Tetragonal FeSe Research Articles

Selective orbital reconstruction in tetragonal FeS: A density functional dynamical mean-field theory study

Transport properties of tetragonal iron monosulfide, mackinawite, show a range of complex features. Semiconductive behavior and proximity to metallic states with nodal superconductivity mark this d-band system as unconventional quantum material. Here, we use the density functional dynamical mean-field theory (DFDMFT) scheme to comprehensively explain why tetragonal FeS shows both semiconducting and metallic responses in contrast to tetragonal FeSe which is a pseudogaped metal above the superconducting transition temperature. Within local-density-approximation plus dynamical mean-field theory (LDA+DMFT) we characterize its paramagnetic insulating and metallic phases, showing the proximity of mackinawite to selective Mott localization. We report the coexistence of pseudogaped and anisotropic Dirac-like electronic dispersion at the border of the Mott transition. These findings announce a new understanding of many-particle physics in quantum materials with coexisting Dirac-fermions and pseudogaped electronic states at low energies. Based on our results we propose that in electron-doped FeS substantial changes would be seen when the metallic regime was tuned towards an electronic state that hosts unconventional superconductivity.

S/Te co-doping in tetragonal FeSe with unchanged lattice parameters: Effects on superconductivity and electronic structure

Previous study suggested that Se-site doping with Te or S in tetragonal FeSe adjusts the superconductivity only by expanding or compressing the lattice parameters. However, three chalcogens X (X = S, Se, Te) show different electronegativity, which could directly change the interaction between Fe &X, and their electronic structures. In order to understand this underlying mechanism, the samples with different anion compositions but the same lattice parameters are required. In this work, two superconductors: FeSe0.95Te0.03S0.02 and FeSe0.9Te0.057S0.043 have been synthesized by solid state reaction. The thus-obtained samples have the same lattice parameters in tetragonal single phase as that of undoped FeSe. It is found that despite the unchanged lattice, S/Te co-doping on Se-site still increased the density of the electronic states (DOS) near the Fermi level, thereby enhanced the superconductivity. Our results suggest that the influence of Se-site doping with the same valence chalcogen in superconducting FeSe is not simply in the view of the lattice size, effects on electronic structures originated from different electronegativity during Se-site doping should also be considered.

Superconductivity in LiOHFeS single crystals with a shrunk c-axis lattice constant

By using a hydrothermal ion-exchange method, we have successfully grown superconducting crystals of LiOHFeS with ${T_c}$ of about 2.8 K. Being different from the sister sample (Li${_{1-x}}$Fe${_x}$)OHFeSe, the energy dispersion spectrum analysis on LiOHFeS shows that the Fe/S ratio is very close to 1:1, suggesting an almost charge neutrality and less electron doping in the FeS planes of the system. Comparing with the non superconducting LiOHFeS crystal, each peak of the X-ray diffraction pattern of the superconducting crystal splits into two, and the diffraction peaks locating at lower reflection angles are consistent with that of non-superconducting ones. The rest set of diffraction peaks with higher reflection angles is corresponding to the superconducting phase, suggesting that the superconducting phase may has a shrunk c-axis lattice constant. Magnetization measurements indicate that the magnetic shielding due to superconductivity can be quite high under a weak magnetic field. The resistivity measurements under various magnetic fields show that the upper critical field is quite low, which is similar to the tetragonal FeS superconductor.

Robustness of superconductivity to competing magnetic phases in tetragonal FeS

We have determined the superconducting and magnetic properties of a hydrothermally synthesized powder sample of tetragonal FeS using muon spin rotation ({\mu}SR). The superconducting properties are entirely consistent with those of a recently published study, showing fully gapped behavior and giving a penetration depth of {\lambda}_{ab} = 204(3) nm. However, our zero-field {\mu}SR data are rather different and indicate the presence of a small, non-superconducting magnetic phase within the sample. These results highlight that sample-to-sample variations in magnetism can arise in hydrothermally prepared phases, but interestingly the superconducting behavior is remarkably insensitive to these variations.

Critical current density and vortex pinning in tetragonalFeS1−xSex(x=0,0.06)

We report critical current density ($J_c$) in tetragonal FeS single crystals, similar to iron based superconductors with much higher superconducting critical temperatures ($T_{c}$'s). The $J_c$ is enhanced 3 times by 6\% Se doping. We observe scaling of the normalized vortex pinning force as a function of reduced field at all temperatures. Vortex pinning in FeS and FeS$_{0.94}$Se$_{0.06}$ shows contribution of core-normal surface-like pinning. Reduced temperature dependence of $J_c$ indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path ($\delta$$l$), in contrast to K$_x$Fe$_{2-y}$Se$_2$ where spatial variations in $T_{c}$ ($\delta$$T_{c}$) prevails.

Strong-coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

We investigate the electronic properties of the tetragonal FeS superconductor by using scanning tunneling microscope/spectroscopy. It is found that the typical tunneling spectrum on the top layer of sulfur can be nicely fitted with an anisotropic s-wave or a combination of two superconducting components in which one may have a highly anisotropic or nodal like superconducting gap. The fittings lead to the maximum superconducting gap $\Delta_{max}\approx$ 0.90$\;$meV, which yields a ratio of 2$\Delta_{max}/k_BT_c\approx$ 4.65. This value is larger than that of the predicted value 3.53 by the BCS theory in the weak coupling limit, indicating a strong coupling superconductivity. Two kinds of defects are observed on the surface, which can be assigned to the defects on the S sites (four-fold image) and Fe sites (dumbbell shape). Impurity induced resonance states are found only for the former defects and stay at zero-bias energy.

Synthesis of large FeSe superconductor crystals via ion release/introduction and property characterization**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574370, 11274358, and 11190020), the National Basic Research Program of China (Grant No. 2013CB921700), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020100).

Large superconducting FeSe crystals of (001) orientation have been prepared via a hydrothermal ion release/introduction route for the first time. The hydrothermally derived FeSe crystals are up to 10 mm×5 mm×0.3 mm in dimension. The pure tetragonal FeSe phase has been confirmed by x-ray diffraction (XRD) and the composition determined by both inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive x-ray spectroscopy (EDX). The superconducting transition of the FeSe samples has been characterized by magnetic and transport measurements. The zero-temperature upper critical field Hc2 is calculated to be 13.2–16.7 T from a two-band model. The normal-state cooperative paramagnetism is found to be predominated by strong spin frustrations below the characteristic temperature Tsn, where the Ising spin nematicity has been discerned in the FeSe superconductor crystals as reported elsewhere.

Coexistence of low-moment magnetism and superconductivity in tetragonal FeS and suppression ofTcunder pressure

We report local probe ($\mu$SR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low moment ($10^{-2}-10^{-3}\mu_\mathrm{B})$ disordered magnetism with a transition temperature of $T_\mathrm{N}\approx 20$ K microscopically coexists with bulk superconductivity below $T_\mathrm{c}=4.3(1)$ K. From transverse field $\mu$SR we obtain an in-plane penetration depth $\lambda_\mathrm{ab}(0)=223(2)$ nm for FeS. The temperature dependence of the corresponding superfluid density $\lambda_\mathrm{ab}^{-2}(T)$ indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting $T_\mathrm{c}$ of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa.

Fabrication and Characterization of Sintered Iron-Chalcogenide Superconductors

Iron-chalcogenide superconductors are appealing for high-magnetic-field applications due to their promising superconducting properties, i.e., extremely high upper critical and irreversibility field.In this paper, the first achievements obtained from sintering polycrystalline samples are presented. The sintering of FeSe polycrystalline powders and pellets was obtained in several steps starting from stoichiometric quantities of freshly polished powders shots; after grinding, powders were loaded into cleaned and dried silica tubes sealed under vacuum for subsequent heat treatments in order to obtain the superconducting phase. Samples were then characterized from the structural, transport, and magnetic points of view. From the structural characterization, it has been discovered that, in addition to the presence of the desired tetragonal FeSe phase, hexagonal phase and several impurities are present in the samples; the lack of homogeneity inside the samples is confirmed by the superconductive characterization: despite the small undeniable enhancement of the results with the application of subsequent HTs and, nearby, a resistive superconductive onset of 12 K, the transition is still broad after the third HT.

Strong anisotropy in nearly ideal tetrahedral superconducting FeS single crystals

Mackinawite, the tetragonal form of FeS, is the newest iron-based superconductor that generates excitement in the field, as it represents a direction away from the purely arsenide- and selenide-based systems. Other sulfides have been shown recently to have superconducting properties, including BaFe${}_{2}$S${}_{3}$ under high pressures and H${}_{2}$S under extreme pressures, but tetragonal FeS is perhaps the simplest sulfide to exhibit superconductivity behavior at ambient pressure. The authors of this work present a novel method for preparing high-quality single crystals of FeS and investigate its superconducting properties. They find that the electronic and magnetic properties of FeS are highly anisotropic. In particular, the level of anisotropy characterized by the ratio of the upper critical fields in two orthogonal directions was found to be the largest of all the iron-based superconductors.

Effects of High-Energy Ball Milling Time on the Sintering Process of FeSe Superconductors

FeSe superconducting bulks were prepared with high energy ball milling (HEBM) aided sintering process, within which process, tetragonal β-FeSe superconducting phase could be formed directly with one step sintering process, and the formation of hexagonal δ-FeSe non-superconducting phase was effectively avoided. The influences of HEBM time on the sintering process of FeSe bulks were systematically investigated. With different HEBM time, the phase composition and morphology of precursor powders changed correspondingly, which thus influenced the final phase composition and superconducting properties of FeSe superconducting bulks. Due to the formation of FeSe bulks with larger tetragonal phase content and higher superconducting transition temperature, HEBM time of 6.0 h was recognized as the optimal parameter. Shorter HEBM time could lead to the insufficient decrease of particle size and low density. While longer HEBM time caused the formation of amorphous hexagonal δ-FeSe, which crystallized during sintering process. Thus no more tetragonal FeSe could be obtained. The FeSe superconducting bulk with the critical temperature Tc(onset) of 8.0 K was obtained with the HEBM time of 6 h, and sintering temperature of 700 oC for 12 h.

Observation of Superconductivity in Tetragonal FeS.

The possibility of superconductivity in tetragonal FeS has attracted considerable interest because of its similarities to the FeSe superconductor. However, all efforts made to pursue superconductivity in tetragonal FeS have failed so far, and it remains controversial whether tetragonal FeS is metallic or semiconducting. Here we report the observation of superconductivity at 5 K in tetragonal FeS that is synthesized by the hydrothermal reaction of iron powder with sulfide solution. The obtained samples are highly crystalline and less air-sensitive, in contrast to those reported in the literature, which are meta-stable and air-sensitive. Magnetic and electrical properties measurements show that the samples behave as a paramagnetic metal in the normal state and exhibit superconductivity below 5 K. The high crystallinity and the stoichiometry of the samples play important roles in the observation of superconductivity. The present results demonstrate that tetragonal FeS is a promising new platform to realize high-temperature superconductors.

Transition metal incorporation into mackinawite (tetragonal FeS)

Naturally occurring mackinawite (tetragonal FeS) with incorporated transition metals is an important precursor to the formation of metal sulfides in ore deposits, but experimental results have not been sufficient to establish clear trends in the structure and stability of the transition-metal-enriched mineral. Using density functional theory with dispersion corrections, we report the first systematic examination of the relationship between composition and structure for FeS incorporating bivalent transition metals. Our method was validated by successful calculations of the structures of FeS, FeSe, FeSe 1-x S x , Fe 1-x Me x Se (Me = Co, Ni, Cu), and FeNi x Te. Two classes of transition-metal-incorporated FeS structures then were investigated: Fe 1-x Me x S (metal-substituted FeS) and FeMe x S (FeS intercalated by a metal at either a tetrahedral or square-pyramidal interstitial site), where Me = Co, Ni, and Cu, and x ≤ 0.25. We find that incorporated transition metals can both increase and decrease lattice parameters, depending on the metal and how it is incorporated into the FeS structure. As the mole fraction of substituting metal increases, the FeS unit-cell volume decreases for Co, is nearly constant for Ni, but increases for Cu. Metal substitution changes the c lattice parameter, which is sensitive to interactions between the mackinawite sheets, as much as it does the a and b lattice parameters. Upon intercalation, the unit-cell volume and c parameter increase but the a and b parameters decrease. Experimental structural data are consistent with our results for metal-substituted FeS. We determined the thermodynamic stability of metal-incorporated FeS by computing the free energy involved in the metal incorporation reactions as a function of chemical potential of sulfur. The thermodynamic results lead to the general conclusions that metal incorporation into mackinawite most likely occurs via substitution, which may be important to influence phase transformation pathways of mackinawite.

Metallic, oxygen-containing reaction products after polarisation of iron in H2S saturated saline solutions

Corrosion of Fe in H2S containing solutions is a general problem in oil production. Here, layers rich in the the tetragonal FeS modification mackinawite were grown by polarisation of iron 200 mV above the corrosion potential in H2S saturated acetic saline solution. These layers have no band gap above room temperature thermal energy, hence behave metallic. The sulfide layers are duplex layers, with an inner, oxide-rich and an outer, oxide-poor region. Strain induced by differences in oxygen content may be responsible for the poor adhesion between the two layers, which leads to eventual loss of the outer layer.

Antiferromagnetic ground state with pair-checkerboard order in FeSe

Monolayer FeSe thin film grown on SrTiO_{3}(001) (STO) shows the sign of T_{c}> 77 K, which is higher than the T_{c}-record of 56 K for the bulk FeAs-based superconductors. However, little is known about the magnetic ground state of FeSe, which should be closely related to its unusual superconductivity. Previous studies presume the collinear stripe antiferromagnetic (AFM) state as the ground state of FeSe, same to that in FeAs superconductors. Here we find a novel magnetic order named "pair-checkboard AFM" as the magnetic ground state of tetragonal FeSe. The novel pair-checkboard order results from the interplay between the nearest, the next-nearest and the unnegligible next-next-nearest neighbor magnetic exchange couplings of Fe atoms. The monolayer FeSe in pair-checkbord order shows an unexpected insulating behavior with a Dirac-cone-like band structure related to the specific orbital order of d_{xz} and d_{yz} characters of Fe atoms, which could explain recently observed insulator-superconductor transition. The present results cast new insights on the magnetic ordering in FeSe monolayer and its derived superconductors.

Synthesis of mackinawite FeS thin films from acidic chemical baths

The growth of monophasic iron sulfide thin films onto glass substrates has been achieved by chemical bath deposition at acidic values of pH. Powder X-ray diffraction (p-XRD) confirms the deposition of tetragonal FeS (mackinawite) with preferred orientation along (001) plane. The crystallite size estimated by Scherrer equation was found to be 14nm. Scanning electron microscopy (SEM) shows the formation of nanoflakes as base layer and nanoflowers as top layer. Energy Dispersive X-ray (EDX) analysis of the deposited iron sulfide thin films shows the iron to sulfur ratio close to 1:1 confirming the deposition of FeS. UV–vis absorption spectroscopy showed a blueshift due to the nanosize crystallites FeS with a band gap of 1.87eV.

Arsenic Mobility during Flooding of Contaminated Soil: The Effect of Microbial Sulfate Reduction

In floodplain soils, As may be released during flooding-induced soil anoxia, with the degree of mobilization being affected by microbial redox processes such as the reduction of As(V), Fe(III), and SO4(2-). Microbial SO4(2-) reduction may affect both Fe and As cycling, but the processes involved and their ultimate consequences on As mobility are not well understood. Here, we examine the effect of microbial SO4(2) reduction on solution dynamics and solid-phase speciation of As during flooding of an As-contaminated soil. In the absence of significant levels of microbial SO4(2-) reduction, flooding caused increased Fe(II) and As(III) concentrations over a 10 week period, which is consistent with microbial Fe(III)- and As(V)-reduction. Microbial SO4(2-) reduction leads to lower concentrations of porewater Fe(II) as a result of FeS formation. Scanning electron microscopy with energy dispersive X-ray fluorescence spectroscopy revealed that the newly formed FeS sequestered substantial amounts of As. Bulk and microfocused As K-edge X-ray absorption near-edge structure spectroscopy confirmed that As(V) was reduced to As(III) and showed that in the presence of FeS, solid-phase As was retained partly via the formation of an As2S3-like species. High resolution transmission electron microscopy suggested that this was due to As retention as an As2S3-like complex associated with mackinawite (tetragonal FeS) rather than as a discrete As2S3 phase. This study shows that mackinawite formation in contaminated floodplain soil can help mitigate the extent of arsenic mobilization during prolonged flooding.

Pressure-dependent magnetization and magnetoresistivity studies on tetragonal FeS (mackinawite): revealing its intrinsic metallic character

The transport and magnetic properties of the tetragonal FeS were investigated using magnetoresistivity and magnetization within K, 70 kOe and 3.0 GPa. In addition, room-temperature x-ray diffraction and photoelectron spectroscopy were also applied. In contrast to previously reported nonmetallic character, FeS is intrinsically metallic but due to a presence of a weak localization such metallic character is not exhibited below room temperature. An applied pressure reduces strongly this additional resistive contribution and as such enhances the temperature range of the metallic character which, for ∼3 GPa, is evident down to 75 K. The absence of superconductivity as well as the mechanism behind the weak localization will be discussed.

Effects of Mn doping on the normal-state transport of tetragonal FeSe superconductor up to 700 K

A series of Fe1−xMnxSe polycrystalline samples with , 0.02, 0.05 and 0.08 is prepared by conventional solid-state reaction. Normal-state transport of electrical resistivity and thermopower are investigated up to 700 K. The electrical resistivity monotonically increases with increasing Mn content. The temperature dependence of resistivity is analyzed using a power-law equation . At , the electrical resistivity behavior resembles misfit-layered cobalt oxides Ca3Co4O9+δ. The electrical resistivity follows a T2-dependence between 150 and 220 K with the Fermi-liquid transport coefficient having the size between and , followed by a transition to an incoherent metal in the vicinity of Mott transition. Metal-nonmetal transition is observed at 315 K, 370 K, 304 K and 275 K for , 0.02, 0.05 and , respectively. The resistivity follows a power law for , which might comply with the prediction of the spin fluctuation theories for the behavior in the vicinity of an antiferromagnetic transition. Thermopower exhibits unconventional behavior. Above room temperature, the thermopower shows a sign crossover to positive at 352 K, 537 K, and 374 K for , 0.05, and 0.08, respectively. Unlike the undoped and Bi-doped FeSe, the thermopower of Mn-doped samples increases with increasing temperature in the high-temperature regime up to 700 K, which might arise from the thermally activated conduction of hole carriers. Being consistent with earlier reports, the sharp dip anomaly of thermopower occurs between 100 and 120 K and seems to be associated with the excitations of a pseudogap ground state.

Mackinawite (FeS) reduces mercury(II) under sulfidic conditions.

Mercury (Hg) is a toxicant of global concern that accumulates in organisms as methyl Hg. The production of methyl Hg by anaerobic bacteria may be limited in anoxic sediments by the sequestration of divalent Hg [Hg(II)] into a solid phase or by the formation of elemental Hg [Hg(0)]. We tested the hypothesis that nanocrystalline mackinawite (tetragonal FeS), which is abundant in sediments where Hg is methylated, both sorbs and reduces Hg(II). Mackinawite suspensions were equilibrated with dissolved Hg(II) in batch reactors. Examination of the solid phase using Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Hg(II) was indeed reduced in FeS suspensions. Measurement of purgeable Hg using cold vapor atomic fluorescence spectrometry (CVAFS) from FeS suspensions and control solutions corroborated the production of Hg(0) that was observed spectroscopically. However, a fraction of the Hg(II) initially added to the suspensions remained in the divalent state, likely in the form of β-HgS-like clusters associated with the FeS surface or as a mixture of β-HgS and surface-associated species. Complexation by dissolved S(-II) in anoxic sediments hinders Hg(0) formation, but, by contrast, Hg(II)–S(-II) species are reduced in the presence of mackinawite, producing Hg(0) after only 1 h of reaction time. The results of our work support the idea that Hg(0) accounts for a significant fraction of the total Hg in wetland and estuarine sediments.