37 research outputs found
A stable Algebraic Spin Liquid in a Hubbard model
We show the existence of a stable Algebraic Spin Liquid (ASL) phase in a
Hubbard model defined on a honeycomb lattice with spin-dependent hopping that
breaks time-reversal symmetry. The effective spin model is the Kitaev model for
large on-site repulsion. The gaplessness of the emergent Majorana fermions is
protected by the time reversal (TR) invariance of this model. We prove that the
effective spin model is TR invariant in the entire Mott phase thus ensuring the
stability of the ASL. The model can be physically realized in cold atom systems
and we propose experimental signals of the ASL.Comment: Published in PR
Non-Abelian bosonization of the frustrated antiferromagnetic spin-1/2 chain
We study the spin-1/2 chain with nearest neighbor () and
next-nearest neighbor () interactions in the regime , which is equivalent to two chains with a `zig-zag' interaction. In
the continuum limit, this system is described in term of two coupled level-1
WZW field theories. We illustrate its equivalence with four off-critical Ising
models (Majorana fermions). This description is used to investigate the opening
of a gap as a function of and the associated spontaneous breakdown
of parity. We calculate the dynamic spin structure factor near the wavevectors
accessible to the continuum limit. We comment on the nonzero string order
parameter and show the presence of a hidden
symmetry via a nonlocal transformation on the microscopic Hamiltonian. For a
ferromagnetic interchain coupling, the model is conjectured to be critical,
with different velocities for the spin singlet and spin triplet excitations.Comment: 20 pages, RevTeX, 1 postscript figure. Minor corrections added,
resulting in different velocity renormalizations; no qualitative change in
conclusion
Strong-Coupling Expansion for the Hubbard Model
A strong-coupling expansion for models of correlated electrons in any
dimension is presented. The method is applied to the Hubbard model in
dimensions and compared with numerical results in . Third order expansion
of the Green function suffices to exhibit both the Mott metal-insulator
transition and a low-temperature regime where antiferromagnetic correlations
are strong. It is predicted that some of the weak photoemission signals
observed in one-dimensional systems such as should become stronger as
temperature increases away from the spin-charge separated state.Comment: 4 pages, RevTex, 3 epsf figures include
Primary homework in England: the beliefs and practices of teachers in primary schools
This study examines teachers’ views about and practices in homework in primary schools, based on questionnaire data from 235 primary teachers and 19 in-depth interviews. Findings suggest that teachers prioritise contradictory goals and act in ways that support only some of these. Reading with parents is a universal form of homework and other homework focuses either on English or mathematics or takes a project-led approach. Integration of homework into class learning is problematic. Teachers are concerned about the possible effects of homework on educational inequality and questions are raised about teachers’ perceptions of homework as a signifier of good parenting
Sensitized near-infrared lanthanide luminescence from Nd(III)- and Yb(III)-based cyclen-ruthenium coordination conjugates.
The development of novel mixed lanthanide-transition-metal (f-d) based supramolecular self-assemblies made from neodymium- and ytterbium-based tetraamide-functionalized cyclen complexes bearing a single 1,10-phenanthroline moiety coordinating to a RuII(bipy)2 (bipy = bipyridine) unit is described. Excitation of the Ru(II) metal-to-ligand charge-transfer band in water gave rise to long-wavelength sensitized emission from the Yb(III) or Nd(III) centers, observed in the near-infrared
Combining organic photochromism with inorganic paramagnetism—optical tuning of the iron(II) electronic structure
The crystalline photochromism of a diarylethene pyridyl ligand is applied to the modulation of the electronic environment of a high-spin FeII metal ion.
Synthesis, structural studies, theoretical calculations, and linear and nonlinear optical properties of terpyridyl lanthanide complexes: New evidence for the contribution of f electrons to the NLO activity
The synthesis and structural, photophysical, and second-order nonlinear optical (NLO) properties of a novel lanthanide terpyridyl-like complex family LLn(NO3)3 (Ln = La, Gd, Dy, Yb, and Y) are reported. The isostructural character of this series in solution and in the solid state has been established on the basis of X-ray diffraction analysis in the cases of yttrium and gadolinium complexes, theoretical optimization of geometry (DFT), and NMR spectroscopy. The absorption, emission, and solvatochromic properties of the free terpyridyl-like ligand L were thourougly investigated, and the twist intramolecular charge transfer (TICT) character of the lowest energy transition was confirmed by theoretical calculation (TDDFT and CIS). The similar ionochromic effect of the different lanthanide ions was evidenced by the similar UV−visible spectra of the complete family of complexes. On the other hand, the quadratic hyperpolarizability coefficient β, measured by the harmonic light scattering (HLS) technique, is clearly dependent on the nature of the metal, and a careful examination of the particular case of yttrium unambiguously confirms the contribution of metal f electrons to the NLO activity
CCDC 614028: Experimental Crystal Structure Determination
Related Article: K.Senechal-David, N.Zaman, M.Walko, E.Halza, E.Riviere, R.Guillot, B.L.Feringa, M.-L.Boillot|2008|Dalton Trans.||1932|doi:10.1039/b715017b,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.