Abstract:We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS 2 /WSe 2 and MoSe 2 /MoS 2 heterobilayers as well as twisted MoS 2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30 ~ 40 cm -1 ) that arise from the layerbreathing mode (LBM) vibrations between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and atomically clean interface. Its frequency also evolves systematically with the relative orientation between of the two layers. Our research demonstrates that LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.2

Photoemission studies show the presence of a high-energy anomaly in the observed band dispersion for two families of cuprate superconductors, Bi2Sr2CaCu2O8+delta and La 2-x BaxCuO4. The anomaly, which occurs at a binding energy of approximately 340 meV, is found to be anisotropic and relatively weakly doping dependent. Scattering from short range or nearest neighbor spin excitations is found to supply an adequate description of the observed phenomena.

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.

Kidd et al. Reply: In the preceding Comment [1], Petersen, Ismail, and Plummer raised two interesting and possibly controversial points about our recent work on the phase transition in Sn͞Ge͑111͒ [2]. Their first point is that the calculated amount of charge transfer needed to raise the Fermi level for nesting over the entire surface is more than what the impurities can provide. The second point is that highly electronegative TCNQ molecules adsorbed on the surface do not suppress the transition, and that is, as their argument goes, at variance with our model. We will explain below that these points merit careful consideration, but do not invalidate our model.A short answer to the first point is that our model does not require a full shift in Fermi level over the entire surface. As explained in our earlier work [2], a shift of just 10 meV (half of the amount cited in [1] and less than k B T 25 meV at room temperature) already leads to a noticeable ͑3 3 3͒ response based on a calculation of the response function in reciprocal space. A more detailed study, based on a real-space calculation of the response function, reveals that the qualitative features of the ͑3 3 3͒ response are actually insensitive to the magnitude of the charge transfer. The calculation involves an N 3 N array of Sn atoms within a tight-binding formalism [1 -3]. The potential energy at the central atom is shifted by DV 10.001 eV to create an electron deficiency, thus simulating the effect of a defect as an electron donor. The value of DV is chosen to be very small in order to keep the system in the linear response regime. The Hamiltonian, with the impurity potential included, is diagonalized, and the resulting charge redistribution on the lattice is calculated.The results for N 41 at a temperature of 300 K are shown in Fig. 1 for a region near the central impurity atom. Each circle represents an atom, and a star marks the central atom. Red (blue) color is used to indicate a positive (negative) charge, and neutral atoms are shown in white. The intensity of the red or blue color is an indication of the amount of the excess charge. The response to the impurity is fairly local, and we have verified that N is large enough to avoid boundary effects. The main features of the charge response include six blue atoms in the first shell surrounding the central atom and six red atoms in the next shell. Beyond that, the charge response decays rapidly. The results look very similar to what STM shows at room temperature [4]. Thus, the local response is indeed ͑3 3 3͒ even for a very small impurity potential.Regarding the second point, we note that not all impurities or adsorbates can act as charge donors or acceptors. In fact, highly electropositive or electronegative species tend to form deep impurity levels (traps) that are irrelevant to doping in the host. This is likely the case for the TCNQ molecules. The chemical bonds between TCNQ and the substrate can give rise to a dipolar layer and an altered work function as observed in the experiment, and this is not...