FeAs-based Compounds Research Articles

Spin reorientation inBa0.65Na0.35Fe2As2studied by single-crystal neutron diffraction

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.

In-plane optical spectroscopy study on FeSe epitaxial thin film grown on SrTiO3substrate

We perform in-plane optical spectroscopy measurement on (00l) FeSe thin film grown on SrTiO${}_{3}$ substrate by the pulsed laser deposition method. The study indicates that the low-frequency conductivity consists of two Drude components: a broad one which takes up most of the spectral weight and a very narrow one roughly below 100--150 cm${}^{\ensuremath{-}1}$. The narrow Drude component locates at so low frequencies that no such behavior was observed in iron pnictides. The overall plasma frequency is smaller than that of the FeAs-based compounds, suggesting a stronger correlation effect. Similar to iron pnictides, a temperature-induced spectral weight transfer is observed for FeSe. However, the relevant energy scale is smaller. Additionally, different from a recent angle-resolved photoemission spectroscopy measurement which revealed a spin density wave (SDW) order at low temperature for FeSe thin films grown on SrTiO${}_{3}$ substrate, no signature of the band structure reconstruction arising from the formation of the SDW order is seen by optical measurement in the thick FeSe films.

High-pressure study of the thermoelectric properties of various oxides (ZnO, Ti2 O3 , Fe x O y ) and FeAs-based compounds

In the present work the results of investigation of the thermoelectric (TE) properties of some Zn, Ti and Fe oxides and FeAs-based compounds under compression up to 20 GPa are reported. The data obtained of thermoelectric power S dependence on pressure P for the materials under consideration revealed some new features of the electron properties of both initial and high-pressure phases (RS-ZnO). For FeAs-based compounds the complex behavior of S on P was found depending probably on the contribution of d-states in the conductivity. The decreasing of the semiconductors gap has been established under pressure for the narrow-gap semiconductors Ti2O3 and Fe2O3; the sign inversion being observed in the first oxide pointing to the change of dominant type of the charge carries. Pressure cycling of Fe2O3 led to a nanostructured state with the modified electronic properties. TE data at high pressure allowed the electronic properties of the materials with the different type of the electron structure to be detailed.

Universal pair-breaking in transition-metal-substituted iron-pnictide superconductors

The experimental transport scattering rate was determined for a wide range of optimally doped transition-metal-substituted FeAs-based compounds with the ThCr${}_{2}$Si${}_{2}$ (122) crystal structure. The maximum transition temperature ${T}_{c}$ for several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of suppression with increasing scattering rate indicative of a common pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a limit of $\ensuremath{\sim}$26 K on the maximum ${T}_{c}$ for all transition-metal-substituted 122 systems, in agreement with experimental observations, and sets a critical scattering rate of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ s${}^{\ensuremath{-}1}$ for the suppression of the superconducting phase. The observed critical scattering rate is much weaker than that expected for a sign-changing order parameter with strong interband scattering, providing important constraints on the nature of the superconducting gap in the 122 family of iron-based superconductors.

Inelastic Neutron-Scattering Measurements of Incommensurate Magnetic Excitations on Superconducting LiFeAs Single Crystals

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron scattering experiments. There is no indication for static magnetic ordering but inelastic correlations appear at the incommensurate wave vector (0:5+-d 0:-+d 0) with d=0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

Magnetic Orientation of Superconductors With Layered Crystal Structures

Tri-axial orientation of twin-free powder of Y2Ba4Cu7O15-δ (Y247) and uni-axial orientation of various FeAs-based compounds were attempted using modulated rotating and rotating magnetic fields, respectively. Tri-axial orientation with high degrees of inplane and c -axis orientation of Y247 was achieved under magnetic fields above 5 T, while magnetic fields at which reduction of the degrees began were different. We found from these crossover fields that quantitative magnetic anisotropy could be obtained. This determination method is particularly useful in that single crystalline samples are not required. From the magnetic orientation using the rotating field, all the obtained FeAs-based compounds showed a hard axis of magnetization parallel to the c-axis direction. On the other hand, the degrees of c-axis orientation were dependant on the types of ions in the blocking layers, and almost complete c-axis orientation was achieved for the compounds with transition-metal ions containing 3d electrons. Magnetic orientation using powder samples is a strong tool for the clarification of physical properties of Cooper pairs and critical current properties in FeAs-based superconductors.

In-plane anisotropy of electrical resistivity in strain-detwinned SrFe2As2

Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe${}_{2}$As${}_{2}$ above and below the temperature of the coupled structural/magnetic transition, ${T}_{\mathrm{TO}}$. Resistivity is smaller for electrical current flow along the orthorhombic ${a}_{o}$ direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the ${b}_{o}$ direction (direction of ferromagnetic chains), which is similar to CaFe${}_{2}$As${}_{2}$ and BaFe${}_{2}$As${}_{2}$ compounds. A strongly first-order structural transition in SrFe${}_{2}$As${}_{2}$ was confirmed by high-energy x-ray measurements, with the transition temperature and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above ${T}_{\mathrm{TO}}$. With the increase of strain, the resistivity anisotropy starts to develop above ${T}_{\mathrm{TO}}$, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs-based compounds in which the structural transition is strongly first order.

Optical response of FeAs-based compounds

We investigated the optical properties of FeAs-based compounds. The optical conductivity spectrum of undoped CaFe2As2 shows an abrupt opening of a gap below the magnetic/structural transition temperature (TN). The low-energy spectrum above TN is composed of two components: one is a coherent Drude term and the other is an incoherent one. Below TN, the Drude term becomes very narrow without changing its weight, and a gap seems to open in the incoherent term. The two-component feature also exists in the spectrum of superconducting Ba(Fe0.92Co0.08)2As2 and a superconducting gap opens in both components. The London penetration depth calculated from the spectral weight analysis is ≃2770Å consistent with other experiments.

Striped antiferromagnetic order and electronic properties of stoichiometric LiFeAs from first-principles calculations

We use state-of-the-arts first-principles method to investigate the structural, electronic, and magnetic properties of stoichiometric LiFeAs. We optimize fully all the structures, including lattice constants and internal position parameters, for different magnetic orders. We find the magnetic ground state by comparing the total energies among all the possible magnetic orders. Our calculated lattice constants and As internal position are in good agreement with experiment. The experimental fact that no magnetic phase transition has been observed at finite temperature can be attributed to the tiny inter-layer spin coupling. Our results show that stoichiometric LiFeAs has almost the same striped antiferromagnetic spin order as other FeAs-based parent compounds and tetragonal FeSe do, which may imply that all Fe-based superconductors have the same mechanism of superconductivity.

Optical study of the spin-density-wave properties of single-crystallineNa1−δFeAs

We report an optical investigation on the $ab$ plane charge dynamics for ${\text{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\text{FeAs}$ single crystal. Formation of a partial energy gap is clearly observed in the magnetic ordered phase. Comparing with the optical response of other FeAs-based parent compounds with high magnetic transition temperatures, both the gap value $2\ensuremath{\Delta}$ and the energy scale for the gap-induced spectral weight redistribution are smaller in ${\text{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\text{FeAs}$. The experimental results favor an itinerant origin of the antiferromagnetic transition.

Successive Phase Transitions under High Pressure in FeTe0.92

We performed magnetization and electrical resistivity measurements under high pressures of up to 19 GPa for FeTe0.92. The compound shows an anomaly in magnetization and resistivity at atmospheric pressure due to a structural distortion accompanied by a magnetic transition. We also observed magnetic and resistive anomalies under high pressure, suggesting that two pressure-induced phases exist at a low temperature. Unlike in FeAs-based compounds, no superconductivity was detected under high pressures of up to 19 GPa, although the anomaly at atmospheric pressure was suppressed by applying pressure.

Band structure, Fermi surface, and superconducting gap in FeAs-based superconductors revealed by angle-resolved photoemission spectroscopy

In this paper, we present a brief review on our angle-resolved photoemission measurements on the band structure, Fermi surface, and superconducting gap of the newly-discovered FeAs-based high temperature superconductors. (1) The Fermi surface of the FeAs-based compounds are characterized by the hole-like Fermi surface sheets near Γ (0, 0) and the existence of singular Fermi spots near M (tp, π). However, the origin of the Fermi spots in the magnetically-ordered state of the parent compound BaFe2As2 and optimally-doped (Ba0.6K0.4)Fe2As2 superconductor may be different. (2) The magnetically-ordered state in the parent compound, BaFe2As2, shows unusual electronic signatures that are distinct from the conventional spin-density-wave system. (3) Multiple nodeless superconducting gap has been observed in (Ba0.6K0.4)Fe2As2 superconductor which is consistent with s-wave superconducting order parameter.

Temperature-dependent striped antiferromagnetism of LaFeAsO in a Green’s functionapproach

We use a Green’s function method to study the temperature-dependent average momentand magnetic phase-transition temperature of the striped antiferromagnetism of LaFeAsO,and other similar compounds, as the parents of FeAs-based superconductors. We considerthe nearest and the next-nearest couplings in the FeAs layer, and the nearest couplingfor inter-layer spin interaction. The dependence of the transition temperatureTN and thezero-temperature average spin on the interaction constants is investigated. We obtain an analytical expressionfor TN and determine our temperature-dependent average spin from zero temperature toTN in terms of unified self-consistent equations. For LaFeAsO, we obtain a reasonableestimation of the coupling interactions with the experimental transition temperatureTN = 138 K. Our results also show that a non-zero antiferromagnetic (AFM)inter-layer coupling is essential for the existence of a non-zeroTN, and the many-body AFM fluctuations reduce substantially the low-temperature magneticmoment per Fe towards the experimental value. Our Green’s function approachcan be used for other FeAs-based parent compounds and these results shouldbe useful to understand the physical properties of FeAs-based superconductors.

High-TC superconductivity related to deep inner orbital coupling in FeAs-based compounds

The discovery of new iron-based high temperature superconductors REOFeAs (RE=La, Ce, Pr, etc.) led to a great deal of interest in study of superconductivity mechanism. The relation in electronic structure between arsenic atom and its nearest neighbor cation A for various Fe–As superconductor systems is studied. The result shows that the energy levels of 5p orbitals of all cation A occupy in a narrow energy extent of −14 to −26 eV, which are close to that of As 4s orbital. The calculated electronic structures show that there is an inner orbital coupling between As and near cation A. The distance of A-As has an approximately proportional relation with the critical temperature of superconductivity (TC). The larger intensity of the inner orbital coupling corresponds to the higher TC. We conclude the high-TC superconductivity originates from the inner orbital coupling between As and its near cation A atom for this kind of Fe-based REOFeAs superconductors.

Heat capacity measurements on FeAs-based compounds: a thermodynamic probe of electronic and magnetic states

We report heat capacity measurements of the pnictide materials SmFeAsO1-xFx, NdFeAsO, LaFeAsO1-xFx and LiFeAs. For SmFeAsO1- xFx, with x close to 0.1, we use 3He measurements to demonstrate a transfer of entropy from the peak at TN to a previously unidentified ∼2 K feature, which grows with increasing doping. Our results on the Sm samples are compared with a similarly doped La sample to elucidate the crystal field levels of the Sm3+ ion at 0, 20 and 45 meV, which lead to a Schottky-like anomaly, and also show that there is a significant increase in the Sommerfeld coefficient γ when La is replaced by Sm or Nd. The lattice contribution to the heat capacity of the superconducting oxypnictides is found to vary negligibly with chemical substitution. We also present a heat capacity measurement of LiFeAs showing the feature at Tc, which is significantly rounded and much smaller than the BCS value.