Session C10: Fe-based Superconductors III

Phase diagram of a hetero-structured iron-based superconductor Sr2VO3FeAs

The interplay of spin and orbital degrees of freedom induces various magnetic or nematic phases, coexisting with the superconductivity, and leads to rich phase diagrams in iron-based superconductors (FeSCs). Among various iron-based superconductors, Sr₂VO₃FeAs holds a unique position as a naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium oxide, in which the unusual C₄ symmetric phases are recently identified and found to be highly distinct from those found in other FeSCs. In this talk, I will present our experimental explorations on these unusual phase transitions in the FeAs layers of Sr₂VO₃FeAs, and discuss the possible role of the magnetic proximity coupling and the resulting frustration of the otherwise dominant Fe stripe and V Neel fluctuations for stabilizing hidden orders in FeSCs.   

Self-induced magnetic flux structure in magnetic superconductor

We image the temperature evolution of the distribution of the magnetic induction in single crystals of the magnetic superconductor EuRbFe4As4. In contrast to the traditional Meissner flux expulsion, the samples cooled in constant magnetic fields show unusual vortex patterns revealing a strong enhancement of the internal magnetic flux density upon approaching the magnetic transition temperature. The observed patterns demonstrate a cooperative response of the magnetic subsystem, which acts as an internal pump of the magnetic flux, and the superconducting system, which controls the delivery of vortices into the bulk of the sample. We suggest possible current distributions responsible for the observed flux patterns.
  

STM/STS study of superconducting properties modified by Ca deposition on Ca10(Pt4As8)(Fe2As2)5

Ca₁₀(Pt₄As₈)(Fe₂As₂)₅ contains both superconducting building blocks (Fe₂As₂ layers) and conducting spacers (Pt₄As₈ layers), unlike insulating spacers in other Fe-based superconductors. Prior work showed the important role played by the charge environment and surface structures in its superconducting properties: evidence for superconductivity is absent in bare Pt₄As₈ surface but recovers when Ca atoms are atop [1]. Different surface reconstructions also lead different superconducting features. We use scanning tunneling microscopy and spectroscopy (STM/S) to directly confirm the role of charge balance and surface structures on superconductivity in this compound. Cleaving provides large areas of bare Pt₄As₈ layer, which originally does not show superconducting features. Depositing additional Ca atoms on the bare Pt₄As₈ surface, while the sample is on the STM scanning stage, results in changes in spectral features. We will discuss the relationship between surface structures and superconducting features based on the Ca structures formed by controlling the deposition temperature.
[1] J. Kim et al., Sci. Rep. 6, 35365 (2016).   

Prediction of (Li0.8Co0.2OH)CoSb as A New Superlatticed Superconductor

We study theoretically the superlattice material of (Li_0.8Co_0.2OH)CoSb as a new candidate superconductor, in which the superconducting layer CoSb has the same crystal structure as FeSe, but with stronger spin-orbit coupling (SOC). Through density functional theory calculations, we find that substitution of Li by Co in the spacing layer not only minimizes the lattice mismatch between the spacing and superconducting layers in the ab plane, but also helps to stabilize the overall structure of the system. Moreover, we have investigated the detailed nature of charge transfer and found that Co in the spacing layer can significantly enhance the interlayer charge transfer, which is essential for achieving superconductivity. The predicted system is not only likely to add a new member into the superconducting family, but may also offer new opportunities for realizing topological superconductivity because of the stronger SOC.   

Temperature- and pressure-dependent electronic structure evolution of quasi-one-dimensional iron chalcogenide ladder compounds, BaFe2S3, and BaFe2Se3: optical studies and DFT calculations

Since the discovery of a pressure-induced iron-based superconductor, BaFe₂S₃ (BFS), intensive studies have been performed on BFS and a similar compound BaFe₂Se₃ (BFSe). Both compounds are antiferromagnetic insulators, with increasing pressure they show metallic phases and eventually become superconductors under very high pressure. We measured reflectance spectra of those two compounds by controlling two experimental tuning parameters: temperature and pressure. From our temperature-dependent measured reflectance spectra of both samples using polarized lights, we obtained anisotropic optical conductivity along two crystal axes. We observed bandgaps in both samples along the ladder direction at low temperature. We also observed pressure-induced insulator-metal transitions (IMT) in both samples from our pressure-dependent measured mid-infrared reflectance spectra of both samples at 300 K using diamond anvil cells. The density functional theory calculations were also performed to understand the measured experimental spectra. In this presentation, we will present our results and discuss on them.   

Melting of vortex lattice in magnetic iron-pnictide superconductor RbEuFe4As4

The iron-based superconductors are characterized by sizable thermal fluctuations due to high transition temperatures and small coherence lengths. We report calorimetric and transport observations for the vortex-lattice melting in the magnetic iron-pnictide superconductor RbEuFe₄As₄ with T_c ≈ 37K and dH_c2/dT ≈ 4T/K. The melting transition is seen as a sharp drop of the resistivity and a step of the specific heat at the magnetic-field-dependent temperature. The typical step height is ∼ 4-5% of the zero-field jump, similar to YBa₂Cu₃O₇ and in agreement with theoretical estimates. The specific-heat peak due to the latent heat expected for very clean materials, however, was not observed. The melting line lies in the temperature/magnetic-field plane noticeably below the upper-critical-field line, and the vortex-liquid state occupies a significant portion of the phase diagram. The location of the melting transition is in quantitative agreement with theoretical predictions without fitting parameters.   

Influence of magnetic ordering on the vortex matter in RbEuFe4As4 single crystals

Eu-containing Fe-based superconductors are ideal systems to study the interplay of superconductivity and magnetic order. The superconductor RbEuFe₄As₄ has T_c~37K and undergoes a magnetic ordering of the Eu at T_m~15K. We studied vortex pinning and dynamics in RbEuFe₄As₄ single crystals as a function of temperature (T), magnetic field and orientation (Θ) by angular dependent magnetization. The critical current density (J_c) of the pristine crystals is low, indicative of small disorder, and weakly anisotropic. Introduction of aligned columnar defects (CDs) by heavy ion irradiation increases J_c, but produces almost no changes in T_c and T_m. In the crystals with CDs, J_c(Θ) exhibits a lockin phase, the fingerprint of correlated pinning. Both pristine and irradiated crystals show clear evidence of the influence of the magnetic order on the vortex matter. In pristine crystals, the rate of decrease in J_c with T slows down in the proximity of T_m. Irradiation enhances this effect, with J_c(T) becoming nonmonotonic and developing a local maximum near T_m. In both cases, flux creep rates exhibit local minima around the magnetic transition, indicating an increase in the vortex pinning energy.
  

Comparison of S- and Te-substitution effects in FeSe thin films

We have successfully grown single crystalline films of FeSe_1−xS_x with x ≤ 0.43 via pulsed laser deposition. As x increases, the structural transition temperature decreases and the superconducting transition temperature, T_c, shows a gradual decrease even when the structural transition disappears. We observed a new kink structure in the resistivity-temperature curves for films with large x, which is likely due to a magnetic transition. The obtained phase diagram of FeSe_1−xS_x is in contrast to that of FeSe_1−yTe_y films, which shows a sudden increase of Tc at the composition where the structural transition disappears[1]. These results demonstrate that the relationship between the nematicity and the superconductivity is not universal in FeSe, suggesting that the nematicity does not play a primary role in the superconductivity in this system[2].
[1] Y. Imai et al., Sci. Rep. 7, 46653 (2017).
[2] F. Nabeshima et al., J. Phys. Soc. Jpn. 87, 073704 (2018).   

Verwey-like Transition in β-Fe4+δSe5 Nanowires of Fe-vacancy Ordering

The magnetotransport and the source-drain frequency dependence of β-Fe_4+δSe₅ nanowires are investigated. The nanowires feature Fe-vacancy ordering and mixed valence of Fe. The resistance shows a first-order metal-insulator transition of transition temperature at ~30K in the absence of magnetic fields. Colossal positive magnetoresistance is observed at temperatures around 30K. The metal-insulator transition presents uniaxial anisotropy with the c-axis in field-orientation dependence. The frequency dependence in resistance below the metal-insulator transition temperature is similar to features in magnetite nanowires at temperatures lower than the Verwey temperature. Our results reveal that the Fe-vacancy-ordered β-Fe_4+δSe₅ nanowires undergo a magnetic first-order phase transition strongly involving spin-orbital coupling with d_z2 orbitals .   

Observation of low-energy phonon anomalies in Ca0.73La0.27FeAs2

Couplings between lattice and charge or spin degrees of freedom underpin a wide range of phenomena in strongly correlated electronic systems. Fe-based superconductors provide an important platform in probing the phonon effect on high temperature superconductivity, where spin-phonon interaction is expected to dominate. Ca_0.73La_0.27FeAs₂, considered as a parent compound of the 112 family, has an antiferromagnetic ground state, which gives way to a superconducting state at high pressure. We have performed a systematic inelastic X-ray scattering study on single crystals of Ca_0.73La_0.27FeAs₂, at both ambient and high pressure, and compared the results with density functional theory (DFT) calculations. A low-energy phonon anomaly was observed, and its nature will be discussed.   

STM and Nernst measurements on tetragonal and orthogonal phase of Fe(Se1-xSx)

The crossover from the Bardeen-Cooper-Schrieffer (BCS) state for weakly paired fermions to the Bose Einstein condensation (BEC) state of tightly-bound pairs has attracted much attention as a new paradigm of superconductivity. In the BCS-BEC crossover regime, exotic phenomena, such as pseudogap formation owing to preformed pairs above superconducting transition temperature T_c, have been predicted.
It has been reported that multiband iron-based superconductor FeSe with the small Fermi energies is located at the BCS-BEC crossover regime. Moreover, very recent heat capacity and ARPES measurements have suggested a possibility that the tetragonal phase of Fe(Se_1-xS_x) (x > 0.17) is located at the state closer to the BEC limit. Pairing fluctuation and pseudogap formation above T_c, however, have not been reported so far.
In this study, we perform STM and Nernst measurements on FeSe_0.87S_0.13 and FeSe_0.75S_0.25, which are in the orthogonal and tetragonal phases, respectively. In our Nernst measurements, well exceeding Gaussian fluctuations are observed above T_c in FeSe_0.75S_0.25, while pseudogap formation is not observed by the STM measurements. We will discuss the present results in terms of the BCS-BEC crossover in multiband superconducting systems.