Repositório RCAAP

The long-range bond-percolation problem, on a linear chain (d = 1), in the presence of diluted sites (with an occupancy probability p s for an active site) is studied by means of a Monte Carlo simulation. The occupancy probability for a bond between two active sites i and j, separated by a distance r ij is given by p ij = <img width=32 height=32 id="_x0000_i1026" src="../../img/revistas/bjp/v33n3/a32img01.gif" align=absbottom>, where p represents the usual occupancy probability between nearest-neighbor sites. This model allows one to analyse the competition between long-range bonds (which enhance percolation) and diluted sites (which weaken percolation). By varying the parameter a (a > 0), one may find a crossover between a nonextensive regime and an extensive regime; in particular, the cases a = 0 and a ® ¥ represent, respectively, two well-known limits, namely, the mean-field (infinite-range bonds) and first-neighbor-bond limits. The percolation order parameter, P¥, was investigated numerically for different values of a and p s. Two characteristic values of a were found, which depend on the site-occupancy probability p s, namely, a1(p s) and a2(p s) (a2(p s) > a1(p s) > 0). The parameter P¥ equals unit, "p > 0, for 0 < a < a1(p s) and vanishes, "p < 1, for a > a2(p s). In the interval a1(p s) < a < a2(p s), the parameter P¥ displays a familiar behavior, i.e., 0 for p < p c(a) and finite otherwise. It is shown that both a1(p s) and a2(p s) decrease with the inclusion of diluted sites. For a fixed p s, it is shown that a convenient variable, p* º p*(p, a, N), may be defined in such a way that plots of P¥ versus p* collapse for different sizes and values of a in the nonextensive regime.

For certain orientations of Josephson junctions between two p x-wave or two d-wave superconductors, the subgap Andreev states produce a 4pi-periodic relation between the Josephson current I and the phase difference <FONT FACE=Symbol>f:</FONT> I <FONT FACE=Symbol>µ</FONT> sin( <FONT FACE=Symbol>f/</FONT>2). Consequently, the ac Josephson current has the fractional frequency eV/$\hbar $, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current is carried by single electrons, rather than by Cooper pairs.

We study the structure of resonance states localized around nonmagnetic impurities in the CuO2 planes of the cuprate superconductors within a potential scattering formalism. In particular we show that strong quantum interference effects arise between several impurities. This interference can be utilized to distinguish the dwave superconducting state from the phase with d-density wave order. This is important if the origin of the pseudo-gap state in the underdoped regime of the High Tc superconductors is caused by preformed Cooper pairs or staggered orbital currents. Furthermore impurity interference can be utilized to reveal subdominant superconducting order parameters and to pose further constraints on the potential scattering scenario.

The effect of nonlocal electrodynamics is considered in finite samples. It is found for such samples that nonlocality modifies not only the in-plane penetration depth lambda||, as predicted by Kosztin and Leggett, but also the out-of-plane penetration depth lambda<FONT FACE=Symbol>^</FONT>. The Sr2RuO4 data are reinterpreted taking into account the contribution from lambda<FONT FACE=Symbol>^</FONT>.

Numerical simulations based on Monte Carlo dynamics are used to investigate the resistivity behavior of granular superconductors containing a random distribution of pi junctions, as in superconducting materials with d-wave symmetry. The presence of pi junctions leads to quenched in circulating currents (chiralities) and to chiral glass behavior at low temperatures, even without an external magnetic field. An XY spin glass model in the phase representation is used to determine the current-voltage characteristics and critical exponents of the resistivity transition. In two dimensions, the linear resistivity is nonzero at finite temperatures and the dynamic scaling analysis of the nonlinear resistivity is consistent with a phase transition at zero temperature. In three dimensions, we find a transition at finite temperatures below which the linear resistivity vanishes and the corresponding critical exponents are determined from the scaling analysis. The results are in good agreement with Langevin simulations in the phase representation. The dynamic exponent z is significantly different from previous results obtained in the vortex representation.

We have investigated the field-temperature (H - T) diagram of the superconducting and pseudogapped states of Bi2Sr2CaCu2O8+y over a wide range of hole doping (0:10 < p < 0:225). Using interlayer tunneling transport in magnetic fields up to 60 T to probe the density-of states (DOS) depletion at low excitation energies we mapped the pseudogap closing field Hpg. We found that Hpg and the pseudogap onset temperature T* are related via a Zeeman relation gmB Hpg ~ kB T*, irrespective of whether the magnetic field is applied along the c-axis or parallel to CuO2 planes. In contrast to large anisotropy of the superconducting state, the field anisotropy of Hpg is due solely to the g-factor. Our findings indicate that the pseudogap is of singlet-spin origin, consistent with models based on doped Mott insulator.

Within the quasiclassical approximation we have studied the thermodynamics and the thermal conductivity in the vortex state in nodal superconductors (sc). Recent angle dependent magnetothermal conductivity results indicate a gap function delta(k) corresponding to f- wave sc in Sr2RuO4 and d- wave sc in CeCoIn5 and k-(ET)2Cu(NCS)2 respectively. More recently it is shown that delta(k) in both YNi2B2C and PrOs4Sb12 have point nodes described by hybrid s+g wave gap function.

The widely held notion that high-temperature superconductivity originates in the cuprate-planes is proven to be faulty. In the cuprates such as YBa2Cu3O7, we argue that the superconductivity resides in the BaO layers. This superconductivity is s-wave, not d-wave, in the bulk. The trio of ruthenate compounds, doped Sr2YRuO6, GdSr2Cu2RuO8, and Gd2-zCe zSr2Cu2RuO10 all superconduct in their SrO layers, which is why they have almost the same ~49 K onset temperatures for superconductivity.

We have performed a systematic study of the structural, transport, and magnetic properties of polycrystalline samples of the magnetic superconductors Ru1-xIr xSr2GdCu2O8; x = 0, 0.02, 0.05, 0.10, and 0.15. The samples were prepared by solid state reaction and sintered at 1060 ºC for 72 h under O2 flow. X-ray diffraction analysis shows that all samples are nearly single phase and that the lattice parameters are independent of Ir content. Transport properties measurements revealed that the Ru substitution by Ir results in a decrease of Tc,onset from ~50 K (x = 0) to ~30 K (x = 0.10). Further addition of Ir (x > 0.10) causes an evolution from metallic to nonmetallic behavior of r(T). We have also found that magnetic order develops in the undoped Ru-1212 materials near T M ~ 130 K. This temperature decreases linearly with increasing Ir content at the rate of ~ -1.6 K / Ir at.%, suggesting that Ir effectively substitutes Ru in the RuO planes. A subtle drop in r(T) is observed close to T M, probably due to the suppression of the spin-flip scattering. The magnetoresistivity measurements revealed that the temperature Tc,zero, in which r(T) ~ 0, decreases rapidly for low applied magnetic fields (H < 2 T), and that this drop becomes much less pronounced in higher magnetic fields (2 < H < 18 T). The appreciable broadening of r(T) curves at low magnetic fields is reminiscent of the behavior in high-Tc materials showing granular behavior, as for example in Sm1.85Ce0.15CuO4-y.

Alloy studies in the p - d organic conductor l-(BETS)2Fe xGa1-xCl4 have given new insight into the nature of field induced superconductivity (FISC), since the mechanism of the FISC involves cancellation of the p - d exchange field by the external field. Alloying on the Fe xGa1-x site allows tuning of the exchange field, thereby influencing the FISC phase boundary. A brief review of the low temperature phases are given, and new high magnetic field thermoelectric and mm wave results that probe the low temperature ground state are presented.

That very strong coupling of the B-B bond-stretching E2g branch of phonons to the B 2psigma bonding hole states is responsible for the remarkable superconductivity in MgB2 is well established. This entirely new manner of driving the superconducting Tc to high levels requires additional analysis. Here recent findings, such as how this strong coupling is related to possible structural instability, are discussed, and investigation into the practical limits of such coupling is initiated.

In this paper a brief overview of anomalous behavior resulting from the two-gap superconductivity in MgB2 is given. We focus on two characteristic effects: an anomalous enhancement of the upper critical field by nonmagnetic impurities and nonequilibrium interband phase textures which appear as a result of interband breakdown caused by electric field. Both effects distinguish MgB2 from the existing low-Tc and high-Tc superconductors.

We have investigated the low-temperature (T < Tc=10) mixed-state current-voltage (IV) response of magnesium diboride films beyond the point where the superconductivity is completely destroyed and the system enters the normal state. The resistance-versus-current R(I) curves are extremely steep and featureless, with a critical current density j c, marking the onset of dissipation, that is unusually high (j c>j d/10) with respect to the depairing current density j d. At large flux densities Hc2/10 $\lessim$ B $\lessim$Hc2, the R(I) curve has a functional shape that is largely independent of B, indicating that the rise in resistivity with increasing current occurs mainly due to pair-breaking rather than flux motion. The macroscopic destruction current I*, which drives the system normal, has a B/Hc2: $\sqrt{B/H_{c2}}$ flux-density dependence, suggesting that the vortices mainly reduce the effective cross section over which a current of effective density j ~ j d flows.

The first direct measurement of an anisotropic superconducting property in MgB2 was achieved for the bulk nucleation field Hc2, in a sample of aligned crystallites. It was found a ratio gamma(T) = Hab c2/ Hc c2= 1.6 - 1.9, for T varying from 32 K to 26K, between H applied parallel to the ab plane, and along the c direction. The anisotropy of the induced critical current density was evaluated through the Bean model to be ab c/ Jc c~ 1.5. We present here a brief review of these studies in connection with current results found in the literature.

We propose a two band model for superconductivity. It turns out that the simplest nontrivial case considers solely interband scattering, and both bands can be modeled as symmetric (around the Fermi level) and flat, thus each band is completely characterized by its half-band width Wn (n=1,2). A useful dimensionless parameter is d, proportional to W2 - W1. The case delta = 0 retrieves the conventional BCS model. We probe the specific heat, the ratio gap over critical temperature, the thermodynamic critical field and tunneling conductance as functions of d and temperature (from zero to Tc). We compare our results with experimental results for MgB2 and good quantitative agreement is obtained, indicating the relevance of interband coupling. Work in progress also considers the inclusion of band hybridization and general interband as well as intra-band scattering mechanisms.

We examine the electronic structure of Na xCoO2.yH2O within the local density approximation. The parametrization of the band which forms the largest hole-Fermi surface centered at gamma shows significant deviations from what is frequently assumed in recent sophisticated theoretical studies. In particular, the commonly used nearest neighbor approaches in the framework of single band pictures are found to be unrealistic. The special role of H2O in screening the disorder in the charge reservoir is briefly discussed and compared with the case of Y1-xCa xCu3O6+delta.

Glassy carbon (GC) has been high-pressure high-temperature treated. An interesting morphology evolution from the pristine sample to the high pressure products was observed. It is found that GC can be graphitized under pressure at a temperature much lower than that at ambient condition. Furthermore the in-situ structure and electrical measurements of GC and graphitized glassy carbon (GGC) under high temperature and high pressure have been investigated up to 30 GPa. We particularly emphasize the unusual magnetic properties of GC treated under high pressures and high temperatures. A paramagnetic to ferromagnetic-like, and then to superconducting (a diamagnetic signal with hysteresis magnetic response) -like behavior, which can be observed at temperatures as high as 80 K, appears as a successive evolution from the initial GC to GGC in accordance with three regions distinguished by the graphitization temperature. This interesting evolution of magnetic properties probably evokes the new understanding of carbon element.

We discuss two methods used for the study of unconventional superconductors: ultrasound attenuation and collective modes. These two methods, as well as microwave absorption, turn out to be coupled and have become very important now. Within models built by path integration technique we analyze some recent ideas concerning possible realization of the mixture of different d-wave states in high temperature superconductors (HTSC).We specifically consider the mixture of d x2-y2 and d xy states. We study the collective mode spectrum and the ultrasound attenuation in the mixed state, and show that each of the two methods allows us to distinguish a pure d-wave state from a mixed one. They also allow us to identify the type of pairing and order parameter in unconventional superconductors, including the presence and topology of gap nodes, the magnitude of the gap, and degree of admixture in the mixed state.

Millimeter-sized single crystals of CaLaBaCu3O7-delta were grown by the conventional self-flux technique. In this report we present a characterization of the structural and morphological properties of our first time grown single crystals. X-Ray diffraction patterns show that the unit cell of the samples corresponds to a tetragonal structure. SEM images evidence excellent morphological properties without presence of twins. EDX measurements show the difficulty of Ca to enter in the structure. The implications of our results are extensively presented and discussed.

Although a considerable amount of the research work has been done on graphite, its physical properties are still not well understood, and novel phenomena such as the magnetic-field-driven metal-insulator transition (MIT), the quantum Hall effect, ferromagnetic and superconducting correlations have recently been revealed. Theoretical analysis suggests that the MIT in graphite is the condensed-matter realization of the magnetic catalysis phenomenon known in relativistic theories of (2 + 1) - dimensional Dirac fermions (DF), i. e. that the applied field opens an insulating gap in the spectrum of DF, associated with the electron-hole pairing. On the other hand, we demonstrate in this paper that a two parameter scaling analysis proposed by Das and Doniach [D. Das and S. Doniach, Phys. Rev. B 64, 134511 (2001)] to characterize the magnetic-field-tuned Bose metal - insulator transition can be well applied to the MIT observed in graphite. We discuss the possibility that the MIT in graphite is associated with the transition between Bose metal and excitonic insulator states.