Os compostos \'LN IND.2\'\'CU\'\'O IND. 4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\') apresentam supercondutividade quando o lantanídeo (\'LN\') é parcialmente substituído por \'CE\' ou \'TH\' e em seguida são submetidos a um tratamento térmico de redução para a remoção de uma pequena quantidade de oxigênio da estrutura. Desta forma, compostos policristalinos \'LN IND. 2-x\'\'M IND.X\'\'CU\'\'O IND.4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\'; \'M\' = \'CE\', \'TH\'; 0 \'< OU =\' X \'< OU =\' 0,20) foram preparados a partir de precursores sol-gel. A vantagem desta técnica de preparação é que os tratamentos térmicos são feitos em temperaturas abaixo da tempertura eutética, evitando a presença de uma fase líquida durante a sinterização dos materiais em estudo. A investigação da cinética de formação destes compostos foi feita através de medidas de difração de raios X e análise térmica diferencial e mostrou que os materiais preparados por processo sol-gel apresentam uma distribuição homogênea dos cátions. As propriedades supercondutoras foram investigadas através de medidas de magnetização e resistência elétrica em diversas amostras e uma ênfase foi dada às séries de \'EU IND. 2-X\'\'CE IND. X\'\'CU\'\'O IND. 4-Y\' (0 \'< OU =\' X \'< OU =\' 0,18) e o desaparecimento da supercondutividade em \'(\'SM IND. 1-X\'\'EU IND. X\') IND. 1,85\'\'CE IND. 0,15\'\'CU\'\'O IND. 4-Y\' (0 \'< OU =\' X \'< OU =\' 1). Os resultados foram interpretados a partir da adoção de um modelo de supercondutividade granular. As propriedades estruturais foram analisadas em mais de 50 amostras desta família de óxidos através de medidas de difração de raios X. O refinamento de estrutura pelo método de Rietveld permitiu concluir que a introdução de portadores em \'LN IND. 2-x\'\'M IND.X\'\'CU\'\'O IND.4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\'; \'M\' = \'CE\', \'TH\'; 0 \'< OU =\' X \'< OU =\' 0,20) ocorre através da substituição parcial do lantanídeo por \'CE\' ou \'TH\' e influencia diretamente as distâncias das ligações entre \'CU\' e \'O\'nos planos de \'CU\'\'O IND. 2\', responsáveis diretos pelas propriedades supercondutoras nesta família de óxidos supercondutores. / Polycrystalline samples of \'LN IND.2\'\'CU\'\'O IND. 4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\') exhibit superconductivity when subject to two different steps: i) substitution of \'CE\' or \'TH\' for the Ln element in the Ln2Cu04-y T\' phase parent compound; and ii) reduction of the resultant material. Polycrystalline samples of \'LN IND. 2-x\'\'M IND.X\'\'CU\'\'O IND.4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\'; \'M\' = \'CE\', \'TH\'; 0 \'< OR =\' X \'< OR =\' 0,20) were prepared through a sol-gel precursor. This chemical route was employed for producing homogeneous, pure, and microstructurally controlled ceramics at sufficiently low sintering temperatures, below the eutectic temperature. The kinetics of the phase formation was studied by x ray powder diffraction and thermal analysis. These results showed that all samples are homogeneous under the cationic point of view. Measurements of electrical resistance and magnetic susceptibility were performed mostly on polycrystalline samples belonging to the frontier where superconducting properties are suppressed in this family as \'EU IND. 2-X\'\'CE IND.X\'\'CU\'\'O IND. 4-Y\' (0 \'< OR =\' X \'< OR =\' 0,18) and \'(\'SM IND. 1-X\'\'EU IND. X\') IND. 1,85\'\'CE IND. 0,15\'\'CU\'\'O IND. 4-Y\' (0 \'< OR =\' X \'< OR =\' 1). The results were discussed within a superconducting granular scenario. Over 50 compounds of this family of oxides were characterized by x ray powder diffraction and all diffractograms were refined through Rietveld analysis. The results revealed that carriers are injected into \'CU\'\'O IND. 2\' planes through a partial substitution of \'CE\' or \'TH\' for the \'LN\' in \'LN IND. 2-x\'\'M IND.X\'\'CU\'\'O IND.4-Y\' (\'LN\'= \'PR\', \'ND\', \'SM\', \'EU\'; \'M\' = \'CE\', \'TH\'; 0 \'< OR =\' X \'< OR =\' 0,20). The dopings are accompanied by an expansion of the \'CU\'-\'O\' bond length within the \'CU\'\'O IND. 2\' planes, an essential feature for the appearance of superconductivity in these series.

In this thesis, we study the effects of doping in two-dimensional quantum antiferromagnets. We consider cases where the undoped parent compound is a Mott insulator with long-range antiferromagnetic order and focus on the low-doping situations. The limit of localized impurities is studied in a system consisting of a host magnet and two additional weakly coupled spins. We derive the effective Hamiltonian describing the interaction between these impurities as a function of their distance and show that it exhibits xyz anisotropy, leading to NMR and EPR line broadening. We calculate the magnetization disturbance in the host magnet induced by a single impurity and find that it always enhances Neel order. Relaxing the localization constraint, we investigate the single-hole dynamics of the t-J model on the honeycomb lattice. Using exact diagonalizations, series expansion and the self-consistent Born approximation, we calculate the quasi-particle dispersion, bandwidth and residues and compare our findings with the well-established results for the square lattice. Similar to the latter case, we find an almost flat band along the edges of the magnetic Brillouin zone and well-defined hole pockets around the corners. The most important part of this thesis is devoted to the magnetic properties of lightly doped La2-xSrxCuO4, the simplest and by far most studied cuprate superconductor. Starting from the undoped parent compound, we calculate the spin-wave spectrum and the spin-flop transitions in a uniform magnetic field at zero temperature. We then consider the low-doping regime and derive the effective field theory describing the spin dynamics in insulating La2-xSrxCuO4, x ≤ 0.055, at low temperature. The spin structure resulting from the spiral solution of the extended t-J model, obtained by taking into account the Coulomb trapping of holes by Sr ions, is confined in the copper-oxide planes. Our solution explains why the incommensurate structure is directed along the orthorhombic b axis and allows us to calculate the positions and shapes of the neutron scattering peaks numerically. These results are in perfect agreement with experimental data. We also show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the spin-glass ground state.

Graduation date: 2000

Heterogeneous ferromagnetic-superconducting systems such as a regular array of ferromagnetic nano dots deposited on the top of a superconducting thin film have attracted many research teams both experimental and theoretical. The interest in these systems does not only stem from being good candidates for technological applications, but also because they represent a new class of physical systems where two competing order parameters can coexist. This work focuses on the theoretica laspects of these systems by studying the static and dynamics of few model systems. In the first part, the static properties of a superconducting thin film interacting with a ferromagnetic texture are considered within the London approximation. In particular, the ferromagnetic textures considered here are a circular dot of submicrometer size with in-plane magnetization, an elliptical dot magnetized in the direction perpendicular to the superconductor, and a ferromagnetic dot magnetized in the direction normal to the superconducting film and containing non magnetic cavities. I also consider the interaction of vortices in the superconductor with a ferromagnetic columnar defect which penetrates the supercondcting film. In each case the vector potential and magnetic field of the ferromagnet in the presence of the superconductor are calculated. Afterward the presence of vortices in the superconductor is assumed and the energy of vortex-texture system is found. The pinning potential and force supplied by the texture are then derived from the energy of interaction between the ferromagnet and superconductor. I show that if the magnetization of the ferromagnet exceeds a critical value then vortices are spontaneously created in the ground state of the system. Such spontaneous creation of vortices is possible mostly in a close vicinity of the superconducting transition temperature Ts. For every case, the threshold value of the magnetization at which vortices start to be spontaneously created in the SC is calculated as a function of the parameters of the texture geometry. The phase diagrams for transitions from vortexless regime to regimes with one or more vortices are determined for all cases. In the second problem, the transport properties of a ferromagnetic superconducting bilayer with alternating magnetization and vortex density are studied within a phenomenological model. I show that pinning forces do not appear for continuous distribution of vortices, so a discrete model for the bilayer system is constructed. Afterward, I calculate the pinning forces acting on vortices and antivortices resulting from highly inhomogeneous distribution of flux lines and prove that this system has strong transport anisotropy. In the absence of random pinning, the system displays a finite resistance for the current in the direction perpendicular to the domains while its resistance vanishes for the parallel current. The transport anisotropy strongly depends on temperature. I study this dependence and show that the ratio of parallel to perpendicular critical current is largest close to the superconducting transition temperature Ts and the vortex disappearance temperature Tv while it has a minimum in between them.

This dissertation describes experimental studies of the superconducting proximity effect in single-crystal Pb, Sn, and Zn nanowires of lengths up to 60 um, with both ends of the nanowires in contact with macroscopic electrodes that are either superconducting (Sn or Pb) or non-superconducting (Au). The Pb, Sn, and Zn nanowires are fabricated using a template-based electrochemical deposition method. Electric contacts to the nanowires are formed in situ during electrochemical growth. This method produces high transparency contacts between a pair of macroscopic electrodes and a single nanowire, circumventing the formation of oxide or other poorly conducting interface layers. Extensive analyses of the structure and the composition of the nanowire samples are presented to demonstrate that (1) the nanowires are single crystalline and (2) the nanowires are clean without any observable mixing of the materials from the electrodes. The nanowires being investigated are significantly longer than the nanowires with which electrode-induced superconductivity was previously investigated by other groups. We have observed that in relatively short (~6 um) Sn and Zn nanowires, robust superconductivity is induced at the superconducting transition temperatures of the electrodes. When Sn and Pb nanowires are in contact with a pair of Au electrodes, superconductivity is suppressed completely. For nanowires of 60 um in length, although the suppression of superconductivity by Au electrodes is only partial, the induced superconductivity at the higher transition temperatures of the electrodes remains full and robust. Therefore, an anomalous superconducting proximity effect has been observed on a length scale which far exceeds the expected length based on the existing theories of the proximity effect. The measured current-voltage characteristic of the nanowires reveals more details such as hysteresis, multiple Andreev reflection, and phase-slip centers. An interesting relation between the proximity effect and the residual-resistance-ratio of the nanowires has also been observed. Possible mechanisms for this proximity effect are discussed based on these experimental observations.

The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

This dissertation describes experimental studies of the superconducting proximity effect in single-crystal Pb, Sn, and Zn nanowires of lengths up to 60 um, with both ends of the nanowires in contact with macroscopic electrodes that are either superconducting (Sn or Pb) or non-superconducting (Au). The Pb, Sn, and Zn nanowires are fabricated using a template-based electrochemical deposition method. Electric contacts to the nanowires are formed in situ during electrochemical growth. This method produces high transparency contacts between a pair of macroscopic electrodes and a single nanowire, circumventing the formation of oxide or other poorly conducting interface layers. Extensive analyses of the structure and the composition of the nanowire samples are presented to demonstrate that (1) the nanowires are single crystalline and (2) the nanowires are clean without any observable mixing of the materials from the electrodes. The nanowires being investigated are significantly longer than the nanowires with which electrode-induced superconductivity was previously investigated by other groups. We have observed that in relatively short (~6 um) Sn and Zn nanowires, robust superconductivity is induced at the superconducting transition temperatures of the electrodes. When Sn and Pb nanowires are in contact with a pair of Au electrodes, superconductivity is suppressed completely. For nanowires of 60 um in length, although the suppression of superconductivity by Au electrodes is only partial, the induced superconductivity at the higher transition temperatures of the electrodes remains full and robust. Therefore, an anomalous superconducting proximity effect has been observed on a length scale which far exceeds the expected length based on the existing theories of the proximity effect. The measured current-voltage characteristic of the nanowires reveals more details such as hysteresis, multiple Andreev reflection, and phase-slip centers. An interesting relation between the proximity effect and the residual-resistance-ratio of the nanowires has also been observed. Possible mechanisms for this proximity effect are discussed based on these experimental observations.

Low temperature specific heat (LTSH) is a powerful tool to investigate the physical properties of bulk samples. For superconductivity, LTSH can probe the pairing state in superconductors and provides additional information under magnetic fields. In this thesis, I present comprehensive specific-heat studies of superconductivity in YNi2B2C, NbSe2, and CeRu2. (1) Single crystalline YNi2B2C was found to be superconducting at the superconducting transition temperature Tc ~ 13.77 K. The superconducting specific heat Ce(T) can be described by either the point-node or the two-gap model. (2) Single crystalline NbSe2 has a two-dimensional crystalline structure showing the Tc ~ 6.7 K and the anisotropy in the critical fields, Hc2¡æ/Hc2// ~3. We investigated the Ce(T) and the electronic specific heat £^(H) by the two-gap model. Obtained fitting parameters, such as gap values and the relative ratio of two gaps in both analyses (Ce(T) and £^(H)), are comparable meaning that the superconductivity of NbSe2 can be described by the two-gap scenario. (3) Finally, we studied the DC magnetic susceptibility and the specific heat of polycrystalline CeRu2 in different magnetic fields. In the bulk CeRu2, the amount of the possible impurity phase or nano-clusters was reduced after annealing. Based on the analysis results of zero-field and in-field specific heat, CeRu2 is a BCS-like superconductor with an anisotropic gap.

Thesis (Ph. D.)--University of Chicago, Dept. of Physics, 2001. / Includes bibliographical references. Also available on the Internet.