Pairing symmetry of superconducting graphene

The possibility of intrinsic superconductivity in alkali-coated graphene monolayers has been recently suggested theoretically. Here, we derive the possible pairing symmetries of a carbon honeycomb lattice and discuss their phase diagram. We also evaluate the superconducting local density of states (LDOS) around an isolated impurity. This is directly related to scanning tunneling microscopy experiments, and may evidence the occurrence of unconventional superconductivity in graphene.Comment: Eur. Phys. J. B, to appea

Proposed lower bound for the shear viscosity to entropy density ratio in some dense liquids

Starting from relativistic quantum field theories, Kovtun et al. (2005) have quite recently proposed a lower bound eta/s >= hbar /(4 pi kB), where eta is the shear viscosity and s the volume density of entropy for dense liquids. If their proposal can eventually be proved, then this would provide key theoretical underpinning to earlier semiempirical proposals on the relation between a transport coefficient eta and a thermodynamic quantity s. Here, we examine largely experimental data on some dense liquids, the insulators nitrogen, water, and ammonia, plus the alkali metals, where the shear viscosity eta(T) for the four heaviest alkalis is known to scale onto an `almost universal' curve, following the work of Tankeshwar and March a decade ago. So far, all known results for both insulating and metallic dense liquids correctly exceed the lower bound prediction of Kovtun et al.Comment: to appear in Phys. Lett.

Electronic beam shifts in monolayer graphene superlattice

Electronic analogue of generalized Goos-H\"{a}nchen shifts is investigated in the monolayer graphene superlattice with one-dimensional periodic potentials of square barriers. It is found that the lateral shifts for the electron beam transmitted through the monolayer graphene superlattice can be negative as well as positive near the band edges of zero-$\bar{k}$ gap, which are different from those near the band edges of Bragg gap. These negative and positive beam shifts have close relation to the Dirac point. When the condition $q_A d_A= -q_B d_B= m \pi$ ($m=1,2,3...$) is satisfied, the beam shifts can be controlled from negative to positive when the incident energy is above the Dirac point, and vice versa. In addition, the beam shifts can be greatly enhanced by the defect mode inside the zero-$\bar{k}$ gap. These intriguing phenomena can be verified in a relatively simple optical setup, and have potential applications in the graphene-based electron wave devices.Comment: 5 pages, 4 figures, submitted on Oct. 15, 201

Photon pair production by STIRAP in ultrastrongly coupled matter-radiation systems

Artificial atoms (AAs) offer the possibility to design physical systems implementing new regimes of ultrastrong coupling (USC) between radiation and matter [C. Ciuti et al., Phys. Rev. B 72, 115303 (2005)], where previously unexplored non-perturbative physics emerges. While experiments so far provided only spectroscopic evidence of USC, we propose the dynamical detection of virtual photon pairs in the dressed eigenstates, which is a smoking gun of the very existence of USC in nature. We show how to coherently amplify this channel to reach 100% efficiency by operating advanced control similar to stimulated Raman adiabatic passage (STIRAP) [N.V. Vitanov et al., Rev. Mod. Phys. 89, 015006 (2017)]