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The Effect of Pressure on the Coexistence of Superconductivity and Magnetism in RuSr2GdCu2O8 and RuSr2Gd1.4Ce0.6Cu2O10-δ

Fallahi, Saeed | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42332 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Akhavan, Mohammad
  7. Abstract:
  8. The coexistence of long-range magnetic order and superconductivity in the ruthenocuprate families, Ru1212 and Ru1222 has been studied both theoretically and experimentally. Historically these two different phases are incompatible with each other and in the most previous research reported on the coexistence of these phases, there have been observed separated magnetic and superconducting phases. However in the ruthenocuprate families, there is a single phase with both magnetic and superconducting phase which coexist with each other. It has been determined that superconductivity arises in CuO2 planes, and magnetic orders occur in RuO2. From experimental point of view, we have investigated the effect of Ca substitution for Sr on structural, magnetic and superconducting properties of RuSr2−xCaxGd1.4Ce0.6Cu2O10−δ system. In this system, the magnetic coupling of RuO2 and CuO2 plays an important role in the magnetic and superconducting states. X-ray diffraction analysis shows that all samples are single phase and the lattice parameters decrease continuously by increasing Ca content. As Ca concentration increases, rotation of the RuO6 octahedron increases and Ru-O(1)-Ru angle decreases. These variations strengthen the magnetic moments in the RuO2 planes. The onset superconducting transition temperature is found to decrease with Ca substitution. The enhancement of weak ferromagnetic component and hole trapping by Ru magnetic moments in RuO2 planes reduces the electrical conduction, and destroys the superconducting state in the system. Analysis of the resistivity data based on the hoping conduction mechanism indicates a variation of the hoping exponent (p) across the magnetic transition at Tm. The hoping exponent p is not affected sharply by Ca concentration. From computational point of view; we have performed a first-principle calculation of the structural, electronic and high pressure properties of RuSr2GdCu2O8, a ferromagnetic superconductor, by employing a full-potential linearized augmented plane-wave method within the density-functional theory. The effect of pressure was achieved by varying the volume of the unit cell with constant a:b:c ratio. The experimentally observed anti-phase rotation of RuO6 octahedra has been attributed to the residual forces on ORu which results in shear strain in the RuO2 layer. Partial charge analysis shows that applying pressure up to 6 GPa leads to hole creation in the CuO2 sheets which causes increase in the superconducting transition temperature. We have estimated the Curie temperature TM of this compound in the mean-field approximation using Heisenberg model with first-nearest neighbor exchange interactions determined from DFT calculations for parallel and anti-parallel spin configurations of Ru moment in RuO2 planes. The effect of pressure causes the magnetic moment of Ru atoms to decrease due to the increase of hybridization between the adjacent Ru atoms. The calculated exchange splitting in Cu dx2−y2 states increases slightly with pressure but it is still very small that it does not affect superconductivity, and the hole doping mechanism is dominant
  9. Keywords:
  10. Ruthenucuprate ; Electron Structure ; First Principles Method ; Electrical Transport Properties ; Pressure Effect

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