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Recently obtained single-crystal structure of a thiolate-protected gold cluster shows that all thiolate groups form "staple" motifs on the cluster surface. To find out the driving force for such a formation, we use first-principles density functional theory simulations to model formation of "staple" motifs on an Au38 cluster from zero to full coverage. By geometry optimization, molecular dynamics, and simulated annealing, we show that formation of "staples" is strongly preferred on a cluster surface and helps stabilize the cluster by pinning the surface Au atoms and increasing the HOMO-LUMO gap. We devise a method to generate initial structural models for thiolate-protected gold clusters by adding "staples" to the cluster surface. Using this method, we obtain a staple-covered, low-energy structure for Au38(SCH3)24, a much studied cluster whose structure is not yet known. Optical band-edge energy computed from time-dependent DFT for our Au38(SCH3)24 structure shows good agreement with experiment.

A search for a heavy neutral lepton N of Majorana nature decaying into a W boson and a charged lepton is performed using the CMS detector at the LHC. The targeted signature consists of three prompt charged leptons in any flavor combination of electrons and muons. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV, with an integrated luminosity of 35.9  fb^{-1}. The search is performed in the N mass range between 1 GeV and 1.2 TeV. The data are found to be consistent with the expected standard model background. Upper limits are set on the values of |V_{eN}|^{2} and |V_{μN}|^{2}, where V_{ℓN} is the matrix element describing the mixing of N with the standard model neutrino of flavor ℓ. These are the first direct limits for N masses above 500 GeV and the first limits obtained at a hadron collider for N masses below 40 GeV.

The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.complex oxide | heterostructure | interface physics | electronic reconstruction | polar discontinuity O ver the past few years, precisely constructed, atomically sharp perovskite oxide heterointerfaces have become focal points for condensed-matter-physics and materials science research (1-5). The incorporation and reconstruction of spin (6, 7), charge (8-10), and orbital (11) degrees of freedom across the heterointerfaces have led to novel electronic properties that are different from those inherent to the individual components. For example, pioneering work on the LaAlO 3 and SrTiO 3 (STO) heterostructures has revealed a nontrivial two-dimensional electron gas (2DEG) (10,12,13) at the interface, which also exhibits magnetic (14) and even superconductivity properties (15) that are induced by the polar discontinuity (16) (valence mismatch) across the interface.Motivated by this, research nowadays is primarily focused on probing and understanding the novel interfacial phenomena observed in complex-oxide heterostructures. However, the focus on interfacial properties sidesteps possible macroscopic implications of the interfacial atomic-scale control on the broad range of properties that are present in bulk complex oxides. On the other hand, in the semiconductor industry, atomic-scale interface engineering has long been used to improve the performance of devices through control of the threshold voltage (17), channel carrier mobility (18), Schottky barrier height (19), and so on. This forms the fundamental premise for this work: Can we control the bulk properties of a heterostructured system through the emergent state of matter at the interface? Such an approach could be particularly intriguing if one of the layers is highly polar and electrically switchable, i.e., ferroelectric in nature. Because functional ferroelectric systems, such as the nonvolatile memory (20), ferroelectric field effect transistor (21, 22), ferroelectric tunnel junction (23-27), and switching photo-diode (28), are strongly correlated with the interface electronic structures, it is of great importance to study how the interface atom...

In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.

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Topological insulators are new states of matter with a bulk gap and robust gapless surface states protected by time-reversal symmetry. When time-reversal symmetry is broken, the surface states are gapped, which induces a topological response of the system to electromagnetic field-the topological magnetoelectric effect. In this paper we study the behavior of topological surface states in heterostructures formed by a topological insulator and a magnetic insulator. Several magnetic insulators with compatible magnetic structure and relatively good lattice matching with topological insulators Bi2Se3, Bi2Se3, Sb2Te3 are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in Bi2Se3/MnSe superlattices and obtain the surface state bandstructure. The magnetic exchange coupling with MnSe induces a gap of ∼54 meV at the surface states. In addition we tune the distance between Mn ions and TI surface to study the distance dependence of the exchange coupling.PACS numbers: 73.20.-r, 85.75.-d Topological insulators (TI) are new states of quantum matter which has the same symmetry as the conventional insulators and semiconductors but cannot be adiabatically deformed to them without going through a phase transition. Recently, time-reversal invariant (TRI) TI's are theoretically predicted and experimentally realized in both two and three dimensions (2D and 3D). 1-3A TRI TI is characterized by robust surface states and unique, quantized response properties, just like the quantized Hall conductance in 2D quantum Hall states. For 3D TI the topological response is the topological magnetoelectric (TME) effect 4 , which is a magneto-electric effect with magnetization M generated by electric field E with a quantized coefficient. The TME effect occurs when the surface states of TI become gapped due to timereversal symmetry breaking, and is a generic property of 3D topological insulators, which can be obtained theoretically from generic models and from an effective field theory approach 4,5 , independently of microscopic details. Various consequences of the TME effect have been proposed, including Faraday/Kerr rotation of linear polarized light 4,6-8 , the image monopole effect 9 , the charge carried by a mangnetic monopole 10,11 ,and other types of coupling between the charge and spin degree of freedom at the TI surface 12,13 . Experimental progress has been made recently on the Faraday/Kerr effect in 3D TI 14-17 , but the quantized effect predicted have not been observed yet.To realize the TME effect it is essential to introduce time-reversal symmetry breaking (T-breaking) at the surface of TI to make the surface insulating. There are two possible physical ways to open the T-breaking gap at the surface. The first approach is to introduce magnetic impurities such as Mn or Fe to topological insulators. Both the Dirac-type surface states 18 and the bulk states 19 can mediate ferromagnetic coupling between magnetic impurities and thus induce ferromagnetic...

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density functional theory how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by interfacial charge generated, for example, by oxygen vacancies. PACS numbers: 77.55.hj, 77.22.Ej, 68.37.Ma,Interfaces between dissimilar materials, especially those with polar discontinuities, often exhibit unusual phenomena. For example, interfacial roughening and atomic diffusion can relieve the diverging electrostatic energy in semiconductor heterointerfaces [1]. In ferroelectrics, ionic displacements from the nominal highsymmetry lattice sites cause a permanent electrical polarization. As a result, electrical charge appears at the surfaces or interfaces of ferroelectric films that must be compensated by a form of screening [2][3][4][5][6][7][8][9]. In ferroelectric films sandwiched between electrodes with perfect metallic screening, conduction electrons screen the surface charges. When the contacting materials are nonideal metals or insulators, other compensation mechanisms are triggered. One possibility is the formation of domains of opposite polarization [10,11]. Another possibility, ionic screening, was recently suggested by theory, but has not been observed [5,6]. When a top electrode is not present, it has been found that absorbates or surface point defects provide charge compensation at the free surface of ferroelectric films [12][13][14][15][16]. Moreover, it was demonstrated that the polarity of the films could be reversed by varying the oxygen partial pressure over their surfaces [17]. It is now clear that ferroelectric properties can be controlled by the mechanism by which surface charge is compensated [9,[12][13][14]18].Current understanding of ferroelectric interfaces is based primarily on theory [4][5][6][7][19][20][21][22][23][24][25][26] because few experimental techniques have the ability to obtain atomicallyresolved measurements of the local atomic displacements that give rise to electric polarization. Only in recent years has it been possible to use phase-contrast high-resolution transmission electron microscopy to obtain local polarization displacements [27][28][29]. Measurements by Jia et al. were used to raise questions about the widely accepted notion that atomic displacements and the tetragonality of the unit cell are directly coupled [28][29][30]. Fong et al.[31] used X-ray scattering to obtain atomic positions and shed light on the ferroelectric aspects of PbTiO 3 on SrTiO 3 .In this paper we report phase-contrast images of ferroelectric interfaces obtained simultaneously with h...