*Nat Commun ; 12(1): 6837, 2021 Nov 25.*

##### RESUMO

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.

*Nat Commun ; 11(1): 3997, 2020 Aug 10.*

##### RESUMO

A current challenge in condensed matter physics is the realization of strongly correlated, viscous electron fluids. These fluids can be described by holography, that is, by mapping them onto a weakly curved gravitational theory via gauge/gravity duality. The canonical system considered for realizations has been graphene. In this work, we show that Kagome systems with electron fillings adjusted to the Dirac nodes provide a much more compelling platform for realizations of viscous electron fluids, including non-linear effects such as turbulence. In particular, we find that in Scandium Herbertsmithite, the fine-structure constant, which measures the effective Coulomb interaction, is enhanced by a factor of about 3.2 as compared to graphene. We employ holography to estimate the ratio of the shear viscosity over the entropy density in Sc-Herbertsmithite, and find it about three times smaller than in graphene. These findings put the turbulent flow regime described by holography within the reach of experiments.

*Phys Rev Lett ; 123(20): 206403, 2019 Nov 15.*

##### RESUMO

We investigate the effects of external dielectric screening on the electronic dispersion and the band gap in the atomically thin, quasi-two-dimensional (2D) semiconductor WS_{2} using angle-resolved photoemission and optical spectroscopies, along with first-principles calculations. We find the main effect of increased external dielectric screening to be a reduction of the quasiparticle band gap, with rigid shifts to the bands themselves. Specifically, the band gap of monolayer WS_{2} is decreased by about 140 meV on a graphite substrate as compared to a hexagonal boron nitride substrate, while the electronic dispersion of WS_{2} remains unchanged within our experimental precision of 17 meV. These essentially rigid shifts of the valence and conduction bands result from the special spatial structure of the changes in the Coulomb potential induced by the dielectric environment of the monolayer.

*ACS Nano ; 13(9): 10210-10220, 2019 Sep 24.*

##### RESUMO

For quasi-freestanding 2H-TaS2 in monolayer thickness grown by in situ molecular beam epitaxy on graphene on Ir(111), we find unambiguous evidence for a charge density wave close to a 3 × 3 periodicity. Using scanning tunneling spectroscopy, we determine the magnitude of the partial charge density wave gap. Angle-resolved photoemission spectroscopy, complemented by scanning tunneling spectroscopy for the unoccupied states, makes a tight-binding fit for the band structure of the TaS2 monolayer possible. As hybridization with substrate bands is absent, the fit yields a precise value for the doping of the TaS2 layer. Additional Li doping shifts the charge density wave to a 2 × 2 periodicity. Unexpectedly, the bilayer of TaS2 also displays a disordered 2 × 2 charge density wave. Calculations of the phonon dispersions based on a combination of density-functional theory, density-functional perturbation theory, and many-body perturbation theory enable us to provide phase diagrams for the TaS2 charge density wave as functions of doping, hybridization, and interlayer potentials, and offer insight into how they affect lattice dynamics and stability. Our theoretical considerations are consistent with the experimental work presented and shed light on previous experimental and theoretical investigations of related systems.

*J Phys Condens Matter ; 31(46): 465603, 2019 Nov 20.*

##### RESUMO

The theory of correlated electrons is currently moving beyond the paradigmatic Hubbard U, towards the investigation of intersite Coulomb interactions. Recent investigations have revealed that these interactions are relevant for the quantitative description of realistic materials. Physically, intersite interactions are responsible for two rather different effects: screening and bandwidth renormalization. We use a variational principle to disentangle the roles of these two processes and study how appropriate the recently proposed Fock treatment of intersite interactions is in correlated systems. The magnitude of this effect in graphene is calculated based on cRPA values of the intersite interaction. We also apply the variational principle to benzene and find effective parameters comparable to those obtained by ab initio density matrix downfolding.

*Nano Lett ; 19(5): 3182-3186, 2019 05 08.*

##### RESUMO

The observation of quantum light emission from atomically thin transition metal dichalcogenides has opened a new field of applications for these material systems. The corresponding excited charge-carrier localization has been linked to defects and strain, while open questions remain regarding the microscopic origin. We demonstrate that the bending rigidity of these materials leads to wrinkling of the two-dimensional layer. The resulting strain field facilitates strong carrier localization due to its pronounced influence on the band gap. Additionally, we consider charge carrier confinement due to local changes of the dielectric environment and show that both effects contribute to modified electronic states and optical properties. The interplay of surface wrinkling, strain-induced confinement, and local changes of the dielectric environment is demonstrated for the example of nanobubbles that form when monolayers are deposited on substrates or other two-dimensional materials.

*Nat Commun ; 10(1): 180, 2019 01 14.*

##### RESUMO

Charge transfers resulting from weak bondings between two-dimensional materials and the supporting substrates are often tacitly associated with their work function differences. In this context, two-dimensional materials could be normally doped at relatively low levels. Here, we demonstrate how even weak hybridization with substrates can lead to an apparent heavy doping, using the example of monolayer 1H-TaS2 grown on Au(111). Ab-initio calculations show that sizable changes in Fermi areas can arise, while the transferred charge between substrate and two-dimensional material is much smaller than the variation of Fermi areas suggests. This mechanism, which we refer to as pseudodoping, is associated with non-linear energy-dependent shifts of electronic spectra, which our scanning tunneling spectroscopy experiments reveal for clean and defective TaS2 monolayer on Au(111). The influence of pseudodoping on the formation of many-body states in two-dimensional metallic materials is analyzed, shedding light on utilizing pseudodoping to control electronic phase diagrams.

*Nano Lett ; 18(4): 2725-2732, 2018 04 11.*

##### RESUMO

The electronic and optical properties of monolayer transition-metal dichalcogenides (TMDs) and van der Waals heterostructures are strongly subject to their dielectric environment. In each layer, the field lines of the Coulomb interaction are screened by the adjacent material, which reduces the single-particle band gap as well as exciton and trion binding energies. By combining an electrostatic model for a dielectric heteromultilayered environment with semiconductor many-particle methods, we demonstrate that the electronic and optical properties are sensitive to the interlayer distances on the atomic scale. An analytic treatment is used to provide further insight into how the interlayer gap influences different excitonic transitions. Spectroscopical measurements in combination with a direct solution of a three-particle Schrödinger equation reveal trion binding energies that correctly predict recently measured interlayer distances and shed light on the effect of temperature annealing.

*J Phys Condens Matter ; 30(6): 065502, 2018 02 14.*

##### RESUMO

The electronic structure of thin films of FeTe grown on Bi2Te3 is investigated using angle-resolved photoemission spectroscopy, scanning tunneling microscopy and first principles calculations. As a comparison, data from cleaved bulk Fe1.08Te taken under the same experimental conditions is also presented. Due to the substrate and thin film symmetry, FeTe thin films grow on Bi2Te3 in three domains, rotated by 0°, 120°, and 240°. This results in a superposition of photoemission intensity from the domains, complicating the analysis. However, by combining bulk and thin film data, it is possible to partly disentangle the contributions from three domains. We find a close similarity between thin film and bulk electronic structure and an overall good agreement with first principles calculations, assuming a p-doping shift of 65 meV for the bulk and a renormalization factor of around two. By tracking the change of substrate electronic structure upon film growth, we find indications of an electron transfer from the FeTe film to the substrate. No significant change of the film's electronic structure or doping is observed when alkali atoms are dosed onto the surface. This is ascribed to the film's high density of states at the Fermi energy. This behavior is also supported by the ab initio calculations.

*Nano Lett ; 17(11): 6721-6726, 2017 11 08.*

##### RESUMO

We analyze the interplay of spin-valley coupling, orbital physics, and magnetic anisotropy taking place at single magnetic atoms adsorbed on semiconducting transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se). Orbital selection rules turn out to govern the kinetic exchange coupling between the adatom and charge carriers in the MX2 and lead to highly orbitally dependent spin-flip scattering rates, as we illustrate for the example of transition metal adatoms with d9 configuration. Our ab initio calculations suggest that d9 configurations are realizable by single Co, Rh, or Ir adatoms on MoS2, which additionally exhibit a sizable magnetic anisotropy. We find that the interaction of the adatom with carriers in the MX2 allows to tune its behavior from a quantum regime with full Kondo screening to a regime of "Ising spintronics" where its spin-orbital moment acts as classical bit, which can be erased and written electronically and optically.

*Phys Rev Lett ; 115(23): 236101, 2015 Dec 04.*

##### RESUMO

We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene, a case of pure van der Waals interaction, strengthens with n and weakens with p doping of graphene. Density-functional theory calculations that include the van der Waals interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the van der Waals interaction is modified by changing the spatial extent of graphene's π orbitals via doping.

*Phys Rev Lett ; 114(12): 125503, 2015 Mar 27.*

##### RESUMO

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E_{1u} lattice vibration at 6.3 µm. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme-ultraviolet (XUV) pulses, we measure the response of the Dirac electrons near the K point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the E_{1u} vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.

*Phys Rev Lett ; 114(4): 047403, 2015 Jan 30.*

##### RESUMO

In this Letter, we study the electronic structures and optical properties of partially and fully fluorinated graphene by a combination of ab initio G0W0 calculations and large-scale multiorbital tight-binding simulations. We find that, for partially fluorinated graphene, the appearance of paired fluorine atoms is more favorable than unpaired atoms. We also show that different types of structural disorder, such as carbon vacancies, fluorine vacancies, fluorine vacancy clusters and fluorine armchair and zigzag clusters, will introduce different types of midgap states and extra excitations within the optical gap. Furthermore, we argue that the local formation of sp3 bonds upon fluorination can be distinguished from other disorder inducing mechanisms which do not destroy the sp2 hybrid orbitals by measuring the polarization rotation of passing polarized light.

*Phys Rev Lett ; 113(24): 246601, 2014 Dec 12.*

##### RESUMO

Hydrogen adatoms and other species covalently bound to graphene act as resonant scattering centers affecting the electronic transport properties and inducing Anderson localization. We show that attractive interactions between adatoms on graphene and their diffusion mobility strongly modify the spatial distribution, thus fully eliminating isolated adatoms and increasing the population of larger size adatom aggregates. Such spatial correlation is found to strongly influence the electronic transport properties of disordered graphene. Our scaling analysis shows that such aggregation of adatoms increases conductance by up to several orders of magnitude and results in significant extension of the Anderson localization length in the strong localization regime. We introduce a simple definition of the effective adatom concentration x*, which describes the transport properties of both random and correlated distributions of hydrogen adatoms on graphene across a broad range of concentrations.

*Nano Lett ; 13(12): 6251-5, 2013.*

##### RESUMO

Topological insulators (TIs) represent a new quantum state of matter characterized by robust gapless states inside the insulating bulk gap. The metallic edge states of a two-dimensional (2D) TI, known as the quantum spin Hall (QSH) effect, are immune to backscattering and carry fully spin-polarized dissipationless currents. However, existing 2D TIs realized in HgTe and InAs/GaSb suffer from small bulk gaps (<10 meV) well below room temperature, thus limiting their application in electronic and spintronic devices. Here, we report a new 2D TI comprising a graphene layer sandwiched between two Bi2Se3 slabs that exhibits a large intrinsic bulk band gap of 30-50 meV, making it viable for room-temperature applications. Distinct from previous strategies for enhancing the intrinsic spin-orbit coupling effect of the graphene lattice, the present graphene-based TI operates on a new mechanism of strong inversion between graphene Dirac bands and Bi2Se3 conduction bands. Strain engineering leads to effective control and substantial enhancement of the bulk gap. Recently reported synthesis of smooth graphene/Bi2Se3 interfaces demonstrates the feasibility of experimental realization of this new 2D TI structure, which holds great promise for nanoscale device applications.

##### Assuntos

Grafite/química , Nanoestruturas/química , Propriedades de Superfície , Bismuto/química , Eletrônica , Membranas Artificiais , Pontos Quânticos/química , Selênio/química , Temperatura*Nano Lett ; 13(11): 5013-9, 2013 Nov 13.*

##### RESUMO

The ease by which graphene is affected through contact with other materials is one of its unique features and defines an integral part of its potential for applications. Here, it will be demonstrated that intercalation, the insertion of atomic layers in between the backside of graphene and the supporting substrate, is an efficient tool to change its interaction with the environment on the frontside. By partial intercalation of graphene on Ir(111) with Eu or Cs we induce strongly n-doped graphene patches through the contact with these intercalants. They coexist with nonintercalated, slightly p-doped graphene patches. We employ these backside doping patterns to directly visualize doping induced binding energy differences of ionic adsorbates to graphene through low-temperature scanning tunneling microscopy. Density functional theory confirms these binding energy differences and shows that they are related to the graphene doping level.

*Phys Rev Lett ; 110(8): 086111, 2013 Feb 22.*

##### RESUMO

Intercalation of Eu under graphene on Ir(111) results in patterns oriented along the graphene moiré and quantized in size by its unit mesh. The patterns are formed by stripes, compact islands, and channels. Over a wide range of intercalated amounts the step concentration of the pattern has a rather constant saturation value. These findings are explained by the chemically modulated binding of graphene to the substrate and the preexisting strain in graphene due to its cooldown from the growth temperature. Local variations in the intercalation step density appear to reflect local variations in the preexisting strain.

*Nanoscale ; 5(8): 3306-14, 2013 Apr 21.*

##### RESUMO

Using density functional theory calculations, we show that recently synthesized carbon nanocomposites of graphene nanoribbons encapsulated in a carbon nanotube (GNR@CNT) possess rich emergent electronic and magnetic properties that offer new functionality and tunability and display systematic trends that are sensitive to the matchup of constitutive GNRs and CNTs. The encapsulation of H-passivated GNRs in metallic armchair CNTs always leads to a metallic complex while those in semiconducting zigzag CNTs can be either metallic or semiconducting depending on the chirality of GNRs. In particular, the complex of armchair GNRs in a zigzag CNT exhibits an oscillating electronic band gap with changing GNR width and a well-separated spatial distribution of electrons and holes localized in the CNT and GNR components, respectively. When bare large zigzag GNRs are encapsulated in an armchair CNT, the resulting complex shows strong structural stability and enhanced magnetism and, most interestingly, such GNR@CNT configurations can be tuned to be metallic or semiconducting depending on relative bond position and magnetic order. These results offer key insights for understanding and predicting emergent properties of GNR@CNT, which establish a roadmap for guiding design and synthesis of specific nanocomposite configurations with tailor-made properties for nanoelectronic, photovoltaic and spintronic applications.

*Nano Lett ; 13(3): 1073-9, 2013 Mar 13.*

##### RESUMO

In Dirac materials, like graphene or topological insulators, massless pseudorelativistic electrons promise new, very fast electronic devices by utilizing the partial suppression of backscattering. However, the semimetal nature of graphene makes the realization of practical field effect transistors difficult, due to small on-off current ratios. Here, we propose a new concept, based on Dirac states inside the conduction (or valence) band of a lightly doped wide band gap semiconductor. With the application of a gate voltage, the Dirac states become populated; that is, the Fermi level is switched between the "classical" high-resistivity semiconducting and the relativistic high-mobility metallic range. We demonstrate by theoretical calculations that such a transition can be realized, for example, in thin anatase nanowires, which have been synthesized before. Ta-doped anatase nanowires offer an excellent possibility to build field effect transistors with high speed and good on-off ratio. Guidelines for finding similar "Dirac semiconductors" are provided.

*Science ; 339(6115): 55-9, 2013 Jan 04.*

##### RESUMO

The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.