Using angle-resolved photoemission spectroscopy (ARPES), we studied the effect of the impurity potential on the electronic structure of FeTe$_{0.5}$Se$_{0.5}$ superconductor by substituting 10\% of Ni for Fe which leads to an electron doping of the system. We could resolve three hole pockets near the zone center and an electron pocket near the zone corner in the case of FeTe$_{0.5}$Se$_{0.5}$, whereas only two hole pockets near the zone center and an electron pocket near the zone corner are resolved in the case of Fe$_{0.9}$Ni$_{0.1}$Te$_{0.5}$Se$_{0.5}$, suggesting that the hole pocket having predominantly the $xy$ orbital character is very sensitive to the impurity scattering. Upon electron doping, the size of the hole pockets decrease and the size of the electron pockets increase as compared to the host compound. However, the observed changes in the size of the electron and hole pockets are not consistent with the rigid-band model. Moreover, the effective mass of the hole pockets is reduced near the zone center and of the electron pockets is increased near the zone corner in the doped Fe$_{0.9}$Ni$_{0.1}$Te$_{0.5}$Se$_{0.5}$ as compared to FeTe$_{0.5}$Se$_{0.5}$. We refer these observations to the changes of the spectral function due to the effect of the impurity potential of the dopants.
By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based superconductor $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the en…
We have carried out a systematic study of the PbO-type compound FeSe_{1-x}Te_x (x = 0~1), where Te substitution effect on superconductivity is investigated. It is found that superconducting transition temperature reaches a maximum of Tc=15.2K at about 50% Te substitution. The pressure-enhanced Tc of FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses su…
The influence of a partial substitution with sulphur into Te sites on the crystal, electronic and magnetic structures of FeTe is investigated by DFT calculations. The results reveal a phase transition from the antiferromagnetic double-stripe order for pure FeTe to the single-stripe order for S-substituted samples, which coincides with the previously observed appearance of the superconducting state. The magnetic transition is caused by the variations of the average chalcogen position in the unit cell. The analyzed normal-state properties of Fe(Te,S) and Fe(Se;S) compounds allow a detection of t…
Angle-resolved photoemission spectroscopy (ARPES) reveals effects of electron doping, which is realized by Co and Ni substitution for Fe in FeTe$_{1-y}$Se$_{y}$ (y$\sim$0.35) superconductor. The data show consistent band shifts as well as expansion and shrinking of electron and hole Fermi surface, respectively. Doping of either element leads to a Lifshitz transition realized as a removal of one or two hole pockets. This explains qualitatively a complex behavior of Hall coefficient observed before [Bezusyy, et al., Phys. Rev. B 91, 100502 (2015)], including change of sign with doping, which tak…
Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe1-xSx; (x = 0-30 %) system and study the impact of low temperature oxygen (O2) annealing as well. Rietveld analysis of room temperature x-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at aro…
We performed angle-resolved photoemission spectroscopy studies on a series of FeTe$_{1-x}$Se$_{x}$ monolayer films grown on SrTiO$_{3}$. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication th…
We report comparative substitution effects of nonmagnetic Ti^(4+) and magnetic Ir^(4+) impurities for Ru^(4+) in the spin-triplet superconductor Sr2RuO4. We found that both impurities suppress the superconductivity completely at a concentration of approximately 0.15%, reflecting the high sensitivity to translational symmetry breaking in Sr2RuO4. In addition, a rapid enhancement of residual resistivity is in quantitative agreement with unitarity-limit scattering. Our result suggests that both nonmagnetic and magnetic impurities in Sr2RuO4 act as strong potential scatterers, similar to the nonma…
We report electronic Raman scattering measurements on optimally doped YBa2Cu3O7 where Zn or Ni impurities have been substituted by Cu. Using Raman selection rules, we have probed the superconducting gap in the nodal and antinodal regions. We show that under impurity substitutions, the energy of the antinodal peak detected in the superconducting state is not related to the critical temperature Tc and that signatures of superconductivity disappear in the nodal regions. Our experimental findings advocate in favor of gapless arcs around the nodes. The breakdown of the relationship between the anti…
We report an impurity effect in the organic superconductor $κ$-(ET)$_2$Cu(NCS)$_2$ by substitution of the ET molecule with an analogue, bis(methyleneditio)tetrathiafulvalene (MT). The superconducting transition temperature decreases with increasing substitution. The in-plane magnetic penetration depth is enhanced with substitution, which is quantitatively attributed to the decrease in the in-plane mean free path. The enhancement of the penetration depth can also explain the reduction of the effective pinning in terms of the condensation energy.
The Hall effect is investigated in thin-film samples of iron-chalcogenide superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1-xTex) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes i…
We report neutron scattering measurements, which reveal spin-liquid polymorphism in a '11' iron chalcogenide superconductor, a poorly-metallic magnetic FeTe tuned towards superconductivity by substitution of a small amount of Tellurium with iso-electronic Sulphur. We observe liquid-like magnetic dynamics, which is described by a competition of two phases with different local structure, whose relative abundance depends on temperature. One is the ferromagnetic (FM) plaquette phase observed in the non-superconducting FeTe, which preserves the C$_4$ symmetry of the underlying square lattice and is…
The suppression of superconductivity by substitution effects has been measured in high quality single crystals of U(Pt_{1-x} Pd_x)_3 with 0 <= x <= 0.002. While the superconducting transition temperature T_c varies linearly with residual resistivity r_0, consistent with pair-breaking by impurity potential scattering, the rate of suppression of T_c with r_0 is much larger for Pd substitution than for other impurity substitutions or by increased defect density. This effect is correlated with an increase in the inelastic scattering coefficient, and may be related to Pd-induced changes in the magn…
Ni-substitution effects on the superconductivity and Cu-spin correlation have been investigated in La2-xSrxCu1-yNiyO4 from the electrical resistivity and muon spin relaxation measurements, taking into account the hole trapping by Ni recently suggested. It has been found that the Ni-substitution suppresses the superconductivity, induces the localization of holes and develops the Cu-spin correlation to the same degree as the Zn substitution. These suggest that Ni with a trapped hole tends to give rise to potential scattering of holes in the CuO2 plane to the same degree as Zn, being discussed in…
We investigated Fe-substitution effects on ferromagnetic fluctuations in the superconducting overdoped and non-superconducting heavily overdoped regimes of the Bi-2201 cuprates by the magnetization and electrical-resistivity measurements. It was found that the spin-glass state was induced at low temperatures by the Fe substitution. The Curie constant and the effective Bohr magneton, estimated from the magnetic susceptibility, as well as the dimensionality of the ferromagnetic fluctuations from the resistivity, suggest the enhancement of the ferromagnetic fluctuations owing to the Fe substituti…
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…
In this study, Si and C were incorporated into polycrystalline MgB2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40K, Rho A(40K), is higher for the SiC reacted samples regardless of carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the SiC reacted sample…
Precise neutron-scattering experiments on La$_{1.88-y}$Sr$_{0.12+y}$Cu$_{0.99}$R$_{0.01}$O$_4$ ($y$=0 for R=Cu, Zn and $y$=0.01 for R=Ga, Fe) were performed to examine the stability of stripe order in cuprate oxides through impurity substitution effects. For all the impurity-substituted samples a static charge-density-wave order similar to that reported in the low-temperature-tetragonal phase (LTT) was discovered in the low-temperature-orthorhombic (LTO) phase for the first time. Fe substitution significantly enhanced both the charge- and spin-density-wave orders. These results indicate that d…
The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indis- tinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensu- rate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitu…
A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy (STM) experiments on the interstitial Fe impurity (IFI) and its nearest-neighboring (nn) sites in $\mathrm{FeTe_{0.5}Se_{0.5}}$ superconducting (SC) compound. By introducing a new impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubove-de Gennes (BDG) equations we investigated the low-lying energy states of the IFI and the underlying Fe-plane. We found the peak of ZBS does not shift or split in a magnetic field as long as the tunneling parameter between I…
The study of the anion ordered (TMTSF)\_2ClO\_4\_(1-x)ReO\_4\_x, solid solution in the limit of a low ReO\_4- substitution level (0<=x<=17%) has revealed a new and interesting phase diagram. Superconductivity is drastically suppressed as the effect of ReO\_4- non magnetic point defects increases following the digamma behaviour for usual superconductors in the presence of paramagnetic impurities. Then, no long range order can be stabilized above 0.1K in a narrow window of substitution. Finally, an insulating SDW ground state in ReO\_4- -rich samples is rapidly stabilized with the decrease of th…
Suppression of T_c in bilayer bismuth and thallium cuprates, by substitution of Co impurities at Cu sites, are taken for examination. T_c suppression data on differently doped Bi2212 and Tl2212 are analysed within the unitary pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to a phenomenological relation valid for weak coupling d-wave superconductors. Values of the pseudogap magnitude at each doping are thereby estimated within a "fermi-level density of states suppression" picture. Pseudogap magnitude from our estimation is observed to have a correspondence with a re…
We have performed transverse- and zero-field muon spin rotation/relaxation experiments, as well as magnetometry measurements, on samples of Fe1-xVxSe and their Li+NH3 intercalates Li0.6(NH2)0.2(NH3)0.8 Fe1-x Vx Se. We examine the low vanadium substitution regime: x = 0.005, 0.01, and 0.02. The intercalation reaction significantly increases the critical temperature (Tc) and the superfluid stiffness for all x. The nonintercalated samples all exhibit Tc = 8.5 K while the intercalated samples all show an enhanced Tc > 40 K. Vanadium substitution has a negligible effect on Tc, but seems to suppress…
Non-magnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can influence significantly the transport and the superconducting behaviour. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states, and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small …
We predict a resonance impurity state generated by the substitution of one Cu atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c superconductor. The precise microscopic origin of the pseudogap is not important for this state to be formed, in particular this resonance will be present even in the absence of superconducting fluctuations in the normal state. In the presence of superconducting fluctuations, we predict the existence of a counterpart impurity peak on a symmetric bias. The nature of impurity resonance is similar to the previously studied resonance in the d-…
Impurity substitution of Zn in La-214 and (Y,Ca)-123 high-T_c superconductors suppresses T_c but does not affect appreciably the onset of the pseudogap phase in the underdoped region nor optimal doping or the position of the inferred quantum critical point. Based on a 1/N expansion of the t-J model we explain these findings as well as the similar dependence on a magnetic field in terms of a quantum critical point scenario where a flux phase causes the pseudogap.
Nonmagnetic impurity effect was studied on the n-type Fe-based superconductor BaFe1.87Co0.13As2 (Tc = 25 K) by a successful Zn substitution for Fe up to 7 at.%. Magnetic susceptibility, electrical resistivity, specific heat, and Hall coefficient measurements indicated that Tc linearly decreases with the Zn concentration and disappears at 7 at.%. The result is quantitatively comparable with what was observed for YBCO, while it disagrees with a recent report for the p-type Ba0.5K0.5Fe2As2. Fragile SC against a nonmagnetic impurity was first confirmed for the n-type 122 Fe-based superconductor.
We study theoretically the current debatable issue about the effect of transition-metal (TM) substitution in iron-based superconductors through treating all of the TM ions as randomly distributed impurities. The extra electrons from TM elements are localized at the impurity sites. In the mean time the chemical potential shifts upon substitution. The phase diagram is mapped out and it seems that the TM elements can act as effective dopants. The local density of states (LDOS) is calculated and the bottom becomes V-shaped as the impurity concentration increases. The LDOS at the Fermi energy $ρ(ω=…
An unexpected insensitivity of the Fermi surface to impurity scattering is found in Ru substituted BaFe$_2$As$_2$ from first-principles theory, offering a natural explanation of the unusual resilience of transport and superconductivity to a high level of disordered substitution in this material. This robustness is shown to originate from a coherent interference of correlated on-site and inter-site impurity scattering, similar in spirit to the microscopic mechanism of superdiffusion in one dimension. Our result also demonstrates a strong substitution dependence of the Fermi surface and carrier …
Based on the five-orbital model, we derive the reduced impurity scattering rate $g=zγ/2πT_{c0}$ in Sm(Fe$_{1-x}$Ru$_{x}$)AsO$_{0.85}$F$_{0.15}$ from the residual resistivity. At $x=0$, the transition temperature is $T_{c0}=50$ K. For $0.05<x<0.36$ ($0.84>T_{c}/T_{c0}>0.3$) the obtained value of $g$ ranges from 1.5 to 2.9, which suggests that the $s_\pm$-wave state cannot survive. We point out that the magnetoresistance frequently gives an underestimated value of $g$ in correlated electron systems.
The nonmagnetic impurity effect is studied on the Fe-based BaFe1.89Co0.11As2 superconductor (Tc = 25 K) with Zn substitution for Fe up to 8 at. %, which is achieved by means of high-pressure and high-temperature heating. Tc decreases almost linearly with increasing the Zn content and disappears at ~8 atomic %, being different in the shared phenomenology of the early Zn doping studies, where Tc decreases little. The Tc decreasing rate, however, remains much lower (3.63 K/%) than what is expected for the s(+-)-wave model, implying the model is unlikely. Another symmetry model such as the non-sig…
Neutron scattering experiments have been carried out to explore Ni-impurity effects on static spin correlations in La2-xSrxCuO4 (LSCO) in the vicinity of the superconductor-insulator boundary where both parallel and diagonal spin-density modulations (SDM) coexist at low temperature. Upon dilute Ni substitution the incommensurability decreases for both types of SDM, while the volume fraction of the diagonal (parallel) SDM increases (decreases). Subsequent Ni doping induces a bulk three-dimensional antiferromagnetic (AF) order when x ~ Ni concentration. TN of such the AF order depends on x and s…
The electronic ground state in high-$T_{\rm c}$ cuprates where the superconducting state is suppressed by Ni substitution has been investigated in La$_{2-x}$Sr$_x$Cu$_{1-y}$Ni$_y$O$_4$ from the specific heat and muon spin relaxation measurements. It has been found that the ground state changes from a magnetically ordered state with the strong hole-trapping by Ni to a metallic state with the Kondo effect of Ni with increasing hole-concentration. Moreover, the analysis of the results has revealed that a phase separation into the magnetically ordered phase and the metallic phase occurs around the…
A short review of the optical and far-infrared measurements on MgB$_{2}$ is given. Multiband and multigap effects are analyzed by comparing optical properties with other experiments and {\em ab initio} calculations. The covered topics are: the plasma frequency, electron-phonon interaction, impurity scattering, the effects of C and Al substitution, interband transitions and the far-infrared signatures of the superconducting gaps.
We present a theory of combined nonmagnetic and magnetic impurity scattering in anisotropic superconductors accounting for the momentum-dependent impurity potential. Applying the model to the d-wave superconducting state, we obtain a quantitative agreement with the initial suppression of the critical temperature due to Zn and Ni substitutions as well as electron irradiation defects in the cuprates. We suggest, that the unequal pair-breaking effect of Zn and Ni may be related to a different nature of the magnetic moments induced by these impurities.
We have investigated the suppression of the superconducting transition temperature, Tc, with an increase of the residual resistivity, ρ_0, through the substitution of M (M = Co, Ni, Zn) for Fe in Fe1-yMySe0.3Te0.7 single crystals, in order to clarify the symmetry of the superconducting gap in FeSe1-xTex. Small, large and very small suppression of Tc have been observed through the Co, Ni and Zn substitution, respectively. The magnitude of the suppression rate is hardly explained in terms of the pair-breaking effect due to potential scattering calculated based on the Abrikosov-Gor'kov theory, ev…
Using first principle electronic structure calculations we investigated the role of substitutional doping of B,N,P,Al and vacancies (${\cal V}$) in diamond (X$_α$C$_{1-α}$). In the heavy doping regime, at about $\sim 1-6%$ doping an impurity band appears in the mid gap. Increasing further the concentration of the impurity substitution fills in the gap of the diamond host. Our first principle calculation indicates that in the case of vacancies, a clear single-band picture can be employed to write down an effective {\em one band} microscopic Hamiltonian, which can be used to further study variou…
Angle-resolved photoemission measurements were performed on Zn-doped La2-xSrxCuO4 (LSCO) to investigate the effects of Zn impurities on the low energy electronic structure. The Zn-impurity-induced increase in the quasi-particle (QP) width in momentum distribution curves (MDC) is approximately isotropic on the entire Fermi surface and energy-independent near the Fermi level (EF). The increase in the MDC width is consistent with the increase in the residual resistivity due to the Zn impurities if we assume the carrier number to be 1-x for x=0.17 and the Zn impurity to be a potential scatterer cl…
We investigated the doping effects of magnetic and nonmagnetic impurities on the single-crystalline p-type Ba0.5K0.5Fe2-2xM2xAs2 (M = Mn, Ru, Co, Ni, Cu and Zn) superconductors. The superconductivity indicates robustly against impurity of Ru, while weakly against the impurities of Mn, Co, Ni, Cu, and Zn. However, the present Tc suppression rate of both magnetic and nonmagnetic impurities remains much lower than what was expected for the s\pm-wave model. The temperature dependence of resistivity data is observed an obvious low-T upturn for the crystals doped with high-level impurity, which is d…
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…
$^{75}$As and $^{139}$La NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO$_{0.85}$ have been performed to investigate the Zn-impurity effects microscopically. Although superconductivity in LaFeAsO$_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties,…
Effects of magnetic impurity on the superconductivity has been investigated in Mg1-xMnB2. With increase of Mn concentration x, the lattice constant c (perpendicular to the boron sheet) decreases at a rate of -1.4 % (= dlnc/dx), while a remains nearly unchanged. The transition temperature Tc steeply decreases with $x$ at a rate of dTc/dx = - 159 K. These results suggest that superconducting state of the parent MgB2 is amenable to the magnetic impurities, i.e., Mn2+.
The $ab$-plane resistivity and Hall effect are studied in Fe$_{1-y}$M$_y$Te$_{0.65}$Se$_{0.35}$ single crystals doped with two transition metal elements, M = Co or Ni, over a wide doping range, $0 \leq y \leq 0.2$. The superconducting transition temperature, $T_{c}$, reaches zero for Co at $y \simeq 0.14$ and for Ni at $y \simeq 0.032$, while the resistivity at the $T_{c}$ onset increases weakly with Co doping, and strongly with Ni doping. The Hall coefficient $R_H$, positive for $y$ = 0, remains so at high temperatures for all $y$, while it changes sign to negative at low $T$ for $y > 0.135$ …
A Yu-Shiba-Rusinov (YSR) state is a localized in-gap state induced by a magnetic impurity in a superconductor. Recent experiments used an STM tip to manipulate the exchange coupling between an Fe adatom and the ${\rm FeTe}_{0.55}{\rm Se}_{0.45}$ superconductor possessing a $Z_2$ nontrivial band structure with topological surface states. As the tip moves close to the single Fe adatom, the energy of the in-gap state is modulated and exhibits two unusual zero-energy crossings, which cannot be understood by coupling the magnetic impurity to the superconducting topological surface Dirac cone. Here,…
Zero-field muon-spin-relaxation measurements have been carried out in order to investigate the Zn- and Ni-substitution effects on the Cu-spin dynamics in the electron-doped Pr_0.86_LaCe_0.14_Cu_1-y_(Zn, Ni)_y_O_4+\alpa-δ_ with y = 0, 0.01, 0.02, 0.05 and different values of the reduced oxygen content δ(δ\le 0.09). For the samples with y = 0 and very small δvalues of δ< 0.01, a muon-spin precession due to the formation of a long-range antiferromagnetic order has been observed at low temperatures below \~ 5 K. For the moderately oxygen-reduced samples of 0.01 \le δ\le 0.09, on the contrary, no m…
We report the results of an ab initio modeling of substitutional impurities such as zinc, copper and zirconium ions incorporated into the magnesium sublattice of MgB2. The goal of computational studies was to gain an atomistic understanding of how the structural and bonding properties of the local ionic environment are affected by a change in the host cation. The simulations performed for the given set of substituents indicate that at a low doping level, the induced lattice distortions and additional forces are noticeable small. At the same time, the electron redistributions around the impurit…
The effect of Mn substitution, acting as a magnetic impurity for Fe, on the Dirac cone was investigated in Ba(Fe$_{1-x}$Mn$_x$As)$_2$. Both magnetoresistance and Hall resistivity studies clearly indicate that the cyclotron effective mass ($m^{\ast}$) of the Dirac cone is anomalously enhanced at low temperatures by the impurity, although its evolution as a function of carrier number proceeds in a conventional manner at higher temperatures. Kondo-like band renormalization induced by the magnetic impurity scattering is suggested as an explanation for this, and the anomalous mass enhancement of th…
We present an ab-initio study of Ru substitution in two different compounds, BaFe2As2 and LaFeAsO, pure and F-doped. Despite the many similarities among them, Ru substitution has very different effects on these compounds. By means of an unfolding technique, which allows us to trace back the electronic states into the primitive cell of the pure compounds, we are able to disentangle the effects brought by the local structural deformations and by the impurity potential to the states at the Fermi level. Our results are compared with available experiments and show: i) satisfying agreement of the ca…
Tailoring the normal and superconducting state properties of ternary scandium tellurides, Sc$_6M$Te$_2$ ($M = $ Fe, Ru and Ir) through chemical substitution
The intragrain pinning in high-$T_c$ superconductor compounds Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements close to the diameter of Abrikosov vortices in YBCO.
We study striped morphologies induced by magnetic impurities in d-wave superconductors (DSCs) near optimal doping by self-consistently solving the Bogoliubov-de Gennes equations based on the $t-t^{\prime }-U-V$ model. For the single impurity case, it is found that the stable ground state is a modulated checkerboard pattern. For the two-impurity case, the stripe-like structures in order parameters are induced due to the impurity-pinning effect. The modulations of DSC and charge orders share the same period of four lattice constants (4$a$), which is half the period of modulations in the coexisti…
In this paper we summarize the effects of magnetic and nonmagnetic impurities on the spin dynamics in Febased superconductors and their parent compounds. The effects of chemical substitution, vacancies, and disorder on the suppression or stabilization of superconductivity and spin-density-wave phases are reviewed in the context of recent neutron-spectroscopy measurements of spin excitations. We also present new results on the structure of magnetic fluctuations in BaFe2As2 single crystals doped with Mn local moments and discuss them in relationship to the previously reported (pi, pi) branch of …
Although their crystal structures are complex, all high temperature superconductors contain some crystal planes consisting of only Cu and O atoms in a square lattice. Superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substitution of a single impurity atom at a Cu site creates a simple but powerful perturbation to these interactions. Detailed knowledge of the effects of such an impurity atom on the superconducting order parameter and on the quasi-particle local density of states (LDOS) could allow competing theories of high temperature supercon…
In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurements on Ba(Fe_{1-x}Zn_x)_2As_2 (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around T~135 K, indicating anti…
We review the progress of nonmagnetic impurity doping study on the Fe-based superconductors. On the theoretical side, two highly potential candidates for the pairing symmetry order parameter, i.e. the multi-gap s++ and s+- wave models, have been proposed but continuously debated. The debate arises because of the complex gap structure, exceptive magnetic and metallic behaviors of Fe-based superconductors which may make the influences of nonmagnetic defects varied in chemical potential, impurity disorder, inter- and intra-band scattering strength, and electron localization, and hence difficulty …
