FeAs Layers Research Articles

Copper doping effects on the superconducting properties of Sm‐based oxypnictides

AbstractA systematic investigation has been performed by synthesis and comprehensive characterization of a series of SmFe1−xCuxAsO0.8F0.2 bulks (x = 0–0.2). These samples are well characterized by structural, Raman spectroscopy, microstructural, transport, magnetic measurements, and supplementary calculations within density functional theory (DFT). The parent compound, SmFeAsO0.8F0.2 (Sm1111), exhibits a superconducting transition temperature (Tc) of approximately 54 K. The lattice volume (V) is increased with Cu substitution (x) without observing any impurity phase related to copper, which confirms the successful incorporation of Cu at Fe sites in the superconducting FeAs layers. These analyses are also well in agreement with Raman spectroscopy measurements and relevant DFT results. The superconducting transition is decreased systematically with copper doping and completely suppressed for 7% Cu‐doped Sm1111 (x = 0.07). A large amount of Cu substitution (x ≥ 0.07) has demonstrated the metal to insulate transition in the low‐temperature range, and no impurity phase was observed even at high Cu doping levels (x = 0.2). The calculated critical current density of the parent sample is suppressed with copper substitution, suggesting the reduced pinning centers, sample density, and grain connections, as confirmed by the microstructural analysis. Our studies suggest that the substitution of Cu in the superconducting FeAs layer, resulting the enlargement of the lattice volume, is a source of strong disorder scattering, leading to the suppression of Tc and the emergence of metal‐to‐insulator, unlike the more successful carrier doping by nickel (Ni) or cobalt (Co), as previously reported.

Superconductivity and pseudogap features of an isolated FeAs layer in KCa2Fe4As4F2 unraveled by STM/STS

Superconductivity and pseudogap features of an isolated FeAs layer in KCa2Fe4As4F2 unraveled by STM/STS

Interlayer coupling in the superconducting state of iron-based superconductors

High-temperature superconducting paring is generally believed to be mediated by the magnetic fluctuations that mostly occur within the two-dimensional conducting layers (Cu-O, Fe-As, or Fe-Se) in copper oxides, iron pnictides, iron chalcogenides, etc. Here, on the basis of inelastic neutron scattering measurements on a superconducting iron pnictide ${\mathrm{Ca}}_{0.33}{\mathrm{Na}}_{0.67}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, we have discovered a highly three-dimensional spin resonance mode with upward V-shape dispersions both in the $ab$ plane and along the $c$ axis. The superconducting gaps exhibit strong ${k}_{z}$ modulations involved with significant changes on the size of one hole pocket and the density state of the ${d}_{{z}^{2}}$ orbital of Fe. The resonance dispersions don't always seem confined by the total gaps summed on those imperfectly nesting Fermi sheets. It is demonstrated that the $c$-axis dependence of both the resonance energy and its intensity in iron pnictides can be universally scaled with the distance between two adjacent Fe-As layers ($d$), and the ${k}_{z}$ modulation of the gap is also anticorrelated with $d$. These results highlight the role of interlayer coupling in iron-based superconductivity, suggesting that the interlayer pairing may also be driven by magnetic fluctuations under certain spin-orbit couplings.

Role of Lanthanides and Bilayer Fe2As2 in the Electronic Properties of RbLn2Fe4As4O2 (Ln = Gd, Tb, and Dy) Superconductors.

The superconducting transition temperatures (Tc) of RbGd2Fe4As4O2, RbTb2Fe4As4O2, and RbDy2Fe4As4O2 are 35 K, 34.7 K, and 34.3 K without doping, respectively. For the first time, we have studied the high-temperature nonmagnetic state and the low-temperature magnetic ground state of 12442 materials, RbTb2Fe4As4O2 and RbDy2Fe4As4O2, using first principles calculations and comparing them with RbGd2Fe4As4O2. We also performed a detailed study of the effects of lanthanides and bilayer Fe2As2. We predict that the ground state of RbLn2Fe4As4O2 (Ln = Gd, Tb, and Dy) is spin-density-wave-type, in-plane, striped antiferromagnets, and the magnetic moment around each Fe atom is about 2 μB. We also found that the structural differences caused by the simple ionic radius have little effect on the properties of these three materials. Different lanthanide elements themselves play a major role in the electronic properties of the materials. It can be confirmed that the effect of Gd on RbLn2Fe4As4O2 is indeed different from that of Tb and Dy, and the presence of Gd is more conducive to interlayer electron transfer. This means that Gd can transfer more electrons from the GdO layer to the FeAs layer compared to Tb and Dy. Therefore, RbGd2Fe4As4O2 has a stronger internal coupling strength of the bilayer Fe2As2 layer. This can explain why the Tc of RbGd2Fe4As4O2 is slightly higher than that of RbTb2Fe4As4O2 and RbDy2Fe4As4O2.

The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca10(Pt4As8)((Fe0.97Pt0.03)2As2)5 superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A□) gradually diminishes. At a critical pressure, A□ and superconducting transition temperature (Tc) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A□ with Tc obeys the relation of Tc ~ (A□)0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-Tc superconductivity.

Electronic structure studies of RbLn2Fe4As4O2 (Ln = Sm, Tb, Dy and Ho) compounds

We report a comprehensive density functional theory based first principles studies on newly discovered iron-based RbLn2Fe4As4O2 (Ln = Sm, Tb, Dy, Ho) superconducting compounds. A dominant Fe-d state, along with small arsenic state contribution is found in the low energy regime of all the four compounds, that constitutes conducting Fe-As layer. Orbital selective evolution of electronic structure is evident in hybrid RbLn2Fe4As4O2 compounds under the effect of chemical pressures induced by various lanthanide substitutions, and moderate on-site electron correlation. The characteristic electronic structure with multi-orbital derived multi-band nature undergoes an orbital-selective evolution. In all the four compounds, an orbital selective Lifshitz transition as well as an orbital-selective bandwidth renormalization is predicted due to on-site electron correlation. The calculated Fermi surfaces show evidence of bilayer splitting due to the interlayer inter-orbital interaction between the two Fe-As layers in a bilayer block. The splitting is found to be minimum along the Brillouin zone centre to the corner and is maximum along the zone centre to (π,0) direction. The largest significant effect due to spin-orbit coupling is the splitting of dxz and dyz derived bands above the Fermi level. The size of spin-orbit coupling is found to be consistent with earlier theoretical calculations on 122 materials. Splitting increases with substitutions of lanthanides in decreasing order of radii. Spin-orbit coupling influences the size of the Fermi radii and thus likely to influence nesting conditions and superconductivity as well. Predictions on possible pairing symmetry based on orbital characters of the Fermi surfaces together with qualitative estimation of nesting conditions for Fermi pockets are provided. The calculated large Fe-As hopping amplitudes based on 32-band Wannier function tight-binding Hamiltonian indicates that electrons mainly hop through As atoms.

Role of Doping Effect and Chemical Pressure Effect Introduced by Alkali Metal Substitution on 1144 Iron-Based Superconductors.

CaAFe4As4 with A = K, Rb, and Cs are close to the doped 122 system, and the parent material can reach a superconducting transition temperature of 31-36 K without doping. To study the role of alkali metals, we investigated the induced hole doping and chemical pressure effects as a result of the introduction of alkali metals using density-functional-based methods. These two effects can affect the superconducting transition temperature by changing the number of electrons and the structure of the FeAs conductive layer, respectively. Our study shows that the dxz and dyz orbitals, which are degenerate in CaFe2As2, become nondegenerate in CaAFe4As4 due to two nonequivalent arsenic atoms (As1 and As2). The unusual oblate ellipsoid hole pocket at Γ point in CaAFe4As4 results from a divalent cation Ca2+ replaced by a monovalent cation A+. It shows one of the main differences in fermiology compared to a particular form of CaFe2As2 with reduced 1144 symmetry, due to the enhancement of As2-Fe hybridization. The unusual band appears in CaFe2As2 (1144) and gradually disappears in the change of K to Cs. Further analysis shows that this band is contributed by As1 and has strong dispersion perpendicular to the FeAs layer, suggesting that it is related to the peculiar van Hove singularity below the Fermi level. In addition, various aspects of CaFe2As2 (1144) and CaAFe4As4 in the ground state are discussed in terms of the influence of hole doping and chemical pressure.

Observations of Nematicity, Dopants, and Zero-Bias Conductance Peaks for the Ca0.9La0.1FeAs2 Superconductor

Ca1-xLaxFeAs2 (CLFA112) belongs to a new family of Fe-based superconductors (FeSCs) and has a unique crystal structure featuring an arsenic zigzag chain layer, which has been proposed to be a possible two-dimensional topological insulator. This suggests that CLFA112 is a potential topological superconductor-a platform to realize Majorana fermions. Up to now, even a clear superconducting (SC) gap in CLFA112 has never been observed, and the SC properties of CLFA112 remain largely elusive. In this letter, we report the results of an atomic-scale investigation of the electronic structure of CLFA112 crystals using low-temperature scanning tunneling microscopy (STM). We revealed four different types of surfaces exhibiting distinct electronic properties, with all surfaces displaying dominating 2 × 1 surface reconstructions. On a Ca/La layer on top of an FeAs layer, a clear SC gap of ~12 mV was observed only at the crevices (vacancies) where the FeAs layer can be directly accessed. Remarkably, the FeAs termination layer displayed a dispersing nematic modulation both in real and q space. We also present peculiar zero-bias conductance peaks for the very As chain layer that is believed to exhibit a topological edge state as well as the influence of La dopants on the As chain layer.

Anomalous metallic oxygen band in the potential superconductor KCa[formula omitted]Fe[formula omitted]As[formula omitted]O[formula omitted]: A DFT study

The electronic structure, magnetism and Fermi surface of a hole self-doped potential iron-based superconductor KCa2Fe4As4O2 (12442) were studied by a first-principles DFT/GGA approach. The self-doping value is obtained from DFT is 0.75 hole on Fe-3d states for KCa2Fe4As4O2. Surprisingly, for the KCa2Fe4As4O2 it was found that except the Fe-3d bands the O-2p bands also cross the Fermi level, which is quite anomalous behavior for O-2p states. In general, the O-2p states are usually fully occupied in transition metal oxides. Here it is not the case due to the fact that CaO layer gives some of the electrons to the FeAs layer, thus leaving O-2p states partially occupied. The O-2p states form additional Fermi surface sheet with propeller-like shape around Γ-point and modify the Fe-3d Fermi surface sheets due to the hybridization. To prove the presence of anomalous metallic O-2p band in KCa2Fe4As4O2 the comparative study with the same family systems KCa2Fe4As4F2 and RbGd2Fe4As4O2 (where it is not the case) was performed. Finally, we discuss the main differences between the emergence of the metallic O-2p band in case of KCa2Fe4As4O2 and those in case of cuprate superconductors LaBaCuO4 and YBa2Cu3O7. Therefore, we expect that the KCa2Fe4As4O2 might be a potential superconductor with unusual properties.

Obtaining tetragonal FeAs layer and superconducting KxFe2As2 by molecular beam epitaxy

Obtaining tetragonal FeAs layer and superconducting KxFe2As2 by molecular beam epitaxy

Coexistence of magnetism and superconductivity in 112-type iron pnictides EuFeAs[formula omitted] doped with Co

Polycrystalline samples of Eu(Fe1−xCox)As2 with x = 0.05, 0.10 and 0.13 were systematically studied by the 57Fe Mössbauer spectroscopy as well as with magnetization and electrical resistivity measurements. The obtained results deliver interesting information about the interplay of magnetism and superconductivity in the studied compounds at low temperatures. As it was shown, all three compounds exhibit the Eu2+ related long-range antiferromagnetic order below 40 K and the superconductivity in FeAs layers which emerges deep in the antiferromagnetic phase. In the case of Eu(Fe0.95Co0.05)As2, the 57Fe Mössbauer spectroscopy data gives clear evidence that this system undergoes the SDW transition at 80 K thus the superconductivity observed below 18 K coexists with SDW and 4f-europium magnetic order within the same volume. In the samples with higher Co doping, the SDW order is fully suppressed and the superconductivity occurs at a much higher critical temperature close to 28 K.

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5 with T c ≃ 12 K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature T* ≃ 7 K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers θ = 0°. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below T*, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above T* expected for Abrikosov vortices. Furthermore, when θ is larger than 0.5°, the flux-flow resistance and critical current have no anomalous behaviors across T*. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers ξ ⊥ becomes shorter than half of the interlayer distance d/2. These experimental findings indicate the existence of intrinsic Josephson junctions below T* and thus quasi-two-dimensional superconductivity in Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5, similar to those in the cuprate superconductors.

Strong antiferromagnetic proximity coupling in the heterostructure superconductor Sr2VO3−δFeAs

We report the observation of strong magnetic proximity coupling in the heterostructure superconductor ${\mathrm{Sr}}_{2}{\mathrm{VO}}_{3\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$, determined by upper critical field ${H}_{c2}(T)$ measurements up to 65 T. Using the resistivity and the radio-frequency measurements for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$, we found a strong upward curvature of ${H}_{c2}^{c}(T)$, together with a steep increase in ${H}_{c2}^{ab}(T)$ near ${T}_{c}$, yielding an anisotropic factor ${\ensuremath{\gamma}}_{H}={H}_{c2}^{ab}/{H}_{c2}^{c}$ up to $\ensuremath{\sim}20$, much higher than those of other iron-based superconductors. These are attributed to the Jaccarino-Peter effect, rather than to the multiband effect, due to strong exchange interaction between itinerant Fe spins of the FeAs layers and localized V spins of Mott-insulating ${\mathrm{SrVO}}_{3\ensuremath{-}\ensuremath{\delta}}$ layers. These findings provide evidence for strong antiferromagnetic proximity coupling that is comparable to the intralayer superexchange interaction of the ${\mathrm{SrVO}}_{3\ensuremath{-}\ensuremath{\delta}}$ layer and sufficient to induce magnetic frustration in ${\mathrm{Sr}}_{2}{\mathrm{VO}}_{3\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$.

Mn-induced Fermi-surface reconstruction in the SmFeAsO parent compound

The electronic ground state of iron-based materials is unusually sensitive to electronic correlations. Among others, its delicate balance is profoundly affected by the insertion of magnetic impurities in the FeAs layers. Here, we address the effects of Fe-to-Mn substitution in the non-superconducting Sm-1111 pnictide parent compound via a comparative study of SmFe_{1-x}Mn_{x}AsO samples with x(text{Mn})= 0.05 and 0.10. Magnetization, Hall effect, and muon-spin spectroscopy data provide a coherent picture, indicating a weakening of the commensurate Fe spin-density-wave (SDW) order, as shown by the lowering of the SDW transition temperature T_text{SDW} with increasing Mn content, and the unexpected appearance of another magnetic order, occurring at T^{*} approx 10 and 20 K for x=0.05 and 0.10, respectively. We attribute the new magnetic transition at T^{*}, occurring well inside the SDW phase, to a reorganization of the Fermi surface due to Fe-to-Mn substitutions. These give rise to enhanced magnetic fluctuations along the incommensurate wavevector varvec{Q}_2 =(pi pm delta ,pi pm delta ), further increased by the RKKY interactions among Mn impurities.

Vortex phase diagram in 12442-type RbCa2Fe4As4F2 single crystal revealed by magneto-transport and magnetization measurements

The recently discovered 12442-type ACa2Fe4As4F2 (A = K, Rb, Cs) compounds are the only iron-based superconductors (IBSs) with double FeAs layers between neighboring insulating layers, analogous to the double CuO2 layers in some high-T c cuprates. Here, we report the study of vortex phase diagram of RbCa2Fe4As4F2 single crystal via magneto-transport and magnetization measurements. The resistive transition under magnetic fields shows a foot-like kink at a characteristic temperature, T s, followed by a resistive tail in nearly zero resistivity region. Such behavior is ascribed to a vortex slush transition at T s, below which the vortex state has short-range vortex lattice correlation, and then a second-order transition into the vortex glass phase occurs with further decreasing temperature. Above T s, the Arrhenius plot of resistivity shows two linear regions that are separated by a crossover line T cr(B), which is associated with a crossover from collective to plastic pinning or different flux pinning behaviors resulted from different types of defect. In addition, the magnetic hysteresis loops reveal a second magnetization peak (SMP), which is shifted to lower fields with increasing temperature for T< 12 K. However, the SMP unexpectedly moves back to a higher field at T= 12 K, and then gradually turns into a shoulder or kink that moves to higher fields at high temperatures, such anomalous behavior has never been observed in IBSs. According to the magneto-transport and magnetization data, the vortex phase diagram of RbCa2Fe4As4F2 is finally constructed. Details on the different vortex phase transitions and relevant physical scenarios are given and discussed.

Physical properties of the layered oxypnictide Sr2ScFeAsO3: a Mössbauer study down to 1.7 K

A polycrystalline sample of Sr2ScFeAsO3 was studied by 57Fe Mössbauer spectroscopy down to 1.7 K. In contrast to the earlier Mössbauer data, the obtained in this work results indicate that Sr2ScFeAsO3 is in paramagnetic state down to 10 K, while the spectra recorded at 4.6 K and 1.7 K show a weak magnetic order of Fe moments in the Fe2As2 layers. Temperature dependences of isomer shift and quadrupole splitting/shift are compared with specific heat and electrical resistivity data from earlier investigations revealing different local Debye temperatures for the Fe2As2 and perovskite-related Sr2ScO3 layers. Finally, a fast decrease of the carrier density was observed below 80 K and this effect seems to be responsible for the absence of superconductivity in the studied compound.

Pulsed laser deposition of Fe-oxypnictides: Co- and F-substitution

The majority of thin film studies that were devoted to Fe-oxyarsenides has focused so far on F-substituted (i.e. indirectly electron doped) LnOFeAs (Ln = La, Nd, Sm). Here we turn our attention towards Co-substituted (i.e. directly electron doped) LaOFeAs and SmOFeAs in order to investigate its growth on different substrate materials by using pulsed laser deposition (PLD). We detected dominant LnOFeAs phase formation and discuss the occurrence of minor impurity phases in the different films on different substrates. The lack of a superconducting transition in LnOFe0.85Co0.15As films on MgO(100) could be due to strain, since we observe an onset of superconductivity in SmOFe1 − xCoxAs (x = 0.07, 0.15) films on other oxide substrates. In addition, Co-substitution (i.e. within the Fe2As2 layers) and F-substitution (i.e. within the Ln2O2 layers) leading to direct and indirect electron doping respectively, appears for films deposited on CaF2 substrates. In contrast to the F-substituted but Co-free Fe-oxyarsenides, the co-doped SmO1 − xFxFe0.85Co0.15As film has experimentally accessible upper critical fields down to the lowest temperatures and may serve as an ideal test bed for further theoretical modeling of Fe-oxyarsenides.

Spectroscopic evidence of bilayer splitting and strong interlayer pairing in the superconductor KCa2Fe4As4F2

In high temperature cuprate superconductors, the interlayer coupling between the CuO$_2$ planes plays an important role in dictating superconductivity, as indicated by the sensitive dependence of the critical temperature (T$_C$) on the number of CuO$_2$ planes in one structural unit. In Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconductor with two CuO$_2$ planes in one structural unit, the interaction between the two CuO$_2$ planes gives rise to band splitting into two Fermi surface sheets (bilayer splitting) that have distinct superconducting gap. The iron based superconductors are composed of stacking of the FeAs/FeSe layers; whether the interlayer coupling can cause similar band splitting and its effect on superconductivity remain unclear. Here we report high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) measurements on a newly discovered iron based superconductor, KCa$_2$Fe$_4$As$_4$F$_2$ (T$_C$=33.5\,K) which consists of stacking FeAs blocks with two FeAs layers separated by insulating Ca$_2$F$_2$ blocks. Bilayer splitting effect is observed for the first time that gives rise to totally five hole-like Fermi surface sheets around the Brilliouin zone center. Band structure calculations reproduce the observed bilayer splitting by identifying interlayer interorbital interaction between the two FeAs layers within one FeAs block. All the hole-like pockets around the zone center exhibit Fermi surface-dependent and nodeless superconducting gap. The gap functions with short-range antiferromagetic fluctuations are proposed and the gap symmetry can be well understood when the interlayer pairing is considered. The particularly strong interlayer pairing is observed for one of the bands. Our observations provide key information on the interlayer coupling and interlayer pairing in understanding superconductivity in iron based superconductors.

19F NMR Study of the Bilayer Iron-Based Superconductor KCa2Fe4As4F2**Supported by the National Key Research and Development Program of China under Grant No 2017YFA0302901, the National Basic Research Program of China under Grant No 2015CB921304, the National Science Foundation of China under Grant Nos 11674375 and 11634015, and the Strategic Priority Research Program and Key Research Program of Frontier Sciences of

We report a 19F nuclear magnetic resonance (NMR) study on single-crystal KCa2Fe4As4F2 (Tc ∼ 33.3 K). The 19F NMR spectral shape of KCa2Fe4As4F2 is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/19T1 shows a hump below Tc, however the 1/75T1 decreases just below Tc, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa2Fe4As4F2.

Electronic structure and H−T phase diagram of Eu(Fe1−xRhx)2As2

The iron-based superconductors represent a promising platform for high-temperature superconductivity, but the interactions underpinning their pairing present a puzzle. The EuFe$_2$As$_2$ family is unique among these materials for having magnetic order which onsets within the superconducting state, just below the superconducting transition. Superconductivity and magnetic order are normally antagonistic and often vie for the same unpaired electrons, but in this family the magnetism arises from largely localized Eu moments and they coexist, with the competition between these evenly-matched opponents leading to reentrant superconducting behavior. To help elucidate the physics in this family and the interactions between the magnetic order and superconductivity, we investigate the $H$--$T$ phase diagram near optimal Rh doping through specific heat, resistivity, and magnetization measurements, and study the electronic structure by angular-resolved photoemission spectroscopy. The competition between the Eu and FeAs layers may offer a route to directly accessing the electronic structure under effective magnetic fields via ARPES, which is ordinarily a strictly zero-field technique.