Zigzag Bonds Research Articles

Spectrum, Lifshitz transitions and orbital current in frustrated fermionic ladders with a uniform flux

Ultracold Fermi gases with synthetic gauge field represent an excellent platform to study the combined effect of lattice frustration and an effective magnetic flux close to one flux quantum per particle. The minimal theoretical model to accomplish this task is a system of spinless noninteracting fermions on a triangular two-chain flux ladder. In this paper we consider this model close to half-filling, with interchain hopping amplitudes alternating along the zigzag bonds of the ladder and being small. In such setting qualitative changes in the ground state properties of the model, most notably the flux-induced topological Lifshitz transitions are shifted towards the values of the flux per a diatomic plaquette ($f$) close to the flux quantum ($f\sim 1/2$). Geometrical frustration of the lattice breaks the $k \to \pi - k$ particle-hole spectral symmetry present in non-frustrated ladders, and leads to splitting of the degeneracies at metal-insulator transitions. A remarkable feature of a translationally invariant triangular ladder is the appearance of isolated Dirac node at the Brillouin zone boundary, rendering the ground state at $f=1/2$ semi-metallic. We calculate the flux dependence of the orbital current and identify Lifshitz critical points by the singularities in this dependence at a constant chemical potential $\mu$ and constant particle density $\rho$. The absence of the particle-hole symmetry in the triangular ladder leads to the asymmetry between particle ($\rho>1$) and hole ($\rho<1$) doping and to qualitatively different results for the phase diagram, Lifshitz points and the flux dependence of the orbital current in the settings $\rho={\rm const}$ and $\mu={\rm const}$.

Electrolyte adsorption in graphene and hexagonal boron nitride nanochannels

Understanding the dynamics of confined electrolytes through graphene-based channels is essential in multiple fields of science including: (a) developing novel artificial membranes for drug delivery, DNA sequencing and water desalination and (b) developing quantum nanoelectronic devices. In this work, All-Atom molecular dynamics simulations are used to study the adsorption of confined NaCl electrolyte solutions in graphene (GR), hexagonal boron nitride (hBN) and combined GR-hBN-GR nano-channels over a wide range of electrolyte concentrations c∈(0.1,2) M. The behavior of electrolytes confined between these surfaces is analyzed by evaluating the ionic density profiles, Potential of Mean Force (PMF), net charge density and integrated charge. Although neither graphene nor hBN has net electric charge, we observed significant changes in the ionic structure including the enhanced accumulation of sodium and the depletion in chloride density close to the hBN interfaces at low salt concentration. However, increasing the salt concentration leads to accumulation of chloride close to hBN surfaces. Two different types of hBN-GR junctions are considered in this study: a) shuffle (zigzag bonds) and (b) glide (parallel bonds). High adsorption of sodium and chloride ions near shuffle GR-hBN-GR surfaces is observed due to electrostatic attraction forces between the sodium and nitrogen atoms and between chloride and boron atoms at hBN–graphene junctions across zigzag bonds. This study reveals that GR-hBN-GR with shuffle interface is highly advantageous in adsorbing sodium and chloride ions than graphene and hBN alone. To the best of our knowledge, this is the first study that elucidates the interaction between monovalent electrolytes and GR-hBN-GR heterostructures in a confined medium.

Poly[dipotassium [(μ6-2,2',2'',2'''-{[pyrazine-2,3,5,6-tetra-yltetra-kis-(methyl-ene)]tetra-kis-(sulfanedi-yl)}tetra-acetato)-disilver(I)] 5.2-hydrate

The reaction of AgNO3 with the ligand 2,2',2'',2'''-{[pyrazine-2,3,5,6-tetra-yltetra-kis-(methyl-ene)]tetra-kis-(sulfanedi-yl)}tetra-acetic acid in the presence of a potassium acetate buffer lead to the formation of a silver(I)-potassium-organic framework, poly[dipotassium [(μ6-2,2',2'',2'''-{[pyrazine-2,3,5,6-tetra-yltetra-kis(methyl-ene)]tetra-kis-(sulfanedi-yl)}tetra-acetato)-disilver(I)] 5.2-hydrate], {K2[Ag2(C16H16N2O8S4)]·5.2H2O} n , (I). The asymmetric unit is composed of half a binuclear silver complex located about a center of symmetry, a potassium cation and 2.6 disordered water mol-ecules. The whole binuclear silver complex is generated by inversion symmetry with the pyrazine ring being located about an inversion centre. The ligand coordinates in a bis-tetra-dentate manner. The binuclear silver complex anions are linked via bridging Ag⋯S⋯Ag zigzag bonds, forming a network lying parallel to the bc plane. The networks are linked by Ocarboxyl-ate⋯K +⋯Ocarboxyl-ate bridging bonds to form a framework. The disordered water mol-ecules are present near to the K+ cations.

Coexisting Z-type charge and bond order in metallic NaRu2O4

How particular bonds form in quantum materials has been a long-standing puzzle. Two key concepts dealing with charge degrees of freedom are dimerization (forming metal-metal bonds) and charge ordering. Since the 1930s, these two concepts have been frequently invoked to explain numerous exciting quantum materials, typically insulators. Here we report dimerization and charge ordering within the dimers coexisting in metallic NaRu2O4. By combining high-resolution x-ray diffraction studies and theoretical calculations, we demonstrate that this unique phenomenon occurs through a new type of bonding, which we call Z-type ordering. The low-temperature superstructure has strong dimerization in legs of zigzag ladders, with short dimers in legs connected by short zigzag bonds, forming Z-shape clusters: simultaneously, site-centered charge ordering also appears. Our results demonstrate the yet unknown flexibility of quantum materials with the intricate interplay among orbital, charge, and lattice degrees of freedom.

A Roadmap for the Chalcogenide-graphene Composites Formation Under a Glassy Regime

Background: Nano-composite is an innovative material having nano in which fillers dispersed in a matrix. Typ-ically, the structure is a matrix- filler combination, where the fillers like particles, fibers, or fragments are surrounded and bound together as discrete units by the matrix. The term nano-composite encompasses a wide range of materials right from three dimensional metal matrix composites to two dimensional lamellar composites. Therefore, the physical, chemical and biological properties of nano materials differ from the properties of individual atoms and molecules or bulk matter. The chalcogenide – graphene composites in glassy regime is the growing novel research topic in the area of composite material science. It is obvious to interpret such materials different physicochemical mechanism. Objective: The key objective of this research work to explore the internal physicochemical mechanism of the chalcogenide – graphene composites under the glassy regime. Including the prime chalcogen alloying element selenium amorphous atomic structure and their fullerene like bonding nature. By accommodating the essential properties of the stacked layers of bilayer graphene. The diffusion, compression and dispersion of the bilayer graphene in selenium rich ternary (X(1-x-y)-Y(x)- Z(y) + GF (bilayer graphene); X = Se, Y = Semimetal or metalloid, Z = None metal) alloys under the complex regime on and after thermal melting process are addressed. Materials and Methods: To synthesize the composite materials the well-known melt quenched method had adopted. More-over, to interpret the amorphous selenium (Se8) chains and rings molecular structures we had used vista software with an available CIF data file. While to show the armchair and zig-zag bonds with bilayer graphene structure the nanotube modeler simulation software has used. Results: Outcomes of this study reveals the chalcogenide -graphene nano composite formation under a glassy regime changes the individual materials structural and other physical properties that is reflecting in different experimental evi-dences, therefore, the modified theoretical concepts for the different properties of such composite materials are interpreted in this study. Discussion: The dispersion and diffusion of the high stiff graphene bonds in low dimension chalcogen rich alloys has been interpreted based on their quadric thermal expansion behaviour. In addition to this, a possible bond angle modification in the formation of X(1-x-y)-Y(x)- Z(y) + GF composites are also addressed. To interpret the distinct optical property behavior of the formed X(1-x-y)-Y(x)- Z(y) + GF composites and parent chalcogenide glassy alloys a schematic model of the energy levels is also addressed. Conclusion: To make a better understating on the formation mechanism such composites, the diffusion and deformation of high stiff graphene σ and π bonds in a low dimension chalcogenide alloy basic mechanism are discussed on basis of novel “thermonic energy tunneling effect” concept, which could result in quadratic thermal expansion of graphene. Moreover, the structural unit modifications of such composite materials are described in terms of their bond angle modifications and in-fluence of the coordination defects. The energy levels suppression and creation of addition sub energy levels in such com-posite materials are discussed by adopting the viewpoint impact of the foreign alloying elements and surface π-plasmonic resonance between the graphene layers in the honeycomb band structure. Thus, this study has described various basic aspects of the chalcogenide system – bilayer graphene composites formation under a glassy regime.

Anomalous thermodynamics of a quantum spin system with large residual entropy

In contrast to strongly frustrated classical systems, their quantum counterparts typically have a non-degenerate ground state. A counterexample is the celebrated Heisenberg sawtooth spin chain with ferromagnetic zigzag bonds $J_1$ and competing antiferromagnetic basal bonds $J_2$. At a quantum phase transition point $|J_2/J_1|=1/2$, this model exhibits a flat one-magnon excitation band leading to a massively degenerate ground-state manifold which results in a large residual entropy. Thus, for the spin-half model, the residual entropy amounts to exactly one half of its maximum value $\lim_{T\to\infty} S(T)/N = \ln2$. In the present paper we study in detail the role of the spin quantum number $s$ and the magnetic field $H$ in the parameter region around the transition (flat-band) point. For that we use full exact diagonalization up to $N=20$ lattice sites and the finite-temperature Lanczos method up to $N=36$ sites to calculate the density of states as well as the temperature dependence of the specific heat, the entropy and the susceptibility. The study of chain lengths up to $N=36$ allows a careful finite-size analysis. At the flat-band point we find extremely small finite-size effects for spin $s=1/2$, i.e., the numerical data virtually correspond to the thermodynamic limit. In all other cases the finite-size effects are still small and become visible at very low temperatures. In a sizeable parameter region around the flat-band point the former massively degenerate ground-state manifold acts as a large manifold of low-lying excitations leading to extraordinary thermodynamic properties at the transition point as well as in its vicinity such as an additional low-temperature maximum in the specific heat. Moreover, there is a very strong influence of the magnetic field on the low-temperature thermodynamics including an enhanced magnetocaloric effect.

Controlling the thermal conductance ofgraphene/h−BNlateral interface with strain and structure engineering

Although phonon-mediated thermal conduction in pristine graphene and hexagonal boron nitride is well understood, less is known about phonon transport in single-sheet graphene-hexagonal boron nitride ($\mathrm{Gr}/h\ensuremath{-}\mathrm{BN}$) lateral heterostructures, where the thermal resistance of the interfaces plays an important role in the overall thermal conductivity. We apply the newly developed extended atomistic Green's function method to analyze with detail the effect of strain and structure engineering on the thermal conductance ${G}_{\mathrm{int}}$ of the $\mathrm{Gr}/h\ensuremath{-}\mathrm{BN}$ interface. Our calculations show that longitudinal tensile strain leads to significant ${G}_{\mathrm{int}}$ enhancement (up to $25%$ at 300 K) primarily through the improved alignment of the flexural acoustic phonon bands, despite the reduction in the longitudinal acoustic (LA) and transverse acoustic phonon velocities. In addition, we find that alternating C-N zigzag bonds along the zigzag interface lead to a greater ${G}_{\mathrm{int}}$ than C-B bonds through more effective transmission of high-frequency LA and transverse optical phonons, especially at high strain levels. We also demonstrate how the interfacial structure dramatically affects the orientation of the transmitted optical phonons, a phenomenon that is neither seen for acoustic phonons nor predictable from conventional acoustic wave scattering theory. Insights from our paper can provide the basis for manipulating the interfacial thermal conductance in other two-dimensional heterostructures.

First principles analysis of atomic configurations of group IV elements in Ge crystal for solar cells

AbstractWe focused on group IV semiconductors, i.e., Ge (Si), containing C, Si (C, Ge), and/or Sn atoms as alternative candidates for poly‐Si crystals to achieve higher engineering efficiency in solar cells in this research. The main target of this study was to obtain basic data on atomic configurations, such as bond strain and stable sites for substituted C, Si, and Sn atoms in Ge and for C, Ge, and Sn atoms in Si crystals through first principles analysis. We found that ((C, Si)/C) atoms with smaller covalent radii than (Ge/Si) formed tensile local strain along zigzag bonds in the &lt;110&gt; direction while (Sn/(Ge, Sn)) atoms with larger covalent radii than (Ge/Si) formed compressive local strain. Another important result was that local strains except for zigzag bonds had opposite signs to strains along zigzag bonds. The main results to summarize stable configurations of substitutional IV elements were: (1) C and Sn (Si) atoms were stable at positions next to one another, while Si and Sn atoms were stable at mostly separated positions in Ge crystals. (2) C (Ge) and Sn atoms were stable at positions next to one another, while C and Ge atoms were stable at mostly separated positions in Si crystals. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of magnetism and light sp-dopants on chain creation in Ir and Pt break junctions

Applying the generalization of the model for chain formation in break-junctions (Di Napoli et al 2012 J. Phys.: Condens. Matter 24 135501), we study the effect of light impurities on the energetics and elongation properties of Pt and Ir chains. Our model enables us to develop a tool ideal for detailed analysis of impurity-assisted chain formation, in which zigzag bonds play an important role. In particular we focus on H (s-like) and O (p-like) impurities and assume, for simplicity, that the presence of impurity atoms in experiments results in a ..M-X-M-X-... (M: metal, X: impurity) chain structure in between the metallic leads. Feeding our model with material-specific parameters from systematic full-potential first-principles calculations, we find that the presence of such impurities strongly affects the binding properties of the chains. We find that, while both types of impurities enhance the probability of chains being elongated, the s-like impurities lower the chain's stability. We also analyze the effect of magnetism and spin-orbit interaction on the growth properties of the chains.

Structural and electronic properties of armchair graphene nanoribbons under uniaxial strain

We theoretically investigate the structures, relative stabilities and electronic properties of the armchair graphene nanoribbons (AGNRs) under uniaxial strain via first-principles calculations. The results show that, although each bond length decreases (increases) with increasing compression (tension) strain especially for the axial bonds a1, a4 and a7, the ribbon geometrical width d increases (decreases) with increasing compression (tension) strain due to the rotation of the zigzag bonds a2, a3, a5 and a6. For each nanoribbon, as expected, the lowest average energy corresponds to the unstrained state and the larger contract (elongate) deformation corresponds to the higher average energy. At a certain strain, the average energy increases with decreasing the ribbon width n. The average energy increases quadratically with the absolute value of the uniaxial strain, showing an elastic behavior. The dependence of the band gap on the strain is sensitive to the ribbon width n which can be classified into three distinct families n=3I, 3I+1 and 3I+2, where I is an integer. The ribbon width leads to oscillatory band gaps due to quantum confinement effect.

Modeling impurity-assisted chain creation in noble-metal break junctions

In this work we present the generalization of the model for chain formation in break junctions, introduced by Thiess et al (2008 Nano Lett. 8 2144), to zigzag transition-metal chains with s and p impurities. We apply this extended model to study the producibility trends for noble-metal chains with impurities, often present in break junction experiments, namely, Cu, Ag and Au chains with H, C, O and N adatoms. Providing the material-specific parameters for our model from systematic full-potential linearized augmented plane-wave first-principles calculations, we find that the presence of such impurities crucially affects the binding properties of the noble-metal chains. We reveal that both the impurity-induced bond strengthening and the formation of zigzag bonds can lead to a significantly enhanced probability for chain formation in break junctions.

Phase diagrams of the metallic zigzag carbon nanotube

We investigate a metallic zigzag carbon nanotube by means of a Hubbard model, which includes both on-site and nearest-neighbor interactions. Assuming weak interactions, a renormalization group analysis of the equivalent two-leg ladder followed by bosonization and refermionization results in a Gross-Neveu model with an enlarged symmetry relative to the original Hamiltonian. For the undoped case the symmetry of the Gross-Neveu model is SO(8), but for the doped case the particle-hole symmetry is broken and the symmetry reduces to SO(6). Four ground-state phases are found in the undoped carbon nanotube with repulsive interactions, a $d$-wave Mott insulator, an $s$-wave Mott insulator, a $p$-density wave, and a charge density wave. The doped case has two ground-state phases, a $d$-wave superconductor and a phase where a $p$ density wave and a charge density wave coexist. We also explore the global phase diagram with a general interaction profile and find several additional states, including a chiral current phase where current flows around the nanotube along the zigzag bonds.

RVB mean-field analysis of the Heisenberg model on the quasi-one-dimensional triangular lattice

Stimulated by recent experiments on an organic conductor κ -(BEDT-TTF) 2Cu 2 (CN) 3, we investigate the S = 1 2 Heisenberg model on the triangular lattice with quasi-one-dimensional anisotropy. This model is equivalent to the one-dimensional (1D) chains coupled with frustrated zigzag bonds. Using the resonating valence bond mean-field approximation, we calculate one-particle excitation spectra of this system. By changing the anisotropy parameter J ′ / J from the decoupled 1D chains ( J ′ / J = 0 ) to the isotropic triangular lattice ( J ′ / J = 1 ), we find almost gapless excitations in the wide range from the 1D limit. Relations to the observed “gapless” spin excitation in κ -(BEDT-TTF) 2Cu 2(CN) 3 are discussed.

Possibility of Gapless Spin Liquid State by One-Dimensionalization

Motivated by the observation of a gapless spin liquid state in κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , we analyze the anisotropic triangular lattice S =1/2 Heisenberg model with the resonating valence bond mean-field approximation. Paying attention to the small quasi-one-dimensional anisotropy of the material, we take an approach from one-dimensional (1D) chains coupled with frustrating zigzag bonds. By calculating one-particle excitation spectra changing anisotropy parameter J '/ J from the decoupled 1D chains to the isotropic triangular lattice, we find almost gapless excitations in the wide range from the 1D limit. This one-dimensionalization by frustration is considered to be a candidate for the mechanism of the gapless spin liquid state.

Phase separation, spin polarization, and triplet superconductivity in the Hubbard chains coupled with ferromagnetic zigzag bonds

Double Hubbard chains coupled with zigzag bonds, which consist of ferromagnetic exchange interaction and hopping terms, is studied by the density-matrix renormalization group method. We establish the phase diagram based on the results of pair binding energy, spin gap, and ground-state total spin as well as local density. We also calculate the spin-triplet pair correlation functions and suggest the importance of interchain hopping integrals to the formation of spin-triplet pairing.

Spin-triplet f-wave superconductivity in one-dimensional triangular-lattice Hubbard models

The Hubbard model at quarter filling defined on the one-dimensional (1D) triangular lattice (i.e., three chains coupled with interchain zigzag bonds) is studied by the density-matrix renormalization group method in order to clarify the symmetry of the Cooper-pair wave function of our proposed spin-triplet superconductivity induced by the ring-exchange mechanism. We calculate the rotational symmetry of the pair correlation function and show that the pairing occurs predominantly in the spin-triplet f-wave channel. Possible experimental relevance is discussed.

Oxygenation of carbon nanotubes: Atomic structure, energetics, and electronic structure

This paper presents an extensive and systematic analysis of the oxygenation of semiconducting and metallic single-wall carbon nanotubes by using the first principles pseudopotential plane wave method. Our study involves the physisorption of oxygen molecules, chemisorption of oxygen atoms and formation of an oxide, and deals with the equilibrium binding geometry and corresponding electronic energy structure. The binding energies of an oxygen molecule physisorbed at different sites are determined by calculating short and long range interactions. The triplet state of the physisorbed oxygen molecule is energetically favorable, whereas the nonmagnetic (spin paired) state yields a relatively stronger binding energy. An oxygen atom is adsorbed on top of the carbon-carbon bond. The zigzag bonds of the nanotubes are weakened and eventually are replaced by a carbon-oxygen-carbon bridge bond. Chemisorption of atomic oxygen and physisorption of an oxygen molecule modify the electronic energy structure of the bare tube in different ways. For a different coverage and pattern, self-consistent field electronic energy structure calculations using the optimized physisorption geometry corresponding to the triplet ground state result in a small energy gap between unoccupied oxygen levels and the top of the valence band of the semiconducting carbon nanotube. These results invalidate the hole doping of the semiconducting carbon nanotube upon the physisorption of oxygen.