Nearest-neighbor Sites Research Articles

Long Jumps of an Organic Molecule Induced by Atomic Force Microscopy Manipulation

Non‐contact atomic force microscopy at cryogenic temperatures is used for the controlled lateral manipulation of individual 3,4,9,10‐perylene‐tetracarboxylicacid‐dianhydride (PTCDA) molecules on the Ag(111) surface. The molecules are moved along the [‐110] direction of the Ag lattice in the regime of repulsive tip‐molecule forces performing discrete jumps that span distances from single to multiple lattice spacings. The analysis of the two‐dimensional force field measured before and during the manipulation reveals that the displacement beyond nearest neighbor sites cannot be explained by long range tip‐molecule forces but instead has to involve an energy transfer to translational modes of the molecule. Combined with the results of the simultaneous measurement of the energy dissipation, these findings allow to identify a likely manipulation mechanism and provide insight into the process of energy transfer between excited large molecules and metal surfaces. Furthermore, implications for the theoretical treatment of NC‐AFM based molecule manipulation are discussed.

Spectral properties of a two-particle Hamiltonian on a lattice

We consider a system of two arbitrary quantum particles on a three-dimensional lattice with some dispersion functions (describing particle transport from a site to a neighboring site). The particles interact via an attractive potential at only the nearest-neighbor sites. We study how the number of eigenvalues of a family of operators h(k) depends on the particle interaction energy and the total quasimomentum \(k \in \mathbb{T}^3\), where \(\mathbb{T}^3\) is a three-dimensional torus. We find the conditions under which the operator h(0) has a double or triple virtual level at zero depending on the particle interaction energy.

Hard-core bosons in a zig-zag optical superlattice

We study a system of hard-core bosons at half-filling in a one-dimensional optical superlattice. The bosons are allowed to hop to nearest and next-nearest neighbor sites producing a zig-zag geometry and we obtain the ground state phase diagram as a function of microscopic parameters using the finite-size density matrix renormalization group (FS-DMRG) method. Depending on the sign of the next-nearest neighbor hopping and the strength of the superlattice potential the system exhibits three different phases, namely the bond-order (BO) solid, the superlattice induced Mott insulator (SLMI) and the superfluid (SF) phase. When the signs of both hopping amplitudes are the same (the "unfrustrated" case), the system undergoes a transition from the SF to the SLMI at a non-zero value of the superlattice potential. On the other hand, when the two amplitudes differ in sign (the "frustrated" case), the SF is unstable to switching on a superlattice potential and also exists only up to a finite value of the next nearest neighbor hopping. This part of the phase diagram is dominated by the BO phase which breaks translation symmetry spontaneously even in the absence of the superlattice potential and can thus be characterized by a bond order parameter. The transition from BO to SLMI appears to be first order.

Metastability and Phase Separation in a Simple Model of a Superconductor with Extremely Short Coherence Length

We present studies of the atomic limit of the extended Hubbard model with pair hopping for arbitrary electron density and arbitrary chemical potential. The Hamiltonian consists of (i) the effective on-site interaction $U$ and (ii) the intersite charge exchange term $I$, determining the hopping of electron pairs between nearest-neighbour sites. In the analysis of the phase diagrams and thermodynamic properties of this model we treat the intersite interactions within the mean-field approximation. In this report we focus on metastable phases and determine their ranges of occurrence. Our investigations in the absence of the external magnetic field show that the system analysed exhibits tricritical behaviour. Two metastable phases (superconducting and nonordered) can exist inside the regions of the phase separated state stability and a first-order transition occurs between these metastable phases.

Discontinuous properties of current-induced magnetic domain wall depinning

The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with x-ray magnetic circular dichroism. In the vicinity of the threshold current density (Jth = 4.2 × 1011 A.m−2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of “Barkhausen jumps”. A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime.

Neutron powder diffraction and molecular dynamics study of superionic SrBr2

The nature of the dynamic ionic disorder within the high-temperature superionic phase of strontium bromide, β-SrBr2, has been investigated using reverse Monte Carlo (RMC) modelling of neutron powder diffraction data and complementary ab initio molecular dynamics (MD) simulations. The RMC and MD results are in good agreement and indicate the presence of extensive dynamic disorder within the Br− sublattice of the cubic fluorite structure. Rapid anion diffusion predominantly occurs as hops between nearest neighbour sites in the 〈100〉 directions, though the trajectories are markedly curved and pass through the peripheries of the octahedral voids in the cation sublattice. In addition, there are extensive correlations between the motions of individual Br−, often leading to the formation of a short-lived square antiprism co-ordination environment around the Sr2+. Such polyhedra are observed within the (ambient temperature) ordered tetragonal crystal structure of α-SrBr2. The nature of the ionic disorder in SrBr2 is of particular interest because it is the only known example of a Br−-ion superionic. Owing to the large size of this anion, a comparison with the behaviour of other superionic phases gives an insight into the role of ionic size on the conducting properties within these materials.

AC Susceptibility of the Dipolar Spin Ice Dy2Ti2O7: Experiments and Monte Carlo Simulations

Experimental data of the frequency-dependent ac susceptibility for the dipolar spin ice Dy2Ti2O7 has been analyzed by Monte Carlo simulations on the basis of the single-spin-flip Metropolis algorithm. We have directly evaluated the ac susceptibility by applying an ac magnetic field. We found that the simulated behaviors reproduces the experimental behavior of the ac susceptibility in the temperature range from 0.6 to 1.0 K, where dilute magnetic monopoles diffusively move. The conversion factor from simulation time to real time, i.e., the rate of hopping of monopoles to nearest-neighbor sites, strongly depends on temperature.

Designer spin systems via inverse statistical mechanics

In this work, we extend recent inverse statistical-mechanical methods developed for many-particle systems to the case of spin systems. For simplicity, we focus in this initial study on the two-state Ising model with radial spin-spin interactions of finite range (i.e., extending beyond nearest-neighbor sites) on the square lattice under periodic boundary conditions. Our interest herein is to find the optimal set of shortest-range pair interactions within this family of Hamiltonians, whose corresponding ground state is a targeted spin configuration such that the difference in energies between the energetically closest competitor and the target is maximized. For an exhaustive list of competitors, this optimization problem is solved exactly using linear programming. The possible outcomes for a given target configuration can be organized into the following three solution classes: unique (nondegenerate) ground state (class I), degenerate ground states (class II), and solutions not contained in the previous two classes (class III). We have chosen to study a general family of striped-phase spin configurations comprised of alternating parallel bands of up and down spins of varying thicknesses and a general family of rectangular block checkerboard spin configurations with variable block size, which is a generalization of the classic antiferromagnetic Ising model. Our findings demonstrate that the structurally anisotropic striped phases, in which the thicknesses of up- and down-spin bands are equal, are unique ground states for isotropic short-ranged interactions. By contrast, virtually all of the block checkerboard targets are either degenerate or fall within class III solutions. The degenerate class II spin configurations are identified up to a certain block size. We also consider other target spin configurations with different degrees of global symmetries and order. Our investigation reveals that the solution class to which a target belongs depends sensitively on the nature of the target radial spin-spin correlation function. In the future, it will be interesting to explore whether such inverse statistical-mechanical techniques could be employed to design materials with desired spin properties.

Solute drag by vacancies in body-centered cubic alloys

Transport coefficients, the elements of the so-called Onsager matrix, are essential quantities for modeling solid-state kinetics controlled by diffusion. Focusing on diffusion in binary alloys with a body-centered cubic crystal structure, we investigate the drag of solute atoms by vacancies, an effect induced by kinetic correlations. To accomplish this, an analytic method---the self-consistent mean field method---is extended to take into account interactions between the solute atom and a vacancy up to the third nearest neighbor sites. We identify kinetic effects involving one or more frequencies. Analytic results are compared with select atomic kinetic Monte Carlo simulations. We show that (1) solute drag is a more general phenomena than previously assumed, (2) it can induced by association and dissociation exchanges, and (3) we identify the mechanisms involved.

Identification of a tri-carbon defect and its relation to the ultraviolet absorption in aluminum nitride

We report on the identification of a tri-carbon defect in AlN bulk crystals grown by physical vapor transport. The defect gives rise to a single infrared absorption line at 1769 cm−1 in unintentionally carbon doped crystals. This line splits into eight lines in crystals enriched with the carbon isotope 13C. The observed line patterns can unambiguously be assigned to a local vibrational mode of a defect that contains exactly three carbon atoms. The most probable arrangement of the three carbon atoms is on nearest-neighbor substitutional sites, replacing two nitrogen atoms and one aluminum atom, whereby one carbon-carbon bond is directed non-parallel and the other parallel to the crystal's c axis. It is suggested that the tri-carbon defect can exist in three different charge states (neutral, singly negative, and doubly negative) and hence possesses two transition levels within the band gap. This energy level scheme explains the appearance and disappearance of the local vibrational mode in dependence on the Fermi level position as well as a similar appearance-disappearance behavior of a strong ultraviolet absorption band at 4.7 eV that has been repeatedly reported in the literature. We propose that the singly negative charge state of the tri-carbon defect essentially contributes to that ultraviolet absorption.

An in-situ neutron diffraction study of the crystal structure of PrBaCo2O5+δ at high temperature and controlled oxygen partial pressure

Abstract Neutron powder diffraction was used to characterize the layered perovskite PrBaCo 2 O 5 + δ (PBCO) under in-situ conditions from 573 to 852 °C and at p O 2 = 10 − 1 –10 − 4 atm. The data were fit to a tetragonal model in space group P4/mmm . The oxygen vacancies were found to be localized within the Pr layer, with total oxygen stoichiometry between 5.57(1) and 5.17(2). This was verified by coulometric titration. The location of these vacancies and the anisotropic displacement of the surrounding oxygen anion sites indicate that ion transport occurs via a hopping mechanism between O sites in the Pr layer and the nearest neighbor sites in the Co layer. While the direct hopping distance between these sites was found to be greater for PBCO than previously reported for NdBaCo 2 O 5 + δ , the larger radius of Pr 3 + stabilizes a higher concentration of mobile anions within the Ln layer.

Electronic and Oxygen Migration Properties of Monoclinic La2GeO5-δ

We investigated the electronic and oxygen migration properties of monoclinic lanthanum germanate La2GeO5-δ by first-principles calculation based on the density functional theory. Based on analysis of the electronic density of states, it was confirmed that an oxygen-deficient crystal with a composition of La2GeO4.75 retains the insulating property with a band gap of 3.519 eV as well as the perfect crystal with a gap of 4.044 eV. The contribution of each atomic orbital to electronic densities of states was evaluated from the partial densities of states and partial charge distribution of a defect-originated state. By using climbing image nudged elastic band method, we obtained the activation barrier for oxygen hopping between nearest-neighbor sites and this value was comparable with experimental one.

In-plane anisotropy of the magnetic and the electric properties of the Fe pnictide Ba(Fe1−x Co x )2As2

The anisotropy of the electronic and the magnetic properties within the FeAs planes of Co(2%)-doped BaFe2As2 has been investigated by using 75As nuclear magnetic resonance (NMR) measurements on the As (As0) sites with all four nearest neighbor sites occupied by Fe. Even in the tetragonal phase at temperatures above 100 K, two sets of NMR spectra with twofold symmetry of in-plane Knight shifts and electronic nuclear quadrupole frequencies are observed. They originate from two domains with their symmetry axes lying at right angles to each other. The anisotropies of the electronic and the magnetic properties become pronounced at ∼140 K, below which an in-plane anisotropy is reported to appear in electrical resistivity under a pressure along one of Fe-Fe directions. 75As NMR measurements have also been carried out for the As (As1) sites surrounded by one Co and three Fe. From the data, we conclude that the spin susceptibility of As1 is ∼1/3 that of As0 and that the antiferromagnetic spin fluctuation of As1 is fairly suppressed as compared with that of As0. We discuss these behaviors of the FeAs layer in connection with the impurity-induced local orbital order model proposed by theoretical studies.

Realizing non-Abelian gauge potentials in optical square lattices: an application to atomic Chern insulators

We describe a scheme to engineer non-Abelian gauge potentials on a square optical lattice using laser-induced transitions. We emphasize the case of two-electron atoms, where the electronic ground state g is laser-coupled to a metastable state e within a state-dependent optical lattice. In this scheme, the alternating pattern of lattice sites hosting g and e states depicts a chequerboard structure, allowing for laser-assisted tunnelling along both spatial directions. In this configuration, the nuclear spin of the atoms can be viewed as a ‘flavour’ quantum number undergoing non-Abelian tunnelling along nearest-neighbour links. We show that this technique can be useful to simulate the equivalent of the Haldane quantum Hall model using cold atoms trapped in square optical lattices, offering an interesting route to realize Chern insulators. The emblematic Haldane model is particularly suited to investigate the physics of topological insulators, but requires, in its original form, complex hopping terms beyond nearest-neighbouring sites. In general, this drawback inhibits a direct realization with cold atoms, using standard laser-induced tunnelling techniques. We demonstrate that a simple mapping allows us to express this model in terms of matrix hopping operators that are defined on a standard square lattice. This mapping is investigated for two models that lead to anomalous quantum Hall phases. We discuss the practical implementation of such models, exploiting laser-induced tunnelling methods applied to the chequerboard optical lattice.

Effect of trivalent dopants on local coordination and electronic structure in crystalline and amorphous ZnO

Density functional theory calculations are used to investigate the structure and binding energies of clusters formed between oxygen vacancies and trivalent dopant atoms (indium, gallium and aluminium) substituted into zinc oxide. Our results show that indium atoms form stable nearest neighbour pairs with oxygen vacancies, while gallium and aluminium atoms associate with them at next nearest neighbour sites. Using a combination of classical molecular dynamics and reverse Monte Carlo methods, models of amorphous indium zinc oxide at different compositions up to 25at.% indium are created. Analysis of these models indicates that, in contrast with the trend observed in the crystal phase, indium does not tend to be undercoordinated in the amorphous phase. The value of the band gap obtained for the amorphous compositions is smaller than that of crystalline undoped ZnO by about 0.8eV and is largely independent of the indium concentration. Electron-effective masses calculated in all the amorphous models decrease with increasing amount of indium due to the larger dispersion of the In-dominated conduction bands. This trend is compared to resistivity measurements on amorphous indium zinc oxide, which also decrease with increasing indium concentration.

Conserved Sandpile with A Variable Height Restriction

We study a restricted-height version of the one-dimensional Oslo sandpile with conserved density, using periodic boundary conditions. Each site has a limiting height which can be either two or three. When a site reaches its limiting height it becomes active and may topple, loosing two particles, which move randomly to nearest-neighbor sites. After a site topples it is randomly assigned a new limiting height. We study the model using mean-field theory and Monte Carlo simulation, focusing on the quasi-stationary state, in which the number of active sites fluctuates about a stationary value. Using finite-size scaling analysis, we determine the critical particle density and associated critical exponents.

Klein tunneling of two correlated bosons

Reflection of two strongly interacting bosons with long-rage interaction hopping on a one-dimensional lattice scattered off by a potential step is theoretically investigated in the framework of the extended Hubbard model. The analysis shows that, in the presence of unbalanced on-site and nearest-neighbor site interaction, two strongly correlated bosons forming a bound particle state can penetrate a high barrier, despite the single particle can not. Such a phenomenon is analogous to one-dimensional Klein tunneling of a relativistic massive Dirac particle across a potential step.

Anisotropy and universality in finite-size scaling: Critical Binder cumulant of a two-dimensional Ising model

We reanalyze transfer-matrix and Monte Carlo results for the critical Binder cumulant U* of an anisotropic two-dimensional Ising model on a square lattice in a square geometry with periodic boundary conditions. Spins are coupled between nearest-neighboring sites and between next-nearest-neighboring sites along one of the lattice diagonals. We find that U* depends only on the asymptotic critical long-distance features of the anisotropy, irrespective of its realization through ferromagnetic or antiferromagnetic next-nearest-neighbor couplings. We modify an earlier renormalization-group calculation to obtain a quantitative description of the anisotropy dependence of U*. Our results support our recent claim towards the validity of universal finite-size scaling for critical phenomena in the presence of a weak anisotropy.

Strain-induced topological insulator phase and effective magnetic interactions in Li2IrO3

We present an effective tight-binding Hamiltonian for Li${}_{2}$IrO${}_{3}$ based on maximally localized Wannier functions for states near the Fermi level as obtained from first-principles electronic structure calculations. The majority of the Wannier orbitals are positioned on the center site with dominant ${j}_{\mathrm{eff}}=1/2$ character, while relatively small ${j}_{\mathrm{eff}}=3/2$ tails lie on the three nearest-neighbor sites. Interestingly, the spin quantization axis of the ${j}_{\mathrm{eff}}=1/2$ components deviates from the local octahedral axis and points toward the nearest-neighbor Ir direction. In our tight-binding model, there are relatively strong next-nearest- and the third-nearest-neighbor hopping terms within the two-dimensional Ir honeycomb lattice in addition to the relatively small but significant interlayer hopping terms. The ratio between the nearest-neighbor and the third-nearest-neighbor hoppings, which can be controlled by the lattice strain, plays a critical role in determinating the ${Z}_{2}$-invariant character of Li${}_{2}$IrO${}_{3}$. From our tight-binding model, we also derive an effective Hamiltonian and its parameters for the magnetic exchange interactions. Due to the complex spin-dependent next-nearest-neighbor hopping terms, our pseudospin Hamiltonian includes significant next-nearest-neighbor antiferromagnetic Kitaev terms as well as Dzyaloshinskii-Moriya and Heisenberg interactions. From our model Hamiltonian we estimate classical energies of collinear magnetic configurations as functions of the Hund's coupling of the Ir atom, from which zigzag-type magnetic order gives the lowest energy.

Interplay and Competition Between Superconductivity and Charge Orderings in the Zero-Bandwidth Limit of the Extended Hubbard Model with Pair Hopping and on-Site Attraction

We present studies of an effective model which is a simple generalization of the standard model of a local pair superconductor with on-site pairing (i.e., the model of hard core bosons on a lattice) to the case of finite pair binding energy. The tight binding Hamiltonian consists of (i) the effective on-site interaction U, (ii) the intersite density-density interactions W between nearest-neighbors, and (iii) the intersite charge exchange term I, determining the hopping of electron pairs between nearest-neighbor sites. In the analysis of the phase diagrams and thermodynamic properties of this model, we treat the intersite interactions within the mean-field approximation. Our investigations of the U<0 and W>0 case show that, depending on the values of interaction parameters, the system can exhibit three homogeneous phases: superconducting (SS), charge-ordered (CO), and nonordered (NO) as well as the phase separated SS–CO state.