Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on an iron-based superconductor LaFePO. A steep increase in superconducting transition temperature (Tc) of LaFePO with dTc/dP > 4 K/GPa to a maximum of 8.8 K for P = 0.8 GPa was observed. These results are similar to isocrystalline LaFeAsO1-xFx system reported previously. X-ray diffraction measurements were also performed under high pressure up to 10 GPa, where linear compressibility ka and kc are presented.
The superconducting gap structure of LaFePO ($T_c=7.4 $K) is studied by thermal conductivity ($κ$) at low temperatures in fields $H$ parallel and perpendicular to the c axis. A clear two-step field dependence of $κ(H)$ with a characteristic field $H_s(\sim 350$ Oe) much lower than the upper critical field $H_{c2}$ is observed. In spite of large anisotropy of $H_{c2}$, $κ(H)$ in both $H$-directions is nearly identical below $H_s$. Above $H_s$, $κ(H)$ grows gradually with $H$ with a convex curvature, followed by a steep increase with strong upward curvature near $H_{c2}$. These results indicate …
We report an optical spectroscopy study on the single crystal of Na$_2$Ti$_2$As$_2$O, a sister compound of superconductor BaTi$_2$Sb$_2$O. The study reveals unexpectedly two density wave phase transitions. The first transition at 320 K results in the formation of a large energy gap and removes most part of the Fermi surfaces. But the compound remains metallic with residual itinerant carriers. Below 42 K, another density wave phase transition with smaller energy gap scale occurs and drives the compound into semiconducting ground state. These experiments thus enable us to shed light on the compl…
A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by solid-state reaction. This material has an alternating layer stacking structure of anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group of the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543 A, respectively. The interlayer Fe-Fe distance corresponding to the c-axis length is the longest ever reported in the iron-based oxypnictide systems. In both magnetization and resistivity measurements, the present compound exhibited superconductivity below 17 K, which is much higher than that…
Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high-temperature superconductors. Spin fluctuations have been observed in iron-based superconductors as well. However, in some iron-based superconductors such as LaFePO$_{0.9}$, they have not been observed by inelastic neutron scattering (INS). LaFePO$_{0.9}$ is an iron-based superconductor with a low superconducting transition temperature ($T_{\rm c}$= 5 K), where line nodes are observed in the superconducting gap function. The line-node symmetry typically originates from sign r…
Highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition (IBAD)-MgO/Y2O3/Hastelloy substrates by molecular beam epitaxy. The oxypnictide coated conductors showed a superconducting transition temperature (Tc) of 43 K with a self-field critical current density (Jc) of 7.0 x 104 A/cm2 at 5 K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires. Albeit higher Tc as well as better crystalline quality than Co-doped BaFe2As2 coated conductors, in-field Jc of NdFeAs(O,F) was lower than that of Co-doped BaFe2As2. These results suggest that grain bound…
We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdoped sample (T_c=20 K), an optimally doped sample (T_c=28 K), and an overdoped sample (T_c=22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO_{1-y}$. A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, 1/T_1T in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe …
The discovery of about 50 K superconductivity in the tetragonal Fe-based pnictides has stimulated the search for superconductivity in a wide class of materials with similar structure. Copper forms compounds isostructural to LaOFeAs. Single phase BiOCuS can be prepared by a solid state reaction at temperature lower than 500 C from a mixture of Bi2O3, Bi2S3 and Cu2S. The samples have been characterized by means of EDX analysis, X-ray diffraction, magnetic and electrical measurements. The cell parameters are a = 3.8708 A, c = 8.565 A. Charge carrier doping can be realized either by F substitution…
High-temperature (high-$T_{\rm c}$) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state. On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates. Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in i…
In the family of the iron-based superconductors, the $RE$FeAsO-type compounds (with $RE$ being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures ($T_{\mathrm{c}}$) up to $55\ \textrm{K}$ and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe$_{0.92}$Co$_{0.08}$AsO ($T_{\mathrm{c}}=18\ \textrm{K}$) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to th…
Topological superconductors have long been predicted to host Majorana zero modes which obey non-Abelian statistics and have potential for realizing non-decoherence topological quantum computation. However, material realization of topological superconductors is still a challenge in condensed matter physics. Utilizing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations, we predict and then unveil the coexistence of topological Dirac semimetal and topological insulator states in the vicinity of Fermi energy ($E_F$) in the titanium-based oxypnictide supercon…
A family of titanium oxypnictide materials BaTi2Pn2O (Pn = pnictogen) becomes superconducting when a charge and/or spin density wave is suppressed. With hole doping, isovalent doping and pressure, a whole range of tuning parameters is available. We investigate how charge doping controls the superconducting transition temperature Tc. To this end, we use experimental crystal structure data to determine the electronic structure and Fermi surface evolution along the doping path. We show that a naive approach to calculating Tc via the density of states at the Fermi level and the McMillan formula sy…
We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-Tc$ superconductivity (HTSC) for a disorder-free CuO$_2$ plane based on an evaluation of local hole density ($p$) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO$_2$ plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO$_2$ pla…
The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 10^5 A/cm^2 at 45 T and 4.2 K for both ma…
We successfully synthesized new layered iron oxypnictide Sr2CrO2(FeAs)2 by solid-state-reaction techniques. This is the first report on the Fe-substituted Sr2Mn3As2O2-type oxypnictides to our knowledge. A pellet of stoichiometric mixtures of SrO, Cr and FeAs wrapped with tantalum foil was sintered at 900°C for 96 h. Results of x-ray diffraction measurement and Rietveld refinements show that the structure of the sample belongs to the space group of I4/mmm with the lattice constants of a = 3.9948(1) Å and c = 18.447(1) Å. Temperature dependence of the resistivity for Sr2CrO2(FeAs)2 shows semicon…
In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc=55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-base…
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm$_{1-x}$Th$_x$OFeAs with various thorium doping and critical temperatures $T_C = 21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2Δ_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2Δ_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2Δ_S/k_BT_C = 1.2-1.6$ remain nearly constant within all the $T…
The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $\leq x \leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at T$\rm{_{S}}$~=~130~K which is rapidly suppressed by $x \simeq$ 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120~K. The temperature dependence of the thermal expansion coefficient $α_{V}$ reveals a rich phase diagr…
Fluorine-substitution-dependent phase diagram and superconducting properties of Sm-based oxypnictides synthesized by a high-pressure growth technique
Institute of High Pressure Physics (IHPP), Polish Academy of Sciences（波兰科学院高压物理研究所）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；National Centre for Nuclear Research（国家核研究中心）；Faculty of Physics, Warsaw University of Technology（华沙理工大学物理学院）；Core Electronics Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST)（产业技术综合研究所核心电子技术研究所）
We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAsO0.6F0.35 and thin films SmFeAsO1-xFx by means of bulk-sensitive conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical con…
We report the results from resistivity and magnetic measurements on polycrystalline Ce oxypnictide (CeFeAsO1-xFx) samples where x spans from 0.13 to 0.25. We find that the orbital limiting field is as high as 150 T and it systematically decreases with increasing doping. The Maki parameter is greater than one across the phase diagram and the large Maki parameter suggests that orbital and Pauli limiting effects contribute to the upper critical field. The broadening of the superconducting transition in the resistivity data was interpreted using the thermally activated flux flow (TAFF) model where…
Effects of high-pressure synthesis on phase formation and superconducting properties of PrFeAsO1-xFx
Institute of High Pressure Physics (IHPP), Polish Academy of Sciences（波兰科学院高压物理研究所）；Faculty of Physics, Warsaw University of Technology（华沙理工大学物理学院）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）
