UXSS 2019 at CFEL Hamburg

Michael is organizing UXSS 2019, taking place in Hamburg, June 16-21, 2019. The ultrafast x-ray summer seminar (UXSS) is an annual school aimed at PhD students and junior postdocs working on or interested in learning about x-ray methods at free-electron laser sources. Topics range from basics of x-ray sources via atomic and molecular physics and chemical dynamics to structural biology and inelastic light scattering in correlated materials. Registration will open in early 2019.


Paper published in Science Advances

Our paper on cavity materials – how to manipulate matter with pure vacuum fluctuations of light – was published in Science Advances. Here is the press release.

The vacuum fluctuations of light (yellow wave) are amplified in an optical cavity (upper and lower reflecting mirrors). Crystal lattice vibrations (red atoms) at a two-dimensional interface surf this strong light wave. The thus mixed light-vibrational waves couple particularly strongly to electrons in a two-dimensional atomically thin material (green and yellow atoms), changing its properties.
© Jörg M. Harms / MPSD

Gabriel’s paper in Nature Communications

A wave of laser light hits the magnetic material, shaking the electron spins (arrows). This weakens magnetism and induces Weyl fermions in the laser-shaken material. © J.M. Harms / MPSD

Gabriel’s paper is out in Nature Communications: G. E. Topp et al., Nature Communications 9, 4452 (2018). Here is a link to the press release “Shedding light on Weyl fermions” on the MPSD website (click here for a German version).

Congratulations, Gabriel!

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.