We have measured the temperature dependences of Rho and Chi for Fe-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ in the overdoped regime, in order to investigate Fe-substitution effects on electronic properties around x=0.22.
From the Rho measurements, it has been found around x=0.22 that the values of Rho are large at room temperature and that Rho exhibits a pronounced upturn at low temperatures. Moreover, from the Rho and Chi measurements, it has been found that T_c_ is anomalously depressed around x=0.22. These results indicate that the electronic anomalies around x=0.22 are enhanced by Fe substitution, which might be related to the development of stripe correlations by Fe substitution.
The experimental transport scattering rate was determined for a wide range of optimally doped transition metal-substituted FeAs-based compounds with the ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc 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 \sim26 K on the maximum Tc for all transition metal-substituted 122 systems, in agreement with experimental observations, and sets a critical sc…
We investigated the evolution of the temperature-composition phase diagram of Fe$_{1+y}$Te upon Se substitution. In particular, the effect of Se substitution on the two-step, coupled magneto-structural transition in Fe$_{1+y}$Te single crystals is investigated. To this end, the nominal Fe excess was kept at $y$ = 0.12. For low Se concentrations, the two magneto-structural transitions displayed a tendency to merge. In spite of the high Fe-content, superconductivity emerges for Se concentrations, $x \geq$ 0.1. We present a temperature-composition phase diagram to demonstrate the interplay of str…
To reveal the relative importance of charge doping and defect scattering in substitutionally modified 122 iron pnictides, we perform a systematic first principles study on selected bands at the Fermi level. Disorder effects are induced by various substitutions using an orbital based coherent potential approximation (CPA). Pronounced level shifts of individual bands suggest that transition metal substitutions introduce mobile charge carriers into the system. However, important deviations from such a rigid band scenario as well as spectral broadenings due to impurity scattering correlate with th…
Recent experiments on Zn-substituted 122-type iron-based superconductors (FeSCs) at electron- and hole- doped region provide us with a testing ground for understanding the effect of Zn impurities in these systems. Our first-principle calculations of the electronic structure reveal that the Zn 3d orbitals are far below the Fermi level and chemically inactive, while the Zn 4s-orbital is partially occupied and its wave function overlapping with those 3d-orbitals of neighboring Fe-ions. This suggests that the impurity effect is originating in the Zn 4s-orbital, not its 3d-orbitals. Employing a phe…
We systematically investigated the anisotropic in-plane resistivity of the iron telluride including three kinds of impurity atoms: excess Fe, Se substituted for Te, and Cu substituted for Fe. Sizable resistivity anisotropy was found in the magneto-structurally ordered phase whereas the sign is opposite ($ρ_a$ $>$ $ρ_b$, where the $b$-axis parameter is shorter than the $a$-axis one) to that observed in the transition-metal doped iron arsenides ($ρ_a$ $<$ $ρ_b$). On the other hand, our results demonstrate that the magnitude of the resistivity anisotropy in the iron tellurides is correlated with …
Recent experiments demonstrated that isovalent doping system gives the similar phase diagram as the heterovalent doped cases. For example, with the phosphorous (P)-doping, the magnetic order in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ compound is first suppressed, then the superconductivity dome emerges to an extended doping region but eventually it disappears at large $x$. With the help of a minimal two-orbital model for both BaFe$_{2}$As$_{2}$ and BaFe$_{2}$P$_{2}$, together with the self-consistent lattice Bogoliubov-de Gennes (BdG) equation, we calculate the phase diagram against the P content …
We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.
We study an inhomogeneous three-orbital Hubbard model for the Cu-substituted iron pnictides using an extended real-space Green's function method combined with density functional calculations. We find that the onsite interactions of the Cu ions are the principal determinant of whether an electron dopant or a hole dopant is caused by the Cu substitution. It is found that the Cu substitution could lead to a hole doping when its onsite interactions are smaller than a critical value, as opposed to an electron doping when the interactions of Cu ions are larger than the critical value, which may expl…
Emergent magnetism, heavy electrons and pressure-induced reentrant superconductivity in the iron substituted 4d transition-metal sulfides
Zhejiang University of Technology（浙江工业大学）；Fudan University（复旦大学）；Qufu Normal University（曲阜师范大学）；Nanjing University of Posts and Telecommunications（南京邮电大学）；Suzhou University of Technology（苏州科技大学）；Hangzhou Dianzi University（杭州电子科技大学）；National Taiwan Normal University（台湾师范大学）；Academia Sinica（中央研究院）；Zhejiang University（浙江大学）；Ningbo University（宁波大学）；Hangzhou Normal University（杭州师范大学）；Shanghai Jiao Tong University（上海交通大学）
We study the in-plane anisotropy of the thermoelectric power and electrical resistivity on detwinned single crystals of isovalent substituted EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_2$. Compared to the resistivity anisotropy the thermopower anisotropy is more pronounced and clearly visible already at temperatures much above the structural and magnetic phase transitions. Most remarkably, the thermopower anisotropy changes sign below the structural transition. This is associated with the interplay of two contributions due to anisotropic scattering and orbital polarization, which dominate at high- and low…
A unified theory is outlined for the cuprates, Fe-based, and related superconductors. Their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, and the introduction of a Lagrange Bose field enables their treatment as bosons or fermions. This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T^* and of "marginal-FL" behavior above it. The anomalous behavior of numerous physical quantities is…
Using angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe$_{1-x}$Co$_x$As from an optimally doped superconducting compound ($x=0.028$) to a heavily overdoped non-superconducting one ($x=0.109$). Similar to the case of "122" type iron pnictides, our data suggest that Co dopant in NaFe$_{1-x}$Co$_x$As supplies extra charge carriers and shifts the Fermi level accordingly. In the $x=0.109$ compound, the hole-like bands around the zone center $Γ$ move to deeper binding energies and an electron pocket appears instead. The overall band renormalization remains basica…
In contrast to SrFe2As2, where only the iron possesses a magnetic moment, in EuFe2As2 an additional large, local magnetic moment is carried by Eu2+. Like SrFe2As2, EuFe2As2 exhibits a spin-density wave transition at high temperatures, but in addition the magnetic moments of the Eu2+ order at around 20 K. The interplay of pressure-induced superconductivity and the Eu2+ order leads to a behavior which is reminiscent of re-entrant superconductivity as it was observed, for example, in the ternary Chevrel phases or in the rare-earth nickel borocarbides. Here, we study the delicate interplay of the …
The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for lanthanide) is the first material with a Tc above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.05<x<0.20) and a second dome (SC2, 0.2<x<0.5). Here, we report the Tc for the whole hydrogen-doping range in LaFeAsO1-xHx under pressures of up to 19 GPa. Tc rises to 52 K at 6 GPa for the Tc-valley composition between the two Tc domes. This is the first instance of the Tc exceeding 50 K in La-1111-type iron-based supe…
The detailed temperature dependence of the infrared-active mode in Fe$_{1.03}$Te ($T_N\simeq 68$ K) and Fe$_{1.13}$Te ($T_N\simeq 56$ K) has been examined, and the position, width, strength, and asymmetry parameter determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below $T_N$ there is an anomalous decrease in frequency and the mode shows litt…
The pairing mechanism in the iron-pnictide superconductors is still unknown. However, similarities to the cuprate high-temperature superconductors suggest that a similar mechanism may be at work. Recently, careful experimental studies of the spin excitation spectrum revealed, like in the cuprates, a strong temperature dependence in the normal state and a resonance feature in the superconducting state. Motivated by these findings, we develop a model of electrons interacting with a temperature dependent magnetic excitation spectrum based on these experimental observations. We apply it to analyse…
Iron-based oxypnictides substituted with yttrium have been prepared via a conventional solid state reaction. The product after first 50 hours of reaction showed diamagnetic-like transition at around 10 K but was not superconducting, while additional 72 hours of high temperature heat treatment was required to yield superconducting sample which was doped with fluoride. Temperature dependence of the susceptibility shows both screening and Meissner effect at around 10 K, while resistance as a function of temperature displayed a drop at around the same temperature.
Temperature-dependent London penetration depth, $λ(T)$, was measured in optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K, RRR=$ρ(300K)/ρ(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4) single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from $λ(0) = 300 \pm 10$ nm in as-grown sample to $λ(0) = 275 \pm 10$ nm. At low temperatures, $λ(T) \sim T$ indicating superconducting gap with line nodes. Analysis of the full - temperature range superfluid density is consisten…
The nature of superconducting fluctuation effects in the isovalently-substituted iron pnictide BaFe$_2$(As$_{1-x}$P$_x$)$_2$ (x=0.35) is probed through measurements of the magnetization and magnetoconductivity around the superconducting transition. The results, obtained with magnetic fields up to 9 T applied in the two main crystal directions, are consistent with anisotropic Ginzburg-Landau (GL) approaches for finite applied magnetic fields. The analysis allowed to determine with accuracy the out-of-plane, $ξ_c(0)$, and in-plane, $ξ_{ab}(0)$, GL coherence lengths. Significant differences are f…
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving displacement of Fe atoms, show sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400-1200 cm-1 are attributed to the elect…
Local structure of SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ ($x$ = 0.0, 0.05, 0.25 and 0.5) superconductors has been investigated by temperature dependent As $K$-edge extended x-ray absorption fine structure. The effect of Ru substitution remains confined to the iron-arsenide layer but neither the static disorder nor the Fe-As bond strength suffers any change for $x \le$ 0.25. With further Ru substitution the static disorder increases while the Fe-As bond strength remains unchanged. Also, the Ru-As distance ($\sim$2.42 Å), different from the Fe-As distance ($\sim$2.39 Å), does not show any cha…
We use high resolution angle-resolved photoemission to study the electronic structure of the iron based high-temperature superconductors Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ as a function of Ru concentration. We find that substitution of Ru for Fe is isoelectronic, i. e., it does not change the value of the chemical potential. More interestingly, there are no measured, significant changes in the shape of the Fermi surface or in the Fermi velocity over a wide range of substitution levels ($0<x<0.55$). Given that the suppression of the antiferromagnetic and structural phase has been associated with th…
We study the crystalline and electronic properties of the $\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$ system ($x=0$, 0.25, 0.5, 0.75, and 1.0) using X-ray diffraction, X-ray spectroscopy and density functional theory. We show that the introduction of Co $3d$ states in FeSe relaxes the bond strengths and induces a structural transition from tetragonal to hexagonal whose crossover takes place at $x\approx0.38$. This structural transition in turn modifies the magnetic order which can be related to the spin state. Using resonant inelastic X-ray spectroscopy we estimate the spin state of the system…
The influence of a partial substitution with S, Te, Co, Ni and Cu atoms on the electronic structure of the FeSe superconductor has been investigated within the density functional theory. The results of the supercell calculations reveal distinct changes of electronic structures of the substituted FeSe systems, which can be responsible for their superconducting properties. The replacement of Se atoms by Te or S ones yields imperfect nesting between the holelike and electronlike Fermi surface (FS) sheets, which enhances magnetic fluctuations responsible for superconducting pairing, thus leading t…
Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_…
The upper critical field $H_{c2}$ of optimally doped iron-based superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$ = 0.35, $T_c$ = 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H \parallel c$ and $H \parallel a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion irradiated single crystals, with columnar defect density corresponding to a matching field $Bϕ$ = 25 T. The $H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for $…
We report the complex dielectric function of high-quality $A\textrm{Fe}_2\textrm{As}_2$, ($A$=Ca, Sr, Ba) single crystals with $T_{\mathrm{N}}\approx150\ \textrm{K}$, $200\ \textrm{K}$, and $138\ \textrm{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\leq T\leq300\ \textrm{K}$ and wavenumbers from $100\ \textrm{cm}^{-1}$ to $52000\ \textrm{cm}^{-1}$. In $\textrm{CaFe}_2\textrm{As}_2$ we identify the optical spin-density--wave gap $2Δ_{\mathrm{SDW}}\approx1250\ \textrm{cm}^{-1}$. The $2Δ_{\mathrm{SDW}}/(k_{\mathrm{B}}T_{\mathrm{N}})$ ratio, characterizi…
This article reviews recent experimental investigations on two binary Fe-chalcogenides: FeSe and Fe$_{1+y}$Te. The main focus is on synthesis, single crystal growth, chemical composition, as well as on the effect of excess iron on structural, magnetic, and transport properties of these materials. The structurally simplest Fe-based superconductor Fe$_{1+x}$Se with a critical temperature $T_c \approx$ 8.5 K undergoes a tetragonal to orthorhombic phase transition at a temperature $T_s \approx$ 87 K. No long-range magnetic order is observed down to the lowest measured temperature in Fe$_{1+x}$Se. …
We have systematically studied the substitution of nonmagnetic Zn and magnetic Ni at iron sites in Ce based oxypnictide. The parent compound (CeOFeAs) shows an anomaly in resistivity around 150 K due to structural transition from tetragonal (space group: P4/nmm) to orthorhombic structure (space group: Cmma). Substitution of Zn suppresses this anomaly to lower temperature (~130 K) but Ni substitution does not show any anomaly around this temperature and the compound behaves like a metal. Further, we find that non magnetic (Zn) doping leads to higher impurity scattering as compared to magnetic N…
Evolution of Fermi surface (FS) states of NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_{\rm c}$ and the power law exponent ($n$) of temperature-dependent resistivity ($ρ(T) = ρ_0 + AT^n$) show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced with increasing bosonic fluctuation in the same type of FS state. Around $x=$0.2, all the transport properties show anomalies, indicating that $x$$\sim$0.2 is the critical composition of drastic FS change. The angle resolved photoemission spectrosco…
We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enhancement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) d…
The effects of electron-electron correlations on the low-energy electronic structure and their relationship with unconventional superconductivity are central aspects in the research on the iron-based pnictide superconductors. Here we use soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study how electronic correlations evolve in different chemically substituted iron pnictides. We find that correlations are intrinsically related to the effective filling of the correlated orbitals, rather than to the filling obtained by valence counting. Combined density functional theory (DFT)…
Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides
University of Oxford（牛津大学）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Institute of High-Pressure Physics, Polish Academy of Sciences（波兰科学院高压物理研究所）；Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院量子材料与技术研究所）
We have performed Raman-scattering measurements on high-quality single crystals of the superconductors K$_{0.8}$Fe$_{1.6}$Se$_2$ ($T_c$ = 32 K), Tl$_{0.5}$K$_{0.3}$Fe$_{1.6}$Se$_2$ ($T_c$ = 29 K), and Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.6}$Se$_2$ ($T_c$ = 31 K), as well as of the insulating compound KFe$_{1.5}$Se$_2$. To interpret our results, we have made first-principles calculations for the phonon modes in the ordered iron-vacancy structure of K$_{0.8}$Fe$_{1.6}$Se$_2$. The modes we observe can be assigned very well from our symmetry analysis and calculations, allowing us to compare Raman-active pho…
Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large 6.5% hardening of the B_1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature T_s and its change below T_s is discussed in relation with the opening of an energy gap between l…
Strong electron interactions in solids increase effective mass, and shrink the electronic bands [1]. One of the most unique and robust experimental facts about iron-based superconductors [2-4] is the renormalization of the conduction band by factor of 3 near the Fermi level [5-9]. Obviously related to superconductivity, this unusual behaviour remains unexplained. Here, by studying the momentum-resolved spectrum of the whole valence band in a representative material, we show that this phenomenon originates from electronic interaction on a much larger energy scale. We observe an abrupt depletion…
Hydrogen is the simplest bipolar element and its valence state can be controlled from +1 to -1. We synthesized the 1111-type CaFeAsH and LnFeAsO1-xHx (Ln = lanthanide) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H-), which is isovalent with the fluoride ion (F-), in the 1111-type iron arsenides. When some …
We report superconductivity in single crystals of the new iron-pnictide system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized via x-ray, transport, magnetic and thermodynamic measurements. The magnetic ordering associated with a structural transition at 140 K present in BaFe2As2 is completely suppressed by substitution of 5% Fe with Pt and superconductivity is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk nature of the superconducting phase. All properties of th…
Taking cue from the increase in the superconducting transition temperature (Tc) of Fe1+xSe via nominal (2 wt%) substitution of Cr instead of excess Fe, we have now extended our study with nominal substitution (<=5 wt%) with other transition metals (Ni, Co, Fe, Mn, Cr, V and Ti) in place of excess iron. The Tc is found to increase (maximum ~11 K) or get suppressed depending on the substituted transition metal. Our studies indicate that the superconducting transition temperature depends on various parameters like the ionic size of the transition metal, its magnetic moment as well as the amount o…
The recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of Ca atoms. We show that high Tc's above 15 K only emerge if the iron-arsenide layers are at most free of platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by charge doping of pure FeAs layers. We point out, that two different negatively charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the …
Fluoride-doped iron-based oxypnictides containing rare-earth gadolinium (GdFeAsO0.8F0.2) and co-doping with yttrium (Gd0.8Y0.2FeAsO0.8F0.2) have been prepared via conventional solid state reaction at ambient pressure. The non-yttrium substituted oxypnictide show superconducting transition as high as 43.9 K from temperature dependent resistance measurements with the Meissner effect observed at a lower temperature of 40.8 K from temperature dependent magnetization measurements. By replacing a small amount of gadolinium with yttrium Tc was observed to be lowered by 10 K which might be caused by a…
We report an enhancement of superconducting transition temperature ($T_{\rm c}$) when Chromium (Cr) is substituted in excess at the Iron (Fe) site (FeCr$_x$Se, $x=$0.01, 0.02 and 0.03). There is a corresponding increase in the superconducting volume fraction with $T_{\rm c}$ attaining a value of 11.2 K on 2 $%$ Cr substitution when compared to a $T_{\rm c}$ of 8.5 K for the conventional tetragonal Fe-excess sample Fe$_{1+x}$Se. The results point to the role of chemical pressure (introduced via ionic size variation at the Fe site upon Cr substitution in excess) on superconducting properties.
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…
Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x=0.065$) single crystal, in magnetic fields up to 9 T and with different orientations $θ$ relative to the crystal $c$ axis. A significant $ρ(T)_{H,θ}$ rounding is observed just above the superconducting critical temperature $T_c$ due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us …
The effect of selenium substitution by sulfur or tellurium in the Tl1-yFe2-zSe2 antiferromagnet was studied by x-ray and electron diffraction, magnetization and transport measurements. Tl0.8Fe1.5(Se1-xXx)2 (nominal composition) solid solutions were synthesized in the full x range (0<x<1) for X=S and up to x=0.5 for X=Te, using the sealed tube technique. No superconductivity was found down to 4.2K in the case of sulfur despite the fact that the optimal crystallographic parameters, determined by Rietveld refinements, are reached in the series (i.e. the Fe-(Se,S) interplane height and (Se,S)-Fe-(…
