Strong-coupling effects in high-T c superconductors

It is conceivable that the high-T _c superconducting perovskites are conventional electronphonon superconductors. In this case one expects significant strong-coupling effects because of the unusually high ratiok _B T _c / of the order 0.1 and greater. We use a set of reasonable models for the Eliashberg function ² F() (which takes into account available information on the phonon spectra and which fit the measuredT _c 's) and calculate strong-coupling effects in the specific heatc _s (T)/T _c , the ratio ₀/k _B T _c , the critical fieldsH _c (T) andH _c2 (T) including Pauli limiting, and other measurable quantities. Strongcoupling corrections turn out to be in the range of 0 to about 100%, depending on the quantity of interest. We discuss the perspectives of using strong-coupling effects as indicators for conventional electron-phonon superconductivity in the new materials.     

Superconductivity-induced phonon self-energy effects in high-T c superconductors

The superconductivity-induced self-energy of phonons has been calculated in the lowest-order conserving approximation using conventional strong-coupling theory and also including scattering at nonmagnetic impurities. Results for the dependence of the shift and the width of phonons on temperature and scattering rates are presented and their sensitivity on the strength of the electron-phonon coupling and on impurities is pointed out. The theory is applied to optical data in YBa₂Cu₃O₇ using the experimental one-phonon density, the value ofT _c and [^*=0.25 as inputs. Neglecting anisotropies and the momentum dependence of the electron-phonon coupling the resulting strong-coupling model with 2.9 yields results which are in good agreement with the data, in particular, if impurity scattering is taken into account.     

Tunneling between dissimilar high-T(c) oxide superconductors

We report the first successful fabrication and measurement of high-T(c) heterojunctions with different oxide electrodes, YBa(2)Cu(3)O(7-y) and Bi(2)Sr(2)CaCu(2)O(y). Different kinds of junction characteristics are observable according to the magnitude of the tunnel resistance. With higher tunnel resistance, gap structures corresponding to two gaps are clearly observed, ensuring that the conventional tunneling scheme is also valid for this geometry. Peculiar behavior for the zero bias conductance peak is also observable. Josephson current is found to flow between these dissimilar superconductors.     

Van Hove singularity in Raman scattering spectra of high-T c superconductors

Experimental data are presented which supporting observation of an extended van Hove singularity in Raman spectra of high-T _c superconductors. Excitation of large-momentum electron-hole pairs is proposed as a possible mechanism of the scattering. Fiz. Tverd. Tela (St. Petersburg) 40, 27–31 (January 1998)     

Possible relevance of lattice effects in high-T c superconductors

The relevance of the lattice-mediated superconducting pairing in a system of quasilocalized polarons dressed by local lattice deformations is considered. The spin correlations are taken into account using thet-J model expressed in terms of holon and spin operators. The Holstein-like Hamiltonian for holons with the transport term depending on spin correlations is transformed by the generalized Lang-Firsov transformation which implies the spread of the charge and the deformations to the nearest neighbours. The analytical formula for the superconducting transition temperatureT _c is deduced using the assumption of an extremely narrow polaron band. The hole-concentration dependence ofT _c and the isotope exponent are discussed using the classical approximation for incommensurate spiral spin correlations. This work was supported by the Grant Agency of the Czech Republic, project No. 202/96/0864.     

On the phase transition d→(s±id) in high-T c superconductors

A model with an attractive potential in the s and d channels is studied. It is found that in a definite interval of the ratio of the s and d components of the potential the model undergoes a second-order phase transition from the d to the (s±id) state, with breaking of time-reversal symmetry. The transition temperature and the jump in the specific heat are calculated. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 8, 570–574 (25 October 1996)     

Robustness of the Van Hove scenario for high-T(c) superconductors

The pinning of the Fermi level to the Van Hove singularity and the formation of flat bands in the two-dimensional t-t' Hubbard model is investigated by the renormalization group technique. The "Van Hove" scenario of non-Fermi-liquid behavior for high-T(c) compounds can take place in a broad enough range of the hole concentrations. The results are in qualitative agreement with the recent angle-resolved photoemission spectroscopy data on La 2CuO (4).     

Optical and dc conductivities in high-T c superconductors: Spin-fluctuation scattering in the Emery model

On the basis of the Emery model for the CuO₂ plane, the optical conductivity and resistivity due to the inelastic scattering of oxygen holes by antiferromagnetic fluctuations of copper spins are calculated. For moderate hole doping, the electrical conductivity obeys a generalized Drude law. Using a phenomenological model for the dynamic spin susceptibility, the in-plane resistivity reveals a crossover from a quadratic to a linear temperature dependence at the scale of the spin-fluctuation energy. The frequency dependence of the scattering rate changes from a quadratic to a linear increase over a wide frequency range. The theory is compared with experiments on LSCO and YBCO compounds, where the spin dynamics is described within the model by Millis et al. A good quantitative agreement (in particular of the frequency-dependent scattering rate) with experiments is found. We conclude that the spin-fluctuation scattering may play a dominant role in the transport properties of Cu-oxide superconductors.     

Nickel impurity-induced enhancement of the pseudogap of cuprate high-T(c) superconductors

The influence of magnetic Ni and nonmagnetic Zn impurities on the normal-state pseudogap (PG) in the c-axis optical conductivity of (Sm,Nd)Ba(2){Cu(1-y)(Ni,Zn)(y)}(3)O(7-delta) crystals was studied by spectral ellipsometry. We find that these impurities, which strongly suppress superconductivity, have a profoundly different impact on the PG. Zn gives rise to a gradual and inhomogeneous PG suppression while Ni strongly enhances the PG. Our results challenge theories that relate the PG either to precursor superconductivity or to other phases with exotic order parameters, such as flux phase or d-density wave states, that should be suppressed by potential scattering. The apparent difference between magnetic and nonmagnetic impurities instead points towards an important role of magnetic correlations in the PG state.     

What does NMR tell us about the electronic state of high-T c superconductors?

An introduction to the models which are usually applied, when interpreting the NMR (nuclear magnetic resonance) parameter Knight shift (K _s) and spin-lattice relaxation rate 1/T ₁ in the normal and the superconducting state of high-temperature superconductors, is given. The different hyperfine interaction parameters involved, as well as the static and dynamic susceptibility χ(q,ω) will be discussed. I will point at those highlights as antiferromagnetic correlations, spin gap and vortex lattice dynamics which have emerged from the analysis of the NMR data.     

Spin and charge order around vortices and impurities in high-T(c) superconductors

A comparative study is made for the spin and charge structure around superconducting vortices and unitary impurities, by solving self-consistently an effective Hamiltonian including interactions for both antiferromagnetic spin-density wave (SDW) and d-wave superconducting orderings. Around vortices, we show the induction of an SDW two-dimensionally modulated with a period of eight lattice constants (8a(0)) and an associated charge-density wave (CDW) with a period of 4a(0), which explains very well recent experimental observations. In the case of unitary impurities, an SDW modulation with identical periodicity, but without an associated CDW, is also predicted.     

On the optical conductivity of high-T c superconductors

We have solved the self-consistent equation for self-energy of a hole in a quantum antiferromagnet. The optical conductivity is estimated. The results are in good agreement with experiments and numerical simulations.     

Tunneling and Andreev spectroscopies of high-T c superconductors

Tunneling spectroscopic studies of high-temperature superconductors (HTSC) are overviewed. It is demonstrated, in particular, that the superconducting gaps determined from Andreev reflections and tunneling current-voltage characteristics coincide at all doping levels. Facts are presented evidencing the strong electron-phonon interaction in the HTSC systems. The nature of the so-called pseudogap, which is observed in some tunneling experiments, is briefly discussed.     

Synthesis and properties of Hg-based high-T c superconductors

Il Nuovo Cimento D - The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures...     

New mean-field theory of the tt't'J model applied to high-T(c) superconductors

We introduce a new mean-field approach to the tt't'J model that incorporates both electron-like quasiparticle and spinon excitations as suggested by some experiments and numerical studies. It leads to a mean-field phase diagram which is consistent with that of hole and electron doped cuprates. Moreover, it provides a framework to describe the observed evolution of the electron spectral function from the undoped insulator to the overdoped Fermi metal for both hole and electron doping. The theory also provides a new non-BCS mechanism leading to superconductivity.     

Nernst coefficient and magnetoresistance in high-T(c) superconductors: the role of superconducting fluctuations

In high-T(c) cuprates, the Nernst coefficient (nu) as well as the magnetoresistance (Deltarho/rho) increases drastically below the pseudogap temperature, T(*), which attracts much attention as a key phenomenon in the pseudogap region. We study these transport phenomena in terms of the fluctuation-exchange+T-matrix approximation. In this present theory, the d-wave superconducting (SC) fluctuations, which are mediated by antiferromagnetic (AF) correlations, become dominant below T(*). We especially investigate the vertex corrections both for the charge current and the heat one to keep the conservation laws. As a result, the mysterious behaviors of nu and Deltarho/rho are naturally explained as the reflection of the enhancement of the SC fluctuation, without assuming thermally excited vortices. The present result suggests that the pseudogap phenomena are well described in terms of the Fermi liquid with AF and SC fluctuations.     

BCS-like Bogoliubov quasiparticles in high-T(c) superconductors observed by angle-resolved photoemission spectroscopy

We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-T(c) cuprate Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta). We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below E(F) as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.