Observation of Cu Spin Fluctuations in High-Tc Cuprate Superconductor Nanoparticles Investigated by Muon Spin Relaxation

The nano-size effects of high-Tc cuprate superconductor La2−xSrxCuO4 with x = 0.20 are investigated using X-ray diffractometry, Transmission electron microscopy, and muon-spin relaxation (μSR). It is investigated whether an increase in the bond distance of Cu and O atoms in the conducting layer compared to those of the bulk state might affect its physical and magnetic properties. The μSR measurements revealed the slowing down of Cu spin fluctuations in La2−xSrxCuO4 nanoparticles, indicating the development of a magnetic correlation at low temperatures. The magnetic correlation strengthens as the particle size reduces. This significantly differs from those observed in the bulk form, which show a superconducting state below Tc. It is indicated that reducing the particle size of La2−xSrxCuO4 down to nanometer size causes the appearance of magnetism. The magnetism enhances with decreasing particle size.

Direct visualization of a static incommensurate antiferromagnetic order in Fe-doped Bi2Sr2CaCu2O8+δ

In cuprate superconductors, due to strong electronic correlations, there are multiple intertwined orders which either coexist or compete with superconductivity. Among them, the antiferromagnetic (AF) order is the most prominent one. In the region where superconductivity sets in, the long-range AF order is destroyed. Yet the residual short-range AF spin fluctuations are present up to a much higher doping, and their role in the emergence of the superconducting phase is still highly debated. Here, by using a spin-polarized scanning tunneling microscope, we directly visualize an emergent incommensurate AF order in the nearby region of Fe impurities embedded in the optimally doped Bi2Sr2CaCu2O8+δ (Bi2212). Remarkably, the Fe impurities suppress the superconducting coherence peaks with the gapped feature intact, but pin down the ubiquitous short-range incommensurate AF order. Our work shows an intimate relation between antiferromagnetism and superconductivity.

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>