Session S5: New Fe-based Superconductors and Related Materials I

\textbf{Coexistence of superconductivity and antiferromagnetism in (Li}$_{\mathrm{\mathbf{0.8}}}$\textbf{Fe}$_{\mathrm{\mathbf{0.2}}}$\textbf{)OHFeSe}

In this talk, we report the synthesis of an air-stable material, (Li$_{\mathrm{0.8}}$Fe$_{\mathrm{0.2}})$OHFeSe, which shows superconducting transition temperature up $T_{\mathrm{c}}$ to \textasciitilde 40 K, by means of a novel hydrothermal method [1]. The crystal structure is unambiguously determined by a combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. Moreover, antiferromagnetic order is found to coexist with superconductivity. We also grew single crystals of (Li,Fe)OHFeSe, and observed a first-order transition from superconductor to AFM insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal [2]. $T_{\mathrm{c}}$ is continuously enhanced with electron doping by ionic gating up to a maximum $T_{\mathrm{c}}$ of 43 K, and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest $T_{\mathrm{c}}$. A novel phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out, indicating that the superconductor-insulator transition is a common feature for unconventional superconductivity. References: [1] X. F. Lu, N. Z. Wang, H. Wu, Y. P. Wu, D. Zhao, X. Z. Zeng, X. G. Luo, T. Wu, W. Bao, G. H. Zhang, F. W. Huang, Q. Z. Huang, X. H. Chen, Nature Mater. \textbf{14}, 352 (2015). [2] B. Lei, Z. J. Xiang, X. F. Lu, N. Z. Wang, J. R. Chang, S. Chang, A. M. Zhang, Q. M. Zhang, X. G. Luo, T. Wu, Z. Sun, and X. H. Chen, arXiv: 1503.02457.   

Atomic scale visualization of novel magnetic phase transitions in Fe-based superconductor Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$

Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ consists of superconducting FeAs layers and Mott insulating Sr$_{\mathrm{2}}$VO$_{\mathrm{3}}$ layers, and exhibits superconductivity with T$_{\mathrm{c}}$ near 30 K despite being a parent compound material. Unlike normal Fe-based superconductors, the magnetism of Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ has complexity due to the presence of two magnetic atomic layers of V and Fe; therefore, the issue of magnetism has been actively debated. In this work, we studied the orbital and magnetic phase transitions in the range of 4 K to 180 K using spin-polarized scanning tunneling microscope. We directly observed the changes of charge density waves of V atomic layer related to the nematicity at 150 K, and spin density waves of V atomic layer resulting from spin ordering of underlying Fe atomic layer below 50 K. Moreover, controlling the sample bias voltage, the hysteresis of magnetic domain is observed at 4 K. Our results show key clues to solve controversy about the magnetism of Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$.   

Observation of coexistence of itinerant electronic states and local moments in parents compound superconductor Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$

Using variable temperature scanning tunneling spectroscopy (STS) and quasi-particle interference (QPI) analysis, we studied coexistence of itinerant electronic states and local moments in Sr$_{\mathrm{4}}$V$_{\mathrm{2}}$O$_{\mathrm{6}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Temperature dependent STS measurements showed Fano resonances resulting from the hybridization between local moments (V) and itinerant electrons (Fe) below 100 K, and the formation of Fano lattice implying collective spin excitations between local moments of V atoms below 50 K. QPI analysis showed replica bands and kink features in Fe-itinerant band, implying the existence of Bosonic modes between Sr$_{\mathrm{2}}$VO$_{\mathrm{3}}$ layers and FeAs layers. Our results show the collective behaviors of itinerant electrons and local moments, and the possibility of local moments contributing to superconductivity.   

Bipartite bosonic modes and magnetic memory effects in superconducting $\mbox{Sr}_{\mbox{4}} \mbox{V}_{\mbox{2}} \mbox{O}_{\mbox{6}} \mbox{Fe}_{\mbox{2}} \mbox{As}_{\mbox{2}} $

Using a homemade variable temperature high field spin-polarized STM, we have performed spectroscopic-imaging STM measurement on the parent-state superconductor $\mbox{Sr}_{\mbox{4}} \mbox{V}_{\mbox{2}} \mbox{O}_{\mbox{6}} \mbox{Fe}_{\mbox{2}} \mbox{As}_{\mbox{2}} $ with each unit cell composed of superconducting FeAs layer sandwiched by two nearly Mott-insulating $\mbox{Sr}_{\mbox{2}} \mbox{VO}_{\mbox{3}} $ layers. The hybridization between the localized V electrons and the itinerant Fe electrons causes electron transfer to the FeAs bands and generates a Gamma-centered electron pocket, as well as a Fano resonance at -18 meV with signature of Fano lattice. In the QPI measurement, we observed two distinct bosonic modes, i.e. the kinks and the partial replicas of the QPI dispersion with characteristic mode energies around 14 meV and 20 meV respectively, which agree with the self-energies due to two distinct electron-boson mode coupling functions in Migdal approximation. In spin-polarized STM mode, we observed atomic scale magnetic memory effect of the V atoms controlled with low energy (around 50 meV) spin-polarized tunneling current and used it to reveal underlying magnetic domains in the FeAs layer. Variable temperature spin-polarized STM measurements on some known antiferromagnetic materials will also be presented and discussed.   

Unusual phase transition in a natural heterostructure of iron pnictides and vanadium oxides

We report the unusual phase transition in Sr$_2$VO$_3$FeAs single crystal, where the Mott-insulating vanadium oxides and the high-$T_c$ superconducting iron pnictides form a natural heterostructure. Clear evidence of the phase transition at $T_0$ = 155 K was observed in the iron pnictide layer, not in the vanadium oxide layer, using bulk and NMR measurements. Neither magnetic ordering with sufficient spin moment nor symmetry change in the crystal structure has been detected at $T_0$. At $T_{mag}$ $\approx$ 45 K, far below $T_0$, magnetic transition occurs in the iron pnictide layer, while the vanadium oxide layer remains nonmagnetic at low temperatures. The complex evolution of various phases in Sr$_2$VO$_3$FeAs is drastically distinct from the phase transitions found in other iron pnictides or vanadium oxides, highlighting the importance of the additional interlayer coupling between the layers.   

Linear Magnetoresistance of $Ca_{10}Pt_nAs_8(Fe_2As_2)_5$ ($n=3$ and $4$)

We report the normal-state magnetoresistance (MR) of superconducting $Ca_{10}Pt_nAs_8(Fe_2As_2)_5$ ($n = 3$ and $4$) as a function of temperature ($50 - 300\: K$) and magnetic field ($0 - 14$ Tesla). It is found that MR is positive in a wide temperature range in both transverse ($H \perp I$) and longitudinal ($H \parallel I$) cases. At a fixed temperature and field, we observe MR ($H \perp I$) $>$ MR ($H \parallel I$), suggesting spin-orbital coupling in addition to charge-spin interaction. Remarkably, MR shows linear field dependence between $0$ and $14$ Tesla in a wide temperature range for both $n = 3$ and $4$. The implication of such unusual field dependence of MR will be discussed.   

Correlation induced self-doping in the iron-pnictide superconductor Ba2Ti2Fe2As4O

The electronic structure of the intercalated iron-based superconductor Ba2Ti2Fe2As4O (Tc \textasciitilde 21.5 K) has been investigated by using ARPES and combined LDA $+$ DMFT calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacing layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect, i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state, which is in contrast with the LDA prediction of an electron-doped FeAs layer. This exotic behavior is successfully reproduced by the LDA $+$ DMFT calculations, in which the self-doping effect is attributed to the electronic correlations in the Fe 3d shell. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.   

Numerical Study of a Multiorbital Hubbard Model for the Two-Leg Ladder BaFe$_{2}$S$_{3}$ High-Tc Superconductor Using the Density Matrix Renormalization Group

Iron based high-Tc superconductors have attracted considerable attention because of its unconventional superconducting properties. Here, we analyze the magnetic and pairing characteristics of the recently discovered two-leg ladder material BaFe$_{2}$S$_{3}$ that becomes superconducting by applying pressure [1], using a two-orbital Hubbard model studied via the Density Matrix Renormalization Group technique. The hopping parameters, which spans up-to the $2^{nd}$ nearest-neighbor rungs, were obtained from the ab-initio downfolded band structure at ambient and high pressures [2]. The magnetic phase diagram at a realistic Hund coupling $J/U = 0.25$ is presented varying the Hubbard $U$, at select values of the electronic fillings. At half-filling, we find a robust magnetic order in excellent agreement with experiments [1] i.e. antiferromagnetic (ferromagnetic) along the leg (rung) directions. We also discuss a possible tendency for this system to form a paired bound state of holes in a small but finite window of Hubbard $U$. The symmetries of this tentative paired ground state will be discussed. \\[4pt] [1] Hiroki Takahashi et al., \textit{Nature Materials \textbf{14}, 1008 (2015)}\\[0pt] [2] Ryotaro Arita et al., \textit{Phys. Rev. B \textbf{92}, 054515 (2015)}   

Enhancement of Superconductivity Near a Structural Instability in Ba(Ni$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2}}$

We present a comprehensive study of single crystals of Ba(Ni$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{\mathrm{2}}$As$_{\mathrm{2\thinspace }}$synthesized using a flux method. With cobalt substitution, we track the evolution of the structural triclinic phase of BaNi$_{\mathrm{2}}$As$_{\mathrm{2}}$ and the superconducting ground state with heat capacity and resistivity measurements. We will present our study of the systematic suppression of the low temperature triclinic state with increasing Co concentration as well as a more than threefold enhancement in the superconducting critical temperature, discussing its relation to iron-based superconductors.   

\textbf{Magnetism in}\textbf{\textit{ Ln}}\textbf{MnSbO (}\textbf{\textit{Ln }} $=$\textbf{ La or Ce)}

Neutron diffraction of polycrystalline (PND) \textit{Ln}MnSbO (\textit{Ln }$=$ La or Ce) reveals differences between the magnetic ground state of the two compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds adopt the \textit{P4/nmm} space group down to 1.5 K and whereas magnetization measurements do not show any anomaly at high temperatures, PND reveals a C-type antiferromagnetic (AFM) order below $T_{\mathrm{N}} \quad =$ 255 K for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at $T_{\mathrm{SR}} \quad =$ 5 K in CeMnSbO due to a spin-reorientation (SR) transition of the Mn$^{\mathrm{2+}}$ from pointing along the $c$-axis to the \textit{ab}-plane is found. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments. This indicates that the Mn SR transition is driven by the Ce-Mn coupling similar to recent observation in the isostructural CeMnAsO. The ordered moments are found to be somewhat smaller than those expected for Mn$^{\mathrm{2+}}$ (S $=$ 5/2) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower $T_{\mathrm{N\thinspace }}$found in this compound compared to the As-based counterpart ($T_{\mathrm{N}} \quad =$ 347K for CeMnAsO) indicates that the Mn-\textit{Pn} (\textit{Pn} $=$ As or Sb) hybridization that mediates the exchange Mn-Mn coupling is weaker for the Sb-based compounds.   

Collinear antiferromagnetism in trigonal SrMn$_{2}$As$_{2}$ revealed by single crystal neutron diffraction

FeAs-based compounds and related materials have been an area of intense research in understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn$_{2}$As$_{2}$ that crystallizes in a trigonal structure (P$\bar{3}$m1) and undergoes an antiferromagnetic (AFM) transition at $T_N$ $\approx$ 120 K. The temperature dependence of the magnetic susceptibility remains nearly constant below $T_N$ with $H\parallel c$ while it decreases significantly with $H\parallel ab$. This shows that the local Mn moments order and lie in the $ab$ plane instead of aligning along the $c$ axis as in BaMn$_{2}$As$_{2}$. Single crystal neutron diffraction measurements on SrMn$_{2}$As$_{2}$ determined that the Mn moments are collinearly aligned in a G-type AFM order with AFM alignments between a moment and all nearest neighbors in the basal plane and also perpendicular to it. This manifests that G-type AFM order is robust for Mn122 systems despite different symmetries, i.e. tetragonal for BaMn$_{2}$As$_{2}$ and trigonal for SrMn$_{2}$As$_{2}$.\\ Work at Ames Laboratory was supported by the DOE, BES, Division of Materials Sciences \& Engineering, through DE-AC02-07CH11358. This research used resources at University of Missouri Research Reactor.   

Anomalous frequency dependent diamagnetism in metal silicide

Discovery of superconductivity in PbO-type FeSe has generated a lot of interest. Among the samples we synthesize with similar structure, NiSi has showed anomalous but very interesting results. Nickel silicides are important electronic materials that have been used as contacts for field effect transistors, as interconnects and in nanoelectronic devices. The magnetic properties of NiSi are not well known, however. In this presentation, we report a highly unusual magnetic phenomenon in NiSi. The ac susceptibility measurements on NiSi reveal strong frequency dependence of static and dynamic susceptibilities that are primarily diamagnetic at room temperature. The static susceptibility is found to exhibit a strong frequency dependence of the diamagnetic response below 100K, while dynamic susceptibility showed peak type feature at 10KHz frequency around 50K. Detailed neutron scattering measurements on high quality powder sample of NiSi on SPINS cold spectrometer further revealed an inelastic peak around 1.5meV, even though no magnetic order is detected. The inelastic peak dissipates above 100K, which is where the static susceptibility starts to diverge with frequency.   

The Long Forgotten Compound: CoTe, and its Epitaxial Film Growth and Properties

As part of our investigation of Co-doped, Fe-chalcogenide superconductors, we have synthesized films of CoTe; a long forgotten binary compound. Using pulsed laser deposition, we have grown epitaxial films on MgO, CaF$_2$, and SrTiO$_3$ and have found that careful control of growth conditions allows for the synthesis of either (001) or (101) oriented films. X-ray diffraction shows the structure of the films is hexagonal. However, we also find the surprising presence of the nominally disallowed (001) peak. We also report temperature dependent transport and magnetic properties. This material may be of interest as a magnetic semiconductor and for its relationship to chemically doping Fe-based superconductors.