Ordering kinetics and steady states of XY-model with ferromagnetic and nematic interaction

The two-dimensions XY model, undergoes the Berezinskii Kosterlitz Thouless (BKT) transition through unbinding of defect pairs of opposite signs. When the interaction between spins is purely ferromagnetic, these defects have +-1 charge, whereas for pure nematic interaction between spins, they have charge +-1/2. Two-dimensional XY-model in the presence of both ferromagnetic and nematic interactions has been studied both theoretically and experimentally. In this paper, we have studied dynamics of defects in the presence of both ferromagnetic and nematic interactions on a square lattice. Varying the strength of ferromagnetic and nematic interactions, we have observed behavior of both integer and half integer defects and based on that we propose a phase diagram which exhibit three distinct regions in the phase diagram below the critical TBKT : polar phase, nematic phase and coexistence phase and a disordered regions above it. Also, for pure polar and pure nematic case, our results show that the exponent, for algebraic decay of number of defects with time, decays linearly with temperature

Current reversal in polar flock at order-disorder interface

We studied a system of polar self-propelled particles (SPPs) on a thin rectangular channel designed into three regions of order-disorder-order. The division of the three regions is made on the basis of the noise SPPs experience in the respective regions. The noise in the two wide region is chosen lower than the critical noise of order-disorder transition and noise in the middle region or interface is higher than the critical noise. This make the geometry of the system analogous to the Josephson Junction (JJ) in solid state physics. Keeping all other parameters fixed, we study the properties of the moving SPPs in the bulk as well as along the interface for different widths of the junction. On increasing interface width, system shows a order-to-disorder transition from coherent moving SPPs in the whole system to the interrupted current for large interface width. Surprisingly, inside the interface we observed the current reversal for intermediate widths of the interface. Such current reversal is due to the strong randomness present inside the interface, that makes the wall of the interface reflecting. Hence Our study give a new interesting collective properties of SPPs at the interface which can be useful to design devices like switch using active agents

Raman signatures of lattice dynamics across inversion symmetry breaking phase transition in quasi-1D compound, (TaSe4_4)3_3I

Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics reveals three additional Raman active modes in the low-temperature non-centrosymmetric structure. Two vibrational modes become Raman active due to the absence of an inversion center, while the third mode is a soft phonon mode resulting from the vibration of Ta atoms along the \{-Ta-Ta-\} chains. Furthermore, the most intense Raman mode display Fano-shaped asymmetry, inferred as the signature of strong electron-phonon coupling. The group theory and symmetry analysis of Raman spectra confirm the displacive-first-order nature of the structural transition. Therefore, our results establish (TaSe$_4)_3$I as a model system with broken inversion symmetry and strong electron-phonon coupling in the quasi-1D regime.Comment: Main text - 6 figures, 11 pages, supplementary - 10 figures, 13 page

Cage Encapsulated Gold Nanoparticles as Heterogeneous Photocatalyst for Facile and Selective Reduction of Nitroarenes to Azo Compounds

A discrete nanoscopic organic cage (OC1(R)) has been synthesized from a phenothiazine based trialdehyde treating with chiral 1,2-cyclohexanediamine building block via dynamic imine bond formation followed by reductive amination. The cage compound has been characterized by several spectroscopic methods, which advocate that OC1(R) has trigonal prismatic shape formed via 2 + 3] self-assembled imine condensation followed by imine reduction. This newly designed cage has aromatic walls and porous interior decorated with two cyclic thioether and three vicinal diamine moieties suitable for binding gold ions to engineer the controlled nucleation and stabilization of ultrafine gold nanoparticles (AuNPs). The functionalized confined pocket of the cage has been used for the controlled synthesis of AuNPs with narrow size distribution via encapsulation of Au(III) ions. Inductively coupled plasma mass spectrometric (ICP-MS) analysis revealed that the composite Au@OC1(R) has very high (similar to 68 wt %) gold loading. In distinction, reduction of gold salts in absence of the cage yielded structureless agglomerates. The fine-dispersed cage anchored AuNPs (Au@OC1(R)) have been finally used as potential heterogeneous photocatalyst for very facile and selective conversion of nitroarenes to respective azo compounds at ambient temperature in just 2 h reaction time. Exceptional chemical stability and reusability without any agglomeration of AuNPs even after several cycles of use are the potential features of this material. The composite Au@OC1(R) represents the first example of organic cage supported gold nanoparticles as photocatalyst

Anomalous Ca Content Dependence of Dielectric Properties of Charge-Ordered Pr_1−<i>x</i>Ca<i>_x</i>MnO₃ as a Signature of Charge-Ordered Phase Modulation

Low-temperature dielectric properties of charge/orbital-ordered manganite, Pr1−xCaxMnO3 for 0.40 ≤ x ≤ 0.50, was investigated systematically as a function of Ca content, x. The Ca content dependence of dielectric permittivity and dissipation factor exhibited distinct maxima near x~0.45. The overall dielectric response of charge-ordered Pr1−xCaxMnO3 was dominated by dielectric polarization induced by polaron hopping and exhibited thermally activated relaxation behaviour. The thermally activated dielectric relaxation behaviour over the investigated temperature range was further analysed with the help of two models: the small polaron hopping model and the Mott three-dimensional variable range hopping model. The estimated polaron transport parameters also displayed non-monotonic variation with x and exhibited a broad minima between x = 0.425 and 0.45. Considering the previous work reported so far, the charge order pattern of Pr1−xCaxMnO3 below x = 0.425 was most likely to be of Zener-polaron type, while near x = 0.50 was checker-board type and for in-between compositions; neither pure checker-board type nor pure Zener-polaron type can be considered a ground state. The observed results suggest that a modulation of the checkerboard-type charge/orbital ordering pattern in Pr1−xCaxMnO3 possibly takes place in the Ca content range of investigation, 0.40 ≤ x ≤ 0.50

Three New Cu−Azido Polymers and Their Systematic Interconversion: Role of the Amount of the Blocking Amine on the Structural Diversity and Magnetic Behavior

Three new coordination polymers $[Cu_5(N_3)_{10}(en)_2]_n$ (1), $[Cu_6(N_3)_{12}(en)_4]n$ (2), and $[Cu_4(N_3)_8(en)_4]_n$ (3) have been synthesized in a controlled manner by treatment of a 1:2 mixture of $Cu(NO_3)_2$ and $NaN3$ with varying amount of ethylenediamine (en). Single-crystal structure analyses clearly indicated that the puckered $Cu_4$ biscubane unit in 1 gradually opens to a slightly more open $Cu_4$ macrocyclic unit in 2 when more en approaches to the $Cu_4$ core. Upon addition of further en, an open $Cu_4$ linear secondary building unit was obtained in complex 3. Complex 1 contains four different kinds of bridging modes of the azide anion and is a complicated 3D polymer. Similarly, complexes 2 and 3 are 3D and 2D polymers, respectively, containing three different kinds of bridging azides. Complex 3 contains two very rare cis end-to-end (EE) and single-end-on (EO) azido modes. Structural transformation from 1 to 3 was monitored and explained qualitatively. Variable-temperature magnetic studies in the temperature range of 300−2 K reveal the existence of dominant ferromagnetic behavior in all the three cases with a metamagnetic-type behavior in complex 1 with the critical field of transition at 0.8 T. The purity of all the complexes were established by elemental analyses, as well as by the powder XRD patterns that matched well with the expected patterns from the single-crystal structure analysis

Tuning nuclearity of clusters by positional change of functional group: Synthesis of polynuclear clusters, crystal structures and magnetic properties

Four neutral polynuclear magnetic clusters, (Mn6Mn2Na2I)-Mn-III-Na-II(N-3)(8)(mu(1)-O)(2)(L-1)(6)(CH3OH)(2)] (1), (Mn6Na2I)-Na-III(N-3)(4)(mu(4)-O)(2)(L-2)(4)(CH3COO)(4)] (2), Ni-5(II)(N-3)(4)(HL1)(4)(HCOO)(2)(CH3OH)(2)(H2O)(2)]center dot 2CH(3)OH (3) and (Ni4Na2I)-Na-II(N-3)(4)(HL2)(6)]center dot 2CH(3)OH (4) have been synthesized using tetradentate ligands H2L1-2 along with azide as a co-ligand. H2L1-2 are the products formed in situ upon condensation of 2-hydroxy-3-methoxybenzaldehyde with 1-aminopropan-2-ol and 1-aminopropan-3-ol, respectively. Single crystal X-ray diffraction and bond valence sum calculation showed that complex 1 is composed of both Mn-III and Mn-II. Complex 3 contains coordinated formate, which was formed upon in situ oxidation of methanol. The magnetic study over a wide range of temperatures of all the complexes (1-4) showed that 1 and 2 are antiferromagnetic whereas other two (3-4) are predominantly ferromagnetic. The estimated ground states of the complexes are S approximate to 3(1), S = 4(2), S = 5(3) and S approximate to 4(4), respectively. (C) 2014 Elsevier B.V. All rights reserved