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We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AF) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce a Q-resolved effective lattice temperature, and identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ~0.3 ps and persists for 25 ps. We associate this with transient changes to the AF exchange striction-induced lattice distortion, supported by the observation of a transient Q-asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order., Manuscript and supplement combined in one file

We performed angle-resolved photoemission spectroscopy (ARPES) of bulk2H-WSe$_2$ for different crystal orientations linked to each other bytime-reversal symmetry. We introduce a new observable called time-reversaldichroism, which quantifies the modulation of the photoemission intensity uponeffective time-reversal operation. We demonstrate that the hiddenorbital-texture of the crystal's electronic structure leaves its imprint ontothe time-reversal dichroism, due to multiple orbitals interference effects inphotoemission. Our experimental results are in quantitative agreement withstate-of-the-art fully relativistic calculations performed using the one-stepmodel of photoemission. While spin-resolved ARPES probes the spin component ofentangled spin-orbital texture in multiorbital systems, we demonstrate thattime-reversal dichroism is sensitive to its orbital-texture counterpart.

Using femtosecond time-resolved x-ray diffraction we investigate the structural dynamics of the coherently excited A(1g) phonon mode in the Fe-pnictide parent compound BaFe2As2. The fluence dependent intensity oscillations of two specific Bragg reflections with distinctly different sensitivity to the pnictogen height in the compound allow us to quantify the coherent modifications of the Fe-As tetrahedra, indicating a transient increase of the Fe magnetic moments. By a comparison with time-resolved photoemission data, we derive the electron-phonon deformation potential for this particular mode. The value of Delta mu/Delta z = -(1.0-1.5)eV/angstrom is comparable with theoretical predictions and demonstrates the importance of this degree of freedom for the electron-phonon coupling in the Fe pnictides.

Strain is a leading candidate for controlling magnetoelectric coupling in multiferroics. Here, we use x-ray diffraction to study the coupling between magnetic order and structural distortion in epitaxial films of the orthorhombic (o-) perovskite LuMnO(3). An antiferromagnetic spin canting in the E-type magnetic structure is shown to be related to the ferroelectrically induced structural distortion and to a change in the magnetic propagation vector. By comparing films of different orientations and thicknesses, these quantities are found to be controlled by b-axis strain. It is shown that compressive strain destabilizes the commensurate E-type structure and reduces its accompanying ferroelectric distortion.

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ around the high-symmetry points $\mathrm{\ensuremath{\Gamma}}$ and $M$. A global oscillation of the Fermi level at the frequency of the ${A}_{1g}(\mathrm{As})$ phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the ${A}_{1g}(\mathrm{As})$ phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.

Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$. Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.

The nonequilibrium state of the high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode, since their decay toward the nodal region through $e\mathrm{\text{\ensuremath{-}}}e$ scattering is blocked by phase space restrictions. The observed lack of momentum dependence in the decay rates is in agreement with relaxation dynamics dominated by Cooper pair recombination in a boson bottleneck limit.

Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP ${\mathrm{CsPbBr}}_{3}$ by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of $0.26\ifmmode\pm\else\textpm\fi{}0.02\text{ }{m}_{e}$, 50% heavier than the bare mass ${m}_{0}=0.17\text{ }{m}_{e}$ predicted by density functional theory. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fr\"ohlich coupling parameter $\ensuremath{\alpha}=1.81$. A good agreement with our experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.