In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal–insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state.

We investigate the magnetic and electronic properties of europium cyclooctatetraene nanowires (EuCot) by means of lowtemperature x-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as a wide-band-gap semiconductor with a band gap of 2.3 eV. By means of Eu M 5,4 edge XMCD, orbital and spin magnetic moments of (−0.1 ± 0.3) µ B and (+7.0 ± 0.6) µ B , respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M 5 edge show hysteresis for grazing x-ray incidence at 5 K, thus confirming EuCot as a ferromag-netic material. Our density functional theory 17 calculations reproduce the experimentally ob-18 served bandgap. Modelling the experimental 19 results theoretically, we find that the effective 20 interatomic exchange interaction between Eu 21 atoms is of the order of meV, that magne-22 tocrystalline anisotropy energy is roughly half 23 as big and that dipolar energy is approximately 24 ten times lower.

The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or non-local relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, non-thermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with non-equilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these non-local, non-equilibrium dynamics.

Sandwich-molecular wires consisting of europium and cyclooctatetraene (Cot) were grown in situ on the moiré of graphene with Ir(110). The moiré templates a uniaxial alignment of monolayer EuCot nanowire carpets and multilayer films with the EuCot wire axis along the [001] direction of the Ir substrate. Using angle-resolved photoemission spectroscopy, we investigate the band structure of the wire carpet films. While π-derived bands were not observed experimentally, we find a flat band 1.85 eV below the Fermi energy. Using density-functional theory and X-ray photoelectron spectroscopy and replacing europium through barium in the sandwich-molecular wires, it is concluded that the flat band is derived from Eu 4f states weakly mixed with Eu 5d states and slightly overlapping with Cot π states. X-ray magnetic circular dichroism is employed to characterize the magnetic properties of the EuCot wire carpet films at low temperatures. Clear evidence for an easy-axis magnetization along the wires is found.

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