Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of α-RuCl3, in which the Ru ions are in a d 5 configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that α-RuCl3 is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto J ef f states. This makes α-RuCl3 a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10Dq and the charge transfer energy ∆ are evaluated. Furthermore, we observe significant Cl photodesorption with time, which must be taken into account when interpreting photoemission and other surface sensitive experiments.

We report on the energy- and momentum-resolved optical response of black phosphorus (BP) in its bulk form. Along the armchair direction of the puckered layers, we find a highly dispersive mode that is strongly suppressed in the perpendicular (zigzag) direction. This mode emerges out of the single-particle continuum for finite values of momentum and is therefore interpreted as an exciton. We argue that this exciton, which has already been predicted theoretically for phosphorene-the monolayer form of BP-can be detected by conventional optical spectroscopy in the two-dimensional case and might pave the way for optoelectronic applications of this emerging material.

A quantum spin-liquid might be realized in α-RuCl3, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, α-RuCl3 is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K0.5RuCl3. This gives rise to a peculiar charge disproportionation into formally Ru 2+ (4d 6 ) and Ru 3+ (4d 5 ). Every Ru 4d 5 site with one hole in the t2g shell is surrounded by nearest neighbors of 4d 6 character, where the t2g level is full and magnetically inert. Thus, each type of Ru sites forms a triangular lattice and nearest-neighbor interactions of the original honeycomb are blocked.

We combine electron energy-loss spectroscopy and first-principles calculations based on densityfunctional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of ∼ 100 meV and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the ΓK-direction of the hexagonal Brillouin zone.Our data support the recent report by Cassabois et al. [1] who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor. 1 arXiv:1706.04806v1 [cond-mat.mes-hall] 15 Jun 2017Hexagonal Boron Nitride (h-BN) is the binary analog of graphite. Like its carbon counterpart, it consists of sp 2 -hybridized hexagonal layers that are stacked along the crystallographic c-axis. This leads to many similarities between these two materials like the applicability as dry lubricants or the possibility to wrap the hexagonal sheets into nanotubes [2,3].More recently, h-BN came into focus in the field of 2D semiconductors, either as a substrate to boost the performance of graphene devices [4] or as an important ingredient in so called van der Waals heterostructures [5]. These overall similarities between the carbon and BN-case notwithstanding, the electronegativity difference between the boron-and nitrogen-atoms has profound implications which show up most prominently in the electronic structure, in particular the fundamental band gap. Though its apparent simplicity in terms of structure and electronic properties, there has been an ongoing argument about the nature and size of the band gap. For a long time, there has been a strong controversy about whether the fundamental gap is direct or indirect and reported values on the size of the gap E G range from below 4 eV to more than 7 eV, depending on the sample quality and the employed method (see e.g. [6,7] for extended compilations of available data on E G ).With time, in particular in the theoretical community, consensus emerged that the gap is indirect [8-10] but still, calculated values differed substantially between 3.9 eV and 5.95 eV.These predictions were, however, strongly challenged by the remarkable observation of a strong photoluminescence (PL) signal in h-BN reported in 2004 [11] which promised great potential for lasing in the deep ultraviolet. Recently, Cassabois et al. [1] took an important step forward in resolving this long-standing debate. They inferred -although only indirectly -the existence of the lowest indirect exciton (iX in their nomenclature) at an energy of 5.955 eV from a careful analysis of detailed PL data thereby establishing h-BN as an indirect semiconductor also from an experimental perspective. They also could show that the presence of sharp excitonic absorption (and/or emission) in a material with an indirect band gap -which appears to contradict the traditional understanding [12] -occurs as a conseq...

We have investigated the electronic excitations in bulk 2H-MoS2 using electron energy-loss spectroscopy. The electron energy-loss spectra in the ΓM and ΓK directions were measured for various momentum transfer values. The results allow the identification of the A1 and B1 exciton peaks and in particular their energy-momentum dispersion. The dispersions exhibit approximately quadratic upward trends and slight anisotropies in the ΓM and ΓK directions. The fitted energy-momentum transfer functions allow the estimation of the effective masses of the excitons which are in close proximity to predicted values.