Miodrag L. Kulic

ANISOTROPIC IMPURITIES IN ANISOTROPIC SUPERCONDUCTORS

Physical properties of anisotropic superconductors like the critical temperature and others depend sensitively on the electron mean free path. The sensitivity to impurity scattering and the resulting anomalies are considered a characteristic feature of strongly anisotropic pairing. These anomalies are usually analyzed in terms of s-wave impurity scattering which leads to universal pair breaking effects depending on only two scattering parameters, the mean free path and the impurity cross section. We investigate here corrections coming from anisotropies in the scattering cross section, and find not only quantitative but also qualitative deviations from universal s-wave isotropic pairbreaking. The properties we study are the transition temperature, the density of states, quasiparticle bound states at impurities, and pinning of flux lines by impurities.

Statics and dynamics of charge fluctuations in the t-J model.

The equation for the charge vertex {gamma} of the {ital t}-{ital J} model is derived and solved in leading order of a 1/{ital N} expansion, working directly in terms of Hubbard operators. Various quantities which depend crucially on {gamma} are then calculated, such as the lifetime and the transport lifetime of electrons due to a charge coupling to other degrees of freedom and the charge-charge correlation function. Our results show that the static screening of charges and the dynamics of charge fluctuations depend only weakly on {ital J} and are mainly determined by the constraint of having no double occupancies of sites. {copyright} {ital 1996 The American Physical Society.}

Why is d-wave pairing in hts robust in the presence of impurities?

Abstract In the recent theory of strong correlations by [1] – [3] it was shown that by lowering doping concentration a forward peak is developed in the charge scattering channel. Physically, this means that short-range impurity scattering potential is transformed into long-range one. Accordingly, near the optimal doping the nonmagnetic scattering is pronounced in the d -channel and d -wave pairing is robust against defects and impurities. It is reflected in a decrease of the slope of the critical temperature T c ( Γ s ) at Γ s → 0 by factor (1 − β ), where β is the scattering anisotropy parameter and Γ s is an average scattering rate. For large doping, β is small [1], [2] and d -wave pairing loses its robustness. The theory is generally formulated for the bi-layer model by including: (1) intra- and inter-plane pairing; (2) intra- and inter-plane impurities.

Self-assembly of colloidal superstructures in coherently fluctuating fields.

From microscopic fluid clusters to macroscopic droplets, the structure of fluids is governed by the van der Waals force, a force that acts between polarizable objects. In this Letter, we derive a general theory that describes the nonequilibrium counterpart to the van der Waals force, which emerges in spatially coherently fluctuating electromagnetic fields. We describe the formation of a novel and complex hierarchy of self-organized morphologies in magnetic and dielectric colloid systems. Most striking among these morphologies are dipolar foams--colloidal superstructures that swell against gravity and display a high sensitivity to the applied field. We discuss the dominance of many-body forces and derive the equation of state for a material formed by the coherent van der Waals force. Our theory is applied to recent experiments in paramagnetic colloidal systems and a new experiment is suggested to test the theory.

Theory of interplay of nuclear magnetism and superconductivity in AuIn{sub 2}

The recently reported [S. Rehmann, T. Herrmannsd{umlt o}rfer, and F. Pobel, Phys. Rev. Lett. {bold 78}, 1122 (1997)] coexistence of a nuclear magnetic order, with the critical temperature T{sub M}=35 {mu}K, and superconductivity, with the critical temperature T{sub S}=207 mK, in AuIn{sub 2} is studied theoretically. It is shown that superconducting (S) electrons and localized nuclear magnetic moments interact predominantly via the contact hyperfine interaction, giving rise to a spiral or domainlike magnetic order in the superconducting phase depending on the strength of magnetic anisotropy. In clean samples (l{gt}{xi}{sub 0}) of AuIn{sub 2} the oscillatory magnetic order should produce a line of nodes in the quasiparticle spectrum of S electrons. The critical field H{sub c}(T=0) in the coexistence phase is reduced by a factor of 2 with respect to its bare value. {copyright} {ital 1997} {ital The American Physical Society}

Novel 1/N Expansion for Self-Energy and Correlation Functions of the Hubbard Model

The novel slave-free (i.e. without slave bosons and fermions) approach to the problem of strong correlations is used in studying the Hubbard (t-J) model, where the self-energy is expressed in terms of charge and spin vertex functions of Hubbard operators. A systematic 1/N expansion (N is the number of spin components) of the self-energy is carried out. In O(1) it gives the same quasiparticle spectrum as the slave boson theory. The next order O(1/N) of the self-energy Σ(k, ω) is calculated exactly by solving analytically the integral equations for the charge and spin vertices. As an example for the feasibility of the proposed method explicit results for Σ(k, ω) are given in the case J=0. Dealing only with observable quantities our method is transparent and easily applicable to models which exclude double occupancy on lattice sites, such as the Anderson model and the Emery model for high temperature superconductors.

NMR relaxation rates in superconductors with (in)commensurate magnetic order a model inspired by high-Tc superconductors

Abstract We propose a model for s wave superconductors (the antiferromagnetic superconductor (AFS) model) in the presence of nonuniform (in)commensurate magnetic order and analyse the NMR properties of this model. It is found that the superconducting quasiparticle spectrum has nodes in the gap when the exchange energy h is larger than the superconducting order parameter Δ. This leads to a power-law dependence of vaious quantities on temperature and frequency. By using the RPA susceptibility X ij ( k ,ω) the longitudinal and transverse NMR relaxation rates 1 T 1 and 1 T 2 , as well as the Knight shift K ( T ) are calculated in the AFS model. As an example we study an HTS-like system with a t−t′ quasiparticle spectrum. The results obtained are compared with experimental results on YBa 2 Cu 3 O 6+ x . A reasonable fit for K ( T ) is obtained for the ratio 2Δ T c = 6−8 and h ≈ (0.1−0.2) t . At low T one has 1 T 1 ∼ T 3 for both 17 O and 63 Cu, while (1) the coherence Hebel-Slichter peak is absent, (2) the slope at T c fits rather well by the quasiparticle damping rate T=0.8T c ( T T c ) 3 . The temperature dependence of the rate 1 T 2 is similar to case of the d wave pairing. The proposed AFS theory is general and it is applicable to systems where the coexistence of (in)commensurate magnetic order and superconductivity takes place: magnetic superconductors, heavy-fermion superconductors, and rare-earth nickel boride-carbides RNi 2 B 2 C.

Theoretical analysis of NMR experiments in normal and superconducting states of high-Tc superconductors

Abstract The Knight shift and T 1 - and T 2 -rates of YBa 2 Cu 3 O 6+ x in the normal and superconducting state are modeled by calculating the magnetic susceptibility in the bi-layer Hubbard model within various approximations. An optimal set of parameters (OSP) is found in the RPA approximation which fits experiments on YBCO for optimal and nearly optimal doping. The analysis of the self-consistent FLEX approximation for the particle self-energy and susceptibility shows that the latter is renormalized quantitatively but not qualitatively. The differences in the oxygen and copper T 1 -rates are explained by using the OSP parameters and assuming the finite hyperfine coupling C ′ between 17 O and next-nearest neighboring Cu spins. The numerical analysis of T 1 −1 and T 2 −1 and the ratio 63 T 1 ab −1 / 63 T 1 c −1 in the superconducting state supports strongly the idea of d -wave pairing in YBa 2 Cu 3 O 7 with much stronger intraplane rather than interplane pairing. It is also shown that the simple RPA or FLEX approximations are inadequate in explaining NMR data in underdoped YBCO systems, where antiferromagnetic fluctuations are very pronounced.

Electron-phonon coupling in the presence of strong correlations

Abstract The leading term for the electronic self-energy in a 1/N expansion due to an on-site electron-phonon interaction is calculated for Hubbard models with infinite U. The resulting modifications of the Eliashberg function are discussed and calculated for a square lattice in the continuum approximation.

Percolation magnetoresistance in ferromagnetic superconductors

Abstract It is shown that the magnetoresistance (as a function of magnetic field H) in polycrystalline magnetic superconductors has the percolation character which is the consequence of the anisotropy of magnetic susceptibility. The magnetoresistance Rm(H), and the upper critical field Hc2(T) of ErRh4B4 are evaluated and compared with the experimental data.