Search

We acknowledge financial support from Ministerio de Ciencia e Innovacion MCIN/AEI/10.13039/501100011033 (Spain) under Grants No. PID2020-113565GB-C22 and No. PID2020-113565GB-C21, and from Junta de Andalucia group PAIDI-205. M.C.G. acknowledges funding from Fondo Europeo de Desarrollo Regional (FEDER) and Consejeria de Economia, Conocimiento, Empresas y Universidad de la Junta de Andalucia, en marco del programa operativo FEDER Andalucia 2014-2020, Objetivo especifico 1.2.3, "Fomento y generacion de conocimiento frontera y de conocimiento orientado a los retos de la sociedad, desarrollo de tecnologias emergentes" under Grant No. UPO-1380159, Porcentaje de cofinanciacion FEDER 80%. J.B. acknowledges financial sup-port from Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya, co -funded by the European Union Regional Development Fund within the ERDF Operational Program of Catalunya (project QuantumCat, Ref. No. 001-P-001644). We also acknowledge the use of the C3UPO computer facilities at the Universidad Pablo de Olavide., The phase diagram of a para-H2 monolayer absorbed on an experimentally synthesized amorphous carbon sheet was calculated using a diffusion Monte Carlo technique. We found that the ground state of that system changed drastically from a perfectly flat substrate to a situation in which the carbon atoms were allowed a certain degree of disorder in the z direction. In the first case, at zero pressure we have a glass of density 0.056 +/- 0.003 angstrom-2 in equilibrium with an incommensurate solid of 0.068 +/- 0.002 angstrom-2. At the equilibrium density, the glass was found to have a tiny, but non-negligible superfluid fraction of less than 1% (0.44 +/- 0.05%). In the z-disordered substrate, we observe a significant enhancement of the superfluid fraction in the glass phase as well as a smaller but not zero value in the incommensurate crystal., Ministry of Science and Innovation, Spain (MICINN) PID2020-113565GB-C21 PAIDI-205, Junta de Andalucia UPO-1380159, European Commission Spanish Government, Consejeria de Economia, Conocimiento, Empresas y Universidad de la Junta de Andalucia, en marco del programa operativo FEDER Andalucia 2014-2020, Objetivo especifico 1.2.3, "Fomento y generacion de conocimiento frontera y de conocimiento orientado a los retos de la sociedad, desarrollo de tecnologías emergentes” UPO-1380159, Generalitat de Catalunya, Porcentaje de cofinanciacion FEDER 80%., European Union Regional Development Fund within the ERDF Operational Program of Catalunya (project QuantumCat, Ref. No. 001-P-001644), PID2020-113565GB-C22

Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how noncollinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI_{3}, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models., We acknowledge financial support from the Swedish Research Council (VR—2018-06807). M. S. acknowledges the support of Ministerio de Ciencia e Innovación (MICINN-Spain) through Grant No. PID2019–108573GBC22, and Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S); of Generalitat de Catalunya (Grant No. 2017 SGR1506); and of the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). P. B. and S. P. acknowledge financial support from Italian MIUR under the PRIN project “Tuning and Understanding Quantum Phases in 2D Materials— Quantum2D,” Grant No. 2017Z8TS5B and “TWEET: Towards Ferroelectricity in Two Dimensions,” Grant No. 2017YCTB59, respectively. D. A. and S. P. acknowledge financial support by the Nanoscience Foundries and Fine Analysis (NFFA-MIUR Italy) project. The authors thankfully acknowledge the computer resources at Pirineus and the technical support provided by CSUC (RES-FI- 2021-1-0034). Additionally, computational work was done on resources at PDC, Stockholm, via the Swedish National Infrastructure for Computing (SNIC)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Quantum simulations with ultra-cold atoms in optical lattices open up an exciting path towards understanding strongly interacting quantum systems. Atom gas microscopes are crucial for this as they offer single-site density resolution, unparalleled in other quantum many-body systems. However, currently a direct measurement of local coherent currents is out of reach. In this work, we show how to achieve that by measuring densities that are altered in response to quenches to non-interacting dynamics, e.g., after tilting the optical lattice. For this, we establish a data analysis method solving the closed set of equations relating tunnelling currents and atom number dynamics, allowing to reliably recover the full covariance matrix, including off-diagonal terms representing coherent currents. The signal processing builds upon semi-definite optimization, providing bona fide covariance matrices optimally matching the observed data. We demonstrate how the obtained information about non-commuting observables allows to lower bound entanglement at finite temperature which opens up the possibility to study quantum correlations in quantum simulations going beyond classical capabilities., 19 pages, 17 figures. Scheme simplified, substantial material added

Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultracompact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of the bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra., Work at Fudan is supported by NSFC (11825403, 11991061), Program for Professor of Special Appointment (Eastern Scholar), and Qing Nian Ba Jian Program. J. S. F. is supported by Anhui Provincial Natural Science Foundation (1908085MA10), and the Opening Foundation of State Key Laboratory of Surface Physics Fudan University (Grant No. KF2019_07). Work at CNR-SPIN supported by Nanoscience Foundries and Fine Analysis (NFFA-MIUR Italy) and by the PRIN-2017 project “TWEET: Towards Ferroelectricity in two dimensions” (IT-MIUR Grant No. 2017YCTB59). X. H. thanks the support by the EU H2020-NMBP-TO-IND-2018 project “INTERSECT” (Grant No. 814487).

We develop a fundamental theory of the long-range electrostatic interactions in two-dimensional crystals by performing a rigorous study of the nonanalyticities of the Coulomb kernel. We find that the dielectric functions are best represented by 2×2 matrices, with nonuniform macroscopic potentials that are two-component hyperbolic functions of the out-of-plane coordinate z. We demonstrate our arguments by deriving the long-range interatomic forces in the adiabatic regime, where we identify a formerly overlooked dipolar coupling involving the out-of-plane components of the dynamical charges. The resulting formula is exact up to an arbitrary multipolar order, which we illustrate in practice via the explicit inclusion of dynamical quadrupoles. By performing numerical tests on monolayer BN, SnS2, and BaTiO3 membranes, we show that our method allows for a drastic improvement in the description of the long-range electrostatic interactions, with comparable benefits to the quality of the interpolated phonon band structure., We are grateful to N. Marzari and S. Ponc´e for bringing this problem to our attention and for their critical reading of an early version of the manuscript. We also thank the two anonymous reviewers for their careful reading and helpful suggestions. We acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECOSpain) through Grants No. MAT2016-77100-C2-2-P and No. PID2019–108573 GB-C22 and Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S) and of Generalitat de Catalunya (Grant No. 2017 SGR1506). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia (CESGA)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases (1H,1T, and 1T′) of Janus WXO (X=S, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles mods. Phonon spectra analysis reveals that while the 1H phase is dynamically stable, the 1T phase exhibits imaginary frequencies and transforms to the distorted 1T′ phase. Ab initio molecular dynamics simulations confirm that 1H- and 1T′−WXO monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of 1H−WXO are found to be isotropic, they are orientation dependent for 1T′−WXO. It is also shown that 1H−WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T′-WSO, 1T′−WXO monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus WXO monolayers together with their large piezoelectric response imply that these systems are interesting for several anoelectronic applications.

Y In alignment with the efforts on miniaturizing the components of electronic devices with enhanced performance, we investigate a dielectric nanocapacitor (DNC) based on metallic borophene electrodes separated with insulating hexagonal boron nitride (h-BN) monolayers (n = 1-5). The capacitive performance of the proposed DNC as a function of applied electric field ((E) over right arrow) and thickness of the dielectric material is examined by using ab initio methods. The borophene plates and h-BN monolayers are commensurate and coupled only with van der Waals interaction, which constitutes an ideal configuration as a DNC. It is found that a single h-BN layer is not thick enough as a spacer to hinder quantum tunneling effects, and similar to the case with no insulating layer, borophene electrodes are shorted. Being effective from two h-BN layers, the charge separation on borophene plates is attained via (E) over right arrow in the vertical direction. The capacitance of the DNC rapidly saturates at (E) over right arrow greater than or similar to 0.1 V/angstrom and reaches its maximum value of 0.77 mu F/cm(2) for n = 2. The capacitance decreases with an increasing number of insulating layers as the distance between electrodes enlarges and shows a similar trend that is expected from the classical Helmholtz model. Our results suggest metallic and lightweight borophene and insulating h-BN monolayers as ideal constituents for the DNC design.

Physical review / B 104(16), L161104 (2021). doi:10.1103/PhysRevB.104.L161104, We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrational excitations. This interpretation is supported by a quantitative line-shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in good agreement with electronic temperature values obtained from valence band data. Furthermore, we show how the momentum change of the outgoing core electrons leads to a detectable but very small change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the presence of a narrow �� band in the C 1s state., Published by Inst., Woodbury, NY

We studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-B2N2) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-B2N2 can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-B2N2 were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-B2N2 can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-B2N2 prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-B2N2 with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-B2N2 and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.

The potential energy surface (PES) of interlayer interaction of twisted bilayer graphene with vacancies in one of the layers is investigated via density functional theory (DFT) calculations with van der Waals corrections. These calculations give a non-negligible magnitude of PES corrugation of 28 meV per vacancy and barriers for relative sliding of the layers of 7 - 8 meV per vacancy for the moir\'e pattern with coprime indices (2,1) (twist angle 21.8$^\circ$). At the same time, using the semiempirical potential fitted to the DFT results, we confirm that twisted bilayer graphene without defects exhibits superlubricity for the same moir\'e pattern and the magnitude of PES corrugation for the infinite bilayer is below the calculation accuracy. Our results imply that atomic-scale defects restrict the superlubricity of 2D layers and can determine static and dynamic tribological properties of these layers in a superlubric state. We also analyze computationally cheap approaches that can be used for modeling of tribological behavior of large-scale systems with defects. The adequacy of using state-of-the-art semiempirical potentials for interlayer interaction and approximations based on the first spatial Fourier harmonics for the description of interaction between graphene layers with defects is discussed., Comment: 11 pages, 4 figures

Building on recent developments in electronic-structure methods, we define and calculate the flexoelectric response of two-dimensional (2D) materials fully from first principles. In particular, we show that the open-circuit voltage response to a flexural deformation is a fundamental linear-response property of the crystal that can be calculated within the primitive unit cell of the flat configuration. Applications to graphene, silicene, phosphorene, boron nitride, and transition-metal dichalcogenide monolayers reveal that two distinct contributions exist, respectively of purely electronic and lattice-mediated nature. Within the former, we identify a key metric term, consisting in the quadrupolar moment of the unperturbed charge density. We propose a simple continuum model to connect our findings with the available experimental measurements of the converse flexoelectric effect., We thank Raffaele Resta and Konstantin Shapovalov for their critical read of the manuscript and helpful comments. We acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grants No. PID2019—108573 GB-C22 and Severo Ochoa FUNFUTURE center of excellence (CEX2019- 000917-S) and of Generalitat de Catalunya (Grant No. 2017 SGR1506). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia (CESGA)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The realization of integrated quantum circuits requires precise on-chip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and classical calculations we demonstrate how the coherent and ballistic dynamics depend on the details of the QPC confinement potentials., Uploaded final version published on PRL

We present a Raman-scattering study of the interlayer and intralayer E2g Raman-active modes of hexagonal boron nitride (h−BN) under hydrostatic pressure for pressures up to the transition to the wurtzite phase (10.5 GPa). Pressure coefficients and Grüneisen parameters are determined for both modes, and are compared to ab initio calculations based on density functional perturbation theory. The pressure coefficient of the low-energy interlayer mode is higher than that of the high-energy intralayer mode owing to the large compressibility of the h−BN crystal along the c direction. Both modes exhibit a sublinear phonon frequency increase with pressure, which is more marked in the case of the low-energy mode. The intensity of the low-energy mode increases with pressure, suggesting an enhancement of the mode polarizability as the honeycomb layers become closer. The Raman spectrum of the metastable wurtzite phase is observed at ambient pressure in the transited sample after decompression., This work has been supported by the Spanish MINECO/FEDER under Contracts No. MAT2015-71035-R and No. MAT2016-75586-C4-1-P. Support for the h-BN crystal growth from the Materials Engineering and Processing program of the National Science Foundation, Grant No. CMMI 1538127, is greatly appreciated.

We introduce the concept of valley-layer coupling (VLC) in two-dimensional materials, where the low-energy electronic states in the emergent valleys have valley-contrasted layer polarization such that each state is spatially localized on the top or bottom super-layer. The VLC enables a direct coupling between valley and gate electric field, opening a new route towards electrically controlled valleytronics. We analyze the symmetry requirements for the system to host VLC, demonstrate our idea via first-principles calculations and model analysis of a concrete 2D material example, and show that an electric, continuous, wide-range, and switchable control of valley polarization can be achieved by VLC. Furthermore, we find that systems with VLC can exhibit other interesting physics, such as valley-contrasting linear dichroism and optical selection of the electric polarization of interlayer excitons., 6 pages, 4 figures

This is a Reply to the Comment from F. Cinti and M. Boninsegni on our recent work on the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a two-dimensional dipolar system [Bombín, Mazzanti, and Boronat, Phys. Rev. A 100, 063614 (2019)]. The main criticism about our work expressed in that Comment is that we did not explicitly report the two spatial contributions to the total superfluid fraction. Here we analyze our results for a point of the phase diagram corresponding to the stripe phase, close to the gas-to-stripe transition line, and for a temperature below the BKT critical temperature. The scaling with the system size of the contribution to the superfluid fraction, coming from the direction in which spatial order appears, shows that it remains finite in the thermodynamic limit, as we already stated in our original work. This allow us to state that the stripe phase is superfluid at low temperatures. Furthermore, we offer some comments that help to understand where the differences between the results of Cinti and Boninsegni and ours come from., This work has been supported by the Ministerio de Economia, Industria y Competitividad (MINECO, Spain), under Grant No. FIS2017-84114-C2-1-P.

The realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, Al2XX′ (X/X′=O,S,Se,Te) by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all Al2XX′ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of Al2XX′ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, Al2XO monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary Al2XX′ structures but also point out them as promising materials for optoelectronic applications.

Among transition-metal dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling.

International audience; Following the emergence of two-dimensional (2D) materials, a large amount of work has been dedicated to this class of materials. Numerical simulations have proven to be powerful to analyze and predict structure-properties relationships. However, a recurrent issue that arises in some 2D compounds is the appearance of a dynamical instability as an unstable phonon branch in a small pocket close to the Brillouin zone center. The origin (numerical and/or physical) of this instability is hardly discussed. Here, using a rising 2D material, GaSe, as a case study, this issue is tackled by discussing the numerical techniques that may be used to lift the instability but also by understanding the fundamental origin of it. The interlayer distance is the crucial parameter and the ionicity of the compounds is the key physical property governing the propensity for instability. In many works, this distance is arbitrarily fixed to a value for which the absence of interactions between the periodic images of the layers is assumed. A careful control of the effect of this distance on the stability is required prior to subsequent calculations of physical properties.

Surface-state contributions to the dc conductivity of most homogeneous metals exposed to uniform electric fields are usually as small as the system size is large compared to the lattice constant. In this work, we show that surface states of topological metals can contribute with the same order of magnitude as the bulk even in large systems. This effect is intimately related to the intrinsic anomalous Hall effect, in which an applied voltage induces chiral surface-state currents proportional to the system size. Unlike the anomalous Hall effect, the large contribution of surface states to the dc conductivity is also present in time-reversal invariant Weyl semimetals, where the surface states come in counter-propagating time-reversed pairs. While the Hall voltage vanishes in the presence of time-reversal symmetry, the twinned chiral surface currents develop similarly as in the time-reversal broken case. For this effect to occur, the relaxation length associated with scattering between time-reversed partner states needs to be larger than the separation of contributing surfaces, which results in a characteristic size dependence of the resistivity and a highly inhomogeneous current-density profile across the sample., 6+1 pages, 4 figures

The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient Q33 as high as -3.2 m4/C2 and a resulting bulk piezoelectric coefficient d33 up to -85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few uC/cm2. In the paraelectric state, readily accessible owing to low transition temperatures, CIPS exhibits large dielectric tunability, similar to widely-used barium strontium titanate, and as a result both giant and continuously tunable electromechanical response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials and might facilitate the quest towards novel ultrathin functional devices incorporating electromechanical response., Comment: Published version in Phys. Rev. Materials https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.3.024401

International audience; We present a single-phase double-sided pillared metamaterial with effective mass density and elastic modulus simultaneously negative. The negative effective mass density is achieved owing to the combination of the symmetric bending resonance and the antisymmetric compressional resonance of both pillars, whereas the negative effective elastic modulus results from their symmetric compressional resonance. Comparisons with the single-sided pillared structure are made. The dependence of the width of the double-negative band against the geometric parameters of the unit cell is also investigated. In addition, the negative refraction and the zero-index refraction of the symmetric Lamb wave in the deep-subwavelength scale are observed, while at the same frequency, the propagation of the antisymmetric Lamb wave is forbidden.

High-energy electronic excitations of graphene and MoS2 heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/MoS2/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack., acceptedVersion

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk 2H-MoS2 hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of A and B excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of 2H-MoS2. Also, the pressure dependence of the indirect optical transitions of bulk 2H-MoS2 has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk MoS2 may occur at pressures higher than 26 GPa., Work inValenciawas supported by the SpanishGovernment (Grants No. TEC2014-53727-C2-1-R and No. MAT2016- 75586-C4-1-P) and the Generalitat Valenciana (Grant No. PROMETEOII/2014/059). Work in Bellaterra was supported by Spanish MINECO (Grants No. FIS2015-64886-C5-3-P and No. FIS2015-64886-C5-4-P), and the Severo Ochoa Centers of Excellence Program (Grants No. SEV-2013-0295 and No. SEV-2015-0496), Generalitat de Catalunya (Grant No. 2017SGR1506 and the CERCA Programme) and by the European Union H2020-EINFRA-5-2015 MaX Center of Excellence (Grant No. 676598). M.B.G. acknowledges Fellowship No.UVINV-PREDOC13-110538 under the program “Atracció de Talent, VLC-CAMPUS” of the University of Valencia.

We provide a theoretical framework to analyze the properties of frontal collisions of two growing interfaces considering different short range interactions between them. Due to their roughness, the collision events spread in time and form rough domain boundaries, which defines collision interfaces in time and space. We show that statistical properties of such interfaces depend on the kinetics of the growing interfaces before collision, but are independent of the details of their interaction and of their fluctuations during the collision. Those properties exhibit dynamic scaling with exponents related to the growth kinetics, but their distributions may be non-universal. These results are supported by simulations of lattice models with irreversible dynamics and local interactions. Relations to first passage processes are discussed and a possible application to grain boundary formation in two-dimensional materials is suggested., Paper with 12 pages and 2 figures; supplemental material with 4 pages and 3 figures

Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs) with graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2 on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements (ARPES) and Density Functional Theory (DFT) calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2 valence band at K points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single layer WS2. . Our results provide an important reference for future studies of electronic properties of WS2 and its applications in valleytronic devices., Comment: 11 pages, 3 figures + SI 7 pages 8 figures

Sevim, Koray/0000-0002-6150-8954; Sevincli, Haldun/0000-0002-1896-2588; Ozbal, Gozde/0000-0002-6265-964X; Ozdamar, Burak/0000-0001-7185-1978, WOS: 000439974200007, Using first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula A(2)B(2), where A and B belong to groups IV and V, respectively (A = C, Si, Ge, Sn, Pb; B = N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices P (6) over bar m2 and P (3) over bar m1 are shown to be dynamically stable, named as alpha- and beta-phases correspondingly. Both phases have similar cohesive energies, and the a phase is found to be energetically favorable for structures except CP, CAs, CSb, and CBi, for which the beta phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi, and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a mexican-hat shape. SnAs and PbAs have purely quartic valence band edges, i.e., E - alpha k(4), a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.

Assuming that two-component Fermi gases with opposite artificial magnetic fields on a square optical lattice are well-described by the so-called time-reversal-symmetric Hofstadter-Hubbard model, we explore the thermal superfluid properties along with the critical Berezinskii-Kosterlitz-Thouless (BKT) transition temperature in this model over a wide-range of its parameters. In particular, since our self-consistent BCS-BKT approach takes the multi-band butterfly spectrum explicitly into account, it unveils how dramatically the inter-band contribution to the phase stiffness dominates the intra-band one with an increasing interaction strength for any given magnetic flux., Comment: 9 pages with 3 figures; to appear in PRA

We report on the hydrogenation of (3×3)/(4×4) silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of (3×3)/(4×4) silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed sp2-sp3 to a dominating sp3 state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries., Deutsche Forschungsgemeinschaft (VO1261/4-1); ESF Nachwuchsforschergruppe "E-PISA"; TUBITAK (116C073); The Science Academy, Turkey, under the BAGEP program

Recent angle-resolved photoemission spectroscopy measurements of single-layer $1H\ensuremath{-}{\mathrm{TaS}}_{2}$ grown on Au(111) suggest strong electron doping from the substrate. In addition, scanning tunneling microscopy/spectroscopy measurements on this system show suppression of the charge-density-wave (CDW) instability that occurs in bulk $2H\ensuremath{-}{\mathrm{TaS}}_{2}$. We present results from ab initio density functional theory calculations of free-standing single-layer $1H\ensuremath{-}{\mathrm{TaS}}_{2}$ to explore the effects of doping on the CDW. In the harmonic approximation, we find that a lattice instability along the $\mathrm{\ensuremath{\Gamma}}M$ line occurs in the undoped monolayer, consistent with the bulk $3\ifmmode\times\else\texttimes\fi{}3$ CDW ordering vector. Electron doping removes the CDW instability, in agreement with the experimental findings. The doping and momentum dependences of both the electron-phonon coupling and of the bare phonon energy (unscreened by metallic electrons) determine the stability of lattice vibrations. Electron doping also causes an expansion of the lattice, so strain is a secondary but also relevant effect.

The charge dynamical response function of the Hubbard model is investigated on the square lattice in the thermodynamical limit. The obtained charge excitation spectra consist of a continuum, a gapless collective mode with anisotropic zero-sound velocity, and a correlation induced high-frequency mode at $\omega\approx U$. The correlation function is calculated from Gaussian fluctuations around the paramagnetic saddle-point within the Kotliar and Ruckenstein slave-boson representation. Its dependence on the on-site Coulomb repulsion $U$ and density is studied in detail. An approximate analytical expression of the high frequency mode, that holds for any lattice with one atom in the unit cell, is derived. Comparison with numerical simulations, perturbation theory and the polarization potential theory is carried out. We also show that magnetic instabilities tend to vanish for $T\gtrsim t/6$, and finite temperature phase diagrams are established., Comment: 17 pages, 15 figures with expanded discussion