Nearest-neighbor Bonds Research Articles

Photocatalytic and electrochemical investigation of Mn and Sn co-doped ZnO nanoparticles: Effect of doping concentration

The chemical precipitation method is employed to prepare the Mn and Sn co-doped ZnO nanoparticles with various doping concentrations. Debye Scherrer’s formula, Williamson-Hall equation and Halder-Wagner methods are used to calculate the crystallite size of the prepared nanoparticles. The addition of dopants enhances the periodicity of the atoms. The bond lengths, second nearest neighbor distances and bond angles are examined. The purity of the samples is confirmed using energy dispersive X-ray analysis technique. The process of doping reduces the maximum optical absorption. The dopants reduce the band gap of the prepared nanoparticles and the band gap ranges from 3.07 eV to 3.03 eV. The charge carrier concentrations are calculated from the optical band gap. The Hall coefficient of the prepared nanoparticles increases due to the addition of the dopants. The refractive index of the material is also examined. The quenching is observed in the photoluminescence spectrum due to the dopants. The dielectric properties and electric modulus depend on doping concentrations of Mn and Sn. The degradation of the Congo red and methylene blue dyes with prepared nanoparticles as catalysis is reported. The specific charge of the Mn and Sn co-doped ZnO-based electrode is analyzed using cyclic voltammetric study and galvanostatic charge–discharge technique.

Off-stoichiometry and molybdenum substitution effects on elastic moduli of B1-type titanium carbide

B1-type MX ceramics are composed of transition metals (M) and C, N, and/or O (X) occupying the M and X sites, respectively, and having M–X nearest neighbor (NN) bonds and M–M and X–X next nearest neighbor (NNN) bonds. Substitution of the elements and the formation of structural vacancies in B1-type ceramics change the numbers and strengths of the bonds, leading to novel properties. The change in elastic modulus of off-stoichiometric TiC in equilibrium with a Ti–Mo solid solution phase was experimentally investigated based on the rule of mixtures from the Voigt model. The experimentally obtained values agreed well with the results of density functional theory calculations. The bulk modulus (K) of TiC increased from 205.6 to 239.2 GPa as the fraction of Ti sites occupied by Mo increased from 0.11 to 0.33, whereas the Young’s modulus (E) and the shear modulus (G) remained nearly constant. On the other hand, all three elastic moduli decreased with increasing vacancy fraction at the C sites. These results suggest that the M–X bond strength should be the dominant factor in these moduli and the effect of M–M bond on K is greater than that of G and E.

Structural and Electrical Properties of "Cadmium Sulfide- Cobalt" Compound and Using it as a Gas Sensor

Background: Both the cubic (zinc blend) and hexagonal (wurtzite) phases of cadmium sulphide are semiconductors due to the presence of a row of four equally spaced Cd atoms surrounded by one S atom. Both the cubic and hexagonal crystal forms of Cd-S have very similar nearest neighbor bond lengths. Material and Methods: The cadmium sulfide compound was prepared at a concentration of 1 M by dissolving the percentage by weight of each of cadmium(13.326g) and sulfur (3.806g) in 100 ml of distilled water. Cobalt was prepared at a concentration of 0.5M by dissolving a percentage by weight of Cobalt acetate (14.55g) in 100 ml of distilled water. Films CdS and CdS:Co were prepared using the chemical method, which is the thermal chemical spraying technique, which deposited a solution of cadmium sulfide mixture of cobalt solution with cadmium sulfide on a glass floor.
 Results: The thickness of the prepared films was 100 mm. The surfaces of the prepared films were studied by measuring transmission electron microscopy. The results showed that the prepared films are homogeneous and have crystalline granules of spherical shape, and the addition of cobalt to the cadmium sulfide compound led to an increase in the particle size values. The electrical properties represented by measuring the voltaic- current were studied, and the results showed that the relationship between the voltaic- current is linear, and this indicates that the prepared films behave as ohmic.
 Conclusion: The continuous electrical conductivity was also calculated, and the results showed that the addition of cobalt acetate to the cadmium sulfide compound led to an increase in the electrical conductivity values. The gas sensitivity of the prepared films towards hydrogen sulfide gas was studied, and the results showed that the prepared films had good sensitivity, and adding cobalt acetate to the cadmium sulfide compound led to a decrease in the sensitivity values.

Magnetism of the S=12 J1−J2 square-kagome lattice antiferromagnet

The spin-$1/2$ Heisenberg antiferromagnet on the square-kagome (SK) lattice has attracted growing attention as a model system of highly frustrated quantum magnetism. A further motivation for theoretical studies comes from the recent discovery of SK spin-liquid compounds. The SK antiferromagnet exhibits two non-equivalent nearest-neighbor bonds $J_1$ and $J_2$. One may expect that in SK compounds $J_1$ and $J_2$ are of different strength. We present a numerical study of finite systems by means of the finite-temperature Lanczos method. We discuss the temperature dependence of the specific heat $C(T)$, the entropy $S(T)$, and of the susceptibility $X(T)$ of the $J_1$-$J_2$ SK Heisenberg antiferromagnet varying $J_2/J_1$ in the range $0 \le J_2/J_1 \le 4$. We also discuss the zero-field ground state of the model. We find indications for a magnetically disordered singlet ground state for $0 \le J_2/J_1 \lesssim 1.65$. Beyond $J_2/J_1 \sim 1.65$ the singlet ground state gives way for a ferrimagnetic ground state. In the region $0.77 \lesssim J_2/J_1 \lesssim 1.65$ the low-temperature thermodynamics is dominated by a finite singlet-triplet gap filled with low-lying singlet excitations leading to an exponentially activated low-temperature behavior of $X(T)$. On the other hand, the low-lying singlets yield an extra maximum or a shoulder-like profile below the main maximum in the $C(T)$ curve. For $J_2/J_1 \lesssim 0.7$ the low-temperature thermodynamics is characterized by a large fraction of $N/3$ weakly coupled spins leading to a sizable amount of entropy at very low temperatures. In an applied magnetic field the magnetization process features plateaus and jumps in a wide range of $J_2/J_1$.

Structural and thermodynamic properties of quasi-2D Mo(1–x)W x (S, Se, Te)2 monolayer alloys: a statistical first principle study

In this work, we report an ab initio study of the structural and thermodynamic properties of two-dimensional transition-metal dichalcogenides (2D-TMDC) alloys, Mo(1–x)W x (S, Se, Te)2, using the cluster expansion framework to compute the Helmholtz free energy of alloys as a function of alloy composition and temperature, in the framework of the generalized quasi-chemical approximation. We consider alloying only on the metal sublayer. Our results indicate a weak dependence of the structural properties (lattice constants, nearest-neighbor bond lengths, and layer width) on the alloy composition (i.e. concentrations of W and Mo atoms), in line with the very similar values of the atomic radii of Mo and W atoms. A stronger dependence on the chalcogen is obtained, a trend that reflects the larger variations in atomic radii among the three chalcogen species. As a function of composition, the structural parameters we examined show similar trends, with negligible bowing (i.e. deviations from a Vegard’s law interpolation between end compounds), for the three alloys. Moreover, already at 300 K the behavior of these structural features as a function of composition is very similar to that of the standard-regular-solution (SRS) high-temperature limit. In contrast, the electronic band gaps of the the three alloys as a function of composition show small but significant bowing, as high as −1% to −2% near the x = 0.5 alloy composition. Similarly to the structural features, the band gaps attain the high-temperature SRS limit already at 300 K. Regarding thermodynamic properties, we obtain negative values of the internal energy of mixing for the three alloys over the full range of compositions. Therefore, the theoretical alloying phase diagram for the three alloys is featureless, with stability of a fully-mixed alloy at all temperatures and compositions, with no miscibility gap (hence no bimodal nor spinodal decomposition lines). The thermodynamic potentials (mixing internal energy, mixing entropy, and mixing free energy) reach the high-temperature limit at ∼1000 K, the temperature range of synthesis of 2D-TMDC alloys. These trends of structural and electronic properties of the 2D-TMDC alloys are due to the very similar atomic radii and the nearly identical coordination chemistry of Mo and W. Our results are in agreement with experimental work on the alloying of Mo and W atoms, for samples of Mo(1–x)W x S2 monolayer alloys, that found that the random mixed alloy is the thermodynamically stable state for this alloy, with no segregation or phase separation.

Loop-current charge density wave driven by long-range Coulomb repulsion on the kagomé lattice

Recent experiments on vanadium-based nonmagnetic kagom\'e metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) revealed evidence for possible spontaneous time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered state. The long-sought-after quantum order of loop currents has been suggested as a candidate for the TRS breaking state. However, a microscopic model for the emergence of the loop-current CDW due to electronic correlations is still lacking. Here, we calculate the susceptibility of the real and imaginary bond orders on the kagom\'e lattice near van Hove filling, and reveal the importance of next-nearest-neighbor Coulomb repulsion $V_2$ in triggering the instability toward imaginary bond ordered CDW. The concrete effective single-orbital $t$-$V_1$-$V_2$ model on the kagom\'e lattice is then studied, where $t$ and $V_1$ are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram in the plane spanned by $V_1$ and $V_2$ at van Hove filling. The region dominated by $V_1$ is occupied by a $2a_0 \times 2a_0$ real CDW insulator with the inverse of Star-of-David (ISD) bond configuration. Increasing $V_2$ indeed drives a first-order transition from ISD to stabilized loop-current insulators that exhibit four possible current patterns of different topological properties, leading to orbital Chern insulators. We then extend these results away from van Hove filling and show that electron doping helps the stabilization of loop currents, and gives rise to doped orbital Chern insulators with emergent Chern Fermi pockets carrying large Berry curvature and orbital magnetic moment. Our findings provide a concrete model realization of the loop-current Chern metal at the mean-field level for the TRS breaking normal state of the kagom\'e superconductors.

Assessing the prospect of XAFS experiments of metalloproteins under in vivo conditions at Indus-2 synchrotron facility, India.

The feasibility of X-ray absorption fine-structure (XAFS) experiments of ultra-dilute metalloproteins under in vivo conditions (T = 300 K, pH = 7) at the BL-9 bending-magnet beamline (Indus-2) is reported, using as an example analogous synthetic Zn (0.1 mM) M1dr solution. The (Zn K-edge) XAFS of M1dr solution was measured with a four-element silicon drift detector. The first-shell fit was tested and found to be robust against statistical noise, generating reliable nearest-neighbor bond results. The results are found to be invariant between physiological and non-physiological conditions, which confirms the robust coordination chemistry of Zn with important biological implications. The scope of improving spectral quality for accommodation of higher-shell analysis is addressed.

Determinant quantum Monte Carlo for the half-filled Hubbard model with nonlocal density-density interactions

We design a novel formalism of determinant quantum Monte Carlo method for the half-filled Hubbard model with on-site Hubbard interaction $U$ and nearest neighbor density-density interaction $V$ on the square lattice. The formalism is free of sign problem for $|U|\ge 4|V|$, and is achieved by introducing discrete auxiliary fields on the nearest-neighbor bonds alone. Based on this formalism, we study the ground state phase diagram of the model systematically using the projector algorithm. Within the sign-problem free parameter space $|U|\ge 4|V|$, we obtain antiferromagnetism for $U\ge 4|V|>0$, charge density wave for $U0$, s-wave superconductivity for $U<0$ and small negative $V$, and phase separation for $U<0$ and a larger negative $V$. We also obtain the single particle gap and spin excitation spectra, and by comparison to mean field results, as well as available literatures, we discuss the possible phase boundary beyond the sign-problem free region. The unbiased numerically exact results can be taken as benchmarks in future studies. Finally, we discuss possible extension for longer-range interactions in the model.

Molecular dynamics simulation of the ferroelectric phase transition in GeTe: Displacive or order-disorder character

Experimental investigations of the phase transition in GeTe provide contradictory conclusions regarding the nature of the phase transition. Considering growing interest in technological applications of GeTe, settling these disputes is of great importance. To that end, we present a molecular dynamics study of the structural phase transition in GeTe using a machine-learned interatomic potential with ab initio accuracy. First, we calculate the asymmetric shape of the radial distribution function of the nearest-neighbor bonds above the critical temperature, in agreement with previous studies. However, we show that this effect is not necessarily linked to the order-disorder phase transition and can occur as a result of large anharmonicity. Next, we study in detail the static and dynamic properties of the order parameter in the vicinity of the phase transition and find fingerprints of both order-disorder and displacive phase transition.

Two-Dimensional Ionic Crystals: The Cases of IA-VII Alkali Halides and IA-IB CsAu

The alkali halides, known as ionic crystals, have the NaCl-type or CsCl-type structure as the ground state. We study the structural, vibrational, and electronic properties of two-dimensional (2D) ionic crystals from first-principles. Two potential structures that are hexagonal and tetragonal are investigated as structural templates. Through phonon dispersion calculations, 4 and 12 out of 20 alkali halides in the hexagonal and tetragonal structures are dynamically stable, respectively. The electronic band gaps range from 6.8 eV for LiF to 3.9 eV for RbI and CsI in the tetragonal structure within the generalized gradient approximation. By considering the Madelung energy and the core–core repulsion, we propose a hard sphere model that accounts for the nearest-neighbor bond length and the cohesive energy of 2D alkali halides. We also predict that the band gap of 2D CsAu is larger than that of the 3D counterpart by a factor of more than two, whereas the 2D structures are unstable for the long wavelength phonons in the acoustic branch.

Noncoplanar magnetic orders and gapless chiral spin liquid on the kagome lattice with staggered scalar spin chirality

Chiral three-spin interactions can suppress long-range magnetic order and stabilize quantum spin liquid states in frustrated lattices. We study a spin-1/2 model on the kagome lattice involving a staggered three-spin interaction J_\chiJχ in addition to Heisenberg exchange couplings J_1J1 on nearest-neighbor bonds and J_dJd across the diagonals of the hexagons. We explore the phase diagram using a combination of a classical approach, parton mean-field theory, and variational Monte Carlo methods. We obtain a variety of noncoplanar magnetic orders, including a phase that interpolates between cuboc-1 and cuboc-2 states. In the regime of dominant J_{\chi}Jχ, we find a classically disordered region and argue that it may harbor a gapless chiral spin liquid with a spinon Fermi surface. Our results show that the competition between the staggered three-spin interaction and Heisenberg exchange interactions gives rise to unusual ground states of spin systems.

Another exact ground state of a two-dimensional quantum antiferromagnet

We present the exact dimer ground state of a quantum antiferromagnet on the maple-leaf lattice. A coupling anisotropy for one of the three inequivalent nearest-neighbor bonds is sufficient to stabilize the dimer state. Together with the Shastry-Sutherland Hamiltonian, we show that this is the only other model with an exact dimer ground state for all two-dimensional lattices with uniform tilings.

Phonon-induced rotation of the electronic nematic director in superconducting Bi2Se3

The doped topological insulator ${A}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, with $A={\mathrm{Cu},\mathrm{Sr},\mathrm{Nb}}$, becomes a nematic superconductor below ${T}_{c}\ensuremath{\approx}3$--4 K. The associated electronic nematic director is described by an angle $\ensuremath{\alpha}$ and is experimentally manifested in the elliptical shape of the in-plane critical magnetic field ${H}_{c2}$. Because of the threefold rotational symmetry of the lattice, $\ensuremath{\alpha}$ is expected to align with one of three high-symmetry directions corresponding to the in-plane nearest-neighbor bonds, consistent with a ${Z}_{3}$-Potts nematic transition. Here, we show that the nematic coupling to the acoustic phonons, which makes the nematic correlation length tend to diverge along certain directions only, can fundamentally alter this phenomenology in trigonal lattices. Compared to hexagonal lattices, the former possesses a sixth independent elastic constant ${c}_{14}$ due to the fact that the in-plane shear strain doublet $({\ensuremath{\epsilon}}_{xx}\ensuremath{-}{\ensuremath{\epsilon}}_{yy},\ensuremath{-}2{\ensuremath{\epsilon}}_{xy})$ and the out-of-plane shear strain doublet $(2{\ensuremath{\epsilon}}_{yz},\ensuremath{-}2{\ensuremath{\epsilon}}_{zx})$ transform as the same irreducible representation. We find that, when ${c}_{14}$ overcomes a threshold value, which is expected to be the case in doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, the nematic director $\ensuremath{\alpha}$ unlocks from the high-symmetry directions due to the competition between the quadratic phonon-mediated interaction and the cubic nematic anharmonicity. This implies the breaking of the residual in-plane twofold rotational symmetry (${C}_{2x}$), resulting in a triclinic phase. We discuss the implications of these findings for the structure of nematic domains, for the shape of the in-plane ${H}_{c2}$ in ${A}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, and for the presence of nodes inside the superconducting state.

Magnetic order and spin liquid behavior in [Mo3]11+ molecular magnets

Molecular magnets with one unpaired electron per molecule offer promise in the ongoing search for exotic states of matter, including quantum spin liquids. Here, the authors investigate a series of molecular magnets based on Mo trimer building blocks with one unpaired electron each and find that the magnetic ground states are very sensitive to small changes in the breathing parameter---the ratio between first and second nearest neighbor bond lengths in the breathing kagome lattice that they form. When this parameter is sufficiently close to 1, these materials show an absence of magnetic order and other hallmarks of quantum spin liquid behavior, which may be correlated with plaquette charge ordering.

Laughlin topology on fractal lattices without area law entanglement

Laughlin states have recently been constructed on fractal lattices, and the charge and braiding statistics of the quasiholes were used to confirm that these states have Laughlin type topology. Here, we investigate density, correlation, and entanglement properties of the states on a fractal lattice derived from a Sierpinski triangle with the purpose of identifying similarities and differences compared to two-dimensional systems and with the purpose of investigating whether various probes of topology work for fractal lattices. Similarly to two-dimensional systems, we find that the connected particle-particle correlation function decays roughly exponentially with the distance between the lattice sites measured in the two-dimensional plane, but the values also depend on the local environment. Contrary to two-dimensional systems, we find that the entanglement entropy does not follow the area law if one defines the area to be the number of nearest neighbor bonds that cross the edge of the selected subsystem. Considering bipartitions with two bonds crossing the edge, we find a close to logarithmic scaling of the entanglement entropy with the number of sites in the subsystem. This also means that the topological entanglement entropy cannot be extracted using the Kitaev-Preskill or the Levin-Wen methods. Studying the entanglement spectrum for different bipartitions, we find that the number of states below the entanglement gap is robust and the same as for Laughlin states on two-dimensional lattices.

Anomalous magnetic exchange in a dimerized quantum magnet composed of unlike spin species

We present here a study of the magnetic properties of the antiferromagnetic dimer material CuVOF$_4$(H$_2$O)$_6\cdot$H$_2$O, in which the dimer unit is composed of two different $S = 1/2$ species, Cu(II) and V(IV). An applied magnetic field of $\mu_0H_{\rm c1} = 13.1(1)~\rm T$ is found to close the singlet-triplet energy gap, the magnitude of which is governed by the antiferromagnetic intradimer, $J_0 \approx 21~\rm K$, and interdimer, $J' \approx 1~\rm K$, exchange energies, determined from magnetometry and electron-spin resonance measurements. The results of density functional theory (DFT) calculations are consistent with the experimental results and predicts antiferromagnetic coupling along all nearest-neighbor bonds, with the magnetic ground state comprising spins of different species aligning antiparallel to one another, while spins of the same species are aligned parallel. The magnetism in this system cannot be accurately described by the overlap between localized V orbitals and magnetic Cu orbitals lying in the Jahn-Teller (JT) plane, with a tight-binding model based on such a set of orbitals incorrectly predicting that interdimer exchange should be dominant. DFT calculations indicate significant spin density on the bridging oxide, suggesting instead an unusual mechanism in which intradimer exchange is mediated through the O atom on the Cu(II) JT axis.

Quantum paramagnetism and magnetization plateaus in a kagome-honeycomb Heisenberg antiferromagnet

A spin-1/2 Heisenberg model on a honeycomb lattice is investigated by doing triplon analysis and quantum Monte Carlo calculations. This model, inspired by ${\mathrm{Cu}}_{2}{(\mathrm{pymca})}_{3}({\mathrm{ClO}}_{4}$), has three different antiferromagnetic exchange interactions (${J}_{A}, {J}_{B}, {J}_{C}$) on three different sets of nearest-neighbor bonds which form a kagome superlattice. While the model is bipartite and unfrustrated, its quantum phase diagram is found to be dominated by a quantum paramagnetic phase that is best described as a spin-gapped hexagonal-singlet state. The N\'eel antiferromagnetic order survives only in a small region around ${J}_{A}={J}_{B}={J}_{C}$. The magnetization produced by the external magnetic field is found to exhibit plateaus at 1/3 and 2/3 of the saturation value, or at 1/3 alone, or no plateaus. Notably, the plateaus exist only inside a bounded region within the hexagonal-singlet phase. This study provides a clear understanding of the spin-gapped behavior and magnetization plateaus observed in ${\mathrm{Cu}}_{2}{(\mathrm{pymca})}_{3}({\mathrm{ClO}}_{4}$), and also predicts the possible disappearance of 2/3 plateau under pressure.

Evidence of ytterbium doping in YbxZn1-xO nanoparticles synthesized by polymer precursor method

In this work, we report the synthesis of YbxZn1-xO nanoparticles (0.000 ≤x≤ 0.100) by polymer precursor method and the study of their vibrational and structural properties. Thermal analysis of the polymeric precursor showed that the thermal decomposition occurs in few stages, with the crystallization of the wurtzite structure taking place at a temperature below 500 C. Fourier transform infrared spectroscopy indicated the presence of Zn-O and Yb-O bonds. X-ray diffraction data showed the formation of the ZnO wurtzite phase for all samples. The application of the Rietveld method revealed a decrease in the average particle size and an increasing trend in unit cell volume as the Yb3+ content increased. Additionally, the nearest-neighbor bond lengths along and off the c-direction, as well as the bond angles, were calculated. The results obtained provided additional evidence on the efficiency of Yb3+ doping by the polymer precursor method.

Eg[formula omitted]t2g Sub band splitting via crystal field and band anticrossing interaction in NixCd1-xO thin films

In the present work, full composition range of NixCd1-xO thin films, from Cadmium oxide to Nickel oxide are prepared with sol-gel chemical synthesis route on corning glass substrates. The Fourier transform infrared spectra provide a direct indication of compositional dependence for different vibrational modes with increasing Ni doping. The microscopy images reflect the reduced crystallinity and closely packed morphology with increasing Ni doping which endorses the changes reflected in electronic structure. The O K edge, soft X-ray absorption spectra (XAS) for thin films indicate a clear evidence of eg and t2g sub band splitting and has been explained via crystal field splitting and three level band anti-crossing (BAC) interaction phenomena. At higher doping fraction, the inhomogeneous electric field of neighboring ions surrounding the unfilled Ni 3d level results in rupturing of L−S coupling and further crystal field splitting. The BAC interaction brings about partially unoccupied localized Ni 3d and empty d acceptor levels above conduction band minima (CBM). As saturated Fermi level goes below CBM with increasing Ni doping, transitions to antibonding orbital [eg(π*) and t2g(σ*)] with finite energy difference, is reflected as separated eg-t2g peaks in XAS spectra. On the other hand, extended X-ray absorption fine structure for Ni K edge fitting provides a complete information for nearest (Ni-O) and next nearest (Ni-Ni) neighbor bond length.

Unusual Force Constants Guided Distortion-Triggered Loss of Long-Range Order in Phase Change Materials.

Unusual force constants originating from the local charge distribution in crystalline GeTe and Sb2Te3 are observed by using the first-principles calculations. The calculated stretching force constants of the second nearest-neighbor Sb-Te and Ge-Te bonds are 0.372 and −0.085 eV/Å2, respectively, which are much lower than 1.933 eV/Å2 of the first nearest-neighbor bonds although their lengths are only 0.17 Å and 0.33 Å longer as compared to the corresponding first nearest-neighbor bonds. Moreover, the bending force constants of the first and second nearest-neighbor Ge-Ge and Sb-Sb bonds exhibit large negative values. Our first-principles molecular dynamic simulations also reveal the possible amorphization of Sb2Te3 through local distortions of the bonds with weak and strong force constants, while the crystalline structure remains by the X-ray diffraction simulation. By identifying the low or negative force constants, these weak atomic interactions are found to be responsible for triggering the collapse of the long-range order. This finding can be utilized to guide the design of functional components and devices based on phase change materials with lower energy consumption.