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We report calculations of phonon dispersions of iron (Fe) at its γ-δ phase transition using a self-consistent ab initio lattice dynamical method in conjunction with an effective magnetic force approach via the antiferromagnetic approximation. Our results show that anharmonic phonon-phonon interactions play a crucial role in stabilizing the δ-Fe phase in the open bcc lattice. In contrast, the lattice dynamics of the close-packed fcc γ-Fe phase are dominated by magnetic interactions. Simultaneous considerations of the lattice anharmonic and magnetic interactions produced temperature-dependent phonon dispersions for δ-Fe and γ-Fe phases in excellent agreement with recent experimental measurements. The present results highlight the key role of lattice anharmonicity in determining the structural stability of iron at high temperatures, which has significant implications for other high-temperature paramagnetic metals like Ce and Pu. [ABSTRACT FROM AUTHOR]

Many TiO2 applications (e.g., in heterogeneous catalysis) involve contact with ambient atmosphere and/or water. The resulting hydroxylation can significantly alter its surface properties. While the behavior of a single, isolated OH species on the model metal oxide surface of rutile TiO2(110) is relatively well understood, much less is known regarding highly hydroxylated surfaces and/or whether TiO2(110) could be fully hydroxylated under ultrahigh vacuum conditions. Here we report the in situ formation of a well-ordered, fully hydroxylated TiO2(110)-(1 × 1) surface using an enhanced photochemical approach, the key parts of which are predosing of water and multistep dissociative adsorption and subsequent photolysis of the carboxylic (trimethyl acetic) acid. Combining scanning tunneling microscopy, ultraviolet photoelectron spectroscopy, and density functional theory results, we show that the attained "super OH" surface is also fully reduced, as a result of the photochemical charging of electron traps associated with the OH groups. [ABSTRACT FROM AUTHOR]

Besides the conventional bosons and fermions, in synthetic two-dimensional (2D) materials there could exist more exotic quasiparticles with non-Abelian statistics, meaning that the quantum states in the system will be transformed by noncommuting unitary operators when we adiabatically braid the particles one around another. Here, we propose an experimental scheme to observe non-Abelian statistics with cold atoms in a 2D optical lattice. We show that the Majorana-Schockley modes associated with line defects can be braided with non-Abelian statistics through adiabatic shift of the local potentials. We also demonstrate that the braiding operations are robust against typical experimental imperfections and the readout of topological qubits can be accomplished by local measurement of the atom number. [ABSTRACT FROM AUTHOR]

BaSi2-type disilicides and digermanides hold great promise for solar-cell applications, but their structural stability and phase transition mechanisms remain unresolved. Here we present ab initio calculations of pressureinduced structural phase transitions of BaSi2, BaGe2, and SrGe2 and show that Si tetrahedra in orthorhombic BaSi2 tend to convert to corrugated layers in the trigonal phase under high pressure with bond breaking along the b axis, and a three-dimensional Si net in the cubic phase is stabilized energetically at low pressure. The orthorhombic semiconductor-to-trigonal metal conversion is also preferred for SrGe2 both energetically and kinetically. However, Ge tetrahedra in BaGe2 tend to convert to a ThSi2-type tetragonal net with bond breaking around the c axis. The kinetic barriers are large for both the reaction (∼0.43 eV under compression) and the counter-reaction (∼0.39 eV under decompression) for BaSi2, which explains the stability of the trigonal and cubic phases at room temperature and the high-temperature requirement for the phase transitions. [ABSTRACT FROM AUTHOR]

We investigate string correlations in an infinite-size spin-1/2 bond-alternating Heisenberg chain. By employing the infinite matrix product state representation with the infinite time evolving block decimation method, a finite string correlation for extremely large lattice distances is directly observed, in contrast to an extrapolated extreme value for finite size chains. We find that a topological quantum phase transition occurs between two different phases separated and characterized by two different long-range string orders in the space of bond-alternating interactions. Also, the critical exponent ß from the long-range string orders is obtained as ß = 1/12 and the central charge at the critical point is obtained as c ≃ 1, which shows that the topological phase transition belongs to the Gaussian universality class. In addition, it is shown that, for the topological quantum phase transition, the phase boundary can be captured by the singular behavior of the von Neumann entropy and the pinch point of the fidelity per site. [ABSTRACT FROM AUTHOR]

We identify by ab initio calculations a new cubic carbon phase in Pa&3macr; (T6h) symmetry, which has a unit cell that comprises eight sp3-connected C3 triangle rings (thus termed cubic C3). The transformation from graphite occurs via a novel sheet-rumphng mechanism under anisotropic compression along the a[100] axis, which is distinct from the usual c-axis compression mechanism for the graphite-to-diamond transition. Calculated structural parameters and simulated x-ray diffraction of cubic C3 match the experimental data, thus resolving the long-standing problem of structural determination of a new carbon phase [Science 140, 817 (1963)]. The sheet-rumpling pathway unveiled here estabhshes a new mechanism for structural transformation in layered covalent materials. [ABSTRACT FROM AUTHOR]

We identify by ab initio calculations an orthorhombic carbon (O-carbon) in Pbam (D92h) symmetry for compressed graphite in AA stacking, which is formed via a distinct one-layer by one-layer slip and buckling mechanism along the [210] direction. It is dynamically stable and energetically more favorable than oilier known compressed graphite phases, albeit its slightly higher kinetic barrier. The O-carbon is comparable to diamond in ultralow compressibility, has a band gap wider than that of diamond, and is compatible with experimental x-ray diffraction data. The present results offer insights for understanding the complex structural landscape of compressed graphite and the versatile nature of carbon in forming a rich variety of structures under pressure. [ABSTRACT FROM AUTHOR]

The atomistic mechanism for direct conversion of graphite to diamond is a long-standing problem in condensed matter physics. Here, we establish by ab initio calculations bond reconstruction pathways from graphite to a basic series of diamond polytypes of 2H, 3C, 4H, and 12R. The conversion proceeds through two newly identified compressed-graphite phases of orthorhombic and monoclinic carbon with odd-membered (5 + 7) rings toward the diamond structures via a local-bond-rotation mechanism. The rhombohedral l2R phase represents a new crystal form of diamond with an alternating four-layered hexagonal (h) and cubic (c) close-packed structure in (hcch)3 stacking. These results resolve the fundamental questions about the graphite-to-diamond phase transformation at high pressure and high temperature. [ABSTRACT FROM AUTHOR]

We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an out-going, circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with the NPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional. [ABSTRACT FROM AUTHOR]

We study the phase diagram of the Bose-Hubbard model on the kagome lattice with a broken sublattice symmetry. Such a superlattice structure can naturally be created and tuned by changing the potential offset of one sublattice in the optical generation of the frustrated lattice. The superstructure gives rise to a rich quantum phase diagram, which is analyzed by combining quantum Monte Carlo simulations with the generalized effective potential Landau theory. Mott phases with noninteger filling and a characteristic order along stripes are found, which show a transition to a superfluid phase with an anisotropic superfluid density. Surprisingly, the direction of the superfluid anisotropy can be tuned by changing the particle number, the hopping strength, or the interaction. Finally, we discuss characteristic signatures of anisotropic phases in time-of-flight absorption measurements. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe) is a direct band-gap semiconducting material with broad applications in optoelectronic devices. Here we report on a high-resolution angle-resolved photoemission (ARPES) study of CdTe(111) surfaces prepared by sputtering and annealing that show a (2 × 2) reconstruction as observed by electron diffraction. The ARPES maps along high-symmetry directions show prominent features with their intensities modulated by varying the incident photon energy, thus suggesting important matrix element effects associated with photoemission. The results are in excellent agreement with first-principles calculations of the bulk band structure and one-dimensional density of states. A prominent surface state is observed that exhibits a (2 × 2) periodicity in agreement with the symmetry of the surface reconstruction. [ABSTRACT FROM AUTHOR]

Recent shock compression experiments produced clear evidence of a new carbon phase, but a full structural identification has remained elusive. Here we establish by ab initio calculations a body-centered cubic carbon phase in Ia3¯d(O10h) symmetry, which contains twelve atoms in its primitive cell, thus termed BC12, and comprises all-sp3 six-membered rings. This structural configuration places BC12 carbon in the same bonding type as cubic diamond, and its stability is verified by phonon mode analysis. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak found in shock compression experiments. Electronic band and density of states calculations reveal that BC12 is a semiconductor with a direct band gap of ~2.97eV. These results provide a solid foundation for further exploration of this new carbon allotrope. [ABSTRACT FROM AUTHOR]

Cadmium telluride (CdTe), a compound widely used in devices, is a key base material for the experimental realization of the quantum spin Hall phase. We report herein a study of the electronic structure of CdTe by angle-resolved photoemission spectroscopy from well-ordered (110) surfaces. The results are compared with first-principles calculations to illustrate the topological distinction between CdTe and a closely related compound HgTe. Through a theoretical simulation a topological phase transition as well as the Dirac-Kane semimetal phase at the critical point was demonstrated in the mixed compound HgxCd1-xTe. [ABSTRACT FROM AUTHOR]