Nonmagnetic scattering induced suppression of superfluid density in Bi-2212

Nonlinear current-voltage (IV) characteristics of Bi-2212 observed in the presence of the nonmagnetic impurity have been explained incorporating the idea of Berezinskii-Kosterlitz - Thouless (BKT). An exponent (η) is extracted as a function of temperature (T) for several Bi2-xSr2 CaCu2-x ZnxO8+δ (Bi-2212) superconducting samples. Within the framework of the Ambegaokar-Halperin-Nelson-Siggia (AHNS) theory we have extracted the superfluid phase stiffness (SPS) as a function of T. A scaling between the SPS and critical temperature is observed. Strong suppression by the nonmagnetic impurity has been explained using the idea of localized phase fluctuations in the superconducting planes.

On the nature of self-field critical current in superconductors and its use as a probe of the superfluid density

<p>Superconductors are used in many applications where large electrical currents are needed. This is due to their ability to transport an electric current without resistance. There is however a limit to the magnitude of current that can be conducted before dissipation starts to occur. This is known as the critical current and is a topic of great interest in applied superconductivity. For type II superconductors, it is well known that vortex motion plays a role in the determination of the in-field critical current. This has led great effort in engineering the microstructure of these superconductors to hinder the motion of vortices and enhance their critical currents. However the self-field critical current (when there is no applied external field) generally does not see any enhancement due to efforts to pin vortex motion. The work here examines the behaviour of the self-field critical current in thin-film and cylindrical wire superconductors of many different superconductor types and sizes. It is found that a critical state is reached when the current density at the surface of the sample reaches the magnitude of Bc/μ₀λ for type I and Bc₁/μ₀λ for type II superconductors regardless of the size and material type. This finding shows that there is a fundamental limit to the self-field current density that cannot be enhanced by engineering the microstructure and is essentially of thermodynamic origin. The result also sets up the self-field critical current density as a probe of the superfluid density. This was explored in many different superconductor types by considering the temperature dependence of the self-field critical current. The ground-state magnetic penetration depth, groundstate energy gap and specific heat jump at the critical temperature were key thermodynamic parameters extracted from the critical current data. For a very large number of superconductors the extracted parameters in general matched well with literature values measured using conventional but much more complex techniques. A result inferred from the critical state was that the current distribution across the width of a rectangular superconductor would be uniform, contrary to expectations of the Meissner state. This was tested by measuring the perpendicular magnetic field resulting from a transport current in a superconducting tape as it reached the critical state. It was indeed found that the current distribution is uniform across the width. The self-field critical current was also measured in YBa₂Cu₃Oy samples with Zn impurities to measure the superfluid density and further test the self-field critical current as a measure of superfluid density and in particular explore whether it follows the canonical dependence on the transition temperature observed for superconductors with d-wave symmetry. Here the critical current was found to reduce as more impurities were added and indeed this matched its expected canonical reduction, following the superfluid density as Jc(sf) ∝p³/². These results taken together support the unexpected existence of a fundamental limit in the self-field critical current, which is thermodynamic in origin.</p>

On the nature of self-field critical current in superconductors and its use as a probe of the superfluid density

<p>Superconductors are used in many applications where large electrical currents are needed. This is due to their ability to transport an electric current without resistance. There is however a limit to the magnitude of current that can be conducted before dissipation starts to occur. This is known as the critical current and is a topic of great interest in applied superconductivity. For type II superconductors, it is well known that vortex motion plays a role in the determination of the in-field critical current. This has led great effort in engineering the microstructure of these superconductors to hinder the motion of vortices and enhance their critical currents. However the self-field critical current (when there is no applied external field) generally does not see any enhancement due to efforts to pin vortex motion. The work here examines the behaviour of the self-field critical current in thin-film and cylindrical wire superconductors of many different superconductor types and sizes. It is found that a critical state is reached when the current density at the surface of the sample reaches the magnitude of Bc/μ₀λ for type I and Bc₁/μ₀λ for type II superconductors regardless of the size and material type. This finding shows that there is a fundamental limit to the self-field current density that cannot be enhanced by engineering the microstructure and is essentially of thermodynamic origin. The result also sets up the self-field critical current density as a probe of the superfluid density. This was explored in many different superconductor types by considering the temperature dependence of the self-field critical current. The ground-state magnetic penetration depth, groundstate energy gap and specific heat jump at the critical temperature were key thermodynamic parameters extracted from the critical current data. For a very large number of superconductors the extracted parameters in general matched well with literature values measured using conventional but much more complex techniques. A result inferred from the critical state was that the current distribution across the width of a rectangular superconductor would be uniform, contrary to expectations of the Meissner state. This was tested by measuring the perpendicular magnetic field resulting from a transport current in a superconducting tape as it reached the critical state. It was indeed found that the current distribution is uniform across the width. The self-field critical current was also measured in YBa₂Cu₃Oy samples with Zn impurities to measure the superfluid density and further test the self-field critical current as a measure of superfluid density and in particular explore whether it follows the canonical dependence on the transition temperature observed for superconductors with d-wave symmetry. Here the critical current was found to reduce as more impurities were added and indeed this matched its expected canonical reduction, following the superfluid density as Jc(sf) ∝p³/². These results taken together support the unexpected existence of a fundamental limit in the self-field critical current, which is thermodynamic in origin.</p>

Ion-size effects on cuprate High Temperature Superconductors

<p>The cuprates are a family of strongly electronically-correlated materials which exhibit high-temperature superconductivity. There has been a vast amount of research into the cuprates since their discovery in 1986, yet despite this research effort, the origins of their electronic phases are not completely understood. In this thesis we focus on a little known paradox to progress our understanding of the physics of these materials. There are two general ways to compress the cuprates, by external pressure or by internal pressure as induced by isovalent-ion substitution. Paradoxically, they have the opposite effect on the superconducting transition temperature. This thesis seeks to understand the salient difference between these two pressures. We study three families of cuprates where the ion size can be systematically altered; Bi₂(Sr₁.₆₋xAx)Ln₀.₄CuO₆₊δ, ACuO₂ and LnBa₂−xSrxCu₃O₇₋δ where Ln is a Lanthenide or Y and A={Mg,Ca,Sr,Ba}. We utilise a variety of techniques to explore different aspects of our paradox, for example; Raman spectroscopy to measure the antiferromagnetic superexchange energy and energy gaps, Density Functional Theory to calculate the density of states, Muon Spin Relaxation to measure the superfluid density as well as a variety of more conventional techniques to synthesize and characterise our samples. Our Raman studies show that an energy scale for spin fluctuations cannot resolve the different effect of the two pressures. Similarly the density of states close to the Fermi-energy, while an important property, does not clearly resolve the paradox. From our superfluid density measurements we have shown that the disorder resulting from isovalent-ion substitution is secondary in importance for the superconducting transition temperature. Instead, we find that the polarisability is a key property of the cuprates with regard to superconductivity. This understanding resolves the paradox! It implies that electron pairing in the cuprates results from either (i) a short-range interaction where the polarisability screens repulsive longer-range interactions and/or (ii) the relatively unexplored idea of the exchange of quantized, coherent polarisation waves in an analogous fashion to phonons in the conventional theory of superconductivity. More generally, we have also demonstrated the utility of studying ion-size effects to further our collective understanding of the cuprates.</p>

Ion-size effects on cuprate High Temperature Superconductors

<p>The cuprates are a family of strongly electronically-correlated materials which exhibit high-temperature superconductivity. There has been a vast amount of research into the cuprates since their discovery in 1986, yet despite this research effort, the origins of their electronic phases are not completely understood. In this thesis we focus on a little known paradox to progress our understanding of the physics of these materials. There are two general ways to compress the cuprates, by external pressure or by internal pressure as induced by isovalent-ion substitution. Paradoxically, they have the opposite effect on the superconducting transition temperature. This thesis seeks to understand the salient difference between these two pressures. We study three families of cuprates where the ion size can be systematically altered; Bi₂(Sr₁.₆₋xAx)Ln₀.₄CuO₆₊δ, ACuO₂ and LnBa₂−xSrxCu₃O₇₋δ where Ln is a Lanthenide or Y and A={Mg,Ca,Sr,Ba}. We utilise a variety of techniques to explore different aspects of our paradox, for example; Raman spectroscopy to measure the antiferromagnetic superexchange energy and energy gaps, Density Functional Theory to calculate the density of states, Muon Spin Relaxation to measure the superfluid density as well as a variety of more conventional techniques to synthesize and characterise our samples. Our Raman studies show that an energy scale for spin fluctuations cannot resolve the different effect of the two pressures. Similarly the density of states close to the Fermi-energy, while an important property, does not clearly resolve the paradox. From our superfluid density measurements we have shown that the disorder resulting from isovalent-ion substitution is secondary in importance for the superconducting transition temperature. Instead, we find that the polarisability is a key property of the cuprates with regard to superconductivity. This understanding resolves the paradox! It implies that electron pairing in the cuprates results from either (i) a short-range interaction where the polarisability screens repulsive longer-range interactions and/or (ii) the relatively unexplored idea of the exchange of quantized, coherent polarisation waves in an analogous fashion to phonons in the conventional theory of superconductivity. More generally, we have also demonstrated the utility of studying ion-size effects to further our collective understanding of the cuprates.</p>

Superfluid density, Josephson relation and pairing fluctuations in a multi-component fermion superfluid

In this work, a Josephson relation is generalized to a multi-component fermion superfluid. Superfluid density is expressed through a two-particle Green function for pairing states. When the system has only one gapless collective excitation mode, the Josephson relation is simplified, which is given in terms of the superfluid order parameters and the trace of two-particle normal Green function. In addition, it is found that the matrix elements of two-particle Green function is directly related to the matrix elements of the pairing fluctuations of superfluid order parameters. Furthermore, in the presence of inversion symmetry, the superfluid density is given in terms of the pairing fluctuation matrix. The results of the superfluid density in Haldane model show that the generalized Josephson relation can be also applied to a multi-band fermion superfluid in lattice.

Superfluid density, Josephson relation and pairing fluctuations in a multi-component fermion superfluid

In this work, a Josephson relation is generalized to a multi-component fermion superfluid. Superfluid density is expressed through a two-particle Green function for pairing states. When the system has only one gapless collective excitation mode, the Josephson relation is simplified, which is given in terms of the superfluid order parameters and the trace of two-particle normal Green function. In addition, it is found that the matrix elements of two-particle Green function is directly related to the matrix elements of the pairing fluctuations of superfluid order parameters. Furthermore, in the presence of inversion symmetry, the superfluid density is given in terms of the pairing fluctuation matrix. The results of the superfluid density in Haldane model show that the generalized Josephson relation can be also applied to a multi-band fermion superfluid in lattice.