Nader Zaki

Direct Measurement of the Tunable Electronic Structure of Bilayer MoS2 by Interlayer Twist

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS2). Our measurements, performed on arbitrarily stacked bilayer MoS2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center (Γ̅ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ∼40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle. While our results at Γ̅ agree with recently published photoluminescence data, our measurements of the quasiparticle spectrum over the full 2D Brillouin zone reveal a richer and more complicated change in the electronic structure than previously theoretically predicted. The electronic structure measurements reported here, including the evolution of the effective mass with twist-angle, provide new insight into the physics of twisted transition-metal dichalcogenide bilayers and serve as a guide for the practical design of MoS2 optoelectronic and spin-/valley-tronic devices.

Tuning the electronic structure of monolayer graphene/ Mo S 2 van der Waals heterostructures via interlayer twist

We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer MoS2, the resulting band structure reveals the absence of hybridization between the graphene and MoS2 electronic states. Further, the graphene-derived electronic structure in the heterostructures remains intact, irrespective of the twist angle between the two materials. In contrast, however, the electronic structure associated with the MoS2 layer is found to be twist-angle dependent; in particular, the relative difference in the energy of the valence band maximum at {\Gamma} and K of the MoS2 layer varies from approximately 0 to 0.2 eV. Our results suggest that monolayer MoS2 within the heterostructure becomes predominantly an indirect bandgap system for all twist angles except in the proximity of 30 degrees. This result enables potential bandgap engineering in van der Waals heterostructures comprised of monolayer structures.

Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H–NbSe2

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe2 that we measure by scanning tunneling spectroscopic imaging. We show, from the momentum and energy dependence of the quasiparticle interference, that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe2. We demonstrate that, by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wave vector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions. In 2H-NbSe2, we use this combination to confirm that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the charge density wave ordering wave vector.

Trapping surface electrons on graphene layers and islands

(Received 9 September 2011; revised manuscript received 26 January 2012; published 13 February 2012)We report the use of time- and angle-resolved two-photon photoemission to map the bound, unoccupiedelectronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of thelowest three image-potential states are measured. In addition, the weak interaction between Ir and graphenepermits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrateto graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of midgapclean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands isprovided by coverage-dependent measurements.DOI: 10.1103/PhysRevB.85.081402 PACS number(s): 73

Experimental observation of spin-exchange-induced dimerization of an atomic one-dimensional system

Using low-temperature scanning tunneling microscopy, we demonstrate an unambiguous 1-D system that surprisingly undergoes a CDW instability on a metallic substrate. Our ability to directly and quantitatively measure the structural distortion of this system provides an accurate reference for comparison with first principles theory. In comparison to previously proposed physical mechanisms, we attribute this particular 1-D CDW instability to a ferromagnetic state. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.

Atom-wide Co Wires on Cu(775) at Room Temperature

We report on a surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300 K. Scanning tunneling microscopy (STM)-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases. The formation of relatively long laterally unencapsulated one- and two-atom wires also differentiates it from past experimentally observed step-island formation. Furthermore, our experiments also show directly that the Co wires coexist with another Co phase that had been previously predicted for growth on Cu(111). Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model.

Surface states on vicinal Cu(775): STM and photoemission study

We report angle-resolved photoemission spectroscopy (ARPES) and a set of in situ scanning tunneling microscopy (STM) measurements on a narrow-terrace-width vicinal Cu(111) crystal surface, Cu(775), whose vicinal cut lies close to the transition between terrace and step modulation. These measurements show sharp zone-folding (or umklapp) features with a periodicity in

Failure of DFT-based computations for a stepped-substrate-supported correlated Co wire

{k}_{\ensuremath{\parallel}}

Cuprate phase diagram and the influence of nanoscale inhomogeneities

, indicating that the predominant reference plane is that of Cu(775), i.e., that the surface is predominately step-modulated. Our measurements also show variation in umklapp intensity with photon energy, which is consistent with prior ARPES experiments on other vicinal Cu(111) surfaces and in agreement with our designation of the state as being step-modulated. The measurements also show a weak terrace-modulated state, which, based on several characteristics, we attribute to the presence of terrace widths larger than the ideal terrace width. By measuring the intensity ratio of the two distinct surface-state modulations from photoemission and the terrace-width distribution from STM, we derive a value for the terrace width at which the surface state switches between the two modulations.

Photoemission band mapping with a tunable femtosecond source using nonequilibrium absorption resonances

Density functional theory (DFT) has been immensely successful in its ability to predict physical properties, and, in particular, structures of condensed matter systems. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, notch, and embedded in bulk. The results demonstrate that increasing Co coordination extinguishes the dimerization, indicating that the failure of DFT for Co on the Cu step arises from excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that including local interactions via DFT+U or DFT+DMFT does not restore the dimerization for the step-substrate supported wire, though the Co wire does dimerize in DFT+DMFT for the isolated vacuum case. This system can serve as a benchmark for future electronic structure methods.