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, even if errors in the estimation of the carrier concentration, effective mass and ρ_0 were taken into account. Accordingly, these results suggest that the superconducting symmetry is the s++-wave in FeSe1-xTex.
Effects of impurities and disorder on quasiparticle spectrum in superconducting iron pnictides are considered. Possibility for occurrence of localized energy levels due to impurities within the superconducting gap and the related modification of band structure and of superconducting order parameter are discussed. The evolution of superconducting state with impurity doping is traced.
We propose a new model for impurity center formed by a cathion substitute for Cu, like Zn, in CuO_{2} planes. Its main effect on superconducting electrons is due to the non-zero exchange field on O sites, neighbors to the (non-magnetic) impurity. We discuss a strong suppression of d-wave order parameter, a zero- energy resonance in local density of states, and spin polarization of charge carriers, which can be related to the experimentally observed effects in Zn-doped copper oxides. These results are obtained \emph{without} using the unitary scattering limit.
Recent experiments on Sr_2RuO_4 have indicated the presence of linear nodes in the superconducting order parameter. Among the possible spin triplet states, 2D p-wave superconductivity with E_u symmetry appears to be inconsistent with the experiments, whereas the 2D f-wave order parameter with B_1g x E_u symmetry turns out to be a more likely candidate. Here it is shown how the quasi-particle bound state wave functions around a single impurity provide a clear signature of the symmetry of the underlying superconducting order parameter.
We study the influence of superconductivity on the formation of the localized magnetic moment for a single-level quantum impurity embedded in an s-wave Bardeen-Cooper-Schrieffer (BCS) superconducting medium, modeled by single-impurity Anderson Hamiltonian. We have combined Bogoliubov transformation with Green's function method within self-consistent Hartree-Fock Mean Field approximation to analyze the conditions necessary in metal (in the superconducting) for the formation of the magnetic moment at the impurity site for the low-frequency limit $|ω|<<Δ_{sc}$ as well as for the finite supercondu…
Motivated by recent experimental progress in high-resolution scanning tunneling microscopy (STM) techniques, we propose to investigate the local quasiparticle density of states around a unitary impurity in the heavy fermion superconductor CeCoIn$_{5}$. Based on the T-matrix approach we obtain a sharp nearly zero-energy resonance state (ZERS) in the strong impurity potential scattering localized around the impurity, and find qualitative differences in the spatial pattern of the tunneling conductance modulated by the nodal structure of the superconducting gap. These unique features may be used a…
The superconducting state around a well isolated nonmagnetic impurity in the high-Tc superconductors is studied using the two-dimensional t-J model. The spatial dependence of the order parameter and the local density of states are obtained from the numerical diagonalization of the Bogoliubov-de Gennes equation derived using the Gutzwiller approximation. We find a zero-energy peak in the local density of states around the impurity. Different from the previous results on a vortex or surfaces in the t-J model, the splitting of zero-energy peak is not found. The zero-energy states corresponding to…
It has previously been found that a magnetic impurity in a conventional $s$-wave superconductor can give rise to a local $π$-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the $π$-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When non-resonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a $π$-phase shift. We a…
Experiments observe an enhanced superconducting gap over impurities as compared to the clean-bulk value. In order to shed more light on this phenomenon, we perform simulations within the framework of Bogoliubov-deGennes theory applied to the attractive Hubbard model. The simulations qualitatively reproduce the experimentally observed enhancement effect; it can be traced back to an increased particle density in the metal close to the impurity site. In addition, the simulations display significant differences between a thin (2D) and a very thick (3D) film. In 2D pronounced Friedel oscillations c…
The impurity effects on the transition temperature Tc and the isotope effect are examined in multiband superconductors with magnetic and nonmagnetic impurities, where the effect of Coulomb repulsion is considered. It is shown that an internal transition of the superconducting state is induced by impurity doping, and that the transition is accompanied by a jump of the isotope-effect coefficient alpha. In particular, the transition is illustrated in a system with two electron bands. In some special cases, extended Abrikosov and Gor'kov (AG) equations for Tc and the expressions of the isotope-eff…
The family of multi-layered superconductors derived from the doped topological insulator Bi$_2$Se$_3$ has been found to be unusually robust against non-magnetic disorder. Recent experimental studies have highlighted the fact that the location of impurities could play a critical role for this puzzling robustness. Here we investigate the effects of four different types of impurities, on-site, interstitial, intercalated and polar, on the superconducting critical temperature. We find that different components of the scattering potential are active depending on the impurity configuration and choice…
Superconducting quantum phase transitions tuned by disorder (d), paramagnetic impurity (MI) and perpendicular magnetic field (B) have been studied in homogeneously disordered ultrathin a-Pb films. The MI-tuned transition is characterized by progressive suppression of the critical temperature to zero and a continuous transition to a weakly insulating normal state with increasing MI density. In all important aspects, the d-tuned transition closely resembles the MI-tuned transition and both appear to be fermionic in nature. The B-tuned transition is qualitatively different and probably bosonic. I…
We derive the generalized Ginzburg-Landau free energy functional for conventional and unconventional singlet superconductors in the presence of paramagnetic, orbital and impurity effects. Within the mean field theory, we determine the criterion for appearence of the non uniform (Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state, with vortex lattice structure and additional modulation along the magnetic field. We also discuss the possible change of the order of transition from normal to superconducting state. We find that the superconducting phase diagram is very sensitive to geometrical …
We investigate the local density of states in the vicinity of a native dumbbell defect arising from an Fe vacancy in FeSe single crystals. The tunneling spectra close to the impurity display two bound states inside the superconducting gap, equally spaced with respect to zero energy but asymmetric in amplitude. Using spin-polarized density functional theory (DFT) calculations on realistic slab models with Fe vacancy, we show that such a defect does not induce a local magnetic moment. Therefore, the dumbbell defect is considered as non-magnetic. Thus, the in-gap bound states emerging from a non-…
The influence of SiC-impurity layer and growth temperature on microstructure and superconducting properties were studied for MgB2 superconducting tapes. The pulsed laser deposition (PLD) system was used for the deposition of amorphous SiC-impurity layers on the flexible metallic Cu (001) tapes. The MgB2 superconducting tapes were fabricated by growing MgB2 films on the top of SiC/Cu tapes over a wide temperature range of 460 - 600 °C by using hybrid physical-chemical vapor deposition (HPCVD) system. Among all tapes, the MgB2/SiC/Cu tape deposited at a temperature of 540 °C has the highest Tc o…
Weak impurity scattering produces a narrow band with a finite density of states near the phase difference $ϕ=π$ in the mid-gap energy spectrum of a macroscopic superconducting weak link. The equivalent distribution of transmission coefficients of various cunducting quantum channels is found.
Impurities with the pronounced forward scattering (FS impurities) change analytical properties of the quasiparticle Green's function substantially compared to the isotropic scattering. By assuming that the superconducting pairing is due to the forward E-P scattering (FEP pairing) it is shown that the critical temperature of clean systems T_{c0} depends linearly on the E-P coupling constant λand the isotope effect αis small. The FS impurities make α=1/2 in the dirty limit and affect in the same way the s- and d-wave FEP pairing. The FS impurities are pair-weakening in both pairing channels. The…
We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induce…
We theoretically study the effect of a nonmagnetic impurity in heavy fermion superconductor CeCoIn$_{5}$ within a realistic band model and the T-matrix approximation approach. By considering two known possible pairing symmetry candidates $d_{x^{2}-y^{2}}$ and $d_{xy}$, we find that although both total density of states exhibit a similar V-shaped gaplike feature, only the $d_{x^{2}-y^{2}}$-wave pairing symmetry gives rise to robust intragap impurity resonance states reflected by the resonance peaks near the Fermi energy in the local density of states. These features can be readily probed by sca…
Multiband superconductivity with dominant two-gap features are recently proposed to challenge the earlier accepted nodal $d$-wave pairing in the first unconventional superconductor CeCu$_{2}$Si$_{2}$. Here we obtain multiband Fermi-surface topology of CeCu$_{2}$Si$_{2}$ via first-principles calculations, and study the problem within an effective two hybridization band model including detailed band-structure. Within T-matrix approximation, our calculations reveal that different pairing candidates could yield qualitatively distinct features characterised by impurity-induced bound states. Except …
We discuss the effects of magnetic impurities on d-wave superconductors. We calculate the electron mediated RKKY interaction between the impurity spins in a d-wave superconductor and find that it decays as $r^{-3}$ at large distances. We argue that this interaction leads to the formation of a spin glass at low temperature $T \ll T_c$. It was previously shown that a local complex $Δ^1 \sim d_{xy}$ order parameter is induced around each impurity spin. We consider the pair tunneling resulting in the Josephson interaction between different patches of induced $d_{xy}$ order parameter. Due to the lo…
Within the two-orbital model for BiS$_{2}$-based superconductors, the effect from a single nonmagnetic impurity scattering on the superconducting-state is investigated in terms of the T-matrix approach. By considering three kinds of the typical Fermi surface topology which characterize the essential features of the doping dependence of the electronic structure in BiS$_{2}$-based superconductors, it is found that the impurity scattering on the superconducting-state with the conventional $s$-wave, the extended $s$-wave, the $\pm s$-wave, and the $d_{x^{2}-y^{2}}$-wave symmetries induces \textit{…
Effects of impurities and disorder on transport properties by electronic quasiparticles in superconducting iron pnictides are theoretically considered. The most prominent new features compared to the case of pure material should appear at high enough impurity concentration when a specific narrow band of conducting quasiparticle states can develop within the superconducting gap, around the position of localized impurity level by a single impurity center. The predicted specific threshold effects in the frequency dependent optical conductivity and temperature dependent thermal conductivity and al…
A theory is developed to study shot noise in superconducting (SAS) and hybrid (SAN) junctions with singly occupied Anderson impurity (A) as a weak link. The zero-frequency DC component of the shot noise spectral density is calculated at zero temperature as a function of the bias at different Coulomb repulsion strengths U, and show a remarkable structure resulting from combination of electron-electron interaction and Andreev reflections.
We address the effect of MgO impurity on the superconducting properties of MgB2. The synthesis of MgB2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and ac applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB2 having more am…
By measuring the spatial distribution of differential conductance near impurities on Fe sites, we have obtained the quasi-particle interference (QPI) patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [arXiv:1710.09089] which is specially designed for the impurity induced bound states. It is found that the supercond…
Superconductors with noncentrosymmetric crystal structures such as Li2T3B(T:Pd,Pt) have been the focus of in-depth research with their parity mixing nature. In this study, we focused our research on non-magnetic impurity effect in Li2T3B (T: Pd, Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We prepared different quality samples of Li2Pd3B and Li2Pt3B by changing conditions in synthesizing, and sample qualities were estimated by residual resistivity. Spin singlet dominant superconductor Li2Pd3B exhibits the weak Tc sup…
We report a nontrivial transition in the core structure of vortices in two-band superconductors as a function of interband impurity scattering. We demonstrate that, in addition to singular zeros of the order parameter, the vortices there can acquire a circular nodal line around the singular point in one of the superconducting components. It results in the formation of the peculiar "moat"-like profile in one of the superconducting gaps. The moat-core vortices occur generically in the vicinity of the impurity-induced crossover between $s_{\pm}$ and $s_{++}$ states.
Heterostructures allow the realization of electronic states that are difficult to obtain in isolated systems. Exemplary is the case of quasi-one-dimensional heterostructures formed by a superconductor and a semiconductor with spin-orbit coupling in which Majorana zero-energy modes can be realized. We study the effect of a single impurity on the spectrum of superconducting heterostructures. We find that the coupling between the superconductor and the semiconductor can strongly affect the impurity-induced states and may induce additional subgap bound states that are not present in isolated unifo…
Impurity scattering in a superconductor may serve as an important probe for the nature of superconducting pairing state. Here we report the impurity effect on superconducting transition temperature $T_\mathrm{c}$ in the newly discovered Cr-based superconductor K$_2$Cr$_3$As$_3$. The resistivity measurements show that the crystals prepared using high-purity Cr metal ($\geq$99.99\%) have an electron mean free path much larger than the superconducting coherence length. For the crystals prepared using impure Cr that contains various \emph{nonmagnetic} impurities, however, the $T_\mathrm{c}$ decrea…
We investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. Employing the two-orbital model where 3$d$ electron and hole conduction bands are hybridizing with the localized $f$-orbital of the impurity spin, we investigate how various symmetries of the superconducting gap and its nodal structure influence the quasiparticle excitations and impurity bound states. We show that the bound states behave qualitatively different for each symmetry. Most importantly we find that the impurity-induced bound states can be used to identi…
Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali doped iron chalcogenide superconductors K_xFe_{2-y}Se_2. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest neighbor d-wave and next-nearest neighbor s-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that sub-gap bound states exist only for d-wave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such di…
Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. As such, YSR states have been very successfully modeled with an effective scattering potential (Kondo impurity model). Using a scanning tunneling microscope, we exploit the proximity of the tip to a surface impurity and its influence on the YSR state to make a quantitative connection between the YSR state energy and the impurity-substrate hybridization. We corroborate the coupling between impurity and substrate as a key energy scale for surface …
A combined effect of the orthorhombic crystal field and potential impurity scattering on several superconducting states of a tetragonal symmetry is studied within a weak-coupling mean field approach. It is shown that the nonmagnetic impurities stabilize the states belonging to the identity irreducible representation. The electronic specific heat jump at the phase transition is analyzed. Its dependence on the potential scattering rate for large impurity concentration is shown to be remarkably different for the states with a nonzero value of the Fermi surface averaged order parameter than for th…
We propose a model of a hypothetical superconductor which includes impurities with retarded interaction with quasiparticle and superconducting matrix where these impurity can be applied with large concentration. Interaction between the impurity and a conduction electron in such a system has been calculated. We have found the critical temperature of the system matrix+impurity essentially exceeds critical temperature of the pure superconducting matrix.
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 …
A dirty BCS superconductor with magnetic impurities is studied. Asymptotic solution of the thermodynamics of such superconductor with spin $1/2$ and $7/2$ magnetic impurities, is found. To this end, the system's free energy $f(H, β)$ is bounded from above and below by mean-field type bounds, which are shown to coalesce almost exactly in the thermodynamic limit, provided the impurity concentration is sufficiently small. The resulting mean-field equations for the gap $Δ$ and a parameter $ν$, characterizing the impurity subsystem, are solved and the solution minimizing $f$ is found for various va…
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.
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 study the effect of magnetic impurities in the d_{x^2-y^2}-wave superconducting (SC) state of the two dimensional t-J model.The spin-orbit and the spin-exchange interactions are examined by treating the impurity as a classical spin. The Bogoliubov de Gennes equation derived within a slave-boson mean-field approximation is solved numerically at T = 0. The spin-exchange scattering induces spin-triplet p-wave SC order parameters near the impurity, while a SC state with broken time-reversal symmetry and a spontaneous current appears in the presence of the spin-orbit interaction. When both inter…
We study the non-local superconducting pairing of two interacting Anderson impurities, which has an instability near the quantum critical point from the competition between the Kondo effect and an antiferromagnetic inter-impurity spin exchange interaction. As revealed by the dynamics over the whole energy range, the superconducting pairing fluctuations acquire considerable strength from an energy scale much higher than the characteristic spin fluctuation scale while the low energy behaviors follow those of the staggered spin susceptibility. We argue that the glue to the superconducting pairing…
The optical response of the two-band superconductor MgB$_2$ has been studied in the 0.7-4 THz range on films with very low impurity level. The effect of the high-energy $σ$-gap is observed in the ratio $R_S/R_N$ between the normal and superconducting state reflectance, while in a neutron irradiated film with a slightly higher impurity level mainly the effect of the $π$-gap is evident as reported in previous experiments. At terahertz frequencies, the electrodynamic of MgB$_2$ can be well described by the two-band parallel conductivity model and is dominated by the $π$-bands when the impurity le…
We study sharp low-energy resonance peaks in the local density of states (LDOS) induced by Zn impurities or possible Cu vacancies in superconducting Bi_2Sr_2CaCu_2O_{8+delta}. The measured structure of these near-zero-bias resonances is quantitatively reproduced by an extended impurity potential without invoking internal impurity states or sophisticated tunneling models. The Zn potential extends at least to the nearest-neighbor Cu sites, and the range of order parameter suppression extends at least 8 Åaway from the Zn site. We further show that the local spin susceptibilities near Zn impuritie…
We study the effect of impurities in superconducting graphene and discuss their influence on the local electronic properties. In particular, we consider the case of magnetic and non-magnetic impurities being either strongly localized or acting as a potential averaged over one unit cell. The spin dependent local density of states is calculated and possibilities for visualizing impurities by means of scanning tunneling experiments is pointed out. A possibility of identifying magnetic scatters even by non spin-polarized scanning tunneling spectroscopy is explained.
We report a study in which the effect of defects/impurities, growth process, off-stoichiometry, and presence of impurity phases on the superconducting properties of noncentrosymmetric CePt3Si is analysed by means of the temperature dependence of the magnetic penetration depth. We found that the linear low-temperature response of the penetration depth -indicative of line nodes in this material- is robust regarding sample quality, in contrast to what is observed in unconventional centrosymmetric superconductors with line nodes. We discuss evidence that the broadness of the superconducting transi…
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…
We apply the unbiased weak-coupling continuous time quantum Monte Carlo (CTQMC) method to review the physics of a single magnetic impurity coupled to s-wave superconducting leads described by the BCS reduced Hamiltonian. As a function of the superconducting gap $Δ$, we study the signature of the first order transition between the singlet and doublet (local moment) states on various quantities. In particular we concentrate on the Josephson current with 0 to $π$ phase shift, the crossing of the Andreev bound states in the single particle spectral function, as well as the local dynamical spin str…
Superconducting transition temperature Tc of NazCo1-xMxO2.yH2O has been studied for M=Ir and Ga, which can substitute for Co. It has been found that Tc is suppressed by the M-atom doping. The decreasing rates |dTc/dx| are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the Tc suppression.
We study in-gap states of a single magnetic impurity embedded in a honeycomb monolayer which is deposited on superconducting substrate. The intrinsic spin-orbit coupling induces the quantum spin Hall insulating (QSHI) phase gapped around the Fermi energy. Under such circumstances we consider the emergence of Shiba-like bound states driven by the superconducting proximity effect. We investigate their topography, spin-polarization and signatures of the quantum phase transition manifested by reversal of the local currents circulating around the magnetic impurity. These phenomena might be importan…
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-…
Recently Chakravarty {\em et al.} proposed an ordered $d$-density wave (DDW) state as an explanation of the pseudogap phase in underdoped high-temperature cuprates. We study the competition between the DDW and superconducting ordering based on an effective mean-field Hamiltonian. We are mainly concerned with the effect of the DDW ordering on the electronic state around a single nonmagnetic impurity. We find that a single subgap resonance peak appears in the local density of state around the impurity. In the unitary limit, the position of this resonance peak is always located at $E_r=-μ$ with r…
Quantum impurity models are prevalent throughout many body physics, providing some prime examples of strongly correlated systems. Aside from being of great interest in themselves they can provide deep insight into the effects of strong correlations in general. The classic example is the Kondo model wherein a magnetic impurity is screened at low energies by a non interacting metallic bath. Here we consider a magnetic impurity coupled to a quantum wire with pairing interaction which dynamically generates a mass gap. Using Bethe Ansatz we solve the system exactly finding that it exhibits both scr…
Magnetic impurities at surfaces of superconductors can induce bound states referred to as Yu-Shiba-Rusinov (YSR) states within superconducting gaps. Understanding of YSR states with spin-orbit coupling (SOC) plays a pivotal role in studies of Majorana zero modes. Spin polarization of a zero-bias peak (ZBP) is used to determine its topological nature. Here we investigate the YSR states of single magnetic impurities at the surface of Pb using the fully relativistic first-principles simulations including band structure of Pb and five 3$d$ orbitals of the impurity in the superconducting state. We …
The microscopic mechanism of circular currents induced in the vicinity of a non-magnetic impurity is analyzed for the (time-reversal symmetry breaking) chiral superconducting state ${\bf d}({\bf k}) = \hat{\bf z} (k_x \pm i k_y)$ which is very likely realized in Sr_2RuO_4. From the analytic, not self-consistent solution of the Bogolyubov-de Gennes equations we find two types of quasiparticle states, a bound state and the continuum state. Through impurity scattering the condensate transfers angular momentum to the quasiparticle states which generate a circular current. At low temperature the co…
A model describing the Anderson impurity in the Bardeen-Cooper-Schriffer superconductor is proven to exhibit hidden integrability and is diagonalized exactly by the Bethe ansatz.
We study the effect of impurity scattering in the unitarity limit on the A and B phase superconductivity in PrOs4Sb12. We take the triplet superconducting order parameters and the impurity scattering is treated within the standard theory. We find the quasiparticle density of states and thermodynamics are very sensitive to the impurity scattering. The impurity scattering dependence of the superconducting transition temperature and the order parameter at T=0 are very similar to those in d-wave superconductors. Some of these characteristics will provide a sensitive test of the symmetry of the und…
Effects of the superconducting pair potential on the impurity scattering processes in metallic carbon nanotubes are studied theoretically. The backward scattering of electrons vanishes in the normal state. In the presence of the superconducting pair correlations, the backward scatterings of electron- and hole-like quasiparticles vanish, too. The impurity gives rise to backward scatterings of holes for incident electrons, and it also induces backward scatterings of electrons for incident holes. Negative and positive currents induced by such the scatterings between electrons and holes cancel eac…
