The two critical temperatures conundrum in La1.83_{1.83}Sr0.17_{0.17}CuO4_4

The in-plane and out-of-plane superconducting stiffness appear to vanish at different transition temperatures, which contradicts thermodynamical expectation. In addition, we observe a surprisingly strong dependence of the out-of-plane stiffness transition on sample width. With evidence from Monte Carlo simulations, this effect is explained by a crossover from three dimensions into quasi one dimensional behavior of the finite sample. Due to an essential singularity at the thermodynamic transition temperature $T_c$, the out-of-plane stiffness appears to vanish below $T_c$ by an amount which is proportional to the inverse width. A fit of our prediction to the data is used to determine the value of the anisotropy parameter. Analogy to studies of helium superfluids in nanopores is made

Effects of lattice geometry on thermopower properties of the repulsive Hubbard mode

We obtain the Seebeck coefficient or thermopower $S$, which determines the conversion efficiency from thermal to electrical energy, for the two-dimensional Hubbard model on different geometries (square, triangular, and honeycomb lattices) for different electronic densities and interaction strengths. Using Determinantal Quantum Monte Carlo (DQMC) we find the following key results: (a) the bi-partiteness of the lattice affects the doping dependence of $S$; (b) strong electronic correlations can greatly enhance $S$ and produce non-trivial sign changes as a function of doping especially in the vicinity of the Mott insulating phase; (c) $S(T)$ near half filling can show non-monotonic behavior as a function of temperature. We emphasize the role of strong interaction effects in engineering better devices for energy storage and applications, as captured by our calculations of the power factor $PF=S^2 \sigma$ where $\sigma$ is the dc conductivity.Comment: 10 pages, 8 figure

Tuning superinductors by quantum coherence effects for enhancing quantum computing

Research on spatially inhomogeneous weakly-coupled superconductors has recently received a boost of interest because of the experimental observation of a dramatic enhancement of the kinetic inductance with relatively low losses. Here, we study the kinetic inductance and the quality factor of a strongly-disordered weakly-coupled superconducting thin film. We employ a gauge-invariant random-phase approximation capable of describing collective excitations and other fluctuations. In line with the experimental findings, in the range of frequencies of interest, we have found that an exponential increase of the kinetic inductance with disorder coexists with a still large quality factor $\sim 10^5$. More interestingly, on the metallic side of the superconductor-insulator transition, we have identified a range of frequencies and temperatures, well below the critical one, where quantum coherence effects induce a broad statistical distribution of the quality factor with an average value that increases with disorder. We expect these findings to further stimulate experimental research on the design and optimization of superinductors for a better performance and miniaturization of quantum devices such as qubits circuits and microwave detectors.Comment: 15 pages, 10 figure