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A model for the collision between a sand grain and a body of yardang material is established by using the principles of classical mechanics. A new nondimensional parameter, the "abrasion number" An, is derived from the model. The volume removed per impact for different targets is proportional to An approximately. As an indicator of the energy ratio of the impacting material to the target material, An not only reflects the previous "abrasion law" but also introduces two dominant mechanical parameters, Young's modulus and yield stress. We propose An to be a similarity criterion for abrasion experiments. The practicability of the model under natural conditions is examined by an example. It is concluded that the abrasion rates of yardangs in the western Qaidam basin, China, are 0.011-0.398 mm/yr in the period of 1986 to 2010 and remarkable abrasion events generally correspond with strong wind conditions. The model will be helpful for reconstructing the history of aeolian activities from the morphologies of yardangs in the arid regions on Mars. [ABSTRACT FROM AUTHOR]

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]

Light top superpartners play a key role in stabilizing the electroweak scale in supersymmetric theories. For R-parity conserved supersymmetric models, traditional searches are not sensitive to the compressed regions. In this Letter, we propose a new method targeting this region, with top squark and neutralino mass splitting ranging from m~t -- mχ ≳ mt to about 20 GeV. In particular, we focus on the signal process in which a pair of top squarks are produced in association with a hard jet, and we define a new observable RM whose distribution has a peak in this compressed region. The position of the peak is closely correlated with m~t We show that for the 13 TeV LHC with a luminosity of 3000 fb-1, this analysis can extend the reach of the top squark in the compressed region to m~t around 800 GeV. [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]

Silicon and germanium transform from diamond to β-tin structure under compression, but upon decompression they turn into metastable BC8 Si and ST12 Ge phases, respectively, instead of returning to the lowest-enthalpy diamond structure. Here we explore by first-principles calculations the atomistic mechanism underlying this intriguing phenomenon. We identify a body-centered tetragonal structure in I41/a (C4h6) symmetry as a precursory state of the BC8 Si phase formed via a double cell bond-rotation mechanism with a low kinetic barrier. Kinetics also play a central role in selecting the decompression pathway in Ge via a trinary cell bond-twisting reconstruction process toward the ST12 Ge phase. In both cases, transformation back to energetically more favorable diamond structure is inhibited by the higher enthalpy barrier. These results explain experimental findings and highlight the kinetic origin of the divergent decompression pathways in Si and Ge. [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]

Scanning tunneling microscopy results reveal a pronounced site selectivity in the hole-mediated photooxidation of trimethyl acetate (TMA) on TiO2(110), wherein the reaction readily occurs at regular Ti sites but is completely inhibited at oxygen vacancy (Vo) defects. Utilizing electron energy loss spectroscopy and density functional theory, we show that the lack of reactivity of TMA groups adsorbed at Vo's cannot be attributed to either a less active adsorption conformation or electron transfer from the Vo defect. Instead, we propose that the excess unpaired electrons associated with the Vo promptly recombine with photoexcited holes approaching the surface, effectively "screening" TMA species at the Vo site. We also show that this screening effect is predominately localized at the Vo, only mildly affecting TMA's at adjacent Ti sites. [ABSTRACT FROM AUTHOR]

We establish a quantum Otto engine cycle in which the working substance contacts with squeezed reservoirs during the two quantum isochoric processes. We consider two working substances: (1) a qubit and (2) two coupled qubits. Due to the effects of squeezing, the working substance can be heated to a higher effective temperature, which leads to many interesting features different from the ordinary ones, such as (1) for the qubit as working substance, if we choose the squeezed parameters properly, the positive work can be exported even when TH < TL, where TH and TL are the temperatures of the hot and cool reservoirs, respectively; (2) the efficiency can be higher than classical Carnot efficiency. These results do not violate the second law of thermodynamics and it can be understood as quantum fuel is more efficient than the classical one. [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]

The quantum system under periodical modulation is the simplest path to understand the quantum nonequilibrium system because it can be well described by the effective static Floquet Hamiltonian. Under the stroboscopic measurement, the initial phase is usually irrelevant. However, if two uncorrelated parameters are modulated, their relative phase cannot be gauged out so that the physics can be dramatically changed. Here, we simultaneously modulate the frequency of the lattice laser and the Rabi frequency in an optical lattice clock (OLC) system. Thanks to the ultrahigh precision and ultrastability of the OLC, the relative phase could be fine-tuned. As a smoking gun, we observed the interference between two Floquet channels. Finally, by experimentally detecting the eigenenergies, we demonstrate the relation between the effective Floquet Hamiltonian and the one-dimensional topological insulator with a high winding number. Our experiment not only provides a direction for detecting the phase effect but also paves a way in simulating the quantum topological phase in the OLC platform. [ABSTRACT FROM AUTHOR]

Comprehensive study on parity-time (PT) symmetric systems demonstrates the novel properties and innovative application of non-Hermitian physics in recent years. In the quantum regime, PT symmetric physics exhibits unique quantum dynamical behaviors such as spontaneous state-distinguishability oscillation. However, the construction and control of a PT symmetric quantum system are still challenging, that and restrict the experimental investigation of PT symmetric quantum nature and application. In this Letter, we propose and construct a recycling-structure PT symmetric quantum simulator for the first time, which can effectively simulate the discrete-time dynamical process of a PT symmetric quantum system in both unbroken and broken phases, to be different from our previous work [J.-S. Tang, et al., Nat. Photonics 10, 642 (2016)]. We investigate the dynamical features of quantum state distinguishability based on the PT symmetric simulator. Our results demonstrate the novel PT symmetric quantum dynamics characterized by the periodical oscillation of state distinguishability in the unbroken phase, and the monotonic decay of that in the broken phase. This work also provides a practical experimental platform for the future intensive study of PT symmetric quantum dynamics. [ABSTRACT FROM AUTHOR]

In the string melting version of a multiphase transport model, the hadronization algorithm has been improved by favoring parton combinations close in not only coordinate space but also momentum space. Formation probabilities of mesons, baryons, and antibaryons during hadronization are determined by their corresponding Wigner functions for the valence parton combinations with no free parameters, and the net baryon, electric, and strangeness charges are conserved during quark coalescence. Effects of the hadronization on anisotropic flows, collision energy dependence of the proton directed flow, relative particle yield ratios, as well as dihadron correlations in relativistic heavy-ion collisions at the energies ranging from Relativistic Heavy Ion Collider beam energy scan to Large Hadron Collider have been extensively discussed. [ABSTRACT FROM AUTHOR]

We report an experimental demonstration of a machine learning approach to identify exotic topological phases, with a focus on the three-dimensional chiral topological insulators. We show that the convolutional neural networks--a class of deep feed-forward artificial neural networks with widespread applications in machine learning--can be trained to successfully identify different topological phases protected by chiral symmetry from experimental raw data generated with a solid-state quantum simulator. Our results explicitly showcase the exceptional power of machine learning in the experimental detection of topological phases, which paves a way to study rich topological phenomena with the machine learning toolbox. [ABSTRACT FROM AUTHOR]

The spectral theorem of von Neumann has been widely applied in various areas, such as the characteristic spectral lines of atoms. It has been recently proposed that dynamical evolution also possesses spectral lines. As the most intrinsic property of evolution, the behavior of these spectra can, in principle, exhibit almost every feature of this evolution, among which the most attractive topic is non-Markovianity, i.e., the memory effects during evolution. Here, we develop a method to detect these spectra, and moreover, we experimentally examine the relation between the spectral behavior and non-Markovianity by engineering the environment to prepare dynamical maps with different non-Markovian properties and then detecting the dynamical behavior of the spectral values. These spectra will lead to a witness for essential non-Markovianity. We also experimentally verify another simplified witness method for essential non-Markovianity. Interestingly, in both cases, we observe the sudden transition from essential non-Markovianity to something else. Our work shows the role of the spectra of evolution in the studies of non-Makovianity and provides the alternative methods to characterize non-Markovian behavior. [ABSTRACT FROM AUTHOR]

Topological semimetals are characterized by the nodal points in their electronic structure near the Fermi level, either discrete or forming a continuous line or ring, which are responsible for exotic properties related to the topology of bulk bands. Here we identify by ab initio calculations a distinct topological semimetal that exhibits nodal nets comprising multiple interconnected nodal lines in bulk and have two coupled drumheadlike flat bands around the Fermi level on its surface. This nodal net semimetal state is proposed to be realized in a graphene network structure that can be constructed by inserting a benzene ring into each C─C bond in the bct-C4 lattice or by a crystalline modification of the (5,5) carbon nanotube. These results expand the realm of nodal manifolds in topological semimetals, offering a new platform for exploring novel physics in these fascinating materials. [ABSTRACT FROM AUTHOR]

Heat management in thermoelectric and power devices as well as in random access memories poses a grand challenge and can make the difference between a working and an abandoned device design. Despite the prevalence of dislocations in all these technologies, the modeling of their thermal resistance is based on 50-year-old analytical approximations, whose simplicity was driven by practical limitations rather than physical insight. We introduce an efficient ab initio approach based on Green's functions computed by two-dimensional reciprocal space integration. By combining elasticity theory and density functional theory, we calculate the scattering strength of a 90° misfit edge dislocation in Si. Because of the breakdown of the Born approximation, earlier literature models fail, even qualitatively. We find that a dislocation density larger than 109cm-2 is necessary to substantially influence thermal conductivity at room temperature and above. We quantify how much of the reduction of thermal conductivity measured in nanograined samples can be explained by realistic dislocation concentrations. [ABSTRACT FROM AUTHOR]

We introduce an intermediate quantum computing model built from translation-invariant Ising-interacting spins. Despite being nonuniversal, the model cannot be classically efficiently simulated unless the polynomial hierarchy collapses. Equipped with the intrinsic single-instance-hardness property, a single fixed unitary evolution in our model is sufficient to produce classically intractable results, compared to several other models that rely on implementation of an ensemble of different unitaries (instances). We propose a feasible experimental scheme to implement our Hamiltonian model using cold atoms trapped in a square optical lattice. We formulate a procedure to certify the correct functioning of this quantum machine. The certification requires only a polynomial number of local measurements assuming measurement imperfections are sufficiently small. [ABSTRACT FROM AUTHOR]

Three-dimensional topological Weyl semimetals can generally support a zero-dimensional Weyl point characterized by a quantized Chern number or a one-dimensional Weyl nodal ring characterized by a quantized Berry phase in the momentum space. Here, in a dissipative system with particle gain and loss, we discover a new type of topological ring, dubbed a Weyl exceptional ring consisting of exceptional points at which two eigenstates coalesce. Such a Weyl exceptional ring is characterized by both a quantized Chern number and a quantized Berry phase, which are defined via the Riemann surface. We propose an experimental scheme to realize and measure the Weyl exceptional ring in a dissipative cold atomic gas trapped in an optical lattice. [ABSTRACT FROM AUTHOR]

Quantum coherence is the most distinguished feature of quantum mechanics. It lies at the heart of the quantum-information technologies as the fundamental resource and is also related to other quantum resources, including entanglement. It plays a critical role in various fields, even in biology. Nevertheless, the rigorous and systematic resource-theoretic framework of coherence has just been developed recently, and several coherence measures are proposed. Experimentally, the usual method to measure coherence is to perform state tomography and use mathematical expressions. Here, we alternatively develop a method to measure coherence directly using its most essential behavior--the interference fringes. The ancilla states are mixed into the target state with various ratios, and the minimal ratio that makes the interference fringes of the "mixed state" vanish is taken as the quantity of coherence. We also use the witness observable to witness coherence, and the optimal witness constitutes another direct method to measure coherence. For comparison, we perform tomography and calculate l1 norm of coherence, which coincides with the results of the other two methods in our situation. Our methods are explicit and robust, providing a nice alternative to the tomographic technique. [ABSTRACT FROM AUTHOR]

The weak-value-based metrology is very promising and has attracted a lot of attention in recent years because of its remarkable ability in signal amplification. However, it is suggested that the upper limit of the precision of this metrology cannot exceed that of classical metrology because of the low sample size caused by the probe loss during postselection. Nevertheless, a recent proposal shows that this probe loss can be reduced by the power-recycling technique, and thus enhance the precision of weak-value-based metrology. Here we experimentally realize the power-recycled interferometric weak-value-based beam-deflection measurement and obtain the amplitude of the detected signal and white noise by discrete Fourier transform. Our results show that the detected signal can be strengthened by power recycling, and the power-recycled weak-value-based signal-to-noise ratio can surpass the upper limit of the classical scheme, corresponding to the shot-noise limit. This work sheds light on higher precision metrology and explores the real advantage of the weak-value-based metrology over classical metrology. [ABSTRACT FROM AUTHOR]

The design and synthesis of three-dimensional denser carbons are one of the hot issues in condensed matter physics because of their fascinating properties. Here we identify by ab initio calculations several tetragonal and monoclinic polymorphs of carbon that adopt the t32, t32*, m32, and m32* structures in P421c, P43212, P21/c, and C2 symmetry, respectively. These carbon polymorphs have large 32-atom unit cells in all-sp3 bonding networks comprising five- and six-membered rings that are dynamically stable, as verified by a phonon mode analysis. Electronic band structure calculations show that they are insulators with band gaps in the range of 5.19-5.41 eV, close to the calculated band gap of 5.34 eV for diamond. Remarkably, these carbon phases possess an extremely high atom number density exceeding that of diamond. The present results establish different types of carbon phases and offer insights into their outstanding structural and electronic properties. [ABSTRACT FROM AUTHOR]

We identify by ab initio calculations a novel topological semimetal carbon phase in all-sp² bonding networks with a 16-atom body-centered orthorhombic unit cell, termed bco-C16. Total-energy calculations show that bco-C16 is comparable to solid fcc-C60 in energetic stability, and phonon and molecular dynamics simulations confirm its dynamical stability. This all-sp² carbon allotrope can be regarded as a three-dimensional modification of graphite, and its simulated x-ray diffraction (XRD) pattern matches well a previously unexplained diffraction peak in measured XRD spectra of detonation and chimney soot, indicating its presence in the specimen. Electronic band structure calculations reveal that bco-C16 is a topological node-line semimetal with a single nodal ring. These findings establish a novel carbon phase with intriguing structural and electronic properties of fundamental significance and practical interest. [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]

We reveal a set of novel decay topologies for the 125 GeV Higgs boson in supersymmetry which are initiated by its decay into a pair of neutralinos, and discuss their collider search strategies. This category of exotic Higgs decays is characterized by the collider signature: visible objects +ET, with ET dominantly arising from escaping dark matter particles. Their benchmark arises naturally in the Peccei-Quinn symmetry limit of the minimal supersymmetric standard model singlet extensions, which is typified by the coexistence of three light particles: singletlike scalar h1 and pseudoscalar a1, and singlinolike neutralino χ1, all with masses of ≲10 GeV, and the generic suppression of the exotic decays of the 125 GeV Higgs boson h2→h1h1, a1a1 and χ1χ1, however. As an illustration, we study the decay topology: h2→χ1χ2, where the binolike χ2 decays to h1χ1 or a1χ1, and h1/a1→ff, with f̄f=μ+μ-, b̄b. In the dimuon case (mh1/a1~1 GeV), a statistical sensitivity of S/√B>6σ can be achieved easily at the 8 TeV LHC, assuming σ(pp→Wh2)/σ(pp→WhSM)Br(h2→μ+μ-χ1χ1)=0.1. In the b√b case (mh1/a1~45  GeV), 600 fb-1 data at the 14 TeV LHC can lead to a statistical sensitivity of S/√B>5σ, assuming σ(pp→Zh2)/σ(pp→ZhSM)Br(h2→b̄bχ1χ1)=0.5. These exotic decays open a new avenue for exploring new physics couplings with the 125 GeV Higgs boson at colliders. [ABSTRACT FROM AUTHOR]

The deviations of the glass transition temperature (Te) in thin films of an amorphous conjugated polymer poly(9,9-dioctylfluorene-co-/V-(4-butylphenyl)diphenylamine) (TFB) are reported. Monotonie and nonmono-tonic rs deviations are observed in TFB thin films supported on Si-SiOx and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOTtPSS), respectively. A three-layer model is developed to fit both monotonie and nonmonotonic Tg deviations in these films. A 5-nm PEDOT:PSS capping layer was not found to be effective to remove the free-surface effect in Si-SiOx supported TFB films. [ABSTRACT FROM AUTHOR]