of the structural/spin-density-wave transition at T ₀, and of the antiferromagnetic (AFM) ordering temperature of the Eu moments at T _N. We found out that while T _N decreases monotonically from 19.1 K (for x  =  0) to below 2 K (for [Formula: see text]), T ₀ remains almost constant up to x  =  x _c and decreases steadily for higher values of x. Annealing at low temperatures for several days leads to enhancement of T _N and T ₀ by a few kelvin and sharpened the anomalies associated with these transitions. However, annealing did not change the variation of T _N and T ₀ across the series. The observation that T ₀ is almost constant until the long-range AFM ordering of the Eu²⁺ moments gets destroyed, suggests a subtle interrelationship between the Eu²⁺ and Fe²⁺ magnetic sublattices.

Emergent electronic structure of CaFe2As2

CaFe₂As₂ exhibits collapsed tetragonal (cT) structure and varied exotic behaviour under pressure at low temperatures that led to debate on linking the structural changes to its exceptional electronic properties like superconductivity, magnetism, etc. Here, we investigate the electronic structure of CaFe₂As₂ forming in different structures employing density functional theory. The results indicate that the stability of the cT phase under pressure arises from the enhancement in hybridization induced effects and shift of the energy bands towards lower energies. The Fermi surface centered around Γ point gradually vanishes with the increase in pressure. Consequently, the nesting between the hole and electron Fermi surfaces associated to the spin density wave state disappears indicating a pathway to achieve the proximity to quantum fluctuations. The magnetic moment at the Fe sites diminishes in the cT phase consistent with the magnetic susceptibility results. Notably, the hybridization of Ca 4s states (Ca-layer may be treated as a charge reservoir layer akin to those in cuprate superconductors) is significantly enhanced in the cT phase revealing its relevance in its interesting electronic properties.

Improving superconductivity in BaFe2As2-based crystals by cobalt clustering and electronic uniformity

Quantum materials such as antiferromagnets or superconductors are complex in that chemical, electronic, and spin phenomena at atomic scales can manifest in their collective properties. Although there are some clues for designing such materials, they remain mainly unpredictable. In this work, we find that enhancement of transition temperatures in BaFe₂As₂-based crystals are caused by removing local-lattice strain and electronic-structure disorder by thermal annealing. While annealing improves Néel-ordering temperature in BaFe₂As₂ crystal (T_N = 132 K to 136 K) by improving in-plane electronic defects and reducing overall a-lattice parameter, it increases superconducting-ordering temperature in optimally cobalt-doped BaFe₂As₂ crystal (T_c = 23 to 25 K) by precipitating-out the cobalt dopants and giving larger overall a-lattice parameter. While annealing improves local chemical and electronic uniformity resulting in higher T_N in the parent, it promotes nanoscale phase separation in the superconductor resulting in lower disparity and strong superconducting band gaps in the dominant crystal regions, which lead to both higher overall T_c and critical-current-density, J_c.

Lattice parameters guide superconductivity in iron-arsenides

The discovery of superconducting materials has led to their use in technological marvels such as magnetic-field sensors in MRI machines, powerful research magnets, short transmission cables, and high-speed trains. Despite such applications, the uses of superconductors are not widespread because they function much below room-temperature, hence the costly cooling. Since the discovery of Cu- and Fe-based high-temperature superconductors (HTS), much intense effort has tried to explain and understand the superconducting phenomenon. While no exact explanations are given, several trends are reported in relation to the materials basis in magnetism and spin excitations. In fact, most HTS have antiferromagnetic undoped 'parent' materials that undergo a superconducting transition upon small chemical substitutions in them. As it is currently unclear which 'dopants' can favor superconductivity, this manuscript investigates crystal structure changes upon chemical substitutions, to find clues in lattice parameters for the superconducting occurrence. We review the chemical substitution effects on the crystal lattice of iron-arsenide-based crystals (2008 to present). We note that (a) HTS compounds have nearly tetragonal structures with a-lattice parameter close to 4 Å, and (b) superconductivity can depend strongly on the c-lattice parameter changes with chemical substitution. For example, a decrease in c-lattice parameter is required to induce 'in-plane' superconductivity. The review of lattice parameter trends in iron-arsenides presented here should guide synthesis of new materials and provoke theoretical input, giving clues for HTS.

Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe2As2 single crystals

Superconductivity has been reversibly induced/suppressed in undoped CaFe₂As₂ (Ca122) single crystals through proper thermal treatments, with T_c at ∼25 K at ambient pressure and up to 30 K at 1.7 GPa. We found that Ca122 can be stabilized in two distinct tetragonal (T) phases at room temperature and ambient pressure: PI with a nonmagnetic collapsed tetragonal (cT) phase at low temperature and PII with an antiferromagnetic orthorhombic (O) phase at low temperature, depending on the low-temperature annealing condition. Neither phase at ambient pressure is superconducting down to 2 K. However, systematic annealing for different time periods at 350 °C on the as-synthesized crystals, which were obtained by quenching the crystal ingot from 850 °C, reveals the emergence of superconductivity over a narrow time window. Whereas the onset T_c is insensitive to the anneal time, the superconductive volume fraction evolves with the time in a dome-shaped fashion. Detailed X-ray diffraction profile analyses further reveal mesoscopically stacked layers of the PI and the PII phases. The deduced interface density correlates well with the superconducting volume measured. The transport anomalies of the T-cT transition, which is sensitive to lattice strain, and the T-O transition, which is associated with the spin-density-wave (SDW) transition, are gradually suppressed over the superconductive region, presumably due to the interface interactions between the nonmagnetic metallic cT phase and the antiferromagnetic O phase. The results provide the most direct evidence to date for interface-enhanced superconductivity in undoped Ca122, consistent with the recent theoretical prediction.

Fermi-Surface Topological Phase Transition and Horizontal Order-Parameter Nodes in CaFe2As2 Under Pressure

Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe₂As₂, are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe₂As₂ crystals under quasi-hydrostatic pressure, and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). The almost perfect agreement between PCARS results at different pressures and theoretical predictions highlights the intimate connection between the changes in the lattice structure, a topological transition in the holelike Fermi surface sheet, and the emergence on the same sheet of an order parameter with a horizontal node line.

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.