Versatile control of superconducting transition temperature in anti-ThCr2Si2-type Y2O2Bi via H, Li, or F doping

In Y2O2Bi with Bi square net, H substitution and Li intercalation led to higher superconducting transition tempareture (Tc), while F substitution led to lower Tc, where Tc is universally scaled...

Elastic behavior, pressure-induced doping and superconducting transition temperature of GdBa2Cu3O7-x

Doping superconductors are known to vary the superconducting transition temperature TC depending on the degree of holes or electrons introduced in a system. In this study, we report how pressure-induced hole doping influences the TC of GdBa2Cu3O7-x superconducting perovskite. The study was carried out in the framework of density functional theory (DFT) using the Quantum espresso code. Ultrasoft pseudopotential with generalized gradient approximation (GGA) and local density approximation (LDA) functional was used to calculate the ground state energy using the plane waves (PW). The stability criterion was satisfied from the calculated elastic constants. The BCS theory and the Mc Millan’s equation was used to calculate the TC of the material at different conditions of pressure. The underdoped regime where the holes were less than those at optimal doping was found to be below 20 GPa of doping pressure. Optimal doping where the material achieved the highest TC (max) ~ 20 GPa of the doping pressure. Beyond the pressure of ~20 GPa was the over doping regime where a decrease in TC was recorded. The highest calculated TC (max) was ~141.16 K. The results suggest that pressure of ~20 GPa gave rise to the highest TC in the study.

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>

Moiré Superconductivity and the Roeser-Huber Formula

As shown previously, a relation between the superconducting transition temperature and some characteristic distance in the crystal lattice holds, which enables the calculation of the superconducting transition temperature, Tc, based only on the knowledge of the electronic configuration and of some details of the crystallographic structure. This relation was found to apply for a large number of superconductors, including the high-temperature superconductors, the iron-based materials, alkali fullerides, metallic alloys, and element superconductors. When applying this scheme called Roeser-Huber formula to Moir&eacute;-type superconductivity, i.e., magic-angle twisted bi-layer graphene (tBLG) and bi-layer WSe2, we find that the calculated transition temperatures for tBLG are always higher than the available experimental data, e.g., for the magic angle 1.1∘, we find Tc&asymp; 4.2&ndash;6.7 K. Now, the question arises why the calculation produces larger Tc&rsquo;s. Two possible scenarios may answer this question: (1) The given problem for experimentalists is the fact that for electric measurements always substrates/caps are required to arrange the electric contacts. When now discussing superconductivity in atomically thin objects, also these layers may play a role forming the Moir&eacute; patterns. The consequence of such substrate-induced super-Moir&eacute; patterns is that the resulting Moir&eacute; pattern always will show a larger cell size, and thus, a lower Tc of the final structure will result. (2) A correction factor to the Roeser-Huber formalism may be required to account for the low charge carrier density of the tBLG. Here, we test both scenarios and find that the introduction of a correction factor &eta; enables a proper calculation of Tc, reproducing the experimental data. We find that &eta; depends exponentially on the value of Tc.

Thickness Dependence of Superconductivity in Layered Topological Superconductor β-PdBi2

We report a systematic study on the thickness-dependent superconductivity and transport properties in exfoliated layered topological superconductor β-PdBi2. The superconducting transition temperature Tc is found to decrease with the decreasing thickness. Below a critical thickness of 45 nm, the superconductivity is suppressed, but followed by an abrupt resistance jump near Tc, which is in opposite to the behavior in a superconductor. We attribute suppressed Tc to the enhanced disorder as the thickness decreases. The possible physical mechanisms were discussed for the origination of sharply increased resistance in thinner β-PdBi2 samples.

Cd additive effect on self-flux growth of Cs-intercalated NbS2 superconducting single crystals

Single crystals of Cs-intercalated NbS2 (Cs x NbS2) were synthesized using a CsCl/KCl self-flux. The size and Cs content of Cs x NbS2 single crystals increased upon adding Cd metal into the starting materials. When 10–30 at% of Cd per Nb was provided in the starting materials, plate-like Cs x NbS2 (x ∼ 0.3) single crystals with 1–2 mm in size and 10–100 μm in thickness were obtained. The superconducting transition temperature of these Cs x NbS2 single crystals was 1.65 K.