Evidence for a diamondlike electronic band structure of Si multilayers on Ag(111)

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a sp3 diamondlike structure. This refutes the interpretation o silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers

Absence of Dirac cones in monolayer silicene and multilayer Si films on Ag(111)

Monolayer silicene and multilayer silicon films on Ag(111) have been the subject of many investigations within the last few years. For both systems, photoemission data have been interpreted in terms of linearly dispersing bands giving rise to the characteristic Dirac cone features, similar to graphene. Here we demonstrate, on the basis of angle-resolved valence band and core level photoemission data that this assignment is not correct. The bands previously attributed to states with Dirac fermion character are shown to derive from Ag(111) interface and bulk states in the silicene monolayer and from the well-known Ag-View the MathML source(3×3)R30°-Si(111) structure in Si multilayers. These results question the validity of the claim that graphene-like silicene and silicene multilayers are in fact formed on Ag(111)

Quasi-free-standing single-layer WS2 achieved by intercalation

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single-layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single-layer's native properties. This dilemma can potentially be solved by decoupling the single-layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer WS2-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer WS2

Phonon collapse and van der Waals melting of the 3D charge density wave of VSe2_2

Among transition metal dichalcogenides (TMDs), VSe$_2$ is considered to develop a purely 3-dimensional (3D) charge-density wave (CDW) at T$_{CDW}$=110 K. Here, by means of high resolution inelastic x-ray scattering (IXS), we show that the CDW transition is driven by the collapse of an acoustic mode at the critical wavevector \textit{q}$_{CDW}$= (2.25 0 0.7) r.l.u. and critical temperature T$_{CDW}$=110 K. The softening of this mode starts to be pronounced for temperatures below 2$\times$ T$_{CDW}$ and expands over a rather wide region of the Brillouin zone, suggesting a large contribution of the electron-phonon interaction to the CDW formation. This interpretation is supported by our first principles calculations that determine a large momentum-dependence of the electron-phonon interaction, peaking at the CDW wavevector, in the presence of nesting. Fully anharmonic {\it ab initio} calculations confirm the softening of one acoustic branch at \textit{q}$_{CDW}$ as responsible for the CDW formation and show that van der Waals interactions are crucial to melt the CDW. Our work also highlights the important role of out-of-plane interactions to describe 3D CDWs in TMDs

Systematics of electronic and magnetic properties in the transition metal doped Sb2_2Te3_3 quantum anomalous Hall platform

The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically-doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here we report on a detailed and systematic investigation of transition-metal (TM)-doped Sb$_2$Te$_3$. By combining density functional theory (DFT) calculations with complementary experimental techniques, i.e., scanning tunneling microscopy (STM), resonant photoemission (resPES), and x-ray magnetic circular dichroism (XMCD), we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM-doped TIs. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V- and Fe-doped Sb$_2$Te$_3$ display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity-dependent and can vary from in-plane to out-of-plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM-doped Sb$_2$Te$_3$ in the ferromagnetic state.Comment: 40 pages, 13 figure