# Archive 2017

Here you can find past events of the MPSD institute.

_{4,5}-edge (~30 eV). In the experiment, a 5 fs VIS-NIR pump pulse excites carriers across the direct band gap and the dynamics are probed with a time-delayed broadband extreme ultraviolet pulse generated by high harmonic generation in xenon spanning ~20-45 eV. The observed transient absorption signal contains the energetic distribution of both carriers, electrons and holes, due to state blocking as well as spectroscopic features induced by bandshifts (e.g. due to band gap renormalization) and broadening (e.g. due to many body effects). By iterative procedures the measured signal can be successfully decoupled into these contributions resolving the carrier and band dynamics with excellent time and energy resolution. Hot carrier relaxation on a 100-fs time scale and carrier recombination on a 1-ps time scale are observed in nanocrystalline Germanium. Going from bulk semiconductor to two-dimensional layers, long-lived core-exciton states are observed at the MoN

_{2,3}edge between 32 and 35 eV in MoS

_{2}. Comparing the XUV absorption spectra of bulk and monolayer MoS

_{2}, a ~4 eV red-shift suggests a tightly bound core-exciton. The lifetime of the core-exciton states can be directly measured in the time domain. Furthermore, transient Stark shifts, coherences, and coherent population transfer between different core-exciton states are observed. [more]

_{2}Se

_{3}/BSCCO [1] and graphene/h-BN [2,3]. Iwill show how the proximity effect (in Bi

_{2}Se

_{3}/BSCCO) and moire superlatticepotential (in graphene/h-BN) tune the electronic properties and further lead tothe realization of many novel quantum phenomena. The variety of 2D materialsgenerates great possibilities in 2D heterostructures which are waiting for moreresearch investigations. [more]

_{2}, and show that its success is the result of error compensation between semi-local and non-local exchange, resulting in a proper derivative discontinuity (reproduction of the band gap) and a total energy which is a linear function of the fractional occupation numbers (removing most of the electron self-interaction). As it is well known, however, HSE06 does not work equally well for all materials. On the example of Ga

_{2}O

_{3}, I will show that tuning both the mixing and the screening parameter of HSE for the given material allows to ensure the same error compensation. Unless the electronic screening is strongly direction- or orbital-dependent (as in ZnO), the optimized HSE hybrid is nearly self-interaction free and provides a band structure on par with GW. Since the total energy can also be calculated, the real equilibrium structure of a defect can be found and the levels are in good agreement with experimental observations. [more]

_{n+1}IrnO

_{3n+1}realizes an interesting analogue to the cuprate high Tc superconductor. When electron-doped, the single-layer Sr

_{2}IrO

_{4}is shown to manifest a pseudogap phase and possibly d-wave superconductivity. Using second harmonic generation rotational anisotropy, we show that near the antiferromagnetic transition there exists a magnetic multipolar order that breaks inversion symmetry in hole-doped Sr

_{2}IrO

_{4}. One candidate for this order is the loop current order that is predicted to exist in the pseudogap region of cuprate. In bilayer Sr

_{3}Ir

_{2}O

_{7}, electron-doping has been found to induce an insulator-to-metal transition in addition to a structural phase transition at low temperature. There has been no prior experimental evidence of electronic instabilities in the bilayer system. Using time-resolved optical reflectivity, we detect a charge-density-wave-like instability in metallic samples of electron-doped Sr

_{3}Ir

_{2}O

_{7}. The absence of signatures of a new spatial periodicity from diffraction, scanning tunnelling and photoemission based probes suggests an unconventional and possibly short-ranged nature of this density wave order. [more]

_{2}, it has been found that the spin randomization is characteristically faster than the time scales for inter- and intra-valley scatterings. In this talk, I present our recent study of the ultrafast non-collinear spin dynamics of the electron in a valley of monolayer MoS2 by using real-time propagation time-dependent density functional theory. We found that the spin precession is sharply selectively coupled only with the particular optical phonon that lifts the in-plane mirror symmetry. We suggest that the observed spin randomization can be attributed to this spin-phonon interaction. Further, our results imply that flipping of spins in a spin-orbit-coupled system can be achieved by the control over phonons. In a later part of the talk, I would also describe the feature of the computational package we have developed and have used for the spin-phonon dynamics, which is based on the plane-wave basis set and various types of pseudopotentials. [more]

*g*" is generically the lowest order symmetry-allowed direct coupling of an IR-active phonon coordinate

_{2}n_{el }x^{2}_{IR}*x*to the electronic density

_{IR}*n*in systems with inversion symmetry. In this talk I will present model evidence for light-enhanced electron-phonon coupling and light-induced effective attraction between electrons based on nonlinear electron-phonon coupling [3], the latter of which was already discussed in a similar context in [4]. [1] M. Mitrano et al., Nature 530, 461 (2016) [2] E. Pomarico et al., Phys. Rev. B 95, 024304 (2017) [3] M. A. Sentef, arXiv: 1702.00952 [4] D. M. Kennes et al., Nature Physics (2017) , doi:10.1038/nphys4024, arXiv:1609.03802 [more]

_{el}_{3}Fe

_{5}O

_{12}(YIG) and find a quenching of magnetic order on a time scale as short as 1 ps. This observation attests to a highly efficient coupling of crystal lattice and electron spins in this material. [more]