Charge pumping in graphene

Riku’s paper with Mike Ridley on charge pumping in ac-driven graphene nanoribbons has been published in Physical Review B. Congrats!

Popular summary: Typically, electronic current flowing through a conductor needs a net voltage to be applied across the conductor. However, applying an alternating voltage, which is zero on average, may induce a direct current. This mechanism is known in the engineering literature as AC-DC conversion or rectification. Here we investigated this mechanism in a quantum transport setup consisting of graphene nanoribbons, and derived some general “rules of thumb” for quantum pumping.

How to make a material more correlated with light

From top to bottom, electronic spectra show more and more coherence-incoherence spectral weight transfer, indicative of enhanced electron-lattice coupling in the strongly driven system.

Our preprint “Light-enhanced electron-phonon coupling from nonlinear electron-phonon coupling” is available on arXiv. In this work, it is shown how one can amplify electron-lattice coupling by using lasers that are tuned to a phonon, that is coupled quadratically to the electrons of the material. Such enhanced electron-lattice coupling can lead to the formation of polarons – electrons coupled to a “cloud” of lattice distortion – or even make the system superconducting. It has recently been debated how possible light-induced superconductivity in carbon football molecular crystal (“fullerenes”) may come about, and nonlinear electron-phonon coupling might play an important role. Similarly, more direct signatures of light-enhanced electron-lattice coupling have been observed in metallic bilayers of the carbon flatland material graphene. Now experiments have to be performed to check the hypothesis of our theory paper.

Elementary-particle physics in laser-driven materials

Dancing Weyl cones: When excited by tailored laser pulses (white spiral), the cones in a Dirac fermion material dance on a path (8-shape) that can be controlled by the laser light. This turns a Dirac material into a Weyl material, changing the nature of the quasiparticles in it. One of the cones hosts right-handed Weyl fermions; the other cone hosts left-handed ones. [less] © Jörg M. Harms/MPSD
Dancing Weyl cones: When excited by tailored laser pulses (white spiral), the cones in a Dirac fermion material dance on a path (8-shape) that can be controlled by the laser light. This turns a Dirac material into a Weyl material, changing the nature of the quasiparticles in it. One of the cones hosts right-handed Weyl fermions; the other cone hosts left-handed ones. © Jörg M. Harms/MPSD

Our work “Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials” was published in Nature Communications (doi:10.1038/ncomms13940).

Further reading:
Studying fundamental particles in materials

Understanding the energy flow in a high-temperature superconductor

Microscopic image of one of the bismuth strontium calcium copper oxide samples the scientists studied using a new high-speed imaging technique. Color changes show changes in sample height and curvature to dramatically reveal the layered structure and flatness of the material. Credit: Brookhaven National Laboratory Read more at: http://phys.org/news/2016-12-laser-pulses-scientists-complex-electron.html#jCp
Microscopic image of one of the bismuth strontium calcium copper oxide samples the scientists studied using a new high-speed imaging technique. Color changes show changes in sample height and curvature to dramatically reveal the layered structure and flatness of the material. Credit: Brookhaven National Laboratory

Our work “Energy Dissipation from a Correlated System Driven Out of Equilibrium” was published in Nature Communications (doi:10.1038/ncomms13761).

Further reading:
Laser pulses help scientists tease apart complex electron interactions
Energiefluss im Supraleiter
Laserpulse helfen Forschern, komplexe Elektronenwechselwirkungen zu entflechten

Control of competing orders

In a new theory published on arXiv today we show how a laser beam can exert control in a system with competing superconducting and charge orders. The underlying mechanism with a striking resonance for photon frequencies near the gap edge may even be used to understand light-induced superconductivity. The above figure illustrates how a symmetry between the competing orders allows for low-energy excitations corresponding to a rotation of a composite order parameter towards charge order (CDW, left) or towards superconductivity (SC, right).
Reference: arXiv:1611.04307