Hiroya Sakurai, 2, ∗ Naohito Tsujii, Osamu Suzuki, Hideaki Kitazawa, Giyuu Kido, Kazunori Takada, Takayoshi Sasaki, and Eiji Takayama-Muromachi Superconducting Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan International Center for Young Scientists, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan NanoMaterials Laboratory, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0044, Japan Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan (Dated: March 23, 2022)

We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As1-x′P x′)O exhibits the re-emergent antiferromagnetic (AFM) order at x′ ~ 0.6 (AFM2) separated from the parent AFM order at x′ =0 (AFM1). Systematic 31P-NMR study on [Sr4Sc2O6]Fe2(As1−x P x )2 (SrSc42622), which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As1−x′/P x′)O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around x ~ 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer. In order to elucidate the further universality and diversity, we have carried out 77Se-NMR measurement on Li x (NH3)yFe2−δ Se2 (Tc = 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (T) dependences of Knight shift and nuclear relaxation rate (1/T 1 T). We discuss the universality and diversity of the relationship between the Tc and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.

A triangular-lattice antiferromagnet of Ag2FeO2 was synthesized under high pressure. Its magnetism was studied in terms of electrical resistivity, magnetic susceptibility, heat capacity, powder neutron scattering, and Mossbauer spectroscopy. The magnetic state of Ag2FeO2 changes successively through a second-order phase transition at T-p = 36 K and a crossover at T-c = 20 K. A partially disordered (PD) state appears below T-p, in which approximate to 2/3 spins are ordered and the remaining approximate to 1/3 spins fluctuate, which state persists at least down to 5 K. The spin correlation length starts to grow at T-p; however, it remains very short (approximate to 27 angstrom) below T-c. This exotic magnetism is concerned with strong frustration in the classic antiferromagnetic triangular lattice.

We report a systematic NMR study on [Sr4Sc2O6]Fe-2(As1-xPx)(2), for which the local lattice parameters of the iron-pnictogen (FePn) layer are similar to those of the series LaFe(As1-x'Px')O, which exhibits two segregated antiferromagnetic (AFM) order phases, AFM1 at x' = 0 - 0.2 and AFM2 at x' = 0.4 - 0.7. Our results reveal that the parent AFM1 phase at x = 0 disappears at x = 0.3 - 0.4, corresponding to a pnictogen height (h(pn)) from the Fe plane of 1.3-1.32 angstrom, which is similar to that of LaFe(As1-x'Px')O and various parent Fe pnictides. By contrast, the AFM2 order reported for LaFe(As0.4P0.6)O does not appear at x similar to 0.8, although the local lattice parameters of the FePn layer and the microscopic electronic states are quite similar. Despite the absence of the static AFM2 order, reemergent dynamical AFM spin fluctuations were observed at approximately x similar to 0.8, which can be attributed to the instability of the AFM2 phase. We suggest this re-enhancement of AFM spin fluctuations plays a significant role in enhancing the T-c to 17 K for x = 0.8 - 1. Finally, we discuss the universality and diversity of the complicated magnetic ground states from a microscopic point of view, including the difference in the origins of the AFM1 and AFM2 phases, and their relations with the high superconducting transitions in Fe pnictides.

Structural phase transitions in BiMnO3 were studied by means of energy-dispersive X-ray diffraction in the pressure 0-4 GPa and temperature 300-900 K ranges, and also by means of angle-dispersive X-ray diffraction and Raman spectroscopy at high pressures up to 50 GPa and ambient temperature. The P-T phase diagram of BiMnO3 was constructed. A suppression of the transition temperatures between monoclinic C2/c and orthorhombic Pnma phases was observed under pressure. The temperature and pressure dependencies of lattice parameters were obtained. A new orthorhombic Imma phase was observed at P > 20 GPa. The lattice dynamics calculations were performed for the analysis of the Raman spectra of BiMnO3. (C) 2013 Elsevier B. V. All rights reserved.

Polycrystals of Ba0.5K0.5Fe1.9M0.1As2 (M = Fe, Co, Ni, Cu, and Zn) synthesized at high pressures show quasi-periodic spikes in the magnetization loop at 2 K owing to successive flux jumps, while the single crystals are observed as absence of jumps. The Zn-doped crystal indicates higher frequency flux jumps than that of Zn-free samples, because of the reduction of grain size and enhancement of grain boundary effect. However, the Co-, Ni- and Cu-doped crystals show less jumps duo to the increased granularity. Our present results suggest that the flux jump in the textured polycrystals is controlled largely by the grain boundary as the flux pining effect rather than the intrinsic dopants.

Heavily potassium-substituted barium iridates Ba2-xKxIrO4 (x <= 0.5) and lanthanum-substituted Ba2-yLayIrO4 (y <= 0.05) were prepared for the study of the carrier doping effects on the transport properties of the spin-orbit Mott state. The carrier type is holelike for the nondoped (x = y = 0) and K-doped (x>0) phases, while it is electronlike for the La-doped (y>0) phases. It was found that electron doping is more effective in decreasing the electrical resistivity. This suggests an asymmetry of the density of states between the upper and lower energies of the Fermi level. A semimetallic state emerges for the K-doped phases with x >= 0.3 at ambient pressure. More conducting metallic states (rho similar to 10(-2) Omega cm, d rho/dT>0) were achieved under high pressure for both the K- and La-doped phases. Notwithstanding, no superconductivity was observed in the metallic states down to 4.2 K. The experimental results are discussed with respect to the electronic phase diagram calculated by Watanabe et al. [Phys. Rev. Lett. 110, 027002 (2013)].

We have used the ac magnetic susceptibility to investigate the vortex state in an optimally doped p-type Ba0.5K0.5Fe2As2 single crystal under various ac and dc fields. A peak effect is observed in the temperature dependence of the in-phase ac susceptibility, indicating an order-disorder transition on the vortex phase diagram. The peak effect displays an anomalous history effect compared with other type-II superconductors, which we ascribe to the strong pinning existing in the material. We observe the development of a small dissipation peak at the temperature T-p2 slightly below the peak effect region. Similar to the peak effect boundary, T-p2 delimits a region in the H-T phase diagram which is independent on the ac field amplitude. We argue that this small peak may arise from the softening of the vortex lattice, leading to a collective pinning of the whole vortex lattice. This effect assists and further enhances the peak effect occurring in the Ba0.5K0.5Fe2As2 superconductor.

Terahertz (THz) emission has been recently detected from intrinsic Josephson junction (IJJ) stacks made of the high critical temperature superconductor Bi2Sr2CaCu2O8+delta (BSCCO). The most employed structure is a mesa standing on a big pedestal of a single crystal with a thin gold layer as its top electrode. In this work, a large (300 x 50 x 1.2 mu m(3)) IJJ stack with superconducting electrodes was fabricated and studied. The stack consisted of N approximate to 800 IJJs. It was prepared with a double-sided fabrication process, and significant THz emission was detected. The output power is comparable to the emission power detected from mesa structures, obviously not weakened by the superconducting upper electrode. The observation of THz emission from the double-sided structure suggests that off-chip THz emission from IJJs can be obtained not only from mesa structures and, most importantly, that the emission power can be potentially enhanced in integrated multi-stack radiation sources.

Magnetic properties in the novel spin-orbit Mott insulator Ba2IrO4 were studied using muon spin rotation (μSR) technique. Zero-field μSR experiments revealed that Ba2IrO4 shows an antiferromagnetic transition at TN ∼ 240 K without any spontaneous magnetization. The most stable μ+ site was determined by the electrostatic (Madelung) potential calculation. The effective magnetic moment of the iridium ions (|μ|) in the antiferromagnetic ordered state was calculated using a dipolar-field model, with an internal field obtained by μSR experiments. The magnetic moment is significantly reduced (|μ| ∼ 0.34 μB) due to a low-dimensional quantum spin fluctuation with a large intra-plane correlation. The magnetic ground state of the spin-orbit Mott insulator Ba2IrO4 is quite similar to those in parent materials of high-TC cuprate superconductors such as La2CuO4.

We report a Zn-doping effect on BaFe1.92Pt0.08As2 (T-c =24.5 K) single crystal. T-c rapidly decreases with increasing the Zn content in the lattice, revealing a distinct feature which is highly contrasted with the intrinsic scattering feature induced by Pt. Doped Zn completely kills the superconductivity by no more than 8 atomic %, while intrinsic scattering induced by doped Pt shows a good coexistence with the superconductivity. The results are discussed in connection with the s(+/-)- and s(++)-wave models.

Resonant soft x-ray powder diffraction experiments on orthorhombic TmMnO3 and LuMnO3 are presented. Experiments were performed in the vicinity of the Tm M-5 and the Mn L-2,L-3 edges to study the Tm and Mn magnetic moments, respectively. These experiments show that for the heavy rare-earth perovskite with an E-type ordered ground state, the Tm magnetic moments order already in the nonferroelectric incommensurate magnetic phase. Additionally, deviations from the collinear E-type Mn magnetic structure at low temperatures are found for both compounds. These experiments show the power of resonant soft x-ray diffraction, extended to polycrystalline 4f materials.

We investigated magnetic domain structures of a multiferroic manganite, BiMnO3, by Lorentz transmission electron microscopy. Ferromagnetic domains were observed below similar to 105 K, close to the ferromagnetic Curie temperature, T-C. The spontaneous magnetization aligns distinctly along the [010] direction, suggesting that the magnetic easy direction is along the b axis. Inflection and merging of the domain walls was observed at twin boundaries. This indicates pinning of the magnetic domain walls at crystallographic twin boundaries. Furthermore, we observed narrow magnetic domain walls, suggesting strong magnetocrystalline anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4742747]

X-ray near edge absorption spectroscopy was used to probe the electronic structure of multiferroic orthorhombic LuMnO3 polycrystalline samples and strained, twin-free orthorhombic (1–10) LuMnO3 films grown by pulsed laser deposition on (1–10) YAlO3 substrates. For all o-LuMnO3 samples x-ray near edge absorption spectroscopy spectra reveal that the pre-edge structure is influenced by the increase in MnO6 distortion as a result of the smaller Re-ion or film strain. Furthermore there is clear evidence of anisotropic Mn-O bonding and Mn orbital ordering along the c- and [110] direction. The experimental film and bulk data are in agreement with ab initio simulations.

As-75 nuclear spin-lattice relaxation rate 1/T-1 was measured in Zn-free and Zn-substituted La(Fe1-xZnx) AsO0.85 (x = 0, and 0.05), T-c of which is 25 K and 0 K, respectively. Although the temperature dependence of 1/T1T above T-c is the same between two samples, 1/T1T of x = 0 shows T-3.0 +/- 0.1 dependence (1/T-1 proportional to T-4) in the superconducting (SC) state, but 1/T1T of non-SC x = 0.05 sample continuously decreases below 50 K. The normalized 1/T1T [(1 = T/T-1)(x = 0)/(1/T1T)(x = 0).(05)], which is related to the SC gap structure, is proportional to T-2.4 +/- 0.1 below T-c. These results indicate that the temperature dependence of 1/T1T in the SC state can be affected by the temperature dependence of the normal-state 1/T1T.

The spin-orbit Mott state in the novel quasi-2D iridate Ba2IrO4 was studied on crystal structure, electrical resistivity, magnetic susceptibility, and mu SR experiments. Ba2IrO4 crystallizes in a K2NiF4-type structure (I4/mmm, a = 4.030(1) angstrom and c = 13.333(4) angstrom) which includes IrO2 square planer lattices with straight Ir-O-Ir bonds. The minimal Mott-gap size is similar to 70 meV. The magnetic ground state is antiferromagnetic long-range order (T-N similar to 240 K) without spin canting. The magnetic moment (vertical bar mu vertical bar similar to 0.34 mu(B)/Ir) is much reduced by a low-dimensional quantum spin fluctuation with a large intra- plane correlation vertical bar J vertical bar. The critical exponent (beta similar to 0.18) suggests that the magnetic state is classified into 2-D X-Y or (anisotropic) Heisenberg spin systems with weak 3-D interlayer coupling vertical bar J'vertical bar.

A novel metallic silver chromate, Ag 2 CrO 2 , was synthesized using a high-pressure technique. Ag 2 CrO 2 crystallizes in trigonal symmetry with lattice parameters of a = 2.9271(1) A and c = 8.6721(4) A. The structure consists of CrO 2 and double Ag layers stacked alternately along the c -axis. The former realizes an S = 3/2 triangular-lattice Heisenberg system, while the latter provides itinerant electrons. Ag 2 CrO 2 exhibits an antiferromagnetic long-range order at T N = 24 K with the weak ferromagnetic moment. The resistivity shows a sudden drop at T N , suggesting a large s – d interaction (RKKY interaction) between the Cr 3 d localized spins on the triangular lattice and the Ag 5 s itinerant electrons. The RKKY interaction is responsible for releasing the magnetic frustration and the three-dimensional long-range ordering at T N .

Additional charge carriers were introduced to the iron oxyarsenide Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of electrical resistivity, Hall coefficient, and magnetic susceptibility. The host compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to show a semiconducting-like conductivity accompanied by a positive magneto-resistance (22% at 70 kOe). Although the carrier density is comparable at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no superconductivity appears down to 2 K. This is primarily because the net carrier density decreases over 3 orders of magnitude on cooling and additionally a possible magnetic order at ~120 K prevents carriers from pairing. The properties were altered largely by introducing the additional carriers.

Since the discovery of the high-Tc superconductor in the La-Ba-Cu-O system [1], a great deal of experimental and theoretical effort have been made to clarify the nature of the Cu-based oxides. In order to elucidate mechanism of the high-Tc superconductivity, discovery of a new type of superconductor is no doubt of great importance. Recently, Akimitsu et al. found a new oxide superconductor in the Nd-Ce-Sr-Cu-O system [2]. Soon after their discovery, the superconducting phase was isolated and identified [3]. It has a tetragonal cell with space group P4/nmm and has a structure closely related to but different from the K2NiF4− or T’-Nd2CuO4− -type structure. Although, Tc of the Nd-Ce-Sr-Cu oxide is not so high (ca. 20 K) compared with the 1–2–3 or Bi(Tl)-based superconductors, it has aroused interest widely due to a very simple crystal structure. In this article, I will discuss superconductivity and crystal chemistry of the Nd-Ce-Sr-Cu oxide. Also, various compounds isostructural to it will be presented.

Nonmagnetic impurity effect was studied on the n-type Fe-based superconductor BaFe1.87Co0.13As2 (Tc = 25 K) by a successful Zn substitution for Fe up to 7 at.%. Magnetic susceptibility, electrical resistivity, specific heat, and Hall coefficient measurements indicated that Tc linearly decreases with the Zn concentration and disappears at 7 at.%. The result is quantitatively comparable with what was observed for YBCO, while it disagrees with a recent report for the p-type Ba0.5K0.5Fe2As2. Fragile SC against a nonmagnetic impurity was first confirmed for the n-type 122 Fe-based superconductor.