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.

Among the many theoretical mechanisms proposed for higher \(T_c\), the interfacial mechanism provides not only continual inspiration but also hope. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discovery of superconductivity in naturally assembled rare-earth(R)-doped CaFe\(_2\)As\(_2\) (Ca122) with \(T_c\) up to 49 K, has brought an alternate route to tackle the problem. Detailed magnetization, resistivity, chemical composition, specific heat, and annealing studies have been systematically carried out on R-doped Ca122 single crystals with R = La, Ce, Pr, and Nd. The experimental observations lead us to the conjecture that the \(T_c\) enhancement may be related to naturally occurring chemical interfaces associated with defects. Most recently, we successfully induced superconductivity in undoped Ca122 with a \(T_c\) up to 25 K through proper thermal treatment. We later conducted systematic annealing for different time periods at a constant temperature of 350 \(^{\circ }\)C on the a singe crystal Ca122 sample quenched rapidly from 850 \(^{\circ }\)C to room temperature. The room temperature X-ray diffraction (XRD) shows an initial tetragonal structure (PI) with a \(c_\mathrm{I}\) = 11.547(1) Å prior to any annealing and remains to be a tetragonal phase (PII) with only a slightly larger \(c_\mathrm{II}\) = 11.702(2) Å following prolong annealing. Neither PI or PII phase is superconducting above 2 K. However, superconductivity was detected after intermediate annealing, when mixture of the two phase appears. The evolution of mixed phase as suggested by the XRD results is consistent with that shown by the XRD simulations with microstructures of stacking of the two phases. The observation has provided the most direct evidence of interfacial induced superconductivity in Ca122 to date and possibly in R-doped Ca122.

mong the many theoretical mechanisms proposed for higher T_c , the interfacial mechanism provides not only continual inspiration but also hope. Difficulties do exist, as most of the materials proposed to exhibit the interfacial mechanism are artificially formed heterostructures and are by nature delicate and easily disturbed by strain and change in the stoichiometry at the interface. The discovery of superconductivity in naturally assembled rare-earth(R)-doped CaFe _2 As _2 (Ca122) with T_c up to 49 K, has brought an alternate route to tackle the problem. Detailed magnetization, resistivity, chemical composition, specific heat, and annealing studies have been systematically carried out on R-doped Ca122 single crystals with R = La, Ce, Pr, and Nd. The experimental observations lead us to the conjecture that the T_c enhancement may be related to naturally occurring chemical interfaces associated with defects. Most recently, we successfully induced superconductivity in undoped Ca122 with a T_c up to 25 K through proper thermal treatment. We later conducted systematic annealing for different time periods at a constant temperature of 350 ^∘ C on the a singe crystal Ca122 sample quenched rapidly from 850 ^∘ C to room temperature. The room temperature X-ray diffraction (XRD) shows an initial tetragonal structure (PI) with a c_I = 11.547(1) Å prior to any annealing and remains to be a tetragonal phase (PII) with only a slightly larger c_II = 11.702(2) Å following prolong annealing. Neither PI or PII phase is superconducting above 2 K. However, superconductivity was detected after intermediate annealing, when mixture of the two phase appears. The evolution of mixed phase as suggested by the XRD results is consistent with that shown by the XRD simulations with microstructures of stacking of the two phases. The observation has provided the most direct evidence of interfacial induced superconductivity in Ca122 to date and possibly in R-doped Ca122.

Superconductivity has been reversibly induced/suppressed in undoped CaFe2As2 (Ca122) single crystals through proper thermal treatments, with Tc 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 Tc 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.

We have systematically grown large single crystals of layered compound beta-PdBi2, both the hole-doped PdBi2-xPbx and the electron-doped NaxPdBi2, and studied their magnetic and transport properties. Hall-effect measurement on PdBi2, PdBi1.8Pb0.2, and Na0.057PdBi2 shows that the charge transport is dominated by electrons in all of the samples. The electron concentration is substantially reduced upon Pb-doping in PdBi2-xPbx and increased upon Na-intercalation in NaxPdBi2, indicating the effective hole-doping by Pb and electron-doping by Na. We observed a monotonic decrease of superconducting transition temperature (Tc) from 5.4K in undoped PdBi2 to less than 2K for x > 0.35 in hole-doped PdBi2-xPbx. Meanwhile, a rapid decrease of Tc with the Na intercalation is also observed in the electron-doped NaxPdBi2, which is in disagreement with the theoretical expectation. In addition, both the magnetoresistance and Hall resistance further reveal evidence for a possible spin density wave (SDW)-like transition below 50K in the Na-intercalated PdBi2 sample. The complete phase diagram is thus established from hole-doping to electron-doping. Meanwhile, high pressure study of the undoped PdBi2 shows that the Tc is linearly suppressed under pressure with a dTc/dP coefficient of -0.28K/GPa.

Recently, the detection of non-bulk superconductivity with unexpectedly high onset-T c s up to 49 K in Pr-doped CaFe 2 As 2 [(Ca,Pr)122] single crystals and the report of a T c up to 65 K in one-unit-cell (1UC) FeSe epi-films, offer an unusual opportunity to seek an answer to the question posed in the title. Through systematic compositional, structural, resistive, and magnetic investigations on (Ca,Pr)122 single crystals, we have observed a doping-level-independent Tc, the simultaneous appearance of superparamagnetism and superconductivity, large magnetic anisotropy, and the existence of mesoscopic-2D structures in these crystals, thus providing clear evidence consistent with the proposed interface-enhanced Tc in these naturally occurring rare-earth-doped Fe-based superconductors, (Ca,R)122. Similar resistive and magnetic measurements were also made on the 3–4UC FeSe ultrathin epi-films. We have detected weak links in the Meissner state below 20 K, weakly coupled small superconducting patches between 20–45 K, and collective excitations of spin and/or superconducting nature between 45–80 K. The unusual frequency dependences of the diamagnetic moment observed in the films in different temperature ranges will be presented and their implications discussed.

A metal-rich ternary phosphide, SrPt6P2, with a unique structure type was synthesized at high temperatures. Its crystal structure was determined by single-crystal X-ray diffraction [cubic space group Pa (3) over bar; Z = 4; a = 8.474(2) angstrom, and V = 608.51(2) angstrom(3)]. The structure features a unique three-dimensional anionic (Pt6P2)(2-) network of vertex-shared Pt6P trigonal prisms. The Sr atoms occupy a 12-coordinate (Pt) cage site and form a cubic close-packed (face-centered-cubic) arrangement, and the P atoms formally occupy tetrahedral interstices. The metallic compound becomes superconducting at 0.6 K, as evidenced by magnetic and resistivity measurements.