Quasi-One-Dimensional Superconductors: From Weak to Strong Magnetic Field

We discuss the possible existence of a superconducting phase at high magnetic field in organic quasi-one-dimensional conductors. We consider in particular (i) the formation of a Larkin–Ovchinnikov–Fulde–Ferrell state, (ii) the role of a temperature-induced dimensional crossover occurring when the transverse coherence length _z (T) becomes of the order of the lattice spacing, and (iii) the effect of a magnetic-field-induced dimensional crossover resulting from the localization of the wave functions at high magnetic field. In the case of singlet spin pairing, only the combination of (i) and (iii) yields a picture consistent with recent experiments in the Bechgaard salts showing the existence of a high-field superconducting phase. We point out that the vortex lattice is expected to exhibit unusual characteristics at high magnetic field.     

Magnetic Resonance in ZnGeP2 and (Zn,Mn)GeP2

Electron paramagnetic (EPR) and ferromagnetic resonance (FMR) have been studied in the system of (Zn,Mn)GeP₂ ferromagnetic layer grown on undoped ZnGeP₂ single crystal. Strong FMR signals are registered in the wide temperature range up to room temperature. EPR and photo-EPR of intrinsic defects are observed in ZnGeP₂ substrate. EPR spectra characteristic of Mn²⁺ ions on Zn²⁺ sites in the bulk appear after the growth of the ferromagnetic layer on ZnGeP₂ crystal indicating the efficient Mn-diffusion into the bulk crystal by the annealing treatments.     

Atomic Scale Elastic Textures Coupled to Electrons in Superconductors

We present an atomic scale theory of lattice distortions using strain-related variables and their constraint equations. Our approach connects constrained atomic length scale variations to continuum elasticity and can describe elasticity at all length scales. We apply the general approach to a two-dimensional square lattice with a monatomic basis, and find the atomic scale elastic textures around a structural domain wall and a single defect, as exemplar textures. We clarify the microscopic origin of anisotropic gradient terms, some of which are included phenomenologically in Landau–Ginzburg theory. The obtained elastic textures are used to investigate the effects of elasticity-driven lattice deformation on the nanoscale electronic structure in superconductor by solving the Bogliubov–de Gennes equations with the electronic degrees of freedom coupled to the lattice ones. It is shown that the order parameter is depressed in the regions where the lattice deformation takes place. The calculated local density of states suggests the electronic structure is strongly modulated as a response to the lattice deformation—the elasticity propagates the electronic response over long distances. In particular, the trapping of low-lying quasiparticle states around the defects is possible. These predictions could be directly tested by STM experiments in superconducting materials.     

Effect of the External Magnetic Field on the Resonance Scattering in Cuprate Superconductors

Within the kinetic energy driven superconducting mechanism, the effect of a uniform external magnetic field on the resonance scattering in cuprate superconductors in the superconducting state is studied. It is shown that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying a large enough external magnetic field. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime.     

Tunnelling in Organic Superconductors

We present a simple scheme for computing the full current-voltage characteristics for tunnelling experiments within the framework of the non-equilibrium Keldysh Green function formalism. This formalism is flexible enough to address different pairing symmetries combined with magnetic fields at arbitrary bias voltages. We show how to apply these results to probe for the symmetry of the superconducting order parameter in the Bechgaard salts using tunnelling experiments.     

Layered Superconductors in an External Magnetic Field

Presence of localized magnetic moments and inhomogeneites leads to an unconventional behavior of the upper critical field for layered superconductors. In addition, one can observe a large diamagnetic moment at H H _c2 and in the normal resistive state.     

Real and Marginal Isotope Effects in Cuprate Superconductors

We critically review recent and earlier results on isotope effects in cuprate superconductors and emphasize that the sample preparation and the isotope exchange and back exchange are crucial in understanding and interpreting the data. Only extremely careful preparation techniques yield reliable results and permit differentiation between real isotope effects and marginal ones. The former are substantial and highlight the lattice vibrational importance in cuprate superconductors.     

Level Statistics of Quasiparticles in Disordered Two-Dimensional Superconductors

We study level statistics in physical systems consisting of noninteracting quasiparticles in disordered superconductors that have neither time-reversal nor spin-rotation invariance. These systems are representatives of class D within the recent classification scheme of random matrix ensembles (RME). Different phase diagrams are exposed, which strongly depend on specific models of disorder. In the metallic regime the nearest neighbor spacing distribution P(s) in different models reproduces (with high accuracy) the Wigner surmise for the Gaussian unitary ensemble (GUE) expected in the presence of magnetic field whereas in the localized regime level repulsion is suppressed, reflecting only basic symmetries of the system and ignoring particle-hole symmetries.     

rf-SQUID Effect in Bulk Quaternary Borocarbide Superconductors

This paper reports observation of rf-SQUID effect due to natural grain boundary junctions in YNi₂B₂C (T _c 15.5 K), ErNi₂B₂C (T _c 11.5 K, T _N 6.5 K), and DyNi₂B₂C (T _c 6.5 K, T _N 11K), LuNi₂B₂C (T _c 16.5 K) and YPd₅B₃C_0.35 (T _c 23 K) bulk borocarbide superconductors. The observation of rf-SQUID effect due to natural grain boundary junctions in all the five borocarbide superconductors clearly indicates that natural grain boundary junctions in these superconductors behave as Josephson junctions, and this behavior of natural grain boundary junctions in the quaternary borocarbides appear to be universal in this class of superconductors. Observation of rf-SQUID effect in magnetic borocarbide superconductors (ErNi₂B₂C and DyNi₂B₂C) at 4.2 K also indicates that antiferromagnetic ordering does not destroy SQUID effect, as T _N > 4.2 K in both cases. Rapid increase in flux noise due to thermally activated flux hopping has been observed in all the SQUIDs as temperature approaches T _c.     

Thermodynamic Properties of High-Temperature Superconductors in the Pseudogap Regime

Experimental data extracted from thermodynamic measurements in underdoped high-temperature superconductors show significant deviations from the results predicted by the BCS theory. In these particular compounds, the anomalies observed in the superconducting phase are associated also to the presence of a pseudogap in their normal state. On the basis of a simple model, which treats the pseudogap phenomenologically, we performed a Ginzburg–Landau expansion to extract the main properties of the superconducting phase. Our results for the specific-heat-coefficient jump at the transition point, (T _c), coherence length, T(T), and penetration depth, (T), qualitatively recover the experimental data.     

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.     

The Effect of the Phase Separation in the Phase Diagram of Cuprate Superconductors

We show that the main features of the cuprates superconductors phase diagram can be derived considering the disorder as a key property of these materials. Our basic point is that the high pseudogap line is an onset of phase separation which generates compounds made up of regions with distinct doping levels. We calculate how this continuous temperature dependent phase separation process occurs in high critical temperature superconductors (HTSC) using the Cahn–Hilliard approach. Calculations with the BdG approach and an analysis of the local density of states (LDOS), yield good agreement with the measured values of T ^* and T _c . This work has been partially supported by CAPES, CNPq and CNPq-Faperj Pronex E-26/171.168/2003.     

Calculation of the Critical Field and the Nernst Effect in Cuprate Superconductors

A recent experiment which measured the Nernst effect, the diamagnetic signal and the H _c2 field was interpreted as a support to the scenario which the pseudogap has the pair condensate without long-range phase coherence. We present here calculations which qualitatively reproduces the onset of the Nernst signal temperature T _ν (ρ) and T _c (ρ) as function of the doping level ρ. Together with our previous calculations to the magnetization and H _c2, we conclude that the phase separation scenario supports also the new Nernst effect experiments. This work has been partially supported by CAPES and CNPq.     

On The Theory of Isotope-effect in the d-wave Superconductors

The isotope-effect in a two-dimensional model of the d-wave phonon-exchange superconductor is studied. It is shown that the mean-field critical temperature, the Berezinskii–Kosterlitz–Thouless critical temperature and the superconducting gap strongly depend on the phonon frequency only in some range of the model parameters. These dependencies have different from the BCS theory forms. A qualitative comparison of the results with experimental data on some high-temperature superconductors is made. In particular, it is shown that the isotope-effect is significant in the case of strong electron–phonon coupling, when the phonon frequency is small compared to the free electron bandwidth.     

Admixture of an s-Wave Component to the d-Wave Gap Symmetry in High-Temperature Superconductors

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa₂Cu₃O_6.56, HoBa₂Cu₄O₈, and La_1.81Sr_0.15Ho_0.04- CuO₄ are reviewed. By this bulk-sensitive technique, information on the gap function is obtained from the relaxation behavior of crystal-field transitions associated with the Ho³⁺ ions which sit as local probes close to the superconducting copper-oxide planes. The relaxation data exhibit a peculiar change from a convex to a concave shape between the superconducting transition temperature T _c and the pseudogap temperature T ^* which can only be modeled satisfactorily if the gap function of predominantly d-wave symmetry includes an s-wave component of the order of 20–25%, independent of the doping level. Moreover, our results are compatible with an unusual temperature dependence of the gap function in the pseudogap region (T _c≤T≤T ^*), i.e., a break up of the Fermi surface into disconnected arcs.     

Pressure Effect in High-Temperature Superconductors and Fullerides

The explanation for the pressure effect in high-temperature superconductors and fullerides is offered. Besides the dependence of the pressure derivative of the critical temperature T _c on doping, the direct dependence of T _c on pressure and the universal dependence of the relative change of T _c ^max–T _c with pressure are obtained for high-temperature superconductors. The unity of the model of the pressure effect in high-temperature superconductors and fullerides is justified. The dependence of T _c on the lattice constant and the connection between the pressure effect and the chemical pressure effect in fullerides are discussed.     

Microwave Nonlinearities in High-Tc Superconductors: The Truth Is out There

This paper discusses some of the major experimental features of microwave nonlinearity in high temperature superconductors, both intrinsic and extrinsic. The case is made for solving the problem of extrinsic nonlinearity through the use of localized measurements of microwave surface impedance and electromagnetic fields. Along these lines, a brief introduction is given to our work on scanning near-field microwave microscopy.     

Pseudo-Gap Phenomenon in the Sn-Doped Mercury-Based Cuprate Superconductors

The Sn-doped mercury-based copper oxide high temperature superconductors, with nominal composition (Hg_{1 – x} Sn _x )Ba₂Ca₂Cu₃O_{8 +} , have been successfully fabricated. These samples were used to study the pseudo-gap phenomenon in their normal phase. By means of ln[1/(T) – 1/ _N (T)] versus 1/T plots we found three characteristic temperatures T ^*, T _S, and T _F, which describe the role of pseudo-gap phenomenon. From the stated plots, we are able to estimate the magnitude of the pseudo-gap for our samples. The pseudo-gap _PG changes into the spin pairing dipolar superconducting gap _SPDS, when the Fermi kinetic energy shrinks to zero at the critical temperature T _c. It is proved that the spin pairing dipolar superconducting gap , where, _T being the total superconducting gap of the cuprate superconductors, which has the value determined by . It is also proved that the spin pairing dipolar superconducting gap is smaller than the dipolar pseudo-gap, _PG.     

Flicker Noise and Telegraph Noise in High-Temperature Superconductors

The mechanism capable to explain the peculiarities of both the telegraph noise and flicker noise in high-temperature superconductors (HTS)—in particular, their common behavior—is proposed. It bases on the impurity mechanism of high-temperature superconductivity and the suggestion that an origin of the flicker noise in most cases is a process that can be described by the equation of diffusion type, with a medium discreteness taken into account. In the transition region, the flicker noise and telegraph noise are explained by fluctuations of the concentration (x _ef ) of the two-level systems each having a localized electron pair. These fluctuations disappear according to the diffusion equation. The telegraph noise is considered as a result of opening and closing superconducting and nonsuperconducting channels due to the x _ef fluctuations. In the normal state, the flicker noise is explained by concentration fluctuations of different impurities that change the charge carrier mobility.