Quantum interference phenomena between impurity states in d-wave superconductors

We investigate the mutual influence of impurities in two-dimensional d-wave superconductors involving self-consistent solutions of the Bogoliubov-de Gennes equations. The local order parameter suppression, the local density of states as well as the interference of impurity-induced structures are analyzed. We employ an impurity position averaging scheme for the density of states that does not neglect these interference effects, as the commonly used T-matrix approaches do.     

Ginzburg-Landau simulation for a vortex around a columnar defect in a superconducting film

By means of numerical simulations based on Ginzburg-Landau theory, we study the vortex depinning from a columnar defect in a superconducting film. We evaluate the limiting thickness of the film, below which the depinning does not occur even under an application of the magnetic field perpendicular to the columnar defect. The limiting thickness is a measure of the pinning strength of the columnar defect. The dependence of this limiting thickness on the magnitude of the applied field is obtained for two types of columnar defects.     

Self-consistent diagrammatics for a correlated superconductor: contrasting two and three dimensions

We use self-consistent diagrammatics to contrast pairing fluctuations in the Attractive Hubbard Model in two and three dimensions. Specifically, we use the self-consistent T-matrix approximation and show that, quite remarkably, this method yields results that are consistent with the exact 2D-XY and 3D-XY critical scaling. We find that dimensionality alone increases the ratio of the mean-field to actual transition temperatures more than 4-fold in 2D, and that the scaling regimes coincide with the normal state pseudogap regime in both 2D and 3D. Together this suggests that the decreasing effective dimensionality when cuprates are underdoped may account for the increasing size of the pseudogap regime.     

A new numerical method for quasi-particle spectroscopy around a pair of vortices in triplet superconductors

Triplet superconductors such as Sr₂RuO₄ and Na_xCoO₂·yH₂O are now found to be p-wave (p_x±ip_y) or f-wave ((p_x±ip_y)coscp_z) superconductors. In conventional singlet superconductors, vortices are quantized because phase of order parameter must rotate by 2π around a vortex. But triplet superconductors have a degree of freedom of spin, which is described by d-vector. The d-vector and phase can rotate by π around a vortex, separately. Therefore appearance of HQVs is predicted. Theoretically, it is found that a pair of HQVs is more stable than a singly quantized vortex, for several parameter regions.In this study, in order to investigate quasi-particle bound states around two vortices in s-wave superconductors, we have developed a new numerical method to solve the BdG equation for two vortices state, using Mathieu functions. We confirmed the validity of this method for two vortices state and applied it in case of a pair of vortices. And we solved it.     

Suppression of Tc by Zn impurity in the electron-type LaFe0.925−y Co0.075ZnyAsO system

The effect of non-magnetic Zn impurity on superconductivity in electron-type pnictide superconductor LaFe_0.925−yCo_0.075Zn_yAsO is studied systematically. The optimally doped LaFe_0.925Co_0.075AsO without Zn impurity exhibits superconductivity at T_c^mid of 13.2 K, where T_c^mid is defiend as the mid-point in the resistive transition. In the presence of Zn impurity, the superconducting transition temperature, T_c^mid, is severely suppressed. The result is consistent with the theoretic prediction on the effect of non-magnetic impurity in the scenario of s_± pairing, but it is in sharp contrast to the previous report on the effect of Zn impurity in the F-doped systems. The possible interpretation of the different effects of Zn impurity on superconductivity in different systems is discussed.     

The orbital characters and kz dispersions of bands in iron-pnictide NaFeAs

The electronic structure and orbital characters of iron-pnictide NaFeAs have been studied by polarization dependent angle-resolved photoemission spectroscopy. Some of the bands are mixed with the orbitals of opposite symmetries, which could be interpreted by the hybridization among the bands. According to the photon energy dependent experiment, the k_z dispersions of the bands that cross the Fermi energy are weak in both paramagnetic and spin density wave states. However, a band well below the Fermi level shows a k_z dispersion of 41 meV, which mainly contains the d_z² orbital.     

Electronic structures of mixed ionic-electronic conductors SrCoOx

Electronic structures of SrCoO_x with x=3, 2.875 and 2.75 were calculated by DFT technique in SLDA approximation. Two kinds of oxygen vacancy ordering with energies of 0.22 and 0.01 eV lower in comparison with random vacancy distribution were revealed. The transition between these ordered vacancy systems with the activation energy 0.34 eV can be a step in the ionic conductivity mechanism. The calculated ion charges, magnetic moments and electron density of states were used to analyze chemical bonding in the crystals. All calculated compounds have metal electronic conductivity.     

Vortex lattice symmetry in hexagonal superconductor CaAlSi probed by small angle neutron scattering

The small angle neutron scattering diffraction patterns from the flux line lattice state in the layered hexagonal superconductor CaAlSi are observed. Under an applied magnetic field (H) parallel to the crystalline c-axis, a hexagonal vortex structure is observed over the entire temperature/field regions. On the other hand, the vortex configuration under H∥a shows an ellipsoidal arrangement of the first-order Bragg peaks due to the anisotropic penetration depth. It was inferred from these results that the vortex state characterized by penetration depth and coherence length in CaAlSi may be described by that of anisotropic uniaxial superconductor using London theory.     

High-resolution photoemission study of FeSr2YCu2O7+δ

We have performed high-resolution photoemission spectroscopy (PES) on FeSr₂YCu₂O_7+δ, of which superconductivity of T_c=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δ_max) to be 10 meV at 15 K. This gap value gives 2Δ_max/k_BT_c∼5, suggesting a strong-coupling nature of superconductivity in FeSr₂YCu₂O_7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.     

Charge and spin dynamics in antiferromagnetic metallic phase of iron-based superconductors

The electronic states, charge dynamics, and spin dynamics in the antiferromagnetic metallic phase of iron-arsenide superconductors are investigated by mean-field calculations for a five-band Hubbard model. Taking into account the difference of observed magnetic moments between LaFeAsO (1111 system) and BaFe₂As₂ (122 system), we investigate the effect of the magnitude of the moments on band dispersion, optical conductivity, and dynamical spin susceptibility. We clarify how the magnitude affects on these quantities and predict different behaviors between the 1111 and 122 systems in the antiferromagnetic metallic phase.     

Al-doped ZnO: Electronic, electrical and structural properties

Changes in structural, electrical and electronic properties of zinc oxide (ZnO) due to Al doping are studied using a quantum-chemical approach based on the Hartree-Fock theory. A periodic supercell of 128 atoms has been exploited throughout the study. The atomic parameters for Zn atom were obtained by reproducing the main properties of ZnO crystal as well as the first three ionization potentials of Zn atom. The perturbation imposed by Al atom incorporation leads to the atomic relaxation, which is computed and discussed in detail. A novel effect of electron density redistribution between different atomic orbitals within the same atom has been found. This phenomenon influences atomic rearrangement near Al impurity. The Al doping generates a free electron in the conduction band, which can be considered as a large radius electron polaron increasing the n-type electrical conductivity in the crystal in agreement with the known experimental data. The obtained small increase in the band-gap width due to the impurity incorporation resolves existing experimental debates on this point.     

Defect properties of CuInSe2 and CuGaSe2

Using first-principles electronic structure theory, we have calculated defect formation energies and defect transition levels in CuInSe₂ and CuGaSe₂. We show that (i) it is easy to form Cu vacancies in CuInSe₂, and (ii) it is also relatively easy to form cation antisite defects (e.g. In_Cu) for this ternary compound. Consequently, defect pairs such as (2V_Cu+In_Cu) have a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn₅Se₈ and CuIn₃Se₅, is explained as a repeat of (2V_Cu+In_Cu) pairs in CuInSe₂. The very efficient p-type self-doping ability of CuInSe₂ is explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe₂ is explained in terms of an electronic passivation of the by . For CuGaSe₂, we find that (i) the native acceptor formation energies and transition energy levels are similar to that in CuInSe₂, but the donor formation energy is larger in CuGaSe₂. (ii) The Ga_Cu donor level in CuGaSe₂ is deeper than In_Cu donor level in CuInSe₂, therefore, Ga_Cu behaves as an electron trap in CuGaSe₂, even when it is passivated by V_Cu. We have also calculated the band alignment between the CPCs and CuInSe₂, showing that it could have significant effect on the solar cell performance.     

Geometric model of a fullerene molecule in the presence of Aharonov–Bohm flux

In this study, we describe a geometric model of a fullerene molecule with Ih symmetry. We combine the well known non-Abelian monopole approach and the geometric theory of defects, where every topological defect is associated with curvature and torsion, to describe a fullerene molecule. The geometric theory of defects in solids is used to consider the topological defects that allow this molecule to form and we apply a continuum formulation to describe this spherical geometry in the presence of an external Aharonov–Bohm flux. We solve a Dirac equation for this model and obtain the eigenvalues and eigenfunction of the Hamiltonian, and we obtain the persistent current for this model and show that it depends on the geometrical and topological properties of the fullerene.     

Site selection NMR study on electronic state in and near vortex core of YBa2Cu4O8

We report a site-selective ¹⁷O spin-lattice relaxation rate T₁⁻¹ in the vortex state of YBa₂Cu₄O₈. We found that T₁⁻¹ at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. Based on T₁⁻¹ in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.     

Ab initio study of 5d-shells Ir substitution for Fe-based SmOFe1−xIrxAs

The lattice and electronic properties for 5d-shells Ir substituted Fe-based superconductor SmOFe_1−xIr_xAs (x=0,0.2,0.25,0.3) are investigated based on the density functional theory (DFT) with a spin generalized gradient approximation SGGA+U method. The electronic density of states (DOS) of SmOFe_1−xIr_xAs is studied and well compared with the results of experimental X-ray photoemission spectroscopy (XPS). The calculation indicated that iridium substitution at the Fe site induced a modification of the FeAs₄ tetrahedron and suppressed the magnetic ordering corresponding to the Fe-3d, which may be the main cause of inducing superconductivity in Ir-doped SmOFeAs system.     

One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM

We have performed low-temperature scanning tunneling microscopy (STM) experiments on the cold-cleaved surface of YBa₂Cu₃O_y single crystals to study the nanoscale electronic order in high-T_c superconductors. STM images measured at low-bias voltage below ∼50 meV show the one-dimensional (1D) electronic modulation along the Cu-O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range ∼2a-4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO₂ plane.     

Quantum chemical study of sulfur and hydrogen at the Σ=5 BCC Fe grain boundary

We investigate the effect of segregated atoms on metal-metal bonding at a grain boundary (GB). The structure and electronic properties of S and H impurities in Fe Σ=5 (013) [100] symmetrical tilt GB are studied using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. A large cluster containing 196 Fe atoms is used to simulate the local environment of the boundary. The results show that impurities induce large relaxation in the GB and that the GB gives rise to an energetically favorable zone for the H and S accumulation. No S-H association is found and the deleterious effect of H is of much less important as compared with S.     

Observation of curious spiral-like features on spray pyrolysed Pb doped Hg-based HTSC films

The surface morphology of spray pyrolysed Hg(Pb)Ba₂Ca₂Cu₃O_8+δ films is investigated by scanning electron microscopy (SEM). By imaging the as-grown surfaces of the films, we have directly observed spiral-shaped growth terraces that emanate from screw dislocations. The high density of screw dislocations (∼10⁶ cm⁻²) observed by SEM micrographs of the films is expected to nucleate during the initial stages of growth due to the bi-phasic nature of the material, providing a continual supply of ledge incorporation sites for the depositing species. A possible mechanism, for the generation of these screw dislocations in terms of incoherent coalescence of growth fronts formed from Hg(Pb):1223 and Hg(Pb):1234 nuclei, is described.     

Superconductivity in new quaternary borocarbide system R–Re–B–C (R=Y, Gd, Tb and Lu)

We report here our observation of superconductivity in a new quaternary borocarbide system R–Re–B–C (R=Y, Gd, Tb and Lu). Samples of the nominal compositions RReBC exhibit superconductivity with T_c≈6 K in LuReBC which is high for an intermetallic. TbReBC and GdReBC exhibit superconductivity at ≈4 K. A magnetic transition is also observed in TbReBC around 30 K. From our measurements, we infer that superconductivity originates from a bulk quaternary superconducting phase in these materials.     

Bogoliubov angle and visualization of particle-hole mixture in superconductors

Superconducting excitations—Bogoliubov quasiparticles—are the quantum mechanical mixture of negatively charged electron (−e) and positively charged hole (+e). Depending on the applied voltage bias in scanning tunneling microscope (STM) one can sample the particle and hole content of such a superconducting excitation. Recent STM experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle-hole dualism of the quasiparticles. We call it a Bogoliubov angle. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We propose that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases locally. This Bogoliubov angle allows one to measure directly the energy and position dependent particle-hole admixture and therefore visualize robustness of superconducting state locally. It may also allow one to measure the particle-hole admixture of excitations in normal state above critical temperature and thus may be used to measure superconducting correlations in pseudogap state.