First-principles study on the heterostructure of twisted graphene/hexagonal boron nitride/graphene sandwich structure

In recent years, the discovery of "magic angle" graphene has given new inspiration to the formation of heterojunctions. Similarly, the use of hexagonal boron nitride, known as white graphene, as a substrate for graphene devices has more aroused great interest in the graphene/hexagonal boron nitride (G/hBN) heterostructure system. Based on the first principles method of density functional theory, the band structure, density of states, Mulliken population, and differential charge density of a tightly packed model of twisted graphene/hexagonal boron nitride/graphene (G/hBN/G) sandwich structure have been studied. Through the establishment of heterostructure models TBG inserting hBN with different twisted angles, it was found that the band gap, Mulliken population, and charge density, exhibited specific evolution regulars with the rotation angle of the upper graphene, showing novel electronic properties and realizing metal-insulator phase transition. We find that the particular value of the twist angle at which the metal-insulator phase transition occurs and propose a rotational regulation mechanism with angular periodicity. Our results have guiding significance for the practical application of heterojunction electronic devices.

Electronic and Transport Properties of Sr-site Substituted: CaxSr(1-x)VO3

The Mott-insulator phase transition behaviour of the superstructure of strongly correlated system, CaxSr(1-x)VO3 (x =0, 0.33, 0.67, 1) have studied using the conventional density functional theory and the dynamical mean field theory. The Mott-Hubbard metal-insulator phase transition of superstructures, Ca0.33Sr0.67VO3 and Ca0.67Sr0.33VO3 formed by the CaVO3 and SrVO3 correlated metals, are obtained at U=4.5eV with β= 6(eV)-1 and U =4.5eV with β= 7(eV)-1 respectively. The values of U and β calculated through the Maximum Entropy model using the Green’s function data, are consistent with the experimental results. The value of Seebeck coefficient (S) of superstructure Ca0.33Sr0.67VO3 and Ca0.67Sr0.33VO3 are found to be +0.0011[V/K] and -0.0011[V/K] within the chemical potential μ = -1.266 eV to μ = -0.938 eV. The figures of merit (ZT) are found to be 0.97 at room temperature for these systems. The variation of electrical and thermal conductivities has also been discussed.

Superconductor–insulator transition in capacitively coupled superconducting nanowires

We investigate superconductor–insulator quantum phase transitions in ultrathin capacitively coupled superconducting nanowires with proliferating quantum phase slips. We derive a set of coupled Berezinskii–Kosterlitz–Thouless-like renormalization group equations demonstrating that interaction between quantum phase slips in one of the wires gets modified due to the effect of plasma modes propagating in another wire. As a result, the superconductor–insulator phase transition in each of the wires is controlled not only by its own parameters but also by those of the neighboring wire as well as by mutual capacitance. We argue that superconducting nanowires with properly chosen parameters may turn insulating once they are brought sufficiently close to each other.