Yuki Takeuchi

Upper critical field reaches 90 tesla near the Mott transition in fulleride superconductors

Controlled access to the border of the Mott insulating state by variation of control parameters offers exotic electronic states such as anomalous and possibly high-transition-temperature (Tc) superconductivity. The alkali-doped fullerides show a transition from a Mott insulator to a superconductor for the first time in three-dimensional materials, but the impact of dimensionality and electron correlation on superconducting properties has remained unclear. Here we show that, near the Mott insulating phase, the upper critical field Hc2 of the fulleride superconductors reaches values as high as ∼90 T—the highest among cubic crystals. This is accompanied by a crossover from weak- to strong-coupling superconductivity and appears upon entering the metallic state with the dynamical Jahn–Teller effect as the Mott transition is approached. These results suggest that the cooperative interplay between molecular electronic structure and strong electron correlations plays a key role in realizing robust superconductivity with high-Tc and high-Hc2.

Electric-Field-Induced Superconductivity Detected by Magnetization Measurements of an Electric-Double-Layer Capacitor

We report evidence for superconductivity induced by the application of strong electric fields onto the surface of a band insulator, ZrNCl, provided by the observation of a shielding diamagnetic signal. We introduced an electric-double-layer capacitor configuration and in situ magnetization measurements at low temperatures as a method to detect the novel electric-field-induced superconducting state. The results showed excellent agreement with a previous report using a transistor configuration, demonstrating that the present technique is a novel method for investigating the nonequilibrium phase induced by electric fields.

Second order accuracy finite difference methods for space-fractional partial differential equations

Abstract Space-fractional partial differential equations are used for simulations of, for example, diffusion of radioactive materials, and financial and other models, which are characterized by heavy-tailed distributions. A number of first order accuracy finite difference methods have been proposed. In the present paper, we introduce second order accuracy finite difference methods with Dirichlet boundary conditions. These methods have a parameter in these schemes, and the parameter stabilizes the schemes. This means that there exist various schemes with second order accuracy, but the stability of each scheme is different. In the present paper, we introduce the most stable scheme for any fractional calculus order by choosing the optimal parameter. In addition, we describe a phenomenon whereby the expected accuracy cannot be obtained if the analytical solution can be expanded to a series having less than second order around boundaries. This also happens in both existing methods and the proposed methods. In the present paper, we develop the stability conditions for the proposed schemes, and numerical examples of second order accuracy and accuracy decay are shown.

Second Order Accuracy Finite Difference Methods for Fractional Diffusion Equations

Finite difference methods for fractional differential equation are ever proposed. However, precise error orders have not been analyzed for the methods higher than first order accuracy. This paper proposes a few finite difference methods for fractional diffusion equations and shows our methods have second order accuracy under the conditions that the solution functions have higher order than second order at boundaries. In addition, we show that the accuracy may decrease in the case that the solution functions have lower order than second order at boundaries when we use second order accuracy scheme. In this paper, we treat schemes based on Grunwald-Letnikov definition and apply them to three kinds of fractional diffusion equations using Riemann-Liouville derivative operator including time-fractional diffusion equation, space-fractional diffusion equation and time-space-fractional diffusion equation. Finally, we show the simulation results which indicate that our methods are stable and have successfully second order accuracy under the assumed conditions.Copyright

Electrostatically and electrochemically induced superconducting state realized in electrochemical cells

We here report the result of in situ magnetization measurements of electrochemical cells at low temperatures. Upon applying voltages between the electrodes of the electrochemical cells, we observed shielding diamagnetic signals from several materials, indicating superconducting transitions. The superconducting states can be induced both electrochemically and electrostatically with appropriate combination of counter electrode materials and electrolytes. The present technique may become a powerful method for searching novel superconductors.

Phase Transition Dynamics of Three Types of Water within Poly(N,N-dimethylacrylamide) Hydrogels

Water in hydrogels has been classified into three types: bound, intermediate, and free water. To investigate the individual phase transition dynamics for each type of water, differential scanning calorimetic (DSC) curves and Raman spectra of poly(N,N-dimethylacrylamide) hydrogels were measured with heating from 130 to 310 K. Bound and intermediate water showed glassy initial states at 130 K, whereas free water became hexagonal ice (Ih) structure. Intermediate water in glassy state undergoes four phase transition steps: glass-to-liquid transition (at 160–190 K), crystallization from liquid state to cubic ice (Ic) (at 200–230 K), Ic–Ih transition (at 240–250 K), and melting (at 250–273 K). It is concluded that pre-melting of ice, which has been observed in various polymer hydrogels, results from the exothermic Ic–Ih transition of intermediate water.