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The phenomenology of resonant scattering has been known since the earliest experiments upon scattering of atomic beams from surfaces and is a means of obtaining experimental information about the fundamentals of weak adsorption systems in the van der Waals regime. We provide an overview of the experimental approach based on new experimental data for the He-Sb2Te3(111) system, followed by a comparative overview and perspective of recent results for topological semimetal and insulator surfaces. Moreover, we shortly discuss the perspectives of calculating helium-surface interaction potentials from ab initio calculations. Our perspective demonstrates that atom-surface scattering provides direct experimental information about the atom-surface interaction in the weak physisorption regime and can also be used to determine the lifetime and mean free path of the trapped atom. We further discuss the effects of elastic and inelastic scattering on the linewidth and lifetime of the trapped He atom with an outlook on future developments and applications.

Metal soaps or metal alkanoates are metal-organic complexes held together with metal cations and the functional groups of hydrocarbon chains. They can be synthesized at a high yield by simply mixing the metal and organic sources, forming crystalline frameworks with diverse topology, and have been studied in the past because of their rich polymorphism-like liquid crystals. Their ability to melt while retaining the crystalline properties upon cooling is unique among nanoporous materials and is especially attractive for membrane fabrication. Herein, metal soaps as a new class of material for molecular separation are reported. Three metal soaps, Ca(SO4C12H25)(2), Zn(COOC6H13)(2), and Cu(COOC9H19)(2), hosting lamellar structure with molecular-sized channels are synthesized. They are processed in thin, intergrown, polycrystalline films on porous substrates by two scalable methods, interfacial crystallization and melting with an extremely small processing time (a minute to an hour). The resulting crystalline films are oriented with the alkyl chains perpendicular to the porous substrate which favors molecular transport. The prepared membranes demonstrate attractive gas separation behavior, e.g., 300-nm-thick Ca(SO4C12H25)(2)membrane prepared in a minute using interfacial crystallization yields H(2)permeance of 6.1 x 10(-7)mol m(-2)s(-1)Pa(-1)with H-2/CO(2)selectivity of 10.5.

The authors present an experimental and theoretical study of the surface phonon dispersion of Bi${}_{2}$ Se${}_{3}$ , together with a determination of the electron-phonon coupling strength \ensuremath{\lambda}. Additional low-energy branches in the dispersion are possibly associated with collective electronic excitations.

Tin selenide (SnSe) is one the most promising materials for flexible electronics. However, experiments on the direct determination of its mechanical properties are still missing. By means of depth-sensing nanoindentation experiments, we directly evaluate the Young's modulus of bulk single crystals of tin selenide (25.3 GPa), as well as their hardness (0.82 GPa). Experimental results are compared with predictions by density functional theory, performed using eleven different functionals. The discrepancies between the experimental results and the thoretical predictions can be ascribed to the oxidation of the SnSe surface, detected by X-ray photoelectron spectroscopy.

We search for novel two-dimensional materials that can be easily exfoliated from their parent compounds. Starting from 108423 unique, experimentally known three-dimensional compounds we identify a subset of 5619 that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van-der-Waals density-functional theory, validated against experimental structural data and calculated random-phase-approximation binding energies, allow to identify 1825 compounds that are either easily or potentially exfoliable, including all that are commonly exfoliated experimentally. In particular, the subset of 1036 easily exfoliable cases---layered materials held together mostly by dispersion interactions and with binding energies up to $30-35$ meV$\cdot\text{\AA}^{-2}$---provides a wealth of novel structural prototypes and simple ternary compounds, and a large portfolio to search materials for optimal properties. For the 258 compounds with up to 6 atoms per primitive cell we comprehensively explore vibrational, electronic, magnetic, and topological properties, identifying in particular 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors., Comment: 15 pages, 5 figures, supplementary material available at https://www.nature.com/articles/s41565-017-0035-5#Sec15. Data available on the Materials Cloud https://www.materialscloud.org/discover/2dstructures/dashboard/ptable

First principles calculations of phonon dispersion relations have been performed to estimate the phononic contribution to the thermal boundary resistance at the interfaces between GeTe, In $$_3$$ SbTe $$_2$$ and In $$_2$$ GeTe $$_3$$ phase change compounds and metals (Al, Pt and TiN). The diffuse mismatch model has been used. The electron–phonon coupling constants provide an estimate of the electron–phonon contribution to the thermal boundary resistance. The electron–phonon and phononic contributions to the thermal boundary resistance turn out to be comparable and they have to be both included for a reliable estimate of the thermal conductance at the interface with phase change compounds.

Two-dimensional materials are emerging as a promising platform for ultrathin channels in field-effect transistors. To this aim, novel high-mobility semiconductors need to be found or engineered. While extrinsic mechanisms can in general be minimized by improving fabrication processes, the suppression of intrinsic scattering (driven e.g. by electron-phonon interactions) requires to modify the electronic or vibrational properties of the material. Since intervalley scattering critically affects mobilities, a powerful approach to enhance transport performance relies on engineering the valley structure. We show here the power of this strategy using uniaxial strain to lift degeneracies and suppress scattering into entire valleys, dramatically improving performance. This is shown in detail for arsenene, where a 2% strain stops scattering into 4 of the 6 valleys, and leads to a 600% increase in mobility. The mechanism is general and can be applied to many other materials, including in particular the isostructural antimonene and blue phosphorene., 14 pages, 14 figures

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, Z(2) topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial Z(2) topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuL2 and (platinum) jacutingaite (P2HgS3). We also identify monolayer Pd2HgSe3 (palladium jacutingaite) as a novel Kane-Mele quantum spin Hall insulator and compare it with platinum jacutingaite. A handful of promising materials are mechanically stable and exhibit Z(2) topological order, either unperturbed or driven by small amounts of strain. Such screening highlights a relative abundance of Z(2) topological order of around 1% and provides an optimal set of candidates for experimental efforts.

Density functional perturbation theory is used to study the phonon dispersion relations at the (111) surface of the topological insulator Sb2Te3. Inelastic Helium atom scattering experiments at the (111) surface of the similar Bi2Se3 topological insulator suggested the occurrence of a deep Kohn anomaly related to the presence of the topological surface metallic state. A slab made of three Sb2Te3 quintuple layers is used here to model the (111) surface. This geometry is shown to be sufficient to generate metallic Dirac surface states in good agreement with experimental data from Angle Resolved Photoemission Spectroscopy. The inclusion of spin-orbit interaction in the calculations of surface phonons at wavevectors corresponding to Fermi surface nesting inside the Dirac cone allow us to address the possible occurrence of a Kohn anomaly in Sb2Te3(111) as well. No evidence is found in our calculations for any Kohn anomaly in Sb2Te3 due to electronic transitions across the Dirac cone. (C) 2018 Elsevier B.V. All rights reserved.

An excellent electrostatic control has been early on identified as one of the most critical ingredients to build band- to- band tunneling field-effect transistors (TFETs) with a steep sub-threshold swing (SS) and a high ON-current (I ON ) [1]. These essential features can be obtained by reducing the thickness of ultra-thin-body structures or the diameter of nanowires. Two-dimensional materials, especially their single-layer (SL) configuration, represent a promising alternative to conventional semiconductors due to their intrinsic sub-1nm thickness. Indeed, a TFET implementing an atomically thin MoS 2 channel combined with a Ge layer was recently shown to exhibit a less than 60 mV/dec SS over several orders of magnitude and a decent I ON [2]. In this experiment, however, MoS 2 had to be grouped with Ge to achieve the desired goal, thus raising the question whether 2-D materials alone can provide a suitable platform for high performance TFETs. Various theoretical studies based on empirical tight-binding models and focusing on SL transition metal dichalcogenides (TMDs) [3] and black phosphorus [4] have come to the conclusion that these compounds, in particular WTe 2 , could deliver ON-currents larger than $100\ \mu\mathrm{A}/\mu\mathrm{m}$ at a supply voltage $V_{DD}=0.5\ \mathrm{V}$ and OFF-current $I_{OFF}=1\ \mathrm{nA}/\mu\mathrm{m}$ . Here, by employing an ab-initio quantum transport simulator, we will demonstrate that none of the usual TMDs reaches a $I_{ON} > > 10\ \mu\mathrm{A}/\mu\mathrm{m}$ , contrary to recently discovered 2-D materials [5] that could pave the way for future, highly efficient TFETs.

The structure and the acoustic phonon branches of graphene on Ru(0 0 0 1) have been experimentally investigated with helium atom scattering (HAS) and analyzed by means of density functional theory (DFT) including Grimme dispersion forces. In-plane interactions are unaffected by the interaction with the substrate. The energy of 16 meV for the vertical rigid vibration of graphene against the Ru(0 0 0 1) surface layer indicates an interlayer effective force constant about five times larger than in graphite. The Rayleigh mode observed for graphene/Ru(0 0 0 1) is almost identical to the one measured on clean Ru(0 0 0 1). This is accounted for by the strong bonding to the substrate, which also explains the previously reported high reflectivity to He atoms of this system. Finally, we report the observation of an additional acoustic branch, closely corresponding to the one already observed by HAS in graphite, which cannot be ascribed to any phonon mode and suggests a possible plasmonic origin.

It is shown that the electron-phonon interaction at a conducting interface between a topological insulator thin film and a semiconductor substrate can be directly probed by means of high-resolution Brillouin light scattering (BLS). The observation of Kohn anomalies in the surface phonon dispersion curves of a 50 nm thick Bi2Te3 film on GaAs, besides demonstrating important electron-phonon coupling effects in the GHz frequency domain, shows that information on deep interface electrons can be obtained by tuning the penetration depth of optically-generated surface phonons so as to selectively probe the interface region, as in a sort of quantum sonar.

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds Ge 2 Sb 2 Te 5 and GeTe in their crystalline form. The related Sb 2 Te 3 compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal Ge 2 Sb 2 Te 5 of about 0.45 W m −1 K −1 measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Phonon dispersion in periodically rippled graphene/Ru(0001) has been studied by high-resolution electron energy loss spectroscopy and density functional theory. We show that, in addition to the usual phonon modes of the free-standing graphene lattice, the phonon spectrum of graphene/Ru(0001) exhibits additional modes arising from the peculiar nanostructured morphology of this system. In particular, the out-of-plane optical phonon has two components at 83 and 105 meV, related to the spectral contribution arising from carbon atoms in the valleys and in the hills of the ripples, respectively.

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically nontrivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator with a sizable band gap of $\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{eV}$ that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesized. This system realizes the paradigmatic Kane-Mele model for quantum spin Hall insulators in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response.

Author(s): Bosoni, E; Campi, D; Donadio, D; Sosso, GC; Behler, J; Bernasconi, M | Abstract: The thermal conductivity of GeTe crystalline nanowires has been computed by means of non-equilibrium molecular dynamics simulations employing a machine learning interatomic potential. This material is of interest for application in phase change non-volatile memories. The resulting lattice thermal conductivity of an ultrathin nanowire (7.3 nm diameter) of 1.57 W m-1 K-1 is sizably lower than the corresponding bulk value of 3.15 W m-1 K-1 obtained within the same framework. The analysis of the phonon dispersion relations and lifetimes reveals that the lower thermal conductivity in the nanowire is mostly due to a reduction in the phonon group velocities. We further predict the presence of a minimum in the lattice thermal conductivity for thicker nanowires.

Scientific reports 6, 32895 (2016). doi:10.1038/srep32895, Published by Nature Publishing Group, London

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission., Impurities and vacancies are commonly found within the crystalline lattice of transition metal dichalcogenides, however they are usually seen as detrimental for their optical properties. Here, the authors demonstrate that sulfur vacancies in MoS2 can give rise to a near-infrared emission peak.

The elastic constants Cij of bulk Bi2Te3 were calculated by density functional theory by different approximations. Computational results were validated by means of nanoindentation tests, performed along the directions parallel and perpendicular to the cleavage plane of Bi2Te3. The indentation modulus was estimated and compared with the experimental one. The generalized gradient Perdew-Burke-Ernzerhof approximation better reproduces experimental results in the direction perpendicular to the cleavage plane, whereas the occurrence of the nanobuckling in the direction parallel to the cleavage plane causes discrepancies between theoretical and experimental findings., Authors acknowledge MaTeRiA project (Grant No. PONa3_00370) for providing equipment to perform nanoindentation experiments. EVC acknowledge funding from the University of Basque Country UPV/EHU (GIC07-IT-756-13), the Departamento de Educación del Gobierno Vasco and the Spanish Ministerio de Ciencia e Innovación (FIS2010-275 19609-C02-01), the Tomsk State University Academic D.I. Mendeleev Fund Program (Grant No. 8.1.05.2015), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), and Saint Petersburg State University (Project No. 11.50.202.2015).

We have studied the electronic excitation spectrum in periodically rippled graphene on Ru(0 0 0 1) and flat, commensurate graphene on Ni(1 1 1) by means of high-resolution electron energy loss spectroscopy and a combination of density functional theory and tight-binding approaches. We show that the periodic moiré superlattice originated by the lattice mismatch in graphene/Ru(0 0 0 1) induces the emergence of an extra mode, which is not present in graphene/Ni(1 1 1). Contrary to the ordinary intra-band plasmon of doped graphene, the extra mode is robust in charge-neutral graphene/metal contacts, having its origin in electron–hole inter-band transitions between van Hove singularities that emerge in the reconstructed band structure, due to the moiré pattern superlattice., The support by the Netherlands National Computing Facilities foundation (NCF) are acknowledged. SY and MIK thank financial support from the European Research Council Advanced Grant program (contract 338957). The research has also received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. RR acknowledges financial support from MINECO (Spain) through grant FIS2014-58445-JIN.

The average elastic properties and the interatomic force constants of a quasi-freestanding graphene sample have been evaluated by analyzing the phonon dispersion measured by means of angle-resolved energy loss spectroscopy. We found a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. Present results indicate that the high crystalline quality of graphene grown on metal substrates leads to macroscopic samples of high tensile strength and bending flexibility to be used for technological applications such as electromechanical devices and carbon-fiber reinforcements.

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD_3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH_3-Ge(111) and CH_3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.

Phonon dispersion relations and electron-phonon coupling of hole-doped trigonal GeTe have been computed by density functional perturbation theory. This compound is a prototypical phase change material of interest for applications in phase change non-volatile memories. The calculations allowed us to estimate the electron-phonon contribution to the thermal boundary resistance at the interface between the crystalline and amorphous phases present in the device. The lattice contribution to the thermal boundary resistance has been computed by non-equilibrium molecular dynamics simulations with an interatomic potential based on a neural network scheme. We find that the electron-phonon term contributes to the thermal boundary resistance to an extent which is strongly dependent on the concentration and mobility of the holes. Further, for measured values of the holes concentration and electrical conductivity, the electron-phonon term is larger than the contribution from the lattice. It is also shown that the presence of Ge vacancies, responsible for the p-type degenerate character of the semiconductor, strongly affects the lattice thermal conductivity of the crystal.

The Ge2Sb2Te5 compound is of interest for applications in phase change non-volatile memories. First-principles calculations of phonon dispersion relations and electron-phonon coupling constant provide an estimate of the electron-phonon contribution to the thermal boundary resistance at the interfaces of Ge2Sb2Te5 with dielectrics (silica) and metal electrodes (Al and TiN). The diffuse mismatch model including full phononic dispersion has been used to compute the phononic contribution to the thermal boundary resistance. The calculated value of the electron-phonon contribution to the TBR at 300 K of about 14 m(2)K GW(-1) would dominate the TBR at the interfaces of hexagonal Ge2Sb2Te5 with the surrounding dielectrics and metals considered here once interdiffusion at the boundaries could be minimized.

High-resolution electron energy loss spectroscopy has been used to study the electronic response of periodically rippled monolayer graphene grown on Ru(0001). A plasmonic mode, assigned to the π plasmon, has been observed at around 6 eV. The dispersion curve of this collective mode indicates plasmon confinement within the hills of the ripples. Moreover, we found that the corrugation of the graphene sheet also significantly affects the damping processes of the π plasmon.

The surface phonon dispersion curves of Bi(111) have been measured by inelastic helium atom scattering (HAS) along the two symmetry directions. The complex set of observed dispersion curves, including several branches in the acoustic region, plus a localized and a resonant branch in the optical region, is interpreted by means of calculations based on density functional perturbation theory (DFPT). It is recognized that the upper phonon branches in the acoustic region starting at about $5.3$ meV and $4.3$ meV at zero wave vector correspond to shear-vertical and longitudinal modes localized on the third surface layer (second bilayer), respectively. The HAS ability of detecting subsurface phonons previously observed for Pb(111) multilayers is attributed to the comparatively strong electron-phonon interaction, and confirmed through a DFPT calculation of the phonon-induced surface charge-density oscillations. A comparison of the integrated HAS intensities with those of Pb(111) multilayers measured under similar kinematic conditions allows for an estimation of the electron-phonon mass-enhancement parameter for the Bi(111) surface. An anomaly at the $\overline{\mathrm{M}}$ point, quite sharp at 123 K but very broad at room temperature, is associated with recombination processes of bulk $\overline{\mathrm{M}}$-point pocket electrons with bulk pocket holes at either $\overline{\ensuremath{\Gamma}}$ or equivalent $\overline{\mathrm{M}}$ points.

The bulk and surface electronic and structural properties of As(111) have been studied with first-principles methods. The inclusion of spin-orbit interaction reveals that As shares the same topologically nontrivial order of the bulk electronic bands of Sb which gives rise to two spin-polarized surface states connecting valence-like and conduction-like states. Bulk and surface phonons have been calculated by means of density functional perturbation theory. The surface phonon bands reveal features related to a remarkable stiffening of the surface bilayer with respect to the bulk ones similarly to what is measured for the Bi(111) and to what is expected for the Sb(111) surface.

The bulk and surface dynamics of Sb(111) and the corresponding electron-phonon interaction have been calculated by density functional perturbation theory. The surface phonon bands reveal features related to a remarkable stiffening of the surface bilayer with respect to the bulk ones. The main contribution to electron-phonon interaction involves transitions between surface and bulk states, mostly driven by bulk phonons, and is found to be in good agreement with the value derived from spin angle-resolved photoemission spectroscopy.

A combined helium atom scattering and density functional perturbation theory study has been performed to elucidate the surface phonon dispersion relations for both the CH3-Si(111)-(1 × 1) and CD 3-Si(111)-(1 × 1) surfaces. The combination of experimental and theoretical methods has allowed characterization of the interactions between the low energy vibrations of the adsorbate and the lattice waves of the underlying substrate, as well as characterization of the interactions between neighboring methyl groups, across the entire wavevector resolved vibrational energy spectrum of each system. The Rayleigh wave was found to hybridize with the surface rocking libration near the surface Brillouin zone edge at both the M̄-point and K̄-point. The calculations indicated that the range of possible energies for the potential barrier to the methyl rotation about the Si-C axis is sufficient to prevent the free rotation of the methyl groups at a room temperature interface. The density functional perturbation theory calculations revealed several other surface phonons that experienced mode-splitting arising from the mutual interaction of adjacent methyl groups. The theory identified a Lucas pair that exists just below the silicon optical bands. For both the CH3- and CD3-terminated Si(111) surfaces, the deformations of the methyl groups were examined and compared to previous experimental and theoretical work on the nature of the surface vibrations. The calculations indicated a splitting of the asymmetric deformation of the methyl group near the zone edges due to steric interactions of adjacent methyl groups. The observed shifts in vibrational energies of the -CD 3 groups were consistent with the expected effect of isotopic substitution in this system. © 2014 AIP Publishing LLC.

Two-dimensional superconductors attract great interest both for their fundamental physics and for their potential applications, especially in the rapidly growing field of quantum computing. Despite intense theoretical and experimental efforts, materials with a reasonably high transition temperature are still rare. Even more rare are those that combine superconductivity with a non-trivial band topology, to potentially host exotic states of matter such as Majorana fermions. Here, we predict a remarkably high superconducting critical temperature of 21-28 K in the easily exfoliable, topologically non-trivial 2D semimetal W2N3 . By studying its electronic and superconducting properties as a function of doping and strain, we find large changes in the electron-phonon interactions that make this material a unique platform to study different coupling regimes and test the limits of current theories of superconductivity. Last, we discuss the possibility of tuning the material to achieve coexistence of superconductivity and topologically non-trivial edge states., 17 pages 10 figures

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi$_2$Te$_3$. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi$_2$Te$_3$(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

The phonon dispersion curves of ultrathin films of Cs(110) on Pt(111) measured with inelastic helium atom scattering (HAS) are reported and compared with density-functional perturbation theory (DFPT) calculations. The combined HAS and DFPT analysis also sheds light on the bulk phonon dynamics of bcc-Cs, on which very little is known from neutron scattering due to its large neutron capture cross-section. Moreover the temperature dependence of the elastic HAS Debye-Waller exponent of Cs(110)/Cu(111) ultrathin films allows for an estimation of the electron-phonon coupling strength λ as a function of the film thickness.