Superconducting gap in the hubbard model and the two-gap energy scales of high-T{c} cuprate superconductors

Recent experiments (angle-resolved photoemission spectroscopy and Raman) suggest the presence of two distinct energy gaps in high-temperature superconductors (HTSC), exhibiting different doping dependences. The results of a variational cluster approach to the superconducting state of the two-dimensional Hubbard model are presented which show that this model qualitatively describes this gap dichotomy. The antinodal gap increases with less doping, a behavior long considered as reflecting the general gap behavior of the HTSC. On the other hand, the near-nodal gap does even slightly decrease with underdoping. An explanation of this unexpected behavior is given which emphasizes the crucial role of spin fluctuations in the pairing mechanism.     

Possible competing order-induced Fermi arcs in cuprate superconductors

We investigate the scenario of competing order (CO) induced Fermi arcs and pseudogap in cuprate superconductors. For hole-type cuprates, both phenomena as functions of temperature and doping level can be accounted for if the CO vanishes at $T^1$ above the superconducting transition $T_c$ and the CO wave-vector Q is parallel to the antinodal direction. In contrast, the absence of these phenomena and the non-monotonic $d$-wave gap in electron-type cuprates may be attributed to $T^1<T_c$ and a CO wave-vector Q parallel to the nodal direction.     

Fermi arcs in the superconducting clustered state for underdoped cuprate superconductors

The one-particle spectral function of a state formed by superconducting (SC) clusters is studied via Monte Carlo techniques. The clusters have similar SC amplitudes but randomly distributed phases. This state is stabilized by competition with the antiferromagnetism expected to be present in the cuprates and after quenched disorder is introduced. A Fermi surface composed of disconnected segments, i.e., Fermi arcs, is observed between the critical temperature T_(c) and the cluster formation temperature scale T*.     

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.     

Polaronic behavior of undoped high-T(c) cuprate superconductors from angle-resolved photoemission spectra

We present angle-resolved photoemission spectroscopy (ARPES) data on undoped La2CuO4, indicating polaronic coupling between bosons and charge carriers. Using a shell model, we calculate the electron-phonon coupling and find that it is strong enough to give self-trapped polarons. We develop an efficient method for calculating ARPES spectra in undoped systems. Using the calculated couplings, we find the width of the phonon sideband in good agreement with experiment. We analyze reasons for the observed dependence of the width on the binding energy.     

Magnetization of high-T c superconductors

We have tried to understand the field dependence of magnetization of high temperature superconductors in the light of phenomenological theory. Especially, the field dependence of dM/d lnB of polycrystalline Bi(2212) is understood by incorporating the overlap of vortices in the London theory.     

Flux diffusion in high-T c superconductors

In type-II superconductors in the flux flow (J J _c ), flux creep (J _c J _c ), and thermally activated flux flow (TAFF) (J J _c ) regimes the inductionB(r,t), averaged over several penetration depths , in general follows from a nonlinear equation of motion into which enter the nonlinear resistivities (B, J ,T) caused by flux motion and (B, J ,T) caused by other dissipative processes.J andJ are the current densities perpendicular and parallel toB,B=|B|, andT is the temperature. For flux flow and TAFF in isotropic superconductors with weak relative spatial variation ofB, this equation reduces to the diffusion equation plus a correction term which vanishes whenJ =0 (this means B××B=0) or when – = 0 (isotropic normal conductor). When this diffusion equation holds the material anisotropy may be accounted for by a tensorial . The response of a superconductor to an applied current or to a change of the applied magnetic field is considered for various geometries. Such perturbations affect only a surface layer of thickness where a shielding current flows which pulls at the flux lines; the resulting deformation of the vortex lattice diffuses into the interior until a new equilibrium or a new stationary state is reached. The a.c. response, in particular the frequency with maximum damping, depends thus on the geometry and size of the superconductor.     

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.     

Magnetostriction of several high-T c superconductors

We report on magnetostriction measurements of superconducting and nonsuperconducting YBa₂Cu₃O_7– and of two Bi samples with the nominal compositions Bi₂CaSr₂Cu₂O _x and Bi_0.8Pb_0.2Sr_0.8CaCu_1.5O _x . Both types of high temperature superconductors show a nearly identical strongly hysteretic field dependence of the magnetostriction (MS) (l/l+(2–6)*10^–6 at 5 T and 1.5 K). In both cases the MS is a linear function of field in the region 1–5 T, which we explain by striction due to surface currents. Between 1.5 and 35 K the hysteresis of the MS decreases strongly with increasing temperature, which implies a decrease of the pinning force. The MS of quenched nonsuperconducting YBa₂Cu₃O_7– is at least two orders of magnitude smaller than that of the superconductor.     

Possible double-pairing effect in high-T c superconductors

Based on a reexamination of experimental facts a simple model called Double-Pairing Superconductivity is set up to describe the high temperature layered superconductors (HTLS). A mean-field scenario is carried out to investigate the model Hamiltonian. Within our model some of the exotic properties of HTLS may be understood, at least qualitatively.     

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.     

Critical temperature of low-dimensional high-T c superconductors

We generalized the Coherent Potential Approximation (CPA) method for the calculation of the critical temperature of superconductors of lower dimensionality. Two dimensional (2d) and one-dimensional (1d) models have been considered. The critical temperature is calculated as function of the electronic concentration and of the position of the Fermi level. The critical temperature can be expressed with either of these two parameters. We also calculate the dependence ofT _c on an additional parameter which is the measure of the interplane coupling.     

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...     

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.     

Magnetic ordering in57Fe-doped high-T c superconductors

The high-temperature superconductor, Tl₂CaBa₂(Cu_1?x Fe _x )₂O_8+δ (the 2122 compound), has been investigated by a number of techniques, including X-ray diffraction, resistance and ac susceptibility measurements, and Mössbauer spectroscopy. The procedures followed to make close to single-phase samples are described. The decrease in the critical temperature for superconductivity,T _c , is less than for the iron-doped 123 compounds. The Mössbauer spectra at 77 K and above consist of an asymmetric doublet. Below about 10 K magnetic hyperfine splitting occurs; relaxation effects are still present at 2.3 K. The spectra can be fitted with two overlapping patterns. Their origin is discussed: comparisons are made with other high-T _c superconductors.     

Fermi-condensate quantum phase transition in high-T c superconductors

The effect of a quantum phase transition associated with the appearance of fermionic condensation in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this Fermi-condensate quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses M _FC * and M _L *. The M _FC * mass characterizes the spectrum up to the binding energy E ₀ , which is of the order of the superconducting gap in magnitude, and determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, E ₀ ?4 T. These results are used to explain some remarkable properties of high-T _c superconductors and are in good agreement with recent experimental data.