104 research outputs found
Gapless spinons and a field-induced soliton gap in the hyper-honeycomb Cu oxalate framework compound [(CH)NH]Cu(CO)
We report a detailed study of the specific heat and magnetic susceptibility
of single crystals of a spin liquid candidate: the hyper-honeycomb Cu oxalate
framework compound [(CH)NH]Cu(CO). The specific
heat shows no anomaly associated with a magnetic transition at low temperatures
down to 180 mK in zero magnetic field. We observe a large linear-in-
contribution to the specific heat , mK/mol K,
at low temperatures, indicative of the presence of fermionic excitations
despite the Mott insulating state. The low- specific heat is strongly
suppressed by applied magnetic fields , which induce an energy gap, , in the spin-excitation spectrum. We use the four-component relativistic
density-functional theory (DFT) to calculate the magnetic interactions,
including the Dzyaloshinskii-Moriya antisymmetric exchange, which causes an
effective staggered field acting on one copper sublattice. The magnitude and
field dependence of the field-induced gap, , are
accurately predicted by the soliton mass calculated from the sine-Gordon model
of weakly coupled antiferromagnetic Heisenberg chains with all parameters
determined by our DFT calculations. Thus our experiment and calculations are
entirely consistent with a model of
[(CH)NH]Cu(CO) in which anisotropic magnetic
exchange interactions due to Jahn-Teller distortion cause one copper sublattice
to dimerize, leaving a second sublattice of weakly coupled antiferromagnetic
chains. We also show that this model quantitatively accounts for the measured
temperature-dependent magnetic susceptibility. Thus
[(CH)NH]Cu(CO) is a canonical example of a
one-dimensional spin-1/2 Heisenberg antiferromagnet and not a
resonating-valence-bond quantum spin liquid, as previously proposed.Comment: 8 pages, 6 figure
All-in-one synthesis of mesoporous silicon nanosheets from natural clay and their applicability to hydrogen evolution
Silicon nanosheets have attracted much attention owing to their novel electronic and optical properties and compatibility with existing silicon technology. However, a cost-effective and scalable technique for synthesizing these nanosheets remains elusive. Here, we report a novel strategy for producing silicon nanosheets on a large scale through the simultaneous molten-salt-induced exfoliation and chemical reduction of natural clay. The silicon nanosheets thus synthesized have a high surface area, are ultrathin (similar to 5 nm) and contain mesoporous structures derived from the oxygen vacancies in the clay. These advantages make the nanosheets a highly suitable photocatalyst with an exceptionally high activity for the generation of hydrogen from a water-methanol mixture. Further, when the silicon nanosheets are combined with platinum as a cocatalyst, they exhibit high activity in KOH (15.83 mmol H-2 per s per mol Si) and excellent photocatalytic activity with respect to the evolution of hydrogen from a water-methanol mixture (723 mu mol H-2 per h per g Si).clos
Polyhedral Oligomeric Silsesquioxane Induced Thermomechanical Reinforcement in Polymer Films Using Only PartsâperâMillion Content
Novel composite nanofibers based on polyamide 66/graphene oxide- grafted aliphatic- aromatic polyamide: preparation and characterization
Metal insertion in a microporous metal-organic framework lined with 2,2'-bipyridine
Reaction of AlCl(3)·6H(2)O with 2,2'-bipyridine-5,5'-dicarboxylic acid (H(2)bpydc) affords Al(OH)(bpydc) (1, MOF-253), the first metal-organic framework with open 2,2'-bipyridine (bpy) coordination sites. The material displays a BET surface area of 2160 m(2)/g and readily complexes metals to afford, for example, 1·xPdCl(2) (x = 0.08, 0.83) and 1·0.97Cu(BF(4))(2). EXAFS spectroscopy performed on 1·0.83PdCl(2) reveals the expected square planar coordination geometry, matching the structure of the model complex (bpy)PdCl(2). Significantly, the selectivity factor for binding CO(2) over N(2) under typical flue gas conditions is observed to increase from 2.8 in 1 to 12 in 1·0.97Cu(BF(4))(2).Eric D. Bloch, David Britt, Chain Lee, Christian J. Doonan, Fernando J. Uribe-Romo, Hiroyasu Furukawa, Jeffrey R. Long, and Omar M. Yagh
Slow spin dynamics in the hyperhoneycomb lattice [(C2H5)(3)NH](2)Cu-2(C2O4)(3) revealed by H-1 NMR studies
We report the results of magnetic susceptibility chi and H-1 nuclear magnetic resonance (NMR) measurements on a three-dimensional hyperhoneycomb lattice compound [(C2H5)(3)NH](2)Cu-2(C2O4)(3) (CCCO). The average value of the antiferromagnetic (AFM) exchange coupling between the Cu2+ (S = 1/2) spins was determined to be J similar to 50 K from the chi measurements. No long-range magnetic ordering has been observed down to T = 50 mK, although NMR lines become slightly broader at low temperatures below 1 K. The broadening of the NMR spectrum observed below 1 K reveals that the Cu spin moments remain at this temperature, suggesting a non-spin-singlet ground state. The temperature and magnetic field dependence of 1/T-1 at temperatures above 20 K is well explained by paramagnetic thermal spin fluctuations where the fluctuation frequency of Cu2+ spins is higher than the NMR frequency of the order of megahertz. However, a clear signature of the slowing down of the Cu2+ spin fluctuations was observed at low temperatures where 1/T-1 shows a thermally activated behavior. The magnetic field dependence of the magnitude of the spin excitation gap suggests that the magnetic behaviors of CCCO are characterized as an AFM chain at low temperatures.This article is published as Ding, Q-P., C. Dissanayake, Santanu Pakhira, W. J. Newsome, F. Uribe-Romo, D. C. Johnston, Y. Nakajima, and Y. Furukawa. "Slow spin dynamics in the hyperhoneycomb lattice [(C 2 H 5) 3 NH] 2 Cu 2 (C 2 O 4) 3 revealed by H 1 NMR studies." Physical Review B 105, no. 10 (2022): L100405.
DOI: 10.1103/PhysRevB.105.L100405.
Copyright 2022 American Physical Society.
Posted with permission.
DOE Contract Number(s): AC02-07CH11358; DMR-1944975
Exceptional chemical and thermal stability of zeolitic imidazolate frameworks
Twelve zeolitic imidazolate frameworks (ZIFs; termed ZIF-1 to -12) have been synthesized as crystals by copolymerization of either Zn(II) (ZIF-1 to -4, -6 to -8, and -10 to -11) or Co(II) (ZIF-9 and -12) with imidazolate-type links. The ZIF crystal structures are based on the nets of seven distinct aluminosilicate zeolites: tetrahedral Si(Al) and the bridging O are replaced with transition metal ion and imidazolate link, respectively. In addition, one example of mixed-coordination imidazolate of Zn(II) and In(III) (ZIF-5) based on the garnet net is reported. Study of the gas adsorption and thermal and chemical stability of two prototypical members, ZIF-8 and -11, demonstrated their permanent porosity (Langmuir surface area = 1,810 m(2)/g), high thermal stability (up to 550°C), and remarkable chemical resistance to boiling alkaline water and organic solvents
Simultaneous Synchrotron X-ray Diffraction and StressâStrain or StressâRelaxation Experiments for the Study of Parallel and Perpendicular Orientation in a Liquid Crystalline Polymer
Synthesis and Characterization of Crystalline Hyperbranched Polysiloxysilane with POSS Groups at the Terminal Position
Parallel and perpendicular orientation in a thermotropic main-chain liquid crystalline polymer
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