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  • Electronic properties of the superconducting and density wave phases in organic anisotropic materials : doctoral thesis
    Pinterić, Marko
    The family of quasi-two-dimensional ▫$\mbox{▫$\kappa$▫-(ET)▫$_2 X$▫}$▫ organic materials distinguishes itself by the rich phase diagram, very similar to the one found in the high-temperature ... superconducting cuprates. Because of this similarity, an enormous effort was made in order to understand the superconducting state in these materials. Despite of that, a long time controversy regarding the symmetry of the superconducting state has persisted to this day. In order to make a breakthrough in the understanding of the problem, we concentrated our attention on three important issues. First, if one wants to understand the superconducting ground state, one has to correlateits properties with the properties of the neighboring insulating spindensity wave ground state and the normal state. Second, sample-dependent and thermal history effects should be accounted for. And finally, the full characterization of the superconducting state should be made on one single crystal and, if possible, using one experimental method. Indeed, in our investigation we successfully addressed all of these issues. Normal state properties point to strong electron correlations, which, depending on the subtle crystal properties, lead to either an antiferromagnetic or a superconducting state. The situation is further complicated by the existence of the important relaxation effects, due to the glass transition, that influence the properties of the superconducting ground state. Our measurements of the insulating spin density wave ground state included the measurement of the single-particle resistivity, the non-ohmic conductivity andthe dielectric spectroscopy. The obtained non-ohmic and dielectric responsecan be explained only by the existence of the collective mode. The behavior of physical parameters, related to the response, suggests that the short-wavelength -- charge domain wall pairs -- excitations are the entities responsible for the observed properties. The existence of domain walls is naturally connected to the existence of the reported weak ferromagnetic ordering, which also implies the existence of domains. Next, the full characterization of the superconducting state was made for samples of different syntheses. Further, every sample was cooled using several different well-controlled conditions in the temperature region that corresponds to the glass transition. Large differences in the properties of the superconducting state were observed and the ground state was assigned to the low-temperature state obtained by the slowest cooling rate. The investigation of the superconducting state was performed by the extraction of the penetration depth. This method has a large advantage as regarding to the fact that the superfluid density can be calculated directly from the penetration depth. The measurements were performed using the ac~susceptometer of high precision, which was inevitable in order to account for small dimensions of the studied samples. The measurements were done in magnetic fields, more than an order of magnitude smaller than the Earthćs field, which is far below the lower critical magnetic field, so the genuine Meissner state was measured and the possible vortex effects were avoided. Finally, a specially developed improvedcalibration was designed in order to obtain the reliable absolute value of the penetration depth for two distinct geometries, with the directionof the magnetic field parallel and perpendicular to the isotropic crystal planes. The qualitative temperature dependence of both the in-plane and the out-of-plane superfluid densities in the genuine superconducting ground state undoubtfully revealed that the symmetry of the superconductivity requires the existence of the linear nodes in the superconducting gap, which is in a good agreement with the ▫$d$▫-wave model. Also, to obtain the temperature dependance of the out-of-plane suprafluid density, Josephson coupling between the conducting planes has to be taken into account. The nodes in the superconducting gap are not surprising if one takes into consideration that an antiferromagnetic state is located in the vicinity of the superconducting one, which points to the unconventional superconductivity. In order to also get a quantitative agreement, the (▫$d+s$▫)-wave model was used, however, a rather large ▫$s$▫-wave component needed to fit our data was not consistent with the recent thermal conductivity results. On the other hand, the superconducting state, obtained by the fastercooling rate, can be well described by the model of the ▫$d$▫-wave superconductor with impurities. Even more important, this state can be correlated fairly well to the ▫$s$▫-wave coupling superconducting model, with nonodes in the superconducting gap. This fact suggests a possible way out of the contradicting results in the scientific community. Finally, few domain effects in the superconducting state, combined with the domain structure in the neighboring insulating spin density wave ground state, point to an important possibility of the existence of a novel ground state. Since ▫$\mbox{▫$\kappa$▫-(ET)▫$_2 X$▫}$▫ materials are in many ways very similar to high-temperature superconducting cuprates, a question appears if this state can be somehow related to newly reported structures -- stripes -- in the latter materials.
    Vrsta gradiva - disertacija
    Založništvo in izdelava - Zagreb : University of Zagreb, Faculty of science, 2003
    Jezik - angleški
    COBISS.SI-ID - 7817238

Knjižnica/institucija Kraj Akronim Za izposojo Druga zaloga
Knjižnica tehniških fakultet, Maribor Maribor KTFMB v čitalnico 1 izv.
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