Intrachain Coupling Research Articles

Extracting structural information from MEH-PPV optical spectra.

The Frenkel-Holstein model in the Born-Oppenheimer regime is used to interpret temperature-dependent photoluminescence spectra of solutions made with the poly(p-phenylene vinylene) derivative MEH-PPV. Using our recently developed structural optimization method and assuming only intrachain electronic coupling, we predict the structure of emissive MEH-PPV chromophores in terms of a mean torsional angle ϕ0 and its static fluctuations σϕ, assuming no cis-trans defects. This allows us to fully account for the observed changes in spectra, and the chromophore structures obtained are consistent with the known phase transition at 180 K between a "red" and "blue" phase.

Random Co(II)-Ni(II) Ferromagnetic Chains Showing Coexistent Antiferromagnetism, Metamagnetism, and Single-Chain Magnetism.

A series of isomorphous compounds of general formula [Co1- xNi x(tzpo)(N3)(H2O)2] n· nH2O ( x = 0, 0.19, 0.38, 0.53, 0.68, 0.84, and 1; tzpo = 4-(5-tetrazolate)pyridine- N-oxide) was prepared. The compounds consist of homometallic or heterometallic chains with simultaneous azide-tetrazolate bridges. The heterometallic systems feature random distribution of metal ions. All compounds across the series exhibit intrachain ferromagnetic coupling, interchain antiferromagnetic (AF) ordering, field-induced metamagnetic transition, and, except the Ni-only compound, single-chain magnetic dynamics. The AF ordering temperature, the metamagnetic critical field, and the relaxation parameters show different composition dependence. Notably, the blocking temperature for the Co-rich materials is higher than the Co-only compound, suggesting synergy between the randomly distributed Co(II) and Ni(II) ions in promoting slow relaxation. The results imply rich physics in the random mixed-metal systems and demonstrate the possibility of improving single-chain relaxation properties by blending metal ions.

S = 1 Tetraazacyclophane Diradical Dication with Robust Stability: A Case of Low-Temperature One-Dimensional Antiferromagnetic Chain

One-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with low local magnetic anisotropy may provide a better understanding of the low-dimensional magnetism. We report solid-state studies, including single crystal X-ray crystallography, of air-stable tetraazacyclophane diradical dication salt 12·2+·2[Al(OC(CF3)2CH3)4]- with a triplet ground state (Δ EST ≈ 0.5 kcal mol-1). The magnetic behavior for 12·2+ at low temperature is best modeled by 1D spin S = 1 Heisenberg chain with intrachain antiferromagnetic coupling of J'/ k = -5.4 K, which is associated with the interaryl C···C contacts, including π-π interactions. Zero-field splitting value, | D/ hc| ≈ 5.6 × 10-3 cm-1, for 12·2+ is rather small; thus, the 1D chains are characterized by the high degree of isotropicity | D/2 J'| ≈ 7.5 × 10-4. The diradical dication salt possesses extraordinary stability with onset of decomposition at temperature of about 180 °C (∼450 K), based on thermogravimetric analysis and EPR spectroscopy.

Symmetry protected topological Luttinger liquids and the phase transition between them

We show that a doped spin-1/2 ladder with antiferromagnetic intra-chain and ferromagnetic inter-chain coupling is a symmetry protected topologically non-trivial Luttinger liquid. Turning on a large easy-plane spin anisotropy drives the system to a topologically-trivial Luttinger liquid. Both phases have full spin gaps and exhibit power-law superconducting pair correlation. The Cooper pair symmetry is singlet dxy in the non-trivial phase and triplet Sz=0 in the trivial phase. The topologically non-trivial Luttinger liquid exhibits gapless spin excitations in the presence of a boundary, and it has no non-interacting or mean-field theory analog even when the fluctuating phase in the charge sector is pinned. As a function of the strength of spin anisotropy there is a topological phase transition upon which the spin gap closes. We speculate these Luttinger liquids are relevant to the superconductivity in metalized integer spin ladders or chains.

Magnetic Slow Relaxation in a Metal-Organic Framework Made of Chains of Ferromagnetically Coupled Single-Molecule Magnets.

We report the study of a Dy-based metal-organic framework (MOF) with unprecedented magnetic properties. The compound is made of nine-coordinated DyIII magnetic building blocks (MBBs) with poor intrinsic single-molecule magnet behavior. However, the MOF architecture constrains the MBBs in a one-dimensional structure that induces a ferromagnetic coupling between them. Overall, the material shows a magnetic slow relaxation in absence of external static field and a hysteretic behavior at 0.5 K. Low-temperature magnetic studies, diamagnetic doping, and ab initio calculations highlight the crucial role played by the Dy-Dy ferromagnetic interaction. Overall, we report an original magnetic object at the frontier between single-chain magnets and single-molecule magnets that host intrachain couplings that cancel quantum tunneling between the MBBs. This compound is evidence that a bottom-up approach through MOF design can induce spontaneous organization of MBBs able to produce remarkable molecular magnetic materials.

Spin-frustrated pyrochlore chains in the volcanic mineral kamchatkite (KCu3OCl(SO4)2)

Search of new frustrated magnetic systems is of a significant importance for physics studying the condensed matter. The platform for geometric frustration of magnetic systems can be provided by copper oxocentric tetrahedra (OCu4) forming the base of crystalline structures of copper minerals from Tolbachik volcanos in Kamchatka. The present work was devoted to a new frustrated antiferromagnetic - kamchatkite (KCu3OCl(SO4)2). The calculation of the sign and strength of magnetic couplings in KCu3OCl(SO4)2 has been performed on the basis of structural data by the phenomenological crystal chemistry method with taking into account corrections on the Jahn-Teller orbital degeneracy of Cu2. It has been established that kamchatkite (KCu3OCl(SO4)2) contains AFM spin-frustrated chains of the pyrochlore type composed of cone-sharing Cu4 tetrahedra. Strong AFM intrachain and interchain couplings compete with each other. Frustration of magnetic couplings in tetrahedral chains is combined with the presence of electric polarization.

Temperature-Tuning of Optical Properties and Molecular Aggregation in AnE-PVstat Copolymer Solution

The correlation between the optical and morphological properties of an anthracene-containing poly(phenylene ethynylene)-alt-(phenylenevinylene) statistical copolymer (AnE-PVstat) in solution has been investigated. As a function of temperature, the molecular aggregation in chloroform/chlorobenzene (CF:CB) solution was examined by means of absorbance and photoluminescence measurements. The study of the evolution of the 0–0 to the 0–1 photoluminescence (PL) ratio with temperature in the framework of the HJ-aggregate model unveiled the presence of both H- and J-like behaviors at room temperature. Upon increasing temperature, the interchain intermolecular coupling decreases, leading to a dissociation of H-aggregates. Hence, molecular torsions are facilitated, polymer planarity is reduced, and the intrachain electronic coupling decreases.

Developing a magnetic metal organic framework of copper bearing a mixed azido/butane-1,4-dicarboxylate bridge: magnetic and gas adsorption properties.

A magnetic metal organic framework {[Cu(but-1,4-dc)0.5(N3)(H2O)]·H2O}n (MFUM-1(Cu)) (but-1,4-dc = butane-1,4-dicarboxylate) was synthesized and characterized structurally and magnetically. In MFUM-1(Cu), each CuII ion has a distorted octahedral geometry with an obvious Jahn-Teller distortion, where the coordination environment is composed of mixed EO-azido/aliphatic based carboxylate/H2O threefold bridges. These bridges extend the structure of MFUM-1(Cu) in two dimensions by covalent connectivity and form square-shaped channels. Also, a study was done to determine the effectiveness of sonochemical synthesis for the preparation of nano-sheets of MFUM-1(Cu) and subsequently the influence of particle size on physical properties such as magnetic behavior and thermal stability. The particles were characterized by elemental analyses, infrared spectroscopy (IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD) analyses. The effects of parameters such as concentration, solvent, and reaction time on the size distribution, morphology, and yield of product were carefully studied. The magnetic properties of MFUM-1(Cu) and corresponding nano-structure were examined which indicated metamagnetism with strong intrachain ferromagnetic coupling versus the weak interchain antiferromagnetic coupling. Finally, the application of MFUM-1(Cu) in the separation of carbon dioxide from nitrogen and also from methane was theoretically investigated. High calculated selectivity of CO2 over N2 and CH4 reveals the potential application of MFUM-1(Cu) in practical systems of gas separation.

Seven-coordinate tetraoxolate complexes

The reaction of TPypA (tris(2-pyridylmethyl)amine) and Fe(BF4)2 and sodium nitraniIate (Na21NO2) or Mn(O2CMe)2 with chloranilic acid (H21Cl) respectively forms [FeIII(TPyA)1NO2]2O and MnII(TPyA)1Cl, respectively. The structures of these 7-coordinate compounds have been determined. The former is a dinuclear FeIII compound possessing equivalent Fe(III) sites, and the latter forms a 1-D zigzag chain based upon the two bridging 1NO22− ions. The isomer shift and quadrupole splitting of the 57Fe Mössbauer spectra of [Fe(TPyA)1NO2]2O are consistent binuclear seven-coordinate high-spin FeIII state. Below 35 K a single quadrupole doublet split onto two doublets of equal intensities and equal isomer shifts as a result of structural non-equivalency of two iron sites within a dimer. The magnetic properties of [FeIII(TPyA)1NO2]2O can be fit the a homonuclear S = 5/2 ± 5/2 dinuclear spin model with strong intradimer antiferromagnetic coupling of J/kB = −146 K (−101 cm−1) and g = 2.00, and interdimer coupling of θ = −5 K (H = −2JSa·Sb). MnII(TPyA)1Cl can be fit the a homonuclear S = 5/2 1-D Fisher chain expression with weak intrachain antiferromagnetic coupling with J/kB = −0.33 K (−0.23 cm−1) and g = 2.06.

Quantum phase transitions in effective spin-ladder models for graphene zigzag nanoribbons

We examine the magnetic correlations in quantum spin models that were derived recently as effective low-energy theories for electronic correlation effects on the edge states of graphene nanoribbons. For this purpose, we employ quantum Monte Carlo simulations to access the large-distance properties, accounting for quantum fluctuations beyond mean-field-theory approaches to edge magnetism. For certain chiral nanoribbons, antiferromagnetic inter-edge couplings were previously found to induce a gapped quantum disordered ground state of the effective spin model. We find that the extended nature of the intra-edge couplings in the effective spin model for zigzag nanoribbons leads to a quantum phase transition at a large, finite value of the inter-edge coupling. This quantum critical point separates the quantum disordered region from a gapless phase of stable edge magnetism at weak intra-edge coupling, which includes the ground states of spin-ladder models for wide zigzag nanoribbons. To study the quantum critical behavior, the effective spin model can be related to a model of two antiferromagnetically coupled Haldane-Shastry spin-half chains with long-ranged ferromagnetic intra-chain couplings. The results for the critical exponents are compared also to several recent renormalization group calculations for related long-ranged interacting quantum systems.

A Series of Chiral, Polar, Homospin Topological Ferrimagnets: M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni)

An isostructural series of transition metal-formate-chloride-hydrate compounds, M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni), have been synthesized using a solvothermal method. These compounds crystallize in the chiral and polar space group P31 and are comprised of three different types of helical chains of edge-sharing M2+-centered octahedra. All three compounds undergo 3D ferrimagnetic ordering at low temperature, and the iron and cobalt analogues exhibit field-induced metamagnetic transitions. The magnetic structure was determined by neutron powder diffraction, revealing ferromagnetic intrachain coupling and antiferromagnetic interchain interactions, with the three chains arranged in a two-up/one-down triangular lattice. As all three chains contain one type of metal in the same spin state, these compounds are rare examples of homospin topological ferrimagnets.

Exchange constants in molecule-based magnets derived from density functional methods

Cu(pyz)(NO3)2 is a quasi one-dimensional molecular antiferromagnet that exhibits three dimensional long-range magnetic order below TN=110 mK due to the presence of weak inter-chain exchange couplings. Here we compare calculations of the three largest exchange coupling constants in this system using two techniques based on plane-wave basis-set density functional theory: (i) a dimer fragment approach and (ii) an approach using periodic boundary conditions. The calculated values of the large intrachain coupling constant are found to be consistent with experiment, showing the expected level of variation between different techniques and implementations. However, the interchain coupling constants are found to be smaller than the current limits on the resolution of the calculations. This is due to the computational limitations on convergence of absolute energy differences with respect to basis set, which are larger than the inter-chain couplings themselves. Our results imply that errors resulting from such limitations are inherent in the evaluation of small exchange constants in systems of this sort, and that many previously reported results should therefore be treated with caution.

Design, structure and magnetic properties of a novel one-dimensional Mn(II) coordination polymer constructed from 4-pyridyl-NH-1,2,3-triazole

A one-dimensional manganese(II) complex, [Mn(L2)]n (1, HL=4-pyridyl-NH-1,2,3-triazole), has been hydrothermally synthesized at 120°C. In the structure of 1, the Mn(II) ions are octahedral coordinated and further bridged by L− ligands, forming a rigid 1D ribbon coordination chain. Varieties of supramolecular interactions, such as hydrogen bonds and CH⋯π interactions, play an important role on the stabilization of the structures. As revealed by magnetic measurement, complex 1 displays weak antiferromagnetic interaction with θ=–4.83K, the best fit using Fisher's 1D Heisenberg model gives the intrachain magnetic coupling constant J=–0.21cm−1 above 30K due to the neighboring Mn(N–N)2Mn rings twisted in the angle of almost 90°.

A Rare Water and Hydroxyl-Extended One-Dimensional Dysprosium(III) Chain and Its Magnetic Dilution Effect.

A novel water and hydroxyl-extended one-dimensional dysprosium(III) chain was hydrothermally obtained, which exhibits a relatively high spin-reversal energy barrier of 88.7 K and intrachain ferromagnetic interaction with the coupling constant Jexch = 3.04 cm-1 calculated by fitting magnetic susceptibilities using POLY_ANISO program based on ab initio calculations. To deeply understand the respective role of the single-ion anisotropy and intrachain exchange on the effective energy barrier, three crystallographically isostructural analogues containing isotropic Gd(III)-, diamagnetic Y(III)-, as well as Y(III)-doped Dy0.05Y0.95 were prepared and characterized structurally and magnetically. Due to the absence of significant intrachain exchange interaction, the effective energy barrier of the Dy0.05Y0.95 decreased by 9.9 K as compared with that of parent dysprosium(III) chain. Thus, it can be concluded that the intrachain ferromagnetic coupling and the magnetic anisotropy of the Dy(III) ion synergistically enhance the effective energy barrier of the dysprosium(III) chain, in which the single-ion anisotropy becomes more predominant.

Slow Magnetic Relaxation in One-Dimensional Azido-Bridged CoII Complexes.

Crystal structures and magnetic properties of three one-dimensional (1D) azido-bridged cobalt(II) chains with different amide ligands (L), [Co2(N3)4(DMF)3] (1), [Co4(N3)8(DEF)5] (2), and [Co2(N3)4(DIPF)2] (3) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, and DIPF = N,N-diisopropylformamide), are reported to investigate the influence of L on their structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that, although 1-3 all consist of cobalt chains bridged by end-on (EO) azides, the coordination geometry of the CoII ions and the repeating units of the 1D structures are quite different. As the size of L increases, the ratio of L to CoII decreases from 6:4 in 1 to 5:4 and 4:4 in 2 and 3, respectively. In 1, two [CoN5O1] and two [CoN4O2] distorted octahedra form the {[CoN5O1][CoN4O2]2[CoN5O1]} tetramers (denoted as Co4A), which are linked to each other by sharing the N-N edge to form the chain. Similarly, the chain structure of 3 is constructed from a similar tetramer unit {[CoN5][CoN4O2]2[CoN5]} (denoted as Co4B), where half of the CoII centers are in the [CoN5] trigonal bipyramid because of the larger steric effect of the DIPF ligand, while, for compound 2 of the medium-sized amide, it has the transition structure between those of 1 and 3. The chain is composed of two different repeating units: Co4A unit similar to that in 1 and Co4B unit similar to that in 3. Because of their similar structures, compounds 1-3 exhibit analogical magnetic properties. Direct-current magnetic measurements demonstrated that all compounds show intrachain ferromagnetic coupling through the EO azides and interchain anti-ferromagnetic interactions. Alternating-current data revealed the slow magnetic relaxation in the anti-ferromagnetic ordered phases. While compound 1 exhibits spin glass behavior, compounds 2 and 3 behave as the single-chain magnets. This difference might come from the interference of the anti-ferromagnetic ordering on the magnetic dynamic of the magnetic chain.

Synthesis, magnetic properties and electronic structure of the S = ½ uniform spin chain system InCuPO5

We have studied the structural and magnetic properties and electronic structure of the compound InCuPO5 synthesized by a solid state reaction method. The structure of InCuPO5 comprises S = ½ uniform spin chains formed by corner-shared CuO4 units. Magnetic susceptibility (χ(T)) data show a broad maximum at about 65 K, a characteristic feature of one-dimensional (1D) magnetism. The χ(T) data are fitted to the coupled S = ½ Heisenberg antiferromagnetic (HAFM) uniform chain model that gives the intra-chain coupling (J/kB) between nearest-neighbor Cu2+ ions as −100 K and the ratio of inter-chain to intra-chain coupling (J′/J) as about 0.07. The exchange couplings estimated from the magnetic data analysis are in good agreement with the values computed from the electronic structure calculations based on the density functional theory + Hubbard U (DFT + U) approach. The combination of theoretical and experimental analysis confirms that InCuPO5 is a candidate material for weakly coupled S = ½ uniform chains. A detailed theoretical analysis of the electronic structure further reveals that the system is insulating with a gap of 2.4 eV and a local moment of 0.70 µB/Cu.

A fragmentation-based approach for evaluating the intra-chain excitonic couplings in conjugated polymers

For computing the intra-chain excitonic couplings in polymeric systems, here we propose a new fragmentation approach. A comparison for the energetic and spatial properties of the low-lying excited states in PPV between our scheme and full quantum chemical calculations, reveals that our scheme can nicely reproduce full quantum chemical results in weakly coupled systems. Further wavefunction analysis indicate that improved description for strongly coupled system can be achieved by the inclusion of the higher excited states within each fragments. Our proposed scheme is helpful for building the bridge linking the phenomenological descriptions of excitons and microscopic modeling for realistic polymers.

One-dimensional magnetic order in the metal-organic framework Tb(HCOO)3

A combination of variable-temperature neutron scattering, reverse Monte Carlo analysis and direct Monte Carlo simulation is used to characterise the emergence of magnetic order in the metal--organic framework (MOF) Tb(HCOO)$_3$ over the temperature range 100 K to 1.6 K $=T_{\rm N}$. We show that the magnetic transition at $T_{\rm N}$ involves one-dimensional ferromagnetic ordering to a partially-ordered state related to the triangular Ising antiferromagnet. In this phase, the direction of magnetisation of ferromagnetic chains tends to alternate between neighbouring chains but this alternation is frustrated and is not itself ordered. In neutron scattering measurements this partial order gives rise to Bragg-like peaks, which cannot be interpreted using conventional magnetic crystallography without resort to unphysical spin models. The existence of low-dimensional magnetic order in Tb(HCOO)$_3$ is stabilised by the contrasting strength of inter- and intra-chain magnetic coupling, itself a consequence of the underlying MOF architecture. Our results demonstrate how MOFs may provide an attractive if as yet under-explored platform for the realisation and investigation of low-dimensional physics.

Tuning of magnetic interaction and transport properties of Ca3Co2O6 through Sm-doping

The structure, magnetic and transport properties of quasi-one-dimensional spin chain compounds Ca3-xSmxCo2O6 (0 ≤ x ≤ 0.8) were investigated. It is found that the intra-chain magnetic exchange coupling constant J and Weiss temperature θ decrease considerably with increasing Sm-doping content x and the sign of J and θ changes from positive to negative as x reaches to 0.6. The result shows that the dominated intra-chain magnetic coupling can be changed from ferromagnetic (FM) to antiferromagnetic (AFM) by Sm-doping. Moreover, the magnetic ground state of Ca3Co2O6 is changed from the spin density wave (SDW) phase to AFM state in Sm-doped samples. The resistivity decreases sharply first with a small amount of Sm-doping and then increases slightly as x increases further, which is a result of the competing effect between carrier concentration variations and lattice distortion. The thermopower has a similar variation trend with that of resistivity. The increased carrier concentration and enhanced spin entropy may be the origin of the change of the thermopower.

Breaking of 1D magnetism in a spin-1 chain antiferromagnet Ni2V2O7: ESR and first-principles studies

We report the breaking of one-dimensional (1D) magnetism in a spin-1 chain antiferromagnet Ni2V2O7. First, basic structure and magnetization data are presented, showing that the compound is ordered at $ T_{N}=7$ K and a field-induced spin-flop transition occurs at $ H_{sf}=2.8$ T. Second, high-field electronic spin resonance (ESR) data demonstrate antiferromagnetic (AFM) resonance modes below T N, which can be well understood by conventional AFM resonance theory with uniaxial anisotropy. Third, the first-principles calculations based on density function theory reveal a much larger interchain AFM interaction than the intrachain couplings. Combining with all these results, we can see that Ni2V2O7 is not a 1D antiferromagnet, although it is a skew chain from the viewpoint of crystallographic structure.