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Quantum Phase Transition from a Spin-liquid State to a Spin-glass State in the Quasi-1D Spin-1 System Sr1-xCaxNi2V2O8
We report a quantum phase transition from a spin-liquid state to a spin-glass
state in the quasi-one dimensional (1D) spin-1 system Sr1-xCaxNi2V2O8, induced
by a small amount of Ca-substitution at Sr site. The ground state of the parent
compound (x = 0) is found to be a spin-liquid type with a finite energy gap of
26.6 K between singlet ground state and triplet excited state. Both
dc-magnetization and ac-susceptibility studies on the highest Ca-substituted
compound (x = 0.05) indicate a spin-glass type magnetic ground state. With
increasing Ca-concentration, the spin-glass ordering temperature increases from
4.5 K (for the x = 0.015 compound) to 6.25 K (for the x = 0.05 compound). The
observed results are discussed in the light of the earlier experimental reports
and the theoretical predictions for a quasi-1D spin-1 system.Comment: 26 pages, 8 figures, 3 table
Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14
Spin-spin correlations and microscopic origin of net magnetization in the
spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder
neutron diffraction. The present study reveals a 3D long-range magnetic
ordering below 16 K where the magnetic structure consists of ferromagnetic
trimers that are coupled ferromagnetically along the spin-chain. The moment
components along the a and c axes arrange antiferromagnetically. Our study
establishes that the uncompensated moment components along the b axis result in
a net magnetization per unit cell. The magnetic structure, determined in the
present study, is in agreement with the results of recent first principles
calculation; however, it is in contrast to a fascinating experimental
prediction of ferrimagnetic ordering based on the periodicity of the exchange
interactions in CaNi3P4O14. Our study also confirms the presence of broad
diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over
a wide temperature range below ~50 K down to a temperature well below the Tc.
Total neutron scattering analysis by the RMC method reveals that the dominating
spin-spin correlation above Tc is ferromagnetic and along the b axis. The
nearest neighbour spin-spin correlations along the a and c axes are found to be
weakly antiferromagnetic. The nature of the trimer spin structure of the
short-range state is similar to that of the 3D long-range ordered state. The
present investigation of microscopic nature of the magnetic ground state also
explains the condition required for the 1/3 magnetization plateau to be
observed in the trimer spin-chains. In spite of the S=1 trimer chain system,
the present compound CaNi3P4O14 is found to be a good realization of 3D magnet
below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and
1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure
AlkoxyalleneâBased LANCA ThreeâComponent Synthesis of 1,2âDiketones, Quinoxalines, and Unique Isoindenone Dimers and a Computational Study of the Isoindenone Dimerization
A series of ÎČâalkoxyâÎČâketoenamides was prepared by the wellâestablished LANCA threeâcomponent reaction of lithiated 1â(2âtrimethylsilylethoxy)âsubstituted allenes, nitriles, and α,ÎČâunsaturated carboxylic acids. The αâtertâbutylâsubstituted compounds were smoothly converted into the expected 1,2âdiketones by treatment with trifluoroacetic acid. A subsequent condensation of the 1,2âdiketones with oâphenylenediamine provided the desired highly substituted quinoxalines in good overall yield. Surprisingly, the αâphenylâsubstituted ÎČâalkoxyâÎČâketoenamides investigated afford not only the expected 1,2âdiketones, but also pentacyclic compounds with an antiâtricyclo[4.2.1.12,5]decaâ3,7âdieneâ9,10âdione core. These interesting products are very likely the result of an isoindenone dimerization which was mechanistically studied with the support of DFT calculations. Under the strongly acidic reaction conditions, a stepwise reaction is likely leading to a protonated isoindenone as reactive intermediate. It may first form a van der Waals complex with a neutral isoindenone before the two regioâ and diastereoselective ring forming steps occur. Interestingly, two neutral or two protonated isoindenones are also predicted to dimerize giving the observed pentacyclic product
Investigation of the magnetic structure and crystal field states of pyrochlore antiferromagnet Nd2Zr2O7
We present synchrotron x-ray diffraction, neutron powder diffraction and
time-of-flight inelastic neutron scattering measurements on the rare earth
pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and
cystal field states. The structural characterization by high-resolution
synchrotron x-ray diffraction confirms that the pyrochlore structure has no
detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals
long range all-in/all-out antiferromagnetic order below T_N ~ 0.4 K with
propagation vector k = (0 0 0) and an ordered moment of 1.26(2) \mu_B/Nd at 0.1
K. The ordered moment is much smaller than the estimated moment of 2.65
\mu_B/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that
the ordering is partially suppressed by quantum fluctuations. The strong Ising
anisotropy is further confirmed by the inelastic neutron scattering data which
reveals a well-isolated dipolar-octupolar type Kramers doublet ground state.
The crystal field level scheme and ground state wavefunction have been
determined.Comment: 12 pages, 15 figures, 2 table
Field Induced Magnetic Ordering and Single-ion Anisotropy in the Quasi-1D Haldane Chain Compound SrNi2V2O8: A Single Crystal investigation
Field-induced magnetic ordering in the Haldane chain compound
SrNiVO and effect of anisotropy have been investigated using
single crystals. Static susceptibility, inelastic neutron scattering,
high-field magnetization, and low temperature heat-capacity studies confirm a
non-magnetic spin-singlet ground state and a gap between the singlet ground
state and triplet excited states. The intra-chain exchange interaction is
estimated to be 0.1 meV. Splitting of the dispersions into two
modes with minimum energies 1.57 and 2.58 meV confirms the existence of
single-ion anisotropy . The value of {\it D} is estimated to be
meV and the easy axis is found to be along the
crystallographic {\it c}-axis. Field-induced magnetic ordering has been found
with two critical fields [0.2 T and
0.5 T at 4.2 K]. Field-induced
three-dimensional magnetic ordering above the critical fields is evident from
the heat-capacity, susceptibility, and high-field magnetization study. The
Phase diagram in the {\it H-T} plane has been obtained from the high-field
magnetization. The observed results are discussed in the light of theoretical
predictions as well as earlier experimental reports on Haldane chain compounds
Spinon Confinement in the One-Dimensional Ising-Like Antiferromagnet SrCo2V2O8
For quasi-one dimensional quantum spin systems theory predicts the occurrence
of a confinement of spinon excitation due to interchain couplings. Here we
investigate the system SrCo2V2O8, a realization of the weakly-coupled
Ising-like XXZ antiferromagnetic chains, by terahertz spectroscopy with and
without applied magnetic field. At low temperatures a series of excitations is
observed, which split in a Zeeman-like fashion in an applied magnetic field.
These magnetic excitations are identified as the theoretically predicted
spinon-pair excitations. Using a one dimensional Schr\"odinger equation with a
linear confinement potential imposed by weak interchain couplings, the
hierarchy of the confined spinons can be fully described.Comment: 4 pages, 3 figure
Collaborative Epistemic Discourse in Classroom Information Seeking Tasks
We discuss the relationship between information seeking, and epistemic beliefs â beliefs about the source, structure, complexity, and stability of knowledge â in the context of collaborative information seeking discourses. We further suggest that both information seeking, and epistemic cognition research agendas have suffered from a lack of attention to how information seeking as a collaborative activity is mediated by talk between partners â an area we seek to address in this paper. A small-scale observational study using sociocultural discourse analysis was conducted with eight eleven year old pupils who carried out search engine tasks in small groups. Qualitative and quantitative analysis were performed on their discussions using sociocultural discourse analytic techniques. Extracts of the dialogue are reported, informed by concordance analysis and quantitative coding of dialogue duration. We find that 1) discourse which could be characterised as âepistemicâ is identifiable in student talk, 2) that it is possible to identify talk which is more or less productive, and 3) that epistemic talk is associated with positive learning outcomes
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